2022, Vol. 26 ›› Issue (15): 2337-2341

Lag screw path for fixation of sacroiliac joint dislocation through S1 pedicle

Li Jinglian1, Zhang Hongfei2, Yan Jiapeng2, Zhou Jiabin2, Yu Hao1   

Abstract: BACKGROUND: Percutaneous sacroiliac screw fixation has become the preferred method for the treatment of sacroiliac joint dislocation. However, at present, the screw is mostly inserted parallel to the horizontal plane. This method of screw placement has relatively high risk, which is easily affected by human pelvis variation, and is difficult to achieve double-screw fixation. Therefore, a new internal fixation method should be developed.  
OBJECTIVE: To simulate the internal fixation of the sacroiliac joint using a lag screw of 7.3 mm in diameter in a three-dimensional model of the pelvises, followed by measurement and statistical analysis, thus to determine the best iliac screw entry point and spatial angle.
METHODS:   Mimics 21.0 software was used for three-dimensional remodeling based on computed tomography data of the pelvis in 60 cases. From the top of the buttocks as close as possible to the upper edge of the iliac crest, a 7.3 mm lag screw was inserted through the S1 pedicle to the midpoint of the upper 1/3 of the S1 vertebral body (the screw was inserted entirely from the caudal direction), and the second lag screw was inserted 5-8 mm below the first screw on the coronal plane. All cases must meet the requirements for safe placement of double screws, then the deviation angles between the axis of the first lag screw and the horizontal plane, sagittal plane, and coronal plane were measured in the software, and the position of the iliac screw entry point was observed and analyzed.  
RESULTS AND CONCLUSION: Screw entry point was slightly outside the vertical tangent to the inner edge of the small pelvis. The optimal angles of the screw placement path to the horizontal, sagittal and coronal planes were α (12.56±6.14)°, β (66.42±5.45)° and γ (18.68±5.09)° in males, respectively, and were α (9.78±5.31)°, β (69.46±5.34)°, and γ (16.86±5.94)° in females, respectively. There were significant differences in α and β between male and female, but not in γ. The sacroiliac screw could be inserted entirely from the caudal direction, at a relatively stable spatial angle. The entry point of the ilia was positioned slightly outside the vertical tangent of the inner edge of the small pelvis on the anteroposterior projection plane, and the variation was small. To conclude, insertion of two parallel screws can be achieved in most patients. Therefore, the caudal insertion of sacroiliac screw can be used as a clinical screw entry method.
Key words: sacroiliac joint, lag screw, three-dimensional reconstruction, Mimics software, computed tomography