Finite Element Analysis On The Effect Of Organic Phase And Inorganic Crystal Orientation On The Mechanical Properties Of Human Tooth Enamel

Some structural materials found in nature exhibit remarkable mechanical performance. It is believed that these are the results of material design over several length scales, organized in a hierarchical fashion. Human tooth is a typical example of such biomaterials. Human teeth are the important masticatory organs, and each tooth possesses a unique structure composing of enamel, dentin and pulp. Enamel, the outer cover of a tooth, is the hardest and stiffest tissue of the body, and can bear a wide range of applied loads and the consequent contact-induced stresses. It should be noted that human tooth enamel is one of those unique natural substances which still cannot be substituted effectively by artificial restorative materials. Hence, it is necessary to study the effects of chemical composition and microstructure of enamel on its mechanical properties. The results would be helpful to the development of new dental materials and the clinical treatments for dental lesions.In this paper, finite element models of humen tooth enamel both at nano-scale and at micro-scale were established, respectively. On this basis, the elastoplastic mechanical behavior of enamel was investigated. The effects of organic phase (which mostly are proteins) and crystal arrangement of inorganic phase on the mechanical properties of enamel were preliminarily analysed. The main results and conclusions were as follows:1. The enamel exhibited anisotropic mechanical properties. As the angle between loading direction and the c-axis of hydroxyapatite (HAP) crystals increased, the compression modulus and yield stress of enamel decreased, and the lowest values occurred in off-axis when the angle was45°. The proteins within enamel had a significant influence on its mechanical properties. With the proteins within enamel being removed, the compression modulus, shear modulus and yield stress of enamel decreased, but its compression yield stress almost remained constant. Also found was that the stress distribution within enamel was closely associated with the proteins. Once the protein was removed, the stress concentration within enamel was aggravated, and plastic deformation happened mainly to brittle HAP crystals. As a result, the fracture toughness of enamel decreased, and therefore the enamel was easily to be damaged.2. The orientation of the HAP crystals within enamel prisms, was reported to be specific, not of uniform arrangement.Compared with the uniform arrangement of HAP crystals, the specific orientation of HAP crystals could result in a slightly lower yield stress, but did not decrease the stiffness of enamel. Due to the specific crystal orientation in enamel prisms the plastic deformation of enamel expanded gradually from prism tail to the entire region of enamel under the external action of loading force. The plastic deformation progression mechanismcan prevent the enamel from fracture by spreading deformation into other regions, and then help keep the structural integrity of enamel. With the orientation of the HAP crystals within enamel prisms, high shear stress and obvious plastic deformation happened to the interod enamel, which was helpful to enhance energy dissipation. It seems that the specific crystal orientation could endow the enamel with a an advantageous mechanism for stress transfer and energy dissipation As a result, the enamel has a high stiffness, but at the same time it also has a certain toughness.