Measurement Techniques and Utility of Hounsfield Unit Values for Assessment of Bone Quality Prior to Spinal Instrumentation A Review of Current Literature

Study Design. A systematic review. Objective. The aim of this study was to evaluate the clinical utility of assessing bone quality using computed tomography (CT) attenuation in Hounsfield units (HU). Summary of Background Data. Assessing bone quality before spine instrumentation is an essential step of preoperative planning. Dual energy x-ray absorptiometry (DEXA) has been the gold standard for assessing bone mineral density (BMD); however, DEXA can result in spuriously elevated BMD measurements in patients with degenerative disease, compression fractures, and/or vascular calcifications. Measuring vertebral HU values has been proposed as an alternate method of assessing BMD and bone quality. Methods. We searched MEDLINE, Cochrane Library, and EMBASE for studies correlating HU to BMD and to spine surgery outcomes. Results. HU measurements correlate with success of lumbar interbody fusion (133.7 vs. 107.3 HU) and posterolateral fusion (167 vs. 139.8HU), cage subsidence (112.4 vs. 140.2 HU), adjacent segment fractures (145.6 vs. 199.4 HU), pedicle screw loosening (116.4 vs. 132.7 HU) and (99.1 vs. 141.2 HU), and risk of incidental durotomy (149.2 vs. 177.0 HU). Intra and interrater reliability coefficients are 0.964 and 0.975. The correlation between HU values and BMD in nondegenerative patients is r(2) = 0.52 compared with HU versus BMD in degenerative patients r(2) = 0.18. Conclusion. HU value measurement is a simple and rapid technique to assess bone quality that should be performed in all patients with pre-existing CT scans. HU measurement has excellent inter and intra-rater reliability and can be performed on axial or sagittal images. L1 HU threshold values of 110HU for detecting osteoporosis, and 135HU for detecting osteopenia are 90% specific. In patients with significant degenerative disease, HU values should be given more credence. Additional high-quality prospective studies comparing HU and DEXA values to patient outcomes are necessary to validate the role of CT measurements in preoperative evaluation.