What is a Cam Lesion?
A cam lesion is a bony deformity of the femoral head-neck junction characterized by a loss of the normal spherical contour of the femoral head and decreased head-neck offset, which causes abnormal contact with the acetabulum during hip motion 1, 2.
Anatomical Description
The cam lesion represents an asphericity of the femoral head where there is a relative increase in the discrepancy between the femoral head and neck offset 2. This deformity is often described as a "pistol-grip deformity" when visualized on anteroposterior radiographs 3. The excess bone at the head-neck junction creates premature mechanical contact between the proximal femur and acetabulum during hip movement, particularly in flexion and internal rotation 1.
Clinical Significance
The cam lesion is the primary osseous abnormality in femoroacetabular impingement (FAI) syndrome, where repetitive mechanical loading of cartilage between the femoral head and acetabular rim contributes to:
- Acetabular labral tears
- Chondral (cartilage) damage, typically in the anterosuperior acetabulum
- Early osteoarthritis 1, 4
The characteristic cartilage injury pattern from cam impingement is a well-defined acetabular articular cartilage lesion located 1 to 1.5 cm from the acetabular rim, initially sparing the labrum but eventually leading to labral detachment 4.
Important Clinical Considerations
Patient Demographics
Isolated cam impingement classically affects young athletic males around 20 years of age 4. The etiology is not fully understood, but recent evidence suggests high-impact sports place stress on the immature physis during growth, potentially playing an important role in development 2.
Diagnostic Pitfall: Soft Tissue Cam Lesions
A critical caveat is that not all cam lesions are osseous. In adolescents, there exists a subset of "soft cam" lesions that are primarily soft tissue (nonosseous) impingement 5. These lesions demonstrate:
- Thickened perichondral ring (71%)
- Periosteal thickening (26%)
- Cartilaginous epiphyseal extension (3%)
MRI is required to detect soft cam lesions, as they are underappreciated on radiographs. The alpha angle on MRI averages 63.5° versus only 51.3° on radiographs in these cases 5. This is clinically important because these patients present with similar symptoms and respond comparably to surgical treatment when conservative management fails.
Measurement and Classification
The cam lesion is quantified using the alpha angle, which measures the deviation from normal sphericity of the femoral head. Classification systems exist based on the shape and position of the lesion (superolateral, anterolateral, or anterior locations) 6, though emerging evidence suggests that cam lesion size may be more prognostically important than location 3.