Average Lumbar Hounsfield Units Predicts Osteoporosis-Related Complications Following Lumbar Spine Fusion

Abstract

Study design: Retrospective Study.

Objective: To compare methods of assessing pre-operative bone density to predict risk for osteoporosis related complications (ORC), defined as proximal junctional kyphosis, pseudarthrosis, accelerated adjacent segment disease, reoperation, compression fracture, and instrument failure following spine fusions.

Methods: Chart review of primary posterior thoracolumbar or lumbar fusion patients during a 7 year period. Inclusion criteria: preoperative dual-energy x-ray absorptiometry (DXA) test within 1 year and lumbar CT scan within 6 months prior to surgery with minimum of 1 year follow-up. Exclusion criteria: <18 years at time of index procedure, infection, trauma, malignancy, skeletal dysplasia, neuromuscular disorders, or anterior-posterior procedures.

Results: 140 patients were included. The average age was 67.9 years, 83 (59.3%) were female, and 45 (32%) had an ORC. There were no significant differences in patient characteristics between those with and without an ORC. Multilevel fusions were associated with ORCs (46.7% vs 26.3%, p = 0.02). Patients with ORCs had lower DXA t-scores (-1.62 vs -1.10, p = 0.003) and average Hounsfield units (HU) (112.1 vs 148.1, p ≤ 0.001). Multivariable binary logistic regression analysis showed lower average HU (Adj. OR 0.00 595% CI 0.0001-0.1713, p = 0.001) was an independent predictor of an ORC. The odds of an ORC increased by 1.7-fold for every 25 point decrease in average HU.

Conclusions: The gold standard for assessing bone mineral density has been DXA t-scores, but the best predictor of ORC remains unclear. While both lower t-scores and average HU were associated with ORC, only HU was an independent predictor of ORC.

Keywords: dual-energy x-ray absorptiometry; hounsfield units; lumbar fusion; osteoporosis-related complications.

Figure 1.

Proportion of patients who developed an osteoporosis related complication by bone density group as determined from t-score impression. Pairwise Chi-squared tests were performed to evaluate for significant differences between groups.

Figure 2.

Overall number, relative frequency, and type of osteoporosis related complications among the entire patient sample.

Figure 3.

Three Hounsfield Unit (HU) measurements were taken at each vertebral body level, from L1-L5, by drawing region of interest circles just inferior to the superior endplate, mid-vertebral body, and just superior to the inferior endplate.