Abstract
Introduction
The optimal goal for cup positioning in hip arthroplasty in individual patients is affected by many factors including surgical exposure, femoral anteversion, and pelvic tilt. Some navigation systems ignore pelvic tilt and are based strictly on the anterior pelvic plane while others incorporate pelvic tilt, as measured in the supine position on the operating table. Neither approach incorporates knowledge of preoperative spino-pelvic flexibility or predictions of the change in spino-pelvic attitude or flexibility following surgery. While prior studies have shown little change in pelvic tilt postoperatively, one recent study based on gait analysis, suggested that changes in pelvic tilt are not predictable. The current study aims to assess changes in pelvic tilt following surgery.
Methods
24 patients, 12 male and 12 female, underwent THA using CT-based navigation. Each patient had supine and standing AP pelvis radiographs both pre-operatively and at a minimum of 1 year post-operatively. Pelvic tilt on each radiograph was measured using a noncommercial two-dimensional/three-dimensional matching application. (HipMatch; Institut for Surgical Technology and Biomechanics, Bern, Switzerland). This software application uses a fully auto- mated registration procedure that can match the three- dimensional model of the preoperative CT with the projected pelvis on a postoperative radiograph. This method has been validated and for measurement of cup position for example showed a mean accuracy of 1.7° +/− 1.7° (rang-4.6° to 5.5°) in the coronal plane and 0.9° +/− 2.8° (rang-5.2° to 5.7°) in the sagittal plane compared with postoperative CT measurements. The software showed a good consistency with an intraclass correlation coefficient (ICC) for inclination of 0.96 (95% confidence interval [CI]: 0.93 to 0.98) and for anteversion of 0.95 (95% CI: 0.91 to 0.98). A good reproducibility and reliability for both inclination and anteversion was found with an ICC ranging from 0.95 to 0.99. No systematic errors in accuracy were detected with the Bland- Altman analysis. Using the HipMatch 2D/3D application, changes in pelvic tilt before and after surgery were assess in both the supine and standing positions.
Results
Preoperatively, the mean standing pelvic tilt was .9 degrees (range 10.9 to −9.2) and the mean supine pelvic tilt was 3.7 degrees (range 11.8 to −7.7). Postoperatively, the mean standing pelvic tilt was 1.1 degrees (range 13.8 to −12.3) and the mean supine pelvic tilt was 5.9 degrees (−4.0 to 16.5). The maximum change following surgery in individual patients was −4.9 degrees standing and −8.5 degrees supine. Pre-operative supine pelvic tilt predicts post-operative supine pelvic tilt with an r2 of .67. Pre-operative standing pelvic tilt predicts post-operative standing pelvic tilt with an r2 of 0.91.
Discussion
Overall, in both the standing and supine positions, pelvic tilt changed very little as a result of total hip arthroplasty in this sample of patients and pre-operative pelvic tilt is clearly predictive of post-operative pelvic tilt. Preoperative assessment of pelvic tilt, as measured either in the supine or standing position, may be useful information when determining optimal cup positioning goals for total hip arthroplasty. We recommend that both preoperative assessment of pelvic tilt and preoperative or intraoperative assessment of femoral anteversion should be considered when determining optimal acetabular component positioning.
