Author: Prof Martin Tyas, Professorial Fellow at the Melbourne Dental School, University of Melbourne, Australia
Historical Evolution of Dental Adhesives
In 1952, Ivor Kramer and John McLean reported a 3-µm thick ‘altered staining layer’ on the dentine surface after demineralization with glycerophosphoric acid, a component of a ‘cavity seal’ for use with a restorative auto-polymerizing resin. The subsequent availability of the scanning electron microscope revealed that this layer is what is now termed the ‘hybrid layer’ or ‘resin-reinforced layer’, consisting of a layer of collagen impregnated by resin and forming a micromechanical bond, the basis of most current resin-based dentine bonding agents (dentine adhesives). Further work, notably by Japanese researchers, resulted in the development of chemically adhesive monomers which are now components of several dentine adhesives, thus enabling both micromechanical and chemical bonding.
Resin-enamel bonding dates back to 1985 when Michael Buonocore demineralized enamel with phosphoric acid, applied an auto-polymerizing restorative resin and established that a strong bond was formed. The ‘acid-etch’ technique for resin-enamel bonding has not changed greatly since Buonocore’s work, however, other acids can be used which allow simultaneous dentine demineralization.
The first inherently adhesive material was polycarboxylate cement, developed by Dennis Smith in the late 1960s. The carboxyl group in the freshly mixed cement forms an ionic bond with the calcium of the enamel and dentine. Poycarboxylate cements are not extensively used, because of their sub-optimal handling properties and their white, opaque appearance. However, they are suitable for lings, bases and temporary restorations.
Glass-ionomer cements (GICs) were developed in the late 1960s by John McLeanand Alan Wilson at the Laboratory of the Government Chemist in London, England. The liquid was modified from that of polycarboxylate cement, and hence is chemically adhesive to enamel and dentine. Glass-ionomer cements have a wide range of uses including pit-and-fissure sealants, linings, bases, temporary and long-term restorative materials and orthodontic bonding cements.
Over the early late 1950s to 1960s, materials research started to become more adopted into clinical practice. For example, Buonocore’s 1955 work on enamel etching only became a clinical reality in the late 1960s, when Ray Bowen synthesized bisGMA and developed resin-based composite materials (‘composites’). In recent times, composite is now used in numerous clinical bonding situations, such as for resin composite restorations following caries, trauma or to improve esthetics, resin-bonded bridges, orthodontic brackets, fixed orthodontic retainers, splinting teeth and altering tooth shape for prosthodontic purposes.
Composite bonding to dentine has presented many more difficulties. Some of the early dentine bonding agents (1980s) were designed to bond chemically to dentinal calcium, but had unacceptable failure rates in non-carious cervical lesions. Current materials rely on a bonding technology described above, which is different from the early materials. Success rates are much higher provided that strict adherence to manufacturers’ instructions is followed. In addition, considerable simplification of clinical techniques has developed, and most current materials can be used to bond composite to enamel and dentine simultaneously. Self-adhesive resin cements have been available for several years, and self-adhesive composites are becoming available.
For GICs, developments include the addition of photo-polymerisable water-soluble resin, commonly hydroxyethylmethacrylate (HEMA), resulting in the resin-modified GICs. These materials show less water sensitivity and improved esthetics. Changes in powder and liquid compositions have improved mechanical and clinical handling properties.
More broadly, the steady improvements in adhesive dental materials have enabled the wider practice of minimum intervention in operative dentistry, when a restoration becomes necessary due to cavitations, esthetics or loss of function.
Usage of Dental Adhesives
Adhesives are generally considered to be intermediary materials used as a very thin layer for the purpose of attaching a non-adhesive material to enamel or dentine, a typical example being bonding of composite to dentine with a dentine adhesive. GICs are not generally considered to be ‘adhesives’ except perhaps for one or two products which are used as a thin layer; rather, they are used as final restorative materials, linings or bases.
As mentioned earlier, GICs adhere via ionic bonding between the carboxyl group in the cement liquid and the calcium in tooth structure. In addition, an ‘ion exchange’ layer, about 2-5 µm thick, develops between the tooth structure and the cement, consisting of calcium, phosphate and carboxyl groups. For the resin-modified GICs, following ‘conditioning’ of the additional tooth structure with a mild acid (usually polyacrylic acid), bonding may also occur via resin penetration into the dentine hybrid layer and into the porous enamel.
As a final restorative material, composites tend to be more suitable for load-bearing areas such as restorations in posterior teeth; however GICs can perform well in small- to medium-sized occlusal cavities. Anecdotally, GIC restorations are associated with less secondary caries than composite restorations, although the clinical evidence is not strong, and may therefore be indicated for high caries risk patients. The esthetics of GICs can occasionally be difficult to achieve, which may indicate a preference for composite.
When placing a composite restoration in a cavity with exposed dentine, it is essentially personal preference whether to use a thin-layer adhesive GIC, a dentine/enamel adhesive or a thick-layer liner/base GIC. Each of these has advantages and disadvantages. In all cases, enamel bonding (if enamel is present) would be practiced.
Accessing Success and Failure
The bond durability of the dental adhesive is extremely important. When deciding which adhesive to purchase, clinicians have available bond strength and microleakage data from laboratory tests. For a composite/dentine adhesive combination, the reported bond strength is highly dependent on the laboratory method, and widely disparate results have been published for the same material. Using bond strength to assess the adhesive properties of GICs is highly misleading, as failure is cohesive in the GIC, not adhesive at the bond interface. Microleakage studies are of little use as there is no apparent clinical correlation; some authors have proposed that such studies be abandoned.
Clinical studies of retention rates and marginal staining incidence of material/adhesive combinations in non-carious cervical lesions are the most commonly reported for assessment of adhesive capability. However, clinical studies may take some years to be designed, executed and published, and therefore may be of limited value. A further issue is that clinical studies are usually undertaken in university or hospital environments, with strict inclusion and exclusion criteria, which are often different from the use of the materials in general practice. This underlines the importance of the practice-based research networks which are being established around the world.
There are not many simple, objective criteria for assessing success or failure of a dental restoration. If the restoration falls out, then it has clearly failed. What if there is some occlusal wear which is not interfering with acceptable function; is this a failure?
In the short-term, tooth-colored adhesive restorations should have acceptable esthetics (which is subjective) and be free of post-operative sensitivity. If relevant, the occlusion should be normal and there should be no food packing between the teeth.
In the long term, the restoration should be retained, esthetics should be satisfactory, pain should be absent and function should not be compromised.
Failure of a dental restoration can be attributed to several factors. For instance, for tooth-colored adhesive restorations (composite and GIC) the main reasons for failure are secondary caries, fracture and loss of retention.
Patient Care and Future Developments
For restorations placed because of caries developed in the patient’s mouth, it is essential to identify the patient-specific risk factors so they can be addressed. Failure to do so will simply result in secondary caries around the restorations. Patients must also be taught by their dentists on how to maintain their restorations.
Improvements in oral health should be accompanied by a reduction in the need to place restorations. There should be a continued emphasis on caries prevention, and hopefully there will be an increased use of preventive pit-and-fissure sealants. Several agents are now available for remineralizing non-cavitated lesions, and research for other agents is ongoing.
In the ageing patient bonding to dentine with dentine adhesives is more challenging, so more clinical data are needed to determine which dentine adhesive system performs best. Caries incidence, especially root caries, is emerging as a key problem in the aged, and GICs would appear to be the restorative material of choice because of the possible potential to prevent secondary caries and their relative moisture tolerance compared to composite. However, in general, there are real difficulties in treating aged patients as they may be in assisted care and have to be treated at the bedside.
Another problem with ageing patients is the maintenance of restorations which have been present for many years, and are deteriorating because the patient’s caries risk profile has changed from low to high. Possible reasons for this change are that manual dexterity has decreased, leading to compromised brushing, and saliva is compromised due to the side effects of drugs such as hypotensive agents. Many aged patients are unable to tolerate extensive ‘replacement dentistry’ and therefore repairs with adhesive materials can provide an efficient and effective solution.