Radiographical cup coverage underestimates in vivo coverage provided by total hip arthroplasty for dysplastic hip

摘要

The accuracy of using 2D anteroposterior (AP) pelvic radiographically determined cup coverage for patients with developmental dysplasia of the hip (DDH) after total hip arthroplasty (THA) to assess the cup coverage in retrospective clinical studies remains unclear. Cementless THA was performed on 20 DDH patients (20 hips). During surgery but after acetabular reconstruction, bone wax was pressed onto the uncovered surface of the acetabular cup. A surface model of the bone wax was generated by 3D scanning. The percentage of the acetabular cup covered in vivo by intact host acetabular bone was calculated by Geomagic Studio software. Acetabular cup coverage was also determined from a postoperative supine anteroposterior pelvic radiograph. The height of the hip center (distance from the center of the femoral head perpendicular to the inter-teardrop line) was also determined from radiographs. The radiographically determined cup coverage was a mean 6.93% (SD 2.47) lower than the in vivo determined cup coverage for 20 DDH patients (P<0.001). However, both methods yielded highly correlated cup coverage measurements (Pearson r=0.761, P<0.001). The size of the acetabular cup (P=0.001) but not the position of the hip center (high vs normal), was significantly associated with the difference between the radiographically determined and in vivo cup coverage. The 2D radiographically determined cup coverage conservatively reflects in vivo cup coverage and remains an important index (taking 7% underestimation errs and effect of greater underestimation of larger cup size into account) for assessing the stability of the cup and monitoring for adequate ingrowth of bone.