I plan to actively pursue my interests in the biology of trabecular bone over the next five to ten years. Specifically, I intend to address the following questions: 1) how do the geometry of trabeculae and the forces exerted on them by normal daily activities change from early post-natal life to the point of acquistion of mature skeletal form?; 2) what is the relationship between patterning of trabecular architecture and joint forces?; 3) is trabecular organization directly dictated by force transmission patterns? 4) how does elimination of normal locomotor loading, a major change in the mechanical environment of bones, alter the structure of trabecular networks? (Swartz, 1994). This approach will facilitates an integration of morphological analyses, developmental considerations, and mechanical modeling.
To address these questions, I have recently begun a multi-year project to examine the ontogeny of trabecular geometry and its relationship to the ontogeny of locomotor force production (Swartz, 1994). In this project, I will take advantage of the tremendous simplicity of trabecular structure in very small mammals (least shrews, Cryptotis parva) to document exhaustively the three-dimensional trabecular architecture of the femoral head and distal radius, and I will employ the sophisticated engineering technique of finite element analysis to document the mechanical behavior of these complex structures (primary engineering collaborator, Dr. Janet Blume, Division of Engineering, Brown University).
Along with this effort, my lab has begun to pursue two parallel questions. First, we are employing in vitro strain gauge analysis to document directly the role of trabecular tissue in load transmission by monitoring femoral strain distributions during naturalistic loading before and after removal of trabecular bone. Our work in a relatively small bodied species (New Zealand white rabbits) shows that removal of successively greater amounts of trabecular bone has no significant effect on the distribution of surface strains in either the regions immediately overlying the neck and proximal shaft trabeculae or the more distal regions that lack trabecular filling (Swartz and Mitchell, in preparation). We plan to continue these analyses over a broader range of body sizes to determine the robusticity of this phenomenon.
Second, we are beginning a phylogenetically based analysis of the nature and distribution of trabecular bone among tetrapods. At present, we have little understanding of why tetrapod bone appears as two distinctive tissue types, and propose that a phylogenetic perspective will clarify the evolutionary origin of organized trabecular tissue and provide insight into the function and evolutionary diversification of the skeleton.
This ongoing work embodies several aspects of my research philosophy. The development of this animal model for studying basic aspects of the biology of trabecular bone arises directly from my interest in the comparative biology of bone, and from an interest in understanding structure and mechanics over a range of size scales. I will employ engineering based analyses to achieve an in-depth understanding of the consequences of the pattern of natural structural design, and I will begin to couple this approach with a developmental perspective. Moreover, the insight I have gained to date and my progress in the future depend on an application of an evolutionary worldview to biomechanical phenomena; by asking why animals possess tissue with the peculiar organization of trabecular bone, and how, historically, extant mammals came to have this pattern, I believe I will gain a different understanding of the biology of bone than is possible by focusing exclusively on how this tissue functions.
