I recently began to approach the organization of trabecular tissue by taking advantage of the natural laboratory provided by mammalian diversity. I proposed that if trabecular organization is largely determined by the mechanics of a bone's environment, then joints that experience dramatically different loading regimes ought to differ in the design of their trabecular tissue. Moreover, I suggested that the changes in loads exerted with increasing body mass would be reflected in the tissue (Swartz et al, submitted).
We studied the size and shape of individual trabeculae in a diversity of bats and non-volant mammals (4 to over 400,000 g) to test these hypotheses, and found that this comparative analysis could not support the view that trabecular morphology is determined by mechanical loads: 1) trabeculae in bat shoulder and hip joints, subject during normal activities to fundamentally different loading intensities and frequencies, are indistinguishable from one another, and 2) trabeculae vary very little in size over a five order of magnitude range of body sizes (Swartz et al, submitted).
This approach also led me to the intriguing insight that individual trabeculae, gross macroscopic structures, are virtually scale-independent. They display allometric patterning that resembles cells rather than whole organs, hence the organization of trabecular tissue is radically different in small and large animals (Swartz et al, submitted). In particular, joints of small animals posses very few, relatively large trabeculae, virtually all connected directly to the surrounding subchondral cortical bone. Moreover, these elements appear in a regular topology, and I have therefore been able to demonstrate that it is possible to homologize individual trabeculae among individuals within a species, and to some extent, among species (Swartz et al, submitted). Our demonstration that individual trabeculae can be homologized among individuals and species implies that either: 1) trabecular architecture has a high degree of genetic determination; or 2) the developmental system that produces adult structure is buffered in such a way as to produce very stereotyped positioning of individual elements despite idiosyncratic individual variation in locomotor behavior and lifelong bone loading.
I have also used a theoretical model to examine the possible mechanical and metabolic consequences of alternative scaling regimes for trabecular tissue (Swartz et al, submitted). These models reveal that increasing trabecular volume by adding many new trabeculae of a given size produces very different allometric patterns than simply increasing the size of individual elements, and that these alternative regimes have differing consequences for changes in surface area and strength with size. We find that our empirical results support the view that surface area-related phenomena may be at least as important in driving the patterning of trabecular tissue as mechanical requirements. Therefore the distinctive functional properties of trabecular tissue may arise because of the metabolic need to increase bone surface area even more than as a response to the mechanical requirement for bone tissues of greatly varying stiffness.
