Performance Comparison between Hounsfield Units and DEXA in Predicting Lumbar Interbody CageSubsidence after Circumferential Lumbar Fusion

Introduction: 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. Studies on opportunistic measurement of computed tomography Hounsfield units
(CTHU) suggest lower values predict interbody cage subsidence, yet cutoff values vary and lack
standardization. This study aims to determine if cutoff value CTHU < 135 is associated with lumbar
interbody cage subsidence and to compare the predictive performance of subsidence between CTHU
and DEXA.

Methods: Circumferential lumbar fusions were retrospectively enrolled if DEXA, CT, and x-rays were
available. Interbody fusions were analyzed for subsidence ≥ 2mm by validated motion detection
software. Lowest DEXAany and DEXAspine T-scores were categorized (normal ≥ -1.0, -1.0 > osteopenia> -2.5,
osteoporosis ≤ -2.5) and L1 CTHUs were measured. 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.

Results: The 127-patient cohort had 96.9% degenerative pathologies, 54.3% females, median age 60
years, 2.4% osteoporosis, 44.1% CTHU < 135, and 13.4% subsidence. 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 statistically 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 if lowest T-score<-1.0 for
DEXAany and DEXAspine (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 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 and is a useful tool for assessing bone quality when planning lumbar
surgery.