We retrospectively reviewed all 143 patients (180 hips) treated with a cementless 28-mm Biolox® forte COC articulation (CeramTec, Plochingen, Germany) between April 1998 and April 2001. During that time, we treated 165 patients (231 hips) with other devices. In 60 of the 180 hips (navigation group), a CT-based navigation system was used. The remaining 120 hips (nonnavigation group) underwent THA without using the navigation system. There were 153 hips in 123 women and 27 hips in 20 men. The mean age at operation was 53 years (range, 27–74 years). The mean BMI was 23 kg/m2 (range, 16–36 kg/m2). The preoperative diagnosis was osteoarthritis in 153 hips, osteonecrosis of the femoral head in 21 hips, and rheumatoid arthritis in six hips. Osteoarthritis in 150 hips occurred secondary to hip dysplasia.
Selection of the patients for navigation was not randomized and navigation was used when the engineer’s support was available. However, there were no differences between the two groups in age, sex, diagnosis, or BMI (Table ). Four patients (four hips) in the navigation group died within 10 years, but all these hips were functioning well until death. The remaining 42 patients (56 hips) were followed for a minimum of 10 years (mean, 11 years; range, 10–13 years). Nine patients (10 hips) in the nonnavigation group were lost to followup. Four patients (five hips) died within 10 years, but all these hips were functioning well until death. The remaining 84 patients (105 hips) were followed for a minimum of 10 years (mean, 11 years; range, 10–13 years). No patients were recalled specifically for this study; all data were obtained from medical records and radiographs.
In the navigation group, transverse CT images from the level of the superior anterior iliac spine to the level of the isthmus of the femoral canal were preoperatively obtained using a helical CT scanner. The slice thickness was 3 mm and the slice pitch was 3 mm, with the exception of a region between 2 cm below the lesser trochanter and the isthmus of the femoral canal, where the slice pitch was 10 mm. Several reference images of the femoral condyles were also taken to measure anteversion, with a slice pitch of 3 mm. Radiation dose of this protocol was calculated to be less than 3 mSV. The navigation system consisted of an optical 3D localizer (Optotrak®
3020; Northern Digital, Waterloo, Canada), a custom-made dynamic reference frame with active light-emitting diodes, custom-made surgical tools designed to work with the dynamic reference frame, an Optotrak®
pen probe, and a UNIX®
-based Sun UltraSPARC®
workstation (Sun Microsystems, Santa Clara, CA, USA). The navigation software was developed in our Medical Image Analysis Division, using an open source software system (Visualization Toolkit; Kitware, Inc, Clifton Park, NY, USA). We reconstructed three-dimensional acetabular and femoral bone surface models from the CT data of each patient. To measure cup orientation (inclination and anteversion), we used the functional pelvic zero position where the pelvis in supine on the CT scan table was axially rotated until the bilateral anterior iliac spines touched the same horizontal plane and then the interteardrop line was used as the mediolateral axis [20
]. We defined the ideal cup orientation as 40° of radiographic inclination and 20° of anatomic anteversion (13.2° of radiographic anteversion or 17.0° of operative anteversion), by considering the ROM without impingement during various daily activities and avoiding a high inclination angle to prevent the acetabular ceramic liner from developing a chipping fracture when the mean average femoral anteversion was 30° [14
]. This was close to the center of the safe zone described by Lewinnek et al. [12
All operations were performed by one of four surgeons (NS, TN, TS, KO) via a posterolateral approach under general anesthesia. In the operating room, shape-based surface registration of the patient’s bones to the previously constructed bone models of the pelvis and femur was performed using 30 surface points [21
]. The accuracy of registration was verified by touching bony landmarks. The navigation system was used to guide the femoral neck osteotomy level, position of the cup center, cup orientation, femoral anteversion, and limb length discrepancy by comparing these parameters to the CT-based preoperative planning. Acetabular osteophyte was removed after cup placement when it overhung the cup rim. A modular Biolox®
forte alumina ceramic liner was inserted in a hemispherical titanium porous-bead-coated shell (AnCA-Fit™ cup; Cremascoli Ortho, Milan, Italy). The modular ceramic head was fixed to a 12/14 taper cone, resulting in 128° of component ROM (oscillation angle) before impingement between the taper and the rim of the liner. The stem position was measured with navigation by using a neck adaptor with light-emitting diode markers. A modular changeable neck was used to change the anteversion to approach 30°. When the stem anteversion ranged from 25° to 35°, a straight modular neck was chosen. When the stem anteversion ranged from 35° to 40° or from 20° to 25°, an 8° retroverted or anteverted neck was used. When the femoral anteversion ranged from 40° to 50° or from 10° to 15°, a 15° retroverted or anteverted neck was used [15
]. The prosthetic head position was measured with navigation and the mean measured femoral neck anteversion was 30° (range, 16°–43°).
Patients were followed up at 3 months, 6 months, 1 year, and annually after surgery. At each visit, we performed clinical assessment using the Merle d’Aubigné-Postel hip score [13
], with a maximum score of 6 points for pain, mobility, and gait function, respectively. An AP radiograph of bilateral hips in supine and a lateral radiograph of the operated side of the proximal femur were taken 1 and 2 weeks after surgery and at each routine followup visit. We radiographically evaluated the mode of acetabular and femoral component fixation, osteolysis, and implant wear or breakage. One of us (MT) who was not a treating surgeon evaluated all radiographs. The interobserver variability was tested using a set of 20 radiographs by two of us (MT, HM) and the Pearson R was 0.929 for inclination and 0.854 for anteversion. There were no missing radiographs. To measure the cup orientation, an ellipse was fitted to the rim of the acetabular component on the early postoperative AP radiographs using computer software (CAM of THA; Kyocera Corp, Kyoto, Japan). To eliminate measurement error due to pelvic axial rotation on AP radiographs, measurements were performed on selected postoperative radiographs taken within 1 month after surgery on which the pubic symphysis and the spinous processes of the sacrum were located along a vertical line. The mode of femoral component fixation was radiographically classified as bone-ingrown stable, stable fibrous, or unstable, according to the criteria of Engh et al. [6
]. This was performed 2 years postoperatively and annually thereafter. We assessed migration of femoral components using the following measurements: the vertical distance from the shoulder of the stem to the midpoint of the lesser trochanter and the varus angle of the stem formed by the stem axis and the proximal femoral axis. Subsidence in a vertical direction of greater than 4 mm [3
] or a change in the varus angle of greater than 2° was considered to indicate stem migration and loosening. Loosening of acetabular components was defined as migration of greater than 2 mm or a change in the abduction angle of the acetabular component of greater than 5° [3
]. Osteolysis was also evaluated by comparing the latest followup radiographs and the postoperative radiographs taken within 3 months of surgery. Osteolysis was defined as a sharply demarcated radiolucent space with a rounded or scalloped appearance that extends away from the surface of the implant [24
We performed Kaplan-Meier survival analysis with revision as the end point. The 95% CI was also calculated. The log-rank test was used to see whether there was a difference in the survival between the navigation and nonnavigation groups. We determined differences in the Merle d’Aubigné-Postel hip score, age, cup inclination, and cup anteversion between the groups using the Mann-Whitney U test. Differences in variance of cup inclination and anteversion between the groups were analyzed using the F-test. We determined differences in dislocation rate between the groups using Fisher’s exact probability test.