Introduction: Cam-type femoroacetabular impingement (FAI) is a pre-osteoarthritic condition causing premature joint degeneration. Cam-deformities are characterised by decreased cranial offset of the femoral head/neck junction and aspherity of the femoral head causing delamination of the acetabular cartilage and detachment of the acetabular labrum. To asses the epidemiological aspects of cam-type FAI we evaluated Nötzlis alpha angle and our own Triangular Index (TI) for use on plain AP pelvic radiographs.

Materials and Methods: Cam malformation was assessed in 2.803 pelvic radiographs by the alpha (α) angle and the TI to define pathological cut off values. The α-angle and TI were assessed in AP and lateral hip radiographs of 164 patients scheduled for THR and the influence of varying rotation on the α-angle and TI was assessed in femoral specimens. The distribution of Cam-deformities was assessed in 3.712 standardized AP pelvic radiographs using the α-angle and TI.

Results: Mean AP α-angle male/female was 55°/45°. The α-angle and TI was highly interrelated, OR 8.6–35 (p< 0.001). Almost all cam-malformations were identifiable in AP projections, sensitivity 88–94% compared to axial view. The TI proved robust for cam identification during rotation (± 20°) compared to the α-angle (−10° to +20°). The distribution of pathologic TI and α-angle (Right/Left) were 11.6/12.5% and 6.1/7.4% in males and 2.2/3.2% and 2.1/3.8% in females. We found a pronounced sexrelated difference in cam-deformity distribution, OR 2.0–6.3 (p< 0.001).

Conclusion: The triangular index and the α-angle were found reliable for epidemiological purpose. Overall prevalence of definite cam-deformity was app. 10% in men and 2,5% in women.