Biomechanical analysis of the effect of Tibial Posterior Fracture and Posterior Soft Tissues in the knee joint

Background: The majority of all tibial plateau fractures (TPF), in detail the 86%, involves a flexion valgus type mechanism leading to fracturing of the postero-latero-central (PLC) region of the tibial plateau. Surgical treatment of this region is technically demanding due to the restricted field of view and area of access: an incorrect surgical management can entail articular incongruity, residual or even secondary depression after the surgical treatment. For this reason, it is occasionally chosen to leave PLC TPF untreated in case of a limited depression. The aim of the study is therefore to numerically check to what extent this choice is justified without clinical consequences (i.e. joint instability) and to analyse posterior Soft Tissues (ST) role in case of this kind of fractures. Methods: Starting from a validated numerical Finite Elements model, different configurations were developed: a native model (Model N); a physiologic model featuring the posterior ST structures (Model P); a model with simulated PLC fractures (osteotomies with 1-2-3 mm step-off) and modification of lateral meniscus, to simulate the partial loss of its functionality (Model NF); a Model P with simulated fractures, i.e. a model NF with posterior ST structures (Model PF). Furthermore, quadriceps muscle, hamstrings and popliteus muscle forces were included. A complete flexion squat from 0° to 90° was simulated for each configuration, with results focusing mostly on tibio-femoral kinematics. Results: The presence of a PLC TPF with increasing step-off caused a progressively deterioration of tibio-femoral biomechanics, especially in the first 35-40 degrees of flexion, which influenced the rest of the flexion. Regarding posterior ST, ligaments and muscles showed to be beneficial in knee adaptation time and first degrees of flexion, when the forces are low. Tibial Antero-Posterior translation (AP) is the most significant parameter, since it shows posterior ST restoring native AP translation, totally for 1-mm step-off fracture and partially for 2-mm step-off fracture, at least in the first 30° of flexion. Conclusions: The obtained results provided further insights on the implications brought about by PLC TPF and on the role of ST on knee biomechanics, especially in case of fracture-related issues. This information should therefore be kept into account by the surgeon when evaluating pros and cons of intervening on the fracture and if the reconstruction of these bony and posterior ST structures is strictly necessary.