Anatomic evaluation of the first three sacral vertebrae and dorsal screw placement.

Although the anthropometric studies of the first sacral vertebra have been previously described, there are few studies that define and compare the morphological features of the S-1 to S-3 vertebrae. Twenty sacra were directly measured, and then digitized images of macroscopic cross-sectional specimens were measured by using Color Image 1.31 software. Measurements, including parameters from sacral vertebral bodies, sacral canal, and auricular articular surfaces of the first three sacral vertebrae, were obtained. The contributions by the first, second, and third sacral vertebrae to the auricular articular surface area were 50%, 35%, and 15%, respectively. The parameters related to the S-1 and S-2 medial and lateral screw placement were also determined. All measured parameters of the upper three sacral vertebral bodies and the sacral canal gradually decreased from S-1 to S-3. This study provides morphometric data of the first three sacral vertebrae that may prove helpful for better understanding the unique anatomy of the sacrum and transpedicular and lateral mass screw placement.