Increasing Diagnostic Accuracy in Orthopedic Implant Related Infections (An in - Vitro and in - vivo Study)

Abstract

The purpose of this study was to investigate the impact of various culturing techniques on improving diagnostic accuracy in bone and orthopedic implant related infections. Braided Co-Cr-Mo cables and sterile discs of rabbit bone were each incubated with a slime producing strain of Staphylococcus epidermidis under different growth conditions: 1) tryptic soy broth (TSB), 2) tryptic soy broth + 50% serum and 3) tryptic soy broth + 50% plasma. Following incubation, cables and bone discs were treated with either trypsin or phosphate buffered saline (PBS) in an attempt to disrupt the biofilm. Incubation of cables and bone discs with bacteria in TSB supplemented with plasma followed by treatment with 0.5% trypsin yielded the highest bacterial counts. Scanning electron microscopic study of cables and bone discs that were incubated in tryptic soy broth with 50% plasma revealed bacteria embedded in the biofilm bacteria within a fibrinous scaffold; this structure was disrupted following enzymatic treatment with trypsin. To determine the clinical value of this technique, an in-vivo rabbit model of orthopedic implant related infection was developed using the same strain of S.Epidermidis. At three weeks cultures of aspirate of joint fluid, swab of joint, swab of the interface between the bone and the implant were performed in broth and on agar plates. Samples of synovium and bone tissue were also obtained and mechanically ground after enzymatic disruption of biofilm with trypsin in half of the samples, than quantitatively cultured. When incubated in broth cultures higher yields than cultures incubated on agar plates. Cultures of bone and synovial tissue yielded higher bacterial counts following trypsin treatment compared to mechanical grinding only (p < 0.001, p < 0.05 respectively). These results suggest that plasma proteins contribute to the structure of the biofilm by forming a plasma derived scaffold which renders it susceptible to enzymatic disruption with trypsin. Higher accuracy of diagnosis of infection in clinical practice may be achieved by culture techniques using enzymatic treatment of implant tissue interface with trypsin and incubation in broth rather than on agar plates.