The local inflammatory environment and microorganisms in "aseptic" loosening of hip prostheses

Long term loosening of hip prostheses remains an important problem in orthopedics. Although various loosening mechanisms have been proposed, the exact process is still unclear. Particle disease and the pressure theory are widely known and generally accepted hypotheses to explain long term implant failure. Each proposed mechanism recognizes a local inflammatory response in which macrophages represent the main cell-type and several proinflammatory and antiinflammatory cytokines (IL-1beta, IL-6, TNFalpha, IL-10, TGFbeta), chemokines (IL-8/CXCL8, MCP-1/CCL2, RANTES/CCL5, MIP-1alpha/CCL3) and other mediators (GM-CSF, M-CSF, MMP-1, PDGF-alpha, PGE(2), IL-11) are identified. The cytokines have different functions and some are capable of stimulating bone resorption in various ways; either directly or indirectly. Even though the implant loosening is thought to be "aseptic", several studies suggested a possible role for bacteria and a bacterial biofilm in implant failure. Biofilm-derived bacteria and bacterial products might have an underestimated and potential role in the loosening process. In this article we will discuss the possible role of a bacterial biofilm and the importance of the local surrounding environment in "aseptic" loosening of hip prostheses.