The Cam Impinging Femur Has Multiple Morphologic Abnormalities

Introduction Femoro-acetabular impingement (FAI) is a common source of impaired motion of the hip, often attributed to the presence of an aspherical femoral head. However, other types of femoral deformity, including posterior slip, retroversion, and neck enlargement, can also limit hip motion. This study was performed to establish whether the “cam” impinging femur has a single deformity of the head/neck junction or multiple abnormalities. Materials and Methods Computer models of 71 femora (28 normal and 43 “cam” impinging) were prepared from CT scans. Morphologic parameters describing the dimensions of the head, neck, and medullary canal were calculated for each specimen. The anteversion angle, alpha angle of Notzli, beta angle of Beaule, and normalized anterior heads offset were also calculated. Average dimensions were compared between the normal and impinging femora. Results Compared to the normal controls, the impinging femora had wider necks (AP: 15.2 vs 13.3 mm, p 2mm), compared to only 14% of normal controls. Average head displacements for the two groups were: FAI: 1.93mm vs Normals: 0.78mm (p Conclusions The CAM impinging femur has many abnormalities apart from the “bump” at the head/neck junction. These femora have increased neck width and head/neck ratio, a smaller spherical bearing surface, and reduced neck offset from the medullary canal. Therefore, surgical treatment limited to localized re-contouring of the head–neck profile may fail to address significant components of the underlying abnormality.