Biomechanical characteristics of fixation methods for floating pubic symphysis

FPS is a life-threatening injury characterized by an unstable anterior pelvic ring and hemodynamic instability. The instability of the pelvic ring is caused by fractures of the bilateral superior and inferior pubic rami and ischial rami. The mortality rate is 8.5–19% in polytrauma patients with pelvic fractures [6]. Shockingly, the mortality rate increases to 18–25% in patients with hemodynamic instability [6]. Moreover, the risk of internal organ injuries increases when there are migrating fracture fragments [5]. Thus, patients with FPS require emergency pelvic stabilization to help minimize the mortality rate. Several fixation methods have been applied under such circumstances, including external fixation, anterior subcutaneous fixation, internal plate fixation, and percutaneous cannulated screw fixation. To choose the optimal fixation method, it is necessary to study the biomechanical performance of the different methods. Therefore, the aim of this study was to compare the biomechanical characteristics of six methods using finite element analysis.

External fixation remains the gold standard in the emergent treatment of anterior pelvic fractures [6], especially in patients with hemorrhagic shock and/or significant soft tissue damage. However, there are disadvantages to using an external fixator, including pin tract infection, loosening, skin impingement by pins, osteomyelitis, inconvenience regarding wound care and mobility, and negative effects on a patient’s quality of life [16, 17]. Particularly in diabetic and obese patients, external fixation is not recommended [16, 18]. External fixation can be a definitive treatment when accurate reduction and adequate fixation is obtained. Unfortunately, our results showed that external fixation did not meet these criteria from a biomechanical point of view. Under both compressive and rotational loads, the construct stiffness of the anterior pelvic ring was the lowest with external fixation. Failure to achieve and maintain stability of the fracture fragments can lead to recurrent bleeding and secondary internal organ injuries. Post-operative fragment dislocation is also a result of deficient fixation, and there are multiple secondary injuries that can be caused by unstable fracture fragments. Therefore, methods achieving increased construct stiffness may be necessary for definitive treatment after emergent stabilization has been performed.

We applied two techniques for achieving subcutaneous anterior fixation, including the use of a rod [18] and a plate [12]. These techniques have been performed in several cases. According to those who developed these methods [10, 17], subcutaneous fixation is appropriate in patients with hemodynamic instability. Operative time is comparable to that seen with external fixation, and the complication rate was reported to be lower. However, different complications including irritation of the lateral femoral cutaneous nerve and heterotopic ossification formation were shown to occur with subcutaneous rod fixation. The plate is not easy to apply in the setting of FPS due to difficulty in achieving reduction. From a biomechanical perspective, our results showed that subcutaneous anterior fixation achieves greater stiffness than external fixation. Interestingly, the subcutaneous rod fixator responded well to compression, while the subcutaneous plate fixator provided favorable anti-rotational capacity. This may be because the rod is ilium-ilium fixation and the plate is ilium-pubis-ilium fixation.

With regard to internal plate fixation methods, our study showed no significant difference between the use of the superior pectineal plate and infrapectineal plate. Both provided adequate stability from a biomechanical perspective. Indications for fixation methods depend on the fracture pattern and should be an area of future clinical research. Cannulated screws are an alternative option that have had good outcomes with use in FPS. This technique avoids extensive surgical exposure and minimizes the complications often experienced with reconstruction plates [14]. Furthermore, cannulated screws can achieve similar biomechanical results to internal fixation. Percutaneous cannulated screw fixation can provide accurate closed reduction and rigid fixation and is an excellent option for the treatment of patients with hemodynamic instability and soft tissue damage [14]. This technique can be performed in both emergent and delayed stabilization. Reduced operative times and diminished blood loss are advantages of percutaneous cannulated screw fixation which help to contribute to lower complication rates. However, frequent intra-operative fluoroscopy using a navigation system is required for accurate screw insertion.

Additionally, it should be mentioned that FPS is often associated with posterior pelvic ring injuries that cannot be restored and stabilized using the fixation methods previously described. Posterior pelvic ring stabilization can be achieved by percutaneous sacroiliac screws or other methods. In the present study, we maintained an intact posterior pelvic ring to minimize confounding factors. This was a limitation of our research.

There were other limitations of this study. The finite element models were based on a skeleton-ligament system, and thus, the muscle forces were not accounted for. This is similar to other finite element studies. A single pelvic model was used for analysis, which may neglect the bony and ligamentous variation seen between people. In the current study, we used the Chinese external fixation system, Constant. Considering that different implants have disparate designs, the results of this research can only be generalized to similar external fixators. The present study was designed to evaluate the biomechanical characteristics of six methods in the treatment of FPS, but pathophysiological and hemodynamic stabilization was not considered. Additional aspects of surgical treatment should be addressed in future clinical studies.