Two internal-fixation plates with large differences in bending stiffness
were applied to pairs of intact femora of six adult mongrel dogs to study
the osteoporosis induced by rigid fixation. After nine and twelve months of
plate application a significant, increased amount of bone atrophy was seen
on the rigidly plated side. Biomechanical measurements of specimens from
various segments of both femoral diaphyses showed the bones to have similar
mechanical properties (as tissue), but different structural properties (as
organ). These findings suggest that the osteoporosis due to rigid plate
fixation occurs by thinning of the cortex rather than by reduction of the
mechanical properties of the osseous tissue.