Teeth Resist Compression and Tension

x ray of a human mouth showing both jaws of teeth

Biological Strategy

Animals

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Manage Tension

When a living system is under tension, it means there is a force pulling on it, like a person pulling on a rope tied to a horse. When applied to a living system, unless the system is completely rigid, the result is that it gets stretched. If stretching exceeds the strength of the living system’s material, it can damage it. Living systems manage tension using materials that are flexible and stretchable enough to survive most tension that occurs in their environment. The ocean’s intertidal zone offers a good example. The waves and incoming and outgoing tides put tension on soft-bodied organisms. Mussels resist tension with flexible threads that hold them onto rocks; in contrast, large algae have stretchy fronds.

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Manage Compression

When a living system is under compression, there is a force pushing on it, like a chair with a person sitting on it. When evenly applied to all sides of a living system, compression results in decreased volume. When applied on two sides, it results in deformation, such as when pushing on two sides of a balloon. This deformation can be temporary or permanent. Because living systems must retain their most efficient form, they must ensure that any deformation is temporary. Managing compression also provides an opportunity to lessen the effects of other forces. Living systems have strategies to help prevent compression or recover from it, while maintaining function. For example, African elephant adults weigh from 4,700 to 6,048 kilograms. Because they must hold all of that weight on their four feet, the tissues of their feet have features that enable compression to absorb and distribute forces.

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Manage Mechanical Wear

A living system is subject to mechanical wear when two parts rub against each other or when the living system comes in contact with abrasive components in its environment, such as sand or coral. Some abrasive components are a constant force, such as finger joints moving, while others occur infrequently, such as a sand storm moving across a desert. Living systems protect from mechanical wear using strategies appropriate to the level and frequency of the source, such as having abrasion-resistant surfaces, replaceable parts, or lubricants. For example, human joints like shoulders and knees move against each other all day, every day. To protect from mechanical wear, a lubricant reduces friction between the cartilage and the joint.

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Animals

Kingdom Animalia (“having breath or soul”): Mammals, reptiles, sponges, corals, insects, birds

This kingdom captures a staggering variety of life—from the smallest coral polyp to the largest elephant. Although animal body shapes and life histories vary, there are a few defining characteristics. Animals are heterotrophs, meaning they gain energy from eating other organisms, and have developed from single-celled ancestors to highly diversified and coordinated multicellular forms. Unlike plant and fungi cells that have rigid cell walls, animal cells are bound by more flexible combinations of proteins. Most animals have cells organized into tissues and are able to move within their environment.

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The teeth of many animals are stiff and resistant to compression due to an outer layer of enamel, but have some resistance to tension due to an inner layer of dentine.

“Consider our teeth (Waters [1980] has a good discussion of their properties), crucial to feeding ourselves, subject to stresses and abrasions over long periods, and…none too reliable. Enamel, the outer layer on teeth (fig. 16.5a), is very stiff and resistant to compression–handy for biting and chewing–but it takes tension poorly and is exceedingly brittle–stiffness and toughness seem in practice as antithetical here as in the various commercial steels. Dentine, the next layer in (and the bulk of the hard stuff in teeth), is less stiff than enamel but not quite so brittle. An off-axis load might safely develop some substantial tension in the dentine–harder dentine would most likely lead to increased functional fragility. Perhaps there’s a general principle involved–limit the hardest material to small pieces at the site of wear, as we do in making carbide-tipped drills and saws. Teeth crack without enormous provocation–their poor behavior in actual fracture is an unpleasant converse of the hardness needed for trituration. Trade-offs seem unavoidable, although it’s no easy thing at this stage of our understanding to give them proper quantitative expression.” (Vogel 2003:332-333)

Last Updated August 18, 2016

References

Book

Comparative Biomechanics: Life's Physical World, Second Edition

Princeton University Press | 17/06/2013 | Steven Vogel

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