Different cytokine expression profiles in metaphyseal and diaphyseal fracture healing may provide new insights in the field of bone regeneration

Abstract

Introduction Fractures are traumatic injuries that mainly occur in bone metaphysis, however most studies of bone healing have focused on diaphyseal bone. This is important because the healing process of trabecular metaphyseal bone has different healing characteristics from the diaphyseal area. Inflammation is thought to play an important, but different role in these two bone fracture types: diaphyseal fractures heal slowly through the formation of callus tissue, and metaphyseal trabecular bone heals faster, with no, or limited callus formation. As cytokines are key modulators of inflammation, the aim of the present study was to define the cytokine profiles at the core of these two conditions with possible implications for the bone healing process. Materials and Methods This study included sixteen patients with long bone metaphyseal and diaphyseal fractures and a healthy control group. Blood samples were taken at two timepoints: i) between the 1 st (the day of the fracture) and the 6 th day after fracture occurrence; ii) between the 7 th and the 21 st day after fracture occurrence. Fractures were treated either conservatively or surgically, depending on specific clinical indications. All participants with diaphyseal fractures were treated surgically. The control group provided blood samples on one occasion. The obtained plasma samples were pooled into 5 different experimental groups and analysed using commercial cytokine arrays. Results Marked differences in cytokine expression profiles were found between the fracture groups and the control group. The diaphyseal group had an "activated" pro-inflammatory cytokine profile with markedly higher levels of cytokines at both timepoints compared to the metaphyseal group, which in contrast had a "silenced" cytokine expression profile. Single cytokine analysis revealed that in both metaphyseal and diaphyseal fracture groups MCP-1 and RANTES showed the most prominent fold change at both timepoints. IL-6 and TNF-α also show similarly elevated levels in both timepoints in the diaphyseal fracture group, whereas this is not observed in the metaphyseal group. Furthermore, IL-3 expression was also elevated in the diaphyseal group, but only in the first timepoint. Conclusion This pilot study indicated chemokines which might be potential crucial drivers of bone healing, as well as painted distinct cytokine plasma profiles evident in metaphyseal and diaphyseal healing.