Internal architecture of the sacrum in the elderly - An anatomic and radiographic study

Study Design. A description of the internal architecture of the sacrum, including its trabecular arrangement, cortical thickness, and overall bone density. Objectives. To determine the strong and weak areas in the sacrum to understand more clearly the sacral structure and its clinical implications. Methods. First, seven cadaveric sacral specimens were sectioned in different planes. Horizontal sections were performed at the upper S1, middle S1, S2, S3, and S4. Sagittal sections were made through the median sacral crest, the sacral foramina, and medial to the articular surface. A coronal section through the whole length of a sacral specimen was produced. All sections were studied radiographically, and the trabecular pattern was analyzed. In the second part of the study, axial computed tomography scans of 40 dry sacrum specimens were analyzed by using the National Institutes of Health Image 1.61 program. The cortical thickness and bone density were determined. Results. In the upper sacrum, three distinctive distributions of bony trabeculae were noted, one extending from the center of the sacral body anterolaterally, and the other two extending from the pedicle toward the auricular surface. A condensation zone was observed at the intersection of these trabeculae and was located at the anterior cortex of the foraminal zone. The junction between S2 and S3 represented a weak area with abrupt disappearance of the condensation zone. Analysis of the bone density of the sacrum using the plot analysis demonstrated that, at S1 and S2, the anterior cortex of the foraminal zone (condensation zone) is the most compact part of the sacrum. Conclusion. These results suggest that the strongest part of the sacrum is the anterior cortex above the foramina in S1 and S2. The weakest point of the sacrum was found to lie at the level of the junction of S2 and S3.