HP29: THE MORPHOLOGY OF THE FEMUR IN FEMORO-ACETABULAR IMPINGEMENT

Abstract

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 and reduced concavity of the anterior head/neck junction. 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 is a unique entity with a single deformity of the head/neck junction or is part of a multi-component continuum of femoral dysmorphia.

Materials and Methods: Computer models of 71 femora (28 normal and 43 “cam” impinging) were prepared from CT scans. Morphologic parameters describing the shape and dimensions of the head, neck, and medullary canal were calculated for each specimen. The anteversion angle, alpha angle of Notzli, beta angle of Beaulé, and normalized anterior heads offset were also calculated. Average dimensions were compared between the normal and impinging femora. A dimensionless model of the femoral neck was also generated to determine whether there is an inherent difference in the shape of the femoral neck in cam impinging and normal femora, independent of any differences in specimen size.

Results: Compared to the normal controls, the impinging femora had wider necks (AP: 15.2 vs 13.3 mm, p< 0.0001), larger heads (diameter: 48.3mm vs 46.0mm, p=0.032) and decreased head/neck ratios (1.60 vs 1.74, p=0.0002). However, there was no difference in neck/shaft angle (125.7° vs 126.5°, p=0.582) or anteversion angle (8.70 vs 8.44°, p=0.866). Most significantly, 53% of impinging femora also had a significant posterior slip (> 2mm), compared to only 14% of normal controls. Average head displacements for the two groups were: FAI: 1.93mm vs Normals: 0.78mm (p< 0.0001). Shape indices derived from individual dimensionless models showed slight AP widening of the abnormal femora (ap/ml ratio: 1.10 abnormal vs. 1.07 normal).

Conclusions: The CAM impinging femur has many abnormalities apart from the morphology of 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. Moreover, the presence of posterior head displacement and reduced anteversion should be appreciated when assessing treatment options, as surgical treatment limited to localized re-contouring of the head–neck profile may fail to address significant components of the underlying abnormality.