S03.KL1 ALLOGRAFTS FOR TREATING INFECTED BONE DEFECTS

Abstract

Infection of bone represents a major challenge in orthopaedic surgery. Chronic cases are distinguished by necrosis of parts of the osseos structures. It is generally accepted, that sequestered bone comprises bacterial colonies that show inherent resistance to both host defence mechanisms and antimicrobial chemotherapy, leaving thorough removal of all necrotic tissue a prerequisite for cure. The resulting dead space needs to be filled – defects require reconstruction. Bone grafting is a well established procedure with well documented success, however, autologous bone is available only in limited amounts and recurrence rates are still high. Availability of allograft bone is unlimited but rarely used in florid osteomyelitis since surgeons fear grafts being at risk to become a focus of ongoing infection. Fresh frozen allogeneic bone contains bone marrow consisting of fat and necrotic cells. Fat is eliciting an inflammatory response that together with immunological reactions against cell membranes may create an environment promoting bacterial growth with the necroses as a growth medium. Removing bone marrow lowers the risk of infection. When using allograft bone in sites with high risks of infection it therefore should be free of all components of bone marrow.

Highly purified allograft bone, consisting of collagen and minerals as autologous bone, is unlikely to initiate or nourish a florid infection; however, the matrix surface may represent a substratum for adherence of bacteria with formation of biofilms. To avoid bacterial adhesion the matrix may be loaded with antibiotics. It has been shown that purified bone is capable of storing huge amounts of antibiotics for prolonged periods of time. This capacity offers the possibility to use allograft bone not only as a filler but the same as a carrier for antibiotic delivery.

In chronic osteomyelitis our most obstinate opponents are not the familiar planktonic pathogens but their phenotypically different sessile forms embedded in biofilms. Those require up to 1000 fold higher concentrations of antibiotics for elimination than their planktonic forms. Debridement removes the predominant amount of bioburden but some colonies disrupted from the biofilm during manipulation may find new habitats in niches of the site and cause recurrence after an indefinite period of time. Levels reached by systemic antibiosis or local therapy with commercially available antibiotic carriers mostly are not effective in eliminating biofilm remnants. Bone impregnated with high loads of vancomycin or tobramycin may provide for high local antibiotic concentations for several weeks (> 1000x MIC) that are likely to eliminate not only planktonic bacteria but also detached biofilm clusters. Allograft bone may be impregnated both in cancellous and cortical form, morsellized or structural. Indications for their use are all forms of chronic bone infection, including osteomyelitis, infected pseudarthroses, deeply infected diabetic feet and infected total joint replacement.

Osteomyelitic lesions and pseudarthroses of long bones may successfully be treated using antibiotic loaded allograft bone, providing dead space management, antibiotic delivery and reconstruction of deficient areas at the same time. As long as local antibiotic levels are higher than the dosage required for eliminating biofilm fragments no contamination of simultaneously implanted alloplastic material needs to be feared.