Posteromedial tibial plateau split fractures are common tibial plateau fractures. Approximately one third of bicondylar tibial plateau fractures have a posteromedial fragment in the coronal plane4. This is often caused by a high-energy injury mechanism. The mechanism involved in this fracture pattern may be one of knee flexion, knee varus, and internal medial femoral condyle rotation5. This type of fracture pattern is worth noting more than others affecting the tibial plateau because this fracture pattern easily causes instability within the knee joint. Previous studies presented that malalignment related to inadequate fixation and the associated soft tissue injuries were the two most important reasons for a poor prognosis5–7.
The operative treatment goal for tibial plateau split fractures are anatomical reduction, especially in articular congruity restoration, stable fixation for early rehabilitation, and avoidance of complications, particularly infection and non-union. The tibial bone plate fixation is a major approach used to fix the fragment. Non-displaced posterior fracture fragments can usually be stabilized through the standard anterolateral approach.8,9 However, in the anteromedial approach10, the fracture site is shown from the lateral side. However, the medial collateral ligament (MCL) is easily injured during dissection. Therefore, a posteromedial approach is widely applied in the treatment of posterior medial condylar fractures.11,12 Satisfactory results have been achieved using this incision to expose the posterior medial condylar tibial plateau fracture. The posteromedial key fragment may displace distally and medially, especially when the knee is flexed. Several reports have illustrated the importance of coronal plane proximal tibial fractures, which are only visible on lateral radiographs or computed tomography scans. If displaced fractures in the coronal plane are not addressed, they may lead to the use of inappropriate fixation techniques. We have papers that proposed using a reverse L-shape incision that allows more space for reduction and easier implant placement. The T-shaped Buttress plate is a conventional implant for posteromedial tibial fracture fixation. Due to insufficient implant stability, nonunion would occur even using our direct approach and fixation. Therefore, a new implant design to provide enough stability that initiates bone union should be a better solution.
In this present study, three different proximate tibial bone plate designs were compared for stability after implantation. In the three plate fixations, an obvious stress concentration surrounding the screw holes was demonstrated. It is known that a smooth round hole in a plate causes a stress concentration.13 The highest peak von Mises stress occurred in the new designed plate. The possible reason might be the oval-shaped screw hole in this implant while the other two designs have a rounded screw hole. The peak von Mises stress among three plate fixations ranged from about 90 to about 110 MPa. There is a big gap between these values and the Titanium alloy fatigue strength (600 Mpa).14 However, the commercialized T-Buttress plate is made of pure titanium. The fatigue strength of pure titanium is 230–280 MPa.15 We can expect the T-Buttress plate would be at high risk of breakage. As for the screw stress distribution, the peak von Mises stress of the screw in all plate designs was almost twice that of the plate. However, the peak screw stress is much lower than the fatigue strength (600 Mpa)14 and the yield strength of Titanium alloy (approximately 800 Mpa).16 These outcomes are in agreement with the in vitro experimental test17 that demonstrated no screw bending in a tibial plateau split fracture, even with loads as high as 900 N. This study suggests no mechanical damage would be expected for the new design and the medial proximal tibial plate because of the simulated physiological load.
The axial construct stiffness from high to low is in the order of medial proximal tibial plate (1459.8 N/mm), the proximal posterior medial plate (1398.6 N/mm) and the T-Buttress plate (1379.3 N/mm) with the corresponding maximum axial displacement of 1.37, 1.43, 1.45 mm, respectively. Direct comparisons of the calculated fragment movement with the experimental data reported in the literature are not appropriate. On one hand, there are limited experimental studies that focused on such comparisons. The found values in this study were similar with those reported by Zeng et al.,18 who used synthetic bone femoral condyles to load tibia specimens with posteromedial tibial plateau split fracture. The measured fragment subsidence ranged from 0.832 mm for the T-shaped buttress plate to 1.559 mm for the lag-screws under 1500 N load. Nevertheless, they indicated that a posterior T-shaped buttress plate produced greater stability in controlling the posteromedial fragment movement than the medial dynamic compression plate and the lateral locking plate. We thought that the medial dynamic compression plate does not have fixed-angle stability while the medial plate and the proximal posterior medial plate involved in the current study both have a locking mechanism to improve the angular and axial stability for fracture fixation. Overall, the maximum fragment movement achieved in the three plate fixations were far below the fragment movement threshold usually considered clinically (3 mm) to evaluate the split tibial plateau fracture reduction success.18
In this study, the PPMT bone plate locking designs have batter stability than the conventional TBP plate. The features of this innovated PPMT bone plate include adequate stability for posteromedial fracture fixation, anatomical design that does not require plate bending for fitting. A conventional hole is located on the posteromedial fracture spike to allow buttressing. Locking designs for angular stability can be placed juxta-articular for subchondral fixation. Therefore, this innovative PPMT bone plate offers an alternative option for surgeons to treat posteromedial tibial plateau fractures.
Some limitations were inevitable. The fibula bone was not included. Only one type of posteromedial fracture was evaluated. Further studies should evaluate additional fracture types. The model used sustained only a static load. Nevertheless, the cyclic load usually occurs during daily activities. This study made use of known parameters and deduced information based on previous literature. Further studies using biomechanical testing models will be needed to establish this information to provide better accuracy.