Abstract:

Objective　To identify the best entry point of the sacroiliac（SI）joint screw  and safe angle by measurement of anatomical parameters of S1 vertebrae and virtual screws fixation of the SI joint in the three-dimensional reconstruction pelvic model.　Methods　50 pelvis including 25 females and 25 males were reconstructed in Three-dimensional form and cut by Mimics software. The anatomical parameters of S1 vertebrae were measured and the virtual screws were placed in each SI joint through different entry points, then the best entry point of the screw was identified and the safe angles of the screw were calculated. The surface soft tissues of the pelvis were reconstructed in three-dimensional form by Mimics software , the surface projection points of entry point P, anterior superior iliac spine’s vertex N, point M were P1, N1 and M1, respectively, and the lengths of P1M1,P1N1 and M1N1 were measured sectionally. 　Results　The lengths between central screw’s entry point P and point N were（95.17±5.59）mm (male) and（98.49±7.71）mm (female). PM was vertical line of PN passing point P and point M was intersect point of iliac crest and PM, the length of PM was（58.71±4.33）mm (male) and（54.59±6.01）mm (female). The distance from point N to point M, which were along the iliac crest, were（129.78±6.25）mm (male) and（130.64±7.79）mm (female). The difference of PM’s length between male and female had statistical significance (P＜0.05) . The central screw’s safe ranges in the forward and backward incline were (10.70±2.00) °(male) and (7.20±1.64) ° (female),and the safe ranges in the cephalic and caudal incline were (12.36±2.68) °(male) and (11.27±3.29) ° (female). The forward and backward incline safe ranges showed statistical difference between male and female (P＜0.05). The length of P1M1 was significantly different between male and female (P＜0.05).  Conclusions  The best entry point of the SI joint screw was determined according to the projection points of anterior superior iliac spine’s vertex N and point M , the screw’s forward and backward incline ranges is about 7°（female） or 10°（male）,the cephalic and caudal incline ranges is about 11°（female）or 12°（male） When the top surface and the posterior surface of S1 vertebrae are regarded as the transverse plane and the coronal plane respectively.

Key words: Sacral Ⅱ region, Iliosacral screw, Image anatomy