Clinical Importance of Hounsfield Unit in Computed Tomography of Sub-Axial Cervical Vertebral Body.

Objective: Computed tomography (CT) is an often-used diagnostic imaging tool during the evaluation of the cervical spine pathology. CT scan can provide one’s bone density by measuring Hounsfield’s unit. In anterior cervical discectomy and fusion (ACDF) surgeries, bone density may play an important role as it may relate to non-union. Therefore, the evaluation and consideration of the Hounsfield’s unit of sub-axial cervical vertebral body is important, especially during multi-level ACDF.Methods: Patients who have taken cervical CT scan pre-operatively to multi-level (3 or 4 levels) anterior cervical discectomy and fusion with anterior plating between June 2010 and August 2018, at our department were retrospectively reviewed. The bone density was estimated by measuring Hounsfield Unit (HU) of sub-axial vertebrae - C3 to T1 - by using PiViewStar imaging software program. Radiological evaluation of fusion was done by assessing postoperative plain X-rays and CT scans of 4 months and 1-year follow-up. The difference in HU were evaluated between different segments of sub-axial vertebrae. Then the fusion rate of the ACDF ending at C6 were compared to C7. All grouped values were tested for normality using the Shapiro-Wilk test. Matched-ANOVA and Fisher’s exact test was used.Results: A total of eighty patients were who underwent 3 or 4 level ACDF with anterior plate fixation were retrospectively reviewed. Bone density estimated from Hounsfield unit (HU) measurement of the sub-axial vertebral bodies ranges from 112.2 to 594.9 with the mean of 280.91. When comparing C5 with 6, 6 with 7, and 7 with T1 they showed significantly decreasing HU value with p-value<0.01 with mean difference of 49.10, 30.98, and 31.25, respectively. ACDF ending at C7 showed significantly lower fusion rate than C6 at 4 months and 1-year follow-up, respectively.Conclusion: The HU show a decreasing trend towards lower sub-axial cervical vertebral body. Furthermore, multi-level ACDF ending at C7 showed higher non-union rate than ending at C6. Although, there is no direct evidence that lower bone density at lower sub-axial cervical vertebrae is causative for non-union, it should be considered for pre-operative planning.