Significance of Measuring Lumbar Spine 3-Dimensional Computed Tomography Hounsfield Units to Predict Screw Loosening

Osteoporosis is a well-known risk factor of screw loosening. Classically, dual-energy x-ray absorptiometry (DEXA) scan is an easy and cost-effective method of detecting bone mineral density (BMD). However, T-score on DEXA scan can be overestimated in patients with degenerative changes of the spine. Our objective was to identify correlation between Hounsfield unit (HU) measured by 3-dimensional computed tomography (3D-CT) and screw loosening.A total of 113 patients treated with lumbosacral spinal fusion were reviewed and categorized into a screw loosening group and a normal group to compare their average values of preoperative CT HU. Screw loosening was defined as radiolucent area around screw that was thicker than 1 mm with a "double halo sign".There were statistically significant differences in patient age and steroid use between screw loosening and non-loosening groups. There was no significant difference in BMD or T-score between the 2 groups. However, HU values measured in axial, coronal, and sagittal images were significantly different between the 2 groups. In the receiver operating characteristic for HU values measured in CT images, the greatest area under the curve was 0.774 and that was in case of Hounsfield unit measured by axial CT images from L1 to L4.Preoperative CT HU is associated with screw loosening. It can be a better predictor of screw loosening than DEXA scan. The best predictor of screw loosening in this study is the average value of HU from L1 to L4 in axial cut.