Female Sex and Longer Fusion Constructs Significantly Increase the Risk of Total Hip Arthroplasty Following Spinal Fusion

Background: Previous studies have noted the progression of arthritis due to increased forces in articular structures adjacent to a fused joint. It is unknown whether spinal fusion generates increased forces at the hip joint causing progression to arthritis leading to total hip arthroplasty (THA). We sought to determine (1) is there a relationship between spinal fusion and THA, (2) what are risk factors for subsequent THA, and (3) is there a time interval from spinal fusion to THA?

Methods: A large patient discharge dataset was utilized to evaluate all patients who underwent spinal fusion and subsequent THA in California from 2004 through 2013. Patients were categorized by age, sex, hospital type, hospital volume, and number of spinal levels fused. Multivariate analysis was performed to investigate the relationship between spinal fusion and THA. Hazard ratios were calculated for risk factors for THA after spinal fusion. Patients were excluded for previous spinal fusion or hip arthroplasty, inflammatory arthropathy, cancer, and an age of ≤40 years, a surrogate for adolescent and neuromuscular scoliosis.

Results: A total of 101,206 patients underwent spinal fusion; 2,803 (2.77%) subsequently underwent THA. In a bivariate analysis comparing 1 to 2 levels versus >2 levels fused, males had a 17% increased relative risk of undergoing subsequent THA (relative risk [RR] = 1.17; 95% confidence interval [CI] = 1.16 to 1.17) and female patients had a 35% increased relative risk (RR = 1.35; 95% CI = 1.34 to 1.35) when the fusion involved >2 levels. For females, the relative risk increased by 119% when >7 levels were fused compared with 1 to 7 levels (RR = 2.19; 95% CI = 2.16 to 2.21). Using multivariate random-effects analysis, significant risk factors for THA after spinal fusion included female sex (hazard ratio [HR] = 1.21; 95% CI = 1.13 to 1.31; p < 0.0001) and spinal fusion of >7 levels (HR = 1.52; 95% CI = 1.03 to 2.24; p = 0.035).

Conclusions: Patients with longer spinal fusion constructs, especially female patients, had a significantly increased risk of undergoing subsequent THA. Patients should be educated regarding the potential for the progression to hip arthritis after spinal fusion and the possibility of future THA.

Level of Evidence: Therapeutic Level IV. See Instructions for Authors for a complete description of levels of evidence.