Long Bone Geometry and Strength in Adult BMP-5 Deficient Mice

Bone morphogenetic l()roteins (BMPs) playa critical role in early skeletal development. BMPs are also potential mediators of bone response to mechanical loading, but their role in later stages of bone growth and adaptation has yet to be studied. We characterized the postcranial skeletal defects in mature mice with BMP deficiency by measuring hind-limb muscle mass and long bone geometric, material, and torsional mechanical pmperties. The animals studied were 26- week-old short ear mke (n = 10) with a homozygous deletion of the BMP-5 gene and their heterozygous control litter mates (n = 15). Gend,er-related effects, which were found to be independent of genotype, were also examined. The femora of short ear mice werl~ 3% shorter than in controls and had significantly lower vallues of many cross-sectional geometric and structural strengtlll parameters (p < 0.05). No significant differences in ash content or material properties were detected. Lower femoral whole bone torsional strength was due to the smaller cross-sectional geometry (16% smaller section modulus) in the short ear mice. The diminished crosssectional geometry may be commensurate with lower levels of in vivo loading, as reflected by body mass (-8%) and quadriceps mass (-n %). While no significant gender differences were found in whole bone strength or cross-sectional geometry, males had significantly greater body mass ( + 18 %) and quadriceps mass (+15%) and lower tibio-fibular ash content (-3%). The d.ata suggest that adult female mice have a more robust skeleton than males, relative to in vivo mechanical demands. FUlrthermore, although the bones of short ear mice are smaller and weaker than in control animals, they appear to be biomechanically appropriate for the in vivo mechanical loads thalt they experience.