Mechanical adaptation of cortical and trabecular bone morphology in the mammalian mandible

The morphology of jaws and teeth can tell us much about how an animal breaks down its food. Herbivorous animals such as cows and sheep have, for example, relatively flat-topped teeth and jaw joints that allow horizontal movements of the lower jaw so that they can grind their food. Meat-eating animals such as cats, on the other hand, have blade-like teeth and jaw joints that allow only vertical movements of the lower jaw. These different ways of breaking down food cause very different forces acting on the teeth and jaw bones, which raise several fundamental questions: How are chewing forces transferred from the teeth through the jaws? What role does tooth shape play in the distribution of these forces? How does the bone in the jaws adapt to these different forces? However, answering these questions is not an easy task, since important information is currently missing. Firstly, little is known about the variation of the internal morphology (geometry) of the jaws between mammalian species, since a comprehensive study of this morphology depends on the availability of high-resolution computed tomography scans and these have become available only recently. Secondly, the strains in the bone resulting from different chewing forces are not well known, and can only be measured on the bone surface. We will address these questions by studying the internal morphology of the lower jaw in mammal species with very different and well-known feeding adaptations. We will apply a combination of state-of-the-art image processing based on high-resolution data, functional computer modelling and mechanical experiments. We will focus on a part of the lower jaw that is known to experience high stresses and strains during chewing, the bone beneath the cheek teeth. The overarching aim is to understand the link between internal bone morphology, tooth morphology and chewing forces. Our group and close collaborators have a very strong research record in the modelling of masticatory function and the study of mechanical adaptations in the skull. We have already provided new insights into the form and function of the skull in mammals (primates including humans) and reptiles (lizards), and in the process developed new methodological approaches for this type of work. Plus we have already examined the mechanical significance of the different shapes of modern human and Neanderthal mandibles. The proposed project will continue and combine these successful lines of research to elucidate the relationships between masticatory loads and the internal and external architecture of the mandible in mammal species with very different dietary habits. With this vital underpinning knowledge, we then plan to extend this work in subsequent proposals to consider clinical and dental applications, especially ageing of the masticatory apparatus, tooth loss and replacement by the latest generation of dental implants which attach directly to the bone. There will be many beneficiaries of our work, including researchers from diverse disciplines (such as, comparative anatomy, functional morphology, palaeontology, dentistry), the UK science base in general (through training of young scientists in an interdisciplinary framework, attraction of overseas students and collaborators), the wider public (public engagement, media interest) and clinicians (dentists and orthodontists).

A significant part of mammalian diversity has arisen through variation in dietary habit and masticatory function. The large differences in mammalian skull and tooth morphology reflect these different feeding adaptations, but the fundamental relationships between bite force and bone and tooth morphology remain largely unexplained. For example: how are chewing forces transferred from the teeth through the jaws? What role does tooth morphology play in the distribution of these forces? How does the bone in the jaws adapt to different forces? We will address these questions by examining the morphology of the mandibular molar region in mammal species with very different and well-known feeding adaptations. The overarching aim is to understand the link between cortical and trabecular bone morphology, tooth morphology and masticatory forces. This will be achieved by in vitro experiments and the use of state-of-the-art image processing and functional computer modelling. Our team has a strong research record in the application of these techniques to the study of functional adaptations in the skull. Recently, we have combined advanced 3D image processing and finite element analysis in novel ways which allow us to modify model geometry and study the mechanical significance of individual morphological variables systematically. In addition, we will use models with modified geometries together with novel adaptive remodelling algorithms to simulate the adaptation of the bone to different loading conditions. This approach allows the study of form-function relationships even in complex structures such as cancellous bone tissue. Our existing BBSRC funded projects focus on large scale morphological adaptations of the skull to masticatory function. This new project builds on their success, and focuses on a smaller anatomical area that is known to experience high strains during masticatory function so that mechanical adaptations of the bone can be studied in detail.

Who are the beneficiaries of this research and how will they benefit? UK LIFE SCIENCES: The BBSRC stresses the need for interdisciplinary approaches to the 'big' questions in biology. As an interdisciplinary team we promote this between colleagues and collaborators, and in the training environment provided for young scientists. ACADEMIC COMMUNITY: Our work is of interest to many disciplines including functional and evolutionary anatomy, palaeontology and developmental biology, here and overseas. It offers new insights into skull structure, function, and evolution. We have introduced novel methodologies (combined MDA/FEA; FEA and adaptive remodelling software; iterative modelling) that have significantly advanced the field and the new project will further refine some of these approaches. THE WIDER COMMUNITY: Animal structure and function interests the public and media and, as such, is a good mechanism for Public Engagement with Science and for fostering interest in science amongst young people. Ultimately this benefits the UK in the development of scientific literacy. CLINICAL RESEARCH: Our results to date have implications for craniofacial medicine and dentistry, and our modelling approaches have wider application (e.g. bone remodelling simulations for examining growth changes and ageing). Fagan is a Royal Society Industry Fellow, working with Smith and Nephew's Research Centre on the modelling of bone, and has many other clinical and industry partnerships. PUBLIC HEALTHCARE: Due to the current demographic changes in industrialised countries age-related tooth and bone loss is becoming an increasing problem for healthcare providers. In addition, the resorption of tooth roots and alveolar bone creates obstacles for orthodontic treatments and the resorption around dental implants is a major problem in dentistry. Our research will provide new insights in the role of mechanical factors on the internal structure of the mandible and thus lead to a better understanding of how healthy internal bone structure can be maintained, to support and secure teeth and dental implants. What will be done to ensure that they have the opportunity to benefit from this research? DISSEMINATION OF RESULTS - ACADEMIC: we publish in high impact journals (PNAS, Proc. Roy. Soc. [see publications and track record]) and speak at international conferences/ seminars/ workshops in the UK and overseas. We also maintain web pages and are planning to extend these to include a database of computer models and other useful data [see Impact Plan]. DISSEMINATION OF RESULTS - WIDER PARTICIPATION: we will continue to engage in public-academic dialogue with schools, open days, festivals, museum talks, and by working with the media offices of our relevant institutions and funding bodies (e.g. BBSRC Business October 2008, pg 24). The University of Hull has public engagement programmes in place. We have research group web pages, but are requesting funds to develop a comprehensive interactive site for the use of colleagues (see above) and the wider public.