Abstract: BACKGROUND: In recent years, biodegradable zinc alloy bone plates could effectively solve the clinical problems such as stress shielding effect and secondary surgical removal. The main factors that affect the stress shielding of biodegradable bone plates include the structural design, material selection, and degradation rate. However, the relationship between the structural design and stress shielding effect of biodegradable zinc alloy bone plates is rarely studied, and there is a lack of scientific basis.
OBJECTIVE: To explore the relationship between the structural design of biodegradable zinc alloy plates and the stress shielding effect.
METHODS: Ordinary zinc-magnesium alloy plates were used to fix tibial fractures in New Zealand rabbits. The plates were removed 3, 6, 9, and 12 months after surgery, and the material degradation rate was analyzed. An orthogonal experiment was designed to compare the four parameters of plate thickness, screw hole diameter, arc diameter, and hole diameter with plate stress. Based on the constructed data set, a prediction model of the four parameters and zinc-magnesium alloy plate stress was established by regression analysis. The optimal parameters of plate design were obtained according to different constraints. Finite element analysis was used to compare the biomechanical properties of zinc-magnesium alloy plates and titanium alloy plates before and after optimization design in a rabbit tibial fracture model.
RESULTS AND CONCLUSION: (1) After implantation in rabbits, the surface of the zinc-magnesium alloy plate showed obvious corrosion, and the degree of corrosion gradually deepened over time. Compared with before implantation, the degradation rates of the zinc-magnesium alloy plate were 11.5%, 17.9%, 21.8%, and 24.5% at 3, 6, 9, and 12 months after implantation, respectively. (2) The optimal theoretical structural parameters of the plate were obtained by regression model: plate thickness 1.1 mm, screw hole diameter 2.4 mm, hole diameter 0.6 mm, arc diameter 6.0 mm. (3) The results of finite element analysis showed that the maximum stresses of the zinc-magnesium alloy plate and titanium alloy plate before and after optimization were less than their yield strength under the composite working conditions. During the initial fixation, the displacement of the fracture end of the titanium alloy plate group before and after optimization was 0.08 mm and 0.12 mm, respectively. The displacement of the fracture end of the zinc-magnesium alloy plate group before and after optimization was 0.10 mm and 0.13 mm, respectively. After 3 months of degradation, the displacement of the fracture end of the zinc-magnesium alloy plate group before and after optimization was 0.11 mm and 0.15 mm, respectively. Both plates were theoretically safe for the treatment of rabbit tibial fractures. Compared with the titanium alloy plate, the zinc-magnesium alloy plate exhibited a lower stress shielding effect. With degradation, the stress shielding rates of the zinc-magnesium alloy plate group before and after optimization decreased by 27.56% and 27.66%, respectively. The results exhibit that the zinc-magnesium alloy plate provides mechanical support in the early stage, and reduces the stress shielding effect in the later stage as the material degraded, which is beneficial to bone healing.

Key words: biodegradable zinc-magnesium alloy, titanium alloy, fracture internal fixation system, regression analysis, finite element analysis, engineered orthopedic material