Does it Matter: Total Hip Arthroplasty or Lumbar Spinal Fusion First? Preoperative Sagittal Spinopelvic Measurements Guide Patient-Specific Surgical Strategies in Patients Requiring Both

Abstract Introduction In patients requiring both total hip arthroplasty (THA) and lumbar spinal fusion (LSF), consideration of preoperative sagittal spinopelvic measurements can aid in the prediction of post-fusion compensatory changes in Pelvic Tilt (PT) and inform adjustments to traditional THA cup anteversion. This study aims to identify relationships between spinopelvic measurements and post-THA hip instability and to determine if procedure order reveals a difference in hip dislocation rate. Methods Patients at a single practice site who received both THA and LSF between 2005-2015 (292; 158=LSF prior to THA, 134=THA prior to LSF) were retrospectively reviewed for incidents of THA instability. Those with complete radiograph series (89) had their sagittal (standing) spinopelvic profiles measured preoperatively, immediately postoperatively, and 3-months-, 6-months-, 1-year-, 1.5 years, and 2-years postoperatively. Measured parameters included Lumbar Lordosis (LL), Pelvic Incidence (PI), PT, and Sacral Slope (SS). Results No significant differences in dislocation rates between operative order groups were elicited (7/73 LSF first, 4/62 THA first; Z= 0.664, P = 0.509). Compared to non-dislocators, dislocators had lower LL (-10.9) and SS (-7.8), and higher PT (+4.3) and PI-LL (+7.3). Additional risk factors for dislocation included sacral fusion (RR = 3.0) and revision fusion (RR = 2.7). Predictive power of the model generated through multiple regression to characterize individual profiles of post-LSF PT compensation based upon peri-operative measurements was most significant at 1-year- (R 2 =0.565, F=0.000456, P=0.028) and 2-years (R 2 =0.741, F=0.031, P=0.001) postoperatively. Conclusion In performing THA after LSF, it is theoretically ideal to proceed with THA at a post-fusion interval of at least one year, beyond which further compensatory PT change is minimal. However, the order of surgical procedure revealed no statistical difference in hip instability rates. In cases characterized by large PI-LL mismatch (larger or less predictable compensation profiles), or large SS or LL loss (considerably atypical muscle recruitment), consideration of full functional anteversion range between sitting and standing positions to account for abnormalities not appreciated with standing radiographic assessment alone may be warranted.