74. Performance comparison between Hounsfield units and DEXA in predicting lumbar interbody cage subsidence after circumferential lumbar fusion.

Bone mineral density assessment is essential for spinal fusion surgical planning, but gold standard dual energy X-ray absorptiometry (DEXA) is affected by degeneration often resulting in falsely elevated scores. The opportunistic measurement of Hounsfield units on computed tomography (CTHU) is gaining in popularity given the access to preoperative CTs obtained for instrumentation guidance systems and the ability to evaluate the region of interest while avoiding adjacent degenerative structures. Emerging studies suggest that lower CTHUs predict interbody cage subsidence, yet cutoff values vary, and lack standardization. To determine if the value CTHU<135 is associated with interbody cage subsidence in the lumbar spine and to compare the predictive performance of subsidence between CTHU and DEXA. Single-center, multisurgeon, retrospective cohort study. Circumferential lumbar fusions ≤ 5 interbody levels with perioperative DEXA, CTs, and X-rays. CTHU at L1, lowest DEXA T-score, and postoperative change in disc space height (cage migration) on X-ray. Circumferential lumbar fusions were retrospectively enrolled if DEXA, CT, and x-rays were available, and minimum follow up was one year. Interbody fusions were analyzed for subsidence ≥ 2mm by validated motion detection software. Lowest DEXA any and DEXA spine T-scores were recorded and categorized (normal ≥ -1.0, -1.0 > osteopenia > -2.5, osteoporosis ≤ -2.5) and L1 CTHUs were measured. Statistical analysis determined the association between CTHU<135 and subsidence. Univariate and multivariate binary logistic regression compared the predictive performance of subsidence between CTHU and DEXA. The 127-patient cohort had 96.9% degenerative pathologies, 54.3% were females, median age was 60 (IQR 51-69) years, 2.4% had osteoporosis, 44.1% had CTHU<135, median length of follow-up was 17 (IQR 12-24) months, and 13.4% developed subsidence on or before the last follow up. CTHU<135 (p=0.004) and age (p=0.016) were significantly associated with subsidence; DEXA lowest T-score (p=0.550) was not. The odds of subsidence were significant if CTHU<135 on crude and adjusted (OR=4.0, 95% CI 1.2-13.9, p=0.029) comparisons. The odds of subsidence were not significant if lowest T-score < -1.0 for DEXA any and DEXA spine (OR=1.8, 95% CI 0.6-4.9, p=0.284 and OR=1.1, 95% CI 0.3-4.1, p=0.920, respectively). CTHU<135 was associated with subsidence while DEXA lowest T-score was not in patients with degenerative pathologies. The odds of subsidence were 4.0 times higher for CTHU<135 after controlling for known risks, supporting this cutoff value. This study suggests that CTHU is a more reliable predictor of subsidence than DEXA when assessing degenerative spinal pathologies and is a useful tool for assessing bone quality when planning lumbar surgery. This abstract does not discuss or include any applicable devices or drugs. [ABSTRACT FROM AUTHOR]