Correlation Between Rod Fracture and Shear Stress: A Novel Parameter

Abstract

We sought to assess the accuracy of a novel parameter proportional to the rod shear stress (RSS) in identifying patients at risk of rod fracture (RF) after surgery for correction of adult spinal deformity. We performed a retrospective medical record review of patients aged ≥18 years treated for adult spinal deformity between 2004 and 2014 with ≥24 months of follow-up. The primary outcome was RFs identified radiographically. Patient weight (w), number of instrumented levels (N), and minimum rod diameter (d) were recorded and used to calculate the RSS parameter (RSS=Nwd2). Receiver operating characteristic curves were produced and the area under the curve (AUC ± 95% confidence interval [CI]) was calculated to compare this parameter's discriminative accuracy to that of its constituent variables. The sensitivity, specificity, and likelihood ratios (LRs) were calculated. A total of 28 RF-positive and 154 RF-negative patients were included. The average age was 59.2 ± 9.6 years, and 93.4% were women. The RSS parameter produced the greatest AUC (0.73 ± 0.11). At an RSS cutoff of 30.1, it achieved a sensitivity of 71.4% and specificity of 71.4% (LR, 2.5; 95% CI, 1.8-3.5). The number of instrumented levels produced the next-greatest AUC (0.65 ± 0.12), with a sensitivity of 78.6% and specificity of 50.0% at a cutoff of 15 (LR, 1.6; 95% CI, 1.2-2.0). The RSS is calculated using easily obtainable information and shows potential as a tool for predicting patient-specific risk of RF after spinal fusion. The number of instrumented levels also correlates strongly with the occurrence of RFs and is not significantly less accurate than the RSS. A larger sample size and prospective validation would be useful in determining with greater confidence which parameter is superior for predicting RFs after spinal fusion.