A Locking Plate Designed With Cluster of Head Screws Would Be Biomechanically Superior Than Conventional Buttress Plate For The Fixation of Posteromedial Tibial Plateau Fractures: A Computational Assessment


 Background: Dealing with high-energy fractures of the tibial plateau remains a challenge despite advances in implants, surgical approaches, and imaging methods. Posterior buttress plate is most commonly used implant but the fixation stability is still a challenge. Recently, a newly designed tibial locking plate was introduced that aims to provide better fixation strength for tibial plateau split fracture. This study compared the biomechanical strength of three different posteromedial tibial plateau split fracture fixation methods. Methods: The tibial plateau fractures were simulated using a human tibiae model. Each fracture model was virtually implanted with one of the three following constructs, proximal medial tibial plate (PMT), proximal posterior medial tibial plate (PPMT), and posterior T-shaped buttress plate (TBP). Posteromedial fragment vertical subsidence was measured under 2000 N joint contact force. The maximum Equivalent stress on the bone plate and bone screw and the construct stiffness were determined.Results: The proximal medial tibial plate (PMT) allowed the least posteromedial fragment subsidence and produced higher construct stiffness than each of the other two constructs. However, the proximal posterior medial tibial plate (PPMT) showed higher stiffness than the T-shaped buttress plate (TBP). The maximum Equivalent stress was the smallest for the proximal medial tibial plate (PMT).Conclusion: This study showed that the proximal medial tibial locking plate or proximal posterior medial tibial locking plate were biomechanically more stable fixation methods for posteromedial split tibial plateau fractures.


Introduction
A posteromedial tibial split fracture is a notable medial plateau injury pattern. The fracture line appears in the coronal plane with a separate posteromedial osteoarticular fragment of variable size. Clinically, ignoring this fragment can lead to distal displacement, with posterior medial femoral condyle subluxation 1 . Bone plate xation is commonly employed to x the fragment bone.
For several years, posteromedial implant popularity has gained some attraction but most surgeons will simply apply the conventional 3.5mm T-plate. Due to insu cient implant stability, bone nonunion sometimes occurs even from direct xation. Moreover, reduction loss and xation loss from indirect xation and insu cient xation have also occurred. Therefore, a more rigid implant that provides enough stability to initiate bone union would be necessary.
A novel anatomical locking plate (Proximal Posterior Medial Tibia Locking Plate) that provides su cient stability for posteromedial tibial plateau fracture xation was designed. This innovative implant is speci c to posteromedial coronal fragment osteosynthesis with a cluster of head screws that is designed to rmly purchase the bone fragment ( Figure 1). The features of this implant include: 1. Anatomical design that does not require plate bending for tting. 2. A elongated screw hole is located on the posteromedial fracture spike to allow buttressing and plate adjustment. 3. Locking mechanism design for angular stability. 4. The plate can be placed juxta-articular for subchondral xation.
The purpose of the study is: (i) to describe the details of an innovative anatomical locking plate for the posteromedial tibial plateau; and (ii) to evaluate the safety and xation e cacy of this innovative plate. We used nite element analysis to provide adequate information on this implant stability. By comparing it to other xation methods, we propose this implant as a possible option for this surgery.

Model preparation
The three-dimensional lower leg nite element model included the tibia. The bony structures were generated using a computed topography data set segmentation from the Visible Human Project 2 . The 3D tibia model was reconstructed via the cortical shell and cancellous core. The fracture model was made based on the fracture morphology described by Higgins 1  The distal end of the tibia was fully xed in all degrees of freedom for the load and boundary conditions in each group. 2000 N force was applied to the proximal tibial plateau to simulate the single leg stand.
The knee contact force is not uniformly shared between the tibia condyles. The compression load was divided into 60% applied to the medial side and 40% to the lateral side, as in previous studies ( Figure 3).

Evaluating parameters
Each group was compared in terms of the maximum von Mises stress in each plate, screw and tibia bone. The plate-bone construct stiffness was calculated. The construct stiffness was derived from the load and vertical displacement data.

Results
For the bone plate equivalent stress, the stress patterns and values on the bone plates in all models are shown in Figure 4. The maximum PPMT plate stress (106.13 MPa) is slightly higher than PMT (87.10 MPa) and TBP plates (93.68 MPa). The PPMT plate stress distribution is similar to that of the TBP plate.
The stress concentration was found around the distal end of the thread hole. The PMT stress is distributed around the distal end of the thread hole. The PPMT stress is distributed on the distal end of the plate around the sliding hole. The TBP stress is distributed around the screw hole. The bone screw stress patterns and values in all models are shown in Figure 5.  4 . This is often caused by a high-energy injury mechanism. The mechanism involved in this fracture pattern may be one of knee exion, knee varus, and internal medial femoral condyle rotation 5 . 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 xation and the associated soft tissue injuries were the two most important reasons for a poor prognosis 5-7 .
The operative treatment goal for tibial plateau split fractures are anatomical reduction, especially in articular congruity restoration, stable xation for early rehabilitation, and avoidance of complications, particularly infection and non-union. The tibial bone plate xation is a major approach used to x the fragment. Non-displaced posterior fracture fragments can usually be stabilized through the standard anterolateral approach. 8,9 However, in the anteromedial approach 10 , 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 exed. 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 xation 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 xation. Due to insu cient implant stability, nonunion would occur even using our direct approach and xation. 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 xations, 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 ovalshaped screw hole in this implant while the other two designs have a rounded screw hole. The peak von Mises stress among three plate xations 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  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 lagscrews 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 xed-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 xation. Overall, the maximum fragment movement achieved in the three plate xations 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 xation, anatomical design that does not require plate bending for tting. 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 xation. Therefore, this innovative PPMT bone plate offers an alternative option for surgeons to treat posteromedial tibial plateau fractures.
Some limitations were inevitable. The bula 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.

Conclusion
This study investigated the structural stability of three different proximate tibial bone plate xation designs for posteromedial tibial plateau split fractures. Previously, the traditional T-shaped buttress plate was considered using a commonly used implant that has superior structural stability to the traditional medial or lateral compression plate. The most important nding in this study is that the innovated proximal posterior medial tibia (PPMT) Locking Plate and proximal medial tibia (PMT) Locking Plate have comparable structural stability with the traditional TBP plate. The PPMT plate has multiple screw hole designs to offer more surgical implantation exibility for surgeons. The plate placements for the PPMT and PMT are different. The surgeon could select the more appropriate bone plate to xation according to different fracture modes.   Load and boundary conditions in each group, the distal end of the tibia was fully xed in all degrees of freedom; the load was applied to the proximal tibial plateau to simulate the single leg stand.