Among other stressors, age and mechanical constraints significantly influence
regeneration cascades in bone healing. Here, our aim was to identify genes
and, through their functional annotation, related biological processes that
are influenced by an interaction between the effects of mechanical fixation
stability and age. Therefore, at day three post-osteotomy, chip-based whole-
genome gene expression analyses of fracture hematoma tissue were performed for
four groups of Sprague-Dawley rats with a 1.5-mm osteotomy gap in the femora
with varying age (12 vs. 52 weeks - biologically challenging) and external
fixator stiffness (mechanically challenging). From 31099 analysed genes, 1103
genes were differentially expressed between the six possible combinations of
the four groups and from those 144 genes were identified as statistically
significantly influenced by the interaction between age and fixation
stability. Functional annotation of these differentially expressed genes
revealed an association with extracellular space, cell migration or
vasculature development. The chip-based whole-genome gene expression data was
validated by q-RT-PCR at days three and seven post-osteotomy for MMP-9 and
MMP-13, members of the mechanosensitive matrix metalloproteinase family and
key players in cell migration and angiogenesis. Furthermore, we observed an
interaction of age and mechanical stimuli in vitro on cell migration of
mesenchymal stromal cells. These cells are a subpopulation of the fracture
hematoma and are known to be key players in bone regeneration. In summary,
these data correspond to and might explain our previously described
biomechanical healing outcome after six weeks in response to fixation
stiffness variation. In conclusion, our data highlight the importance of
analysing the influence of risk factors of fracture healing (e.g. advanced
age, suboptimal fixator stability) in combination rather than alone
