Abstract
Introduction: Plain radiographs are a poor indication of the overall axial and rotational alignment of a total knee replacement. A CT method is described to overcome these problems. Method: A multislice CT scanner took 1mm contingous slices from the acetabular roof to the dome of the talus with the legs immobile in a standard position. The scans are stored and reformatted to allow axial, coronal and sagittal images. The centre of femoral head, distal femur, tibial plateau and ankle joints are identiþed in three dimensions. The mechanical and anatomical axes are identiþed. The alignment of the prostheses is then measured against the mechanical axis in both AP and lateral planes. The rotation of the femoral component is measured relative to the transepicondylar axis. Tibial rotation was measured with reference to the posterior tibial condyles and the tibial tuberosity. Coupled femorotibial rotational alignment was assessed by superimposition of the femoral and tibial axial images. The accuracy of this technique has been checked by using a Ferro Arm which mechanically validates the mechanical axis. Results: The technique provides the only currently available measure of all the alignment characteristics required to assess the quality of a knee replacement. Conclusions: The Perth protocol provides an accurate assessment of alignment of the femoral and tibial components to a measured mechanical axis and rotational measurements of both components both individually and
Theses abstracts were prepared by Professor Dr. Frantz Langlais. Correspondence should be addressed to him at EFORT Central Office, Freihofstrasse 22, CH-8700 Küsnacht, Switzerland.
