Influences of head/neck ratio and femoral antetorsion on the safe-zone of operative acetabular orientations in total hip arthroplasty

ObjectiveTo study the influences of head/neck ratio and femoral antetorsion on the safe-zone of operative acetabular orientations, which meets the criteria for desired range of motion (ROM) for activities of daily living in total hip arthroplasty (THA).MethodsA three-dimensional generic, parametric and kinematic simulation module of THA was developed to analyze the cup safe-zone and the optimum combination of cup and neck antetorsion. A ROM of flexion ≥ 120°, internal rotation ≥45° at 90° flexion, extension ≥30° and external rotation ≥ 40° was defined as the criteria for desired ROM for activities of daily living. The cup safe-zone was defined as the area that fulfills all the criteria of desired ROM before the neck impinged on the liner of the cup. For a fixed stem-neck (CCD)-angle of 130°, theoretical safe-zones fulfilling the desired ROM were investigated at different general head-neck ratios (GR=2, 2.17, 2.37, 2.61 and 2.92) and femoral anteversions (FA=0°, 10°, 20° and 30°).ResultsLarge GRs greatly increased the size of safe-zones and when the CCD-angle was 130°, a GR>2.37 could further increase the size of safe-zones. There was a complex interplay between the orientation angles of the femoral and acetabular components. When the CCD-angle was 130°, the optimum relationship between operative acetabular ante-version (OA) and femoral antetorsion (FA) could be estimated by the formula: OA=-0.80×FA+47.06, and the minimum allowable operative acetabular inclination (OImin) would be more than 210.5xGR2255.ConclusionsLarge GRs greatly increase the size of safe-zones and it is recommended that the GR be more than 2.37 so as to extend the acceptable range of error that surgeons cannot avoid completely during operation. As to the optimum operative acetabular inclination (OI), surgeons need to make a decision combining with other factors, including stress distribution, soft tissue and cup wear conditions, as well as patients' individual situations and demands. The data obtained from this study and the module of THA can be used to assist surgeons to choose and implant appropriate implants.