LONG-BONE GEOMETRY AND STRENGTH IN ADULT BMP-5 DEFICIENT MICE

Bone morphogenetic proteins (BMPs) play a critical role in early skele tal 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 sk eletal defects in mature mice with BMP deficiency by measuring hind-li mb muscle mass and long bone geometric, material, and torsional mechan ical properties, The animals studied were 26-week-old short ear mice ( n = 10) with a homozygous deletion of the BMP-5 gene and their heteroz ygous control litter mates (n = 15). Gender-related effects, which wer e found to be independent of genotype, were also examined. The femora of short ear mice were 3% shorter than in controls and had significant ly lower values of many cross-sectional geometric and structural stren gth parameters (p < 0.05). No significant differences in ash content o r material properties were detected. Lower femoral whole bone torsiona l strength was due to the smaller cross-sectional geometry (16% smalle r section modulus) in the short ear mice. The diminished cross-section al geometry may be commensurate with lower levels of in vivo loading, as reflected by body mass (-8%) and quadriceps mass (-11%). While no s ignificant gender differences were found in whole bone strength or cro ss-sectional geometry, males had significantly greater body mass(+l8%) and quadriceps mass (+15%) and lower tibio-fibular ash content (-3%), The data suggest that adult female mice have a more robust skeleton t han males, relative to in vivo mechanical demands. Furthermore, althou gh 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 that they experience.