Purpose: The alpha angle is used to quantify in a single plane the head-neck junction deformity of cam femoro-acetabular impingement (FAI). When the deformity overlaps the superior retinaculum, femoral head osteoplasty in this area can jeopardise intra-articular vascular structures. This study proposes a new angular measure of the linear radial extension of cam deformity as a planning tool for bone resection and compares the accuracy of femoral head osteoplasty using open and arthroscopic surgery.
Methods: Twenty-five symptomatic patients operated on for FAI were included in this study. Radial magnetic resonance imaging (MRI) was done before and after surgery. Bi-dimensional coordinates of the vascular foramina and radial extension of the deformity (omega angle) were measured. This extension was correlated with the vascular foramina location and alpha-angle value. Accuracy of resection and hip function were evaluated before and after surgery.
Results: The cam lesion frequently extended posteriorly. No relation between values of alpha and omega angles was found. Cam resection was complete in 88 % of cases; there was a significant improvement in outcome score after surgery.
Conclusions: This study showed that alpha angle, measured in one plane, was not a predictor of the radial extension of cam deformity. To achieve a full resection, it was frequently necessary to extend the femoral head osteoplasty over the retinacular area. Pre-operative determination of the omega angle and location of the vascular foramina helped improve cam resection safety and accuracy.
Keywords: Accuracy; Angular extension; Bone resection; Cam lesion; Deformity; Femoroacetabular impingement; Hip arthroscopy; Omega angle; Osteoplasty; Retinaculum; Surgical hip dislocation; Vascular foramina.