Bisphosphonates have been reported to decrease the number of fractures in children with osteogenesis imperfecta (OI). The current study sought to further explore bisphosphonate-associated bone changes in OI by investigating the effects of alendronate (ALN) treatment on bone mechanical and material properties in osteogenesis imperfecta (oim/oim) and wild-type (+/+) mice treated with 26–73 μg kg−1 day−1 of ALN for 8 weeks via subcutaneously implanted pumps. Femoral three-point bend tests to evaluate cortical bone were combined with geometric and material density analysis. Cortical and trabecular architecture of metaphyseal bone were histomorphometrically evaluated and material density assessed by quantitative backscattered electron imaging (qBEI). For the cortical oim/oim bone, which revealed principally inferior biomechanical properties compared to +/+ bone, ALN neither improved cortical strength or any other mechanical property, nor affected cortical width (Ct.Wi.) or material density. In contrast, for the +/+ mice, bone strength was enhanced (+22%, P < 0.05) though coupled with increased brittleness (+28%, P < 0.05). This mechanical improvement was associated with an increase in Ct.Wi. (+17.3%, P = 0.02) and a reduction in heterogeneity of cortical mineralization (CaWidth, −4%, P = 0.04). In the metaphysis, ALN raised cancellous bone volume (BV/TV) significantly in oim/oim as well as in +/+ mice (+97%, P = 0.008 and +200%, P < 0.0001, respectively). This occurred without any change in either material density or trabecular thickness (Tb.Th.) in the oim/oim mice, while in the +/+ mice, material density increased slightly but significantly (+3%, P = 0.004), and Tb.Th. increased by 77% (P < 0.0001). Taken together, these results illustrate the differential effects of ALN on oim/oim vs. +/+ bone, as well as on specific skeletal sites, i.e., cortical vs. trabecular bone. ALN augmented the mechanical, geometrical, and material properties of +/+ cortical and trabecular bone, while the only observable improvement to the oim/oim bone was increased cancellous bone volume. This suggests that in this mouse model of OI, the previously demonstrated bisphosphonate-associated reduction in fractures is primarily attributable to increased metaphyseal bone mass.

Recent studies have reported that bisphosphonates reduce fracture incidence and improve bone density in children with osteogenesis imperfecta (OI). However, questions still persist concerning the effect of these drugs on bone properties such as ultrastructure and quality, particularly in the growing patient. To address these issues, the third-generation bisphosphonate alendronate was evaluated in the growing oim / oim mouse, an animal model of moderate- to-severe OI. Alendronate was administered to 6-weekold mice during a period of active growth at a dosage of 73 mg alendronate/kg/day for the first 4 weeks and 26 μg alendronate/ kg/day for the next 4 weeks. Positive treatment effects included a reduction in the number of fractures sustained by the alendronate-treated oim / oim mice compared with untreated oim / oim mice (2.1 ± 2.0 vs 3.2 ± 1.6 fractures per mouse), increased femoral metaphyseal density (0.111 ± 0.02 vs 0.034 ± 0.04 g/cm 2 ), a tendency towards reduced tibial bowing (4.0 ± 3.7 vs 6.1 ± 5.8°), and towards increased femoral diameter (1.22 ± 0.12 vs 1.15 ± 0.11 mm). Potential negative effects included a persistence of calcified cartilage in the treated oim / oim metaphyses compared with treated wildtype (+/+) (33.8 ± 11.1 vs 22.1 ± 10.2%), and significantly shorter femora compared with nontreated oim / oim mice (14.8 ± 0.67 vs 15.3 ± 0.37 mm). This preclinical study demonstrates that alendronate is effective in reducing fractures in a growing mouse model of OI, and is also an important indicator of potential positive and negative outcomes of third-generation bisphosphonate therapy in children with OI.

Osteogenesis imperfecta (OI), a heritable disease caused by molecular defects in type I collagen, is characterized by skeletal deformities and brittle bones. The heterozygous and homozygous oim mice (oim/+ and oim/oim) exhibit mild and severe OI phenotypes, respectively, serving as controlled animal models of this disease. In the current study, bone geometry, mechanics, and material properties of 1‐year‐old mice were evaluated to determine factors that influence the severity of phenotype in OI. The oim/oim mice exhibited significantly smaller body size, femur length, and moment of area compared with oim/+ and wild‐type (+/+) controls. The oim/oim femur mechanical properties of failure torque and stiffness were 40% and 30%, respectively, of the +/+ values, and 53% and 36% of the oim/+ values. Collagen content was reduced by 20% in the oim/oim compared with +/+ bone and tended to be intermediate to these values for the oim/+. Mineral content was not significantly different between the oim/oim and +/+ bones. However, the oim/oim ash content was significantly reduced compared with that of the oim/+. Mineral carbonate content was reduced by 23% in the oim/oim bone compared with controls. Mineral crystallinity was reduced in the oim/oim and oim/+ bone compared with controls. Overall, for the majority of parameters examined (geometrical, 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 provides 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.

The homozygous oim/oim mouse, a model of moderate-to-severe human osteogenesis imperfecta, contains a G-nucleotide deletion in the Cola-2 gene (the murine pro alpha(I) collagen gene) that results in accumulation of alpha1(I) homotrimer collagen. Although these mice have a distinctive phenotype that includes multiple fractures and deformities, genotyping is necessary to distinguish them from their wildtype (+/+) and heterozygote (oim/+) littermates. In this study, the dye primer and dye terminator chemistry methods, in combination with automated direct DNA sequencing, were compared for accuracy and ease in genotyping. A total of 82 mice from 14 litters were bred and genotyped; this resulted in 18 +/+, 35 oim/+, and 29 oim/oim mice. The dye primer and dye terminator chemistry methods worked equally well for identification of the deletion mutation and thus the genotype of all of the mice. However, the dye terminator method was found to be superior on the basis of the reduced amount of sample handling and reduced quantity of reagent required.