Can combining femoral and acetabular morphology parameters improve the characterization of femoroacetabular impingement?

Abstract

Background: Femoroacetabular impingement (FAI) presupposes a dynamic interaction of the proximal femur and acetabulum producing clinical symptoms and chondrolabral damage. Currently, FAI classification is based on alpha angle and center-edge angle measurements in a single plane. However, acetabular and femoral version and neck-shaft angle also influence FAI. Furthermore, each of these parameters has a reciprocal interaction with the others; for example, a shallow acetabulum delays impingement of the femoral head with the acetabular rim.

Questions/purposes: We introduce the new parameter "omega zone," which combines five parameters into one: the alpha and center-edge angles, acetabular and femoral version, and neck-shaft angle. We sought to determine whether the omega zone could differentiate patients with FAI from (1) normal control subjects (alpha < 55°), but also from (2) control subjects with elevated alpha angles (≥ 55°).

Methods: We evaluated CT data of 20 hips of male patients with symptomatic cam-type FAI and of 35 male hips extracted from 110 anonymized CT scans for vascular diagnosis. We excluded hips with osteoarthritis, developmental dysplasia, or coxa profunda (center-edge angle 20°-45° on AP pelvic view or corresponding coronal CT views). With dedicated software, femoral and pelvic orientation was standardized; we tested the omega zone in four hip positions in three distinct groups: patients with cam-type FAI (alpha > 60°) and control subjects with normal (< 55°) and high alpha angles (≥ 55°).

Results: The omega zone was smaller in patients with cam-type FAI than normal control subjects (alpha angle < 55°) at 60° and 90° of flexion (mean, 12%; 95% confidence interval [CI], 7-17; p = 0.008; Cohen's d = 9%; 95% CI, 4-13; p = 0.003). Furthermore, the omega zone was smaller in all positions in patients with cam-type FAI than control subjects with high alpha angles (0° p = 0.017, 30° p = 0.004, 60° p = 0.004, 90° p = 0.007). In contrast, the omega zone did not differ between control subjects with normal or high alpha angles. In all hips, the omega zone decreased with flexion, corresponding to a decrease in remaining impingement-free motion with flexion.

Conclusions: The omega zone visualizes and quantifies the interaction of the proximal femur and acetabulum. The omega zone differed between patients with cam-type FAI and control subjects with high alpha angles (≥ 55°), who could not be distinguished based on alpha angle alone. For hip-preserving surgery, it can help surgeons decide whether to address the femur, the acetabulum, or both.

Fig. 1

Alpha angle and Center-edge angle measurements are shown in steps of 30° in relation to the anatomical coordinate system according to the International Society of Biomechanics.

Fig. 2A–B

Coverage area (blue), alpha area (red), and omega zone (yellow) in AP (A) and craniocaudal (B) view with the hip in 0° flexion is shown.

Fig. 3A–B

Coverage area (blue), alpha area (red), and omega zone (yellow) in AP (A) and craniocaudal (B) view with the hip in 30°, 60°, and 90° of flexion is shown. With increasing flexion, the circle projected over the femoral head shifts to the anterior femoral head (bottom right).