276. Average lumbar Hounsfield units predicts osteoporosis related complications following lumbar spine fusion

BACKGROUND CONTEXT With an aging population, the number of patients with poor bone quality requiring spinal instrumentation will continue to rise. Both osteoporosis and osteopenia have been associated with increased complications following spinal fusion. While t-scores on dual-energy x-ray absorptiometry (DEXA) have traditionally been used to assess bone quality, its predictive value for postoperative instrumentation failure has not been firmly established. PURPOSE To compare patient characteristics and measures of bone quality (DEXA t-score, Hounsfield units (HU), or FRAX score) as predictors for developing osteoporotic related complications (ORC) following spinal fusion. STUDY DESIGN/SETTING Retrospective review PATIENT SAMPLE Consecutive patients undergoing primary posterior thoracolumbar or lumbar fusion at a single institution by two surgeons were analyzed from 2007 to 2014. Patients were excluded if they were younger than 18 years, with infection, trauma, malignancy, skeletal dysplasia, neuromuscular disorders, or concomitant or staged anterior-posterior procedure. OUTCOME MEASURES NA METHODS ORCs were defined as one of the following categories: revision, compression fracture, proximal junctional kyphosis, pseudarthrosis, or instrumentation failure. Patients with a bone mineral density of the hips and/or spine performed within one year of the index procedure were included. Preoperative CT scans within 6 months of surgery were used to calculate the average Hounsfield units of the lumbar vertebrae. RESULTS A total of 140 patients were included, 45 (32%) had an ORC. Twelve patients (8.6%) developed a pseudarthrosis, 10 patients (7.1%) required reoperation, and 5 patients (3.6%) had a compression fracture. There were no significant differences between patients with and without an ORC in terms of age, sex, ethnicity, body mass index (BMI), diabetes, and active smoking status. However, patients with an ORC were more likely to have been treated with teriparatide within 6 months of surgery (22% vs 7.4%, p=0.02). The location of the fusion, instrumentation, decompression, and interbody fusion did not significantly differ between patients with and without an ORC. Patients with 3 or more levels fused were more likely to have an ORC (46.7% vs 26.3%, p=0.02). There was also a significant difference based on the treating surgeon, (23:73 vs 19:25, p=0.02).  The FRAX score was not significantly associated with an ORC. Patients with ORCs had significantly lower DEXA t-Scores, (-1.62 vs -1.10, p=0.003) and average HU (112.1 vs 148.1, p=0.0001). When analyzed in a multivariable binary regression model, the only factors that were independent predictors of an ORC were treatment with teriparatide (OR 5.20, 95% CI 1.48-18.32, p=0.009) and lower average HU (OR 0.005, 95% CI 0.0001-0.1713, p=0.001). The odds of an ORC increased by 3-fold for every decrease in the average HU of 50 points. CONCLUSIONS Osteoporosis related complications are common following lumbar spine fusion. The gold standard for assessing bone mineral density has been the DEXA t-scores, but the best predictor of ORC has not been established. While both lower t-scores and average HU were associated with ORC, only HU was an independent predictor of ORC. The odds of an ORC increased by 3-fold for every decrease in the average HU of 50 points. FDA DEVICE/DRUG STATUS This abstract does not discuss or include any applicable devices or drugs.