Ultra-structural aspects of human enamel surface pretreatments

Abstract

Enamel acid etching has been studied for almost 60 years, i.e., since successful enamel adhesion was first achieved. It was proposed to be due to the micro-mechanical interlocking based on the use of phosphoric acid to etch the so-called ideal etching pattern etching, at certain conditions. According to the systematically screened 2500 non-duplicated publications, with the inclusion of 175 laboratory studies and 18 clinical trials, phosphoric acid etching was found and is still considered to produce promising bonding on intact enamel surfaces. However, other etching agents, such as poly(acrylic acid), self-etching primers (SEP), non-rinse conditioners and other acids could also contribute to satisfactory bond strengths without creating too high surface roughness, etching depth and/or morphological alteration. Thus, an ideal etching pattern might not be necessary in enamel adhesion. An unduly use of ideal etching pattern in literature to assess the enamel adhesion seems more prone to the flaw observations and vague conclusions. Therefore, a fundamental understanding of enamel adhesion is vital. In this thesis, an ultra-structural investigation based on chemical composition, crystallography, micro-roughness, morphology, micro-hardness and surface tension, on intact human mature enamel with various pretreatment methods were studied, with an ethical approval (HKWC-HKU Cluster REC/IRB document No.: UW13-262). The current study found that phosphoric acid etching by liquids and gels could remove the superficial aprismatic enamel layer. The acid etching could also generate a rougher, softer and porous enamel surface, whilst commercial SEPs that contain 10-methacryloyloxydecyl dihydrogen phosphate (10-MDP) and 2-(methacryloyloxy)ethyl phosphates (HEMA-phosphates) left excess amounts of phosphates onto enamel surfaces and showed a minor demineralization effect. Furthermore, mono-HEMA-phosphate solutions could demonstrate both mild etching and a chemically reactive effect on the intact enamel, particularly when at 40 v/v%. All the pretreatment methods used in this study could effectively increase enamel surface free energy, i.e. physical adsorption deemed to be the critical factor rather than micro-mechanical adhesion in enamel adhesion. Apparently, the measurement of surface free energy was probably more appropriate than morphological examination to evaluate the enamel surface pretreatment method. In addition, due to the insignificant etching effect, the definition of “self-etching” might need to be re-considered. To conclude, the current ultra-structural investigation on various intact human enamel pretreatment methods may offer new insights about the basics and fundamental theory for enamel adhesion. This may yet lead to a better development of primers to serve and enhance clinical dentistry.