Cup position alone does not predict risk of dislocation after hip arthroplasty

Abstract

We used a large prospective institutional registry to determine if there is a 'safe zone' that exists for acetabular component position within which the risk of hip dislocation is low and if other patient and implant factors affect the risk of hip dislocation. Patients who reported a dislocation event within six months after hip arthroplasty surgery were identified, and acetabular component position was measured with anteroposterior radiographs. The frequency of dislocation was 2.1% (147 of 7040 patients). No significant difference was found in the number of dislocated hips among the radiographic zones (±5°,±10°,±15° boundaries). Dislocators <50 years old were less active preoperatively than nondislocators (P=0.006). Acetabular component position alone is not protective against instability.

Keywords: arthroplasty; cup orientation; dislocation; hip; safe zone.

Figure 1

A scatter plot of the inclination angle versus the anteversion angle for the 142 dislocators and the 142 nondislocators to which they were matched.

Figure 2

The same scatter plot is shown as in Figure 1, but with the matched pairs connected with straight lines. If a ‘safe zone’ existed, the nondislocators would be within the zone, the dislocators would lie outside the zone, so the straight lines would all emanate from near the center of the plot.

Figure 3

A color plot shows every pixel at 1° increments; each pixel is a safe zone with boundaries of ±10° anteversion and ± 10° inclination. For example, the red box located at 32° anteversion and 30° inclination reflects the dislocation ratio for the box-shaped zone with boundaries from 22° to 42° anteversion and 20° to 40° inclination. Since the color of the box is orange, the dislocation ratio for the zone is close to 1.5, with more dislocators than nondislocators. The light green and yellow pixels depict ratios close to 1.0 (an equal number of dislocators and nondislocators). The ‘Lewinnek safe zone’ is the pixel at 15° anteversion and 40° inclination; it is light green (ratio = 0.9). If it were a ‘safe zone’, it would be dark green in color (ratio = 0).