So far, the Biomet standard prosthesis (made in the USA) is considered the only officially-approved artificial TMJ prosthesis in China. The bone contact surface of the Biomet standard fossa component is designed as a plane, and the condyle-ramus angle of the condyle component is relatively undiversified. To match the Biomet prosthesis with the jaws of Chinese people, bone grinding is needed in the articular tubercle and mandibular ramus. If there is a large difference between the patient’s anatomical structures and the standard prosthesis, significant bone grinding and bone grafting will be required to complete the prosthesis placement, and in some extreme cases, the prosthesis cannot be installed at all. To overcome such an obstacle, we measured 797 sides of a normal Chinese TMJ with a CT scan, and extracted its features through cluster analysis (5). Aiming for a better fitting, less bone loss, and less operative time. Accordingly, one type of fossa prosthesis and four types of condylar prostheses were designed for different depths of fossa and varied angles of condyle-ramus.
The design of an artificial joint prosthesis is very important for its function. An even stress distribution and stable retention are important preconditions for long-term use. A stress in a part of the implant exceeding the fatigue strength of the material might not cause material damage by one single load, but progressive damage would occur to the material, namely, metal fatigue. Under cyclic loading, the accumulation of damage may lead to fatigue fracture. Therefore, it is very important to conduct biomechanical simulation and analysis before clinical application.
There have been many reports on finite element analysis of TMJ prosthesis (9), mainly focusing on the condyle component (10–13), whereas only few reports focused on the analysis of the fossa component. Ramos et al. (14) compared a Christensen standard fossa component model with a customized one, with a standard condyle prosthesis in both models. They believed that the stress, deformation, and displacement of the customized fossa component were all superior to the standard one, but the screw force in the standard component model was more even. Moreover, the stress of the posterior inferior screw of the condylar component was the strongest. However, the Christensen prosthesis has metal-metal contact, and the metal debris generated by wear and tear can trigger a macrophage reaction and increase the incidence of cancer, which has otherwise been gradually eliminated from clinical practice. Today, the mainstream materials for a TMJ prosthesis are alloy and the ultra-high molecular polymer (UHWPE) (15). Ramos (16) compared the Biomet prosthesis with the Christensen prosthesis, with the UHWPE fossa component (Biomet prosthesis) and contralateral articular disc being added into the model, whose mechanical behaviors were described in detail. They found that the implants influenced the behavior of the mandible by improving the symmetry of the mandible and the strain distribution. The Biomet implant slightly modified the behavior of the mandible, and presented some improvements over the Christensen TMJ model.
Recently, Ackland (17) reported a finite element analysis of a 3D-printed customized TMJ prosthesis. A window structure of a condylar component was designed to facilitate reduction of the masseter muscle. The study analyzed the influences of prosthetic condyle thickness, neck length, head sphericity, and window structure on the biomechanical behavior. There was a fossa component in the model, but its biomechanical behavior was not described.
In this study, we found that the stress values of the condyle component, retaining screws, and jaw bone were far less than their yield strength. However, the local stress value of the fossa component was slightly higher than the yield strength of UHMWPE (19 MPa). Thus, in an extreme bite force condition, the local compression area of the articular socket prosthesis appeared to slightly yield. Chen (7) analyzed a 3D printed customized TMJ prosthesis, and the maximum stress value on the surface of the fossa component reached 20.66 MPa, which was similar to our result. Under tensile stress, the polymer would appear to have a stress concentration in some weak parts, which easily produces cracking. When cracking develops gradually, the crack matter will break down, meaning that a crack emerges, and the material appears destroyed. To avoid cracking, we consider improving the prosthetic structure by thickening the weak part to disperse the stress, or changing the materials to meet mechanical requirements. As this study simulates an extreme situation of maximum muscle force, which is far stronger than the real chewing force, we plan to conduct a dynamic force analysis of the prosthesis in subsequent studies, and to provide a more accurate evaluation of the prosthesis.
Hsu (12) simulated different retention screw numbers and arrangements in the condyle component under bite force loading. At least three screws were necessary for stable retention, and the arrangement of screws greatly influenced the stress distribution. In this study, the fossa component behavior in different screw arrangements of 3–6 screws was analyzed, with scheme #1 (6 screws) as the benchmark. Schemes #6 and #7 (3 screws) showed a slight force increase, while scheme #5 (4 screws) showed the weakest force with the most even distribution. Therefore, it is considered that at least 3 screws are necessary for the fossa component, whereas the best number is 4 screws.
Similarly, the condyle component behavior of 3–8 screws in different schemes was analyzed. It was found that after a reasonable reduction of the number of screws, the force was reduced from that based on scheme #1 (8 screws), and the overall force value of scheme #10 (6 screws) was the minimum. Then, as the number of screws continued to decrease, the contact force gradually increased, but it was still weaker than scheme #1 until schemes #30, #33, and #29 (4 screws). Although 3 screws could accomplish the prosthesis reattachment, the contact force was significantly increased. Thus, it is emphasized that the condyle component needs at least 4 screws for reattachment, 5 screws are better, and the best solution is to use 6 screws.
When arranging the screws' permutations, we should not only consider the stress condition, but also combine it with the actual clinical situation. In the condyle component, the projection position of NO. 2 and NO. 4 screw holes is often closely related to the inferior alveolar nerve. The position of the inferior alveolar nerve should be carefully observed before operation. If there is a distance between the nerve and the prosthesis, we prefer scheme #30 or #33. If the prosthesis is close to the nerve, or even contacts the nerve, scheme #29 is preferred, so as to protect the inferior alveolar nerve.
Moreover, the results of the verification experiment showed that the difference between the actual model and finite element model was within 20%(2%-12%), so the utilized simulation model in the current study was considered to be reliable.
In conclusion, this study advocated good biomechanical properties for a self-designed standard prosthesis that fits the Chinese patients. At least 3 screws were necessary for fossa component retention, and 4 screws were optimal. The condyle component requires at least 4 screws for fixation, and 6 screws for the optimal scheme. As compared with the number of screws, the screws arrangement has a more significant impact on the force. In this study, we selected an optimal screw retention scheme to provide a reference for clinical practice.