There are various methods of internal fixation for fractures around the Vancouver type B1 prosthesis. Still, due to the presence of the intramedullary prosthesis, the double cortical fixation cannot be achieved, the holding power of the internal fixation is reduced, and the conventional internal fixation often fails to achieve a strong fixation, leading to reoperation19,22. The unique locking structure of OBS consists of a pin, rod, and block, which are freely matched to form a plurality of internal fixation complexes. Different from the single position and direction of plate series screws, the position and direction of OBS screws can be adjusted and controlled at will according to the situation, providing wider operability and applicability for the treatment of periprosthetic fractures and well making up for the shortcomings of other internal fixation devices. The multi-rod fixation mode and cross-screw fixation of OBS can achieve three-dimensional fixation and further improve the fixation strength. The rod-block combination can better disperse stress, avoid stress concentration and reduce the fracture risk of the internal fixator23. In addition, intraoperative soft tissue dissection is too much; the blood supply is reduced and can easily make the fracture heal. Therefore, the application of OBS can achieve minimally invasive or limited incision operation, with minimal damage to soft tissue and periosteum. Bridging fixation does not directly compress the periosteum and fracture site, which has little impact on the blood supply of the fracture area and protects the biological environment of the fracture area24. An animal experiment also showed that OBS could effectively reduce the disruption of blood supply at the fracture site and provide a firm fixation25. The OBS connection block can slide axially to achieve compression and has the function of a "reset device," which can be fixed by changing the reduction edge during surgical operation 26.
The use of OBS in the treatment of Vancouver B1 periprosthetic fracture of the femur has been proven to be solid and reliable with satisfactory clinical results 27. Due to the flexibility of the OBS combination mode, this experiment was designed to explore the optimal combination mode. The OBS rod-block structure allows the screw position to be adjusted freely according to the specific situation, and the pressure hole or lock hole can be selected freely. The angular screws have better pull-out resistance and fixed strength15. The three-dimensional fixation of OBS can be achieved well when the three-rod fixation is used. The third rod is about 20mm below the lateral concave of the femur, and the femoral displacement and the maximum Von Mises stress of OBS are both the maximum. The failure possibility of internal fixation is the highest when the third rod is placed there. The lower 35mm femoral deformation and OBS stress are minimal, which is the best third-rod placement position. The intersection Angle of the proximal screw on the femoral displacement and the maximum Von Mises stress of OBS is generally gentle. Still, there are two trough points; namely, the intersection Angle is about 71° and 84°, respectively. We believe that both of these two cases are feasible. To fix the strength, it is recommended to choose a 6mm connecting rod with low femoral displacement and the maximum Von Mises stress of OBS. Although the number of screws was the most stable, considering the complexity of the clinical operation, we believed that the plan D with little difference was the best plan for the number of screws.
Our study has its limitations. The study included subjects that did not reflect actual bone mass in patients with periprosthetic fractures. In addition, due to the stress shielding effect, the proximal femur bone mineral density will decrease, and the Gruen7 area has the greatest decrease28,29. However, bone loss was not considered in this experiment. Muscle and other soft tissues were not considered in this study, and only biomechanical evaluation under the same bone and the same load was performed.