STRAIN IN THE FACIAL BONES OF PRIMATES

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The research proposed here will contribute to a functional understanding of the morphology of the primate facial region. The primate face is dominated by the masticatory apparatus and this research will analyze both the nature of masticatory forces and how these forces are dissipated throughout the primate face. The general approach to be followed is to characterize patterns of in vivo surface bone strain along the facial bones of an anthropoid primate, Macaca fascicularis. Then, using various sorts of anatomical and physiological data (e.g., muscle morphology, jaw- movement and/or jaw-muscle electromyographic data), patterns of internal stress within various (and supposed) structural members of the face are inferred from these strain data. The primary goal is to determine whether and how much and in which direction a given bone or portion of a bone is strained during different behaviors, and then to use these bone strain data to formulate and test hypotheses about loading patterns in various facial bones. That is, to determine how much and in what direction facial bones are bent and/or twisted and/or sheared, etc. during normal function. In vivo strain will be measured along the facial bones of Macaca fascicularis using strain gages. Rosette strain gages (three strain-gage elements with a known orientation to one another) are bonded directly to facial bones with a cyanacrylate adhesive. The macaque then bites a transducer (or chews various foods), and bone strain is monitored on an oscilloscope and recorded on an FM tape recorder. In certain instances the simultaneous electromyographic (EMG) activity of various jaw muscles are also monitored on an oscilloscope and recorded on the FM tape recorder. The magnitude and direction of the principal strains are then determined, as well as a root-mean-square analysis of the EMGs. After surface strains have been characterized along different regions of the face (e.g., the postorbital bar, the anterior root of the zygoma, the postcanine alveolar process and basal portions of the mandible), patterns of internal stress are inferred from patterns of surface strain. This is done in order to understand how macaque facial bones dissipate masticatory stress. This work will help explain why primate (including human) facial bones look the way they do. Moreover, by characterizing the mechanical environment of facial bones, these data may provide insights into how and why facial bones respond to various sorts of altered mechanical stress, and thereby contribute to a better understanding of processes related to oral bone loss, skeletal maintenance of mechanical strength, and perhaps growth modification. Explanations for the morphological diversity of the primate face will of necessity, however, involve more than a consideration of mechanical stress and biomechanical scaling. Nevertheless, this morphological diversity frequently has important structural consequences, and it is these consequences toward which this study is directed.