The relationship of pelvic incidence to post-operative total hip arthroplasty dislocation in patients with lumbar fusion

Purpose To determine if lumbar fusion increases the risk of dislocation following total hip arthroplasty (THA) via a posterior approach and to investigate anatomic variables associated with this increased risk. Methods Five-year retrospective review of THAs performed through a posterior approach identifying cases of post-operative dislocation. Patients were grouped into those with or without previous lumbar spine fusion. Lumbar fusion patients were then further analyzed in terms of cup position, pelvic incidence, sacral slope, and pelvic tilt to determine if there were specific variables associated with the increased risk of dislocation. Results Five hundred nine primary THAs in 460 patients (non-simultaneous bilateral THAs in 41 patients) met inclusion criteria with a dislocation rate of 5.5%. Thirty-one patients were identified as having prior lumbar fusions. The dislocation rate was significantly higher in fusion patients (29 vs 4%; p = 0.009) yielding a relative risk (RR) of dislocation of 4.77 (p = <0.0001). Additionally, cup anteversion was significantly different between groups (26.8 vs 21.42; p = 0.009). Dislocators in the fusion group were also at greater risk of requiring subsequent revision (RR = 3.24; p = 0.003). Subgroup analysis of fusion patients revealed that dislocators had lower pelvic incidence and sacral slope compared to non-dislocators (45.2 vs 58.6 [p = 0.0029] and 26.3 vs 35.6 [p = 0.0384] respectively). Conclusions Patients with lumbar fusion are at increased risk for post-operative dislocations requiring revision. Together, lower pelvic incidence and decreased sacral slope are associated with increased risk of dislocation in these patients.