Treatment of femoral PPFx is a major challenge in modern orthopaedics, and adequate fixation around the prosthesis and the choice of effective internal fixation remains clinically demanding.
This study compared the biomechanics of the LCP + LAP and the OBS for fixation of femoral PPFx. First, a load–displacement curve was obtained through an axial compression test, in which the slope of the stress–displacement curve is considered to demonstrate the stiffness of the internal fixation model as a whole (the fracture end contracts under the load). The initial axial stiffness of the LCP + LAP fixation group was not significantly different from that of the group in Lenz et al.’s study . The stiffness measurements and torsion angles in our study can be used as biomechanical parameters, especially for the OBS. When simulating simple fractures in this study, no apparent difference in stiffness was observed among the four groups in the axial compression experiment (P > 0.05); however, the torsion angle of the LCP + LAP fixation group was higher than that of the OBS group (P < 0.05), which indicated that the biomechanics of the three OBS combinations were better than those of the LCP + LAP fixation group. When simulating the comminuted fracture, the structural stiffness of the three OBS combination groups was higher than that of the LCP + LAP group (P < 0.05), and the torsion angle of the LCP + LAP group was also higher than that of the bridging double-rod and double-cortex group (P < 0.05), indicating that the biomechanics of the OBS combination groups were still better than those of the LCP + LAP fixation group. However, when crushing a fracture, the axial compression stiffness of the comminuted fracture group was significantly lower than that of a simple fracture (P < 0.0125). Lenz et al.  reported that because the support of the cortex at the fracture was reduced, an implant load was needed at this time; therefore, internal fixation was very important for axial stability, especially for crushing a fracture. Finally, the results of the axial compression failure test showed that in OBS, the bridging double-rod and double-cortex fixation and bridging double-rod single cortex fixation have stronger resistance to destructive power than LCP + LAP. However, although the OBS had more fractures on the surface, its failure mode indicated that stress could be dispersed to the femur when the OBS was used in the femur during axial compression. It could be inferred that the failure mode of the locking steel plate was concentrated on the steel plate, which led to the steel plate bearing more stress than the femur. This is consistent with the stress distribution assessment of the finite element analysis of these two systems in the femur by Xiong Ying .
Kammerlander et al.  showed that patients aged ≥ 75 years who received treatment for hip fractures were unable to maintain postoperative weight-bearing limits, with 69% of patients exceeding more than twice the prescribed partial weight-bearing limits; therefore, the goal of treating periprosthetic fractures must be to facilitate immediate full weight-bearing. During gait, an internal fixation system needs to carry 2 to 2.4 times an individual’s weight [28, 29]. The destructive test results in this study were > 3000 N, which was sustainable. However, the concentration of stress on the plate may cause internal fixation fatigue failure due to high-stress cyclic loading during early functional exercise . The OBS can withstand a higher axial load and distributes axial load to the femur, which could make it more suitable for early functional exercise and fracture healing.
Challenges when treating PPFx after cemented hip arthroplasty concern fixation around the prosthesis. Due to the inconvenience of double-cortex screw placement and the relative imbalance between proximal and distal fixation, pull-out of single cortex screws may occur in internal fixation. Fulkerson et al.  observed early failure and axial displacement of the single cortex locking screw under cyclic loading, and other biomechanical studies have shown that double-cortex screw internal fixation has advantages over single cortex screw or cortical ligation alone in proximal fixation of PPFx [26, 32,33,34,35,36]. In this study, double-cortex fixation of the OBS was guided using a C-arm machine. Moreover, because of the orientation and the corresponding angle of the OBS, the connecting block could be adjusted to achieve flexible screw placement, and double-cortex fixation of the OBS could be undertaken around the prosthesis without using additional devices. In the OBS bridging single-cortex fixation group, proximal fractures were fixed with single-cortex screws, but no proximal screw pull-out was observed during the fracture test, and the biomechanics were similar to that of the LCP + LAP fixation group. Using an OBS flexible connecting rod in the design for the bridge single-rod cross-fixation group, two fixed-rod screws can be placed at 90° in spatial distribution to achieve better stability of the fixed-rod screw. However, proximal fractures still occurred with single cortex fixation, although with no screw pull-out damage during the experiment. In the axial compression experiments, there was no significant difference in stiffness between the LCP + LAP fixation group and the bridge single-rod cross-fixation group; however, the anti-torsion performance was better. Insertion of the double-cortex screw was more resistant to pull out under cyclic loading, which is conducive to functional exercise in early rehabilitation and avoids screw pull-out failure .
It has been reported that minimally invasive percutaneous plate osteosynthesis combined with LCP can be used as a surgical method for the treatment of Vancouver type B1 PPFx, with intraoperative advantages (shorter operation time and less blood loss) , and that the OBS can also be combined with Mippo technology for fracture treatment. Liangqi et al.  reported that the OBS combined with Mippo technology to treat ipsilateral proximal femoral and diaphyseal fractures had a high possibility of healing fractures and having a good functional effect. The OBS can also provide personalised internal fixation when combined with 3D printing applications for complex and challenging fractures around the prosthesis, with preoperative preparation having obvious advantages. Moreover, the OBS has many choices of fixed combinations, allowing greater flexibility for the operating surgeon, who can then obtain the best implant position, especially around the prosthetic implants.
This study had several limitations. A standard artificial femur was used in this study, which could simulate good bone reserve. Models for osteoporosis are available; however, this study did not simulate osteoporosis and low-quality bone. Patients with fractures around a femoral prosthesis comprised older adults, and these patients commonly have osteoporosis. Furthermore, in vitro simulation experiments, the influence of soft tissue involvement, the short- and long-term role of soft tissue in fracture stabilization and healing, and the convenience of OBS placement and screw placement could not be evaluated. The effects of cyclic loading were also not tested in this study.