Читать книгу Applied Oral Physiology - Robin Wilding - Страница 9

Оглавление

1 The Origins of Teeth

Abstract

Mammalian teeth have evolved over many millions of years in response to the particular requirements of warm-blooded animals to catch and kill or graze and browse their food. Mammals like the hyena are able to crush bones with their teeth to get at the nutritious marrow. The elephant crushes shoots and leaves. While humans are not capable of such feats, we do have the essential equipment for cracking nuts and crunching raw vegetables. The design of mammalian teeth gives them strength to resist fracture and the sharp edges to slice, grind, and grate through fibrous foods. The strength of teeth comes from the contribution of two quite different materials. One, enamel is very hard but brittle. The other, dentin is softer but very tough. The combination provides what we recognize now as a composite material such as fiberglass. The sharpness of teeth comes from the fractured edges of enamel. For these edges to be exposed, some of the enamel coating on the tooth has to become worn away. In fact, wear is a prerequisite for optimal function of mammalian teeth. This chapter sets out the origins of teeth in mammals, which offer useful insights into the function of the jaws and teeth in modern man.

Keywords: origins of teeth, evolution, teeth as tools, composite structure, tooth strength, tooth design, tooth wear mesial drift

1.1 Evolution

“Man is a fraction of the animal world. Our History is an after-thought, no more tacked on to an infinite calendar. We are not so unique as we would like to believe. And if man in a time of need seeks deeper knowledge concerning himself, then he must explore those animal horizons from which we have made our quick little march.” These words were written by Robert Ardrey in his book African Genesis. In order to understand the origins of teeth, it is worth reviewing their process of evolution.

The origins of teeth can be traced to dermal scales around the mouth which became modified to grasp food. For some animals, it was necessary to reduce the size of a piece of food in order to be able to swallow it. Smaller food particles are also easier to digest and offer rapid access to food energy essential to fuel the active metabolism of small mammals. It was the early mammals who used teeth in this way for the first time (see Appendix A.1 Mastication and Mammals). The work which a mammal’s teeth are required to perform can be compared to tool use in a workshop. There are different processes used in food preparation and the type of teeth required (see Appendix A.2 The Mechanics of Tooth Use).

1.2 The Teeth as Tools

1.2.1 Tooth Strength

The mammalian tooth is particularly well designed to be used as a tool. The surface enamel is the hardest substance produced by any living organism. The brittle nature of the enamel makes it vulnerable to cracking, but this tendency is reduced by the core of underlying dentin which provides the compressive strength and resilience that a solid enamel tooth would lack. The collagen fibers of the dentin run at right angles to the enamel prisms which further discourages the propagation of an enamel crack throughout the tooth. So, the two materials complement each other to form a composite material which is both very hard and resilient (see Appendix H.1 Cracks, Composites, and Teeth).

1.2.2 Tooth Design

Two major requirements of a chewing tool are firstly, that each tooth has sharp cutting or grinding edges and secondly, that it allows the escape of processed food away from the cutting edges to avoid clogging. The mammalian enamel is characterized by long slender prisms which run from the amelodentinal junction to the surface. This is in contrast to the reptilian enameloid which is an amorphous crystalline structure; enameloid has, as a result, a higher compressive strength than enamel. The advantage of the mammalian enamel as a tooth tool is that when it wears, instead of just a few crystals being dislodged, a whole prism fractures away leaving a freshly sharp square edge behind (▶ Fig. 1.1).

The second major requirement of a chewing tool is that it allows the shredded food to escape without build-up, so allowing the cutting surfaces to continue working against each other. The escape of processed food is achieved in the mammalian tooth by the contribution of a hard cutting edge adjacent to a softer, but tougher material which wears at a faster rate. The design feature which provides this function in a variety of mammalian teeth is the cusp. A cusp is a raised ridge or elevation of the enamel surface supported by an inner core of dentin. When the cusp wears sufficiently for the dentin to be exposed, the tool is ready for use. The softer exposed dentin core of the cusp wears at a faster rate than the enamel covering of the cusp. There is, therefore, always a depression next to the cutting edge of enamel, formed by softer dentin, into which food can escape. In this way, the tool does not clog up with cut debris. The design of the mammalian tooth has evolved in a variety of cusp formations which provide either the scissors-like function of carnassial teeth, or the grinding-like function of the molar teeth of an herbivore. Cusps are formed during the development of the tooth by the formation of folds in the tooth crown. In the unerupted sheep molar, these folds may be seen as pointed, smooth, and rounded cusps. As the tooth begins to function, surface wear causes a series of sharp grating surfaces which shred tough grass fibers (▶ Fig. 1.2). Occlusal wear is so vital to the preparation of the mammalian tooth that some animals, such as guinea pigs, start wearing their teeth in utero so as to emerge into the world ready to chew.

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?