Femoroacetabular Impingement (FAI)

Introduction and Conceptual Framework

Femoroacetabular impingement (FAI) is a clinical syndrome in which abnormal mechanical contact between the proximal femur and the acetabular rim during hip motion produces progressive damage to the acetabular labrum, the chondrolabral junction, and the underlying articular cartilage. The concept of FAI, articulated principally by Reinhold Ganz and his collaborators in Bern, Switzerland, in the late 1990s and early 2000s, has transformed the understanding of pre-arthritic hip disease in young adults and has provided a unifying framework that connects the mechanical morphology of the hip to the clinical syndromes of hip pain, labral pathology, and the early development of hip osteoarthritis. The recognition that FAI is a major cause of hip pain in young adults — particularly in athletes and in patients with prior hip pathology such as Perthes disease, SCFE, and developmental dysplasia — has led to the rapid expansion of hip arthroscopy and surgical hip dislocation as treatments for this condition. This chapter, synthesizing content from Operative Hip Arthroscopy (Byrd), Apley & Solomon’s, Miller’s Review, and the Orthopaedic Surgical Approaches text, addresses the morphological subtypes of FAI, the clinical and imaging diagnosis, the natural history and association with osteoarthritis, and the surgical management options including hip arthroscopy and surgical hip dislocation.

Morphological Subtypes

Three principal morphological subtypes of FAI are described, each with distinct mechanical features and clinical implications. Cam Impingement Cam impingement arises from a non-spherical, often “pistol-grip” deformity of the femoral head-neck junction, in which the head-neck offset is reduced and a bony prominence at the anterolateral femoral head-neck junction enters the acetabulum during hip flexion and internal rotation. The aspherical head pinches against the acetabular rim and “shears” against the chondrolabral junction, producing a characteristic pattern of cartilage delamination at the anterosuperior acetabulum with the labrum often relatively preserved early in the disease. The aspherical morphology is quantified by the alpha angle, measured on a radial cross-sectional MRI or oblique-axial CT image: the angle between (1) a line from the center of the femoral head along the long axis of the femoral neck and (2) a line from the center of the femoral head to the point where the bony contour first exceeds the radius of the head. Alpha angles greater than 50-55° are considered abnormal, with higher values indicating more severe deformity. Cam impingement is more common in men, athletes, and patients with a history of childhood hip disorders (Perthes, SCFE, post-traumatic deformity). The aspherical morphology often develops during the adolescent growth spurt, particularly in young male athletes engaged in pivoting sports (football, hockey, soccer), suggesting that the

morphology may be acquired in response to the mechanical demands during the period of physeal maturation. Pincer Impingement Pincer impingement arises from excessive bony coverage of the femoral head by the acetabulum, producing a mechanical block to hip flexion as the femoral neck contacts the over-prominent acetabular rim. The labrum is the principal structure injured, with progressive crushing, ossification, and degeneration of the labrum at the impingement site. Cartilage damage is typically less severe than in cam impingement, and is often restricted to a “contre-coup” pattern at the posteroinferior aspect of the acetabulum from the levering effect of the impingement. Several specific anatomical variants produce pincer impingement: acetabular retroversion (the acetabulum facing more anteriorly than the normal slight anteversion); coxa profunda (a deep acetabulum with the floor extending to or beyond the ilio-ischial line); protrusio acetabuli (the femoral head extending medial to the ilio-ischial line); and focal anterior rim overcoverage from os acetabuli or other rim anomalies. Pincer impingement is more common in women than men, often in middle-aged women, and is often associated with relative inactivity rather than the athletic profile of cam impingement. Combined (Mixed) Impingement Combined impingement — features of both cam and pincer — is the commonest pattern in clinical practice, with most patients showing some degree of both abnormal femoral head- neck offset and abnormal acetabular coverage. The treatment must address both components.

Pathophysiology and Natural History

The repeated mechanical impingement of the femur against the acetabulum during hip motion produces progressive damage to the chondrolabral junction. In cam impingement, the principal pattern is cartilage delamination at the anterosuperior acetabulum, with a relatively well-preserved labrum until later in the disease. In pincer impingement, the labrum is the principal structure damaged, with progressive degeneration, fraying, ossification, and cyst formation in the labral substance. In both patterns, the eventual outcome is progressive joint damage with development of secondary osteoarthritis. The association of FAI with hip osteoarthritis is one of the major clinical insights of the past two decades. Several large cohort studies have demonstrated that morphological features of FAI (alpha angle, crossover sign, acetabular index) in young adults are strongly predictive of the development of symptomatic hip osteoarthritis in middle age. The Copenhagen Osteoarthritis Study and other long-term cohorts have established this risk association, although the absolute proportion of patients with morphological FAI who develop symptomatic OA is variable, and the role of preventive intervention in asymptomatic FAI remains debated.

Clinical Features

The classical presentation is of an active young adult (most commonly in the third or fourth decade) with insidious onset of activity-related groin pain. The pain is typically worse with prolonged sitting (particularly in deep-seated chairs or driving for long periods), with hip flexion activities (squatting, sitting on the floor), and with athletic activities involving pivoting, rotation, or sustained flexion. The “C-sign” — the patient cups the affected hip with the fingers anteriorly and the thumb posteriorly to indicate the location of pain — is a characteristic gesture and pattern of pain localization. Mechanical symptoms (clicking, catching, locking) are common when labral tearing has occurred. The pain is often poorly localized initially; differentiation from lumbar radicular pain, sports hernia, athletic pubalgia, and other causes of hip and groin pain may require careful clinical assessment and targeted imaging. Examination reveals: decreased hip flexion (often by 10-20° compared with the contralateral side); decreased internal rotation in flexion (the most reliable single examination finding); pain on the anterior impingement test (FADIR — flexion, adduction, internal rotation — reproduces the patient’s symptoms); pain on the posterior impingement test (extension and external rotation) in pincer-pattern impingement involving the posterior rim; and sometimes tenderness over the anterior hip joint. Range of motion of the lumbar spine, sacroiliac joints, and femoral nerve should be assessed to exclude other sources of pain.

Imaging

Plain Radiographs The standard imaging assessment begins with two views: a well-centered AP pelvis radiograph (with attention to standardized positioning, since rotation of the pelvis substantially alters the apparent morphology) and a lateral view of the hip (Dunn 45° or 90° view, cross-table lateral, or frog-lateral). The AP pelvis is assessed for: Acetabular morphology and version: The center-edge angle of Wiberg quantifies lateral acetabular coverage (normal 25-40°, with greater than 40° indicating overcoverage and less than 25° indicating undercoverage); the acetabular index (Tönnis angle) quantifies the slope of the acetabular roof (normal 0-10°, with negative values indicating overcoverage); the crossover sign (the anterior wall and posterior wall of the acetabulum crossing on the AP view rather than the normal pattern with the anterior wall lying medial to the posterior wall) indicates focal anterior overcoverage with relative acetabular retroversion; the posterior wall sign (the posterior wall failing to extend to the center of the femoral head) indicates a deficient posterior wall; the ischial spine sign (the ischial spine projecting medial to the ilio-ischial line) is a marker of acetabular retroversion; coxa profunda (the floor of the acetabulum extending to or beyond the ilio-ischial line) and protrusio acetabuli (the femoral head extending medial to the ilio-ischial line) are signs of overcoverage.

Femoral morphology: Reduced head-neck offset, asphericity of the femoral head, “pistol- grip” appearance of the proximal femur, and direct visualization of cam-type bumps at the anterolateral or anterior femoral head-neck junction. The lateral view (typically Dunn 45° lateral) provides better assessment of the femoral head-neck offset and the anterior cam morphology, with quantification by the alpha angle (>50-55° abnormal). Magnetic Resonance Arthrography MR arthrography with intra-articular gadolinium has become the standard imaging study for definitive diagnosis. It provides high-resolution assessment of the labrum (with intra- articular contrast highlighting tears as contrast extending into the labral substance or chondrolabral junction), the cartilage (with characterization of focal cartilage defects and delamination), the femoral head-neck morphology (with measurement of alpha angles on radial cuts), and any associated pathology including osteonecrosis, paralabral cysts, and synovial chondromatosis. Intra-articular injection of local anesthetic at the time of arthrography also serves as a diagnostic test: complete relief of pain after intra-articular anesthetic strongly supports an intra-articular source of pain. CT and 3D Imaging CT with three-dimensional reconstruction provides the most accurate bony morphological assessment and is increasingly used for surgical planning, particularly for complex cam morphology and for assessment of acetabular version and femoral version.

Treatment

Non-Operative Management Initial management of FAI is conservative in most patients. The components include: activity modification (avoidance of provoking positions and activities, with continuation of low-impact alternatives); physical therapy directed at hip range of motion, posterior chain strengthening, core stabilization, and gait retraining; non-steroidal anti-inflammatory medications; and intra-articular corticosteroid injection for inflammatory flares. The success of non-operative management is variable, with some patients achieving long-term symptom control and others progressing to surgical management. Recent randomized trials have provided more definitive guidance on the timing of surgical intervention.

Hip Arthroscopy Hip arthroscopy has become the principal surgical approach for FAI management since the early 2000s. The procedure is performed with the patient supine on a traction table (or lateral with traction) under general anesthesia. Standard arthroscopy portals (anterolateral, anterior, mid-anterior, posterolateral) provide access to the central compartment (cotyloid fossa, ligamentum teres, central acetabulum, central femoral head) and the peripheral compartment (femoral head-neck junction, capsule, labrum). The arthroscopic procedures for FAI include:

Femoral osteoplasty (osteochondroplasty): Resection of the cam bump at the femoral head-neck junction to restore normal head-neck offset. The procedure is the most consequential component of cam-type FAI surgery, and adequate resection is critical to both symptomatic relief and long-term success. Insufficient resection produces persistent impingement; excessive resection risks femoral neck fracture and is the main technical concern. Acetabuloplasty (rim trimming): Resection of the over-prominent acetabular rim in pincer impingement. The procedure is performed in conjunction with labral reattachment, since the labrum is typically taken down to access the rim and must be repaired or refixed. Labral repair or reconstruction: Labral tears are managed by anatomical repair when sufficient labral tissue is preserved; labral reconstruction (using iliotibial band autograft, hamstring autograft, or allograft) is increasingly performed for irreparable labral tears, with reported outcomes comparable to repair in well-selected cases. Capsular plication or closure: The hip capsule, which is typically incised for surgical access, is closed or plicated at the end of the procedure to restore capsular tension and prevent post-operative instability. Treatment of associated pathology: Loose bodies, articular cartilage defects (managed by debridement, microfracture, or in some cases autologous chondrocyte implantation), ligamentum teres tears, and synovial pathology are addressed at the same operation. Surgical Hip Dislocation (Ganz Approach) The surgical hip dislocation, developed by Ganz and colleagues in the 1990s, provides open access to the entire hip joint with preservation of the femoral head vascular supply. The procedure involves: a posterior-based trochanteric flip osteotomy that preserves the attachment of the short external rotators; capsulotomy with Z-shaped or T-shaped incision; dislocation of the femoral head anteriorly; circumferential inspection and treatment of all intra-articular pathology; reduction; and refixation of the trochanteric osteotomy. The approach allows comprehensive treatment of complex FAI, particularly in cases with severe deformity that cannot be adequately addressed arthroscopically, and is the gold standard for surgical training in hip preservation. The disadvantages include the morbidity of the open approach, the prolonged rehabilitation, and the small risk of trochanteric osteotomy complications. Periacetabular Osteotomy The Bernese periacetabular osteotomy (PAO, Ganz osteotomy) is the procedure of choice for symptomatic acetabular dysplasia in the young adult, as discussed in the chapter on developmental dysplasia of the hip. In selected cases of FAI with dysplastic features, PAO is performed alone or in combination with arthroscopic treatment of the femoral cam to address both the under-coverage and the impingement.

Choice Between Arthroscopy and Open Surgery The choice between arthroscopic and open surgical approaches for FAI depends on multiple factors: the morphology of the deformity (severe global pincer impingement and complex multidirectional cam deformity may be better addressed by open surgery); the associated pathology (extensive labral reconstruction, complex chondral lesions, or associated dysplasia may favor open or combined approaches); the surgeon’s experience; the patient’s preference; and the available facilities. Hip arthroscopy is the dominant approach for typical FAI in most contemporary practice, with open surgical hip dislocation reserved for complex cases.

Outcomes

The outcomes of FAI surgery in well-selected patients are generally good. Multiple large series, randomized trials (the FIRST trial, the FASHIoN trial), and registry data have demonstrated meaningful improvements in pain and function for the majority of patients, with arthroscopic hip surgery showing modest to substantial improvements in patient- reported outcomes compared with non-operative management. The proportion of patients who achieve substantial clinical improvement is approximately 70-85% in most series, with the failure rate (defined variably as conversion to total hip arthroplasty, re-operation, or unacceptable symptoms) of approximately 5-15% at 5-year follow-up. The principal predictors of poor outcome are: established hip osteoarthritis at the time of surgery (Tönnis grade 2 or higher; joint space less than 2 mm); older age (>50 years); female sex (in some series); associated dysplasia not corrected by simultaneous PAO; severe labral pathology not amenable to repair or reconstruction; and obesity. Patient selection has emerged as the principal determinant of outcome, with the recognition that FAI surgery is not curative in the patient who has already developed significant osteoarthritis. The long-term effect of FAI surgery on the progression to osteoarthritis remains under investigation. Several observational studies have suggested a delay in arthritis progression in patients who undergo successful FAI surgery, but a definitive demonstration of disease- modifying effect awaits longer follow-up and the maturation of ongoing randomized trials.

Special Considerations

FAI in Specific Populations Athletes: FAI is particularly common in athletes engaged in pivoting sports, with return to sport rates after surgery of 80-90% in many series. The timing of return is typically 4-6 months for non-contact sports and 6-9 months for contact sports. Adolescent athletes: The recognition of cam-type morphology developing during the adolescent growth spurt in athletes has raised questions about preventive measures and the role of early surgery. Current consensus is that surgery is reserved for symptomatic disease, with morphological abnormalities alone not warranting surgical intervention.

Post-SCFE patients: Patients with healed SCFE have a high rate of cam-type FAI with the residual deformity from the original slip. Arthroscopic osteochondroplasty, sometimes combined with proximal femoral osteotomy for severe deformity, addresses the residual impingement. Post-Perthes patients: Healed Perthes disease produces a variable combination of cam (from the deformed femoral head), pincer (from the dysplastic acetabulum), and dysplasia. Treatment is individualized and may combine arthroscopic osteochondroplasty with PAO or femoral osteotomy. Borderline Acetabular Dysplasia The patient with borderline acetabular dysplasia (center-edge angle 20-25°) presents a particularly challenging clinical situation, with features of both dysplasia and FAI. Aggressive labral debridement or rim trimming in this population can produce iatrogenic instability with disastrous outcomes. Modern management favors gentle labral repair without rim trimming, combined with capsular plication; PAO is considered for patients with predominantly dysplastic features. Hip Microinstability Hip microinstability — a syndrome of subtle multidirectional hip instability without frank dislocation — is increasingly recognized as a cause of hip pain that overlaps with FAI. Treatment combines physical therapy, capsular plication, and avoidance of capsular release or wide capsular dissection in arthroscopy.

Summary and Take-Home Points

Femoroacetabular impingement is a clinical syndrome in which abnormal mechanical contact between the proximal femur and acetabular rim produces progressive damage to the labrum, chondrolabral junction, and underlying cartilage. The morphological subtypes — cam impingement from aspherical femoral head-neck junction, pincer impingement from acetabular overcoverage, and combined impingement — are diagnosed by characteristic radiographic features (alpha angle, crossover sign, center-edge angle, ischial spine sign) and confirmed by MR arthrography. The clinical presentation is typically an active young adult with activity-related groin pain, decreased hip internal rotation in flexion, and positive anterior impingement test. Treatment is initially conservative with activity modification, physical therapy, and selective intra-articular injection; surgical management — predominantly by hip arthroscopy with femoral osteochondroplasty, acetabular rim trimming, and labral repair or reconstruction; less commonly by open surgical hip dislocation (Ganz approach) or by periacetabular osteotomy for associated dysplasia — addresses the underlying morphology and produces meaningful improvement in 70-85% of well-selected patients. Patient selection is the principal determinant of outcome, with established osteoarthritis being the strongest predictor of failure. The recognition of FAI as a major cause of pre-arthritic hip disease and a contributor to the development of hip osteoarthritis has transformed the orthopedic understanding of hip pain in young adults over the past two decades.