The talus plays an important role in the biomechanics of the ankle. Abnormal anatomical structures have large effects on the function of the foot. The ankle joint bears a heavy load in the human body. Any injury to its anatomical structure will affects its stability. Talus osteochondral injuries are common ankle joint injuries that considerably affect the ankle joint [18].
Ischemic collapse necrosis of the talus is challenging to treat [19, 20]. To address this clinical challenge, there are currently three main therapeutic approaches: 1. core decompression, which can preserve joint motion and effectively relieve pain, but the disadvantage is that it is suitable for only patients with early talus necrosis and is not effective for end-stage necrosis [21]. 2. Ankle joint fusion surgery has been suggested to relieve pain and is suitable for patients in almost all stages of necrosis, but it will greatly limit the range of motion of the ankle and affect the quality of life of patients [22]. After the talus collapses, structural bone grafting is often performed during fusion to prevent the force lines of the lower limbs from being affected. If the blood supply around the talus is damaged extensively, tibialis calcaneal fusion or posterior ankle arthrodesis is needed. However, for cases of severe collapse and necrosis of the talus, fusion surgery is not suitable, and in earlier studies, fusion surgery has been proven to be inferior to ankle prosthesis replacement in terms of mobility, efficacy and prognosis [23]. 3. Regarding ankle joint replacement for collapse necrosis of the talus, the requirement of the residual bone mass of the talus is very high to reduce the probability of revision or refusion.
Whole talar prosthesis implantation was first performed and reported by Haenroongroj and Vanadurongwan [24] in 1997, but there were many postoperative complications due to limitations of the casting method. In recent years, with the development of modern computer processing technology, 3D printing technology has been widely used in the clinical practice of orthopedics and has yielded good curative effects.
3D printed, personalized all-talar prostheses can be used for collapse necrosis of the talus. The talus, the core of ankle-hind foot movement, has seven joint surfaces, so it is the first choice for personalized treatment. However, there is always controversy about whether the whole talar prosthesis should be fixed after implantation. Regardless of whether the subtalar joint or subtalar joint is fused, the degree of flexibility and range of motion of the foot are affected [13]. If we choose not to fix the talar prosthesis [11, 12] and use the bony structure of the talus between the ankle points and the ankle-foot complex to obtain self-stability, damage to the adjacent articular cartilage and complications such as prosthesis dislocation may occur. The clinical efficacy of different fixation methods has also been assessed in many studies. Due to relatively short follow-up times and limited number of cases in this study, more scientific and objective data cannot be provided. Therefore, biomechanical studies are urgently needed to verify the biomechanical differences between several different fixation methods so that operators can select the best surgical method.
Traditional orthopedic biomechanical experiments are mainly based on animal or cadaver models. Although the results of these experiments are more reliable than those of simulations, it is often very difficult to obtain ideal experimental data without changing the physiological state of the model due to limitations in experimental methods, the need to adhere to ethical standards, and the influence of other factors. In recent years, with the development of medical imaging technology and computer processing technology, finite element analysis, a new biomechanical research method, has been widely used in orthopedic mechanics research. Simulation experiments performed by the finite element method have the advantages of a short experimental time, a low cost, the capability of simulating complex boundary conditions, the ability to provide comprehensive mechanical property testing, and good repeatability [14, 15]. In this study, a finite element model was used to simplify and effectively simulate a normal state and different methods of fixing a talar prosthesis. Previous studies have verified that the finite element model in this study was true and effective [25, 26]. The specific biomechanical results are as follows.
It is traditionally believed that fixation must be performed after the prosthesis is inserted, which is similar to fusion surgery. The subtalar joint [13] needs to be fixed or the scaphoid joint needs to be fixed at the same time to stabilize the prosthesis at the ankle. For the prosthesis-bone interface needing screw fixation, a special coating is often used to achieve the biomechanical effect of bone ingrowth. Professor Tang Kanglai's team conducted a series of studies on this topic and has made clinical progress[27–28]. The operation is similar to fusion surgery. Hindfoot motion is limited to a certain extent, which was shown in the finite element model. When loads in different directions were applied, the range of motion of the talus relative to the screw-fixed joint surface decreased; during a simulated gait cycle, the pressure on the tibialis joint increased, and the pressure on the prosthesis-bone interface decreased. Screw fixation does limit the motion between joints and reduce the pressure between fusion joints; however, the finite element results show that the reduced pressure is completely compensated by the tibialis joint. For the talus joint, there is not only a loss of range of motion and an increase in contact pressure but also an increased probability of osteoarthritis in the talus joint surface and an increased possibility of late prosthesis loosening in the long term. However, the effect on ankle range of motion is similar to that of fusion surgery. Clinical research on total talus replacement with a prosthesis indicates that talar prostheses have a better curative effect in the short term [29]. However, studies on bone ingrowth between the bone and prosthesis interface and long-term clinical follow-up studies are currently underway.
Other scholars have used methods other than screw fixation. The first report of using an unfixed method was published by Assal and Stern [30] in 2004, and good curative effects were achieved within the five-year follow-up period. In the heel-strike phase of the gait cycle, when the ankle is in dorsiflexion, the talus was locked upward in the ankle, and there was a force component exerted vertically downward on the calcaneus, so fixation was not required. In the midstance phase, the upper surface of the talar prosthesis incurred a downward stress from the tibia, which exerted a force against the calcaneus and scaphoid at an angle of 140°. The talus was relatively stable and did not need fixation. During the push-off phase, the ankle began to plantar flex, the hindfoot was locked, and the ankle was in the "unlocked state". The moment arm of the talus against the scaphoid increased, and fixation was not required [31]. The finite element model of this study also confirmed the assumption that talar prosthesis fixation without screws yields stable fixation, with biomechanical and ankle range of motion values closest to normal values, and to the best of our knowledge, no one has conducted relevant studies on self-stability of the talus in the past.
If screw fixation is used, the probability of prosthesis dislocation is relatively low. For cases without screw fixation, dislocation is possible in the following conditions. When the forefoot is off the ground, the Achilles tendon is pulled upward, the hindfoot is plantar flexed, the talus is unlocked forwards, and the anterior ankle is loose. Then, the talus is displaced to large extent forwards and upwards. Due to the containing effect of the scaphoid (bony structure) and the limiting effect of the tibialis anterior and anterior joint capsules (soft tissue) on the prosthesis, the probability of prolapse is relatively low, and the specific biomechanical mechanism needs to be studied further.
Some shortcomings of this study were unavoidable. First, only the bony structures were simulated, and the soft tissues were simplified, which may affect the accuracy of this model to some extent with respect to real conditions. Second, the model was verified by repeating the experiments in previous studies, which does not yield the strongest form of evidence. Therefore, in the future, we plan to verify the results of this finite element study on clinical and cadaveric models. Finally, the surgical method of total talar prosthesis replacement should be carefully considered because the deep layer and the anterior peroneal ligament or the trigonal ligament cannot be reconstructed separately, thus lead to high requirements for the ankle joint bony structure.