One factor that complicates the issue even further is that the tissues involved in dental erosion, both enamel and dentin, are very different (Table 1). While enamel is approximately 85% mineral, combined with a small amount of collagen, organic material and water, dentin is highly organic. Dentin is comprised of about 45% mineral, with the remainder a combination of organic matter and water. Due to the difference in makeup, dentin reacts much differently to erosive activity and wear than enamel (Figure 4).
One way to think about the dental erosion process is to consider how adhesive bonding procedures, using 37% phosphoric acid, condition the tooth surface to enhance retention of the material. After placement of the acid, the result is typically a visibly chalky appearance (Figure 5). The surface has been demineralized, or etched. That is the same thing that happens on a slightly different scale every time there's an acid challenge in someone's mouth, from whatever source. The primary difference is that one challenge is controlled, and limited to a single exposure, while the other can occur over and over in the mouth, over a prolonged period of time. After an erosive acid challenge on enamel, for example, enamel prisms remain (Figure 4a). Once these demineralized prisms of enamel are present, they are highly susceptible to abrasive forces. Micron by micron, the tongue, food, occlusal forces, etc., will break these susceptible areas off and begin a repetitive cycle of tooth surface softening and loss.
Although the same factors are at play on dentin, the overall process of dental erosion on dentin is somewhat different. When dentin is attacked by erosive acids, the result is a demineralized organic matrix. The mineral portion becomes highly demineralized (Figure 4b). This is very important, for example, in adhesive dentistry and when doing bonding type techniques. It is not hard to imagine how erosive acids predispose the dentin to surface loss and wear. These processes also expose open dentinal tubules, which can then lead to tooth sensitivity.
Another complicating factor is that dentin is subject to degradation by proteolytic enzymes, the MMPs, among other things.19 As we try to understand these processes, we have to include this as one of the risk factors in the degradation, or changes, that occur in dentin erosion. If the dentin becomes soft and liquefied, this will have a significant effect on the magnitude, the extent and the rapidity of tooth surface loss.
To briefly summarize, in the case of enamel erosion, we see more demineralization and bulk tissue loss, which is primarily due to the higher mineral content in enamel. Importantly, many of these changes occur at a pH of less than 4. Dentin, on the other hand, has overall less demineralization and bulk tissue loss under an acid challenge, has a softer matrix and is more susceptible to surface loss due to frictional forces. Changes in dentin typically occur at somewhat higher pH, usually above pH 4.
From a clinical standpoint, erosive processes can appear to be contradictory (Figure 6). In reviewing this figure, one might question how there is so much dentin loss, while the enamel appears to be much less affected. Relating this image to the discussion above, it is likely that in this patient the erosive challenge wasn't at a very low pH. It may have been a higher pH, still under 5.5 or so, but at a pH that didn't have a significant impact on the enamel. Because it wasn't a low enough pH, there was a much greater erosive effect on dentin than enamel. At the same time, the area identified by the green arrow shows a much different situation. This area likely involves actual frictional forces from the opposing teeth, which has resulted in more enamel loss. The question becomes: how do you have, in the same mouth, this level of discrepancy? This dilemma serves to highlight the complexity and the clinical challenges that dental professionals face in dealing with this problem. One answer is to make sure that dental professionals are trained to assess this condition from multiple perspectives.
One problem, from an epidemiologic standpoint, is that dental erosion is often not noted on a patient’s chart, especially in the United States. This contrasts significantly with Europe, Australia and some South American countries, where the assessment of dental erosion has become a routine practice. In the past, unless there was pain or some type of cosmetic problem, patients did not seek treatment for erosion, and most dentists didn't offer care for it. As dental erosion has become more of an issue, it is hoped that all dental practitioners develop a greater awareness of the problem and become more equipped to help manage their patients that are either at risk, or are already experiencing some level, of dental erosion.