Upper Instrumented Vertebra Hounsfield Unit Assessment and Association With Mechanical Complications Following Posterior Spinal Fusion.

Abstract

Bone mineral density is often considered in preoperative surgical planning for spinal fusions. CT imaging-based measurement of Hounsfield units (HU) has been suggested as a surrogate marker for dual-energy x-ray absorptiometry scans. However, research establishing a threshold for HU at the uppermost instrumented vertebrae (UIV) that can mitigate the risk of mechanical and junctional complications following posterior spinal fusion is limited. Adults who underwent posterior spinal fusion and had 1-year follow-up data available were included in this retrospective cohort study. HU measurements were collected at various vertebral levels, including at the UIV. Inter- and intrarater reliability were calculated using the using intraclass correlation coefficients (ICCs). HU thresholds for mechanical and junctional complications were established using receiver operating characteristic (ROC) curve analyses, while accounting for age, sex, UIV level, and osteoporosis. Across 70 patients, mean age was 64.4 years, 65.3% were female, and 10.0% had osteoporosis. The intrarater reliability ICCs were classified as excellent (ie, greater than 0.9) for all vertebral levels, whereas the interrater reliability ICCs were classified as good (ie, greater than 0.8) for all vertebral levels. ROC curve analyses identified a HU cutoff of 105. Patients below the HU threshold had markedly higher rates of mechanical and junctional complications (52.0% vs. 8.9%, P < 0.001), specifically adjacent segment disease (32.0% vs. 2.2%, P < 0.001). CT-based measurement of HU at the upper instrumented vertebra has excellent intrarater and good interrater reliability. In this cohort of lumbar and thoracolumbar spinal fusion patients, patients with HU under 105 at the UIV had an increased risk of mechanical and junctional complications following spinal fusions. Future studies may work to refine HU assessment and the association with postoperative complications, as using HU to guide surgical candidacy and UIV selection may be beneficial.