Combining femoral and acetabular parameters in femoroacetabular impingement: the omega surface

The concept of femoroacetabular impingement (FAI) proposes the development of hip osteoarthritis through motion-induced damage to the acetabular cartilage and labrum. Thus, dynamic interaction of the proximal femur and acetabulum is the crux of FAI. Several types of FAI can be distinguished, but FAI classification is mostly done with separate parameters for acetabular and femoral morphology on planar images, without direct representation of the femoroacetabular interaction. Five main parameters influence impingement between the proximal femur and the acetabular rim: alpha and center edge angles, acetabular and femoral version, and neck-shaft angle. We attempted to integrate these five parameters in order to reflect their interaction and derive a signal comprehensive parameter, the omega surface, to characterize the severity of FAI. The omega surface is a CT-based delineation of the femoral head surface that represents the area for impingement-free motion. The omega surface is determined with dedicated software (Articulis™) and can be determined for various positions of the hip joint. We determined the omega surface in a pilot study for five different hip morphotypes and found the omega surface was smaller in FAI morphotypes than in a normal hip. Furthermore, the omega surface was smaller in symptomatic versus control subjects with FAI morphotypes. The omega surface may therefore help in improved differentiation between symptomatic and asymptomatic FAI hips.