Differential Diagnosis for Femoroacetabular Impingement (FAI)
When evaluating a patient with suspected FAI, you must systematically exclude three primary categories of hip-related pain: (1) FAI syndrome itself, (2) acetabular dysplasia and/or hip instability, and (3) other soft-tissue conditions (labral, chondral, and ligamentum teres pathology) without specific bony morphology. 1
Primary Hip-Related Differential Diagnoses
The British Journal of Sports Medicine consensus guidelines establish that hip-related pain in young and middle-aged active adults falls into three distinct categories after imaging 1:
1. FAI Syndrome
- Characterized by symptomatic premature contact between the proximal femur and acetabulum 1
- Presents with cam, pincer, or mixed morphology on imaging 1
- Primary symptom is groin pain, though pain may radiate to back, buttock, or thigh 1
- Repetitive mechanical loading leads to labral and chondrolabral damage 1
2. Acetabular Dysplasia and/or Hip Instability
- Involves misalignment between femoral head and acetabulum due to changes in shape, size, and orientation 1
- Results in instability and acetabular rim overload during normal activities 1
- Requires measurement of center-edge angle on imaging 1
3. Soft-Tissue Conditions Without Specific Bony Morphology
- Labral tears: Can occur independently of bony abnormalities 1
- Chondral lesions: Cartilage damage without characteristic FAI or dysplasia morphology 1
- Ligamentum teres tears: Often overlooked but clinically significant 1
Critical Exclusions: Serious Pathology
Before diagnosing FAI, you must exclude serious musculoskeletal and non-musculoskeletal pathology 1:
- Perthes' disease 1
- Slipped capital femoral epiphysis (SCFE) 1
- Infections 1
- Tumors 1
- Stress fractures 1
- Early osteoarthritis 1
Extra-Articular Hip Conditions
The following conditions can mimic FAI and must be differentiated 2:
Competing Musculoskeletal Conditions
Always screen the lumbar spine and pelvis, as these regions commonly refer pain to the hip 1. The guidelines explicitly state that screening appropriate body regions is mandatory when evaluating hip-related pain 1.
Diagnostic Approach Algorithm
Step 1: Clinical Screening
- Use the FADIR (flexion-adduction-internal rotation) test: A negative test helps rule out hip disease 1
- Note: The FADIR test has good sensitivity but poor specificity, making it useful for screening but not confirmation 1
- Other special tests (Thomas test, prone instability test) have very limited diagnostic utility and are not recommended 1
Step 2: Initial Imaging
Obtain AP pelvis and lateral femoral head-neck radiographs (Dunn view, frog-leg view, or cross-table view) 1. This provides understanding of underlying hip morphology 1.
Step 3: Advanced Imaging When Indicated
Order MRI/MRA or CT scan for three-dimensional morphological assessment or evaluation of intra-articular structures (labrum, cartilage, ligamentum teres) 1.
Step 4: Integrate Clinical and Imaging Findings
Never make a diagnosis based on imaging alone—always combine imaging with symptoms and clinical signs 1. Diagnostic imaging has limited ability to confirm the specific cause of pain, with only small to moderate shifts in post-test probability 1.
Critical Pitfalls to Avoid
Incidental Imaging Findings
Labral, chondral, and ligamentum teres pathology are common on imaging in asymptomatic individuals 1. These should be labeled as "incidental findings" when patients lack corresponding symptoms 1.
Over-Reliance on Clinical Tests
Physical examination maneuvers have very limited ability to confirm FAI syndrome by increasing post-test probability 1. The diagnostic accuracy of symptoms alone for hip-related pain conditions is unknown 1.
Imaging Without Clinical Context
Diagnostic imaging demonstrates post-test probability shifts ranging from very small to substantial for FAI syndrome and acetabular dysplasia, but only small to moderate shifts for labral, chondral, and ligamentum teres pathology 1. The diagnosis must never be made using imaging alone 1.
Nuances in Differential Diagnosis
The distinction between these conditions is not always clear-cut. FAI syndrome is defined as a triad of symptoms, clinical signs, and imaging findings 3, 4. No single pathognomonic finding exists for FAI 3. The comprehensive approach requires detailed assessment of patient history, standardized physical examination with specific orthopedic maneuvers, and appropriate imaging 2, 3.
Bony morphology measurements (alpha angle, center-edge angle) must be clearly defined and reported 1 to distinguish between FAI syndrome, acetabular dysplasia, and soft-tissue conditions without characteristic bony abnormalities.