Effect Of In Vitro Remineralization On Acid-Eroded Human Tooth Enamel:Nanomechanical Properties And Microtribological Behaviour Study

As an important masticatory organ, human teeth are subjected to friction and wear everyday. Enamel, the outer layer of tooth crown, is the hardest tissue in human body and has excellent anti-wear properties. For most people, the enamel is exposed to the occlusal surface and oral chemical environment, and then it often suffers from erosion by acidic agents introduced into the mouth. Erosion not only could result in erosive substance loss of enamel directly due to surface demineralization, but also could reduce the anti-wear properties of enamel because increased acidity in the mouth could decrease both the hardness and elastic modulus of enamel. With the increases in the consumption of acidic beverages and life expectancy, dental erosion is becoming worse, and it has gradually become a main cause for toothwear. Therefore, it has important theoretic and practical significance to study the erosion and remineralization behaviours of human tooth enamel systematically.The effect of remineralization on the mechanical properties and microtribological behaviour of acid-eroded human tooth enamel was studied by nano-indentation/scratch technique. The eroded and remineralized surfaces of enamel were analyzed by means of various microscopic examinations, such as scanning electron microscopy (SEM), energy dispersive spectroscope (EDX), laser confocal scanning microscopy (LCSM), X-ray diffraction (XRD). Main conclusions are drawn as follows:(1) There existed a softened layer beneath the eroded surface of human tooth enamel. When the erosion time in 0.001 M citric acid solution (pH=3.25) was no more than 10 min, the thickness of softened layer was measured to be in the micro-nano scale, about 500-1000 nm. As the erosion time increased, the thickness of softened layer increased nonlinearly. The thickness of softened layer and the erosion lesion of enamel surface had a notable positive correlation.(2) The honeycomb-like structure on the surface of acid-eroded enamel caused by erosion demineralization could be partly filled through 12 h remineralization in artificial saliva. A layer of mineral deposits with a crystal structure as hydraxyapatite (HAP), the major inorganic component of natural enamel, was observed on the remineralized surface. However, the crystals resulting from remineralization had no obvious preferred orientation, and had a lower crystallinity than those crystals of the original enamel. Remineralization in artificial saliva significantly enhanced the nanomechanical and microtribological properties of the acid-eroded enamel surface. However, the loss of the hardness and Young’s modulus of enamel surface by acid erosion could not be totally recovered through remineralization. The wear volume of the acid-eroded enamel decreased obviously after remineralization, but was still much higher than that of the original enamel.(3) CPP-ACP has the potential to repair tooth erosion lesions. After 12 h in vitro remineralization in 2.0% w/v CPP-ACP solution, a layer of uneven amorphous deposition appeared on the acid-eroded surface of enamel. The remineralized surface had higher mineral content, nanoindentation hardness and elasticity modulus, and lower friction coefficient and wear volume compared to the eroded surface. But both the nanomechanical and anti-wear properties of the remineralized surface were obviously inferior to those of the original surface of enamel. The microstructure of the amorphous deposition on the remineralized surface was significantly different from that of the HAP crystal within the sound enamel. Thus, although the calcium and phosphate ions concentration of CPP-ACP solution being larger than that of artificial saliva, the nanomechnical and microtribology properties of the eroded enamel still could not be totally recovered through remineralization in CPP-ACP solution.(4) Due to the microstructure and composition of human primary tooth enamel being different from those of human permanent tooth enamel, primary tooth enamel was more susceptible to acid-erosion under the same condition than permanent tooth enamel. After 12 h in vitro remineralization in 2.0% w/v CPP-ACP solution, the surface morphology damage and nanomechanical and microtribological properties of acid-eroded primary tooth enamel were repaired to some degree. Compared with permanent tooth enamel, primary tooth enamel surface suffered from more significant erosion damage due to its weaker erosion resistance under a same erosion condition, and then the remineralization repair effect of eroded primary tooth enamel was lower.(5) Erosion time, remineralization conditions, and the active substances in body play an important role in the remineralization repair of acid-eroded human tooth enamel. The shorter the erosion time was, the more even and compact the remineralization deposits were, and thus the remineralized surface of enamel seemed to be more smooth. Stirring was not conducive to the remineralization repair of acid-eroded enamel. Under stirring condition, the remineralization deposits were loosely arranged and unevenly distributed on the remineralized surface of enamel, and the honeycomb-like structure due to acid-erosion was still seen indistinctly. Increasing the remineralization time in artificial saliva could be helpful to the formation of flake HAP crystals, which were arranged orderly, on the surface of enamel. The organic components in human whole saliva, such as proteins, could promote the remineralization of acid-eroded enamel. The mineralized crystals were found to be more even and orderly on the remineralized surface of enamel after 12 h in vitro remineralization in human whole saliva than in artificial saliva. Furthermore, the enamel surface remineralized in human whole saliva exhibited better anti-wear properties.