[Accuracy of hip stem migration measurement on plain radiographs: reliability of bony and prosthetic landmarks]

In Germany, more than 150,000 total hip arthroplasties (THA) are performed annually. Early implant migration is supposed to be the best indicator for mechanical failure of femoral stems. Therefore, radiological evaluation of hip stems is routinely done by analyzing plain radiographs of THA.

The purpose of this study was to evaluate the accuracy of implant migration measurement on plain radiographs.

Two observers analyzed 44 anterior-posterior radiographs of the pelvis in 22 pain-free patients at least 2 years after implantation of an anatomical hip stem. The evaluation was performed on digital as well as conventional plain radiographs. Intraobserver reliability was analyzed by double measurements of each radiograph and the evaluation of the consecutive series of each patient. The anatomical structures of interest were the greater trochanter and the minor trochanter. Furthermore, the tip of the prosthesis, the shoulder of the implant, and the center of rotation were determined.

The conventional measurement technique proved higher accuracy compared to the digital measurement technique. The best anatomical structure was the greater trochanter in the conventional technique and the minor trochanter in the digital technique. The best reference structure with regard to the implant was the shoulder of the prosthesis for both techniques.

The recommended reference structures for the evaluation of implant migration on plain radiographs are the greater trochanter and the shoulder of the implant for the conventional measurement technique and the minor trochanter and the shoulder of the implant for the digital technique. Migration of an implant should not assumed before a determined difference of 2 mm.