BTPF, especially when it involves the posterior part, is believed to result from the impingement of femoral condyle accompany extreme force. The morphology of the fracture is determined by the applied force and the position of the knee at the time of the injury. Given the various fracture characteristics and complexity of the knee, current knowledge of the injury mechanism is insufficient to understand the fractures, decide on a surgical plan, choose an approach and reconstruct the joint. In this study, we simulated the position of the knee at the time of the fracture using a computer-assisted 3D technique that enabled us to move the distal femoral condyles to match the displaced fracture fragments of the tibial plateau. After simulation for ninety-six BTPFs involving the posterior fragment, we analysed the fracture line orientation, fracture morphology and the motion of the knee in three planes. Three main injury mechanism types and six sub-types were identified and found to exhibit distinct fracture characteristics. We believe that these findings are meaningful for a greater understanding of the complex fractures.
The kinematics of the knee are complex and sophisticated [13, 14]; with different knee positions, the femoral condyles impinge different areas of the tibial plateau, resulting in different fracture characteristics (Fig. 3). In BTPFs, the femoral condyles impinged both the lateral and medial plateau, leading to fracture in both locations, so the fracture locations in the lateral and medial plateau are interrelated, not isolated [15, 16]. With the extension injury mechanism, the knee joint is extended and stable at the time of injury, and rotation is negligible ; in this scenario, the lateral and medial impingement areas are located in the anterolateral and medial central parts of the tibial plateau , respectively. Under axial force, impingement results in the classic BTPF, with an anterior-to-posterior fracture line. With flexion motion, the lateral and medial femoral condyles exhibit different trajectories in the axial plane, which form flexion rotation positions, and the maximum ranges of internal and external rotation can reach 45 degrees , so the flexion mechanism can divide into two types (internal and external rotation). When the knee is injured during internal rotation and flexion, the lateral femoral condyle rolls back and externally rotates (the knee is internally rotated in relation to it) [20–22], and the lateral condyle compresses the posterolateral part of the tibial plateau, while the medial condyle moves forwards and compresses the anteromedial part of the tibial plateau [18, 22, 23]. A fracture line oriented from anteromedial to posterolateral is observed in flexion-internal rotation-type fractures. By contrast, when the injury occurs during external rotation and flexion, the femoral condyles compress the anterolateral part and posteromedial parts of the tibial plateau, respectively [18, 23]. An anterolateral-to-posteromedial fracture line orientation is found in this type of fracture.
Posterolateral fractures are common in BTPFs, and their morphology has been described in many studies [6, 8, 10, 24]; however, few studies have focused on the mechanism of injury. Currently, posterolateral fracture is believed to result from axial and valgus loading with the knee in flexion, and the femur tends to move in the posterior direction . In the current study, 21 BTPFs with posterolateral compression fracture involvement resulted from this injury mechanism with the knee internally rotated. The compression morphology of posterolateral fracture is a result of the direct impingement between the lateral femoral condyle and an osteoporotic posterior lateral tibial plateau. A different posterolateral fracture morphology, the split type, was observed in 68 BTPFs; this type results from two other injury mechanisms, extension and flexion-external rotation injury mechanisms. With these two types of injury mechanisms, the lateral femoral condyle moves anteriorly and impinges on the anterolateral part of the tibial plateau; the fracture extends to the posterior part, which results in the split morphology of posterolateral fractures. This finding is consistent with the research of Zhu et al. , who also observed these two types of posterolateral fractures with different injury mechanisms in BTPFs and reproduced them in biomechanical experiments. However, in their research, rotational motion was not taken into consideration as a mechanistic factor.
Posteromedial fracture accounts for nearly one-quarter of the surface area of the plateau [4, 26], and the morphology usually belongs to the split type due to the concave geometry and bone mineral density of the medial plateau [14, 16]. Three types of fracture lines in the medial plateau were observed in this series of BTPF cases, corresponding to the three injury mechanisms. With the extension injury mechanism, the medial femoral condyle impinges on the central part of the medial plateau , and the anterior-to-posterior fracture line commonly has a horizontal side branch in medial plateau. With the flexion-internal rotation injury mechanism, the femoral condyle impinges on the anterior part of the medial plateau [14, 18, 19], resulting in an oblique fracture line (directed from anteromedial to posterolateral) and a large posteromedial fragment. These fracture characteristics have been depicted via a “fracture mapping” method by Molenaars , and Yang  reported 42 posteromedial plateau fractures with these characteristics. With the knee flexion-external rotation injury mechanism, the femoral condyle compresses the posterior part of the medial plateau [14, 18, 19], resulting in small fragments and parallel fracture lines. Previous research has deduced that the mechanism involved in this fracture pattern is flexion and internal rotation of the medial femoral condyle (external knee rotation) [4, 27], which is consistent with the current findings.
Patzold et al.  described the difference in medial plateau fracture line orientation in 81 BTPFs and classified the fractures based on the fracture line angle (sagittal fracture line, coronal fracture line, or no fracture line in the medial plateau). As described by Patzold, in the axial plane, a coronal fracture line (60°-120° relative to the sagittal plane) in the medial plateau fracture type was accompanied by a sagittal fracture line in the lateral plateau, which is consistent with the fracture line morphology resulting from the extension injury mechanism in the current study. The sagittal fracture line type consists of two types of fracture line orientations, 0°-30° and 150°-180°, which can result from flexion-internal and external rotation injury mechanisms, respectively.
The posterior fracture fragments resulting from different injury mechanisms may be fixed with different approaches and strategies by reversing the injury mechanism. For BTPFs with an extension injury mechanism, a semi-flexed position (20–40° flexion) and valgus or varus traction during the reduction procedure are recommended to counter the initial injury mechanism. The posterolateral fragment can be fixed to the anterolateral fragment through an extended anterolateral incision via a single 3.5 mm lateral rafting locking plate [29, 30]; a displaced posteromedial fragment always needs a posteromedial approach  with a buttress plate to achieve reduction [4, 31]. Traction in an extended position with external or internal rotation of the tibia is useful for reducing the flexion-internal rotation and flexion-external rotation types of BTPFs, respectively. Luo et al. reported using the posteromedial approach [5, 32], retracting the medial head of the gastrocnemius to expose and fix the posterolateral and posteromedial BTPFs for flexion-internal rotation. For most of the flexion-internal rotation BTPFs, an extended anterolateral incision and a rafting plate are enough to stabilize the posterolateral fragment and a small, nondisplaced posteromedial fragment [33, 34].
This study has some limitations that must be considered. First, the injury mechanisms found in this study are all based on BTPFs with posterior involvement, which are the most complex fractures to understand, the most challenging to devise surgical strategies for, and always inferior in prognosis. Therefore, not all tibial plateau fractures can be explained by these mechanisms; we may further investigate other types of tibial plateau fractures in the future. Second, the findings of our research were based on 3D simulations, and one may argue that the interpretation of injury mechanisms is subjective. We believe that further biomechanical investigations will strengthen the evidence presented herein. Third, for this series of fractures caused by high-energy trauma, the ligaments were difficult to observe and reconstruct by MRI due to the serious displacement of the fragments, and so this study did not investigate the mechanisms of ligament injury.