Malposition of the acetabular component is a risk factor for post-operative dislocation after total hip replacement (THR). We have investigated the influence of the orientation of the acetabular component on the probability of dislocation. Radiological anteversion and abduction of the component of 127 hips which dislocated post-operatively were measured by Einzel-Bild-Röentgen-Analysis and compared with those in a control group of 342 patients.

In the control group, the mean value of anteversion was 15° and of abduction 44°. Patients with anterior dislocation after primary THR showed significant differences in the mean angle of anteversion (17°), and abduction (48°) as did patients with posterior dislocation (anteversion 11°, abduction 42°). After revision patients with posterior dislocation showed significant differences in anteversion (12°) and abduction (40°).

Our results demonstrate the importance of accurate positioning of the acetabular component in order to reduce the frequency of subsequent dislocations. Radiological anteversion of 15° and abduction of 45° are the lowest at-risk values for dislocation.

Aims: Postoperative dislocation of the implant is still an unsolved problem in THR. Malposition of the socket is reported to be one major risk factor. The aim of this study was to investigate the inßuence of cup orientation on the probability of postoperative prosthetic dislocation. Methods: The orientation of the socket was studied in 137 patients with postoperative dislocations, and compared with a control group (n=342). The EBRA-method (Ein-Bild- Roentgen-Analyse) was used to determine anteversion and inclination angles of the cups. A previous study showed that the precision of the method for measuring these parameters was ± 1degree (SD 0.33). Results: Mean values for the orientation of the prosthetic cup in the control group was 44¡ for inclination and 15¡ for anteversion. Patients with anterior dislocations after primary hip replacements showed signiþcant differences of the mean inclination (48¡, Mann-Whitney-Test: p=0.045) and anteversion angle (17¡, p=0.015). For posterior dislocations, mean inclination was 42¡ (p=0.277) and anteversion 11¡ (p=0.002) respectively. After revision hip arthroplasty, only patients with posterior dislocation of the implant showed signiþcant differences of the cup orientation (inclination: 40¡, p=0.03; anteversion:12¡,p=0.017). Conclusions: The results demonstrate the importance of accurate positioning of the socket to minimise the risk of subsequent dislocation. An anteversion of 15¡ and an inclination of 44–45¡ has the lowest at risk for dislocation.