This study evaluated the effect of the implant position on a postoperative dislocation after total hip arthroplasty (THA) using computed tomography.

1171 THAS were performed by one surgeon from March 1993 to September 1996, and from May 1998 to October 2004. There were 41 postoperative dislocations. Of these, 20 cases of the dislocated group and 20 cases of the control group were available for CT to check the acetabular and femoral anteversion of the implanted prostheses. Both groups were matched according to the type and size of the prosthesis, patient factors, and the surgical approach in order to eliminate the possible effect of these factors on a dislocation. All dislocations involved in this study occurred posteriorly. The cup anteversion, stem anteversion, acetabular inclination, vertical off-set, horizontal off-set and leg length discrepancy were measured using the CT and conventional radiographs on the PACS system.

The cup anteversion was significantly lower in the dislocated group than in the non-dislocated group (p<0.001, chi-square test) respectively. The other variables were not different significantly in both groups.

These findings suggests that among the variables of the implant positions, cup anteversion is the most important risk factor for a posterior dislocation after THA.