Characterising the Role of Pin Movement on Infection in External Fracture Fixation

Abstract

External fixation is an essential surgical technique for treating trauma, limb lengthening and deformity correction, however pin-site infection and pin loosening are significant challenges that still need to be overcome. Throughout the clinical literature there is anecdotal evidence which highlights a relationship between excessive movement at the soft tissue pin-site interface and an increased risk of pin-site infection. However the mechanisms by which pin movement may influence pin-site infection and wound healing are poorly understood. Therefore the aim of this doctoral research was to develop and test an in vitro skin equivalent pin-site model to understand and study the role of pin movement on the development of infection and wound healing in external fixation. Three studies were carried out to: first investigate bacterial attachment on pins using the novel pin machine to simulate pin movement in a bacterial culture (Staphylococcus aureus and Staphylococcus epidermidis) and using our optimised bacterial detachment method; a scratch assay with a fibroblast wound model when exposed to mono-species and multi-species bacterial conditioned media (Staphylococcus aureus and Staphylococcus epidermidis); a human skin-equivalent pin-site model using the novel pin machine and using ELISAs to study pro-inflammatory markers in the media (IL-1α, IL-8, TNF-α). No significant increase in bacterial attachment was observed when movement was applied to pins submerged in a bacterial culture, highlighting the fact that pin-site infection is a multi-factor problem involving bacterial colonisation, wound healing of the skin and mechanics of the pin itself. An increase in IL-1α and IL-8 expression was observed when a pin was implanted and left static compared to control (p<0.05), which increased further when movement was applied to the pin (p<0.05), confirming that pin movement has a negative effect on the wound healing of soft tissue around the pin-site. When studying the effect of bacteria on the wound healing in vitro, planktonic and biofilm equivalents of S. aureus and S. epidermidis reduced the rate of migration compared to control (p<0.05). However the most significant impact on wound healing was observed for a planktonic multi-culture of both species, indicating a possible increased virulence of multi-species Staphylococci compared to single species infections. This work highlights the importance of studying multi-species environments in vitro to better understand infection development at pin sites or other wound sites. This research has also confirmed that pin movement is an important factor in maintaining sterile pin-sites however the pin-site skin-equivalent model developed can potentially be used to evaluate the efficacy of pin movement restriction treatments such as wound dressings. The implications of evaluating these clinical interventions for more effective treatments and a reduction in antibiotic use, which helps to tackle the growing global challenge of antimicrobial resistance.