Sacral fracture is a common site of pelvic ring injury. The most common type is a unilateral sacral injury with anterior impaction of the sacrum, a lateral compression type 1 (LC-1) injury, which is usually associated with posterior ring instability and requires clinical treatment. Sacroiliac screw is a conventional internal fixation technique for posterior pelvic ring injury16. Sacroiliac screws are commonly used to stabilize the posterior ring. However, some clinical studies have raised that conventional sacroiliac screw fixation may not provide sufficient stability universally. Keating et al. 24 applied the sacroiliac screws to achieve 84% anatomic or near-anatomic reduction of pelvic fractures, but the final malunion rate was 44%. Damian et al.12 showed that sacroiliac screws are clinically unreliable for vertical sacrum fractures. More recently, lengthened sacroiliac screws have come into use 29 30. Our paper described it as a "lengthened sacroiliac screw." The screw is inserted from the external surface of the ilium across the contralateral sacroiliac joint and exits the ilium. Gardner et al. 25 described the advantages and theoretical basis of using a lengthened sacroiliac screw. First, Lengthening sacroiliac screws has the characteristics of better vertical shear load distribution, lower tip stress, and resistance to displacement. Second, in addition to the absolute length of the screw, the lengthened sacroiliac screw allows for more threads to bind to the bone, which may increase holding power. Third, lengthened sacroiliac screw provides anchorage in the iliac cortical bone, which may increase the role of the screw in maintaining reduction. Our sacral fractures based on biomechanical investigations19,20 showed that the lengthened sacroiliac screws provide better stability than ordinary screws. However, it was not previously reported whether an increased risk of breakage accompanies the application of lengthened sacroiliac screw.
The maximum Von Mises stress, one of the fundamental safety indicators of screws, increased with the increase of screw fracture risk. The higher the stress, the greater the likelihood of screw failure. The following results can be summarized from this study. When comparing different fixation modes with the same kind of screws, we found that the maximum Von Mises stress was the largest, with only fixed S1 segments, and the minor model, with only fixed S2 segments. At least in the model, S1 and S2 are both fixed. Secondly, when fixing the S1 segment and S2 segment simultaneously, the maximum Von Mises stress of the S1 segment screw in the same model was similar to that of the S2 segment screw, regardless of whether it was a lengthened screw model or an ordinary screw model. Thirdly, if considered from the fixed segment, the screw fracture risk of double segment fixation was lower than that of single-segment fixation. In double-segment fixation, the fracture risk of the two screws was similar. When different screws were used to compare the same fixed segment, we found that the maximal Von Mises stress of the lengthened screw was lower than that of the ordinary screw, and the lengthened screw had a lower fracture risk than the ordinary screw.
In summary, from the perspective of screw safety, it is recommended to use lengthened screws for fixation. The safest method is to fix the S1 and S2 segments with extended sacroiliac screws. Normal sacroiliac screws are recommended for S1 and S2, even in the absence of lengthening sacroiliac screws. If only one screw can be used, S2 segment fixation is recommended regardless of the sacroiliac screw. The results of the screw safety analysis are consistent with those of the stability analysis.
The risk of a fatigue fracture in internal fixation can be reduced by avoiding excessive concentration in certain parts through the uniform distribution of stress in internal fixation. However, in this study, When we compared the stress distribution, we found that the stress distribution of screws was not uniform in any models. Compared with the different kinds of screws we used, the stress distribution of the lengthened sacroiliac screw was more uniform. Compared with different fixed segments, the stress distribution of double-segment fixed screws was more uniform. Similarly, these findings are consistent with the stability analysis results and the maximum von mises stress analysis.
The following points need to be pointed out in this study. Firstly, Although anterior ring instability is characteristic of type C pelvic ring injuries, considering the multiple fixation methods of anterior ring fractures may affect the stability of the posterior pelvic ring. This study did not imitate the anterior pelvic ring's injury and fixation but only maintained the anterior pelvic ring's normal state. The anterior pelvic ring had a slight effect on the stability of the posterior pelvic ring and did not affect our comparison of several study models. Secondly, to best simulate pelvic stability, we reserved multiple important pelvic ligaments in our research.Meanwhile, to eliminate any unpredictable forces that might affect the measurements, we did not simulate muscles to simulate the extra stability they would cause. Muscle parameters, joint data parameters, and joint flexibility Settings will qualitatively influence the results. However, the calculation is too complicated to complete the experiment, so they are simplified. 2627Thirdly, it was not feasible to simulate all the features of comminuted sacral fractures accurately. In our study, we used a well-accepted method to imitate a unilateral sacral sagittal fracture through the unilateral sacral foramen, which is considered the typical type of simulated sacral fracture (Denis II).Moreover, our model method had a straight and smooth fracture, which facilitated the standardization of the model and did not affect the accuracy of mesh generation and subsequent calculation. Fourthly, to best mimic the normal state of the pelvis while standing, we position the pelvis so that the upper surface of the symphysis pubis is aligned with the lower surface of the sacrum. Fifthly, the finite element model we studied was bone independent, and our conclusions were theoretically applied to patients who still have cartilage.