BMD and the mechanical competence of bones in the canine skeleton

To determine the usefulness of bone mineral density (BMD) as a consistent marker of bones mechanical competence, the relationships among the BMD and the mechanical properties of bones in the canine model were quantified. The BMD and load-deformation relationships of the long bones (mid-shaft tibiae and mid-shaft femora) and the vertebrae (whole bodies and cancellous cores) from 248 beagles were studied in vitro by using dual energy X-ray absorptiometry and torsional or compressive mechanical testing, respectively. BMD explained half to two-thirds (r² = 0.54-0.64, p < 0.01) of the maximum compressive load-bearing capabilities of the vertebrae, but BMD was unable to explain the torsional mechanics of the long bones. Tibial and femoral BMD data were correlated with each other (r² = 0.66, p < 0.0001) and to a slightly lesser extent (r² = 0.5, p < 0.001) with the BMD of the vertebrae, but long bone torsional mechanics were uncorrelated with vertebral compressive mechanics. Although the mechanical competence of bone is site- specific, BMD of bones from different skeletal sites is correlated. Most importantly, BMD is a useful partial determinant of the vertebrae's compressive mechanical competence (and propensity to fracture), but it cannot predict the torsional mechanical competence of long bones.