Dislocation following hip arthroplasty through the posterior approach – is there a "safe zone" for socket orientation?

Introduction & Aims:\ud Malorientation of the socket is a factor contributing to instability after arthroplasty but the optimal orientation of the cup in relation to the pelvis has not been unequivocally described. Large radiological studies are few and problems occur with film standardisation, measurement methodology used and alternative definitions of describing acetabular orientation.\ud Method:\ud A cohort of 1578 patients from a single institution is studied where all data on the patients was collected prospectively. Risk factors for patients undergoing surgery are analysed. A case is made to justify the use of the terms "Elevation" and "Twist" to describe angular position in relation to the pelvis since they are uniquely defined and can be applied to describe the alignment of the native socket and cup position. Radiological data was compared between dislocating cases and a series of non-dislocating hips matched 2:1 by operation type, age and diagnosis.\ud Results:\ud The overall dislocation rate for all 1578 cases was 3.29% but the rate varied according to the type of surgery performed. The rate in uncomplicated primary cases was 2.46% whilst it increased to 9.26% for cases undergoing second stage implantation for a two stage procedure for infection. Levene’s test for homogeneity of variance indicates that there is no significant difference in the variability of the dislocating and non-dislocating groups for either elevation (p=0.393) or twist (p=0.661).\ud Conclusions: \ud When surgery is carried out through a posterior approach a "safe zone" for socket orientation to avoid dislocation could not be defined. The cause of dislocation is multifactorial, details such as re-establishing the anatomic centre of rotation, balancing soft tissues and avoidance of impingement around the hip are important considerations