Introduction: The success of cemenntless THA (total hip arthroplasty) mainly depends on the choices of stem, its size and accuracy of stem orientation. Selection of the optimal stem judging only by plain X-ray is not so easy. Because deformity varies in each case and it is impossible to obtain profile view of the hip. As osteoarthritic patients tend to develop external rotation contractures, radiographic position of the patients with correct rotation is very difficult. To override these problems, we have been using 3-D preoperative planning system. As for the stem selection, we have been mainly using Revelation stem, because it has a structure called lateral flare that provide proximal physiological load transfer. In the present study, the usefulness of our preoperative planning system especially for the determination of the size and stem orientation with Revelation stem.

Materials and Method: Pre-operative planning was performed in 55 osteoarthritic hips in 50 patients (10 male and 40 females), and the mean age at the operation was 64.05 years old. The 3-dimensinal geometries of the femora femora were reconstructed from the CAT scan DICOM data. The geometry of femur and components were placed on the same coordinate. Cross-sectional images from many directions were observed, and the optimal location and the size of the stem were selected. According to the result, actual operations were done. Planed sizes and selected sizes at the surgeries were compared. For several patients, post-operative CAT scans were performed, then planed stem position and actual stem position were compared.

Result: Stems preoperatively defined were used in 50hips (90.9%),1 size large ones were used in 2 hips (3.6%) and 1 size large ones were used in 3 hips (5.5%).

Discussion: As Revelation stems have very high proximal fit-and-fill, the end point of the stem insertion is very definite. The characteristics made the accuracy of the preoperative planning. So it was not so difficult to perform THA according to the preoperative planning as it had been imagined.