Bone Mineral Density Mapping of Iliosacral Region: The Use of Hounsfield Units to Optimize Transsacral Screw Trajectory

Trauma, degenerative, pediatric, and neuromuscular deformities often require placement of implants into sacrum for construct stability. In these scenarios, fixation to the ilium is often added. To date, multiple studies have validated the correlation between Hounsfield units (HU) as measured on computed tomography and bone mineral density (BMD) on dual-energy X-ray absorptiometry to assess bone quality and fracture risk. We sought to map the BMD of the iliosacral region at the S1 and S2 levels using HU. Materials and Methods: Pelvic CT scans of 100 patients were evaluated. HU measurements were taken from the anterior and posterior ilium, sacral ala, and sacral body using a best-fit circle encompassing a maximal amount of cancellous bone. Following the collection of all data points, an analysis of variance model was created to test the means and standard deviations of each anatomic region. Results: The highest mean BMD was found in the following locations (in descending order): S1 sacral body (279.72 HU, 95% confidence interval [CI], 261.75-297.69), S1 anterior ilium (254.45 HU, 95% CI, 236.64-272.27), S2 anterior ilium (229.88 HU, 95% CI, 211.39-248.36), and the S2 sacral body (191.58 HU, 95% CI, 173.31-209.85). Comparing the anterior ilium to the sacral ala, there was a higher BMD measurement at both the S1 level by 151.7 HU (p < 0.001) and the S2 level by 170.3 HU (p < 0.001). The anterior ilium also showed a significantly higher BMD when compared to measurements taken at the posterior ilium at the same level: at the S1 level by 100.5 HU (p < 0.001) and at the S2 level by 52.2 HU (p = 0.0006). The vertebral body had significantly higher mean BMD measurements when compared to the sacral ala at the S1 level by 176.9 HU (p < 0.001) and at the S2 level by 131.95 HU (p < 0.001). Conclusions: The findings indicate that there is significantly higher BMD density of the anterior ilium and sacral body when compared to the sacral ala at both the S1 and S2 levels. In addition, the anterior ilium appears to provide more dense bone than the posterior ilium, as measured by mean HU.