The art and science of dentistry has continually tried to replace damaged structure that our patients present with to our offices. Over the years, our approach has evolved. Initially it was removal of badly damaged structure, then attempting to replace structure with materials just to replace structure, and eventually to replace structure with materials that mimic the way living structure would actually respond to the environmental stress placed upon it.
A recently published review of the science of bonding by Mazzoni et al., in the Journal of Dental Research (1) has called into question the current science of bonding. The hybrid layer formation, the central and fundamental requirement for our current procedures, is being shown to be an irritant to the living, breathing structure that is the tooth. The odontoblast cellular structure, which directs the health and defensive mechanisms of the dentition (2) throughout the life of the vital tooth, and as it has been pointed out in the Critical Review by Mazzoni et al., (3) the hybrid layer created in dentin bonding systems is unstable in aqueous environments because of hydrolytic degradation of resin and collagen fibrils over time by matrix metalloproteinases (MMPs) and cysteine cathepsins. They go on to state that these collagenolytic and gelatinolytic activities occur in dentin structures treated with either total-etch and/or self-etch adhesives equally.
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Interestingly, there are materials available that will inhibit this “self-destructive” behavior in the restored teeth. Materials that contain polyvinylphosphonic acid will reduce and prevent MMP formation by the odontoblasts. (4) What are these restorative materials? Glass ionomers (GICs)! The chemical fusing and ion exchange without the formation of a hybrid layer allows it to work with moisture as the tooth structure would. Unfortunately, most dentists in the United States are not fully aware of the benefits of GIC. They are, however, aware of the challenges of working with this material. The mixing in a triturator is not acceptable, and the eventual wear against the opposing dentition does take its toll, even though there have been improvements to the material and various coatings to help with this issue. However, unlike composites that ultimately fail from within, GICs fail from without, and the intimate seal to the dentin surface is intricate and long lasting. GICs also assist in the remineralization process through the exchange of calcium and phosphates from the oral cavity through the material into the tooth structure.
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In recent years, some other materials have been developed that call upon this ability to allow moisture and ions to pass through them without compromising the physical properties of the material, thus making these materials very desirable for use in the oral cavity. Products such as Biodentine; Ceramir; ProRoot (an MTA product); Theracal LC; Calcimol LC; the Giomer-containing products in the Beautifil line of restoratives, sealants, and cements; and of course, most recently, Activa. All of these materials have the ability to work with the natural process of providing a conduit for ion and mineral exchange, are biocompatible, and essentially can promote apatite formation and “bring something to the game” in a positive manner. These “reservoir restorations,” as I refer to them, are the basis for our next generation of materials that will work with nature, help with the healing of the tooth structure, and reduce the detrimental response that can be seen with our current bonded restorative process. In my opinion, the future looks bright for dentistry, and for the ways that we help bring our patients back to health.
References
1. Mazzoni A, Tjäderhane L, Checchi V, Di Lenarda R, Salo T, Tay FR, Pashley DH, Breschi L. Role of dentin MMPs in caries progression and bond stability.J DENT RES. February 2015;94(2):241-251, first published on December 22, 2014.
2.Couve E, Osorio R, Schmachtenberg O. The amazing odontoblast activity, autophagy, and aging. JDR, June 26, 2013 (online).
3. Mazzoni A. op.cit.
4.Renson CE. The future for clinical dentistry.Hong Kong Dental Journal. 2004;1:5-12.
