Safe zone for transacetabular screw fixation in prosthetic acetabular reconstruction of high developmental dysplasia of the hip.

BACKGROUND Prosthetic reconstruction of hips with Crowe type-IV developmental dysplasia (a high complete dislocation) is technically demanding. Insufficient osseous coverage and osteopenic bone stock frequently necessitate transacetabular screw fixation to augment primary stability of the metal acetabular shell. We sought to determine whether a previously reported quadrant system for screw fixation of the acetabular cup can be applied in patients with high dislocation of the hip and to define a specialized safe zone for screw fixation in these hips, if needed. METHODS Using volumetric computed tomographic data and image-processing software, we made three-dimensional reconstructions of the osseous and vascular structures in eighteen hips in twelve patients. We virtually reconstructed a cup in the true acetabulum and dynamically simulated transacetabular screw fixation. We mapped the hemispheric cup into several areas and, for each, measured the distance between the virtual screw and the external iliac (femoral) and obturator blood vessels. In the six patients with unilateral high dislocation of the hip and a relatively normal, contralateral hip, the six relatively normal hips served as controls. RESULTS Reconstruction of the cup at the level of the true acetabulum shifted the center of rotation anteroinferiorly in the hips with a high, complete dislocation. Screws guided by the quadrant system frequently injured the obturator blood vessels in the hips with a high dislocation. In these patients, the safe zone shifted as a result of moving the prosthetic cup. CONCLUSIONS The quadrant system, although helpful in determining screw placement in hips with a normal center of rotation, can be misleading and of less value in guiding screw insertion to augment acetabular shells for hips with a high dislocation. We believe that a safe zone specific to hips with a high dislocation should be used to guide transacetabular screw fixation.