P117. Bone density distribution in the cervical spine: Establishing level specific reference values

BACKGROUND CONTEXT Bone density in the lumbar spine is variable by level and can impact surgical strategy. In the cervical spine, low Hounsfield units (HU) have been associated with cage subsidence. The distribution of HU in the cervical spine has not been established. PURPOSE To define level-specific cervical HU reference values and the relationship between cervical bone density, age, BMI, comorbidities and alignment. STUDY DESIGN/SETTING Retrospective review at a single center from May 2015 to December 2019. PATIENT SAMPLE A total of 224 patients, 18 excluded for previous hardware and 5 for known osteoporosis. OUTCOME MEASURES Hounsfield units. METHODS Patients who presented with neck symptoms and had a cervical spine CT for evaluation of pathology or surgical planning were included. Exclusions were hardware on CT, osteoporosis, or cervical deformity. Measurements were performed in 5 regions of each vertebral body (VB) (C2-T1; mid-axial, anterior-axial, posterior-axial, mid-coronal, and mid-sagittal) and 2 regions of the lateral masses (LMs) (C3-C6; mid-cor, mid-sag). The VB measurements were averaged at each level to obtain a composite value of each vertebral body (TotalVB), similarly the mid-cor and mid-sag values were averaged at each LM (TotalLatM). Outliers were excluded pairwise. To evaluate reliability, 6 observers each measured 355 HU values (71 over 5 patients), inter-relater reliability assessed with intraclass correlation coefficients (ICCs). Correlations of composite HU with age, BMI, Charlston Comorbidity Index (CCI) and cervical alignment were evaluated. RESULTS Average age 57±12 years, mean BMI 28±6 kg/m2, 63.5% male, 90.1% white. Diagnoses included radiculopathy (45.8%), myelopathy (37.9%) and facet arthropathy (46.3%). ICC for HU measurements were as follows: VB, 0.82 mid-axial, 0.59 ant-axial, 0.77 post-axial, 0.88 mid-cor, 0.88 mid-sag; LM, 0.46 mid-sag, 0.61 mid-cor. C4 VB and C4 LM had the highest HU (383.6±71.9 and 480.0±103, respectively) while T1 VB and C6 LM had the lowest (232.3±46.5 and 398.3±98.2, respectively). No significant correlations were found between LM HU and age, BMI, CCI, or alignment. Increased kyphosis was weakly correlated with VB HU at all levels (except C2) strongest correlations at C7 (0.25) and weakest correlation at C3 (0.16). Age and CCI showed weak-moderate correlations with VB HU at all levels (age, r= -0.20 to -0.35; CCI r= -0.28 [C2, C7, T1] to -0.17 [C4]). CONCLUSIONS Our level-specific cervical HU values may be referenced when evaluating cervical BMD. The most reliable measurement technique is on the mid-sag or mid-cor VB, measurement of LMs is less reliable. Bone is least dense in the lower cervical spine, which may influence instrumentation strategies. FDA DEVICE/DRUG STATUS This abstract does not discuss or include any applicable devices or drugs.