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Femoroacetabular impingement
Femoroacetabular impingement
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Femoroacetabular impingement
Other namesHip impingement syndrome, femoroacetabular impingement syndrome (FAIS)
Video of the three types of femoroacetabular impingement.
SpecialtyOrthopedics

Femoroacetabular impingement (FAI) is involving one or more anatomical abnormalities of the hip joint, which is a ball and socket joint.[1] It is a common cause of hip pain and discomfort in young and middle-aged adults.[2]

It occurs when the ball shaped femoral head contacts the acetabulum abnormally or does not permit a normal range of motion in the acetabular socket.[3] Damage can occur to the articular cartilage, or labral cartilage (soft tissue, ring-shaped bumper of the socket), or both.[4] The condition may be symptomatic or asymptomatic. It may cause osteoarthritis of the hip.[1]

Treatment options range from conservative to surgery.[5]

Signs and symptoms

[edit]

Pain is the most common complaint in those with FAI.[6] It is experienced in a number of areas, making the diagnosis challenging, but commonly occurs in the groin, upper buttock/lower back, the buttock or beneath the buttock, side of the affected hip and posterior upper leg.[6][3][7] Onset of symptoms has been reported to present in both an acute and more gradual manner.[6] The pain is often significant enough to cause a decrease in activity level and movement.[6] Some will also describe decreased range of motion of the affected hip.[8] Another symptom is groin pain associated with activity and no prior history of trauma.[9] Inability to perform activities such as high hip flexion or prolong sitting can also be seen in individuals with FAI.[9]

Cause

[edit]

FAI is characterized by abnormal contact between the proximal femur and rim of the acetabulum (hip socket). In most cases, patients present with a deformity in the femoral head, or acetabulum, a poorly positioned femoral-acetabular junction, or any or all of the foregoing.[3] The cause of FAI is currently unknown, but both congenital and acquired etiologies have been put forth. Studies have shown an increased incidence in siblings, suggesting a genetic component.[10] At least one study has also shown a predilection in the white population.[10] It has also been reported to be more common in males.[10] However, there is no concrete evidence to suggest a genetic trait and instead, the most favored theory currently supports that FAI (the cam type in particular) is due to repetitive movements involving the hip (e.g. squatting) in young athletes.[10] Aggravating activities that are commonly reported include repetitive or prolonged squatting, twisting movements of the hip, like pivoting during athletics, getting in and out of cars, and even sitting for prolonged periods.[6][11] A combination of these factors may also predispose to a form of FAI; predominantly, a marginal developmental hip abnormality together with environmental factors such as recurrent motion of the legs within a supraphysiologic range.[12]

Three types of FAI are recognized (see title image). The first involves an excess of bone along the upper surface of the femoral head, known as a cam deformity (abbreviation for camshaft, which the shape of the femoral head and neck resembles). The second is due to an excess of growth of the upper lip of the acetabular cup and is known as a 'pincer' deformity. The third is a combination of the two, generally referred to as 'mixed.' The most common type seen, approximately 70% of the time, is the mixed type.[13] A complicating issue is that some of the radiographic findings of FAI have also been described in asymptomatic subjects.[14]

Current literature suggests that the cam type of impingement is associated with the development of hip osteoarthritis.[15] Thus far, no correlation has been seen between the pincer type and development of hip osteoarthritis.[15]

Anatomy

[edit]

The hip joint is classified as a ball and socket joint. This type of synovial joint allows for multidirectional movement and rotation. There are two bones that make up the hip joint and create an articulation between the femur and pelvis. This articulation connects the axial skeleton with the lower extremity. The pelvic bone, also known as the innominate bone, is formed by three bones fused together: the ilium, ischium, and pubis. The musculature of the hip is divided into anterior hip muscles and posterior hip muscles. The major nerve supply that runs through the hip joint is the femoral nerve and the sciatic nerve.[16]

Diagnosis

[edit]

Clinical evaluation is the first step in diagnosis, but will rarely lead to the diagnosis on its own, due to inconsistent and vague nature of the pain.[10] Childhood and current activity should be inquired about. Physical exam should also involve assessing passive internal rotation of the hip during flexion, as range of motion is reduced in proportion to the size of a cam lesion.[10] Flexing the hip to 90 degrees, adducting, and internally rotating the hip, known as the FADDIR test, should also be performed.[10] It is positive when it causes pain. The FABER test should also be performed, this test involves flexing, abducting, and externally rotating the hip. The FABER test is useful when diagnosing concurrently with a labral pathology and is considered positive if the position elicits pain.[17] Additional non-invasive ways to observe possible FAI is changes in gait that include a lower peak hip extension and internal rotation to compensate for bony growth.[18]

X-ray

[edit]
Radiograph of a cam type impingement.
Radiograph of a pincer type impingement.

Projectional radiography ("X-ray") is often considered first line for FAI.[10] Anterior-posterior pelvis and a lateral image of the hip in question should be attained.[10] A 45-degree Dunn view is also recommended.[10][19]

Measurements of impingement on X-ray.[notes 1][20]
Measurement Image Target Normal value
Center-edge angle of Wiberg
The superior-lateral coverage of the femoral head.
  • >20° (<55 years old)[notes 2]
  • <24° (>55 years old)[notes 2]
  • >40° indicates overcoverage
Crossing ratio Percentage of acetabular walls crossing. Normal acetabulum is oriented in anteversion. Its value ranges from 15 to 20° in the equatorial plane of the acetabulum and decreases gradually towards the acetabular roof, where normal values range from 0 to 5°. Retroversion of the upper part of the acetabulum has been related with pincer type impingement. In radiography the presence of a "crossover sign" is produced when the posterior wall of the acetabulum crosses the anterior wall before reaching the acetabular roof. It is a sign of acetabular retroversion and it has been linked with overcoverage and pincer impingement. Nevertheless, this sign has been described in 6% of the normal population. Therefore, more important than its presence is the percentage of crossing. <20%
  • Higher is significant crossing
Alpha angle
Measured in 45° Dunn view. 		Degree of bulging of the femoral head-neck junction: In normal conditions there is a symmetric concave contour at the junction of the femoral head and neck. Loss of this concavity or bone bulging may lead to cam type impingement. The degree of this deformity can be measured by the alpha angle. Although it can be measured in the cross-lateral view, the 45° Dunn view is considered more sensitive and the frog leg view more specific in determining pathologic values.
  • Normal: ≤68° in men, ≤50° in women
  • Borderline: 69° to 82° in men, 51° to 56° in women
  • Pathological: ≥83° in men and ≥57° in women
Femoral head-neck offset
Measured in cross-lateral view. 		Offset of the femoral head with regard to most prominent aspect of the femora neck >10 mm
Offset percentage Femoral head-neck offset related to femoral head diameter >0.18
  • less indicates high risk of cam type impingement
Tönnis angle Slope of the sourcil (the sclerotic weight-bearing portion of the acetabulum) 0 to 10°
  • >10° is a risk factor for instability
  • <0° is a risk factor for pincer impingement
Caput-sourcil angle[21] Superior to the Tönnis angle in cases without joint space narrowing or subluxation.[21] The medial point of the sourcil is at the same height as the most superior point of caput femoris. −6 to 12°[21]
  • >12° is a risk factor for instability
  • <-6° is a risk factor for pincer impingement

Other modalities

[edit]

MRI imaging may follow, particularly if there is no specific evidence on radiographs, producing a three-dimensional reconstruction of the joint for better definition, to evaluate the hip cartilage, or measure hip socket angles (e.g. the alpha-angle as described by Nötzli[22] in 2-D and by Siebenrock in 3-D[23]). MR arthrogram had been used in the past, as it was more sensitive for picking up soft tissue lesions; however, due to improvement in technology, MRI is now considered comparable for picking up such lesions.[6] CT is not usually used due to radiation exposure and no benefit above MRI.[6] It is possible to perform dynamic simulation of hip motion with CT or MRI[24][25] assisting to establish whether, where, and to what extent, impingement is occurring.

The diagnosis is often made in conjunction with a labral tear.[26]

Differential

[edit]

Other conditions that may appear similar include:[8]

Prevention

[edit]

Prevention is currently being investigated.[27] The goal of prevention would be to avoid joint damage and premature hip osteoarthritis.[27] Studies are examining the effectiveness of screening adolescents in school and targeting at-risk individuals for education, physical therapy and decreasing participation in possibly harmful activities/sports as referenced in the epidemiology section.[27]

Treatment

[edit]

Treatment of FAI can be divided into those that are non-operative (conservative) and operative. Conservative treatment is often prescribed for those who have not yet received any therapy.[28] Conservative treatment includes physical therapy, avoidance of those activities that produce pain, and nonsteroidal anti-inflammatory drugs.[6] It may also include joint injections with cortisone or hyaluronic acid, particularly for those who wish to avoid surgery.[6]

Physical therapy is implemented for the purpose of improving joint mobility, strengthening muscles surrounding the joint, correcting posture, and treating any other muscle or joint deficits that may be exacerbating the condition.[11] A movement analysis may also be performed to identify specific movement patterns that may be causing injury.[29] Studies to demonstrate the effectiveness of physical therapy are currently underway, with no conclusive results to date.[28]

Operative treatment is generally recommended to those who continue to have symptoms. It involves the surgical correction of any bony abnormalities causing the impingement and correction of any soft tissue lesions, such as labral tears.[6] The primary aim of surgery is to correct the fit of the femoral head and acetabulum to create a hip socket that reduces contact between the two, allowing a greater range of movement.[30] This includes femoral head sculpting and/or trimming of the acetabular rim.[30][31]

Surgery may be arthroscopic or open.[7] A 2011 study analyzing current surgical methods for management of symptomatic femoral acetabular impingement, suggested that the arthroscopic method had surgical outcomes equal to or better than other methods with a lower rate of major complications when performed by experienced surgeons;[32] consequently, the surgery is now rarely done open.[33]

Outcomes of arthroscopic surgery are currently being studied, but have generally been positive. According to a 2019 meta-analysis, the risk of having surgery fail or need to be re-operated on is about 5.5% whereas the complication rate is 1.7%.[34] Additionally, patient reported outcomes show that approximately three to six months post-operative hip arthroscopy is when pain reduction and activities of daily life are improved. For sport function this timeline is about six months to a year.[35] Failure of hip arthroscopy is more likely to fail in older patients, females, or those who have experienced the symptoms of FAI for a long period of time.[34]

When performed on elite athletes, most are able to return their previous level of competition.[10] These athletes also have a higher rate of return to sport than recreational and collegiate athletes.[36]

Long term, randomized controlled trials evaluating the efficacy of conservative and operative treatments are underway.[37]

Epidemiology

[edit]

There has been limited research on the prevalence of FAI among a general population according to a literature review by Algarni.[38] On the other hand, there are many sources that discuss the prevalence of athletes with the condition especially those that are younger and white.[39] Hockey, tennis, soccer, and equestrian are all sports where the prevalence of a femoral neck abnormalities are higher due to the nature of the sport to force athletes into forced, loaded flexion and internal rotation.[38]

Cam lesions are more common in males, where pincer lesions are more common in females due to differences in anatomical development of the pelvis.[17]

History

[edit]

FAI is a relatively recent discovery. Its original description is credited to orthopedic surgeon Dr. Reinhold Ganz, who first proposed the condition as a cause for hip osteoarthritis in a publication in 2003.[40][41]

While the true diagnosis of FAI can be considered a relatively recent discovery, reports of damage to the femoroaceatabular region date back over a century ago in the orthopedic realm of medicine. It was not until the development of an open surgical dislocation procedure was developed that FAI was discovered as an anatomical difference and cause of osteoarthritis. Orthopedic surgeon Dr. Reinhold Ganz can be credited with this discovery in his 2003 publication that discussed the findings and relation to hip osteoarthritis.[10]

Society and culture

[edit]

Notable persons who have had hip impingement:

Notes

[edit]

References

[edit]

Further reading

[edit]
[edit]
Revisions and contributorsEdit on WikipediaRead on Wikipedia

Femoroacetabular impingement

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Femoroacetabular impingement (FAI), also known as femoroacetabular impingement syndrome (FAIS), is a pathological condition involving abnormal morphology of the joint, where the proximal contacts the acetabular rim prematurely during motion, causing mechanical abutment, labral tears, chondral damage, and progressive joint degeneration. This syndrome typically manifests as or pain in young, active adults, particularly athletes, and is recognized as a leading precursor to early-onset of the . FAI arises from developmental or acquired abnormalities in the -neck junction or , influenced by genetic factors such as variations in genes like GDF5 and FRZB, as well as environmental contributors like repetitive high-impact activities in sports such as soccer or hockey. There are three primary morphological types: cam-type FAI, characterized by an aspherical or reduced femoral head-neck offset leading to anterosuperior abutment; pincer-type FAI, involving acetabular overcoverage or retroversion causing anterolateral impingement; and mixed-type, the most common form combining elements of both, seen in approximately 73% of cases. Epidemiologically, FAI affects 10-15% of the general adult population, with higher prevalence among symptomatic athletes (up to 55%), and cam deformities occurring more frequently in men (9-25%) while pincer deformities predominate in women (up to 67%). Patients commonly present with insidious-onset pain in the , exacerbated by activities involving flexion, adduction, and internal (e.g., or sitting), often accompanied by mechanical symptoms like clicking, catching, or stiffness, and a characteristic "C-sign" posture during examination. relies on a combination of clinical , provocative tests such as the anterior impingement or FADIR (flexion-adduction-internal ) test, and imaging: plain radiographs to identify bony abnormalities (e.g., alpha angle >50-60° for cam, lateral center-edge angle >40° for pincer), with MRI or MR arthrography for assessing soft-tissue involvement like labral pathology. The 2016 Warwick Agreement provides an international consensus framework, defining FAIS as the triad of symptoms, clinical signs, and relevant morphology. Management begins with conservative measures, including patient education, activity modification, focused on strengthening and mobility (effective in up to 70% of adolescent cases over two years), and intra-articular injections such as corticosteroids for short-term relief or for potentially longer benefits. For refractory cases, surgical intervention via —preferred over open procedures due to lower morbidity—is employed to correct bony deformities (e.g., femoroplasty or acetabuloplasty) and repair associated lesions, yielding 90% survivorship at five years and superior outcomes compared to nonoperative care at 12 months in randomized trials. Early recognition and intervention are crucial to mitigate progression to , particularly in younger patients with significant morphological abnormalities.

Clinical Presentation

Signs and Symptoms

Femoroacetabular impingement (FAI) typically presents with an insidious onset of , which is the most common initial symptom, often described as sharp, aching, or intermittent and exacerbated by activities involving flexion, such as , sitting for prolonged periods, or sports participation. This may develop gradually without a specific traumatic event in many cases, though up to 60% of patients report onset following active sports activities. The pain frequently radiates to the buttock, , or , and patients may experience mechanical sensations such as catching, locking, clicking, or popping in the , particularly in more severe cases, which can suggest associated labral pathology. Additional associated symptoms include after periods of inactivity and in the girdle muscles. Functionally, FAI leads to reduced hip range of motion and limitations in daily activities, such as climbing stairs, prolonged sitting, or driving, as well as challenges in athletic performance, especially in pivoting sports like soccer or hockey. These symptoms often interfere with ambulation and high-intensity exercise, contributing to overall decreased in affected individuals, predominantly young, active adults.

Physical Examination Findings

The physical examination for femoroacetabular impingement (FAI) focuses on provocative maneuvers and assessments to elicit pain or mechanical symptoms indicative of intra-articular . A positive finding typically reproduces the patient's characteristic pain, aiding in differentiation from extra-articular conditions. Key tests emphasize reproducibility and are performed with the patient unless otherwise specified. The flexion-adduction-internal rotation (FADIR) test, also known as the anterior impingement test, is a primary provocative maneuver. With the patient , the is flexed to 90 degrees, adducted across the midline, and internally rotated; the test is positive if it elicits sharp anterior pain, suggesting abutment between the and acetabular rim. This test demonstrates high sensitivity (59–100%) and variable specificity (10–100%) for FAI, with one reporting a sensitivity of 99% specifically for intra-articular such as labral tears. Inter-observer reliability is strong, at 96%. Range of motion evaluation reveals characteristic restrictions in FAI. internal rotation is often reduced, with values below 10 degrees associated with concurrent osteoarthritic changes; normal passive hip flexion averages 120 degrees, but impingement-free flexion is approximately 101 degrees, beyond which pain may occur at end-range. Obligate external rotation during passive flexion can indicate intra-articular irritation. These findings help quantify functional deficits but require correlation with provocative tests for specificity. Additional maneuvers include the log roll test and (Stinchfield test) for differential assessment. The log roll involves passive external rotation of the extended hip while ; pain suggests intra-articular involvement and is highly specific for FAI-related pathology. The , performed at 30–45 degrees with extension, tests for hip flexor or capsular irritation; reproduction of pain differentiates FAI from . The flexion-abduction-external rotation (FABER) test, with the ankle placed on the contralateral and gentle downward pressure on the , is positive if it provokes ipsilateral pain, showing moderate sensitivity for FAI. is generally unrevealing for primary FAI but may identify secondary tenderness over the in cases with associated abductor involvement. In advanced FAI, may reveal subtle abnormalities, such as an antalgic pattern or Trendelenburg sign due to abductor weakness from chronic compensation. These observations, noted in approximately 12% of preoperative evaluations, provide context for functional impact but are not diagnostic in isolation. Overall, no single test confirms FAI; a cluster of positive findings enhances diagnostic accuracy.

Pathophysiology

Normal Hip Anatomy

The hip joint is a ball-and-socket formed by the articulation between the rounded and the cup-shaped of the , with both surfaces covered by that facilitates smooth, low-friction movement. This multiaxial design allows for a wide while providing inherent stability through its deep socket configuration. Key stabilizing structures include the , a fibrocartilaginous ring that attaches to the acetabular rim, deepening the socket by approximately 20-30% and enhancing joint congruence and stability via a suction-seal effect around the . The joint is enveloped by a fibrous capsule, which is reinforced anteriorly by the strong , the primary restraint to hip extension that blends with the capsule to limit excessive motion and maintain alignment. The femoral neck-shaft angle, typically ranging from 125° to 135° in adults, optimizes load transmission from the lower limb to the during activities. Relevant bony landmarks encompass the -neck junction, which exhibits a smooth, concave profile in the anterosuperior region to ensure unobstructed motion, and the acetabulum's rim and , where the rim forms the peripheral edge lined by the labrum and the provides superior weight-bearing coverage over the . The vascular supply to the hip joint primarily arises from retinacular vessels branching from the , which course along the to perfuse the , rendering this pathway susceptible to disruption in certain conditions. In normal , the 's geometry permits full flexion exceeding 120° and internal-external of approximately 30-40° without bony , supported by balanced ligamentous tension and muscular control to distribute forces evenly across the .

Mechanisms of Impingement

Femoroacetabular impingement (FAI) arises from abnormal contact between the proximal and the acetabular rim during hip motion, leading to supraphysiologic stress on the structures. This core mechanism disrupts the normal congruence of the , where the femoral head-neck collides with the acetabular margin, particularly in positions that narrow the space. Such contact generates elevated shear and compressive forces, which exceed the physiological limits of the articular surfaces and soft tissues. The kinematic factors contributing to impingement primarily involve hip flexion combined with adduction and internal rotation, motions that drive the proximal toward the acetabular rim and reduce clearance within the . During these dynamic activities, the is limited, and the experiences increased intra-articular pressure, promoting repetitive . Biomechanical studies indicate that these positions, common in daily activities like or in athletic maneuvers, amplify the contact forces compared to neutral postures, initiating a cycle of mechanical overload. This repetitive microtrauma triggers a pathological cascade, beginning with localized and progressing to delamination and labral fraying, as the initially absorbs excess stress but eventually fails under sustained load. The damaged labrum exposes the underlying chondral surfaces to direct shear, fostering progressive wear and synovial irritation that impairs lubrication. Capsular tightening may occur as a compensatory response, further restricting motion and altering load distribution across the . Over time, these changes lead to altered joint loading, with uneven stress concentrations accelerating degenerative processes and increasing the risk of early by 2.5 to 3.2 times in affected individuals. The cumulative effect of persistent impingement promotes subchondral and formation, independent of acute symptoms, ultimately compromising long-term joint stability and function.

Types of Femoroacetabular Impingement

Femoroacetabular impingement (FAI) is classified into three primary morphological subtypes: cam-type, pincer-type, and combined-type, based on the specific bony abnormalities contributing to abnormal contact between the -neck junction and the . Cam-type FAI arises from an aspherical -neck junction, often characterized by a pistol-grip where the appears flattened or convex rather than spherical, leading to impingement primarily on the during motion. This morphology results in outside-in shearing forces on the chondrolabral junction, causing and damage to the articular adjacent to the labrum. Pincer-type FAI, in contrast, involves excessive overcoverage of the by the , such as acetabular retroversion or deepened acetabular depth, which compresses the against the . This subtype can present with variations, including global overcoverage (e.g., coxa profunda or affecting the entire acetabular rim) or focal anterior overcoverage (limited to the anterosuperior rim), as well as subspine impingement where an enlarged contributes to extra-articular contact. Pincer impingement typically spares the initially but leads to labral crushing, intrasubstance tears, and eventual ossification of the labrum. The combined type, featuring both cam and pincer morphologies, is the most prevalent form of FAI, occurring in approximately 60-70% of cases among symptomatic young adults, and results in synergistic damage where cam-related cartilage shearing compounds pincer-induced labral compression. Prevalence estimates vary by population and study, but cam-type FAI accounts for about 25% of cases, often with isolated femoral abnormalities, while pure pincer-type is less common at around 15%, typically involving acetabular overcoverage alone. Gender differences are notable, with cam-type deformities more frequent in males (up to 70% male predominance) and pincer-type more common in females (up to 100% in some cohorts), reflecting potential sex-specific developmental influences.

Etiology and Risk Factors

Causes

Femoroacetabular impingement (FAI) arises primarily from developmental abnormalities in morphology during childhood and adolescence, particularly involving disrupted processes that lead to asphericity of the or retroversion of the . These changes occur as the grows and ossifies, where irregular bone formation at the can result in cam-type deformities characterized by an anterolateral bump on the , or pincer-type deformities due to excessive acetabular overcoverage. Such developmental perturbations are often subtle and progressive, influenced by the timing and pattern of physeal closure during . Genetic factors play a significant role in predisposing individuals to FAI morphologies, with evidence from twin and family studies suggesting , though exact estimates vary and are not conclusively established for FAI. Specific genetic associations, such as variants in the GDF5 gene, have been linked to altered joint development and increased risk of abnormal femoral and acetabular geometries, as GDF5 regulates chondrogenesis and skeletal patterning. Recent studies, including those on GDF5 regulatory sequences, further elucidate genetic influences on shape variations linked to FAI risk. These inherited traits contribute to baseline skeletal variations that may only manifest as impingement under certain conditions. Hormonal influences, particularly estrogen's role in modulating acetabular growth during female adolescence, may further exacerbate retroversion and overcoverage, potentially elevating the risk of pincer-type FAI in women. Acquired contributors, including repetitive microtrauma from high-impact activities, can accelerate or induce FAI in susceptible individuals by causing adaptive or stress fractures at the . For instance, athletes in sports like soccer or hockey often experience cumulative loading that promotes cam lesion formation through shear forces during hip flexion. A notable precursor is (SCFE), where posterior slippage of the femoral epiphysis during growth disrupts normal head sphericity and predisposes to impingement later in life. Overall, FAI follows a multifactorial model integrating skeletal dysplasias, elevated activity levels, and abnormalities, where genetic predispositions interact with environmental stressors to produce symptomatic morphologies.

Epidemiology

Femoroacetabular impingement (FAI) is identified on in approximately 10-15% of adults, with cam-type deformities present in 23-37% of cases depending on the studied. Symptomatic FAI, however, is less prevalent in the general , with an estimated incidence of 54.4 per 100,000 person-years based on clinical diagnoses, though rates vary by study cohort and may reach 3% in specific athletic groups, such as NCAA athletes. This discrepancy highlights that many individuals with morphological abnormalities remain , while symptomatic cases often prompt medical evaluation. FAI typically manifests in young adults, with the highest incidence peaking between ages 20 and 40 years, particularly among physically active individuals. differences are notable in morphological subtypes: cam-type FAI is more common in males, with a prevalence ratio approaching 2:1 compared to females, whereas pincer-type FAI predominates in females. Mixed morphology occurs frequently across both genders, but these patterns underscore sex-specific anatomical variations in development. Athletic populations exhibit markedly higher rates of FAI, especially in sports involving repetitive hip flexion and rotation, such as and soccer. For instance, radiographic cam deformities are present in approximately 70% of elite players, while prevalence reaches 60-70% among professional soccer athletes. These elevated rates reflect the mechanical demands of such activities, which may exacerbate underlying morphologies into symptomatic conditions. Geographic and ethnic variations in FAI prevalence appear limited, with similar radiographic findings reported across Western and Asian populations, ranging from 14% to 67% on imaging. However, underdiagnosis is common in non-athletic groups due to lower utilization of advanced imaging, and overall incidence has risen steadily—doubling in some regions over the past decade—likely attributable to increased awareness and diagnostic imaging.

Diagnosis

Clinical Evaluation

The clinical evaluation of femoroacetabular impingement (FAI) begins with a comprehensive patient history to identify characteristic patterns suggestive of the condition, followed by an integrated to corroborate findings and assess functional impact. This approach allows providers or specialists to raise suspicion for FAI in young, active individuals presenting with hip-related complaints, emphasizing mechanical symptoms over acute trauma. During history-taking, clinicians query the onset of symptoms, which is typically insidious rather than traumatic, often developing over months in patients aged 15 to 50 years who engage in repetitive flexion activities such as sports, squatting, or prolonged sitting. Pain is commonly localized to the (in up to 83% of cases) or anterolateral , exacerbated by activities involving deep flexion and rotation, and may include mechanical sensations like clicking, catching, or instability; associated inquiries cover prior injuries, childhood conditions such as or Legg-Calvé-Perthes disease, previous treatments including physiotherapy or injections, and family history of joint disorders, as these elements inform the likelihood of intra-articular . Patients' activity levels, expectations, and impact on are also documented to gauge baseline function. The physical examination integrates observational and provocative maneuvers to evaluate hip mechanics holistically, starting with to detect antalgic patterns or Trendelenburg sign indicating abductor weakness, followed by assessment of , which often reveals reduced flexion and internal rotation. Key impingement tests include the FADIR (flexion, adduction, internal rotation) maneuver, positive in approximately 88% of FAI cases due to reproduction of groin pain, and the FABER (flexion, abduction, external rotation) test to provoke labral stress; these are combined with strength testing of stabilizers using dynamometry and evaluation for hypermobility via the Beighton score. tools, such as the Hip Outcome Score (HOS), provide a standardized baseline measure of function in and sports-specific tasks, aiding in tracking symptomatic severity. Red flags warranting urgent evaluation include night pain unrelieved by rest, acute onset suggesting or , or systemic symptoms such as fever, , or inflammatory markers, which may indicate alternative pathologies like or rather than isolated FAI. Diagnostic criteria for FAI, as outlined in international consensus guidelines, require a combination of suggestive history (e.g., activity-related ), positive physical exam findings (e.g., impingement signs), and confirmatory imaging, though the initial clinical assessment establishes pretest probability in symptomatic patients without red flags. In young, active patients with positive impingement tests and no red flags, early referral to an orthopedic specialist is recommended to facilitate timely and prevent progression to , aligning with best practice consensus for suspected FAI syndrome.

Radiographic Assessment

Radiographic assessment of femoroacetabular impingement (FAI) begins with plain s as the first-line modality to evaluate bony morphology of the . Standard views include the anteroposterior (AP) pelvis radiograph, obtained with the patient , legs in 15° internal rotation, and the X-ray beam centered between the anterior superior iliac spines and the upper border of the pubis to ensure neutral and rotation, minimizing artifacts from malpositioning. Additional views comprise the frog-leg lateral, with the patient , hips flexed and abducted while knees are extended, and the cross-table lateral, performed with the opposite flexed to avoid overlap. These views allow visualization of the femoral head-neck junction and acetabular coverage without the need for advanced . For cam-type FAI, key measurements focus on femoral head asphericity. The alpha , measured on the frog-leg or cross-table lateral view by drawing a line from the center to the narrowest point of the head-neck junction and extending to the point where the femoral neck axis intersects the subchondral bone, is considered abnormal if greater than 50-55 degrees, indicating loss of sphericity. Visualization of the pistol-grip deformity, characterized by a flattened or convex -neck contour on the AP pelvis view, further supports cam morphology. Pincer-type FAI is assessed through acetabular overcoverage or retroversion on the AP pelvis view. The lateral center-edge angle (LCEA), formed between a vertical line through the femoral head center and a line to the lateral edge of the sourcil (weight-bearing zone of the acetabulum), exceeding 39-40 degrees suggests excessive lateral coverage. The crossover sign, where the anterior acetabular rim projects laterally to the posterior rim on the AP view, indicates acetabular retroversion. Other radiographic signs include the hernation pit, a small ovoid radiolucent lesion at the anterosuperior femoral neck representing a synovial herniation, and subchondral cyst formation in the femoral head or acetabulum, often associated with impingement stress. Labral ossification or abnormal femoral head ossification may also appear as marginal bony proliferations. Plain X-rays demonstrate high sensitivity for detecting cam lesions, ranging from 71% to 96% depending on the lateral view used, but are less reliable for subtle pincer deformities.

Advanced Imaging Modalities

(MRI), particularly MR arthrography (MRA), serves as the gold standard for evaluating soft tissue structures in femoroacetabular impingement (FAI), with MRA demonstrating a sensitivity of approximately 90% for detecting acetabular labral tears compared to arthroscopic findings. MRA enhances visualization by distending the with intra-articular contrast, which improves delineation of labral and associated chondral defects, such as or full-thickness loss, often located anterosuperiorly. Additionally, MRI excels at identifying , a marker of active impingement-related stress, appearing as hyperintense regions on T2-weighted sequences in the or . Computed tomography (CT) provides superior bony detail through three-dimensional (3D) reconstructions, enabling precise assessment of femoral and acetabular morphology, including femoral version angles and asphericity of the femoral head-neck junction. For instance, CT can quantify the alpha angle in multiple planes, aiding in surgical planning for cam-type FAI by simulating and impingement sites. However, its use is limited in younger patients due to ionizing exposure, which can range from 5 to 10 mSv per scan, posing a small but cumulative of . Dynamic techniques, such as MRA combined with motion , replicate to identify impingement zones not evident on static images, using patient-specific 3D models derived from MRI data to avoid . These methods simulate flexion, adduction, and internal , highlighting contact points between the and that correlate with labral and chondral injury locations. Quantitative metrics enhance diagnostic precision; the clock-face positioning system localizes lesions relative to the acetabular rim, with cam-related damage typically occurring between the 1:00 and 3:00 positions on a right hip . T2 mapping, a biochemical MRI , assesses composition by measuring relaxation times, where elevated T2 values (>40 ms) indicate early degeneration in FAI-affected acetabular compared to normal values (25-35 ms). Advanced is indicated when plain radiographs are inconclusive for confirming FAI morphology or to evaluate intra-articular damage prior to , with contrast-enhanced sequences occasionally used to assess synovial vascularity or in complex cases.

Differential Diagnosis

The differential diagnosis of femoroacetabular impingement (FAI) encompasses a range of intra-articular, extra-articular, systemic, and rare conditions that present with overlapping symptoms such as , , or mechanical discomfort, necessitating careful clinical and evaluation to distinguish them. Key differentiation relies on history (e.g., acute versus insidious onset), (e.g., provocative tests), findings, and targeted , as FAI typically features motion-related without systemic signs or acute trauma. Intra-articular mimics include isolated acetabular labral tears, which cause mechanical symptoms like catching or locking but lack the bony morphology of FAI on radiographs; these are confirmed via MRI showing labral disruption without cam or pincer deformities. Loose bodies within the , often from prior trauma or synovial chondromatosis, produce intermittent locking and exacerbated by , distinguishable by their visualization as intra-articular fragments on MRI or CT, unlike the repetitive impingement in FAI. , whether idiopathic or secondary to overuse, presents with and warmth but minimal mechanical symptoms; MRI reveals synovial thickening and fluid without structural abnormalities seen in FAI. Extra-articular conditions frequently overlap with FAI symptoms and include tendinopathy, characterized by anterior groin snapping or pain during flexion, with MRI demonstrating at the iliopsoas insertion rather than intra-articular pathology. , involving or gluteal tendinopathy, localizes pain laterally over the trochanter and responds to resisted abduction, contrasting with the anterolateral groin pain and positive FADIR test in FAI; or MRI shows peritendinous without acetabular or femoral deformities. radiculopathy, often from L2-L4 compression, refers pain to the with associated back symptoms, sensory changes, or positive straight-leg raise, and lacks hip-specific impingement signs on exam or imaging. Systemic disorders such as , including , may mimic FAI through hip involvement causing stiffness and pain, but feature bilateral symptoms, elevated inflammatory markers (e.g., positivity), and changes on MRI, differing from the unilateral, mechanical nature of FAI. of the presents with insidious groin pain and risk factors like use, diagnosed by MRI showing subchondral or collapse, without the impinging bony morphology of FAI. Rare mimics include stress fractures of the , which cause acute, in athletes and appear as linear fractures on MRI, unlike the chronic, activity-related symptoms of FAI. Tumors, such as or , are uncommon but present with night or systemic symptoms, identified by lytic or sclerotic lesions on CT/MRI. In pediatric populations, Legg-Calvé-Perthes disease can simulate FAI through deformity and , but occurs in children aged 4-8 years with avascular changes on MRI, contrasting with the adolescent/adult onset of primary FAI. Differentiation keys include a positive FADIR test and specific radiographic findings (e.g., alpha angle >50° for cam-type FAI) versus negative provocative tests and alternative imaging features in mimics, such as in or normal bony contours in tendinopathies. For instance, infections like exhibit fever, elevated CRP/ESR, and hip effusion on , prompting urgent aspiration, while FAI shows no .

Management

Prevention Strategies

Preventing femoroacetabular impingement (FAI) focuses on modifiable risk factors, particularly in and athletes involved in high-demand , to mitigate the development of symptomatic impingement through early intervention and lifestyle adjustments. Activity modification plays a central role, emphasizing the avoidance of repetitive extreme hip flexion positions that can exacerbate abnormal contact between the and . In , guidelines recommend limiting deep squatting, lunging, and pivoting maneuvers—common in activities like hockey, soccer, and —to reduce overuse and promote balanced training regimens that incorporate rest periods and multi-sport participation. Screening protocols target at-risk populations to enable early detection and intervention before symptoms manifest. Radiographic , including of the on frog-leg lateral views, is advised for symptomatic adolescents and athletes in high-risk sports such as football, where cam-type deformities are prevalent in up to 77% of asymptomatic collegiate players. This approach allows for monitoring morphological changes and recommending activity adjustments to prevent progression to labral or damage. Rehabilitation-oriented strategies emphasize proactive strengthening to enhance pelvic stability and reduce impingement risk. Programs focusing on core and stabilizer muscles, such as the and , improve neuromuscular control and postural alignment, thereby minimizing abnormal joint loading during dynamic activities. Evidence supports incorporating exercises like side-lying hip abduction and planks into routine training for athletes, as these interventions have demonstrated reductions in risk through better hip motor control. Public health initiatives from orthopedic societies promote awareness and early detection in vulnerable groups. The American Academy of Orthopaedic Surgeons (AAOS) and the Academy of Orthopaedic advocate for clinical screening using tests like the FADIR (flexion, adduction, internal rotation) maneuver in dancers and football players, who exhibit elevated FAI morphology due to repetitive hip motions. These guidelines recommend integrating education on activity limits into programs to foster joint-preserving behaviors. Sibling studies indicate heritable components in acetabular and femoral morphology.

Non-Surgical Treatment

Non-surgical treatment for femoroacetabular impingement (FAI) syndrome primarily involves conservative measures aimed at alleviating pain, improving hip function, and delaying or avoiding surgical intervention, particularly in patients with mild symptoms or early disease. These approaches are recommended as the initial management strategy for most cases, with evidence supporting their use in reducing inflammation and enhancing mobility. Pharmacologic options focus on symptom control. Nonsteroidal anti-inflammatory drugs (NSAIDs), such as ibuprofen, are commonly prescribed to reduce and associated with FAI, providing symptomatic relief without addressing the underlying impingement. Intra-articular injections offer short-term benefits by decreasing joint and improving and function for up to 3 months in select cases, though evidence is limited to small case series and they are less effective in severe . These injections are typically image-guided to ensure accurate delivery into the hip joint. Physical therapy forms a cornerstone of non-surgical , emphasizing flexibility, strengthening, and neuromuscular control through structured protocols. Typical programs last 4-12 weeks and are divided into phases: an initial acute phase (0-4 weeks) focusing on range-of-motion exercises, core activation (e.g., transversus abdominis engagement), and basic strengthening like gluteal clam shells and bridges; followed by a subacute phase (4-12 weeks) advancing to functional exercises such as single-leg balances, lunges, and proprioceptive on unstable surfaces. Strengthening targets key muscle groups including the gluteals, hamstrings, and , performed 2-3 times weekly, with isometric exercises prioritized to avoid aggravating impingement. Supervised therapy for 6-8 weeks is often sufficient to assess response. Activity modification is a fundamental, non-invasive strategy recommended in the majority of cases to minimize stress. Patients are advised to rest from high-impact or aggravating activities, such as certain involving deep flexion, while incorporating low-load alternatives like or . is emphasized to reduce intra-articular loads, as excess body weight exacerbates symptoms in FAI. Efficacy of conservative management varies, with short-term improvements in pain and function observed in many patients. A systematic review and meta-analysis reported moderate to large effect sizes for pain reduction (standardized mean difference [SMD] 0.77) and function improvement (SMD 0.65) with combined physical therapy and injections. At 2-year follow-up, approximately 70% of patients respond to core conservative treatments, with an additional 12% benefiting from steroid injections, yielding an overall success rate of 82% in avoiding surgery. However, outcomes are less favorable in cases with structural severity; an alpha angle of 65° or greater on imaging predicts increased risk of chondral damage, potentially leading to conservative treatment failure. Adjunctive measures may enhance outcomes in specific scenarios. Orthotics can address leg length discrepancies contributing to asymmetric hip loading, while image-guided injections support targeted therapy delivery. Patient education on posture and movement patterns is integrated to promote long-term adherence.

Surgical Interventions

Surgical interventions for femoroacetabular impingement (FAI) aim to correct underlying bony abnormalities and associated damage to alleviate , restore mechanics, and prevent further degeneration. These procedures are typically considered after failure of conservative management and are tailored to the morphological type of impingement, such as cam, pincer, or combined lesions. has become the preferred approach for most cases due to its minimally invasive nature, while open techniques are reserved for more complex deformities. Arthroscopic is the standard treatment for the majority of FAI patients, involving small incisions (0.6-1.5 cm) to insert an arthroscope and instruments for visualization and repair. Key components include femoroplasty, which reshapes the femoral head-neck junction to restore and offset by resecting cam lesions; acetabuloplasty, which trims acetabular rim overcoverage in pincer impingement; and labral repair or reconstruction to reestablish the suction-seal mechanism of the hip joint. For chondral defects encountered during , microfracture may be performed by drilling small holes into the subchondral to stimulate repair. These procedures are generally completed in 1-2 hours as an outpatient basis, allowing patients to return home the same day. Open surgical techniques, such as surgical hip dislocation, are indicated for complex cases involving extensive deformities, revision surgeries, or concomitant procedures like periacetabular . This approach, originally described by Ganz and colleagues, utilizes a trochanteric flip and posterior to provide full 360-degree access to the , enabling precise reshaping of both femoral and acetabular morphology while preserving vascular supply to the . It is particularly useful when may not adequately address severe abnormalities. Indications for surgery include persistent symptoms in young, active patients despite at least 3-6 months of non-operative , confirmed by clinical signs (e.g., positive impingement tests) and imaging evidence of FAI morphology without advanced (joint space >2 mm). Contraindications encompass severe , joint space narrowing less than 2 mm, advanced age, or significant comorbidities that increase surgical risk. Postoperative rehabilitation typically spans 3-6 months, beginning with protected weight-bearing using crutches for 1-2 weeks, progressing to focused on restoring , strength, and . Patients often achieve significant pain relief and functional improvement, with success rates of 85-90% in returning to pre-symptom activity levels, particularly in those without preexisting .

Prognosis and Outcomes

Long-Term Prognosis

The natural history of untreated femoroacetabular impingement (FAI) involves progressive deterioration of function and a substantial risk of (OA) development. In nonsurgically managed patients, radiographic OA progression occurs in approximately 35% at a mean follow-up of 12.5 years, with conversion to total arthroplasty (THA) in 10.5% of cases. Cam-type FAI is associated with accelerated progression, conferring a of 3.5 for OA advancement compared to other morphologies. Patient-reported outcomes, such as the Non-arthritic Hip Score, worsen significantly by 6.7 points over 1 to 2 years without intervention, indicating early symptomatic decline. Following treatment, particularly hip arthroscopy, long-term outcomes demonstrate a potential to alter the disease trajectory. Arthroscopic correction of FAI reduces the risk of OA progression by 42% relative to nonsurgical management, with 25% of patients exhibiting less arthritis in the operative hip than the contralateral side at 10-year follow-up. OA progression rates post-arthroscopy are 26.5% at 12.5 years, lower than in untreated cohorts, suggesting a delay in degenerative changes of at least 10 years in select young patients without advanced baseline damage. Approximately 70-80% of athletes return to sport at their preinjury level, with sustained improvements in activity scales like the Sports Activity Scale from 2.3 to 3.0 over 10 years. Prognostic factors significantly influence long-term success. Younger age (under 30 years) predicts superior outcomes, with patients aged 27.2 years achieving higher patient-acceptable symptom states (54.7% success rate) compared to those over 30 (mean age 30.0 years). Minimal damage, reflected by wider preoperative space (4.4 mm vs. 3.9 mm), correlates with better 12-year results, while higher (BMI >25 kg/m²) adversely affects , increasing failure . Quality of life improves markedly post-treatment, though some residual symptoms persist. Modified Harris Hip Scores (mHHS) rise from a preoperative mean of 62.6 to 88.8 at midterm follow-up, reflecting enhanced function and reduced pain (visual analog scale from higher baseline to 1.9). Persistent pain affects 10-20% of patients at 2 years, often linked to preoperative factors like or chondral lesions. Long-term follow-up includes periodic radiographic monitoring to assess OA progression, typically annually in high-risk cases. The lifetime risk of revision is approximately 10%, primarily due to residual impingement or .

Complications and Recurrence

Surgical complications following arthroscopic treatment of femoroacetabular impingement (FAI) are relatively uncommon, with overall rates ranging from 0.5% to 6.4%, though minor issues predominate. Iatrogenic chondral injury, often resulting from instrument contact during resection, occurs in approximately 1-2% of cases and can contribute to accelerated joint degeneration if severe. dysesthesia, particularly involving the lateral femoral due to portal placement or traction, affects about 5% of patients, typically resolving within months but occasionally persisting. Heterotopic , the formation of ectopic bone in soft tissues, is reported in less than 1% of procedures when prophylactic nonsteroidal drugs are used, though rates can rise to 8-44% without such measures. Recurrence of FAI symptoms often stems from incomplete resection of the impinging bone, leading to re-impingement in 5-10% of cases, as evidenced by persistent cam or pincer morphology on postoperative imaging. Revision surgery rates are approximately 5.5% within 5 years, with higher incidences (up to 11-18%) observed in the first 2 years post-procedure, frequently due to residual impingement or iatrogenic . Non-surgical management of FAI carries risks including progression to osteoarthritis if the condition remains untreated, as repetitive impingement can cause cumulative labral and chondral damage leading to joint space narrowing over time. Intra-articular injections, such as corticosteroids for symptom relief, are associated with rare complications like (less than 0.1%), though risks of or tissue weakening exist with repeated use. Key risk factors for complications and recurrence include over-resection of , which may induce hip instability and microinstability, and poor compliance with postoperative rehabilitation, which can impair capsular healing and muscle strengthening. Management of complications typically involves conservative measures initially, such as for or anti-inflammatory prophylaxis for heterotopic ; however, persistent issues like re-impingement often necessitate re-arthroscopy for residual correction, while severe chondral damage or advanced degeneration may require conversion to total hip arthroplasty.

History and Research

Historical Development

Early observations of hip deformities potentially related to femoroacetabular impingement (FAI) date back to the 1930s, when clinicians noted structural abnormalities preceding (OA) in active individuals, including athletes. In 1933, Elmslie described pre-existing deformities in patients with hip OA, suggesting mechanical factors contributed to joint degeneration. Similarly, Smith-Petersen in 1936 recognized impingement-like issues in cases among physically demanding patients and advocated for surgical correction to alleviate mechanical conflict. Building on these insights, mid-20th-century research began linking specific femoral morphologies to idiopathic hip OA. In 1965, Murray identified "tilt deformity" of the femoral head-neck junction—now recognized as cam-type morphology—in approximately 40% of idiopathic OA cases, proposing it as a predisposing factor rather than a consequence. This was further supported by Harris in 1986, who found that pistol-grip deformities preceded OA in 90% of affected hips, emphasizing the role of abnormal contact patterns. The term "femoroacetabular impingement" was formally introduced in 1999 by et al., who described it in the context of complications following periacetabular , marking a shift from viewing such deformities as isolated OA variants to a distinct mechanical . A pivotal milestone occurred in 2003 with the seminal paper by Ganz et al., which systematically defined FAI as a caused by abnormal abutment between the femoral head-neck and , delineating cam, pincer, and combined types as precursors to labral and chondral damage leading to OA in nondysplastic hips. This work unified prior scattered observations and propelled FAI recognition as a treatable entity. Pre-2010 advances included the early adoption of arthroscopic techniques for FAI correction; for instance, Clohisy et al. in 2005 reported on combined arthroscopic and limited open decompression for anterior FAI, demonstrating improved symptoms and morphology restoration. Concurrently, diagnostic paradigms shifted from reliance on open surgical exploration to advanced post-2000, with tools like the alpha (Nötzli et al., 2002) enabling noninvasive identification of cam impingement and the crossover sign (Reynolds et al., 1999) for pincer variants. Nomenclature evolved from "idiopathic OA" to the standardized FAI framework by the mid-2000s, reflecting growing consensus on its etiological role. The formation of the International Society for Hip Arthroscopy (ISHA) in 2008 in formalized international collaboration, fostering research and education on FAI and among specialists. These developments laid the groundwork for viewing FAI not merely as an OA harbinger but as an intervenable condition in young, active populations.

Recent Advances

Recent advances in for femoroacetabular impingement (FAI) have incorporated (AI) to enhance the detection of cam-type deformities through automated extraction of CT image landmarks, enabling faster and more consistent evaluation compared to traditional methods. Dynamic has emerged as a valuable tool for real-time assessment of hip instability and impingement, allowing clinicians to observe femoroacetabular motion during provocative maneuvers and guide interventions with high accuracy. These innovations address limitations in static by providing functional insights into impingement dynamics, particularly in athletes and young adults. In treatment, biologic adjuncts such as (PRP) injections have been explored alongside arthroscopic surgery, though a 2023 indicates mixed evidence for improved outcomes, with no clear benefit in pain relief or function at early or late follow-up. Advances in surgical precision include the integration of robotic-assisted systems in hip arthroscopy, which enhance stability and , potentially reducing complications through better control during femoroplasty and labral repair. These updates aim to optimize recovery while minimizing iatrogenic damage in FAI management. Research on preventing osteoarthritis (OA) progression has shown promising results from early femoroplasty via hip arthroscopy, with a 2024 study reporting a 42% relative risk reduction in OA advancement at minimum 10-year follow-up compared to non-operative cases. Genetic and molecular biomarkers are under investigation for screening high-risk individuals, with a 2025 systematic review identifying potential analytes in serum and synovial fluid that correlate with FAI onset and progression, facilitating earlier intervention. Long-term outcome studies demonstrate sustained functional improvements post-arthroscopy, with a 2022 reporting 90.4% preservation at 10 years and significant gains in patient-reported measures. However, the natural history of untreated FAI remains unclear, with some studies showing conflicting on its direct progression to OA. Emphasis on patient-reported outcomes using the PROMIS system has grown, with 2022 analyses showing strong correlations between PROMIS physical function scores and legacy hip-specific measures, aiding in standardized assessment of satisfaction and . Addressing care gaps, 2025 guidelines from physiatry and emphasize multidisciplinary approaches involving orthopedists, physical therapists, and pain specialists to tailor non-surgical and surgical strategies for FAI. Emerging regenerative therapies, including stem cell-based aspirate concentrates applied during , show adjunctive promise for chondral lesion repair, though high-quality randomized trials are needed to confirm efficacy.

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