According to the current view, the fibula is indirectly involved in the composition of the knee joint. Therefore, the fibula has been neglected in the research on knee osteoarthritis[4]. In this study, it was found that the uneven distribution of bone trabeculae in the lateral condyle of the tibia and the head and neck of the fibula. In the lateral condyle of the tibia, a large number of thick bone trabeculae were distributed longitudinally in clusters beneath the articular surface, which penetrated the epiphyseal line and terminated on the slope of the posterior bone cortex of the metaphysis of the tibia. The cortical slope was a stable arch beam constructed by the union of the fibula and the cortex of the posterolateral side of the metaphysis of the tibia via the tibiofibular joint. This gave us a hint that the tibiofibular arch would play an important role in bearing the stress of the lateral compartment of the knee joint. It was also supported that concentrated reticular bone trabeculae were present in the posteromedial half area of the neck of the fibula, which could provide the internal strength for the brace of the fibular head. As a result, the tibiofibular joint was an indispensable fulcrum for the mechanical arch between the tibia and fibula. Furthermore, the thin and sparse bone trabeculae were located above the tibiofibular joint in the posterolateral marginal region of the tibial plateau. Accordingly, the fibula might not directly bear the stress originated from the knee articular surface.
Previous studies have shown that the proximal tibia was mainly comprised of cancellous bone, the bone density of different parts of proximal fibula and tibia decreased in the following order: fibula, medial platypus, lateral platypus, and middle proximal tibia[5]. There was little chance of cortical thickness loss in the proximal fibula with age but was much more significant in the proximal tibia were pointed out by McNeil et al[15]. This means that the fibula was not subject to bone loss. Accordingly, the strength maintaining of fibula could be superior to the tibial plateau. In the present study, it was observed that the mechanical arch beam was formed by the union of the fibula and the tibial plateau and the tibiofibular joint was the constant fulcrum of the arch beam on the posterior lateral side of the tibial plateau. When upright walking or moving, gravity transmitted by the articular surface of the lateral condyle of the tibia could fall on the top of the arch beam. The dense and reticulated trabeculae in the tibia and the fibula might be combined with each other by the tibiofibular joint to form a situation of tripartite confrontation. Consequently, the fibula probably could play a key role in transmitting the weight of the posterolateral knee joint and kinetic energy in motion.
The process of human aging was accompanied by the decrease of bone mass[16]. Especially, in the load-bearing joints exists varying degrees of the settlement of bone mass, such as the knees, hips, ankles, and spine. A supporting and plugging effect on the lateral tibial plateau through the proximal tibiofibular joint was provided with the strong fibula. It might be the anatomical and pathological mechanism of nonuniform settlement. It was indirectly indicated that the forces of the tibia were uneven in medial and lateral[17–20]. There was evidence that the progression of KOA was related to the tibiofemoral articular stress distribution[21]. Studies found that the slope of the tibial plateau arising from nonuniform settlement results in a transverse shearing force, with the femoral condyle producing ramping during walking and sports[6]. These assumptions should bring the clinician's attention to the role of the factors of the fibula, in the coronal subluxation of the proximal tibia in patients with knee osteoarthritis.
Moreover, the geometry of the tibial plateau has attracted more and more attention from scholars in recent years. Some data have shown the inclination of the tibial plateau could be used as a risk factor for anterior cruciate ligament injury to a certain extent[22]. According to the results of the present study, the fibula more probably affects the inclination of the tibial plateau to some extent, especially for osteoarthritis patients with knee varus. Therefore, for medial KOA, the role of the fibula in supporting the tibial plateau should strongly attract the attention of clinicians.
Collectively, the finding in the present study had revealed a novel correlation between the fibula and tibial, and this might provide an anatomical basis for the further treatment of KOA and understanding the risk factors of cruciate ligament injury.
Limitations
In future research, quantifying the information of trabecular bone at the arched beam will be of important implications for further treatment of KOA.