Applied Anatomical Study Of The Sacral Mass Screw Trajectory

Objectives:To study the anatomical characteristics of the trajectory of sacral lateral mass screw and provide a theoretical basis and reference for the clinical application of the internal fixation technique with aid of sacral lateral mass screw.Methods and materials:1.60 cases (33 male/27 female cases) of adult normal sacral vertebral CT data were collected for this subject with the age ranging from 25 to 78. After the three-dimensional reconstruction of sacrum, the trajectory of sacral lateral mass screw was found and separated from sacral model. Then the anatomical data of the lateral mass screws was measured.2.6 adult cadavers were collected for the placement of sacral mass screw to exam the measurement results.Results:The entry point of S1 mass screw located at 1/3 between superior border and the first sacral hole, which was lateral to facet. The screw diameter was restricted under 13 mm while the length was controlled no more than 33 mm. The screw trajectory was parallel to the S1 endplate on sagittal plane while the mean angle between the screw trajectory and the horizontal plane was 24.05±3.25°.The entry point of S2 mass screw located at the midpoint of connection of the first sacral hole and the second sacral hole. The screw diameter was restricted under 10mm while the length was controlled no more than 36mm.The mean angle between the screw trajectory and the S2 end plate was 11.23±2.53°while the mean angle between the screw trajectory and the horizontal plane was35.79±1.20 (male) and 37.92±3.75°(female).The entry point of S3 mass screw located at the midpoint of connection of lateral edge of the second sacral hole and the third sacral hole. The screw diameter was restricted under 6.5mm while the length was controlled no more than 30mm.The mean angle between the screw trajectory and the S2 end plate was 5.23±1.05°while the mean angle between the screw trajectory and the horizontal plane was 46.26±2.56°.The entry point of S4 mass screw located lmm below the midpoint of connection of lateral edge of the third sacral hole and the forth sacral hole. The screw diameter was restricted under 6mm while the length was controlled no more than 14mm.The mean angle between the screw trajectory and the S2 end plate was 12.15±3.42°while the mean angle between the screw trajectory and the horizontal plane was 48.52±4.36°.The angle between the screw trajectory and the horizontal plane showed significant differences in gender groups.Sacral mass screws were placed in the cadavers’ sacra. Two screws have penetrated the cortex with the rest were all within the sacral bone without penetrating to the sacral hole, canalis sacralis and auricular surface.Conclusions:Sacral mass screw is feasible for sacral internal fixation with the proper direction and length of nails. Sacral lateral mass screw technology can provide reliable way for direct fixation of sacral fractures.