The Impact of Mechanical Instability on Fracture-Related Infection

Fracture-related infection (FRI) remains one of the most significant complications in the surgical treatment of fractured bones. The risk of developing an infection associated with any implanted device is increased by the presence of the device itself, which acts as a foreign body, providing a surface for adhesion and biofilm formation to any microorganism entering the wound. The biomechanical properties of fracture fixation devices play a crucial role in the healing process of surgically treated fractures. It is well established that the biomechanics involved in fracture fixation significantly influence healing outcomes. Additionally, the stability of the fixation is considered a critical factor in the development of FRI. Several preclinical studies provide evidence supporting the link between stable fixation and a reduced risk of infection. In a mouse model of plating osteosynthesis with stable and rigid MouseFix fixation infected with Staphylococcus epidermidis, it was observed that infection rates were influenced by fracture stability. However, no differences were detected when using S. aureus, which caused a more severe infection, resulting in osteolysis and loss of stability in both groups. Additionally, biomechanical instability led to a stronger local inflammatory response, characterized by increased levels of inflammatory markers such as granulocyte colony-stimulating factor (G-CSF), keratinocyte chemoattractant (KC), and interleukin (IL)-6 within the bone. Lately the use of the implant load sensor (fracture monitor) in a sheep model of S. aureus induced FRI revealed the potential of measuring relative implant load (RIL) as a surrogate for fracture healing. The use of sensors for measuring implant load as an objective and quantitative indicator of fracture stability can provide valuable information on the progression of healing. In the sheep study, the fracture monitor was able to track the implant load as the infection was induced and treated. Implant load appeared to correspond to the degree of infection burden, with high correlation values between the bacterial load at revision and euthanasia and the sensor data. The implant load could identify infected animals by measuring the highest RIL values in culture-positive animals at the end of the study and by identifying delayed healing associated with infection. Stability plays a crucial yet not fully understood role in managing FRI. The continued development and application of mechanical load sensors, and the ever-improving control over biomechanical conditions in preclinical fracture models promises significant advances in our understanding. How this may impact patient care remains to be determined, but deeper insights are gradually revealing the connection between biomechanics, immunology and bone healing in infected and non-infected scenarios.