Bonding universal dental adhesive to developmentally hypomineralised enamel

Abstract

Previously referred to as “cheese molars”, “dysmineralised permanent first molars”, “idiopathic hypomineralisation” and “non-fluoride hypomineralisation”, the term “Molar Incisor Hypomineralisation” (MIH) was first introduced by Weerheijm and colleagues in 2001 to describe the characteristic clinical appearance of demarcated opacities, altered translucency, and qualitative defects within dental enamel. A developmental dental defect of systemic origin, MIH invariably affects one to four first permanent molars with or without involvement of the permanent incisors. With global prevalence rates ranging from 0.48% to 40.2%, it is a common condition that affects approximately one in six children in New Zealand.

Despite extensive research efforts, the aetiology and clinical management of MIH remain indeterminate. As MIH-affected teeth have compromised chemomechanical properties, restorative treatment outcomes are often unpredictable, thus requiring multiple re-interventions over the years. The debilitating sequelae and negative impact of MIH on children have been well documented in the literature; therefore, the overarching drive of this research is to alleviate the healthcare burden of MIH on affected individuals.

The first study was an in vitro experiment that investigated the effect of a pretreatment protocol involving the concurrent use of a papain-based deproteinising agent (Papacarie Duo gel) and a contemporary universal dental adhesive resin (3M ESPE Scotchbond Universal Adhesive) on the microshear bond strength of resin composite to hypomineralised enamel. The three primary hypotheses tested were: (1) there would be a difference in bond strength between normal enamel and hypomineralised enamel, (2) deproteinising pretreatment with Papacarie Duo gel would increase the bond strength to resin composite, and (3) there would be a difference in bond strength between etch-and-rinse mode and self-etch mode of Scotchbond Universal Adhesive.

After assessing and confirming the eligibility of each participant, extracted first permanent molars with a known clinical diagnosis of MIH were collected from Paediatric dental specialists across New Zealand over a 13-month period. Upon receipt, the teeth were cleaned, stored, sectioned and prepared in accordance with the approved research methodology. A total of 88 clinically sound “normal” enamel specimens, and 96 hypomineralised enamel (48 creamy/white, 48 yellow/brown) specimens were included in both studies. Following the MIH judgment criteria (EAPD 2003), two independent examiners visually inspected and identified the specimens, which were subsequently randomised and allocated into one of the eight experimental groups.

The results supported both hypotheses and established the conclusive facts that the application of Scotchbond Universal Adhesive in etch-and-rinse mode and the pretreatment of hypomineralised enamel with Papacarie Duo gel led to a marked increase in microshear bond strength values. Analysis of failure modes under scanning electron microscope further reaffirmed the research findings.

The second study was another in vitro experiment which evaluated the surface morphology and nanotopography of 8 representative normal enamel and 16 hypomineralised enamel (8 creamy/white, 8 yellow/brown) specimens. Following the protocol of their respective experimental groups, the enamel specimens were pretreated with different etching modes (i.e., etch-and-rinse or self-etch) of 3M ESPE Scotchbond Universal Adhesive and/or Papacarie Due gel. The investigation involved the quantitative measurement of surface roughness using atomic force microscopy as well as the qualitative examination of enamel surface aberrations under scanning electron microscopy. The two hypotheses tested were (1) deproteinising pretreatment using Papacarie Due gel would lead to qualitative and quantitative changes on the enamel surfaces of all substrates, and (2) there would be morphological and topographic differences between phosphoric acid etching and Scotchbond Universal Adhesive when applied in SE mode.

In spite of limited data, the results supported the hypothesis. Yellow/brown hypomineralised enamel recorded higher surface roughness values than their normal enamel and creamy/white hypomineralised enamel counterparts due to microscopic post-eruptive breakdown of enamel that was not readily discernible. In addition, the deproteinisation of hypomineralised enamel with Papacarie Duo gel followed by acid etching with 37% phosphoric acid produced uniform etching patterns that were comparable to those of normal enamel.

Consolidating the findings from these two studies, it is evident that Papacarie Duo gel and acid etching (i.e., etch-and-rinse mode) are two independent factors that have a profound effect on the in vitro bonding efficacy of hypomineralised enamel to resin-based universal dental adhesives. Although further investigations are warranted, it is expected that the integration of these dental materials into the clinical management of MIH will lead to favourable treatment outcomes. This is in line with the overriding purpose of the research project – to determine a pretreatment protocol that has the potential to reduce the likelihood of traumatic dental experiences and ultimately, improve the quality of life of affected children and families.