Biomechanical stability plays an important role in fracture healing, with unstable fixation being associated with healing disturbances. A lack of stability is also considered a risk factor for fracture-related
infection (FRI), although confirmatory studies and an understanding of the underlying mechanisms are lacking. In the present study, we investigate whether biomechanical (in)stability can lead to altered immune responses in mice under sterile or experimentally inoculated conditions. In non-inoculated C57BL/6 mice, instability resulted in an early increase of inflammatory markers such as
granulocyte-colony stimulating factor (
G-CSF), keratinocyte
chemoattractant (KC) and
interleukin (IL)-6 within the bone. When inoculated with Staphylococcus epidermidis, instability resulted in a further significant increase in
G-CSF,
IL-6 and KC in bone tissue.
Staphylococcus aureus infection led to rapid
osteolysis and instability in all animals and was not further studied. Gene expression measurements also showed significant upregulation in CCL2 and
G-CSF in these mice.
IL-17A was found to be upregulated in all S. epidermidis infected mice, with higher systemic
IL-17A cell responses in mice that cleared the
infection, which was found to be produced by CD4+ and γδ+ T cells in the bone marrow.
IL-17A knock-out (KO) mice displayed a trend of delayed clearance of
infection (P=0.22, Fisher's exact test) and an increase in
interferon (IFN)-γ production. Biomechanical instability leads to a more pronounced local inflammatory response, which is exaggerated by
bacterial infection. This study provides insights into long-held beliefs that biomechanics are crucial not only for fracture healing, but also for control of
infection.