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A biomechanics study about the mechanism of ligaments injury in anterior posterior compression type Ⅱ pelvic injury

Abstract
 Background

The anterior and posterior compression (APC) pelvis fracture is a classic pelvic injury, and APC type II is considered to be a typical one caused by the destruction of pelvic ligaments, while the mechanism of ligaments injury and treatment of which is still controversial. This study aims to explore ligaments injury in anterior posterior compression(APC)type II pelvic injury.
Method:

Fourteen human cadaveric pelvis samples (5 female, 9 male) with the sacrospinous, sacrotuberous, anterior sacroiliac ligaments and partial bone retaining unilaterally were made for this study. To simulate the APC pattern pelvic injury, the samples were divided into two groups randomly, set one group as hemipelvis restricted group (experimental group) and the other one as unrestricted group (control group). According to the biomechanical data, eye observation, motion capture system and real-time video system to record the separation distance of the pubic symphysis and anterior sacroiliac joint, external rotation angle and force when the anterior sacroiliac ligament ruptured. Continuing the external rotation violence, observing the bone and posterior ligaments change since sacrospinous and sacrotuberous ligaments from being damaged to completely ruptured.
Result

When anterior sacroiliac ligament failed, the mean separation distance of pubic symphysis and anterior sacroiliac joint between restricted group and unrestricted group was 28.6 ± 8.4 mm to 23.6 ± 8.2 mm(P = 0.11) and 11.4 ± 3.8 mm to 9.7 ± 3.9 mm (P = 0.30) respectively. In addition, the external rotation angle and force was 33.9 ± 5.5° to 48.9 ± 5.2°(P < 0.01) and 553.9 ± 82.6 N to 756.6 ± 41.4 N (P < 0.01) respectively. The two distances were not significantly different (P > 0.05), however, the external rotation angle and violence was significantly different (P < 0.05), which was bigger in the unrestricted group. In the unrestricted group, when anterior sacroiliac ligament ruptured, no distinct sacrospinous or sacrotuberous ligaments injury was observed, but in the restricted group, all of samples had two ligaments injury and even two samples had ligaments failed. Moreover, with the extreme external rotation violence continuing, there was still no sacrospinous or sacrotuberous ligaments injury in the unrestricted group. But interosseous sacroiliac ligament, posterior sacroiliac ligaments injury and slight sagittal rotation and sacroiliac joint displacement appeared. In the control group, the sacrospinous ligament ruptured firstly and then the sacrotuberous ligament ruptured. When both of the two ligaments failed, the interosseous sacroiliac ligament was damaged while posterior ligament was not. In the restricted group, when all of the anterior sacroiliac ligament, sacrospinous ligament or sacrotuberous ligament failed, mean separation distance of pubic symphysis and anterior sacroiliac joint increased significantly (from 28.6 ± 8.4 to 42.0 ± 7.6 mm, 11.4 ± 3.8 to 16.7 ± 4.2 mm respectively, all P < 0.05).
Conclusion

We have three main findings: First, pelvic external rotation injury can divide into two situations: hemipelvis is restricted and unrestricted, which result into two different outcomes. When anterior sacroiliac ligament rupture, the unrestricted group needs more external rotation angle and force, without obvious sacrotuberous or sacrospinous ligaments injury. But in the restricted group, both of two ligaments injury appear. Second, when anterior sacroiliac ligament fail, pubic symphysis displacement ranges from 14 to 40 mm, which has a high fluctuation. Third, when the anterior sacroiliac ligament is damaged, we dose not observe the inevitable destruction of the pelvic floor ligaments (sacrospinous ligament and sacrotubercular ligament).