Performance Comparison Between Hounsfield Units and DXA in Predicting Lumbar Cage Subsidence in the Degenerative Population.

Background context: Bone mineral density assessment is essential for surgical planning for most spine surgeries, but gold standard dual-energy x-ray absorptiometry (DXA) is affected by degeneration often resulting in falsely elevated scores. Studies of the opportunistic measurement of computed tomography (CT) Hounsfield units (HU) suggest lower CTHU values predict interbody cage subsidence, yet cutoff values vary and lack standardization.

Purpose: This study aimed to determine if value CTHU<135 was associated with lumbar interbody cage subsidence and to compare the predictive performance of subsidence between CTHU and DXA.

Study design/setting: Single-center, multi-surgeon, retrospective cohort study PATIENT SAMPLE: : Adult, circumferential lumbar fusions ≤ 5 interbody levels with DXA, CTs, radiographs, and at least 1 year of follow up.

Outcome measures: CTHU at L1, lowest DXA T-score, and postoperative change in disc space height (cage migration) on radiographs METHODS: : Lowest DXA T-scores overall and of the lumbar spine were recorded and categorized, and L1 CTHUs were measured. Interbody fusions were analyzed for subsidence ≥ 2mm on radiographs by a validated, computer vision algorithmic approach. Analysis determined if an association existed between subsidence and CTHU<135 or DXA lowest T-score. Logistic regression analyzed the performance of predicting subsidence by each method.

Results: The 127-patient cohort had 82.7% degenerative pathologies, 45.7% males, median age of 60 years, 2.4% osteoporosis on DXA, 44.1% CTHU<135, and 13.4% subsidence. CTHU<135 (p=0.004) and age (p=0.016) were significantly associated with subsidence, however DXA lowest T-score (p=0.550) was not. The odds of subsidence were significant if CTHU<135 for crude and adjusted (OR=4.0, 95% CI 1.2-13.9, p=0.029) comparisons. The odds of subsidence were not significant for DXA_any lowest T-score or DXA_spine lowest T-score (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).

Conclusion: CTHU<135 was associated with subsidence while DXA lowest T-score was not in this study of 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 DXA in this primarily degenerative population and is a useful tool for assessing bone quality at the region of interest when planning lumbar surgery.