Other significant sources of inspiration for this bookcame from leading experts and heads of Oral Biology Departments. I was given invaluable advice on the preclinical curriculum by Gerald Roth from the Kentucky School of Dentistry, Don Burnett of the University of British Columbia, and Barry Sessle and Richard Ten Cate from the University of Toronto.
I have attempted to acknowledge the sources I have used throughout the book, but if I have not been adequately thorough, I apologize in advance and ask readers to let the publishers knowof anyomissions, so that corrections can be made in subsequent editions
I would like to express my sincere thanks to Delia DeTurris, the acquisition editor of Thieme publishers, for her belief in this project and her quiet but invaluable advice. I would also like to thank Gaurav Prabhuzantye, the project manager of the production team at Thieme, for his care and patience in dealing with my manuscript.
Finally, I would like to thank my patient wife, Carrie, who has put up with my long periods of silence, when my mind was in my book.
Robin Wilding, BDS, Dip Pros, M Dent, PhD, MSc
About the Author
Robin Wilding grew up in Zimbabwe and obtained bachelor and master’s degrees in Dentistry, with a diploma in Prosthodontics, from the University of the Witwatersrand in Johannesburg, South Africa. He spent 7 years in private practice as a Prosthodontist in Cape Town before joining the staff of the University of Western Cape as Professor of Dental Prosthetics. An interest in oral biology led to the development of a separate department in that school, and he was eventually transferred to the position of Professor and Chairman of Oral Biology. He was awarded a PhD for his thesis on the factors which determine chewing efficiency. He completed a master's degree in Holistic Science at Schumacher College. He has a number of research publications in international journals which have attracted over 500 citations. He has lectured undergraduates, postgraduates, and practitioners in South Africa and the United Kingdom. Robin was registered in the United Kingdom as a Specialist Prosthodontist until his recent retirement. He was the course organizer for the Prosthetics course offered by the Bristol University Open Learning for Dentists.
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