LIEBERMAN, Daniel E.; PONTZER, Herman; MOMIN, Eric; DEVLIN, Maureen; POLK, John; HALLGRIMSSON, Benedikt; COOPER, David; Harvard University; Harvard University; Harvard University; Harvard University; University of Illinois, Urbana-Champaign; University of Calgary; University of Calgary; : An Experimental Test of Wolff�s �Law� in the Knee

Despite more than a century of research and interest, there have been few controlled experiments to test the hypothesis that there is a predictable functional relationship between the orientation of trabecular struts within a joint and the orientation of loads applied to the joint. We tested this hypothesis (often termed Wolff�s Law) by comparing the strut orientation in the spongiosa of the distal femur in two groups of immature guinea fowls that were exercised on treadmills at flat versus 20� inclines. In both groups, the birds were exercised 10 min/day, for 60 days at 1 m/s. Kinematic and force plate analyses found that the orientation of peak force in the knee in the parasagittal plane was approximately 16� more flexed in the incline group. Strut orientation was analyzed from micro-CT scans of the joints using a radon transform analysis that quantifies the orientation of peak trabecular density (OPTD). As predicted by Wolff�s Law, the OPTD was approximately 18� more flexed relative to the long axis of the femur in the incline versus horizontal birds (p<0.05). These results demonstrate a predictable relationship between the orientations of trabecular struts and compressive loads applied to a joint during growth. In this sense, Wolff�s Law appears to work.