Medical Research: Emerging Concepts and Applications Vol. 3

Adhesive dentistry is routinely evaluated as a problem of material chemistry, yet the tissue to which adhesives are applied is neither uniform nor inert. Enamel and dentine vary systematically with anatomical region, developmental history, disease activity, ageing, operative manipulation and systemic exposure, and each of these sources of variation alters the micromechanical and chemical conditions on which hybridisation depends. This critical narrative review examines the extent to which substrate condition governs the effectiveness and durability of resin bonding to dental hard tissues, and evaluates how confidently current evidence supports substrate-specific clinical protocols. Literature was identified through structured searching of biomedical and multidisciplinary scholarly sources, supplemented by backward and forward citation tracking and by inspection of the reference lists of recent syntheses. Evidence was appraised for design adequacy, substrate characterisation, ageing protocol and the strength of the link between laboratory measurement and clinical outcome. Several conclusions are well supported. Sound enamel remains the most predictable bonding substrate, and phosphoric acid conditioning of enamel improves marginal outcomes irrespective of the adhesive class used elsewhere in the cavity. Caries-affected dentine, sclerotic cervical dentine and hypomineralised enamel consistently yield lower and more variable bond strengths than their sound counterparts, and the magnitude of the deficit depends on how the altered tissue was obtained and characterised. Other conclusions remain tentative. The comparative performance of etching strategies on altered dentine is inconsistent across meta-analyses, the biological equivalence of artificially demineralised and naturally altered tissue is not established, and almost no randomised clinical trial has been designed with substrate condition as the primary allocation variable. The principal obstacle to progress is methodological rather than chemical: substrate definition, characterisation and reporting are insufficiently standardised for laboratory findings to be pooled or translated with confidence. Priorities include consensus definitions of altered substrates, mandatory quantitative substrate characterisation in bonding studies, and clinical trials stratified by lesion type.