Lizard's lack of ligament sheds light on jaw mechanics

Using a moving 3D computer model based on the skull and teeth of a New Zealand reptile called tuatara, a research team from the University of Hull, University College London, and the Hull York Medical School has discovered how damage to dental implants and jaw joints may be prevented by sophisticated interplay between our jaws, muscles, and brain.

This research, which was funded by the Biotechnology and Biological Sciences Research Council (BBSRC), will appear in a future print edition of the Journal of Biomechanics (August 27, 2010).

The tuatara is a lizardlike reptile that has iconic status in its homeland of New Zealand because its ancestors were widespread at the time of the dinosaurs. Unlike mammals and crocodiles which have teeth held in sockets by a flexible ligament, tuataras have teeth that are fused to their mandible.

The New Zealand tuatara. Image courtesy of Brian Gratwicke.

The New Zealand tuatara. Image courtesy of Brian Gratwicke.

In other words, the researchers explain, they have no ligament, much like modern dental implants.

"Humans and many other animals prevent damage to their teeth and jaws when eating because the ligament that holds each tooth in place also feeds back to the brain to warn against biting too hard," said Neil Curtis, Ph.D., from the University of Hull and a BBSRC postdoctoral fellow, in a press release.

In the sugar-rich Western world, many people end up losing their teeth and have to live with dentures or dental implants instead, added Marc Jones, Ph.D., from University College London and also a BBSRC postdoctoral fellow. "They've also lost the periodontal ligament that would attach their teeth, so we wanted to know how their brains can tell what's going on when they are eating," he said.

The team created a 3D computer model of the skull of the tuatara to investigate the feedback that occurs between the jaw joints and muscles in a creature that lacks periodontal ligaments.

"Tuataras live happily for over 60 years in the wild without replacing their teeth because they have the ability to unconsciously measure the forces in their jaw joint and adjust the strength of the jaw muscle contractions accordingly," Curtis said.

Although this explains why tuataras and people with false teeth manage not to break their teeth and don't end up with jaw joint disorders, it is still clear that having a periodontal ligament is very useful, in particular for fine tuning chewing movements. This may explain why it has evolved independently in the ancestors of mammals, crocodiles, dinosaurs, and even some fish.

Anecdotal evidence suggests that people with implants and dentures may make food choices related to their lack of periodontal ligament. However, tuataras pursue a broad diet on the islands where they live including beetles, spiders, snails, frogs and occasionally young seabirds.

"To support the extension of health and well-being into old age, it is vital that we appreciate how we as human beings have developed our extraordinary ability to adapt to adverse situations," said Douglas Kell, BBSRC chief executive. "This work allows us to understand some of the complexities of the feedback and responses occurring in healthy human bodies and brains. It is impossible in evolution to predict future innovations such as dental implants, and yet this research indicates a level of redundancy in our biology that opens opportunities to support long term health and well-being."

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