The Etiology of Cam-type Femoroacetabular Impingement: A Cadaveric Study.

Background: There is a dearth of literature examining the causes of cam-type femoroacetabular impingement (FAI) and when such morphology appears. The purpose of the current study was to analyze how the ossific portion of the proximal femur develops over time with respect to standard cam-type FAI parameters. Methods: A collection of 193 femurs from cadavers aged 4 to 21 years were evaluated. The age, sex, ethnicity, and status of the proximal femoral physes (open or closed) of each were recorded. Each specimen was digitally photographed in standardized anteroposterior and modified axial positions. From these photographs, the anterior offset, anterior offset ratio (AOR), and alpha-angle were determined. A cam lesion was defined as an alpha-angle >55 degrees on the lateral view. Results: The mean age of the specimens was 17.5 +/- 4.2 years. The majority were male (69%) and African American (79%) with closed physes (78%). There were significant differences among discrete age groups with respect to alpha-angle (P = 0.01), anterior offset (P < 0.01), and AOR (P < 0.01). In addition, younger femurs with open physes had a significantly higher mean alpha-angle (P < 0.01), lower mean anterior offset (P < 0.01), and higher mean AOR (P < 0.01) compared with older ones with closed physes. Specimens defined as having a cam deformity had a statistically higher alpha-angle (P < 0.01) and lower anterior offset (P < 0.01), but there was no difference in AOR values compared with specimens without a cam lesion (P = 0.1). Conclusions: The apparent decline in alpha-angles as age increases indicates that the traditional alpha-angle in younger patients measures a different anatomic parameter (ossified femur excluding the cartilaginous portion) than in older patients (completely ossified femur). This suggests that the bony alpha-angle is inappropriate in the evaluation of cam lesions in the immature physis. The AOR, rather than the anterior offset, may be more accurate in the evaluation of the growing proximal femur. Clinical Relevance: This study provides novel insight into, and enhances the understanding of, the development of cam-type FAI.