Clinical Evaluation of an Inertial Measurement Unit in Monitoring Pelvic Position During Total Hip Arthroplasty

Introduction: Acetabular cup position is an important factor in successful total hip arthroplasty (THA). Optimal cup placement requires surgeons to possess an accurate perception of pelvic orientation during cup impaction, however, varying pelvic anatomy and limited visual cues in the surgical field may interfere with this process. The purpose of this study was to evaluate the utility of an inertial measurement unit (IMU) in monitoring pelvic position during THA. Materials & Methods: Ten patients scheduled to undergo THA were IRB-approved and consented by four surgeons. A small IMU was placed over the patient9s sacrum pre-operatively and zeroed in standing position. Pelvic orientation data was streamed and captured wirelessly throughout the procedure. Surgeons were blinded to all data throughout the study period. Prior to cup impaction, the surgeon indicated his intended cup abduction angle and the degree to which the cup impactor was manipulated to compensate for perceived AP pelvic tilt. The degree of pelvic tilt as determined by the IMU (angle β) was then recorded (Figure 1). AP-pelvis radiographs were measured in Martell Hip Analysis Suite post-operatively to calculate the cup abduction angle, which was then compared to the surgeon9s intended abduction angle to determine surgeon accuracy. To predict the final cup abduction angle, the degree of pelvic tilt recorded by the IMU (angle β) was subtracted from the abduction angle of the cup impactor (angle α) that was positioned using the OR table as a reference (Figure 1). This value was then compared to the measured post-operative cup abduction angle in order to assess the accuracy of the IMU in measuring pelvic tilt. Surgeon accuracy and IMU accuracy were compared to determine if the IMU was more or less effective than surgeon perception at determining pelvic tilt. Results: The mean intended abduction angle indicated by the surgeons intraoperatively was 43.7° (range 40°–45°), while the mean measured post-operative abduction angle was 40.1° (range 25.9°–49.4°). In five of the cases, the surgeon9s post-operative abduction angle fell within 2° of his intended abduction angle. One cup was placed at a higher than intended abduction angle (4.4°), and four cups were placed in lower than intended abduction angles by an average of 10.8° (range 3.9°–19.1°). Film analysis revealed that surgeons placed the acetabular cup on average 5.4 ± 6.0° from their intended abduction angle (range 0.3°–19.1°). Following analysis of the IMU offset data, it was observed that the IMU deviated on average 3.1 ± 2.6° (range 0.7°–7.2°) from its expected orientation value. The IMU deviated more than 2° from expected pelvic tilt in five cases. Discussion: The IMU was able to ascertain AP pelvic tilt to a higher degree of accuracy than four surgeons using standard surgical techniques. A system in which the pelvis could be monitored and adjusted intraoperatively based on accurate IMU data would allow the surgeon to place the pelvis in optimal position prior to cup impaction, which could potentially increase overall cup positioning accuracy. More data is needed to confirm these results.