Being the hardest organ in body, human teeth have a very unique structure. To understand the wear behavior of human teeth, the properties of materials and structures need to be evaluated. There are several quantitative and qualitative methods to measure or assess these properties. This chapter reviews evaluation techniques to understand their unique properties for both mammal and human teeth. Chapter 6 gives a more detailed discussion on artifi cial dental materials.
Wear Modes and Location
Wear Modes：
Friction and wear can result from direct contact between teeth and from any abrasive particles or devices sandwiched between them during mastication, thegosis, bruxism, toothbrushing, and other functions. Tooth surface loss caused by wear is a common clinical problem, with various epidemiologic studies suggesting prevalence estimates of up to 97 %, with around 7 % of the population showing pathological wear requiring treatment . The main categories of wear that contribute to the destruction of natural teeth and artifi cial materials are classifi ed as follows : physiological wear (vital life functions); pathological wear (disease and abnormal conditions); prophylactic wear (preventive conditions); and fi nishing procedure wear. An overview is provided in Table。
Physiological wear, inevitably due to the function of mastication, is surface degradation that results in a progressive, very slow loss of the convexity of tooth cusps, which manifests as a fl attening of both cusp tips on the posterior teeth and incisal edges on the anterior teeth for mammals. Compared with physiological wear, some pathological factors can cause excessive wear of teeth and restorations. Clinical reports show that wear usually becomes signifi cantly severe due to erosion, bruxism, xerostomia, and so on. Pathological wear can also be caused by detrimental oral habits [ 8 , 12 ], which generally include chewing tobacco; biting on hard objects such as pens, pencils, or pipe stems; opening hairpins with teeth; and biting fi ngernails. In addition, occupational habits may result in the wear of teeth and restorations. For instance, tailors or seamstresses sever thread with their teeth, shoemakers and upholsterers hold nails between their teeth, glassblowers and musicians play wind instruments, and so on.
It was pointed out in Chapter 1 that in normal use, the benefi ts of toothbrushing far outweigh the potential harm; however, wear can occur as a result of overzealous toothbrushing and the improper use of dental fl oss and toothpicks. Scaling and cleaning have been shown to result in minor wear of teeth. Dental treatments usually involve such processes as cutting, fi nishing, and polishing; and they cause the wear of teeth and restorations to some extent . The actual wear situation in the mouth may vary considerably because of different substrates, opposing wear surfaces, lubrication systems involved, and third-party abrasives . The mouth provides an extremely complex tribological system. Therefore, the wear of teeth and restorations is multifactorial in the mouth and dependent on physical and chemical conditions. It has long been recognized that it is diffi cult to ascribe many individual cases to any one category.
