ANATOMIC CONSIDERATIONS OF THE 2ND SACRAL VERTEBRA AND DORSAL SCREW PLACEMENT

Direct measurements and measurements from images of axial cross-sectio ns on 20 cadaveric sacra that had been scanned on computer were used i n this study. The measurements, including parameters from the vertebra l body, lateral mass and spinal canal of the second sacral vertebra (S 2) were performed. The length of the screw path and the optimal angula tion of the screw placement for dorsal sacral internal fixation were a lso included. The mean values of height, anteroposterior diameter, wid th and breadth of the S2 were 25.0 mm, 13.5 mm, 29.4 mm and 83.0 mm, r espectively. The mean values of the mid-sagittal diameter, maximum tra nsverse diameter and area of the S2 spinal canal were 10.3 mm, 23.1 mm and 162.4 mm(2), respectively. The mean transpedicular screw length o f the S2 and optimal medial angle were 25.2 mm and 30.0 degrees, respe ctively. The mean lateral mass screw length of the S2 and optimal late ral angle were 32.8 mm and 22.0 degrees, respectively. The present stu dy provides quantitative anatomic data of the second sacral vertebra. All parameters indicate that, compared with our previous study, S2 is smaller than S1. When S2 lateral mass screw fixation is intended, anch oring the anterior cortex may violate the iliac vessels or lumbosacral trunk; therefore, understanding the unique anatomy of the S2 is imper ative.