Causes of Femoroacetabular Impingement (Cam and Pincer Syndromes)
Femoroacetabular impingement syndrome results from abnormal bony morphology causing symptomatic premature contact between the proximal femur and acetabulum during hip motion, with cam morphology arising from a non-spherical femoral head and pincer morphology from excessive acetabular coverage. 1
Cam Impingement Etiology
Cam morphology is caused by an aspherical femoral head with decreased head-neck offset, creating a "bump" at the femoral head-neck junction that causes abnormal contact with the acetabulum during hip flexion and internal rotation. 2, 3, 4
Developmental Factors
- Cam abnormalities are acquired during skeletal growth and may be influenced by high-impact sporting activities during adolescence. 3, 5
- Young athletes are predisposed to cam morphology due to repetitive stress on the physis during skeletal development, particularly from activities involving repetitive hip loading in maximal flexion. 4, 5
- Evolutionary and developmental factors both contribute to the development of cam morphology. 4
Soft Tissue Cam Lesions
- In adolescents, a subset of cam impingement may be primarily soft tissue (non-osseous), manifesting as thickened perichondral ring (71%), periosteal thickening (26%), or cartilaginous epiphyseal extension (3%). 6
- These soft tissue cam lesions are underappreciated on radiographs and require MRI for detection, with alpha angles on MRI significantly greater than on radiographs (63.5° vs 51.3°). 6
Pathological Sequelae
- Cam impingement causes anterosuperior acetabular cartilage damage through a shearing mechanism where the non-spherical femoral head separates cartilage from bone during flexion while the labrum remains intact. 2
- The repetitive mechanical loading leads to chondrolabral damage and early osteoarthritis. 1
Pincer Impingement Etiology
Pincer morphology results from excessive acetabular coverage that causes the acetabular rim to crush against the femoral neck during hip motion. 2, 7
Anatomical Causes
- Acetabular retroversion creates focal anterior overcoverage. 4
- Global acetabular overcoverage from a deep socket. 4
- Prominent anterior-inferior iliac spine can contribute to impingement. 4
Pathological Pattern
- Pincer impingement causes circumferential cartilage damage in a narrow strip at the acetabular rim. 2
- The labrum is crushed between the acetabular rim and femoral neck, leading to labral degeneration and ossification. 2
- Much less is known about the development of pincer morphology compared to cam morphology. 5
Mixed Morphology
Most cases present with a combination of both cam and pincer mechanisms rather than isolated pathology. 2, 7
- Of 302 analyzed hips, only 26 had isolated cam and 16 had isolated pincer impingement, demonstrating the predominance of mixed morphology. 2
Clinical Significance
Both cam and pincer impingement lead to progressive osteoarthritis of the hip through different but equally damaging mechanisms. 2
- Cam morphology with alpha angle >55° is strongly correlated with subsequent hip osteoarthritis development, with major cam abnormalities conferring high OA risk. 3, 8
- Labral damage indicates ongoing impingement and rarely occurs in isolation. 2
Important Caveat
Anatomic variations causing FAI are common in asymptomatic populations, meaning morphology alone does not equal disease—the syndrome requires the triad of symptoms, clinical signs, and imaging findings. 1, 4