270. Low Hounsfield Units measured at the upper instrumented vertebra are an independent risk factor for density related complications in adult spinal fusion procedures

Abstract

<h3>BACKGROUND CONTEXT</h3> Hounsfield unit values (HU) from computed tomography (CT) have been used to informally assess bone density in patients undergoing spine fusion procedures. HUs are easily obtained from a standard preoperative CT scan, and unlike Dual X-ray Absorptiometry analysis (DEXA) one can focus on specific regions of interest, such as the vertebral body of a planned upper-instrumented vertebrae (UIV). There is still a relative lack of literature on the reliability and utility of HUs to both identify patients with low BMD and guide surgical decision making. <h3>PURPOSE</h3> To determine whether there was a significant difference in preoperative HUs, measured at the UIV, in patients that had a bone-density related complication (DRC) within 2 years of their spinal fusion. <h3>STUDY DESIGN/SETTING</h3> A retrospective comparative study at a single academic institution. <h3>PATIENT SAMPLE</h3> Patients 55 years or older that underwent a spinal fusion procedure in 2017 at a single academic institution. <h3>OUTCOME MEASURES</h3> Occurrence of proximal junctional kyphosis, proximal junctional failure, pseudarthrosis, screw loosening or pullout, hardware failure and adjacent segment disease (ASD). <h3>METHODS</h3> Baseline preoperative demographic information, smoking history, levels fused, UIV and status as a revision procedure were recorded. All postoperative notes and images for 2 years post-procedure were reviewed for the presence of proximal junctional kyphosis, proximal junctional failure, pseudarthrosis, screw loosening or pullout, hardware failure, and ASD. HUs were measured via regions of interest drawn within the cancellous bone of the mid-vertebral body at the UIV of all patients. Patients were divided into 2 groups for comparison, those who experienced a DRC within 2 years and those who did not. Student's t-test was performed to compare HUs between the groups, chi-square analysis was performed for categorical variables. Dichotomous logistical regression was performed to analyze the relationship between density related complications and HU at the UIV, patient BMI, revision procedure, history of smoking, gender, UIV and number of levels fused. Significance was set at p<0.05. <h3>RESULTS</h3> A total of 172 consecutive fusion patients with a preoperative CT scan were reviewed. Of these, 49 were revision procedures. 66 had a UIV in the cervical spine, 10 had a UIV in the thoracic spine and 95 had a UIV in the lumbar spine. Ninety-nine were 1 or 2 level fusions, 49 were 3 or 4 level fusions and 23 were long fusions with 4+ levels involved. Forty-eight patients had a DRC. Baseline demographics were similar between the 2 groups, with the exception of more revision procedures in the DRC group (p<0.001). The mean HUs of the UIV in the cohort that had a DRC was 168.92, as compared to 252.66 in the no-DRC group (p<0.001). Regression analysis revealed that low HUs at the UIV and revision procedures were independent risk factors for a DRC. For every 10 unit decrease in HUs, the odds of a DRC rose by 6%. When thoracic and lumbar fusions were analyzed the mean HUs at the UIV in the DRC group were 108.5 vs 152.6 (p<0.001). When cervical fusions were analyzed separately the mean HUs in the DRC group were 308 vs 383.4 (P=0.014). <h3>CONCLUSIONS</h3> To our knowledge, this is the first study that compares HUs measured at the UIV to the rate of density related complications for single and multilevel fusions in the cervical, thoracic and lumbar spine. This study found that HUs measured at the UIV of a fusion were significantly lower in patients that went on to have a density related complication within 2 years of their index procedure. <h3>FDA DEVICE/DRUG STATUS</h3> This abstract does not discuss or include any applicable devices or drugs.