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Anterior cruciate ligament injury
Anterior cruciate ligament injury
Anterior cruciate ligament injury
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Anterior Cruciate Ligament injury
Diagram of the right knee
SpecialtyOrthopedics
SymptomsAudible "crack" with pain, knee instability, swelling of knee[1]
CausesNon-contact injury, contact injury[2]
Risk factorsAthletes, females[1]
Diagnostic methodPhysical exam, MRI[1]
PreventionNeuromuscular training,[3] core strengthening[4]
TreatmentBraces, physical therapy, surgery[1]
Frequencyc. 200,000 per year (US)[2]

An anterior cruciate ligament injury occurs when the anterior cruciate ligament (ACL) is either stretched, partially torn, or completely torn.[1] The most common injury is a complete tear.[1] Symptoms include pain, an audible cracking sound during injury, instability of the knee, and joint swelling.[1] Swelling generally appears within a couple of hours.[2] In approximately 50% of cases, other structures of the knee such as surrounding ligaments, cartilage, or meniscus are damaged.[1]

The underlying mechanism often involves a rapid change in direction, sudden stop, landing after a jump, or direct contact to the knee.[1] It is more common in athletes, particularly those who participate in alpine skiing, football (soccer), netball, American football, or basketball.[1][5] Diagnosis is typically made by physical examination and is sometimes supported and confirmed by magnetic resonance imaging (MRI).[1] Physical examination will often show tenderness around the knee joint, reduced range of motion of the knee, and increased looseness of the joint.[2]

Prevention is by neuromuscular training and core strengthening.[3][4] Treatment recommendations depend on desired level of activity.[1] In those with low levels of future activity, nonsurgical management including bracing and physiotherapy may be sufficient.[1] In those with high activity levels, surgical repair via arthroscopic anterior cruciate ligament reconstruction is often recommended.[1] This involves replacement with a tendon taken from another area of the body or from a cadaver.[2] Following surgery rehabilitation involves slowly expanding the range of motion of the joint, and strengthening the muscles around the knee.[1] Surgery, if recommended, is generally not performed until the initial inflammation from the injury has resolved.[1] It should also be taken into precaution to build up as much strength in the muscle that the tendon is being taken from to reduce risk of injury[clarification needed].

About 200,000 people are affected per year in the United States.[2] In some sports, women have a higher risk of ACL injury, while in others, both sexes are equally affected.[5][6][7] While adults with a complete tear have a higher rate of later knee osteoarthritis, treatment strategy does not appear to change this risk.[8] ACL tears can also occur in some animals, including dogs.

Signs and symptoms

[edit]

When an individual has an ACL injury, they are likely to hear a "pop" in their knee followed by pain and swelling. They may also experience instability in the knee once they resume walking and other activities, as the ligament can no longer stabilize the knee joint and keep the tibia from sliding forward.[9]

Reduced range of motion of the knee and tenderness along the joint line are also common signs of an acute ACL injury. The pain and swelling may resolve on its own; however, the knee will remain unstable and returning to sport without treatment may result in further damage to the knee.[1]

Causes

[edit]
ACL tear

Tearing occurs when the tibia moves too far forwards or the femur moves too far backwards.[10] Causes may include:

  • Changing direction rapidly (also known as "cutting")
  • Landing from a jump awkwardly
  • Coming to a sudden stop when running
  • A direct contact or collision to the knee (e.g. during a football tackle or a motor vehicle collision)[1]

These movements cause the tibia to shift away from the femur rapidly, placing strain on the knee joint and potentially leading to rupture of the ACL. About 80% of ACL injuries occur without direct trauma.[11] Risk factors include female anatomy, specific sports, poor conditioning, fatigue, and playing on a turf field.[9]

Injuries to the ACL are common, 250,000 ACL injuries occur on an annual basis. This corresponds to a 1 in 3,000 chance of an individual sustaining an ACL injury. Ligaments in the ACL or meniscus are usually torn with an external force being applied to the knee joint. The ACL can be torn without an external force being applied[12]

Female predominance

[edit]

Female athletes are two to eight times more likely to strain their ACL in sports that involve cutting and jumping as compared to men who play the same particular sports.[13] NCAA data has found relative rates of injury per 1000 athlete exposures as follows:[citation needed]

  • Men's basketball 0.07, women's basketball 0.23
  • Men's lacrosse 0.12, women's lacrosse 0.17
  • Men's football 0.09, women's football 0.28

The highest rate of ACL injury in women occurred in gymnastics, with a rate of injury per 1000 athlete exposures of 0.33. Of the four sports with the highest ACL injury rates, three were women's – gymnastics, basketball and soccer.[14]

Differences between males and females identified as potential causes are the active muscular protection of the knee joint, differences in leg/pelvis alignment, and relative ligament laxity caused by differences in hormonal activity from estrogen and relaxin.[13][15] Birth control pills also appear to decrease the risk of ACL injury.[16]

Dominance theories

[edit]
Femur with Q angle: the angle formed by a line drawn from the anterior superior iliac spine through the center of the patella and a line drawn from the center of the patella to the center of the tibial tubercle

Some studies have suggested that there are four neuromuscular imbalances that predispose women to higher incidence of ACL injury. Female athletes are more likely to jump and land with their knees relatively straight and collapsing in towards each other, while most of their bodyweight falls on a single foot and their upper body tilts to one side.[17] Several theories have been described to further explain these imbalances. These include the ligament dominance, quadriceps dominance, leg dominance, and trunk dominance theories.[citation needed]

The ligament dominance theory suggests that when females athletes land after a jump, their muscles do not sufficiently absorb the impact of the ground. As a result, the ligaments of the knee must absorb the force, leading to a higher risk of injury.[18] Quadriceps dominance refers to a tendency of female athletes to preferentially use the quadriceps muscles to stabilize the knee joint.[18] Given that the quadriceps muscles work to pull the tibia forward, an overpowering contraction of the quadriceps can place strain on the ACL, increasing risk of injury.[citation needed]

Leg dominance describes the observation that women tend to place more weight on one leg than another.[19] Finally, trunk dominance suggests that males typically exhibit greater control of the trunk in performance situations as evidenced by greater activation of the internal oblique muscle.[18] Female athletes are more likely to land with their upper body tilted to one side and more weight on one leg than the other, therefore placing greater rotational force on their knees.[20]

Governments and healthcare professionals acknowledge the high incidence of ACL injuries and have dedicated significant research efforts to prevention and rehabilitation. Studies have demonstrated the effectiveness of diverse training methods, such as balance, plyometric, resistance, and technique training, in reducing ACL injury risk among adolescent females. However, evidence supporting this approach for adult sport-active populations, both male and female, is limited. Two underdeveloped areas are the specificity of exercises used in interventions and the consideration of athletes' experiences, including adherence and motivation. Therefore, there is a need for injury prevention researchers to optimize training content and delivery methods to better translate research findings for diverse sport populations of varying ages and genders.[21]

Hormonal and anatomic differences

[edit]

Before puberty, there is no observed difference in frequency of ACL tears between the sexes. Changes in sex hormone levels, specifically elevated levels of estrogen and relaxin in females during the menstrual cycle, have been hypothesized as causing predisposition of ACL ruptures. This is because they may increase joint laxity and extensibility of the soft tissues surrounding the knee joint.[13] Ongoing research has observed a greater occurrence of ACL injuries in females during ovulation and fewer injuries during the follicular and luteal phases of the menstrual cycle.[22]

Study results have shown that female collegiate athletes with concentration levels of relaxin that are greater than 6.0 pg/mL are at four times higher risk of an ACL tear than those with lower concentrations.[23]

Relaxin is increased when estrogen levels are increased during the female menstrual cycle.[24] Estrogen is at its peak just before ovulation, which makes it fall into the follicular phase of the mensural cycle. These hormonal rises often fall within a 2-3 day period of an increase in knee laxity. These times of increased estrogen concentration are subsequently related to times of reduced tendon strength and stability.[25] Additionally, the use of hormonal medication such as birth control may fluctuate the window and level of laxity which should be monitored. The NHI, found that the use of hormonal oral contraceptives reduced the risk of tearing by 68%.[26] Moreover, women are more likely to face an ACL tear or injury when they are experiencing elevated levels of estrogen or progesterone. During these times, females don't necessarily need to limit their activity levels out of fear or precaution but it is beneficial if they participate in the proper warm-ups or strengthening exercises to limit potential risks.

Additionally, female pelvises widen during puberty through the influence of sex hormones. This wider pelvis requires the femur to angle toward the knees. This angle towards the knee is referred to as the Q angle. The average Q angle for men is 14 degrees and the average for women is 17 degrees. Steps can be taken to reduce this Q angle, such as using orthotics.[27] The relatively wider female hip and widened Q angle may lead to an increased likelihood of ACL tears in women.[28]

ACL, muscular stiffness, and strength

[edit]

During puberty, sex hormones also affect the remodeled shape of soft tissues throughout the body. The tissue remodeling results in female ACLs that are smaller and will fail (i.e. tear) at lower loading forces, and differences in ligament and muscular stiffness between men and women. Women's knees are less stiff than men's during muscle activation. Force applied to a less stiff knee is more likely to result in Aitsears.[29]

In addition, the quadriceps femoris muscle is an antagonist to the ACL. According to a study undertaken on female athletes at the University of Michigan, 31% of female athletes recruited the quadriceps femoris muscle first as compared to 17% in males. Because of the elevated contraction of the quadriceps femoris muscle during physical activity, an increased strain is placed onto the ACL due to the "tibial translation anteriorly".[30]

Pathophysiology

[edit]

The knee joint is formed by three bones: the femur (thighbone), the tibia (shinbone), and the patella (kneecap). These bones are held together by ligaments, which are strong bands of tissue that keep the joint stable while an individual is walking, running, jumping, etc. There are two types of ligaments in the knee: the collateral ligaments and the cruciate ligaments.[citation needed]

The collateral ligaments include the medial collateral ligament (along the inside of the knee) and the lateral or fibular collateral ligament (along the outside of the knee). These two ligaments function to limit sideways movement of the knee.[2]

The cruciate ligaments form an "X" inside the knee joint with the anterior cruciate ligament running from the front of the tibia to the back of the femur, and the posterior cruciate ligament running from the back of the tibia to the front of the femur. The anterior cruciate ligament prevents the tibia from sliding out in front of the femur and provides rotational stability.[2]

There are also two C-shaped structures made of cartilage called the medial meniscus and lateral meniscus that sit on top of the tibia in the knee joint and serve as cushion for the bones.[1] 

Right knee, front, showing interior ligaments Left knee, behind, showing interior ligaments

Diagnosis

[edit]

Manual tests

[edit]

Most ACL injuries can be diagnosed by examining the knee and comparing it to the other, non-injured knee. When a doctor suspects ACL injury in a person who reports a popping sound in the knee followed by swelling, pain, and instability of the knee joint, they can perform several tests to evaluate the damage to the knee. These tests include the pivot-shift test, anterior drawer test, and Lachman test. The pivot-shift test involves flexing the knee while holding onto the ankle and slightly rotating the tibia inwards.[31] In the anterior drawer test, the examiner flexes the knees to 90 degrees, sits on the person's feet, and gently pulls the tibia towards themself.[32] The Lachman test is performed by placing one hand on the person's thigh and the other on the tibia and pulling the tibia forward.[33] These tests are meant to test whether the ACL is intact and therefore able to limit the forward motion of the tibia. The Lachman test is recognized by most authorities as the most reliable and sensitive of the three.[34]

Technological innovations like stop-action photography, force platforms, and programmable computers have propelled biomechanics into a key research area within the human sciences. Advances in motion capture, musculoskeletal modeling, and human simulation have deepened our understanding of the mechanical causes of musculoskeletal injuries and diseases. However, measuring force at joint, muscle, tendon, and articular surfaces, especially in the knee, is complex and relies heavily on intricate modeling of motion capture and medical imaging data. This complexity has limited the involvement of biomechanists in designing, implementing, and evaluating prophylactic training interventions and neuromuscular rehabilitation programs.[35]

Medical imaging

[edit]
Anterior cruciate ligament tear seen on MRI. T1 left, right PDW.

Though clinical examination in experienced hands can be accurate, the diagnosis is usually confirmed by magnetic resonance imaging, which provides images of the soft tissues like ligaments and cartilage around the knee.[1] It may also permit visualization of other structures which may have been coincidentally involved, such as the menisci or collateral ligaments.[36] An x-ray may be performed in addition to evaluate whether one of the bones in the knee joint was broken during the injury.[9]

MRI is perhaps the most used technique for diagnosing the state of the ACL, but it is not always the most reliable technique as the ACL can seem healed on chronic cases with the proliferation of synovial scar tissue when treated conservatively.[37]

MRI is particularly useful in cases of partial tear of the ACL. The anteromedial band is most commonly injured compared to the posterolateral band.[38]

Arthrometers/Laximeters

[edit]

Another form of evaluation that may be used in case physical examination and MRI are inconclusive is laximetry testing (i.e. arthrometry and stress imaging), which involve applying a force to the leg and quantifying the resulting displacement of the knee.[39] These medical devices basically replicate manual tests but offer objective assessments.[40] The GNRB arthrometer, for example, is a knee arthrometer that is considered more effective than the Lachman test.[41]

Classification

[edit]

An injury to a ligament is called a sprain. The American Academy of Orthopedic Surgeons defines ACL injury in terms of severity and classifies them as Grade 1, 2, or 3 sprains.[1] Grade 1 sprains occur when the ligament is stretched slightly but the stability of the knee joint is not affected. Grade 2 sprains occur when the ligament is stretched to the point that it becomes loose; this is also referred to as a partial tear. Grade 3 sprains occur when the ligament is completely torn into two pieces, and the knee joint is no longer stable. This is the most common type of ACL injury.[citation needed]

Around half of ACL injuries occur in conjunction with injury to other structures in the knee, including the other ligaments, menisci, or cartilage on the surface of the bones. A specific pattern of injury called the "unhappy triad" (also known as the "terrible triad," or "O'Donoghue's triad") involves injury to the ACL, MCL, and medial meniscus, and occurs when a lateral force is applied to the knee while the foot is fixed on the ground.[42]

Prevention

[edit]

Interest in reducing non-contact ACL injury has been intense. The International Olympic Committee, after a comprehensive review of preventive strategies, has stated that injury prevention programs have a measurable effect on reducing injuries.[43] These programs are especially important in female athletes who bear higher incidence of ACL injury than male athletes, and also in children and adolescents who are at high risk for a second ACL tear.[44][45]

Researchers have found that female athletes often land with the knees relatively straight and collapsing inwards towards each other, with most of their bodyweight on a single foot and their upper body tilting to one side; these four factors put excessive strain on the ligaments on the knee and thus increase the likelihood of ACL tear.[46][18] There is evidence that engaging in neuromuscular training (NMT), which focus on hamstring strengthening, balance, and overall stability to reduce risk of injury by enhancing movement patterns during high risk movements. Such programs are beneficial for all athletes, particularly adolescent female athletes.[47][20]

Injury prevention programs (IPPs), are reliable in reducing the risk factors of ACL inquiries, referring to dominance theories. The ligament dominance theory reduced peak knee abduction moment but should be more focused on prioritizing individualized, task-specific exercises focusing on an athlete's risk profile.[48] It is more beneficial than a generic program. There is an increase in hip and knee flexion angles, such as plyometrics and jump-landing tasks, which reduces the risk of quadriceps dominance. However, there were no changes found for peak vGRF (vertical ground reaction force), which measures for "softer" landings. Unfortunately, there was no conclusive data on how IPPs reduces the risk associated with leg dominance theory.[48]

One effective strategy to lower ACL injury risk is to enhance tissue strength, thereby improving its ability to withstand greater loads. Studies have demonstrated that exercise can stimulate collagen regeneration in medial collateral ligamentous tissues of rabbits and ACL tissues of Rhesus monkeys, restoring them to 79% of healthy tissue strength after a period of immobilization. Surprisingly, there is a lack of published peer-reviewed studies showing that training can significantly increase strength in healthy ACL tissues through collagen regeneration. Moreover, research indicates that collagen concentration and ligament force tolerance in healthy ACL tissues decrease with age, highlighting the importance of reducing ACL loads. This can be achieved by adjusting athletes' technique during sports activities to lessen external joint loading or by enhancing the strength and activation of knee-supporting muscles when external joint loading is high.[49]

Treatment

[edit]

Treatment for ACL tears is important to:[50]

  • Reduce abnormal knee movements and improve knee function
  • Build trust and confidence to use the knee normally again
  • Prevent further injury to the knee and reduce the risk of osteoarthritis
  • Optimise long-term quality of life following the injury

Nonsurgical

[edit]

Nonsurgical treatment for ACL rupture involves progressive, structured rehabilitation that aims to restore muscle strength, dynamic knee control and psychological confidence. A living systematic review with meta-analysis, updated in 2022, showed on the basis of three randomised controlled trials that primary rehabilitation with optional surgical reconstruction produces outcomes similar to early surgical reconstruction.[51] In some cases the ACL may heal without surgery during the rehabilitation process—the torn pieces re-unite to form a functional ligament.[52]

The purpose of exercise treatment is to restore the normal functioning of the muscular and balance system around the knee. Research has demonstrated that by training the muscles around the knee appropriately through exercise treatment, the body can 'learn' to control the knee again, and despite extra movement inside the knee, the knee can feel strong and able to withstand force.[citation needed]

Typically, this approach involves visiting a physical therapist or sports medicine professional soon after injury to oversee an intensive, structured program of exercises. Other treatments may be used initially, such as hands-on therapies in order to reduce pain. The physiotherapist will act as a coach through rehabilitation, usually by setting goals for recovery and giving feedback on progress.

Non-surgical recovery typically takes three to six months, and depends on the extent of the original injury, pre-existing fitness and commitment to the rehabilitation and sporting goals. Some patients may not be satisfied with the outcome of non-surgical management, and opt for surgery later.[citation needed]

Surgery

[edit]
Main article: Anterior cruciate ligament reconstruction

ACL reconstruction surgery involves replacing the torn ACL with a "graft," which is a tendon taken from another source. Grafts can be taken from the patellar tendon, hamstring tendon, quadriceps tendon from either the person undergoing the procedure ("autograft") or a cadaver ("allograft"). Of the three different kinds of autografts, quadriceps tendon grafts have shown to produce less pain at the site of the harvest when compared to patellar tendon and hamstring tendon grafts. Quadriceps tendon grafts have also been shown to produce better results when it comes to knee stability and function.[53]

The surgery is done with an arthroscope or tiny camera inserted inside the knee, with additional small incisions made around the knee to insert surgical instruments. This method is less invasive and is proven to result in less pain from surgery, less time in the hospital, and quicker recovery times than "open" surgery (in which a long incision is made down the front of the knee and the joint is opened and exposed).[1]

Young athletes or anyone opting for ACL surgery should consider delaying their surgery and completing a 4-6 week prehabilitation program. Although there is no consensus on what rehab should consist of, some of the basic parameters include restoring range of motion, decreasing swelling, and ensuring there is adequate quadriceps strength. Patients that received a 4-6 week prehab program had better outcomes in the acute phases of surgery, while the outcomes 3-6+months out are still inconclusive [54]

The American Academy of Orthopedic Surgeons has stated that there is moderate evidence to support the guideline that ACL reconstruction should occur within five months of injury in order to improve a person's function and protect the knee from further injury; however, additional studies need to be done to determine the best time for surgery and to better understand the effect of timing on clinical outcomes.[55] However, delaying ACL reconstruction in pediatric and adolescent populations for more than 3 months has been shown to increase the risk or meniscus injuries significantly.[56]

There are over 100,000 ACL reconstruction surgeries per year in the United States. Over 95% of ACL reconstructions are performed in the outpatient setting. The most common procedures performed during ACL reconstruction are partial meniscectomy and chondroplasty.[57] Asymmetry in the repaired knee is a possibility and has been found to have a large effect between limbs for peak vertical ground reaction force, peak knee-extension moment, and loading rate during double-limb landings, as well as mean knee-extension moment and knee energy absorption during both double- and single-limb landings. Analysis of joint symmetry along with movement patterns should be a part of return to sports criteria.[58]

Tampa Scale of Kinesiophobia, and a question from the Knee injury and Osteoarthritis Outcome Score quality of life subscale. Results showed that nine athletes sustained a second ACL injury Athletes who experienced a second ACL injury had higher scores on the ACL-RSI and on the risk appraisal questions of the ACL-RSI, and they met RTS criteria sooner than athletes who did not sustain a second ACL injury. After reading, all second ACL injuries occurred in athletes who underwent primary ACL with hamstring tendon autografts.[59]

Rehabilitation

[edit]

The goals of rehabilitation following an ACL injury are to regain knee strength and motion. If an individual with an ACL injury undergoes surgery, the rehabilitation process will first focus on slowly increasing the range of motion of the joint, then on strengthening the surrounding muscles to protect the new ligament and stabilize the knee. Finally, functional training specific to the activities required for certain sports is begun. Delaying return to sport is recommended for at least a minimum of nine months, as retear rates become 7x more likely for those returning prior to 9 months. Additionally, it takes around 2 years for the ACL to mature; however, it is unrealistic to expect athletes to wait two years to return to sports. Another factor to consider is that 30% of retear rates occur within the first 30 athletic exposures and 50% within the first 72 athletic exposures. Lastly, a patient reduces their likelihood of a retear with each month they delay return to sport after the 9 month mark.[60] In the pediatric setting, re-ruptures of the ACL post surgically are prevalent, 94.6% of which require a revision surgery. Without proper rehabilitation, growth or angular deformities can occur, also requiring a revision surgery.[61] Patients need to ensure their physical therapist is experienced with treating ACL patients as many therapists can set their patients up for failure. More than half of physical therapists still utilize manual muscle testing techniques to measure leg strength for return to sports which is subjective and not reliable data.[62] In addition, there is no agreed upon criteria for return to sport however there are considerations a therapist should make before clearing their patient. Patients should be put through battery of tests throughout their rehab to ensure their prepared for the demands of their sport. The tests should include a psychological component, plyometric testing, strength symmetry between both lower limbs, and different functional movement assessments that relate to the patients sport.[63]

There are numerous guidelines regarding ACL rehabilitation recommendations and interventions. A Guideline Development Group (GDG), composed of impartial clinical and methodology experts, was formed and tasked with converting evidence into recommendations. Each member graded proposed recommendations anonymously, and the evidence that produced a high-percentage agreement were published.[64]

Post-operative Strategies:

In 2022, a systematic review was conducted on ACL rehabilitation that refutes the usefulness of post-operative bracing. Despite its frequent use in common practive, bracing does not improve any functional outcomes and may in fact limit mobility unnecessarily. Emphasis should instead be placed on neuromuscular electrical stimulation, which has been shown to enhance muscle activation. Furthermore, it may reduce disuse atrophy, especially in the early recovery phase.[65]

Timing and structure of rehabilitation recommendations:

  1. Pre-operative rehabilitation is strongly recommended in order to improve post-operative quadriceps strength, knee range of motion and may decrease the time to return to sport. At least one pre-operative visit can be helpful to determine if there is sufficient voluntary muscle activation and to educate the patient on the post-operative rehabilitation route. Modal agreement: "strongly agree" (mean: 96.1%)[64]
  2. Unsupervised exercise is recommended for patients recovering from ACL reconstructive surgery. Patients who have reduced access to physical therapy and/or have high motivation and are compliant to perform their rehabilitation are encouraged to exercise independently. Patients who are not well educated on body mechanics should have exercise programs individually prescribed and be monitored to ensure proper execution of rehabilitation protocol to prevent progression with adverse events. Modal agreement: "strongly agree" (mean: 84.7%)[64]
  3. Duration of rehabilitation protocol is specific to an individual's needs and demonstration of their ability to safely return to preinjury level. Accelerated timelines can be used under the right conditions and without adverse events. Modal agreement: "strongly agree" (mean: 97%)[64]

Modalities Recommendations:

  1. Continuous passive motion does not offer additional benefit for pain, range of motion, or swelling when compared to active motion exercises. It is recommended against using this modality because of this, as well as it being time-consuming and costly. Modal agreement: "strongly agree" (mean: 75.5%)[64]
  2. Cryotherapy is inexpensive and easy to use. It also results in a high level of patient satisfaction and rarely has adverse events. The use of this modality is warranted in the early phase of postoperative management. Patients should be educated on how to safely apply cryotherapy to prevent injury. Compressive cryotherapy may be more effective, if available. Modal agreement: "strongly agree" (mean: 97%)[64]
  3. Neuromuscular electrical stimulation (NMES) is strongly recommended in the earliest phase after surgery. This modality will stimulate muscle activation while reducing the expected disuse muscle atrophy. In the early phase, NMES can be used during functional activities to further develop strength. Modal agreement: "strongly agree" (mean: 93.4%)[64]
  4. A low-load blood flow restriction (BFR) is encouraged, along with standardized care, in the early phase of rehabilitation. This modality can improve quadriceps and hamstring strength, especially in individuals experiencing increased knee pain and/or cannot tolerate loads applied to the knee joint. Before a clinician decides to incorporate BFR, they should be aware of contraindications. Modal agreement: "strongly agree" (mean: 92.6%)[64]
  5. Dry Needling is not recommended for early rehabilitation phase, as it can increase risk of hemorrhage. Modal agreement "strongly agree" (mean: 67.6%)[64]
  6. Whole-body vibration is effective as an additional intervention to improve quadriceps strength and static balance. However, there have been reported complications of pain and swelling due to use of this intervention. It is recommended to refrain from using this modality due to the complications and high-cost. Modal agreement "agree" (mean: 83.2%)[64]

Return To Sport

Exercise interventions to help prevent secondary injury in return to sport. Exercise interventions consists of neuromuscular training, strength training, agility drills, and plyometrics. Exercises that are chosen are complementary of risk factors that could cause an ACL injury. Some plyometric and balance exercises that help the return to sport protocol include the triple hops, tuck jumps and box jumps, Nordic hamstrings and squats with hip abduction. Neuromuscular training is also very important in return to sport. Progressive perturbations on unstable surfaces that test unilateral and bilateral stance.[66]

A program of five phases of rehab was created to recover and return to sport post ACLR. Phase one consisted of ROM and mobility, diminish pain and swelling, and strengthen the quadriceps muscles. Phase two is jogging on the treadmill. Phase three started agility drills in different body planes, fast bursting movements. Phase 4 starts plyometrics, jumping with both feet. Finally phase five progresses the bilateral jumps to unilateral leg hops. In the later stages the rehabilitation program should be tailored towards the primary sport the athlete is trying to return to.[66]

In ACL-SPORTS programs an athlete must pass a predetermined return-to-sport test. The requirements are limb symmetry, there good sided leg strength is symmetrical to the ACLR side, of 90% in the quadriceps strength test, 90% in each of the four single-legged hop tests, 90% on the Knee Outcomes Survey-Activities of Daily Living scale (KOS-ADLs), and in general rating of self-perceived knee function. All athletes should achieve greater than 90% limb symmetry of good sided 1RM.[66]

Psychological readiness is a crucial factor in determining when an athlete can safely return to sport following ACL reconstruction. Research suggests that objective and subjective psychological assessments should be incorporated into rehabilitation protocols, as athletes with higher psychological readiness scores have lower re-injury rates (Glattke et al., 2022). Additionally, high-intensity plyometric training has been found to be ineffective in ACL recovery and may not contribute to improved functional outcomes (Glattke et al., 2022).[65]

Prognosis

[edit]

The prognosis of ACL injury is generally good, with many people regaining function of the injured leg within months.[2] ACL injury used to be a career-ending injury for competitive athletes; however, in recent years ACL reconstruction surgery followed by physical therapy has allowed many athletes to return to their pre-injury level of performance.[67]

Long term complications of ACL injury include early onset arthritis of the knee and/or re-tearing the ligament. Factors that increase risk of arthritis include severity of the initial injury, injury to other structures in the knee, and level of activity following treatment.[9] Not repairing tears to the ACL can sometimes cause damage to the cartilage inside the knee because with the torn ACL, the tibia and femur bone are more likely to rub against each other.[1] Nevertheless, ACL tears alone increase inflammatory markers in the knee which can have influence on the development of osteoarthritis.[68]

Young female athletes have a significant risk of re-tearing an ACL graft, or tearing the ACL on the other knee after their recovery. This risk has been recorded as being nearly 1 out of every 4 young athletes.[69] Therefore, athletes should be screened for any neuromuscular deficit (i.e. weakness greater in one leg than another, or incorrect landing form) before returning to sport.[17]

Epidemiology

[edit]

There are around 200,000 ACL tears each year in the United States. ACL tears newly occur in about 69 per 100,000 per year with rates in males of 82 per 100,000 and females of 59 per 100,000.[70] When breaking down rates based on age and sex, females between the ages of 14 and 18 had the highest rates of injury with 227.6 per 100,000. Males between the ages of 19 and 24 had the highest rates of injury with 241 per 100,000.[70]

Sports

[edit]

Rates of re-rupture among college athletes were highest in male football players with 15 per 10,000, followed by female gymnasts with 8 per 10,000 and female soccer players with 5.2 per 10,000.[71]

High school athletes are at increased risk for ACL tears when compared to non-athletes. Among high school girls in the US, the sport with the highest risk of ACL tear is soccer, followed by basketball and lacrosse. In the US women's basketball and soccer experience the most ACL tears then all other sports.[72] The highest risk sport for high school boys in the US was basketball, followed by lacrosse and soccer.[73] In basketball, women are 5-8 times more likely to experience an ACL tear than men.[72]

Dogs

[edit]

Cruciate ligament rupture is a common orthopedic disorder in dogs. A study of insurance data showed the majority of the breeds with increased risk of cruciate ligament rupture were large or giant.[74]

References

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Anterior cruciate ligament injury

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The anterior cruciate ligament (ACL) injury is the over-stretching or tearing of the , a vital band of fibrous tissue that connects the thighbone () to the shinbone () and stabilizes the by preventing excessive forward translation and of the . This injury is one of the most common knee traumas, particularly in involving sudden stops, pivots, jumps, or directional changes, such as soccer, , football, and . Partial tears account for 10–27% of ACL injuries, with complete ruptures being more common and often compromising knee stability. ACL injuries affect approximately 200,000 individuals annually in the United States, with an incidence rate of about 68.6 cases per 100,000 person-years, making it a significant concern in both professional and recreational athletics. Women face a higher risk, attributed to factors like anatomical differences (e.g., wider , narrower femoral notch), hormonal influences, and neuromuscular control variations, with female athletes experiencing ACL tears at rates 2–8 times higher than males in comparable sports. Other risk factors include increased , prior ACL reconstruction, familial predisposition, decreased strength relative to , and environmental elements like poor shoe-surface traction or adverse weather. Non-contact mechanisms, such as deceleration or cutting maneuvers, hyperextension, or awkward landings from low heights with the knee extended (for example, jumping off a bed), account for roughly 70% of cases, while direct contact or twisting forces cause the remainder. Symptoms typically manifest immediately or within hours, including a characteristic loud "pop" sensation, intense , rapid swelling due to hemarthrosis ( into the ), instability or "giving way" of the , and limited . Swelling often peaks within 24 hours, and untreated injuries can lead to complications like meniscus tears, cartilage damage, or early . involves clinical tests (e.g., Lachman or pivot-shift maneuvers), imaging such as MRI to confirm the tear's extent (graded I–III based on fiber disruption), and sometimes . Treatment begins conservatively with the protocol (rest, ice, compression, elevation) to manage acute symptoms, followed by physical therapy to restore strength, stability, and proprioception. In patients with chronic ACL deficiency managed non-surgically, rehabilitation particularly emphasizes neuromuscular training and balance exercises to improve proprioception, neuromuscular control, and dynamic knee stability, thereby compensating for ligament laxity and reducing episodes of instability. For complete tears or active individuals, surgical reconstruction using autografts (e.g., from or ) or allografts is common, performed arthroscopically to replace the ligament and restore function, with rehabilitation spanning 6–12 months for return to sport. Outcomes vary, with meta-analyses indicating that approximately 65% of patients return to their pre-injury level of sport, though re-injury risk persists at 5–15%. Prevention strategies, including neuromuscular training programs like FIFA 11+, have shown to reduce incidence by up to 50% in high-risk groups.

Anatomy and Function

Ligament Structure

The anterior cruciate ligament (ACL) is an intra-articular structure within the knee joint, originating from the posteromedial aspect of the lateral femoral condyle and inserting onto the anterior aspect of the tibial plateau in the intercondylar region. It forms a twisted, rope-like band that measures approximately 27 to 38 mm in length and 10 to 12 mm in width, with its obliquity contributing to its overall configuration. The ligament is divided into two primary bundles: the anteromedial bundle, which arises more anteriorly and inferiorly on the femur and inserts anteriorly on the tibia, and the posterolateral bundle, which originates more posteriorly and superiorly on the femur and inserts more posteriorly on the tibia; these bundles exhibit distinct insertion footprints, with the posterolateral bundle often predominant and the anteromedial bundle providing complementary coverage. Histologically, the ACL is composed predominantly of densely packed fibers, accounting for about 90% of its , alongside approximately 10% type III collagen, minor components of , and proteoglycans that support its tensile strength and hydration. These collagen fibers are organized in a hierarchical manner, forming fascicles encased in synovium-like tissue that envelops the ligament, enhancing its intra-articular adaptation. The limited vascularity of the ACL arises primarily from branches of the middle genicular artery, which penetrate the synovial covering to supply the ligament substance, though this sparse perfusion—confined mostly to the periphery—underlies its notoriously poor regenerative capacity following injury. Innervation of the ACL is provided mainly by posterior articular branches of the , incorporating both myelinated and unmyelinated fibers that terminate in free nerve endings for and vascular regulation, as well as specialized mechanoreceptors distributed throughout the . These mechanoreceptors include Ruffini endings that detect ligament stretch, Pacinian corpuscles responsive to rapid acceleration or , and Golgi-like tendon organs that monitor tension, collectively facilitating proprioceptive input to the and contributing to pain signaling pathways.

Biomechanical Role

The anterior cruciate ligament (ACL) serves as the primary restraint to anterior tibial translation relative to the , providing approximately 87% of the restraining force against anterior displacement at 30° of flexion and 85% at 90° of flexion. This function is critical for maintaining stability during dynamic activities, as the ligament's oblique orientation allows it to counteract shear forces that would otherwise displace the forward. In addition, the ACL acts as a secondary restraint to internal tibial , contributing significantly to rotational stability, while playing a minor role in limiting external rotation and varus-valgus angulation. These roles are enhanced through interactions with other stabilizers, such as the (MCL), which provides supplementary resistance to anteromedial laxity, and the lateral collateral ligament (LCL), ensuring multi-planar joint integrity. The ACL also limits knee hyperextension by tautening in terminal extension, preventing excessive posterior femoral rollback and forward tibial migration. Composed primarily of fibers arranged in a hierarchical structure that enables elastic deformation, the ligament exhibits bundle-specific functions: the anteromedial bundle remains taut during knee flexion to primarily resist anterior , while the posterolateral bundle tightens in extension to control and hyperextension. Under load, the ACL can withstand tensile forces up to approximately 2,000 N before , with ultimate strengths reported between 600 and 2,300 N in cadaveric studies, reflecting its capacity to absorb high-impact stresses during activities. Beyond mechanical restraint, the ACL contributes to proprioceptive feedback through mechanoreceptors embedded within its substance, including Ruffini endings for static position , Pacinian corpuscles for dynamic motion detection, and Golgi tendon organ-like structures for tension monitoring. These neural elements facilitate neuromuscular control by relaying position and load information to the , aiding in reflexive stabilization and preventing excessive motion. Disruption of these proprioceptive pathways can impair overall sensorimotor function, underscoring the ligament's integrated role in both passive and active stability mechanisms.

Signs and Symptoms

Acute Presentation

Upon sustaining an (ACL) injury, many patients report an audible or felt "pop" or snap at the moment of injury. This sensation arises from the sudden rupture of the ligament fibers and is often immediately followed by intense pain that can prevent continuation of the activity. The pain is typically described as sharp and deep within the knee joint, reflecting the acute trauma to the highly innervated ligament. A hallmark feature is rapid onset of swelling due to hemarthrosis, which develops within a few hours in approximately 70% of cases as a result of vascular disruption within the ligament's synovium-rich structure. This bloody causes significant knee distension, contributing to discomfort and functional impairment. The initial severe pain often subsides somewhat after the first few minutes but gives way to a profound of , with patients frequently unable to bear full weight on the affected leg or ambulate normally. Additional acute symptoms include markedly limited , primarily due to the and protective muscle guarding, as well as potential bruising over the from extravasated blood. In cases with concomitant injuries, which occur in 50% or more of ACL tears, early signs such as joint line tenderness may indicate meniscal involvement, while medial or lateral could suggest collateral damage. These features collectively facilitate early recognition of the injury in clinical settings.

Chronic Manifestations

One of the primary chronic manifestations of an untreated (ACL) injury is recurrent , characterized by episodes of the knee "giving way" during pivoting, decelerating, or even routine activities such as walking on uneven surfaces. This arises from the loss of the ACL's role in preventing anterior tibial translation and rotational laxity, leading to abnormal that persist without surgical intervention. Studies indicate that up to 86% of patients with untreated ACL tears experience these giving-way episodes, often prompting activity modification to avoid further episodes. Muscle atrophy, particularly of the femoris, is another common long-term effect, resulting from disuse, protective guarding, and altered neuromuscular activation patterns following the injury. This contributes to persistent quadriceps weakness, with significant reductions in cross-sectional muscle area observed in the affected limb compared to the contralateral side even years post-injury. Consequently, individuals often develop gait alterations, such as reduced flexion during stance phase and increased reliance on compensations, which further exacerbate loading imbalances. Persistent pain in untreated ACL injuries typically manifests as low-grade, intermittent discomfort rather than acute episodes, often worsening with high-impact activities. This pain is frequently linked to ongoing synovitis and secondary soft tissue irritation, with patients reporting daily or activity-related symptoms that impact quality of life. Secondary osteoarthritis (OA) represents a significant long-term consequence, driven by accelerated cartilage wear due to repetitive instability and abnormal shear forces on the joint surfaces. Radiographic evidence of OA, including joint space narrowing and osteophyte formation, appears in approximately 50% of untreated cases by 10-15 years post-injury, with higher rates (up to 87%) in those with concomitant meniscal damage. This early-onset posttraumatic OA typically affects individuals in their 30s to 50s, leading to progressive functional decline and increased disability.

Causes and Risk Factors

Injury Mechanisms

The majority of anterior cruciate ligament (ACL) injuries, approximately 70%, occur through non-contact mechanisms, particularly during dynamic sports activities involving rapid changes in direction. A primary non-contact mechanism is pivoting, where the knee undergoes valgus collapse combined with internal tibial rotation, often with the joint near full extension (0° to 30° flexion); this generates excessive anterior tibial translation and rotational torque on the ACL, commonly observed in sports such as soccer and basketball. Contact injuries, comprising about 30% of cases, typically result from a direct blow to the lateral aspect of the proximal , producing varus or valgus stress and hyperextension; such trauma is frequent in collision sports like , where tackles can force the into abnormal alignment. Hyperextension represents another key mechanism, involving excessive straightening beyond the normal range (typically more than 5°-10°), which stretches the ACL under tensile load; this often occurs in high-velocity falls, as seen in or , and can also occur from landing with the knee extended after jumping from low heights, such as off a bed or low platform, representing a recognized non-contact mechanism though more commonly associated with sports-related awkward landings. Deceleration forces during sudden stops with a planted foot also contribute significantly, creating anterior shear forces on the tibia relative to the femur, exacerbated by quadriceps dominance and insufficient hamstring co-contraction; these are prevalent in cutting maneuvers across various athletic contexts.

Intrinsic Predisposing Factors

Females exhibit a 2- to 8-fold higher risk of anterior cruciate ligament (ACL) injury compared to males, particularly in sports involving pivoting and cutting maneuvers. This disparity is attributed to several anatomical differences, including a narrower intercondylar notch width in the femur, which can impinge on the ACL during knee flexion and increase injury susceptibility. Additionally, females typically have an increased quadriceps angle (Q-angle), resulting from a wider pelvis and altered lower limb alignment, which promotes greater knee valgus moments and anterior tibial translation that strain the ACL. Females also possess a smaller ACL cross-sectional area relative to body size, reducing the ligament's load-bearing capacity and making it more prone to rupture under similar forces experienced by males. Hormonal influences, particularly in females, contribute to ACL vulnerability through cyclic variations in levels across the . Elevated concentrations have been shown to increase ligament laxity by inhibiting synthesis and promoting its degradation, thereby weakening the ACL's structural . These fluctuations can enhance instability during high-demand activities, with studies indicating higher rates during the pre-ovulatory phase when peaks. Relaxin, another hormone elevated in females, synergizes with to further modulate turnover and cross-linking, exacerbating . Muscular imbalances, such as relatively weaker s compared to , represent an intrinsic predisposition that diminishes stability. A lower -to- strength impairs co-contraction of these muscle groups, which is essential for countering anterior shear forces on the and protecting the ACL during dynamic movements. In female athletes, this imbalance has been associated with up to 15% reduced strength relative to males, heightening the risk of non-contact ACL tears by allowing excessive dominance. Elevated body mass index (BMI) is associated with increased ACL injury risk, particularly in females, as higher body mass may impose greater compressive and shear forces on the knee joint. Genetic factors, including polymorphisms in collagen genes, influence ACL tissue quality and injury propensity. Variations in the COL1A1 gene, which encodes the alpha-1 chain of type I collagen—a primary component of the ACL—have been linked to reduced ligament strength and higher rupture rates in susceptible individuals. These genetic markers affect collagen fibril assembly and mechanical properties, predisposing carriers to ACL injuries independent of external loads.

Extrinsic Predisposing Factors

Extrinsic predisposing factors for (ACL) injuries encompass modifiable environmental and behavioral elements that elevate risk, particularly in sports involving rapid directional changes and pivoting. These factors include playing surface characteristics, design, athlete , and inadequate physical conditioning, which can compromise stability during high-demand activities. Unlike intrinsic factors such as anatomical variations, extrinsic elements are alterable through equipment selection, training protocols, and environmental management, offering opportunities for injury mitigation. Playing surfaces significantly influence ACL injury risk due to differences in and energy absorption. , commonly used in soccer and , generates higher rotational and friction compared to natural grass, leading to increased noncontact ACL tears. A study of high school soccer and players found ACL injuries were 23% more likely on artificial turf than on natural grass in football (incidence proportion ratio [IPR], 1.23; 95% CI, 1.03-1.47), with similar increased risks in soccer, particularly among females (IPR up to 1.61; 95% CI 1.14-2.26 for lower extremity injuries). This risk is particularly pronounced in female athletes, where meta-analyses indicate up to a 1.5-fold increase in ACL injuries on synthetic surfaces versus grass. Footwear, especially cleated designs, contributes to ACL vulnerability by altering traction dynamics at the shoe-surface interface. Cleats with long, irregular peripheral studs, such as traditional "edge" designs in , create excessive torsional resistance, promoting "foot lock" where the foot remains planted while the body rotates, straining the ACL. In a three-year prospective study of 3,119 high school football players, edge cleats were associated with a 0.017% ACL injury rate, over three times higher than the 0.005% rate for flat, screw-in, or pivot disk alternatives (P < 0.05). Modern recommendations emphasize with balanced traction to reduce rotational forces, particularly on artificial surfaces where cleat-turf interactions exacerbate injury potential. Fatigue from prolonged activity or high training volume impairs neuromuscular coordination, heightening ACL injury susceptibility in the later stages of games or sessions. As muscle endurance wanes, athletes exhibit altered landing mechanics, such as reduced knee flexion and increased valgus moments, which overload the ligament. In a study of 85 youth athletes (aged 14-18), fatigue induced by a standardized protocol significantly elevated ACL injury risk during drop-jump tests (P = 0.001), with 41% of participants shifting to higher-risk biomechanical profiles post-fatigue. Overuse scenarios, including excessive match volume in team sports, further compound this by promoting cumulative microtrauma and diminished proprioceptive feedback. Inadequate conditioning, characterized by insufficient plyometric or balance training, results in poor neuromuscular control that predisposes individuals to ACL tears during dynamic movements. Athletes lacking targeted strength and agility programs demonstrate deficits in muscle co-activation and joint stability, increasing vulnerability to noncontact mechanisms. Meta-analyses of neuromuscular training interventions reveal that without such conditioning, ACL injury rates can be up to twofold higher, as evidenced by odds ratios of 0.51 for injury reduction with proper plyometric and strengthening exercises (95% CI, 0.37-0.69). This underscores the role of modifiable training deficits in extrinsic risk profiles. Adverse weather conditions, such as cold temperatures or wet surfaces, can elevate ACL injury risk by reducing traction, impairing visibility, or altering ligament extensibility; for example, very cold conditions have been linked to a 1.6-fold increase in risk among skiers.

Pathophysiology

Tear Types and Tissue Response

Anterior cruciate ligament (ACL) injuries can be classified as partial or complete tears. Partial tears involve disruption of only a portion of the ligament fibers, often affecting less than 50% of the cross-sectional area, while complete tears result in full discontinuity of the ligament. Mid-substance ruptures, occurring in the central portion of the ligament away from bony attachments, represent the most common type, accounting for approximately 66% of cases. Avulsion fractures, where the ligament pulls off a bone fragment from either the femoral or tibial insertion, are rare in adults due to the stronger bone-ligament interface compared to children. Following injury, the ACL undergoes a phased tissue response similar to other ligaments, but its healing is inherently limited. The initial phase involves hemorrhage and acute inflammation, typically lasting from days 1 to 7, during which hematoma formation occurs and inflammatory cells such as neutrophils and macrophages infiltrate the site to clear debris. This is followed by the proliferative phase from weeks 2 to 6, characterized by fibroblast migration, angiogenesis, and deposition of disorganized type III collagen scar tissue. The remodeling phase, which can extend for months to years, aims to restore organized type I collagen structure but remains deficient in the ACL due to its relative avascularity, particularly in the intra-articular mid-substance, leading to weak and elongated scar formation rather than functional ligament regeneration, although emerging evidence from recent studies indicates that spontaneous healing with MRI-confirmed ligament continuity can occur in a subset of acute complete ruptures managed non-surgically with rehabilitation, challenging the traditional view of uniformly poor healing potential. Biochemically, the injury triggers a cascade of inflammatory mediators that exacerbate tissue damage. Pro-inflammatory cytokines such as interleukin-1 (IL-1) and tumor necrosis factor-alpha (TNF-α) are released by synovial cells and infiltrating leukocytes, promoting the activation of matrix metalloproteinases (MMPs) that degrade the extracellular matrix components like collagen and proteoglycans. This response, while aimed at clearing damaged tissue, contributes to the ACL's poor intrinsic repair capacity, as the ligament's limited vascular supply restricts nutrient delivery and cell proliferation necessary for effective healing. The ACL consists of two functional bundles—the anteromedial (AM) and posterolateral (PL)—each contributing to knee stability at different flexion angles. Isolated tears of the AM bundle, which is more commonly affected in partial injuries, result in less rotational instability compared to combined AM and PL tears, as the intact PL bundle provides restraint against anterior translation in knee extension. Combined bundle involvement in complete tears leads to greater overall laxity and poorer functional outcomes without intervention.

Associated Knee Injuries

ACL injuries often occur in conjunction with damage to other knee structures due to the traumatic mechanisms involved, such as valgus loading combined with internal tibial rotation, and the resulting instability that alters joint biomechanics. Meniscal tears are among the most common associated injuries, affecting 50-80% of cases depending on acuity; lateral meniscal tears predominate in acute injuries (25-70%), resulting from compressive and shear forces on the lateral compartment during the pivot-shift mechanism, while medial meniscal tears (up to 57%) are more frequent in chronic ACL deficiency due to ongoing abnormal loading. Chondral lesions occur in 15-50% of ACL-injured knees, stemming from direct impact at the time of injury or repetitive aberrant shear stresses post-rupture, which accelerate cartilage degradation and contribute to the development of posttraumatic osteoarthritis. Bone contusions, observed in 80-100% of acute ACL tears on magnetic resonance imaging, typically involve the lateral femoral condyle (60-80%) and posterior lateral tibial plateau (70-96%), reflecting the subluxation and reduction of the tibia relative to the femur during injury. Collateral ligament injuries, particularly of the medial collateral ligament (MCL), are seen in up to 41% of cases, arising from concomitant valgus forces that stretch the medial structures. Less commonly, lateral collateral ligament or posterior cruciate ligament injuries may accompany ACL tears in high-energy scenarios. These associated injuries exacerbate knee instability and complicate recovery if not addressed.

Diagnosis

Clinical History and Examination

Patients with an anterior cruciate ligament (ACL) injury typically present with a history of acute knee trauma, most commonly during sports activities involving pivoting or deceleration. The mechanism is non-contact in approximately 70-80% of cases, such as sudden stops, directional changes, or landing from a jump, often accompanied by an audible "pop" sensation, immediate pain, and rapid onset of swelling due to hemarthrosis within 2 to 12 hours. Contact injuries account for the remaining 20-30%, involving direct blows to the knee. In chronic or recurrent presentations, patients report episodes of knee instability, described as "giving way" or buckling, particularly during activities requiring cutting or twisting maneuvers. The physical examination begins with inspection for visible swelling and deformity, followed by palpation to assess for joint effusion, which is present in over 70% of acute ACL injuries. Effusion is evaluated using the patellar ballotment test, where the patella is compressed against the femur to detect a fluid wave or floating patella, indicating intra-articular hemorrhage. Range of motion is often restricted, with flexion limited to 90 degrees or less due to pain and mechanical blockade from effusion, while extension may be mildly affected. A complete neurovascular assessment is essential, including palpation of the dorsalis pedis and posterior tibial pulses, evaluation of sensation in the lower leg, and checking for capillary refill; these are typically normal in isolated ACL tears but must be documented to rule out associated vascular or nerve injuries. If a tense hemarthrosis causes significant discomfort or limits examination, joint aspiration under sterile conditions is performed to relieve pressure and confirm the presence of blood in the joint. The aspirated fluid appears hemorrhagic, and the detection of fat globules on the surface indicates an associated intra-articular fracture, such as a tibial plateau or osteochondral injury, warranting further evaluation. Functional assessment in the clinic may include simple tests to gauge knee stability and patient confidence, such as the single-leg hop for distance, where asymmetry greater than 10% between limbs suggests ongoing impairment. A simulated pivot-shift maneuver can be attempted gently to elicit apprehension or subluxation, though detailed laxity testing is deferred for specialized evaluation.

Manual Laxity Tests

Manual laxity tests are physical examination maneuvers performed by clinicians to evaluate the integrity of the anterior cruciate ligament (ACL) and overall knee stability, relying on manual application of force to detect abnormal anterior tibial translation or rotational instability. These tests are essential in the initial assessment of suspected ACL injuries, particularly when combined with patient history, and are conducted without imaging equipment. The anterior drawer test, , and pivot-shift test are the primary manual tests used, each targeting different aspects of knee laxity. The anterior drawer test assesses anterior tibial translation relative to the femur. The patient is positioned supine with the hip flexed to 45° and the knee to 90°, while the examiner stabilizes the distal femur with one hand and grasps the proximal calf with the other to apply an anteriorly directed force to the tibia. A positive test is indicated by excessive anterior displacement of the tibia, typically greater than 5 mm compared to the contralateral knee, or a soft or absent endpoint suggesting ACL deficiency. This test has variable diagnostic performance, with reported sensitivity ranging from 9% to 62% across studies, reflecting its lower reliability in acute injuries due to muscle guarding or hemarthrosis. Specificity is generally high, often exceeding 86%. The Lachman test is considered the most sensitive manual test for detecting ACL tears, evaluating both anterior translation and the quality of the endpoint. Performed with the patient supine and the knee flexed to 20-30°, the examiner stabilizes the distal femur with one hand and applies an anterior force to the proximal with the other, while assessing for excessive translation (typically >5 mm) and a firm versus soft or absent endpoint; rotation of the may also be incorporated to enhance detection. Positive findings include increased displacement or a mushy endpoint, with pooled sensitivity estimates of 81-96% and specificity around 81-99%, making it superior to the anterior , especially in chronic or partial tears. Its high sensitivity (up to 95% in some cohorts) stems from testing at near-extension, where secondary stabilizers are less engaged. The dynamically reproduces the rotational instability of an ACL-deficient under combined valgus and internal rotation forces. With the patient , the examiner applies axial load, valgus stress, and internal rotation to the starting from full extension, flexing the to observe for anterior of the lateral tibial plateau that reduces with flexion (typically around 20-40°). The test is graded as follows: Grade I (mild) involves a subtle glide or pivot without full ; Grade II (moderate) shows noticeable with a pivot or clunk upon reduction; Grade III (severe) exhibits gross with a pronounced clunk and potential apprehension. While highly specific (94-99%), its sensitivity is lower at 18-48%, attributed to patient relaxation requirements and examiner technique variability, positioning it as a confirmatory rather than screening test.

Imaging Modalities

Magnetic resonance imaging () serves as the gold standard for diagnosing () injuries, offering high diagnostic accuracy with sensitivity ranging from 86% to 95% and specificity from 93% to 98% in detecting . On sagittal knee MRI (typically proton density or T2-weighted images), the normal ACL appears as a continuous, straight or slightly curved low-signal-intensity band (dark) running obliquely from the lateral femoral condyle to the anterior tibial plateau. The fibers are parallel and uniform, with no internal high signal. A complete tear shows discontinuity of the ligament fibers, absence or non-visualization of the ligament, high-signal-intensity changes within the ligament substance (indicating edema/hemorrhage), or an abnormal wavy/irregular appearance. The ligament may appear retracted, horizontal, or the empty notch sign may be present. Partial tears may show focal high signal without complete discontinuity. T2-weighted sequences are particularly valuable, revealing discontinuity, surrounding , and hyperintense signals indicative of acute , while proton and intermediate-weighted images help assess orientation and partial . MRI also identifies associated damage, such as meniscal or bone contusions, providing comprehensive evaluation without . As of 2025, (AI)-assisted MRI analysis has emerged as an advancement, achieving diagnostic performance comparable to radiologists, with sensitivity around 90% and specificity around 91% for ACL tear detection, aiding in faster and more consistent interpretation. Recent MRI grading systems assess tear features relevant to healing potential, such as continuity and patterns, to guide management decisions. Plain radiography () is typically the initial imaging modality to exclude fractures or bony avulsions in suspected ACL injuries, though it cannot directly visualize the ligament itself. A key indirect sign is the , an avulsion of the lateral tibial plateau, which is highly specific (up to 100% in some series) for ACL rupture due to lateral capsular ligament detachment. Other radiographic findings may include the lateral femoral notch sign or , but these lack sufficient sensitivity for confirming ACL tears alone and necessitate advanced imaging. Computed tomography (CT) is not routine for primary ACL diagnosis but excels in evaluating bony avulsions, such as tibial spine fractures, where it delineates fragment size and displacement with high resolution. In preoperative planning for ACL reconstruction, CT with 3D reconstructions assesses bone tunnel positioning and widening, aiding revision surgeries by quantifying malposition risks. CT arthrography, though invasive and involving contrast and radiation, achieves sensitivity of 84-94% and specificity of 91-98% for ligament tears when MRI is contraindicated. Ultrasound is an emerging, non-invasive option for dynamic assessment of ACL integrity, particularly in resource-limited settings or patients with MRI contraindications like metallic implants, where it evaluates anterior tibial translation during stress maneuvers. Its sensitivity for detecting ACL tears ranges from 70% to 97%, with specificity of 87.5% to 98%, though it is operator-dependent and less effective for chronic or small partial tears due to acoustic shadowing and limited depth penetration.

Injury Classification

Anterior cruciate ligament (ACL) injuries are classified by severity to assess the extent of damage and inform clinical decision-making. The most common system uses Roman numeral grades based on integrity and stability. Grade I represents a mild with microscopic damage to fibers but no significant laxity or . Grade II indicates a partial tear involving a portion of the fibers, resulting in moderate laxity. Grade III denotes a complete rupture with full discontinuity of the , leading to marked anterior tibial translation and potential rotatory . The International Knee Documentation Committee (IKDC) provides a standardized grading for evaluating overall function and laxity post-injury, ranging from A (normal) to D (severely abnormal), based on side-to-side differences in anterior translation. Grade A signifies no detectable abnormality in laxity (≤2 mm side-to-side difference), , or symptoms. Grade B (nearly normal) allows minimal laxity (3-5 mm side-to-side difference). Grade C (abnormal) involves moderate laxity (6-10 mm side-to-side difference), while Grade D (severely abnormal) exceeds 10 mm displacement with gross instability. This integrates subjective patient assessment, symptoms, and objective measures to guide management. Anatomic classification further delineates tear location and structure, influencing surgical approaches. Most ACL tears occur in the mid-substance, comprising the bulk of the body. Proximal avulsions involve detachment from the femoral insertion, while distal avulsions detach from the tibial , often seen in younger patients or high-impact trauma. The ACL consists of two functional bundles—the anteromedial (AM) bundle, which tightens in flexion, and the posterolateral (PL) bundle, taut in extension—allowing for bundle-specific tears where one may remain intact. Partial tears, accounting for 10-27% of ACL injuries, carry distinct prognostic implications compared to complete ruptures. These injuries often demonstrate better potential for conservative due to preserved continuity in remaining fibers, with smaller defects (less than 25% cross-section) showing favorable stability outcomes. In contrast, complete tears typically require intervention to restore function, as spontaneous is rare. MRI can aid in visualizing these grades by demonstrating fiber discontinuity and patterns.

Prevention

Neuromuscular Training Programs

Neuromuscular training programs are structured exercise regimens designed to enhance dynamic joint stability, , and movement patterns, thereby reducing the risk of (ACL) injuries, particularly in high-risk populations such as athletes in pivoting . These programs target modifiable risk factors like poor landing mechanics and neuromuscular imbalances through a combination of strength, plyometric, balance, and exercises, typically integrated into warm-ups or dedicated sessions. from randomized controlled trials and meta-analyses indicates that consistent implementation can lead to significant reductions in non-contact ACL injury rates by promoting safer biomechanical patterns during high-demand activities. As of 2025, the National Athletic Trainers' Association (NATA) position statement continues to recommend neuromuscular training programs, with recent meta-analyses confirming risk reductions of 40-70% in athletes when implemented consistently. The FIFA 11+ program, developed by the Fédération Internationale de Football Association (FIFA), is a widely adopted 20-minute warm-up protocol consisting of running drills, strength exercises, , and balance components performed before training sessions and matches. It includes three progressive levels of difficulty to accommodate different age groups and skill levels, with exercises such as straight-line running, hip strengthening, and single-leg balance on unstable surfaces. When implemented at least twice weekly, the program has been shown to reduce overall lower extremity injuries by approximately 30% in soccer players, with specific evidence of decreased ACL injury incidence in competitive settings. The Prevent Injury and Enhance Performance (PEP) program is a targeted intervention tailored for female athletes, focusing on deceleration techniques, proper landing mechanics, and neuromuscular control to address sex-specific risk factors like increased knee valgus during cutting and jumping. Developed for sports such as soccer and basketball, it features a 20-minute routine performed three times per week, incorporating dynamic warm-ups, strengthening, plyometrics, and agility drills to improve hamstring-quadriceps ratios and reduce anterior tibial translation forces. Clinical trials have demonstrated that the PEP program significantly lowers ACL injury rates in adolescent female soccer players, with one prospective study reporting an 88% relative reduction compared to controls. Key components common to effective neuromuscular training programs include single-leg squats to build eccentric strength and stability, Nordic hamstring curls to enhance activation and prevent excessive anterior shear, and drills such as shuttle runs and cutting maneuvers to refine change-of-direction techniques. These exercises are typically prescribed at a of three sessions per week for at least 12 weeks to allow for neuromuscular adaptations, with progression from basic to advanced variations to maintain engagement and efficacy. Such protocols emphasize technique over volume, ensuring athletes learn to land with increased knee flexion and hip abduction to minimize valgus loading. The efficacy of these programs stems from neuromuscular adaptations that improve and reduce biomechanical risk factors, such as decreasing peak vertical ground reaction forces during landing by up to 20% through enhanced muscle co-activation and joint positioning. Meta-analyses confirm that multifaceted neuromuscular reduces ACL risk by 40-70% in female athletes, with greater benefits observed in programs combining strength and performed consistently over multiple seasons. Long-term adherence is crucial, as intermittent implementation diminishes protective effects, highlighting the need for coach education and integration into team routines.

Equipment and Environmental Modifications

Modifications to equipment and environmental factors play a role in reducing the biomechanical stresses that contribute to (ACL) injuries, particularly in involving rapid pivoting, cutting, and maneuvers. These adjustments aim to optimize traction, absorb impact, and maintain neuromuscular function without compromising performance. Evidence from biomechanical and epidemiological studies supports targeted changes, though outcomes vary by sport and individual factors. Appropriate selection is crucial for minimizing ACL strain during high-risk activities. In pivoting sports like soccer and , shoes with lower coefficients at the shoe-surface interface can reduce rotational on the , thereby lowering injury risk during sudden directional changes. For instance, low-friction outsoles facilitate controlled slipping rather than abrupt locking, which has been associated with decreased ACL loading in sidestep cutting tasks. Conversely, aggressive cleats on surfaces increase shear forces and ACL strain compared to natural grass, where such produces less stress; thus, avoiding multi-directional cleats on synthetic fields is recommended to mitigate this risk. Prophylactic knee braces, worn by athletes without prior injury, offer potential protection by altering knee kinematics during dynamic movements. These devices can reduce valgus moments and excessive varus-valgus motion, stabilizing the against inward knee collapse—a common ACL injury mechanism. Studies indicate that certain braces decrease maximum valgus angles during single-leg landings and cutting, potentially lowering ACL loading, though the magnitude of reduction varies (typically in the range of modest biomechanical alterations rather than dramatic shifts). However, evidence on overall is mixed, with some reviews showing neutral or context-dependent effects depending on impact direction and brace design, and no consistent reduction in incidence rates across populations. Evidence on playing surfaces is mixed; while modern with enhanced shock absorption may reduce peak vertical ground reaction forces, several studies indicate higher ACL injury rates on synthetic surfaces compared to natural grass, attributed to increased shoe-surface traction. This is particularly relevant for sports on synthetic fields, where dynamics can affect loading. Additionally, environmental is essential to prevent muscle that impairs protective reflexes; cold exposure reduces flexor force development and shifts neuromuscular activation patterns, increasing ACL strain risk during eccentric contractions, so warming protocols or avoiding play in sub-optimal conditions (below 10°C) are advised.

Treatment

Nonsurgical Approaches

Nonsurgical approaches to (ACL) injury management are suitable for specific patient profiles, including those with partial tears exhibiting mild or no instability, individuals with low demands such as sedentary lifestyles or non-pivoting , and "copers"—patients who demonstrate the ability to maintain stability and return to pre-injury activities through compensatory mechanisms after initial rehabilitation. These indications are determined based on clinical assessment and injury , prioritizing avoidance of in cases where function can be adequately restored without operative intervention. Functional bracing plays a key role in providing dynamic stability for patients pursuing nonsurgical treatment, particularly during activities and gradual return to . Hinged braces are commonly prescribed to limit excessive anterior translation and while allowing controlled motion, with typical use extending 6 to 12 months to support neuromuscular and reduce reinjury risk during high-demand tasks. Physical therapy forms the cornerstone of nonsurgical management, emphasizing early restoration of knee range of motion to prevent stiffness and progressive strengthening of the hamstrings, quadriceps, and surrounding musculature to compensate for the ligament's role in stability. Protocols typically begin with controlled exercises to reduce swelling and improve proprioception, advancing to perturbation training that enhances hamstring activation for dynamic control, enabling patients to achieve functional knee stability without surgical reconstruction. For patients with chronic ACL deficiency, balance exercises are a key component of rehabilitation, focusing on improving proprioception, neuromuscular control, and dynamic knee stability to compensate for ligament laxity and reduce episodes of instability. Common recommended balance/proprioceptive exercises include single-leg stance (progressed from eyes open to closed, on stable to unstable surfaces such as foam pads or BOSU balls), therapist-applied perturbation training while maintaining balance in various positions, balance board or wobble board exercises, single-leg squats or step-ups on unstable surfaces, and advanced drills like single-limb jumps or plyometrics with emphasis on control. These exercises are typically progressed in later phases of rehabilitation after achieving basic strength and range of motion, with evidence supporting neuromuscular training for improved function in ACL-deficient knees without surgery. Outcomes for nonsurgical approaches vary by patient selection, with studies reporting success rates of 40% to 60% in returning to pre-injury activity levels among suitable candidates, particularly copers and low-demand individuals, though higher rates (over 80%) are observed in middle-aged cohorts with structured rehabilitation. Long-term follow-up indicates acceptable subjective function in approximately 56% of patients at 10 years, underscoring the viability of this approach when is minimal and adherence to therapy is high. Although nonsurgical management is generally not recommended for individuals participating in high-demand pivoting sports such as skiing due to the significant risk of recurrent knee instability, buckling, secondary intra-articular damage (including meniscal tears and articular cartilage lesions), persistent pain, and accelerated development of osteoarthritis, rare exceptions exist among elite athletes. These individuals may achieve sufficient functional stability through exceptional quadriceps and hip strength, advanced neuromuscular control, and supportive bracing, as demonstrated by skier Lindsey Vonn, who returned to downhill training and competition shortly after a complete ACL rupture in 2026. Such cases remain exceptional and are not representative of typical outcomes. Most experts advise against attempting return to high-level pivoting activities without surgical reconstruction, particularly for non-elite athletes, and recommend individualized medical consultation, intensive physical therapy, functional bracing, and careful consideration of surgical options to minimize long-term risks. While the ACL has historically been regarded as having poor capacity for spontaneous healing due to limited intra-articular vascularity, recent research has shown that healing can occur in some cases of acute rupture managed conservatively. A secondary analysis of the KANON trial (Filbay et al., 2022) reported MRI evidence of ACL continuity in 53% (95% CI 36-70%) of participants treated with rehabilitation alone (excluding those who crossed over to delayed reconstruction), with such healing associated with superior patient-reported outcomes in sport/recreation and quality of life compared to non-healed or surgically treated groups. These findings suggest potential for non-surgical healing in select patients, though it remains variable and not the typical outcome; surgical reconstruction continues to be recommended for most active individuals with complete tears to restore stability and function.

Surgical Reconstruction Techniques

Surgical reconstruction of the (ACL) aims to restore stability and function by replacing the torn with a graft, typically performed arthroscopically to minimize invasiveness. The choice of graft and technique depends on patient age, activity level, and expertise, with autografts being the most common for younger, active individuals due to lower failure rates compared to allografts. Common autografts include the bone-patellar tendon-bone (BPTB) and (HT) options, each with distinct advantages and morbidities. The BPTB autograft, harvested from the central third of the ipsilateral patellar tendon with bone blocks from the patella and tibia, provides strong initial fixation and rapid incorporation due to the bony interfaces, making it a traditional gold standard for athletes requiring high stability. However, it is associated with higher donor-site morbidity, including anterior knee pain, patellar tendinopathy, and quadriceps weakness, which can persist in up to 20-30% of patients and delay return to sport. In contrast, the HT autograft, typically using the semitendinosus and gracilis tendons in a four-strand configuration, offers lower anterior knee morbidity and better cosmesis but may result in hamstring weakness, increased tibial tunnel widening, and slightly higher laxity in some cases. Meta-analyses indicate comparable overall clinical outcomes, graft survival, and stability between BPTB and HT at medium- to long-term follow-up, with failure rates around 5-10% for both in appropriately selected patients. Allografts, derived from cadaveric tissue such as or BPTB, are preferred for older patients (>40 years) or those with prior surgeries, as they avoid donor-site pain and allow shorter operative times. These grafts exhibit reduced harvest-related morbidity but carry higher failure rates, estimated at 10-20% overall, due to slower and potential immune responses, particularly in younger cohorts; however, outcomes are similar to autografts in low-demand older patients. The procedure is predominantly arthroscopic, involving femoral and tibial tunnel creation for graft passage and fixation with interference screws or suspensory devices. Single-bundle reconstruction, which approximates the anteromedial bundle, is the most widely used due to its technical simplicity and sufficient restoration of anteroposterior stability in most cases. Double-bundle techniques, replicating both anteromedial and posterolateral bundles, may improve rotational stability and pivot-shift reduction but show no significant differences in subjective outcomes or overall failure rates compared to single-bundle in meta-analyses. Tunnel placement is critical for anatomic restoration; the transtibial portal approach, drilled from the tibial tunnel, often results in a more vertical femoral tunnel position, potentially leading to residual laxity, whereas the anteromedial portal technique allows independent, more anatomic drilling for better footprint coverage and knee kinematics. Timing of surgery influences outcomes, with acute reconstruction within 3 weeks of recommended in select cases without significant swelling or to facilitate better graft integration and reduce secondary meniscal , without increasing postoperative or complications compared to delayed .

Rehabilitation Protocols

Rehabilitation protocols following (ACL) or reconstruction aim to systematically restore function, minimize complications, and enable safe return to activity through structured, evidence-based phases. These programs emphasize patient-specific factors, such as graft type and preoperative conditioning, while prioritizing criteria-based advancement over rigid timelines to optimize outcomes. The acute phase, spanning 0-2 weeks post-intervention, centers on controlling swelling, managing pain, and initiating gentle (ROM) exercises to prevent stiffness. Interventions include , compression, elevation, and low-load activities like heel slides and ankle pumps, with partial encouraged using crutches to promote early without excessive stress on the healing tissues. Neuromuscular electrical stimulation may be introduced to activate the early, addressing common inhibition patterns. In the intermediate phase (2-12 weeks), the focus shifts to rebuilding strength and neuromuscular control through progressive resistance exercises, starting with closed kinetic chain movements such as mini-squats and progressing to open kinetic chain activities like leg presses once ROM goals are met. Balance training on stable surfaces advances to unstable ones, incorporating proprioceptive drills to enhance joint stability. Full is typically achieved by week 4-6, with monitoring for and to guide intensity. The advanced phase (3-6 months) emphasizes agility, power, and sport-specific skills, including , cutting drills, and high-speed running to simulate functional demands. Eccentric strengthening and reactive training are integrated to improve dynamic control, preparing patients for return-to-sport testing. Progression requires symmetrical limb loading and no or swelling during activities. The Cross Bracing Protocol, with 2024 updates incorporating refined immobilization durations, promotes early while using a brace locked at 90 degrees flexion initially to approximate ends and facilitate faster graft integration in select post-surgical cases. This approach, building on initial non-operative applications, allows controlled progression to full extension over 4-12 weeks, supported by supervised rehab to monitor healing via . Criteria-based progression ensures safe advancement, with milestones such as achieving 90% strength symmetry (measured via isokinetic testing) before initiating running, and full clearance for pivoting sports only after demonstrating hop test symmetry greater than 90%. Return to sport is generally permitted at 9-12 months, contingent on psychological readiness and sport-specific functional assessments. The Aspetar guidelines advocate for individualized protocols that integrate neuromuscular retraining from the early phases, using exercises like perturbation training and visual feedback to restore sensorimotor function and reduce re-injury risk. These guidelines, graded as strong recommendations based on moderate-quality , stress ongoing monitoring of movement to address asymmetries.

Emerging Biologic Therapies

Emerging biologic therapies for (ACL) injuries focus on regenerative strategies to promote natural healing and tissue repair, aiming to preserve the native structure and improve long-term outcomes compared to traditional reconstruction methods. These approaches leverage biological agents such as growth factors, cells, and scaffolds to enhance , synthesis, and biomechanical integration, particularly in partial tears or as adjuncts to repair. Recent advancements, driven by clinical trials and meta-analyses from 2024-2025, highlight their potential to reduce recovery time and complications, though many remain investigational or in early human application. The Bridge-Enhanced ACL Repair (BEAR) procedure represents a key advancement in preserving the native ligament through suture repair augmented by a blood-soaked implant scaffold. Approved by the FDA via De Novo classification in 2020 for skeletally mature patients aged 14 and older with complete ACL tears, the BEAR implant facilitates clot formation to bridge the ligament ends, promoting endogenous healing without graft replacement. In March 2025, the FDA expanded its indications to include additional patient populations, broadening clinical applicability. Postcommercialization studies from 2025 report satisfactory patient-reported outcome measures, full range of motion, and knee stability comparable to traditional ACL reconstruction, with lower rates of contralateral tears observed in treated patients. A 6-year follow-up analysis published in 2024 confirmed superior isometric hamstring strength with BEAR versus reconstruction, supporting its role in maintaining native biomechanics. However, in August 2025, the U.S. Food and Drug Administration issued a warning letter to Miach Orthopaedics, the manufacturer, citing deficiencies in manufacturing processes, including sterilization, microbial controls, and device hydration protocols, which could compromise the implant's structural integrity and lead to serious adverse health consequences such as infection or device failure. Platelet-rich plasma (PRP) injections and therapies have emerged as minimally invasive options to stimulate and tissue regeneration in ACL injuries, particularly for partial tears. PRP, derived from autologous blood, delivers concentrated growth factors to the injury site, accelerating early healing phases. A 2025 of PRP in ACL reconstruction demonstrated reduced complications and enhanced graft integration, with some trials showing 20-30% improvements in healing rates for partial lesions compared to controls. approaches, including concentrate and mesenchymal stem cells, further promote repair by differentiating into ligament-like tissue; a 2025 found they improved functional outcomes in non-surgical of tears, outperforming exercise alone in pain reduction and stability. However, a 2024 randomized trial noted no significant functional gains from postoperative PRP in full reconstructions, underscoring variability by injury type and timing. These therapies are often combined for synergistic effects in partial tears, with ongoing 2025 trials evaluating long-term efficacy. Experimental applications of growth factors, such as bone morphogenetic protein-2 (), and target collagen synthesis to bolster ACL repair and reduce re-tear risks. , when delivered via gene-modified mesenchymal stem cells, enhances tendon-bone integration in reconstruction models; a 2025 rabbit study showed accelerated and biomechanical strength, suggesting potential for 15% re-tear reduction in early trials. approaches modify cells to overexpress collagen-promoting factors, addressing the ACL's poor intrinsic healing capacity. A 2024 review highlighted 's role in promoting ligament-bone healing when incorporated into scaffolds, with preclinical data indicating improved deposition. These methods remain in early-phase trials as of 2025, primarily as adjuncts to , with applications limited by delivery challenges and safety profiles. Extracorporeal shockwave therapy (ESWT) and (HA) scaffolds offer adjunctive support by improving vascularity and providing matrix frameworks for ACL regeneration. ESWT applies acoustic waves to stimulate blood flow and cellular activity at the injury site, enhancing post-repair; a 2025 reported better patient-reported outcomes when combined with rehabilitation, including improved scores and function. HA scaffolds, biocompatible hydrogels, mimic the to support and ligament bridging, often augmented with aspirate. Clinical studies from 2025 demonstrate HA's efficacy in reducing early postoperative and restoring after ACL procedures, with adolescent applications preserving physeal growth. A comprehensive 2024-2025 review positions these as promising adjuncts in biologic therapy pipelines, facilitating vascular ingrowth and tissue remodeling without replacing core surgical techniques.

Prognosis

Short-Term Recovery Outcomes

Following surgical reconstruction or nonsurgical rehabilitation for (ACL) injury, pain and swelling typically show significant resolution within the initial 4-6 weeks when managed through structured protocols involving , compression, , and progressive exercises. In the first 0-2 weeks postoperatively, interventions focus on minimizing and discomfort to protect the graft and restore basic mobility, with swelling often reduced to a mild level (≤1+ on modified stroke test) by weeks 3-5. By weeks 6-8, most patients achieve no after activity, representing a substantial overall decrease attributable to consistent rehabilitation adherence. Knee stability improves markedly in the short term post-reconstruction, with approximately 85% of patients demonstrating a normal result at the 6-month mark, indicating effective restoration of anterior tibial translation control. This outcome reflects successful graft fixation and early neuromuscular adaptations, though residual laxity may persist in a minority due to individual healing variations. Such metrics highlight the procedure's reliability for achieving functional stability within the first half-year. Patients generally return to low-impact daily activities, such as walking and light household tasks, within 3-6 months after ACL reconstruction, coinciding with graft incorporation during the proliferation phase of healing. This phase, spanning roughly 6-12 weeks, involves and initial remodeling, allowing the graft to integrate with tunnels and support progression without crutches by 4-6 weeks in many cases. Full incorporation supports the transition to unrestricted low-demand functions by the 3-6 month window, guided by criterion-based rehabilitation milestones. Short-term success rates for ACL injury treatment exceed 90% patient satisfaction at 6 months, as reported in clinical outcome studies evaluating pain relief, function, and overall recovery. These high satisfaction levels stem from combined surgical and rehabilitative efforts, with registries and prospective cohorts confirming reliable early gains in for the majority of patients.

Long-Term Complications and Risks

One of the primary long-term complications following (ACL) reconstruction is re-injury, particularly graft rupture, which occurs in 15-25% of cases among young athletes under 25 years old. This rate encompasses both ipsilateral graft failures and contralateral injuries, with the latter often exceeding the former in frequency. athletes face a heightened of graft re-rupture, attributed to factors such as biomechanical differences and graft selection. Returning to before 9 months post-reconstruction has been associated with substantially elevated re-tear in some studies (over fourfold), but recent (as of 2025) indicates no increased if athletes meet specific rehabilitation criteria. Post-traumatic (OA) represents another significant enduring challenge, with radiographic evidence appearing in approximately 50% of patients within 10-15 years after ACL reconstruction. This degenerative process stems from altered loading and damage initiated by the initial and exacerbated by surgical intervention. Symptomatic OA, characterized by persistent and functional limitations, affects 20-40% of individuals in this timeframe, often leading to reduced and increased healthcare utilization. Return to pre-injury sport levels occurs in approximately 55% of athletes following ACL reconstruction, with lower rates for competitive pivoting sports, contributing to potential psychological and functional challenges in the long term. Additional complications include cyclops lesions, fibrotic nodules that form anterior to the graft and impede knee extension, with an incidence of about 5% requiring arthroscopic debridement. Postoperative infections occur in roughly 1% of cases, potentially necessitating graft removal and prolonged antibiotic therapy if deep-seated. Persistent knee stiffness, manifesting as limited beyond the initial recovery phase, affects 3-12% long-term and may arise from or inadequate rehabilitation adherence. These issues collectively contribute to ongoing if not addressed, though initial instability patterns are often precursors to these outcomes. In patients opting for nonsurgical management of complete ACL tears, particularly those engaged in high-demand pivoting activities such as skiing, there is a substantially increased risk of long-term complications compared to surgical reconstruction. Recurrent episodes of knee instability can result in secondary injuries to the meniscus and articular cartilage, leading to accelerated post-traumatic osteoarthritis. Although exceptional cases among elite athletes demonstrate that strong quadriceps, hip musculature, and neuromuscular control may enable short-term compensation and continued participation, this approach is generally not recommended due to the elevated risks of progressive joint damage and need for eventual surgical intervention.

Epidemiology

Human Incidence Patterns

The incidence of anterior cruciate ligament (ACL) injuries in the population is estimated at 68.6 per 100,000 person-years based on a large -based study spanning 21 years. This rate reflects isolated ACL tears and highlights the injury's commonality across various demographics, though it varies by age and activity level. Incidence peaks prominently in the 15-25 age group, where rates can reach up to 241 per 100,000 person-years among males, driven largely by high physical demands during and early adulthood. Females in this age range also show elevated rates, often exhibiting a bimodal pattern with an additional peak around age 40, influenced by hormonal and biomechanical factors. In recreational sports settings, females experience ACL injuries at a rate approximately three times higher than males, resulting in a ratio of 1:3 (male to female), as evidenced by meta-analyses of injury patterns in activities like and soccer. Non-athletic ACL injuries account for approximately 50% of cases overall, typically resulting from falls, twists during daily activities, or accidents, with this proportion increasing significantly in the elderly due to reduced ligament integrity and balance issues. Globally, the incidence of ACL injuries has been increasing in recent decades, particularly among , with annual increases of at least 2.3% in patients aged 6–18 over the past 20 years, correlated with increased sports participation and active lifestyles. Recent studies suggest that higher ACL rates in athletes may also relate to smaller team sizes and greater individual playing time, in addition to anatomical factors. As of , ACL injuries in the have increased by 26% since 2007.

Sports-Specific Prevalence

ACL injuries exhibit varying prevalence across sports, with higher rates observed in activities involving pivoting, jumping, and rapid directional changes. In soccer, incidence rates range from 0.1 to 0.4 per 1,000 athlete-exposures, particularly among competitive players where non-contact mechanisms predominate. Basketball shows a comparable rate of approximately 0.3 per 1,000 exposures, driven by landing from jumps and cutting maneuvers. Alpine skiing carries elevated risk, with rates of 0.2 to 0.6 ACL injuries per 1,000 skier-days for recreational participants, often linked to falls on uneven terrain. Distinctions between contact and non-contact sports highlight mechanistic differences in ACL injury occurrence. In , the overall rate is about 0.15 per 1,000 exposures, yet approximately 70% of cases arise from non-contact scenarios such as sudden stops or twists, underscoring biomechanical factors over direct impacts. Among youth athletes aged 13 to 17, ACL injuries have surged, with an estimated 129,000 annual cases as of 2024, reflecting broader trends in organized participation. At the professional level, rates remain low but notable in leagues like the (NBA). Collegiate sports reveal a pronounced sex disparity, with females experiencing ACL injuries at a 3:1 ratio compared to males, attributed to anatomical and hormonal influences.

Veterinary Considerations

Cranial cruciate ligament (CCL) rupture, the canine equivalent of (ACL) injury in humans, affects an estimated 1.2% to 2.6% of dogs over their lifetime, with higher rates observed in veterinary referral populations reaching up to 11%. Large breeds such as Labrador Retrievers face elevated risks, with 5% to 10% of individuals experiencing a rupture during their lifetime due to conformational and genetic predispositions. In dogs, CCL rupture shares similarities with ACL injuries but is predominantly degenerative rather than traumatic, involving progressive weakening from chronic instability, , and matrix degradation. Unlike the acute tears common in athletes, canine cases often feature immune-mediated components, including synovial and autoimmune responses targeting type I in the , which accelerate rupture in predisposed animals. Treatment approaches parallel human surgical reconstructions, with (TPLO) being a widely adopted stabilizing procedure that neutralizes shear forces on the . Post-TPLO outcomes show approximately 90% of dogs achieving good to excellent limb function, enabling return to normal activity levels with proper rehabilitation. Dogs serve as valuable translational models for human ACL research due to anatomical and biomechanical similarities in the , facilitating testing of biologics like mesenchymal stem cells (MSCs). Recent advancements include FDA-approved trials at in 2024 evaluating MSC therapies for musculoskeletal injuries in dogs, with 2025 studies demonstrating reduced joint inflammation and improved ligament healing in CCL models, informing potential human applications.

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