Abstract:

Objective 　To investigate the safety parameters of S2 transverse screw when the sacroiliac screw cannot be transversely placed into the first sacral(S1) vertebra to fix zone Ⅲ sacral fractures.    Methods    The 3-D models of 96 pelvis were reconstructed by Mimics software based on CT data, and the virtual S1 transverse sacroiliac screws were placed. When the transverse sacroiliac screw cannot be safely inserted in S1, it would be regarded as a dysplasia. There were 34 S1 vertebrae which belonged to the dysplasia group, and the other paired 34 S1 vertebrae according to gender were classified to the normal group. Placement of the S2 transverse sacroiliac screws were simulated in 68 S2 vertebrae, and then the parameters of screw paths were measured respectively. The soft tissues on pelvic surface were generated by Mimics, and then the body surface projections of S2 transverse screw's entry point B, anterior superior iliac spine’s vertex M, iliac crest point N were determined as B1, M1, N1, respectively. At the same time, the lengths of B1M1, B1N1 and M1N1 were measured in section.　Results　There was significant difference in the lengths of the line B1M1, which were (140.94±16.64) mm and (129.37±14.93) mm in male between the normal group and the dysplasia group (P<0.05), and the lengths of the line B1M1 were (143.95±16.27) mm and (132.07±16.84) mm in female with statistical difference between the normal group and the dysplasia group (P<0.05). The lengths of the line B1N1 in the normal group and the dysplasia group were (69.92±6.73) mm and (72.64±7.46) mm in male with no statistical difference (P>0.05), and were (60.80±7.05) mm and (58.85±7.81) mm in female with no statistical difference (P>0.05). The lengths of the line M1N1 were (157.58±16.83) mm and (150.48±13.21)mm in male of the normal group and dysplasia group, which showed no statistical difference (P>0.05). Similarly, there was no significant difference in the lengths of the line M1N1, which were (156.79±15.84) mm and (151.49±16.59) mm in female between the two groups (P>0.05).　Conclusion　When the sacroiliac screw cannot be transversely placed in dysplasia S1 vertebra including lumbosacral transitional vertebrae, sacrum not recessed in pelvis andacute alar slope. A 7.0 mm sacroiliac screw can be inserted in S2 vertebra for fixing zone Ⅲ sacral fractures. Under fluoroscopic monitoring, the body surface entry point of S2 screw can be determined by touching the bony landmarks of the anterior superior iliac spine andiliac crest.

Key words: Sacrum; , Dysmorphic, S2 transversescrew, Zone Ⅲsacral fractures, Anatomical and image