What is Femoroacetabular Impingement?
Femoroacetabular impingement (FAI) syndrome is a motion-related clinical disorder of the hip characterized by symptomatic premature contact between the proximal femur and the acetabulum, defined by a triad of symptoms, clinical signs, and imaging findings (typically cam, pincer, or mixed morphology). 1
Pathophysiology
FAI syndrome results from repetitive mechanical loading of cartilage between the femoral head and acetabular rim due to altered bone morphology at one or both sites. This repetitive trauma progressively damages:
- The acetabular labrum
- The chondrolabral junction
- Articular cartilage
- Can lead to early osteoarthritis if untreated 1, 2
Types of Morphology
There are three distinct anatomic patterns:
- Cam-type: Aspherical femoral head with abnormal head-neck junction
- Pincer-type: Acetabular overcoverage
- Mixed-type: Characteristics of both cam and pincer deformities 3
A third pillar has been identified: abnormal femoral torsion 4
Clinical Presentation
Primary Symptoms
The hallmark symptom is groin pain, though pain may also manifest in the back, buttock, or thigh 1. Notably, the absence of groin pain helps exclude FAI syndrome and/or labral tear findings 1.
Additional symptoms include:
- Clicking or catching sensations
- Hip buckling or giving way
- Stiffness
- Limited range of motion 2
Patient Population
FAI syndrome predominantly affects young and middle-aged active adults, typically between ages 20-40 years 4. It represents the most common hip condition in this demographic presenting with hip-related pain 1.
Diagnostic Approach
A comprehensive approach combining patient symptoms, clinical signs, and diagnostic imaging is essential, as clinical examination and imaging in isolation have limited diagnostic utility 5.
Critical First Step
Before diagnosing FAI syndrome, you must exclude:
- Serious pathology: tumors, infections, stress fractures, slipped capital femoral epiphysis (SCFE), Perthes' disease, osteoarthritis
- Competing musculoskeletal conditions: lumbar spine pathology 1, 5
Clinical Examination
Specific maneuvers aid diagnosis:
- Flexion-adduction-internal rotation (FADIR) test: A negative test helps rule out hip-related pain, though clinical utility is limited 1
- Flexion-abduction-external rotation (FABER) test 2
Important caveat: Most clinical tests have good sensitivity but poor specificity, making them useful for screening but not definitive diagnosis 1.
Imaging Requirements
Standard imaging should include 5, 2:
- Anteroposterior (AP) pelvis radiograph
- Lateral femoral head-neck radiographs (orthogonal views)
- MRI or MR arthrogram for soft tissue evaluation
- CT coxometry with femoral torsion measurement for surgical planning 6
Imaging must always be interpreted alongside symptoms and clinical signs—never in isolation 5.
Clinical Significance
FAI syndrome is increasingly recognized as a major cause of hip pain in young adults and represents the primary etiology of hip osteoarthritis in the young population 3. The condition predisposes affected individuals to developing future hip osteoarthritis if left untreated 7, 8.
The diagnosis has become more common due to increased recognition, with a significant rise in both diagnosis and arthroscopic hip surgery in recent years 1.