OBJECTIVETo study the anatomical and biomechanical features of sacral pedicle and lateral mass so as to provide reference for clinical screw fixation technology of sacral pedicle and lateral mass.

METHODSA total of 60 adult patients'spiral CT images of the sacrum and coccyx were selected randomly. The entry points of sacral pedicle and lateral mass screws were determined, and the screw trajectory was measured using the three dimensional reconstruction method. Meanwhile, the gross anatomy was scrutinized in 15 adult cadaver specimens to determine the sacral pedicle and lateral mass screw entry points. The length, width and angle of sacral pedicle and lateral mass screw trajectory were measured. Eight of 15 cadaver specimens were selected to test the maximal extraction force of sacral pedicle and lateral mass screws. The clinical data of 15 cases treated by pedicle and lateral mass screw technology were collected and analyzed.

RESULTSThe diameter and length of S(1)-S(5) sacral pedicle and lateral mass screw trajectory were regular, with about 20 degree inclination angle. The S(1) pedicle screw entry point was located at the intersection point of the basal lateral part of articular process and median line of transverse process, and no significant difference was found for the maximal extraction force between pedicle and lateral mass screws (P larger than 0.05). The entry points of S(2)-S(5) pedicle screws were located at the intersection point of the line connecting adjacent posterior sacral foramina and median line of the transverse process. The lateral mass screw entry point of S(2)-S(5) was on the median side of intersection point between median line of the transverse process and lateral sacral crest. The maximal extraction force of pedicle screws was significantly greater than that of lateral mass screws (P less than 0.05).

CONCLUSIONBoth the sacral pedicle and the lateral mass screw fixation techniques can offer effective fixation and reconstruction for fracture of the sacrum and coccyx, but pedicle screw fixation may be more convenient, safe and reliable than lateral mass screw fixation.