Mechanical function of the primate craniofacial skeleton

This project aims to build computer models that allow us to emulate and experiment with growth of the facial skeleton in primates, chosen because they are anatomically similar to and therefore informative regarding human facial growth. Although genetic systems regulate the early part of post natal craniofacial growth, later development is strongly regulated by mechanical loading. The craniofacial skeleton responds to its immediate mechanical environment by passively growing where bones meet (the sutures) in response to the expansion of the soft tissues (e.g. brain, tongue, muscles). It is also resculpted by modelling and remodelling mechanisms that add and remove bone from surfaces, being modulated by the mechanical milieu. It is important we understand these mechanically regulated processes because they are essential not only in normal growth but also when things go wrong. Further we need to know how the features of craniofacial form that characterise and vary between related species come about. Commonly our teeth do not fit well to our mouths, yet in the historical past this was not the case; what has happened? The answer likely lies in the change to softer diets that alter the mechanical loading of the growing face and subsequent growth. More rarely sutures may fuse too early or skull cartilages may not grow adequately because of inherited conditions. The subsequent growth of the skull has to adapt to the altered starting conditions and optimise function. The mechanical signals are key in this. Understanding mechanical regulation should lead to better prediction of normal and altered growth and understanding of which features of the facial skeleton are inherited and which adapted to local mechanics. This is important in resolving arguments about the relationships among fossil and living species. One approach to understanding the mechanical regulation of the growth of the face is to carry out experiments in which animals are operated on to cut muscles, move teeth, excise structures etc and observe the outcomes. This has been a very profitable line of research especially in primates, our nearest relatives but now it is ethically and economically difficult to carry on this work in the UK. Our current best sources of information in these areas come from continuing animal studies outside Europe, especially in the USA. Animal experiments are very useful but they are difficult to properly control and lengthy and time consuming to carry out and interpret. They could be replaced if we had a good computer model of facial, and eventually, skull growth. While such a full model is long way off we plan in this project to emulate the mechanical regulation of facial bone adaptation that will allow prediction of the consequences of altered loading. The work will build on computer models that we have developed over the past three years employing engineering techniques for predicting how loads are distributed (finite elements analysis / FEA). We will apply them to two related old world monkey groups, macaques and mangabeys, with similar faces at birth that develop very different features of adult form. Thus macaques develop air sinuses in the maxilla but mangabeys do not, instead they develop deep excavations of the external aspect of the face, the maxillary fossae. We will extend our models by simulating what we know of how bone adapts so that initial loading is used to drive simulated bone deposition or resorption. We will then carry out a series of experiments with our computer models to test ideas about the development of features of facial form and in so doing work to improve our models and our understanding. In this way we will advance knowledge of how the face grows and develop technologies that will underpin future, more complete models of craniofacial development that will eventually underpin predictions of growth with applicability in biology, medicine and studies of human and primate origins.

We propose to extend our work on primate craniofacial mechanics to the prediction and simulation of aspects of growth. We will model mechanical regulation of bony development first by attempting to explain the development of characteristic remodelling fields and then of anatomical features of adult form, not present at birth such as bony thickenings (e.g. pillars) and structures (e.g. the sinuses, fossae processes and prominences). We will capitalise on our novel software tool developments, extending and validating these to address these issues. Multibody dynamics analysis (MDA) will be used to estimate the external forces applied by biorealistic muscle models during normal skull loading. Finite elements analysis (FEA) and a unique adaptive FEA approach from earlier work will be validated against experimental data and incorporated into our present software (Vox-FE). This will be used to model the skulls, predicting their response to changing patterns of stress/strain that result from enlargement of the brain and/or sense organs, variations in sutures and jaw muscles, and the developing dentition. These predicted outcomes will be compared with observed features and discrepancies will guide reformulation of our models and the underlying algorithms for adaptive responses. We will extend our knowledge of craniofacial ontogeny in primates and progress towards the eventual goal of a detailed in silico model that will incorporate the passive expansion of sutures, active growth of cartilages, patterning of bony responsiveness to mechanical conditions and systemic and intrinsic biochemical regulation. To support this work we have access to cadaveric and dried skulls, 3D microCT data and to a supercomputer for highly detailed static and adaptive skull remodelling studies. The work will be carried out by a multidisciplinary team working in state of the art engineering, anatomical and computational facilities and building on years of prior experience and development.