Tibial plateau fracture is a kind of high energy injury, the surgical treatments of which have long been a contentious issue.[23, 24] Current internal fixation treatments of complicated tibial plateau fractures are mainly focused on the operative approach, soft tissue protection and fixation pattern.[23] However, the variable surgical outcomes were observed due to the complex injury, internal fixation failure and variable personal experience. Part of the reasons for the failure of internal fixation was the mismatching of the plates and the non-optimized screw trajectories, which were easily ignored by clinicians. Personalized surgery is recognized as the most appropriate and optimized surgery for individual patient, including personalized surgical methods and personalized implants. As for the internal fixation of tibial plateau fracture, whether the personalized plate and screw system has more superior biomechanical effect than the traditional plate and screw system is still uncertain. Therefore, this study made an effort to verify the biomechanical superiority of the individualized plate and screw system in the internal fixation of tibial plateau fractures, and to discuss the importance of plate matching and optimized screw trajectories in the internal fixation of complex tibial plateau fractures.
According to the AO principle of internal fixation and internal plant design principle,[4, 25] the best biomechanical effect can be obtained by screw implantation of vertical fracture line, and the shape matching plate can better assist in the anatomical reduction of fracture, especially in the case of intra-articular fractures. To meet critical clinical demands, the patient-specific configuration matching and optimized screw trajectory would achieve better fixation effects.[4, 9] However, not all the commercial available anatomical locking plates can match to the bone surface, or the screw trajectories of the locking plates were not always able to fit the fragments satisfactorily. As a result, the forcible placement of mismatching or pre-bending plates with unsatisfactory screw trajectories happened sometimes in the clinical practice, which might have great impact on the fixation and biomechanical effects. Unfortunately, for the surgery of complicated tibial plateau fractures, the fixation with single plate or double plates gains a lot of attention[6, 7, 26], but the biomechanical effect of matching plate and satisfactory screw trajectories on tibial plateau fixation are usually ignored.
In recent years, the clinical application of personalized implants is increasing, especially the 3D printing personalized plates and prosthesis are also gradually applied in clinical practice. However, there is no report of clinical application of personalized tibial plateau plates, and it is not known whether the fixation effect of personalized implants is superior to that of traditional implants. In this study, three different types of locking plate systems were designed to simulate the internal fixation of Schatzker classification I tibial plateau fracture, and the design of the three plates were based on the anatomical locking plate on the lateral tibial plateau of Synthes company. Compared with the original anatomical locking plate system of tibial plateau, the biomechanical properties of the four internal fixation systems were compared by finite element analysis.
The results of the finite element analysis showed that the three kinds of improved plate and screw systems can improve the biomechanical properties of internal fixation. In terms of deformation index, group D (the locking plate with matched anatomical shape and optimized screw trajectories) had the largest reduction in displacement, followed by group B (the locking plate with mismatched anatomical shape but optimized screw trajectories) and group C (the locking plate with matched anatomical shape but non-optimized screw trajectories). This shows that the matching of plate and the optimization of screw trajectories can effectively reduce the deformation index of fracture internal fixation system, and the optimization of vertical fracture line of screw trajectories is stronger than that of matching plate. This result is consistent with the AO fixed principle, which advocates that screws should be fixed vertically to the fracture line to obtain the maximum fixation effect, and the reason can be confirmed from our finite element analysis results.
In terms of stress index, the stress comprehensive index of group B, C, and D was higher than that of group A, but the stresses increase were within the safe range in all groups. The reason why group B, C and D have higher stress is that their deformation index is lower than that of group A, which means under the same force, the displacement of internal fixation or fracture is smaller, then the corresponding stress will be larger. According to our hypothesis, the optimal design of group D should obtain better stress effect than group C, but the results show that the stress of group C is lower than that of Group D. The reason is that in our experimental design, the screw trajectories of group C were not perpendicular to the fracture line, but was fixed to the contralateral cortex along the locking screw trajectories of the original plate, while the screw direction of group D were perpendicular to the fracture line and fixed to the contralateral cortex. From Fig. 1, we can see that the screw length of group C is longer than that of Group D. The longer the screw length is, the less stress is shared per unit area. Therefore, this explains why the stress in group C is smaller than that in Group D. According to the stress of internal fixation, the greater the stress received by internal fixation or fracture end, the more likely it is to cause fracture end, plates and screws cutting, and finally cause internal fixation failure. The stress analysis results seem to suggest that the optimized plate and screw are more likely to cause internal fixation failure, but except for group B, the increased stress ratio of groups C and D is about 50% compared with group A. Previous studies [21, 22] reported that the yield strength of cortical bone and Ti6Al4V implant is about 104 MPa and 795 MPa. Meanwhile, we compared the model with similar structure [14, 15], the stress of bone and internal fixation system were at lower stress level under physiological load. The stress increase in this range mentioned above did not exceed the safe range of plate and will not cause screw rupturing or fracture end cutting. Therefore, to sum up, we think that the matching plate and screw trajectories may not only ensure the safe stress range, but also increase absolute stability of simple fracture fixation.