2001 年 14 巻 3 号 p. 414-422
It is widely known that hydroxyapatite coated implants show a better bone response than titanium implants. However, some shortcomings of hydroxyapatite coated implants, produced by plasma spraying, were reported. A thin physical vapor deposition technique was introduced to overcome such shortcomings. It was reported that thin-hydroxyapatite coated implants showed good biological behaviors in animal experiments or cell culture experiments. But there have been a few reports related to the mechanical behavior of thin coated implants, for example stress distribution around the implants under oral functioning conditions.
The finite element method (FEM) is a powerful tool to analyze the stress distribution on an implant. The purpose of this study was to investigate the influence of thickness and Young's modulus of a hydroxyapatite coated layer on the stress distribution around the implant by the finite element method.
Two finite element models were proposed to simulate the mechanical behavior of implant. One finite element model is called a macro-model. The macro-model is composed of a titanium implant, a hydroxyapatite coating, a cortical bond, and trabecular bone. The goal of the calculation of the macro-model is to simulate the whole stress distribution of the coated implant, including the surrounding bone. The other finite element model is called a micro-model. The micro-model is composed of a titanium implant, a hydroxyapatite coated layer, and cortical bone. The thickness of the coating layer was 5 or 100 μm. The micro-model is calculated in order to obtain the details of the stress distribution on the implant. Using these models, the influences of thickness and Young's modulus of the hydroxyapatite coated layer on stress distribution around implant, were discussed. The results were as follows
1. A vertical load of 100 N was applied on the top of the implants. Stress analysis of the macromodel showed that the maximum stress existed in cortical bone near the implant. Moreover, it was clear that the vertical load corresponded to three-point bending force to the surrounding cortical bone.
2. The micro-model showed that the stress level in the surrounded bone caused by a 5-μm coating, is greater than that of a 100-μm coating. On the other hand, the stress level in the coating layer of a 5-μm coating is less than that of a 100-μm coating. A greater Young's modulus of the hydroxyapatite coated layer gave a higher stress level in the surrounding cortical bone.
3. The results obtained in this study demonstrated that a functionally graded hydroxyapatite coated layer will be suitable for the optimal design of hydroxyapatite coated implants.
