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Patellar dislocation
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| Patellar dislocation | |
|---|---|
| Other names | Kneecap dislocation, dislocated kneecap |
 | |
| X-ray showing a patellar dislocation, with the patella out to the side. | |
| Specialty | Emergency medicine, orthopedics |
| Symptoms | Knee is partly bent, painful and swollen[1][2] |
| Complications | Patella fracture, arthritis[3] |
| Usual onset | 10 to 17 years old[4] |
| Duration | Recovery within 6 weeks[5] |
| Causes | Bending the lower leg outwards when the knee is straight, direct blow to the patella when the knee is bent[1][2] |
| Risk factors | High riding patella, family history, loose ligaments[1] |
| Diagnostic method | Based on symptoms, X-rays[2] |
| Treatment | Reduction, splinting, physical therapy, surgery[1] |
| Medication | Pain medication[3] |
| Prognosis | ~30% risk of recurrence[4] |
| Frequency | 6 per 100,000 per year[4] |
A patellar dislocation is a knee injury in which the patella (kneecap) slips out of its normal position.[5] Often the knee is partly bent, painful and swollen.[1][2] The patella is also often felt and seen out of place.[1] Complications may include a patella fracture or arthritis.[3]
A patellar dislocation typically occurs when the knee is straight and the lower leg is bent outwards when twisting.[1][2] Occasionally, it occurs when the knee is bent and the patella is struck directly.[1] Commonly associated sports include soccer, gymnastics, and ice hockey.[2] Dislocations nearly always occur away from the midline.[2] Diagnosis is typically based on symptoms and supported by X-rays.[2]
Reduction is generally done by pushing the patella towards the midline while straightening the knee.[1] After reduction, the leg is generally splinted in a straight position for a few weeks.[1] This is then followed by physical therapy.[1] Surgery after a first dislocation is generally of unclear benefit.[6][4] Surgery may be indicated in those cases where a fracture occurs within the joint or where the patella has repeatedly dislocated.[3][4][5]
Patellar dislocations occur in about 6 per 100,000 people per year.[4] They make up about 2% of knee injuries.[1] It is most common in those 10 to 17 years old.[4] Rates in males and females are similar.[4] Recurrence after an initial dislocation occurs in about 30% of people.[4]
Signs and symptoms
[edit]People often describe pain as severe and being "inside the knee cap".[3] The leg tends to flex even when relaxed. In some cases, the injured ligaments involved in patellar dislocation do not allow the leg to flex.[2]
Risk factors
[edit]A predisposing factor is tightness in the tensor fasciae latae muscle and iliotibial tract in combination with a quadriceps imbalance between the vastus lateralis and vastus medialis muscles can play a large role, found, mainly, in women involved in sports.[3][7] Moreover, women with patellofemoral pain may show increased Q-angle compared with women without patellofemoral pain.[citation needed]
Another cause of patellar symptoms is lateral patellar compression syndrome, which can be caused from lack of balance or inflammation in the joints.[8] The pathophysiology of the kneecap is complex, and deals with the osseous soft tissue or abnormalities within the patellofemoral groove. The patellar symptoms cause knee extensor dysplasia, and sensitive small variations affect the muscular mechanism that controls the joint movements.[9]
24% of people whose patellas have dislocated have relatives who have experienced patellar dislocations.[2]
Athletic population
[edit]Patellar dislocation occurs mainly in youths (under age 20) engaged in sports that may involve accidental rotation of the knee while in flexion, a movement clinically called valgus, which is the cause of some 93% of patellar dislocation cases.[3] It is more common in females than males and in young in-training military personnel who have a high incidence of patellar dislocation in relation to young athletes and the general population.[3] Direct trauma to the knee displacing the patella is rare.[3]
Displacement of the patella laterally out of its groove strains the medial stabilizing connective tissues, particularly the medial patellofemoral ligament (supporting 50–80% of the knee mechanisms in lateral patellar glide), which is torn usually at its femoral attachment.[3] Traumatic patellar dislocation may cause bleeding into the joint space, ligament and muscle attachment tearing, and fracture of the medial wing of the patella.[3] Fracture of the weight-bearing portion of the lateral femoral condyle occurs in 25% of traumatic patellar dislocations.[3] Surgical repair of the patellar stabilizing structures – the medial patellofemoral ligament and vastus medialis muscle – may be needed for athletes.[3]
Anatomical factors
[edit]People who have larger Q angles tend to be more prone to having knee injuries such as dislocations, due to the central line of pull found in the quadriceps muscles that run from the anterior superior iliac spine to the center of the patella. The range of a normal Q angle for men ranges from <15 degrees and for females <20 degrees, putting females at a higher risk for this injury.[10] An angle greater than 25 degrees between the patellar tendon and quadriceps muscle can predispose a person to patellar dislocation.[11]
In patella alta, the patella sits higher on the knee than normal.[11] Normal function of the VMO muscle (VMO) stabilizes the patella. Decreased VMO function results in instability of the patella.[2]
Forces
[edit]When there is too much tension on the patella, the ligaments will be susceptible to tearing due to shear force or torsion force, which then displaces the patella from its groove.[3] Patellar dislocation may also occur when the trochlear groove is shallow, a condition defined as trochlear dysplasia.[12]
Mechanism of injury
[edit]
Patellar dislocations occur by:
- A direct impact that knocks the patella out of joint
- A twisting motion of the knee, or ankle
- A sudden lateral cut [2]
Anatomy of the knee
[edit]The patella is a triangular sesamoid bone that is embedded in tendon. It rests in the patellofemoral groove, an articular cartilage-lined hollow at the end of the thigh bone (femur) where the thigh bone meets the shin bone (tibia). Several ligaments and tendons hold the patella in place and allow it to move up and down the patellofemoral groove when the leg bends. The top of the patella attaches to the quadriceps muscle via the quadriceps tendon,[2] the middle to the vastus medialis obliquus and vastus lateralis muscles, and the bottom to the head of the tibia (tibial tuberosity) via the patellar tendon, which is a continuation of the quadriceps femoris tendon.[13] The medial patellofemoral ligament attaches horizontally in the inner knee to the adductor magnus tendon and is the structure most often damaged during a patellar dislocation. Finally, the lateral collateral ligament and the medial collateral ligament stabilize the patella on either side.[2] Any of these structures can sustain damage during a patellar dislocation.[citation needed]
Diagnosis
[edit]
To assess the knee, a clinician can perform the Patellar Aprehension Test by moving the patella back and forth while the people flexes the knee at approximately 30 degrees.[14]
The people can do the patella tracking assessment by making a single leg squat and standing, or by lying on his or her back with knee extended from flexed position. A patella that slips laterally on early flexion is called the J sign, and indicates imbalance between the VMO and lateral structures.[15]
On X-ray, with skyline projections, dislocations are readily diagnosed. In borderline cases of subluxation, the following measurements can be helpful:
- The lateral patellofemoral angle, formed by:[16]
- A line connecting the most anterior points of the medial and lateral facets of the trochlea.
- A tangent to the lateral facet of the patella.
- With the knee in 20° flexed, this angle should normally open laterally.[16]
- The patellofemoral index is the ratio between the thickness of the medial joint space and the lateral joint space (L). With the knee 20° flexed, it should measure 1.6 or less.[16]
Prevention
[edit]The patella is a floating sesamoid bone held in place by the quadriceps muscle tendon and patellar tendon ligament. Exercises should strengthen quadriceps muscles such as rectus femoris, vastus intermedius, and vastus lateralis. However, tight and strong lateral quadriceps can be an underlying cause of patellar dislocation. If this is the case, it is advisable to strengthen the medial quadriceps, vastus medialis (VMO), and stretch the lateral muscles.[17] Exercises to strengthen quadriceps muscles include, but are not limited to, squats and lunges. Adding extra external support around the knee by using devices such as knee [orthotics] or athletic tape can help to prevent patellar dislocation and other knee-related injuries.[18] External supports, such as knee braces and athletic tape, work by providing movement in only the desired planes and help hinder movements that can cause abnormal movement and injuries. Women who wear high heels tend to develop short calf muscles and tendons. Exercises to stretch and strengthen calf muscles are recommended on a daily basis.[19]
Treatment
[edit]
Two types of treatment options are typically available:
- Surgery
- Conservative treatment (rehabilitation and physical therapy)
Surgery may impede normal growth of structures in the knee, so doctors generally do not recommend knee operations for young people who are still growing.[20][21] There are also risks of complications, such as an adverse reaction to anesthesia or an infection.[20][21]
When designing a rehabilitation program, clinicians consider associated injuries such as chipped bones or soft tissue tears. Clinicians take into account the person's age, activity level, and time needed to return to work and/or athletics. Doctors generally only recommend surgery when other structures in the knee have sustained severe damage, or specifically when there is:[20]
- Concurrent osteochondral injury
- Continued gross instability
- Palpable disruption of the medial patellofemoral ligament and the vastus medialis obliquus
- High-level athletic demands coupled with mechanical risk factors and an initial injury mechanism not related to contact
Supplements like glucosamine and NSAIDs can be used to minimize bothersome symptoms.[14]
Rehabilitation
[edit]An effective rehabilitation program reduces the chances of re-injury and of other knee-related problems such as patellofemoral pain syndrome and osteoarthritis. Most patella dislocations are initially immobilized for the first 2–3 weeks to allow the stretched structures to heal. Rehabilitation focuses on maintaining strength and range of motion to reduce pain and maintain the health of the muscles and tissues around the knee joint.[14] The objective of any good rehabilitation program is to reduce pain, swelling and stiffness as well as increase range of motion. A common rehabilitation plan is to strengthen both the hip abductors, hip external rotators and the quadricep muscles. Commonly used exercises include isometric quadricep sets, side lying clamshells, leg dips with internal tibial rotation, etc. The idea is that because the medial side is most often stretched by the more common lateral dislocation, medial strengthening will add more stabilizing support. With progression more intense range of motion exercises are incorporated.[22]
Epidemiology
[edit]Rate in the United States are estimated 2.3 per 100,000 per year.[23] Rates for ages 10–17 were found to be about 29 per 100,000 persons per year, while the adult population average for this type of injury ranged between 5.8 and 7.0 per 100,000 persons per year.[24] The highest rates of patellar dislocation were found in the youngest age groups, while the rates declined with increasing ages. Females are more susceptible to patellar dislocation. Race is a significant factor for this injury, where Hispanics, African-Americans and Caucasians had slightly higher rates of patellar dislocation due to the types of athletic activity involved in: basketball (18.2%), soccer (6.9%), and football (6.9%), according to Brian Waterman.[23]
Lateral Patellar dislocation is common among the child population. Some studies suggest that the annual patellar dislocation rate in children is 43/100,000.[25] The treatment of the skeletally immature is controversial due to the fact that they are so young and are still growing. Surgery is recommended by some experts in order to repair the medial structures early, while others recommend treating it non operatively with physical therapy. If re-dislocation occurs then reconstruction of the medial patellofemoral ligament (MPFL) is the recommended surgical option.[26]
In animals, patellar luxation is a common condition in dogs, particularly small and miniature breeds.[27]
References
[edit]- ^ a b c d e f g h i j k l Ramponi D (2016). "Patellar Dislocations and Reduction Procedure". Advanced Emergency Nursing Journal. 38 (2): 89–92. doi:10.1097/TME.0000000000000104. PMID 27139130. S2CID 42552493.
- ^ a b c d e f g h i j k l m n Dath R, Chakravarthy J, Porter KM (2006). "Patella dislocations". Trauma. 8 (1): 5–11. doi:10.1191/1460408606ta353ra. ISSN 1460-4086. S2CID 208269986.
- ^ a b c d e f g h i j k l m n Duthon VB (February 2015). "Acute traumatic patellar dislocation". Orthopaedics and Traumatology, Surgery and Research. 101 (Suppl 1): S59–67. doi:10.1016/j.otsr.2014.12.001. PMID 25592052.
- ^ a b c d e f g h i Jain NP, Khan N, Fithian DC (March 2011). "A treatment algorithm for primary patellar dislocations". Sports Health. 3 (2): 170–4. doi:10.1177/1941738111399237. PMC 3445142. PMID 23016004.
- ^ a b c "Patellar Dislocation and Instability in Children (Unstable Kneecap)". OrthoInfo - AAOS. March 2014. Archived from the original on 18 June 2017. Retrieved 16 October 2017.
- ^ Smith TO, Gaukroger A, Metcalfe A, Hing CB (24 January 2023). "Surgical versus non-surgical interventions for treating patellar dislocation". The Cochrane Database of Systematic Reviews. 1 (1) CD008106. doi:10.1002/14651858.CD008106.pub4. ISSN 1469-493X. PMC 9872769. PMID 36692346.
- ^ Briani RV, de Oliveira Silva D, Pazzinatto MF, Ferreira AS, Ferrari D, de Azevedo FM (February 2016). "Delayed onset of electromyographic activity of the vastus medialis relative to the vastus lateralis may be related to physical activity levels in females with patellofemoral pain". Journal of Electromyography and Kinesiology. 26: 137–42. doi:10.1016/j.jelekin.2015.10.012. hdl:11449/168461. PMID 26617182.
- ^ Ficat RP, Hungerford DS (1977). Disorders of the patello-femoral join. Baltimore: Williams Wilkins. ISBN 978-0-683-03200-0.
- ^ Zaffagnini S, Dejour D, Arendt EA (2010). Patellofemoral pain, instability, and arthritis: clinical presentation, imaging, and treatment. Heidelberg; New York: Springer. ISBN 978-3-642-05423-5.
- ^ Floyd RT (2009). Manual of Structural Kinesiology. Boston: McGraw-Hill Higher Education. ISBN 978-0-07-337643-1.
- ^ a b Buchner M, Baudendistel B, Sabo D, Schmitt H (March 2005). "Acute traumatic primary patellar dislocation: long-term results comparing conservative and surgical treatment". Clinical Journal of Sport Medicine. 15 (2): 62–6. doi:10.1097/01.jsm.0000157315.10756.14. PMID 15782048. S2CID 39224743.
- ^ Dejour H, Walch G, Nove-Josserand L, Guier C (1994). "Factors of patellar instability: an anatomic radiographic study". Knee Surgery, Sports Traumatology, Arthroscopy. 2 (1): 19–26. doi:10.1007/bf01552649. PMID 7584171. S2CID 8223738.
- ^ Saladin KS (2012). Anatomy & physiology: the unity of form and function (6th ed.). New York, NY: McGraw-Hill. p. 268. ISBN 978-0-07-337825-1.
- ^ a b c Brukner P, Khan K (2006). Clinical Sports Medicine (3rd ed.). McGraw-Hill.
- ^ Moses S (10 May 2008). "Patella Tracking Assessment". Family Practice Notebook. Archived from the original on 18 June 2011.
- ^ a b c Saggin PR, Saggin JI, Dejour D (September 2012). "Imaging in patellofemoral instability: an abnormality-based approach". Sports Medicine and Arthroscopy Review. 20 (3): 145–51. doi:10.1097/JSA.0b013e3182553cfe. PMID 22878655. S2CID 1692917.
- ^ Nomura E, Horiuchi Y, Kihara M (April 2000). "Medial patellofemoral ligament restraint in lateral patellar translation and reconstruction". The Knee. 7 (2): 121–127. doi:10.1016/s0968-0160(00)00038-7. PMID 10788776.
- ^ Gerrard DF (May 1998). "External knee support in rugby union. Effectiveness of bracing and taping". Sports Medicine. 25 (5): 313–7. doi:10.2165/00007256-199825050-00002. PMID 9629609. S2CID 41629668.
- ^ Abdulla A (December 2006). "Holiday review. Pills". CMAJ. 175 (12): 1575. doi:10.1503/cmaj.061382. PMC 1660600. PMID 17146100.
- ^ a b c Shea KG, Nilsson K, Belzer J (2006). "Patellar dislocation in skeletally immature athletes". Operative Techniques in Sports Medicine. 14 (3): 188–196. doi:10.1053/j.otsm.2006.08.001.
- ^ a b Nikku R, Nietosvaara Y, Kallio PE, Aalto K, Michelsson JE (October 1997). "Operative versus closed treatment of primary dislocation of the patella. Similar 2-year results in 125 randomized patients". Acta Orthopaedica Scandinavica. 68 (5): 419–23. doi:10.3109/17453679708996254. PMID 9385238.
- ^ Smith TO, Chester R, Cross J, Hunt N, Clark A, Donell ST (2015). "Rehabilitation following first-time patellar dislocation: a randomized controlled trial of purported vastus medialis obliquus muscle versus general quadriceps strengthening exercises" (PDF). The Knee. 22 (4): 313–320. doi:10.1016/j.knee.2015.03.013. PMID 25921095.
- ^ a b Waterman BR, Belmont PJ, Owens BD (March 2012). "Patellar dislocation in the United States: role of sex, age, race, and athletic participation". The Journal of Knee Surgery. 25 (1): 51–7. doi:10.1055/s-0031-1286199. PMID 22624248. S2CID 39546830.
- ^ Fithian DC, Paxton EW, Stone ML, Silva P, Davis DK, Elias DA, White LM (2004). "Epidemiology and natural history of acute patellar dislocation". The American Journal of Sports Medicine. 32 (5): 1114–21. doi:10.1177/0363546503260788. PMID 15262631. S2CID 11899852.
- ^ Sillanpää P. "Treatment of Patellar Dislocation in Children" (PDF). patellofemoral.org. Patellofemoral Foundation. Archived (PDF) from the original on 4 March 2016.
- ^ Palmu S, Kallio PE, Donell ST, Helenius I, Nietosvaara Y (March 2008). "Acute patellar dislocation in children and adolescents: a randomized clinical trial". The Journal of Bone and Joint Surgery. American Volume. 90 (3): 463–70. doi:10.2106/JBJS.G.00072. PMID 18310694. S2CID 29847024.
- ^ Engdahl K, Bergström A, Höglund O, Hanson J (September 2023). "The epidemiology of patellar luxation in an insured Swedish dog population". Preventive Veterinary Medicine. 220 106034. doi:10.1016/j.prevetmed.2023.106034. PMID 37801966.
External links
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Patellar dislocation
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Anatomy
Bony structures of the knee
The knee joint is primarily formed by the distal femur, proximal tibia, and patella, which together provide the bony framework for patellar stability. The distal femur ends in two prominent condyles: the medial condyle, which is smaller and more curved, and the larger, more cylindrical lateral condyle; these condyles articulate posteriorly with the tibia while their anterior surfaces form the trochlea, a shallow groove that guides patellar movement.[5][6] The proximal tibia features a broad, flat tibial plateau divided into medial and lateral condyles by an intercondylar eminence, creating a stable base for weight-bearing and femoral articulation.[7][8] The patella is the largest sesamoid bone in the human body, embedded within the tendon of the quadriceps femoris muscle anterior to the knee joint, where it acts as a fulcrum to optimize force transmission.[9] Its primary roles include increasing the mechanical leverage of the quadriceps during knee extension—enhancing torque production by up to 60% in the final degrees of extension—and protecting the anterior knee joint and underlying structures from compressive forces and direct impact.[10][9][11] The patella's posterior articular surface consists of medial and lateral facets separated by a central ridge, which articulates with the femoral trochlea to form the patellofemoral joint; contact initiates on the distal facets at full extension and shifts proximally as the knee flexes, with maximal area at approximately 90 degrees.[9][12] Relevant bony measurements influencing patellar tracking include trochlear groove depth, typically greater than 3 mm in normal anatomy to ensure containment, and patellar height via the Insall-Salvati ratio—the length of the patellar tendon divided by the patellar length—where values exceeding 1.2 denote patella alta, an elevated position that reduces early engagement with the trochlea.[13][14]Soft tissue stabilizers of the patella
The soft tissue stabilizers of the patella encompass a network of ligaments, tendons, and muscles that provide both static and dynamic restraint to maintain its central tracking in the femoral trochlea, complementing the bony constraints during knee motion. These structures are organized medially and laterally, with the medial components primarily resisting lateral displacement and the lateral ones countering medial shifts. The medial patellofemoral ligament (MPFL) serves as the primary static medial restraint, contributing approximately 50-60% of the resistance to lateral patellar subluxation, particularly effective between 0° and 30° of knee flexion. It originates from the region between the medial femoral epicondyle and the adductor tubercle, superior to the medial collateral ligament, and inserts along the medial border of the patella, typically covering the proximal two-thirds while blending with the vastus medialis obliquus (VMO) fascia distally. Other secondary medial stabilizers include the medial patellotibial ligament (MPTL), which extends from the inferomedial patella to the anteromedial tibia, providing restraint to lateral translation in deeper flexion, and the medial patellomeniscal ligament (MPML), which connects the distal medial patella to the anterior horn of the medial meniscus, further supporting patellofemoral alignment. These ligaments form part of the medial patellar complex, with the MPFL as the dominant structure. On the lateral side, the lateral patellofemoral ligament (LPFL) acts as the main static stabilizer against medial patellar displacement, originating from the lateral femoral epicondyle approximately 13.5 mm anterior and distal to its center and inserting on the lateral patellar border, spanning about 59% of the patella's sagittal length. The iliotibial band (ITB) contributes to lateral stability through its distal fascial expansions into the lateral retinaculum, helping to distribute tensile forces across the lateral knee and resist excessive medial patellar excursion during dynamic activities. Dynamic stabilization is provided by muscular and tendinous elements, including the VMO, which originates from the medial intermuscular septum and distal medial femur and inserts obliquely on the superomedial patella, generating a medial vector to counter lateral forces during knee extension. Superiorly, the quadriceps tendon anchors the patella to the quadriceps muscle group, transmitting extension forces while aiding alignment, and inferiorly, the patellar tendon connects the patella to the tibial tuberosity, completing the extensor mechanism and providing additional directional stability. Retinacular restraints consist of medial and lateral expansions from the patella that integrate with the surrounding fascia, forming layered connections to the VMO medially and vastus lateralis laterally; these structures distribute loads and enhance overall patellar centering without independent ligamentous prominence.Pathophysiology
Normal patellar tracking
The patella glides within the femoral trochlea during knee flexion and extension, maintaining central alignment from full extension (0°) through to approximately 135° of flexion under normal physiological conditions. In early flexion (0° to 30°), the patella initially deviates laterally due to the lateral vector of the quadriceps tendon but is actively countered by medial retinacular forces and dynamic muscle action, ensuring it engages the trochlear groove around 20° of flexion. As flexion progresses beyond 30°, the trochlea deepens, forming a bony constraint that locks the patella medially and enhances stability against lateral displacement.[15][16][17] A key determinant of this balanced tracking is the quadriceps angle (Q-angle), which measures the lateral pull on the patella and is formed by the line from the anterior superior iliac spine (ASIS) to the patellar center and the line from the patellar center to the tibial tuberosity. Normal Q-angle values range from 10° to 15° in males and 15° to 20° in females, reflecting slight sex-based differences in pelvic width and lower limb alignment that influence patellar vector forces without compromising stability.[18][19] Dynamic stabilization is primarily provided by the vastus medialis obliquus (VMO), the medial oblique portion of the quadriceps, which contracts to counter the lateral pull exerted by the vastus lateralis during knee motion. This balanced VMO-vastus lateralis activation ratio ensures medial patellar guidance, particularly in the initial flexion phase where soft tissue restraints predominate before bony engagement.[20]Mechanism of dislocation
Patellar dislocation most commonly occurs laterally, accounting for over 95% of cases, while medial dislocations are exceedingly rare and typically result from severe direct trauma to the medial aspect of the knee.[21][22] The primary mechanism involves a combination of valgus force applied to the knee with internal rotation of the femur relative to a planted foot, often during non-contact activities that disrupt normal patellar tracking within the femoral trochlea.[23][24] This biomechanical event generates laterally directed shear forces on the patella, exceeding the tensile strength of its medial stabilizers.[25] Indirect trauma frequently precipitates lateral dislocation through sudden quadriceps contraction while the foot is fixed on the ground, as seen in sports such as soccer or basketball where pivoting or cutting maneuvers are common.[26] Direct trauma, such as a lateral blow to the medial patella, can also cause immediate displacement by overpowering the medial restraints.[22] In the sequence of injury during a first-time dislocation, the medial patellofemoral ligament (MPFL) typically ruptures first in 90-100% of cases, followed by impaction between the medial patellar facet and the lateral femoral condyle, resulting in characteristic bone bruises on these surfaces.[27][28] The patella often displaces laterally by approximately 2-3 cm before the kinetic energy is absorbed, leading to either spontaneous reduction due to muscular forces or requiring manual intervention.[29] Medial patellar dislocations, comprising less than 5% of incidents, arise from high-energy medial impacts that overcome the stronger lateral stabilizers, such as in motor vehicle accidents or falls with direct force to the lateral patella.[21] These events invert the typical injury dynamics, rarely involving MPFL rupture but potentially damaging lateral structures like the lateral retinaculum.[22]Epidemiology
Incidence and prevalence
Patellar dislocation has an estimated global incidence of approximately 6 per 100,000 person-years in the general population.[2] In the United States, the overall incidence rate is 2.3 per 100,000 person-years, based on national emergency department data.[30] The incidence of first-time dislocations reaches 29 per 100,000 per year among individuals aged 10-17 years.[2] A 2022 analysis of U.S. emergency department data from 2001 to 2020 indicated a significant increase in annual incidence, from 2.61 per 100,000 person-years in 2001 to 3.0 per 100,000 person-years in 2020.[31] Following an initial dislocation, up to 50% of individuals experience recurrent dislocations within 5 years.[32] Higher rates are observed in certain demographics, such as adolescents.[33]Demographic patterns
Patellar dislocations predominantly affect adolescents, with the highest incidence occurring in individuals aged 10 to 18 years, who account for over 50% of cases due to skeletal immaturity during this period.[34] In a population-based study from Olmsted County, Minnesota, the incidence peaked at 147.7 per 100,000 person-years among those aged 14 to 18 years, with rates declining sharply thereafter to 4.0 to 2.1 per 100,000 person-years in individuals aged 46 years and older.[33] Adult and elderly populations experience lower rates overall, reflecting reduced participation in high-risk activities and changes in joint stability with age.[35] Females are affected by patellar dislocation at higher rates than males, with studies reporting a female predominance of approximately 65% of cases, corresponding to a 2:1 ratio in some cohorts.[34] This disparity is particularly pronounced in adolescents, where young females aged 10 to 17 years exhibit the highest incidence, up to 108 per 100,000 person-years in certain populations.[36] However, some analyses, such as those from U.S. emergency department data, indicate no statistically significant overall sex-based difference when stratified by age.[35] Ethnic variations show slightly elevated rates in certain groups; for instance, in the United States, Black and White individuals have significantly higher incidences (4.3 and 4.0 times, respectively) compared to Hispanic individuals (0.43 per 100,000 person-years).[30] Populations with high athletic participation, such as those studied in Scandinavia, report overall incidences of 42 per 100,000 person-years, with peaks in active youth.[36] Bilateral occurrences affect 15-20% of patients with patellar instability, often linked to underlying anatomical predispositions that manifest symmetrically.[37] Socioeconomic patterns reveal increased rates among active youth sports participants, with nearly 52% of dislocations occurring during athletic activities like basketball (18%), soccer (7%), and football (6%).[30]Risk Factors
Anatomical variations
Anatomical variations that predispose the patella to instability and dislocation are primarily congenital or developmental abnormalities affecting the patellofemoral joint's bony and soft tissue structures, leading to impaired tracking and reduced constraint during knee motion. These variations disrupt the normal balance of forces, increasing the likelihood of lateral displacement, particularly in the early degrees of flexion when the trochlea provides minimal bony guidance. Common variations include alterations in patellar height, trochlear morphology, lower limb alignment, and ligamentous integrity. Patella alta, characterized by an elevated position of the patella relative to the femur, delays engagement with the trochlear groove, thereby reducing early stabilization and heightening dislocation risk during quadriceps contraction. This condition is quantified using the Caton-Deschamps index, where a value greater than 1.2 on lateral radiographs or MRI indicates patella alta.[38] Studies have shown that patella alta is present in up to 50% of individuals with recurrent patellar instability, as it allows excessive patellar mobility before trochlear capture.[2] Trochlear dysplasia involves an abnormally shallow, flat, or convex femoral trochlea, which diminishes the bony constraint that normally resists lateral patellar translation. Classified by Dejour's system into types A (shallow groove with crossing sign) through D (severe asymmetry with convex lateral facet), this dysplasia is a key predisposing factor, occurring in 50-85% of cases with patellar instability.[2] The reduced depth, often measured by a sulcus angle exceeding 145° on axial imaging, fails to guide the patella properly, especially in shallow variants (type A), leading to easier subluxation or dislocation.[38] Increased Q-angle, typically exceeding 20°, arises from factors such as genu valgum or excessive femoral anteversion, creating a lateral vector that pulls the patella away from the trochlear midline. This angle, formed by lines from the anterior superior iliac spine to the patella center and from the patella to the tibial tubercle, amplifies the lateral force from the quadriceps, promoting instability.[2] The Q angle is typically larger in women than in men due to anatomical differences, with approximate values of 13-14° in men and 15-17° in women (one study reports means of 13.5° for men and 15.9° for women).[3] The primary anatomical cause is the wider female pelvis, which positions the anterior superior iliac spines farther apart, leading to increased knee valgus alignment and greater lateral pull on the patella by the quadriceps. Other contributing factors may include greater femoral anteversion and differences in lower limb alignment. Femoral anteversion greater than 20° further exacerbates this by directing the quadriceps pull laterally.[22] Increased tibial tuberosity-trochlear groove (TT-TG) distance, typically greater than 20 mm as measured on CT or MRI, indicates lateralization of the tibial tubercle relative to the trochlear groove, increasing the lateral pull on the patella via the patellar tendon. This malalignment contributes to instability and is present in approximately 32% of knees with patellar instability.[39] Genu recurvatum, or hyperextension of the knee beyond 5-10°, alters patellar alignment by increasing lateral tilt and reducing medial contact, which compromises stability during weight-bearing activities. This variation, often linked to posterior capsular laxity, is more prevalent in individuals with connective tissue disorders and contributes to recurrent dislocations by exaggerating valgus stress on the patella.[40] Ligamentous laxity encompasses generalized hypermobility or specific deficiencies, such as shallow medial patellar facets and torsional limb malalignments, which weaken the soft tissue restraints and bony congruence needed for patellar centering. Shallow medial facets, indicated by a reduced medial facet length (less than 15 mm on MRI) or a lateral-to-medial facet ratio greater than 1.5, decrease the patella's medial articular surface, impairing resistance to lateral forces and serving as a strong predictor of recurrent dislocation with 83% discriminatory accuracy.[41] Torsional malalignments, including increased tibial torsion (average 25-35° external) or femoral anteversion, disrupt lower limb alignment and patellar tracking, correlating with instability in 40-60% of affected patients.[42] These laxity-related features often interact with traumatic events to precipitate initial dislocations in predisposed individuals.[2]Activity and trauma-related factors
Patellar dislocations frequently occur in athletic populations, particularly young individuals under 25 years old participating in high-risk sports that involve pivoting, jumping, and sudden directional changes, such as basketball, soccer, and gymnastics. Studies report that 61% to 72% of acute patellar dislocations are sports-related, with the majority affecting adolescents and young adults during athletic activities. The annual incidence in youth aged 10 to 17 can reach 29 per 100,000 person-years, underscoring the vulnerability of this demographic in dynamic sports environments. Direct trauma, typically a blow to the medial aspect of the knee, accounts for approximately 4% of patellar dislocations and is commonly associated with contact sports like football or ice hockey. In contrast, indirect forces predominate, comprising 66% to 96% of cases, and often result from non-contact mechanisms such as sudden deceleration, cutting maneuvers, or twisting motions accompanied by quadriceps contraction. These forces generate valgus stress and internal rotation of the femur relative to the tibia, particularly in pivoting sports. A prior history of patellar dislocation markedly elevates the recurrence risk, with rates ranging from 15% to 60% after the initial episode, especially in younger patients returning to athletic activities. This increased susceptibility persists even with conservative management, highlighting the need for targeted rehabilitation to mitigate repeat injuries. Connective tissue disorders like Ehlers-Danlos syndrome and Down syndrome contribute to patellar dislocation risk by inducing generalized ligamentous laxity, which compromises knee stability during physical activities. In Ehlers-Danlos syndrome, hypermobile joints lead to recurrent instability, with surgical interventions showing higher failure rates compared to non-hypermobile patients. Similarly, patellar instability is prevalent in Down syndrome due to inherent collagen defects and joint laxity, often necessitating early monitoring in active children.Clinical Presentation
Signs and symptoms
Patellar dislocation typically presents with acute, severe pain in the medial aspect of the knee, often described as intense and debilitating immediately following the injury.[2] This pain arises primarily from the tearing of medial stabilizing structures and can persist as soreness even after reduction.[1] Swelling develops rapidly due to hemarthrosis, or bleeding into the joint, resulting in joint effusion that becomes evident within hours of the dislocation.[2] Medial swelling is particularly prominent, contributing to the overall knee distension.[23] Patients often experience significant functional impairment, including an inability to fully extend the knee, limping, and difficulty bearing weight on the affected leg.[1] The knee may feel locked or buckle, limiting normal gait and mobility.[2] In unreduced cases, a visible deformity is apparent, with the patella displaced laterally out of its normal position in the trochlear groove.[2] Such presentations are less common, as many dislocations spontaneously reduce, but the lateral shift remains a hallmark when observed.[1] Sensory changes, such as numbness over the medial knee, may occur due to irritation of the infrapatellar branch of the saphenous nerve during the traumatic event.[43] These symptoms are often linked to common associated injuries like medial patellofemoral ligament (MPFL) tears.[2]Associated injuries
Patellar dislocation frequently occurs alongside various soft tissue injuries, with the medial patellofemoral ligament (MPFL) tear being the most common, affecting approximately 95% of first-time dislocations.[44] These tears typically involve complete rupture and are located at the patellar attachment in about 37% of cases, the femoral origin in 37%, or a combination of sites in 25%.[44] Strains or disruptions of the vastus medialis obliquus (VMO) muscle, a key medial stabilizer of the patella, also commonly accompany these events as part of broader medial retinacular damage, though specific incidence rates are less precisely quantified in the literature.[45] Bony injuries are prevalent, including osteochondral fractures in 10-25% of acute cases, most often affecting the medial facet of the patella or the anterolateral aspect of the lateral femoral condyle.[46] These fractures result from the patella impacting the lateral femoral condyle during dislocation and reduction.[47] Avulsion fractures, particularly at the MPFL's femoral or patellar attachments, occur in conjunction with ligamentous disruptions and contribute to instability if untreated.[46] Chondral damage, such as cartilage lesions or defects, is a frequent sequela, observed in up to 70% of cases via arthroscopy, and predisposes affected individuals to early posttraumatic osteoarthritis due to altered joint mechanics and surface irregularities.[45] Magnetic resonance imaging (MRI) reveals bone bruises or marrow edema in approximately 70-90% of patellar dislocations, predominantly in the medial patella and lateral femoral condyle, reflecting the high-energy impact during the injury mechanism.[48][49] Rare associations include anterior cruciate ligament (ACL) tears in about 5-7% of cases and meniscal injuries in high-energy trauma scenarios, often linked to multiligamentous knee disruptions rather than isolated patellar events.[50]Diagnosis
History and physical examination
The history of patellar dislocation begins with eliciting the mechanism of injury, which typically involves a non-contact twisting motion of the knee with the foot planted, such as external rotation of the tibia on the femur, or a direct blow to the medial patella.[2] Patients often describe an acute sensation of the knee giving way, accompanied by a audible or palpable pop, followed by immediate swelling and inability to bear weight.[51] It is essential to inquire about prior episodes of patellar subluxation or dislocation, as the recurrence rate after an initial event ranges from 15% to 60%.[2] Additionally, a family history of patellar instability should be documented, present in approximately 28% of cases involving recurrent dislocations.[25] Pain assessment reveals an acute onset of severe, sharp discomfort centered medially around the patella, stemming from rupture of the medial patellofemoral ligament (MPFL), which occurs in nearly all dislocations.[2] The pain intensifies with weight-bearing, knee flexion beyond 30 degrees, or any attempt at active extension, often limiting ambulation and requiring the patient to hold the knee in partial extension for relief.[51] Physical examination involves inspection for visible deformity, such as lateral displacement of the patella if unreduced, and assessment of knee effusion, which is common due to hemarthrosis.[51] Palpation identifies tenderness along the medial patellar border and superolateral joint line, with possible crepitus or irregularities at the patellar poles; swelling may obscure these findings acutely.[2] Range of motion is typically restricted, with flexion limited to less than 90 degrees secondary to pain, guarding, and mechanical obstruction, while extension may be incomplete if the patella remains displaced.[2] The patella, if not spontaneously reduced, appears laterally positioned and deformed, with increased lateral glide exceeding 50% of its width.[51] Special tests include the patellar apprehension test, performed with the knee flexed to 20-30 degrees, where medialward pressure on the lateral patella provokes patient apprehension, verbalized fear, and quadriceps contraction indicative of underlying instability.[2] A neurovascular examination is mandatory, evaluating distal pulses, sensation, and perfusion to exclude rare but serious complications such as peroneal nerve injury or evolving compartment syndrome.[2]Imaging and diagnostic tests
Plain radiographs are typically the initial imaging modality for suspected patellar dislocation, consisting of anteroposterior (AP), lateral, and skyline (axial) views to assess for patellar displacement, fractures, loose bodies, and associated abnormalities such as patella alta measured by the Insall-Salvati ratio on the lateral view or trochlear dysplasia on the skyline view.[52][53][14] Magnetic resonance imaging (MRI) serves as the gold standard for evaluating soft tissue injuries following patellar dislocation, including medial patellofemoral ligament (MPFL) tears and chondral damage, with reported sensitivity around 86-95% for detecting cartilage and osteochondral lesions, depending on the study.[54][55] Computed tomography (CT) is utilized for precise bony evaluation in recurrent patellar instability or preoperative planning, particularly to quantify trochlear morphology and dysplasia using metrics like the Dejour classification.[56][14] Ultrasound provides a dynamic assessment of patellar tracking, especially in cases of chronic instability, allowing real-time visualization of lateral subluxation or abnormal glide during knee flexion.[57][58] According to American Academy of Orthopaedic Surgeons (AAOS) recommendations, plain radiographs should be obtained first to confirm the diagnosis and rule out fractures, with MRI reserved for cases where surgical intervention is contemplated to assess soft tissue and cartilage integrity.[59]Treatment
Initial treatment for first-time patellar dislocation typically involves conservative measures: closed reduction if needed, brief immobilization, RICE protocol, and structured physical therapy to strengthen the quadriceps (especially vastus medialis obliquus), improve hip and core stability, and retrain patellar tracking. This approach succeeds in many cases, but recurrence is common (15-60%). For recurrent patellar instability, particularly after a second episode or in adolescent athletes, conservative management alone often has reduced efficacy due to persistent anatomical risks or ligamentous laxity. Guidelines frequently recommend specialist evaluation with imaging (X-rays for alignment, MRI for soft tissues) to identify treatable factors. Surgical stabilization, most commonly medial patellofemoral ligament (MPFL) reconstruction, is increasingly considered to prevent further episodes and protect cartilage, with studies showing significant risk reduction and good return-to-sport rates in young patients.Acute reduction and initial management
The initial management of patellar dislocation focuses on prompt closed reduction to reposition the patella into the femoral trochlear groove, typically performed in the emergency setting. The patient is positioned supine with the hip slightly flexed to relax the quadriceps muscle, followed by gentle extension of the knee while applying steady medial pressure to the lateral border of the dislocated patella; this maneuver often results in a palpable "clunk" as the patella relocates successfully.[60][61] Analgesia is usually unnecessary due to the procedure's low pain level, though procedural sedation may be considered for patients with significant anxiety or discomfort.[60] Closed reduction succeeds in the majority of cases without complications when performed gently.[62] Following reduction, anteroposterior, lateral, and sunrise-view radiographs of the knee are obtained to confirm proper patellar alignment and rule out associated osteochondral fractures or avulsions.[60][61] Recent guidelines, such as the 2024 BOASt standards, recommend avoiding routine aspiration of hemarthrosis, as it provides no proven benefit and carries risks.[63] Immobilization should be brief and used only if necessary for pain relief, with splints allowing knee flexion rather than rigid full-extension bracing, as prolonged immobilization offers no long-term benefit and may cause stiffness.[63][64] Pain and swelling are managed with the RICE protocol (rest, ice, compression, elevation) applied immediately post-injury, along with nonsteroidal anti-inflammatory drugs (NSAIDs) such as ibuprofen for analgesia and to reduce inflammation.[65][66] Weight-bearing is encouraged immediately as tolerated, with crutches provided for comfort if needed, rather than restricted partial loading.[63] Closed reduction is contraindicated in cases of suspected irreducibility, such as when the patella is buttonholed through a tear in the vastus lateralis, requiring surgical intervention; an orthopedic consultation is also warranted if imaging reveals associated fractures.[67][60]Conservative approaches
Conservative management is recommended as the initial approach for first-time patellar dislocations in low-risk patients, such as skeletally mature individuals without significant anatomical risk factors or osteochondral injuries, promoting healing of structures like the medial patellofemoral ligament (MPFL) through protection and rehabilitation.[68] However, it is not considered the gold standard for skeletally immature patients, where recurrence rates can reach up to 70%.[64] A 2023 Cochrane systematic review found insufficient high-quality evidence to favor surgery over non-surgical treatment for initial dislocations, with recurrence rates of approximately 30-70% under conservative care, emphasizing its suitability for low-risk cases.[69] The 2025 ESSKA consensus advocates an individualized approach, considering risk factors like trochlear dysplasia.[64] Brief immobilization may be used in the acute phase if needed for pain, but guidelines recommend transitioning quickly to allow mobility to avoid stiffness.[63][64] Following reduction, early physical therapy is prioritized, with assessment within 3 weeks, focusing on unrestricted weight-bearing and functional recovery.[63] Physical therapy is introduced early, typically within the first 1-2 weeks, starting with isometric quadriceps exercises to activate the vastus medialis obliquus and restore neuromuscular control without stressing the healing MPFL.[68] Progression involves closed-chain exercises, such as mini-squats and weight shifts, to enhance patellar tracking and overall lower limb strength, with evidence showing improved range of motion and function in first-time cases.[70] These protocols prioritize pain-free movement and proprioception to support long-term stability, and are considered essential regardless of operative status.[64][71] Orthotics, including patellar stabilizing braces or taping techniques like McConnell taping, are used to medialize the patella and provide dynamic support, particularly during the intermediate recovery phase and return to activity.[72] Non-rigid braces may be recommended post-acute phase for 1-6 months, especially in sports, to reduce lateral forces and improve immediate stability, with taping offering short-term pain relief and enhanced proprioception.[73] These interventions are tailored to individual biomechanics and discontinued as strength improves.[70] Monitoring involves serial clinical examinations every 2-4 weeks to assess patellar stability, effusion, range of motion, and quadriceps strength, with adjustments to the protocol based on progress and any signs of instability.[74] Follow-up helps evaluate anatomic risk factors and predict recurrence, ensuring timely referral if conservative measures fail.[75] Overall, this approach yields satisfactory outcomes in approximately 30-70% of first-time cases, depending on risk profile, though persistent symptoms warrant reevaluation.[72][64]Surgical options
Surgical options are typically reserved for patients with recurrent patellar dislocations, those at high risk due to anatomical abnormalities (e.g., trochlear dysplasia, patella alta), ongoing symptoms, or cases complicated by significant osteochondral lesions (≥1 cm²) that fail to respond adequately to conservative management.[14][76][64] Recent guidelines, including the 2025 ESSKA consensus and 2024 BOASt, emphasize individualized decisions, with surgery considered earlier for high-risk profiles, including skeletally immature patients, but not routinely for all first-time cases.[64][63] These procedures aim to restore patellar stability by addressing soft tissue deficiencies, bony malalignments, or intra-articular damage. Medial patellofemoral ligament (MPFL) reconstruction is the most commonly performed procedure for restoring medial restraint to the patella, particularly in cases of recurrent instability. This technique involves reconstructing the MPFL using a hamstring autograft, such as the semitendinosus tendon, anchored to the femoral and patellar sites to mimic the native ligament's anatomy and prevent lateral subluxation. It is indicated for patients with two or more dislocations and no severe bony abnormalities, or high-risk first-time cases, with reported success rates ranging from 80% to 95% in reducing recurrent instability; reconstruction is preferred over repair.[77][78][76][64] Bony procedures address underlying skeletal contributions to instability. Tibial tubercle osteotomy (TTO) is utilized for patella alta, involving distalization and medialization of the tibial tubercle to normalize patellar height and improve tracking, thereby reducing lateral forces on the patella. This is particularly effective in adolescents with elevated patellar tendon ratios and associated instability. Trochleoplasty, indicated for high-grade trochlear dysplasia (Dejour types B and D), reshapes the shallow trochlear groove through deepening osteotomy to enhance bony containment of the patella, often combined with MPFL reconstruction in severe cases.[79][80][81][14] Soft tissue realignment techniques, such as vastus medialis obliquus (VMO) imbrication or lateral retinacular release, are less commonly employed due to risks of overcorrection, medial instability, or suboptimal long-term outcomes, and isolated lateral release is not recommended. VMO advancement tightens the medial structures to counter lateral pull, while lateral release alleviates tight lateral restraints, but these are typically adjunctive and avoided as standalone treatments in recurrent cases.[77][82][83][63] Arthroscopic interventions focus on managing intra-articular complications, including removal of loose bodies from osteochondral fractures and repair of chondral defects via debridement, microfracture, or fixation, with fragment refixation preferred for lesions ≥1 cm². These are often performed concurrently with stabilization procedures to prevent further joint damage in dislocated knees.[14][84][85][64] Recent systematic reviews and guidelines indicate that surgical interventions can significantly reduce recurrence rates compared to conservative approaches in high-risk patients, with meta-analyses reporting redislocation rates of approximately 17% after surgery versus 33% non-surgically, though surgery carries higher risks of complications such as graft failure, stiffness, and infection. A 2024 review confirmed lower redislocation with surgery in most studies, but functional outcomes are mixed, with long-term results equalizing beyond five years and emphasizing individualized selection.[77][71][86][87][64]Rehabilitation and Prevention
Rehabilitation protocols
Rehabilitation protocols for patellar dislocation aim to restore knee function, minimize recurrence risk, and facilitate a safe return to activity through structured, phased programs that address pain control, range of motion (ROM), strength, and proprioception. These protocols are tailored based on whether the treatment was conservative or surgical, with immobilization typically used initially in both cases to protect the joint, followed by progressive loading to promote tissue healing and neuromuscular control. Evidence supports early quadriceps activation and hip strengthening to counteract lateral patellar forces, with full recovery often achieved within 3-6 months for most patients.[88] In the initial phase (0-6 weeks post-injury or surgery), the focus is on immobilization using a knee brace or splint to maintain reduction, alongside pain and swelling management through cryotherapy, elevation, and compression. Quadriceps activation exercises, such as straight leg raises and isometric quad sets, are introduced early to prevent atrophy and restore extension, while avoiding open-chain knee extensions beyond 30-45 degrees to limit patellofemoral stress. Patellar mobilizations and gentle ROM exercises (heel slides) help achieve near-full extension by 2-4 weeks, with weight-bearing as tolerated using crutches. Progression criteria include minimal effusion (≤3/10 pain) and the ability to perform a straight leg raise without extensor lag. The intermediate phase (6-12 weeks) emphasizes closed-chain exercises to build strength and stability, including mini-squats, wall sits, and step-ups, which target the vastus medialis obliquus (VMO) for medial patellar tracking. Proprioception training via balance board exercises and single-leg stance improves joint awareness, while hip abductor and external rotator strengthening (e.g., clamshells, side-lying leg lifts) reduces Q-angle stress and enhances overall lower extremity alignment. Full ROM is typically restored by this stage, with criteria for advancement including pain-free 90-degree flexion and controlled single-leg balance for 30 seconds. In the advanced phase (12+ weeks), protocols incorporate sport-specific drills, plyometrics (e.g., double-leg hops progressing to single-leg), and agility training to rebuild dynamic control and endurance. Return to activity occurs at 3-6 months once strength symmetry reaches 85-90% of the uninjured side, with functional tests like single-leg hop for distance confirming readiness. Emphasis on hip abductor strengthening persists to mitigate recurrence by addressing biomechanical imbalances. After full recovery, patients can continue home-based strength training to maintain quadriceps strength, hip stability, and functional knee control, thereby reducing the risk of recurrence. These exercises should be initiated under professional supervision to ensure proper technique and appropriate progression. Key recommended exercises include:- Quad sets: Tighten the thigh muscles while pressing the knee down flat (hold 5-10 seconds).
- Straight leg raises: Lie supine, tighten the quadriceps, lift the straight leg 6-12 inches off the ground (hold 5-6 seconds).
- Hip abduction/adduction: Perform side-lying leg lifts outward (abduction) and inward (adduction).
- Glute bridges (double- or single-leg): Lie supine and lift the hips toward the ceiling while engaging the glutes.
- Mini-squats or wall sits: Perform shallow knee bends or slide down a wall to 30-45 degrees of knee flexion.
- Lunges or step-downs: Execute controlled forward lunges or step-downs from a low step.