Abstract: BACKGROUND: Condylar prosthesis replacement, as one of the surgical methods for the treatment of temporomandibular joint diseases, not only needs to restore the morphology and function, but also needs to ensure long-term stable application.
OBJECTIVE: To design finite element analysis of a customized Gyroid condylar prosthesis. 
METHODS: Gyroid structure specimens with different wall thicknesses (250, 350, 450, 550, 650, and 750 µm) were designed by software. Finite element simulation compression experiments were carried out to test the elastic modulus of the specimens. The Gyroid structure wall thickness range that matches the elastic modulus of mandibular cancellous bone and whose pore size meets the osteogenesis conditions was screened out. This range was subdivided and Gyroid structure specimens were made using 3D printing technology. Mechanical compression experiments were carried out on a universal testing machine. The Gyroid structure wall thickness that meets the mechanical properties of mandibular bone, has an easier osteogenesis and a smaller strength was screened out by elastic modulus and compressive strength, and subsequent experiments were carried out. A three-dimensional model of a customized Gyroid condylar prosthesis was designed, and the finite element analysis of the blade jaw position and cusp interdigitation position of the model under natural occlusion was simulated.
RESULTS AND CONCLUSION: (1) Finite element analysis results showed that with the increase of wall thickness, the elastic modulus of Gyroid structure specimens increased. The elastic modulus of Gyroid structure specimens with wall thickness of 350, 450, 550, 650, and 750 µm matched the elastic modulus of mandibular cancellous bone. Since the subsequent experiments needed to be subdivided into groups and the pore size of the 550, 650, and 750 µm wall thickness group (pore size 800-1 000 μm) was within the osteogenesis range. Gyroid structure specimens with wall thickness of 550, 600, 650, 700, and 750 µm were selected for mechanical compression experiments on a universal testing machine. (2) The results of mechanical compression experiments showed that with the increase of wall thickness, the elastic modulus and compressive strength of Gyroid structure specimens increased. The elastic modulus of Gyroid structure specimens with wall thickness of 550, 600, and 650 µm was within the elastic modulus of the mandibular cancellous bone. Finally, the wall thickness of 650 μm and the pore size of 900 μm were selected to construct the three-dimensional model of the mandibular customized Gyroid condylar prosthesis. (3) The results of finite element analysis of three-dimensional model of the mandibular customized Gyroid condylar prosthesis showed that the stress of the articular disc in the edge-to-edge occlusion was mainly concentrated on the lower surface of the anterior middle band, and the stress of the articular disc in the interposition of tooth tips was mainly concentrated on the lateral surface of the lower surface. The maximum displacement and the maximum equivalent stress of the left and right articular discs in the edge-to-edge occlusion and the interposition of tooth tips were similar. The maximum displacement was 0.031, 0.030, 0.028, and 0.018 mm, and the maximum equivalent stress was 2.87, 2.30, 2.73, and 1.71 MPa, respectively. (4) The results showed that the Gyroid structure with a wall thickness of 650 μm was consistent with the mechanical properties of the mandible, which reduced the strength of the titanium alloy and reduced the damage of the articular disc caused by the customized Gyroid condylar prosthesis. 

Key words: Gyroid structure, wall thickness, condylar prosthesis, finite element analysis, articular disc, temporomandibular joint