Etched enamel structure and topography: Interface with materials

F. R. Tay, D. H. Pashley

Research output: Chapter in Book/Report/Conference proceedingChapter

12 Scopus citations


Experiments on bonding of acrylic resins to enamel and dentine began in the early 1950s in England with Dr. Oskar Hagger. He developed a monomer based on glycerophosphoric acid dimethacrylate that was chemically cured with sulphinic acid [1]. This was shown in a Swiss patent (no. 211116, 1951) to bond to tooth structure.His work led to the development of Sevitron, an early commercial adhesive [2, 3]. In the U.S., Dr. Michael Buonocore made the second, and more important, advance in adhesive dentistry, by demonstrating that acid etching of enamel led to improved resin-enamel bonds using Sevitron-like resin formulations [4]. His rationale for acid-etching enamel was that little adhesion was obtained on unetched enamel, which he correctly surmised lacked microscopic porosities for resin infiltration. He knew that concentrated (85wt%) phosphoric acid was used in industry to pre-treat metal surfaces prior to painting or resin coating; thus, it was logical for him to use 85% phosphoric acid for 30 s to etch enamel, followed by water rinsing.The results of his work were very controversial at the time.Many researchers regarded Dr. Buonocore's approach as unconventional and reckless because he advocated the use of dangerous, industrial-strength acids in the oral cavity. Over the next 10 years, many investigators confirmed the utility of acidetching enamel to increase resin-enamel bond strengths. The concentration of the phosphoric acid was subsequently reduced to 50% [5], and more recently to 32-37%. With the recognition that primary tooth enamel surfaces were largely aprismatic, etching times of 120 s were commonly used for bonding procedures for primary teeth [6]. Those etching times have been reduced to 60 s [7] and, more recently, 20-30s [8-10] for aprismatic enamel for bonding of pit and fissure sealants and orthodontic brackets. Phosphoric acid etching worked so well for retention of pit and fissure sealants that it was natural to adopt the same acid on bur-cut enamel cavosurface margins [11].Both a reduction in the acid concentration as well as etching time [12-14] had been proposed. Despite the availability of alternative enamel etchants such as pyruvic, citric, oxalic, nitric or maleic acid, phosphoric acid still remains the etchant of choice, with the contemporary adoption of a reduced etching time to 15 s for both prismatic and aprismatic enamel. The solutions used to etch enamel were also made into gels to permit better control of these acids, since acid etching of dentine was erroneously thought to devitalize pulps [15]. For bonding to cut enamel, it was further observed that even a 5-s etching time [16, 17], or a phos-phoric acid concentration as low as 3% [18], was sufficient to create adequate retentive patterns and bond strengths in cut enamel. The goals of enamel etching are to clean the enamel of the surface organic pellicle in uncut enamel, to remove the enamel smear layer in cut enamel and to partially dissolve the mineral crystallites to create retentive patterns [19] for the infiltration and retention of resinous materials. There is a general consensus that acid etching increases the surface energy and lowers the contact angle of resins to enamel [20, 21]; however, there is poor correlation between the length of resin tag formation or the depth of resin penetration with the strength of resin-enamel bonds [13, 18, 22].Etching cut enamel for 15 s, for example, has been shown to create sufficient micromechanical retention that is comparable to that achieved with 60 s of etching,without compromising microleakage along the bonded enamel interface. It has been shown that optimal enamel-resin bonds could be achieved as long as the etched enamel surface was clean and free from saliva contamination [23-25]. Increasing the length of the resin tags does not contribute substantially to the increase in cumulative surface area that is created by acid etching of cut enamel [26]. This is attributed to the ability of resin to penetrate the microporosities that are created within the partially demineralized enamel [27, 28]. A marked increase in surface area is achieved via the creation of these microporosities among the apatite crystallites, in which resin can infiltrate and result in the formation of a layer of enamel-resin composite which consists of inter- and intra-crystallite resin encapsulation (Fig. 1.1), as well as resin infiltration into the interprismatic With the advent of contemporary dentine adhesives that contain hydrophilic resin monomers to enhance their coupling with wet dentine substrates, there was a paradigm change by applying these adhesives simultaneously to enamel and dentine [31, 32]. Two main strategies are currently in use for bonding to enamel and dentine: the total-etch technique and the self-etch technique [33]. These adhesives are currently available as three-step, two-step and single-step systems depending on how the three cardinal steps of etching, priming and bonding to tooth substrates are accomplished or simplified [34]. Two-step systems are subdivided into the single bottle, self-priming adhesives that require a separate etching step and the self-etching primers that require an additional bonding step. The recently introduced single-step, self-etch adhesives further simplify bonding procedures into a single-step application. Both the two-step self-etching primers and the single- step (all-in-one) self-etch adhesives contain increased concentration of ionic resin monomers with acidic phosphate or carboxylic functional groups, rendering them aggressive enough to etch through the smear-layer-covered, cut dentine [35] and enamel [36]. Despite a less pronounced enamel-etching pattern, a similar retention mechanism via nanoretention of the partially dissolved apatite crystallites with resin has been observed with the use of self-etch adhesives on enamel [37]. Irrespective of how they are packaged, single-step self-etch adhesives are supplied as two-component assemblies, separating the functional acidic monomers that are liable to hydrolytic degradation from the water component which must be present to effectuate hard-tissue demineralization in order to maintain adequate shelf lives. They are mixed together immediately before use, and the mixture of hydrophilic and hydrophobic resin components is then applied to the tooth substrate. Some of the commercially available single-step, self-etch adhesives are disguised as "single-bottles" by hiding the catalysts in a sponge (AQ Bond/Touch & boundaries (Fig. 1.2). First reported by Gwinnett and Matsui [29], this phenomenon of enamel hybridization parallels what was subsequently reported on dentine by Nakabayashi et al. [30]. Bond, Sun Medical, Shiga, Japan; Parkell, Farmington, N.Y.) or applicator tip (AQ Bond Plus/Brush&Bond,Sun Medical Inc./Parkell) which must be used for activating the adhesive [38]. No-mix, single-step self-etch adhesive is also becoming available (iBond, Heraeus Kulzer, Hanau, Germany) that can accomplish etching, priming and bonding simultaneously to enamel and dentine immediately after dispensing [39].

Original languageEnglish (US)
Title of host publicationDental Hard Tissues and Bonding
Subtitle of host publicationInterfacial Phenomena and Related Properties
PublisherSpringer Berlin Heidelberg
Number of pages31
ISBN (Print)354023408X, 9783540234081
StatePublished - 2005

ASJC Scopus subject areas

  • General Medicine


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