Implant infections: a haven for opportunistic bacteria

The insertion of implants and medical devices has emerged as a common and often life-saving procedure. A current estimate of the rate of total hip replacement in the world is approximately one million a year, and knee replacements more than 250000. More than 30% of hospitalized patients have one or more vascular catheters in place. More than 10% of hospitalized patients have an indwelling urinary catheter. Some patients require multiple joint replacements. In the United States, approximately 2 million nosocomial infections cost nearly $11 billion annually. Exposure to invasive medical devices is one of the most important risk factors.(1)Devices predispose to infection by damaging or invading epithelial or mucosal barriers and by supporting growth of micro-organisms, thus serving as reservoirs. Invasive medical devices impair host defence mechanisms and, when contaminated, can result in resistant chronic infection or tissue necrosis, the major objections to extended use of implant devices. Implant devices today account for approximately 45% of all nosocomial infections.(2)Implant infections are extremely resistant to antibiotics and host defences and frequently persist until the implant is removed, which is the standard therapy. Tissue damage caused by surgery and foreign body implantation further increases the susceptibility to infections, activates host defences and stimulates the generation of inflammatory mediators; these are enhanced by bacterial activity and toxins.(3)The ability of bacteria such as Staphylococcus epidermidis, which are otherwise virtually avirulent, to escape from host defences and antibiotic therapy, has led to the development of alternative methods of control such as infection-resistant materials acting as antimicrobial drug-delivery systems. By these methods, there is a sustained delivery of antimicrobial drugs into the local micro-environment of implants, which avoids systemic side-effects and exceeds usual systemic concentrations by several orders of magnitude. Bioengineering of hybrid implant materials in order to achieve optimal performance and to prevent inflammatory reactions and interface cellular disorganization is a field undergoing rapid development. Hybrid materials that slowly deliver antimicrobial drugs may reduce implant infections in the future.