archives of oral biology 57 (2012) 211–213
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Damaged! A new overview of dental wear Tooth wear is the decrease in tooth volume produced by the accumulated loss of small quantities of material. Whilst both physical and chemical mechanisms can lead to this damage, the final separation of tooth tissue usually requires mechanical analysis because a force is associated with it. Teeth may become worn either because of forceful contacts with food particles and anything accidentally ingested with them, or else with the opposing tooth row. However, although we may understand clearly enough what the definition of dental wear is, there are major problems in evaluating the progress that researchers have made towards understanding it. This is partly because the subject lacks a unified terminology and methodology, but also because of the vastness of the literature, scattered across many vertebrate lineages. Now, a new paper in this issue of Archives of Oral Biology contains a critical review of the literature on the wear of human teeth.1 It deals with two aspects of the subject in detail: the mechanisms of wear on the one hand and the description and classification of wear patterns on the other. The review is likely to be considered a landmark, opening the door for enthusiastic young researchers eager to make progress in what is a rather neglected area of dental research. The aim of this editorial is simply to highlight the appearance of this review and to try to integrate a little of what is known in humans with that from other dentate organisms. Studies of dental wear in human and other vertebrates have focused on a number of different scales of measurement, which can be termed macrowear, mesowear and microwear. Macrowear is that visible to the naked eye. Nothing could highlight the importance of dental macrowear more vividly than the statement that for many mammalian species, it constitutes a major threat to the survival of individuals by jeopardizing their rate of food intake. This old adage is now receiving careful attention from fieldworkers who research mammals in the wild. In a bunodont (low crowned, low cusped) molar tooth type, such as that of the human, wear leads to a gradual eradication of the preformed enamel surface to expose areas of dentine at the cusp tips. Eventually, the working surface of the tooth crown becomes a seamless mixture of primary and secondary dentine surrounded by an enamel rim. In several mammalian species, dental wear that has advanced to this point is associated with a decrease in the efficiency of an individual animal in food breakdown in the
mouth, with various consequences including the time needed for feeding, food choice and also the most vital biological statistic: reproductive fitness.2–8 Macrowear studies are ideal for establishing a link between wear and function like this because food breakdown generally proceeds at a similar ‘millimetre’ scale.9 Such research bears on an old and resolved argument in dentistry about whether the optimal state of the postcanine teeth in the human is completely unworn, or at the other extreme, ‘cuspless’.10,11 The answer may lie in between. It is plausible that it is only after minor adjustments of position that functional teeth make after eruption that the highest rates of breakdown can be achieved. These positional adjustments would be reflected in the tiny wear facets established via interaction between upper and lower tooth rows.12 The angulation of these can be used to reconstruct accurate jaw movement trajectories, with a recent analysis employing this in analyses of stress patterns in molars.13 Modern human populations with high wear rates, such as some of those on the Arabian Peninsula,14 have intriguing potential in testing hypotheses about the most efficient dental state, and also in evaluating suggested schemes of oral rehabilitation in the light of their relative efficiency.15 Coupled to research advocating the inclusion of oral processing in models of digestion, this seems to be an interesting comparative context in which to revive and augment classic studies on the worn dentition. In many human populations, and most mammals, tooth wear is produced by some variable combination of contacts with food (including extraneous material) and opposing teeth. Macroscopic studies cannot really resolve this combination into its causative components, but adherents of mesowear claim that this is possible by looking at the occlusal surface under a light microscope, particularly at functional crown surfaces where both dentine and enamel are exposed as in ungulates.16 The rounding of the edges of enamel facets and the presence of enamel ridges rising above sunken dentinal areas are both clear evidence of the dominance of food abrasion in hypsodont grass-eating mammals.17 In contrast, tooth–tooth contact would tend to level these tissues up and sharpen any edges, as is revealed, for example, by the wear of guinea pig molars in utero.18 More familiar to readers than mesowear may well be microwear studies,18 which aim to resolve and quantify actual wear features in the
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form of pits and scratches on tooth surfaces, usually with SEM,19 but increasingly now with white light confocal microscopy.20 Microwear has been the dominant force in wear studies for over twenty years. Achievements range from demonstrating that conodonts, the ‘teeth’ of enigmatic early chordates that died out 200 million years ago, were actually used to break food particles down21 to the finding that at least one fossil giraffe species ate grass22 whilst some fossil horses did not.23 Current ideas of the diet of human ancestors are largely based on microwear evidence, looking particularly at the degree to which scratches predominate over pits and the overall directionality of wear features (although data are not analysed in quite that way) to show what was being consumed just prior to death.20 In all these ways, knowledge has been extended beyond that possible via comparative anatomy alone. Yet, despite all this research at different scales, none of the above methods really addresses the causes of wear. Partly, this is because wear features are generally so small. Perhaps a whole new field, that of ‘nanowear’ needs to be initiated to investigate this. Such studies have begun, but only with diamond tips that come with nanoindentation equipment,24 not the type of wear particles that cause the actual damage to teeth in the mouth. Many questions can be asked and answered by research at this level. Why are wear features so tiny? To date, the only answer is that of the brittle–ductile transition: the change in behaviour of materials at length scales that depend on the properties of the material (enamel and dentine) being damaged.25 What types of particles can wear tooth tissues? Are phytoliths, the ‘plant glass’ found in many plant tissues, as damaging as quartz grit from soils? Some researchers suggest not.26 And what role does saliva play in preventing tooth wear, not just in terms of reducing dissolution in acid conditions, but in terms of lubrication? Interaction between many of the proteins in saliva and polyphenolic compounds, the most common ‘defence’ chemicals in plants, needs re-examining with respect to dental wear. These proteins are often presumed in the human dental literatures to have evolved as protectors of the enamel surface in humans and higher primates, but their interspecific variability and, in particular, their absence in a grass-eating monkey (grasses lack tannins), suggest that they have an important role in the oral monitoring of these digestion inhibitors.27 Certainly, the binding of the salivary proteins to these tannins results in an increase in oral friction.28,29 Does this affect wear rates? We must stop here because we are not writing a review ourselves: we simply wish to celebrate one.1 All we have tried to argue here is that wear research interests a huge variety of researchers yet seems in some ways to have barely scratched the surface. Funding: None. Competing interests: None declared. Ethical approval: Not required.
9. 10. 11. 12.
references 21. 1. d’Incau E, Couture C, Maureille B. Human tooth wear in the past and the present: tribological mechanisms, scoring
systems, dental and skeletal compensations. Arch Oral Biol 2011. 10.1016/j.archoralbio.2011.08.021. Lanyon JM, Sanson GD. Koala (Phascolarctos cinereus) dentition and nutrition. II. Implications of tooth wear in nutrition. J Zool 1986;209(2):169–81. McArthur C, Sanson GD. Tooth wear in eastern grey kangaroos (Macropus giganteus) and western grey kangaroos (Macropus fuliginosus), and its potential influence on diet selection, digestion and population parameters. J Zool 1988;215(3):491–504. Logan M, Sanson GD. The effects of tooth wear on the feeding behaviour of free-ranging koalas (Phascolarctos cinereus Goldfuss). J Zool 2002;256(1):63–9. DeGusta DM, Everett A, Milton K. Natural selection on molar size in a wild population of howler monkeys (Alouatta palliata). Proc Roy Soc Biol Sci (Biol Lett) 2003;270: S15–7. King SJ, Arrigo-Nelson SJ, Pochron ST, Semprebon GM, Godfrey LR, Wright PC, et al. Dental senescence in a long-lived primate links infant survival to rainfall. Proc Nat Acad Sci USA 2005;102:16579–83. Cuozzo FP, Sauther ML. Severe wear and tooth loss in wild ring-tailed lemurs (Lemur catta): a function of feeding ecology, dental structure, and individual life history. J Hum Evol 2006;51(5):490–505. Galbany J, Altmann J, Pe´rez-Pe´rez A, Alberts SC. Age and individual foraging behavior predict tooth wear in Amboseli baboons. Am J Phys Anthropol 2011;144(1): 51–9. Lucas PW. Dental functional morphology: how teeth work. Cambridge University Press; 2004. Berry DC, Poole DFG. Masticatory function and oral rehabilitation. J Oral Rehabil 1974;1(2):191–205. Luke DA, Lucas PW. The significance of cusps. J Oral Rehabil 1983;10(3):197–210. Osborn JW, Lumsden AGS. An alternative to thegosis and a reexamination of the ways in which mammalian molars work. N Jb Geol Palaont Abh 1978;156:371–92. Benazzi S, Kullmer O, Grosse IR, Weber GW. Using occlusal wear information and finite element analysis to investigate stress distributions in human molars. J Anat 2011;219(3):259–72. Johansson A, Fareed K, Omar R. Analysis of possible factors influencing the occurrence of occlusal tooth wear in a young Saudi population. Acta Odont Scand 1991;49(3):139–45. Johansson A, Johansson A-K, Omar R, Carlsson GE. Rehabilitation of the worn dentition. J Oral Rehabil 2008;35(7):548–66. Fortelius M, Solounias N. Functional characterization of ungulate molars using the abrasion–attrition wear gradient: a new method for reconstructing paleodiets. Am Mus Nov 2000;3301:1–36. Kaiser TM, Brinkmann G. Measuring dental wear equilibriums—the use of industrial surface texture parameters to infer the diets of fossil mammals. Palaeogeogr Palaeoclimatol Palaeoecol 2006;239(3–4):221–40. Teaford MF, Walker A. Dental microwear in adult and stillborn guinea pigs (Cavia porcellus). Archs Oral Biol 1983;28(11):1077–81. Walker A, Hoeck H, Perez L. Microwear of mammalian teeth as an indicator of diet. Science 1978;201:908–10. Scott RS, Ungar PS, Bergstrom TS, Brown CA, Grine FE, Teaford MF, et al. Dental microwear texture analysis shows within-species diet variability in fossil hominins. Nature 2005;436:693–5. Purnell MA. Microwear on conodont elements and macrophagy in the first vertebrates. Nature 1994;374: 798–800.
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22. Solounias N, Teaford M, Walker A. Interpreting the diet of extinct ruminants: the case of a non-browsing giraffid. Paleobiol 1988;1988(14):287–300. 23. MacFadden BJ, Solounias N, Cerling TE. Ancient diets, ecology, and extinction of 5-million-year-old horses from Florida. Science 1999;283:824–7. 24. Guidoni GM, Swain MV, Ja¨ger I. Wear behaviour of dental enamel at the nanoscale with a sharp and blunt indenter tip. Wear 2008;266(1–2):60–8. 25. Lucas PW, Constantino P, Wood BA, Lawn BR. Dental enamel as a dietary indicator in mammals. BioEssays 2008;30:374–85. 26. Sanson GD, Kerr SA, Gross KA. Do silica phytoliths really wear mammalian teeth? J Arch Sci 2007;34(4): 526–31. 27. Mau M, Su¨dekum KH, Johann A, Sliwa A, Kaiser TM. Saliva of the graminivorous Theropithecus gelada lacks proline-rich proteins and tannin-binding capacity. Am J Primatol 2009;71(8):663–9. 28. Prinz JF, Lucas PW. Saliva tannin interactions. J Oral Rehabil 2000;27(11):991–4. 29. de Wijk RA, Prinz JF. The role of friction in perceived oral texture. Food Qual Pref 2005;16(2):121–9.
P.W. Lucas* Department of Bioclinical Sciences, Faculty of Dentistry, HSC Jabriya, PO Box 24923, Safat 11310, Kuwait R. Omar Department of Restorative Sciences, Faculty of Dentistry, HSC Jabriya, PO Box 24923, Safat 11310, Kuwait *Corresponding
author. Tel.: +965 2498 6762; fax: +965 2532 6049 E-mail addresses: [email protected] [email protected]
(P.W. Lucas) Received 19 October 2011 Received in revised form 1 November 2011 Accepted 1 November 2011 0003–9969/$ – see front matter # 2011 Elsevier Ltd. All rights reserved. doi:10.1016/j.archoralbio.2011.11.001