The terms abrasion, attrition and erosion have been adapted from everyday usage and their meanings in the dental context can diverge from those in other fields, in particular from that of tribology, the science of wear, friction and lubrication.
Tooth Wear Mechanisms
Dental Attrition:
Attrition is the physiological wearing away of dental hard tissues through tooth-to-tooth contact, without the intervention of foreign substances. It should in principle occur by two-body wear but mechanistically it cannot be differentiated sharply from dental abrasion, since particles of enamel detached during attrition can act as abrasive particles. In vitro, the rate of enamel/enamel attrition under loads of 0.2–16 kg increases with time and load, and is strongly influenced by the presence and nature of lubricant . Above 10 kg load, water-lubricated wear is greater than between dry or saliva-lubricated surfaces but saliva-lubricated wear only exceeds wear of dry enamel at loads _14 kg. resumably, water or saline can keep detached particles of enamel in suspension and thus facilitate three body abrasion, whereas mucins and other salivary macromolecules reduce frictional forces by coating both the wear surfaces and the particles. At loads of 6 and 10 kg, the rate of dentine/dentine attrition was found to be greater than that of enamel/enamel attrition but at 14 kg the rates were the same. It was suggested that dentine wear was greater at lower loads because of its relatively low mineral content, but that at high loads the fibrous organic matrix would help to reduce fracture, whereas the more highly mineralised enamel would lack this mechanism .
Dental Abrasion:
Dental abrasion is pathological. Imfeld defined abrasion as ‘the pathological wearing away of dental hard tissue through abnormal mechanical processes involving foreign objects or substances repeatedly introduced in the mouth and contacting the teeth’. However, it has been suggested that many dental health problems are caused or exacerbated by almost the complete lack of abrasive wear from the diet in modern Western populations and it is accepted that even normal tooth-cleaning practices produce some abrasion of dentine over a lifetime. Given that tooth-cleaning habits are highly beneficial at the same time as being the most common cause of abrasion, it would seem reasonable to remove the words pathological and abnormal from the above definition. This is not to deny that abusive use of toothbrushes and toothpaste can produce pathological levels of abrasion, as parafunction can with attrition.
Interactions between Tooth Wear Processes
Interaction of Dental Attrition with Erosion:
At loads up to 16 kg, enamel/enamel attrition in vitro is much higher in the presence of than in water . However, this extreme erosive challenge is likely to occur in vivo only in individuals who vomit frequently]. Attrition is much lower in the presence of dilute acetic acid or citric acid , which are much closer to more usual erosive stimuli, than in the presence of water or saline .Enamel surfaces which had been rubbed together in citric acid solution at were smooth, with only slight grooving, and it was suggested that acid softening would both reduce friction between the surfaces and also dissolve potentially abrasive enamel particles fractured off the surfaces . Studies of enamel surface sworn by attrition under higher loads, more representative of chewing or bruxism, and in the presence and absence of acid, would be of interest. There have been no controlled in vitro studies of dentine attrition under acidic conditions.
Interaction of Dental Abrasion with Erosion
Exposure of enamel to acid renders it more vulnerable to abrasion. Rats drinking an acidic drink instead of water showed occlusal and lingual wear of the molars, whether they were consuming soft or hard food . In vitro, softened enamel is more susceptible to abrasion, not only by toothbrush and paste, but even by such mild challenges as tooth brushing without paste or friction from the tongue. Thus, whereas enamel is scarcely abraded by normal tooth brushing, it becomes vulnerable to toothbrush abrasion after erosive challenge .
Saliva and Tooth Wear
The pellicle seems to show some resistance to brushing, since the basal pellicle layer survives 10 s brushing with saliva alone [80]. Brushing with hydrated alumina or saliva/silica slurry was reported to leave a thin pellicle layer on enamel [80, 81] and Hannig [80] suggested that this layer could modify wear. However, brushing with these abrasives would cause no significant wear of sound enamel and this hypothesis needs to be tested using softened enamel. Moreover, the finding that brushing removes the outer pellicle [80] substantiates the suggestion [82] that brushing immediately before eating or drinking might reduce the protective effect of the pellicle against erosion.
Softened enamel exposed to a remineralising solution or to saliva for an adequate time can regain mineral and thus re-acquire mechanical strength [31, 52, 83–85]. In vivo saliva could thus reduce the vulnerability of softened dental hard tissues to mechanical wear. In vitro, resistance to toothbrush abrasion [84] or to ultrasonication [85] was restored after exposure to artificial saliva for 4 and 6 h, respectively. In vivo results over times that would be useful in terms of reducing abrasion of softened enamel have been less encouraging. Experiment, only partial resistance to brushing abrasion was acquired after 60 min exposure to saliva [49], while in another the decrease in abrasion after the same time was not significant [82]. The discrepancy between the in vitro and in situ results might be due to the presence in saliva of proteins (e.g. statherins) known to inhibit hydroxyapatite crystal growth, and their absence from the artificial salivas used in vitro experiments.
