MECHANICAL AND GEOMETRIC CHANGES IN THE GROWING FEMORA OF BMP-5 DEFICIENT MICE

We examined the growth-related changes in femoral geometry and torsion al strength in BMP-5 deficient short-ear mice over a 22-week time inte rval (''long-term'' changes). Four groups of female mice (n = 6 per gr oup) were examined: short-ear animals and their heterozygous control l ittermates at 4 and 26 weeks of age. In agreement with findings previo usly observed in a mixed-gender group of adult mice (26 weeks), the fe mora of short-ear animals were significantly smaller in length and cro ss section at both ages, The magnitudes of the differences between gen otypes were comparable at each age, indicating that the overall rates of appositional and endochondral growth were similar for both genotype s over the 22-week period, In the adult animals, short-ear femora were 27 +/- 7% weaker in torsional strength due to their smaller cross-sec tional geometry, However, bone strength in adult short-ear mice appear ed to be adequate for animal size: No significant difference was detec ted in maximum femoral torque when normalized by body mass, In 4-week old animals, BMP-5 deficiency was associated with a 27 +/- 6% lower bo dy mass, but the torsional strength of the femur was not significantly different from that of controls. Cross-sectional geometry was smaller in 4-week old short-ear mice, but the apparent bone material ultimate shear stress was elevated by 33 +/- 10%, thereby resulting in a whole bone torsional strength equivalent to that of the larger control mice , While the data suggest a higher material strength in the 4-week-old short-ear animals, no significant difference in the level of bone mine ralization was detectable between genotypes at either age.