MORPHOMETRIC EVALUATION OF THE FIRST SACRAL VERTEBRA AND THE PROJECTION OF ITS PEDICLE ON THE POSTERIOR ASPECT OF THE SACRUM

Study Design. This study directly measured the dimensions of the first sacral vertebra based on dry bony specimens. Objectives. To measure the dimensions of the S1 pedicle, vertebral body, and spinal canal, and determine the projections of the S1 pedicle on the posterior aspect of the sacrum. Summary of Background Data. Sacral screw fixation remains a challenging clinical problem because of the unique anatomy of the first sacral vertebra. The anatomic data of S1 have rarely been described in the literature. Methods. Sacrums from 50 carefully prepared bony spines were used. Linear measurements included the dimensions of the vertebral body, spinal canal, and pedicle. Pedicle width was measured in two segments, pedicle zones 1 and 2. Angular measurements of the pedicle relative to the sagittal plane were also included. Results. In pedicle zone 1, the average medial angle of the pedicles was 10.2 degrees for men and 10.7 degrees for women, average width was 10.9 mm for men and 10.4 mm for women, average length of the pedicles was 34.6 mm for men and 34.9 mm for women. In pedicle zone 2, the average medial angle of the pedicles was 39.4 degrees for both men and women, average width was 15.6 mm for men and 14.7 mm for women, average length of the pedicles was 43.7 mm for men, and 41.7 mm for women. The projections of pedicle zones 1 and 2 on the posterior aspect of S1 were determined. Conclusions. The investigators proposed the concept of pedicle zones 1 and 2. The safer area for S1 pedicle screw insertion was found in pedicle zone 2.