Posterolateral tibial plateau fracture with anterior cruciate ligament injury has biomechanical characteristics of anterolateral rotatory instability through finite element analysis

The simultaneous posterolateral tibial plateau fracture (PLTPF) with anterior cruciate ligament (ACL) injury has posed a great challenge to both orthopedic trauma and sports medicine surgeons. This study investigated the biomechanical mechanism of simultaneous PLTPF with ACL injury and demonstrated the consistency with anterolateral rotatory instability (ALRI) of the knee. A healthy male volunteer's right knee CT and MRI images were imported into Mimics software to reconstruct a three-dimensional geometric model of bone, ligament, meniscus and cartilage. The PLTPF were simulated at posterior half of the lateral tibial plateau (LTP) and lateral meniscus posterior horn (LMPH). Three PLTPF depression angles were set at 5 degrees, 10 degrees and 15 degrees. Finite element analysis (FEA) was conducted to observe the displacement of bone and meniscus, ACL strain and LTP stress at 0 degrees and 30 degrees knee flexion with axial and tibial internal rotation loading, in ACL intact and deficient knee. For ACL intact knee, FEA showed axial loading at 0 degrees knee flexion led to abnormal displacements of lateral femoral condyle and LMPH. At 30 degrees flexion, axial 10Nm tibia internal rotation loading resulted in more LTP anterior displacement and ACL strains. The stress of LTP concentrated abnormally on anterolateral part at 0 degrees flexion and posterolateral LTP at 30 degrees flexion. All above parameters tended to increase with the enlargement of depression area and depression angle. In ACL deficient knee, axial 10Nm tibia internal rotation loading led to even more LTP anterior displacement and LTP articular stress at 30 degrees flexion than ACL intact knee, with a tendency of aggravating with increasing depression area and angle. Simultaneous PLTPF with ACL injury has a common flexion valgus and tibial internal rotation injury mechanism with ALRI and should be treated as a special pattern of ALRI. Clinically, high grade PLTPF associated with ACL injury should be addressed by concomitant PLTPF reduction and ACL reconstruction to fully restore LTP articular congruence and knee stability.