Fractures in elderly mice demonstrate delayed ossification of the soft callus: a cellular and radiographic study

OBJECTIVES: The aim of this study was to assess the cellular age-related changes in fracture repair and relate these to the observed radiographic assessments at differing time points. METHODS: Transverse traumatic tibial diaphyseal fractures were created in 12–14 weeks old (young n = 16) and 18 months old (elderly n = 20) in Balb/C wild mice. Fracture calluses were harvested at five time points from 1 to 35 days post fracture for histomorphometry (percent of cartilage and bone), radiographic analysis (total callus volume, callus index, and relative bone mineral content). RESULTS: The elderly mice produced an equal amount of cartilage when compared to young mice (p > 0.08). However, by day 21 there was a significantly greater percentage of bone at the fracture site in the young group (mean percentage 50% versus 11%, p < 0.001). It was not until day 35 when the elderly group produced a similar amount of bone compared to the young group at 21 days (50% versus 53%, non-significant (ns)). The callus area and callus index on radiographic assessment was not significantly different between young and elderly groups at any time point. Relative bone mineral content was significantly greater in the young group at 14 days (545.7 versus -120.2, p < 0.001) and 21 days (888.7 versus 451.0, p < 0.001) when compared to the elderly group. It was not until day 35 when the elderly group produced a similar relative bone mineral content as the young group at 21 days (888.7 versus 921.8, ns). CONCLUSIONS: Elderly mice demonstrated a delay in endochondral ossification which was associated with a decreased relative bone mineral content at the fracture site and may help assess these cellular changes in a clinical setting.