The Impact of Screw Density on Mechanical Complications in Lumbar Spine Deformity Constructs.

Abstract

Retrospective cohort study. To determine the impact of screw density on outcomes in adult spinal deformity surgery. Previous studies have suggested that decreased screw density in long lumbar constructs does not compromise mechanical robustness or increase the risk of complications such as PJK, pseudoarthrosis, or reoperation. We retrospectively identified adults who underwent spinal fusions with an upper instrumented vertebra (UIV) of L2 or above with a minimum 2-year radiographic follow up. We collected data on UIV, Hounsfield units based on CT scan, and radiographic parameters. Outcomes of interest included rod fracture, pseudarthrosis, proximal junctional kyphosis (PJK), proximal junctional failure (PJF), and overall reoperation. We performed a multivariate logistic regression on each outcome inclusive of all variables and dichotomized patients based on mean screw density into high and low screw density cohorts. Multivariable logistic regressions were performed for each outcome of interest to identify predictors of its occurrence. Patients were dichotomized into those with constructs ending at/above T9 (upper thoracic spine) or at/below T10 (lower thoracic spine) to determine if the impact of screw density was altered by construct length. We included 170 patients (mean age 66.8yr; mean screw density 1.83/level; mean radiographic follow-up 4.5 y). Fifty patients underwent reoperation (29%), with refractory pain (N=19) and symptomatic pseudoarthrosis (N=13) being the most common indications. Higher screw density was associated with lower odds of postoperative mechanical complications and reoperation for patients with lower thoracolumbar spine UIVs but not for those with upper thoracolumbar UIVs. Increased screw density may protect against mechanical complications in patients undergoing thoracolumbar fusion with UIV at or below T10. Lower screw density may be safe and cost-effective option for constructs terminating in the thoracic spine for adult spinal deformity surgery though further investigation is warranted. 3.