Does timing of lumbar fusion affect dislocation rate after total hip arthroplasty?

Introduction: The impact of lumbar spinal fusion (LSF) on pelvic mobility and increased risk for THA dislocation are recognized. However, there is still controversy on whether THA should be performed prior or following LSF. This study aims to compare the rates of hip dislocation in patients undergoing THA prior to or following LSF. Methods: We retrospectively reviewed 109 primary THA and LSF. There were 34 men and 75 women with a mean age of 66.9. The cohort was divided into 2 groups: 1) THA prior to lumbar fusion (n = 42) and 2) THA following lumbar fusion (n = 67). Radiographic parameters including acetabular component abduction, anteversion, pelvic incidence (PI), sacral slope, standing lumbar lordosis (LL) and PI-LL mismatch were determined for each patient. The surgical approach and THA parameters were also recorded and compared between the 2 groups. Patients with fracture, malignant disease, and prior hip hardware were excluded. Results: The mean follow up was 14.7 months. Overall, 8 patients (7.3%) had a postoperative hip dislocation (0 in group 1 (0%), and 8 in group 2 (11.9%) (p = 0.022). The mean cup abduction and anteversion in patients that dislocated was 37.7 degrees and 23.4 degrees respectively while patients who did not dislocate had an average cup abduction of 37.6 degrees (p = 0.970) and anteversion of 25.9 degrees (p = 0.367). Patients who dislocated had decreased lumbar lordosis (p = 0.022) and higher PI-LL mismatch (p = 0.0004) compared to that did not dislocate. There were no other significant differences in the spinopelvic parameters between the 2 groups. Neither surgical approach nor dual mobility articulations use had a significant impact on postoperative dislocations. Conclusion: Higher rates of hip dislocation were observed in patients undergoing primary THA following LSF. Dislocators had decreased lumbar lordosis and increased PI-LL mismatch. Patients who require both spinal fusion and THA should undergo hip arthroplasty first to minimize the risk of postoperative instability.