Bone fragility persists in long-term bisphosphonate-treated oim mice despite sex-specific changes in bone quality

Osteogenesis imperfecta (OI) is a group of genetic connective tissue disorders characterized by poor bone matrix and increased bone fragility. Bisphosphonates, a group of antiresorptive drugs FDA-approved for the treatment of osteoporosis, are commonly used to treat OI, but their effects on fracture risk and long-term efficacy in growing children remain unclear. Here, we use the oim mouse model of severe OI to investigate the long-term impact of alendronate, a nitrogen-containing bisphosphonate, on bone fragility and examine sex-specific responses through (i) traditional fracture mechanics testing paired with full-field digital image correlation strain maps around the propagating crack, (ii) porosity measurements using synchrotron microtomography, (iii) collagen organization assessment via second harmonic generation microscopy, and (iv) tissue composition and crosslinking content by Raman spectroscopy and molecular fluorescence. Male and female oim/oim mice exhibit distinct compositional and structural abnormalities that influence disease severity and response to therapy, ultimately affecting fracture resistance. Alendronate treatment does not improve fracture resistance of oim/oim bones, although it increases cortical thickness and cortical porosity, and is associated with the appearance of trabecular-like structures spanning the medullary cavity at mid-shaft, resembling the radiographic zebra lines observed in pediatric OI population. Changes are more pronounced in female oim/oim mice, where load-bearing capacity increases together with canal porosity. These results highlight the continuous need for treatments that rescue OI bone fragility by improving the poor bone matrix and emphasize the importance of considering sexual dimorphism when designing therapies for OI. STATEMENT OF SIGNIFICANCE: Bisphosphonates are widely used anti-resorptive drugs approved for osteoporosis and administered also to children with bone fragility disorders, like osteogenesis imperfecta (OI). Their benefits in young patients, however, have been debated: while some studies report fewer fractures and improved pain, others find limited effects. The impact of long-term bisphosphonate treatment on fracture resistance in growing OI bones remains unclear. Using an interdisciplinary multiscale approach combining advanced imaging and fracture mechanics on OI mouse (oim/oim) bones, this study solves the controversy. We demonstrate that bisphosphonates do not improve bone fragility. However, they enhance bone structural stability, allowing higher loads to be sustained by increasing cortical thickness and generating mid-shaft trabecular bands, resembling the "zebra lines" observed in the bones of children with OI. These changes were particularly evident in female mice.