Fig. 1.1 A diagrammatic representation of the influence of cusps on tooth wear. A cross section of a premolar tooth before wear reveals the core of dentin within each cusp. When the cusp has become worn and the dentin exposed, the dentin wears faster than the enamel and prevents clogging with food debris by providing an escape way for reduced food particles. The enamel edges are kept sharp by wear, as entire enamel prisms break away from the enamel surface leaving a sharp edge. The cusps provide an alternating surface of sharp enamel, and an escape way for food.
Fig. 1.2 The shape and arrangement of mammalian cusps, when worn, determine the required composite chewing surface. (a) The unerupted molar of this immature sheep (left) has several steep and curved cusps. They are not functional chewing elements until they begin to wear down (right). (b) The adult sheep’s molar has worn down to a flat composite shredding surface which does not clog and is self-sharpening.
1.2.3 Tooth Wear in Man
The teeth of modern man usually show little evidence of wear. This is because our modern diet does not require chewing hard foods, and contains little that is rough and fibrous. Dentists have come to accept our unworn dentition as normal, so that when we find tooth wear, which exposes the dentin in our patients, we are concerned. Wear is not necessarily abnormal in a subject who has lived on a course diet, although it may be cosmetically undesirable (▶ Fig. 1.3). There are significant advantages to tooth wear in man. Occlusal wear removes or reduces the enamel fissures between cusps. The occlusal surface is not the only site on the tooth where wear occurs. During chewing, there is a component of the bite force which drives all the teeth forward in an anterior (mesial) direction. This component can be readily illustrated by a simple experiment. A steel shim is placed between any of the posterior teeth. The force to withdraw the shim is measured (also called contact point tightness). The subject then bites firmly with the teeth in maximum intercuspation. The force to withdraw the shim is again measured and will be found to be several times greater than the resting force. The experiment proves that there is a component of the bite force which acts in a mesial direction. This mesial component of the bite force causes the teeth to rub against each other and wears away the approximal (interproximal) surfaces. The approximal contact, which at first is just a point where the two curved surfaces meet, becomes a flattened area of contact between the teeth. This process reduces the area of stagnation between the teeth which is the second most common site for bacteria to accumulate and for dental caries to occur. Occlusal and interproximal tooth wear reduces the risk of caries.
As each tooth loses some approximal enamel, it becomes slightly narrower. A space would develop between the teeth were it not for the mesial component of the bite force which drives the molar and premolar teeth forward like train trucks (see Chapter 7.7.6 Tooth Displacement and Cell Rests of Malassez). This mesial drift may be insignificant in an individual whose diet consists mainly of soft foods which require little masticatory force. In an individual whose diet is course and unrefined, the drift may be as much as 6 mm in the young adult, enough extra space to accommodate the emerging third molar. So, approximal wear can provide sufficient space in the dental arch to prevent overcrowding of the teeth, a major source of malocclusion (▶ Fig. 1.4).
Fig. 1.3 The dentition of a middle-aged hunter–gatherer whose diet was unrefined. Occlusal wear has removed the occlusal fissures, and approximal wear has effectively shortened the dental arch to accommodate all the teeth without crowding. Secondary dentin has been laid down over years of gradual wear to prevent pulpal exposure and the formation of periapical abscess.
Fig. 1.4 A diagrammatic representation of the effects of tooth wear. (a) The unworn dentition has occlusal fissure and deep embrasures which trap food and bacteria. There is insufficient space for the teeth and some may be blocked out of the arch. (b) Tooth wear of a young adult has removed fissures and exposed dentin (yellow shaded). The approximal wear provides enough space to accommodate all the teeth in the arch.
Tooth wear reduces the steepness of cusps, which in turn, alters the way the jaw moves during chewing. Wear allows a more lateral, side-to-side chewing movement than the more vertical chewing movement required by unworn cusps. These changes in the pattern of jaw movements, which accompany wear, are reflected in remodeling of the articular eminence of the temporomandibular joint (see Chapter 9.1.3 Joint Stability).
The mammalian tooth is an achievement in tool design. It is a self-sharpening, nonclogging, lifelong, and multipurpose aid to food processing. Wear of the tooth surface is essential to prepare the tool for use. When paleontologists are uncertain whether a fossil is reptile or early mammal, they look for the telltale signs of tooth wear on the outer surface of the mandibular teeth and the inner surface of the maxillary teeth. They are looking for the hallmark of the mammalian tooth.
Key Notes
The teeth of mammals have evolved to provide the essential function of preparing food. A feature common to all generic types of teeth is cusps. The purpose of cusps is to provide, when worn, a suitable pattern of composite surfaces for a particular type of chewing function.
Review Questions
1. How does the structure of the mammalian tooth provide for both hardness and strength?
2. How does the structure of the mammalian tooth enable it to remain sharp and avoid clogging?
3. What are the functions of cusps in the mammalian tooth?
4. What are the benefits of tooth wear in the human dentition?
Suggested Reading
Kaifu Y, Kasai K, Townsend GC, Richards LC. Tooth wear and the “design” of the human dentition: a perspective from evolutionary medicine. Am J Phys Anthropol 2003 Suppl 37:47–61
2 Dental Hard Tissues