Optimal fixation strategy in unstable pelvic fractures remains controversial topic in orthopaedic trauma care. In the last decade, minimally invasive approaches have shown promising results. The aim of the current study was to evaluate the biomechanical stability of a unilateral and extended unilateral INFIX compared to the a bilateral INFIX for fixation and stabilization of unstable pelvic fractures. The main findings of the current study were that. The biomechanical stability of the extended unilateral INFIX was significantly superior to that of the bilateral INFIX. He addition of a pubic ramus or symphysis pedicle screw to the unilateral INFIX provided significantly better anterior stiffness and decreased anterior fracture fragment displacement.
These results show that, in cases of unilateral pelvic ring instability, the classic bilateral INFIX could be replaced by the extended unilateral INFIX, reducing the risk of nerve damage on the contralateral, uninjured site. Furthermore, we see the potential for using the additional pubic ramus pedicle screw for reduction of the fracture, as it offers good control of the medial fragment. Due to the three-point-attachment, the rod could play a role in reducing the intermediate fragment attached to the screw in the middle, similar to spine surgeries.
Preliminary results of ongoing biomechanical have demonstrated good biomechanical stability of a modified INFIX for acetabular T-type-fractures, simultaneously using this device for the reduction of the anterior acetabular column.
Displaced fractures of the anterior pelvic ring often require open reduction and internal fixation. For this procedure, anterior approaches such as the modified Stoppa approach can be used [11]. These approaches have many disadvantages, such as high blood loss and the risk of nerve damage due to traction [11,12,13].
Minimally invasive treatment options of the anterior pelvic ring are normally performed with a retrograde transpubic screw, a supraacetabular external fixator or, as a new method, with the INFIX. These treatments also have some disadvantages, mainly regarding difficulties to achieve a proper reduction in displaced fractures. An additional supraacetabular external fixator may help to reduce the fracture but may introduce a higher risk of infection of the pin screws, loosening of the screws, loss of reduction, as well as a massive discomfort of the patient who has to cope with this construction in his or her daily routine [14, 15]. The INFIX also seems more effective than external fixation at reducing postoperative surgical site pain [16]. Other disadvantages of the external pelvic fixator include worse outcomes in diabetic and obese patients [17].
Biomechanically, the INFIX is more stable compared to the external fixator and provides better stability [6]. The symphysis-fixed INFIX (extended unilateral INFIX) could provide improved rotational stability compared to the bilateral INFIX with fixation of the symphysis. A finite element study by Song et al. [17] showed better rotational stability of the plate fixation fixed to the symphysis compared to the bilateral INFIX. It was concluded that this could be due to the fixation to the symphysis [17].
Our results also underline that the extended unilateral INFIX could provide better rotational stability than the standard bilateral INFIX, possibly because fixation next to the symphysis provides more anterior stability when using the INFIX for anterior pelvic ring fractures. Further biomechanical studies should follow especially in human pelves to confirm these findings.
We assume that the extended unilateral INFIX has better stability than the bilateral INFIX with the possible advantage of performing a reduction of the fracture with the pre-bend rod.
A problem of the extended INFIX, and also the classic INFIX, is the anterior fixation near the pubic symphysis. Anatomically, the anterior fixation site is located near the urinary bladder, therefore having the potential for irritating it, especially when filled. The potential bladder damage is also a problem when performing INFIX or extended INFIX [2]. In clinical situations, another problem of the INFIX is the potential damage to nerve and vessel structures like the lateral femoral cutaneous nerve, the femoral artery or the femoral vein during the operation. On the other hand, the more invasive Stoppa approach for ORIF is more challenging for the surgeon and has high potential for damaging blood vessel structures or the peritoneum [12].
Our study has a number of potential limitations. The composite bones do not fully resemble the biomechanical conditions in a human body, but they do have the advantage of providing reproducible and comparable biomechanical testing, thus avoiding the variability inherent to cadaveric specimens. In terms of fracture reduction, the synthetic bones are by far easier to reduce than human in vivo pelvic bones. However, previous studies have already demonstrated that the INFIX is a feasible technique of reduction and fixation of fractures in patients with pelvic trauma.
