The material basis for reduced mechanical properties in oim mice bones

Osteogenesis imperfecta (OI), a heritable disease caused by molecular defec ts in type I collagen, is characterized by skeletal deformities and brittle bones. The heterozygous and homozygous oim mice (oim/+ and oim/oim) exhibi t mild and severe OI phenotypes, respectively, serving as controlled animal models of this disease. In the current study, bone geometry, mechanics, an d material properties of 1-year-old mice were evaluated to determine factor s that influence the severity of phenotype in OI, The oim/oim mice exhibite d significantly smaller body size, femur length, and moment of area compare d with oim/+ and wild-type (+/+) controls. The oim/oim femur mechanical pro perties of failure torque and stiffness were 40% and 30%, respectively, of the +/+ values, and 53% and 36% of the oim/+ values. Collagen content was r educed by 20% in the oim/oim compared with +/+ bone and tended to be interm ediate to these values for the oim/+. Mineral content was not significantly different between the, oim/oim and +/+ bones. However, the oim/oim ash con tent was significantly reduced compared with that of the oim/+. Mineral car bonate content was reduced by 23% in the oim/oim bone compared with control s. Mineral crystallinity was reduced in the oim/oim and oim/+ bone compared with controls, Overall, for the majority of parameters examined (geometric al, mechanical, and material), the oim/+ values were intermediate to those of the oim/oim and +/+, a finding that parallels the phenotypes of the mice . This pro,ides evidence that specific material properties, such as mineral crystallinity and collagen content, are indicative and possibly predictive of bone fragility in this mouse model, and by analogy in human OI.