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An anthropological research study is using FEA to simulate the chewing biomechanics of the Macaque, a type of monkey, in order to better understand the evolution and functional anatomy of the primate skull. (Photograph courtesy of William McComas, USC Rossier School of Education.) |
Fossils of primates and early humans exhibit great diversity in the size and shape of the jaw, teeth and facial skeleton. Anthropologists theorize that these different skull forms were evolutionary adaptations to chewing different types of food. David S. Strait, Ph.D., anthropologist and assistant professor at the New York College of Osteopathic Medicine of the New York Institute of Technology, is testing these theories by simulating the chewing biomechanics of living primates with ALGOR finite element analysis (FEA) software. Strait's research may help to explain why there is such diversity among living primates and lay the groundwork for future studies of extinct early humans.
Applying Computer Simulation to Physical Anthropology
Strait serves as project director for a research team that received a National Science Foundation (NSF) grant for a two-year $165,000 study to examine the functional anatomy and evolution of the facial skeleton in primates. "Primates, including humans and their extinct relatives, are characterized by astounding diversity in diet," said Strait. "Primates consume a wide range of food items whose material properties vary greatly. It is thought that some aspects of the primate facial skeleton are designed to resist the loads imposed by chewing. However, the architecture of the face is so complex that it cannot be modeled using simple biomechanical methods. As a result, few studies have been able to test which skeletal features are, in fact, chewing adaptations. We are testing hypotheses about chewing biomechanics and facial function using ALGOR FEA software."
Strait added, "Changes in diet and the use of the teeth, jaws and facial skeleton to acquire and process food have played a major role in shaping the evolutionary histories of many vertebrate groups including humans in particular. An ability to exploit a diet of very hard foods may have allowed some of these early humans to be extremely successful. For example, later members of one extinct group, the Australopithecines, developed massive faces, gigantic teeth and a huge jaw bone. A popular hypothesis is that these modifications in the facial skeleton were adaptations to resist the stress imposed by chewing very hard foods such as nuts or seeds."
Anthropologist David S. Strait of the New York College of Osteopathic Medicine is using ALGOR software to create finite element models of monkey skulls and perform stress analyses to simulate chewing biomechanics. He plans to apply this research to future investigation of extinct early humans. (Photograph courtesy of Chris Sidor.) |
Strait began working with FEA because another member of the research team, Brian G. Richmond, Ph.D., anthropologist and assistant professor at The George Washington University in Washington, D.C., had used ALGOR FEA as part of his Ph.D. dissertation. "He was studying the biomechanical consequences of straight versus curved finger bones in primates," said Strait. Richmond made FEA models of a straight finger bone and a curved finger bone, applied the same set of forces and examined the bone strain and stress. "I realized this is a very powerful technique," said Strait. "One thing that was so impressive to me was that you could use FEA as a virtual experiment to examine the mechanical consequences of having bones of different shapes. I thought, 'We've got to take this and apply it to questions about the evolution of the skull in primates.'"
Why Study Chewing?
According to Strait, chewing presents an interesting engineering problem. "When you chew, forces are being passed up into your face and down into your lower jaw," said Strait. "The bite force is about the same in the upper and lower jaw. The mandible, which is a U-shaped bone, acts essentially as a bent beam. In some species, the mandible becomes very thick and, in other species, is thinner. But, the bones of the face, which are thin sheets of curved bone, don't ever become thicker in primate evolution. Instead, they change the way that they are positioned relative to each other. Hence, different parts of the chewing system have adapted to solve the same stress-resistance problem in different ways."
One challenge facing anthropologists is that a lot of scientific information, such as relative muscle forces and material properties of connective tissues, is unavailable for extinct species. "With fossils, we only have information about the shape of a bone, the muscles aren't preserved," said Strait. "Therefore, we have no direct data about the loads or constraints that should be applied to an FEA model." Another challenge concerns how to validate a model of an extinct species. "None of them are alive to be observed," said Strait, "so I can't do any experimental validation studies."
That is why Strait decided to simulate living primates. "Engineers know that FEA can be a garbage-in, garbage-out scenario. You need to have some type of validation study to make sure that your model is correct. We, as anthropologists, want to do the same thing. First, we have to develop models of living primates and see if we can validate them. If we can, then we'll have an argument that we've demonstrated this can work on a living species and, because of that, we're going to use the same methods on extinct species."
Simulating Primate Chewing
A male Macaque monkey skull was used as the basis for computer-aided modeling and analysis. CT scan information of the skull was used to generate a CAD solid model in SolidWorks (inset). Photograph courtesy of the Smithsonian Institution's National Museum of Natural History. |
As a pilot study, Strait developed an FEA model of the skull of a male Macaque, a type of monkey (Macaca fascicularis), which is widely used in biological and medical research. "The reason why we chose the Macaque is that they have been the subject of a lot of experimental work on facial bone strain in the past," said Strait. "There was already a body of knowledge about how the face of this type of monkey responded to chewing loads. We're using our work on the Macaque model as the vehicle for testing different methodologies. When we find the methodologies that we think are going to give us the best validation data, then we'll have one set of methods to apply to different models of different species in subsequent studies."
Computed tomography (CT) scans of a Macaque skull from the Smithsonian Institution's National Museum of Natural History were digitized and input into the SolidWorks CAD solid modeler to create a three-dimensional solid model of the skull geometry. Strait then used ALGOR's InCAD technology for direct CAD/CAE data exchange between SolidWorks and FEA. Next, he used automatic mesh generation capabilities to create a finite element mesh. "Due to the detailed, irregular, curved shape of the skull geometry, obtaining a satisfactory mesh was quite challenging," said Strait. "ALGOR's technical support staff was very helpful. In particular, they taught me about automatic mesh refinement points, which create a finer mesh in areas of complex geometry or anticipated peak results and a coarser mesh for the rest of the model."
Strait defined custom isotropic material properties to represent the bones of the skull. A variety of nodal and surface forces were used to simulate the muscle groups that act on the skull during chewing. The upper left second molar was fully fixed to represent the point of biting. A static stress with linear material models analysis was performed, which solved for displacements and strains in the model.
An ALGOR static stress with linear material models analysis was performed on this finite element model of the monkey skull to simulate the maximum biting force during chewing. This display shows displacement magnitude results. |
"Our model incorporates data about the material properties of facial bone, and the physiological cross-sectional area of the chewing muscles," said Strait. "In addition, we have obtained actual bone strain data from previous experiments. The experimental results are used to validate the FEA model. Thus far, there is a strong correlation between our data and the FEA results."
Future Work
In the current study, Strait plans to also model another living primate species, a Capuchin (Cebuss capacinus), which is a monkey from South America. The intent is to compare and contrast the Macaque and Capuchin models and demonstrate that they can be validated with experimental data.
In the future, Strait plans to extend the study by modeling several additional species of living primates. Beyond that lies his goal of modeling extinct primate skulls. "What I really want to do is build an FEA model of one of the extinct early humans," he said, "and conduct tests to confirm or reject hypotheses about skeletal traits as chewing adaptations. Computer simulation is the best way to perform this type of study."
Strait's research may also have potential health science applications. "Modeling skull biomechanics could give a better understanding of stresses and strains in the bones of the face and skull," said Strait, "which might then be used, for example, to design safer protective gear, such as motorcycle helmets or car restraints, or to improve surgical techniques."
David S. Strait, Ph.D., earned a Bachelor of Arts degree in Anthropology from Harvard College. He then received masters and doctoral degrees in Anthropology at the State University of New York at Stony Brook. He completed his postdoctoral fellowship at the Center for the Advanced Study of Human Paleobiology at George Washington University. Additionally, he was a research associate in the Human Origins Program at the Smithsonian Institution's National Museum of Natural History. Currently, he is an assistant professor in the Department of Anatomy at the New York College of Osteopathic Medicine, part of the New York Institute of Technology. Dr. Strait's research team includes anthropologists and craniofacial biologists from The George Washington University, Stony Brook University, Baylor College of Dentistry and the University of Colorado.
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