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First published online December 14, 2005
Journal of Experimental Biology 209, 57-65 (2006)
Published by The Company of Biologists 2006
doi: 10.1242/jeb.01971

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Trabecular bone in the bird knee responds with high sensitivity to changes in load orientation

H. Pontzer^1,*, D. E. Lieberman¹, E. Momin¹, M. J. Devlin¹, J. D. Polk², B. Hallgrímsson³ and D. M. L. Cooper⁴

¹ Department of Anthropology, Harvard University, Cambridge MA, 02138, USA
² Department of Anthropology, University of Illinois, Urbana-Champaign, IL, USA
³ Department of Cell Biology and Anatomy and the Bone and Joint Institute
⁴ Departments of Archaeology and Medical Science, University of Calgary, Alberta, Canada

^* Author for correspondence (e-mail: pontzer{at}fas.harvard.edu)

Accepted 4 November 2005

Wolff's law of trajectorial orientation proposes that trabecular struts align with the orientation of dominant compressive loads within a joint. Although widely considered in skeletal biology, Wolff's law has never been experimentally tested while controlling for ontogenetic stage, activity level, and species differences, all factors that may affect trabecular bone growth. Here we report an experimental test of Wolff's law using a within-species design in age-matched subjects experiencing physiologically normal levels of bone strain. Two age-matched groups of juvenile guinea fowl Numida meleagris ran on a treadmill set at either 0° (Level group) or 20° (Incline group), for 10 min per day over a 45-day treatment period. Birds running on the 20° inclined treadmill used more-flexed knees than those in the Level group at midstance (the point of peak ground reaction force). This difference in joint posture enabled us to test the sensitivity of trabecular alignment to altered load orientation in the knee. Using a new radon transform-based method for measuring trabecular orientation, our analysis shows that the fine trabecular bone in the distal femur has a high degree of correspondence between changes in joint angle and trabecular orientation. The sensitivity of this response supports the prediction that trabecular bone adapts dynamically to the orientation of peak compressive forces.

Key words: Wolff's law, trabecular bone, skeletal biology, biomechanics, knee joint, guinea fowl