Mechanical Significance of Femoral Head Trabecular Bone Structure in Loris and Galago Evaluated Using Micromechanical Finite Element Models Timothy M. Ryan 1 * and Bert van Rietbergen 2 1 Department of Anthropology and Center for Quantitative Imaging, The Pennsylvania State University, University Park, Pennsylvania 16802 2 Department of Biomedical Engineering, Eindhoven University of Technology, 5600 MB Eindhoven, The Netherlands KEY WORDS finite element analysis; femur; trabecular bone; biomechanics ABSTRACT Work on the interspecific and intraspe- cific variation of trabecular bone in the proximal femur of primates demonstrates important architectural variation between animals with different locomotor behaviors. This variation is thought to be related to the processes of bone adaptation whereby bone structure is optimized to the mechanical environment. Micromechanical finite element models were created for the proximal femur of the leaping Galago senegalensis and the climbing and quadrupedal Loris tardigradus by converting bone voxels from high- resolution X-ray computed tomography scans of the fem- oral head to eight-noded brick elements. The resulting models had approximately 1.8 million elements each. Loading conditions representing takeoff phase of a leap and more generalized load orientations were applied to the models, and the models were solved using the iterative “row-by-row” matrix-vector multiplication algorithm. The principal strain and Von Mises stress results for the leap- ing model were similar for both species at each load ori- entation. Similar hip joint reaction forces in the range of 4.9  to 12  body weight were calculated for both species under each loading condition, but the hip reaction values estimated for Loris were higher than predicted based on locomotor behavior. These results suggest that functional adaptation to hip joint loading may not fully explain the differences in femoral head trabecular bone structure in Galago and Loris. The finite element method represents a unique and useful tool for analyzing the functional adap- tation of trabecular bone in a diversity of animals and for reconstructing locomotor behavior in extinct taxa. Am J Phys Anthropol 126:82–96, 2005. © 2004 Wiley-Liss, Inc. Recent work on the interspecific variation of tra- becular bone in the proximal femur of extant pri- mates has revealed important architectural differ- ences between taxa with divergent locomotor behaviors (Fajardo and Mu ¨ ller, 2001; MacLatchy and Mu ¨ ller, 2002; Ryan, 2001; Ryan and Ketcham, 2002b; but see Viola, 2002). It is generally thought that, just as with external bone anatomy and diaph- yseal cross-sectional geometry (Anemone, 1990; Connour et al., 2000; Demes and Jungers, 1989), this variation reflects differences in joint loading and limb use during various activities. It is not known, however, whether these structural differ- ences can be fully explained as the result of epige- netic or evolutionary functional adaptation, or whether other nonstructural causes such as genetic, nutritional, hormonal, or gender factors may play a role as well. Experimental and modeling work over the past few years has further developed our understanding of the mechanisms of bone modeling (Huiskes et al., 2000; Sommerfeldt and Rubin, 2001). It is now rel- atively well-established that the local bone modeling process is governed by mechanical feedback where osteoblasts add bone to heavily loaded sites, and osteoclasts remove bone from sites with lower tissue loadings (Hsieh et al., 2001; Huiskes et al., 2000). This response is facilitated by the process of mech- anotransduction, involving the lacuno-canilicular porosity of the bone matrix (Burger and Klein-Nu- lend, 1999). A reasonable hypothesis that follows from these observations, assuming general similar- ity across mechanosensory cells, is that stresses and strains at the trabecular bone tissue level should be distributed rather evenly over the trabecular archi- tecture within a specific mechanically homogeneous Grant sponsor: United States National Science Foundation; Grant number: BCS-9908847; Grant sponsor: Leakey Foundation; Grant sponsor: Texas Advanced Computing Center, University of Texas at Austin. *Correspondence to: Dr. Timothy M. Ryan, Department of Anthro- pology, 409 Carpenter Building, The Pennsylvania State University, University Park, PA 16802. E-mail: tmr21@psu.edu Received 1 May 2003; accepted 9 September 2003. DOI 10.1002/ajpa.10414 Published online 12 May 2004 in Wiley InterScience (www. interscience.wiley.com). AMERICAN JOURNAL OF PHYSICAL ANTHROPOLOGY 126:82–96 (2005) © 2004 WILEY-LISS, INC.