A novel murine model of established Staphylococcal bone infection in the presence of a fracture fixation plate to study therapies utilizing antibiotic-laden spacers after revision surgery

Abstract

Mice are the small animal model of choice in biomedical research due to the low cost and availability of genetically engineered lines. However, the devices utilized in current mouse models of implant-associated bone infection have been limited to intramedullary or trans-cortical pins, which are not amenable to treatments involving extensive debridement of a full-thickness bone loss and placement of a segmental antibiotic spacer. To overcome these limitations, we developed a clinically faithful model that utilizes a locking fracture fixation plate to enable debridement of an infected segmental bone defect (full-thickness osteotomy) during a revision surgery, and investigated the therapeutic effects of placing an antibiotic-laden spacer in the segmental bone defect. To first determine the ideal time point for revision following infection, a 0.7 mm osteotomy in the femoral mid-shaft was stabilized with a radiolucent PEEK fixation plate. The defect was inoculated with bioluminescent Staphylococcus aureus, and the infection was monitored over 14 days by bioluminescent imaging (BLI). Osteolysis and reactive bone formation were assessed by X-ray and micro-computed tomography (micro-CT). The active bacterial infection peaked by 5 days post-inoculation, however the stability of the implant fixation became compromised by 10–14 days post-inoculation due to osteolysis around the screws. Thus, day 7 was defined as the ideal time point to perform the revision surgery. During the revision surgery, the infected tissue was debrided and the osteotomy was widened to 3 mm to place a poly-methyl methacrylate spacer, with or without vancomycin. Half of the groups also received systemic vancomycin for the remaining 21 days of the study. The viable bacteria remaining at the end of the study were measured using colony forming unit assays. Volumetric bone changes (osteolysis and reactive bone formation) were directly measured using micro-CT image analysis. Mice that were treated with local or systemic vancomycin did not display gross pathology at the end of the study. While localized vancomycin delivery alone tended to decrease the bacterial burden and osteolysis, these effects were only significant when combined with systemic antibiotic therapy. This novel mouse model replicates key features of implant-associated osteomyelitis that make treatment extremely difficult, such as biofilm formation and osteolysis, and imitates the clinical practice of placing an antibiotic-laden spacer after infected tissue debridement. In addition, the model demonstrates the limitations of current PMMA spacers and could be an invaluable tool for evaluating alternative antimicrobial treatments for implant-associated bone infection.

Mouse model of hematogenous implant-related Staphylococcus aureus biofilm infection reveals therapeutic targets

“…Prior preclinical models of PJI or orthopedic implant infections have involved direct insertion of an implant with bacteria already adherent to its surface (22,23), direct inoculation of bacteria at the surgical site of an implant (24)(25)(26), or bacteremia resulting in septic arthritis in the absence of an implant (27,28). Although each of these models has features of a hematogenous implant infection, they do not fully recapitulate the spread of the Significance Hematogenous implant-related infections are an important clinical problem because bacteria spread from the bloodstream to a previously well-functioning implant and result in infectious complications and failure of a medical device or prosthesis.…”

A large animal model for a failed two-stage revision of intramedullary nail-related infection by methicillin-resistant Staphylococcus aureus

“…For example, complex device-related infection models have also been described, including osteotomies fixed with IM nailing or external fixation in the goat (Curtis et al, 1995); comparisons of infection after fracture fixation with either primary or secondary bone healing (Rittman and Perren, 1974); comparison of IM nail-related infection secondary to pin tract infection of external fixation (Clasper et al, 2001); measures of neovascularisation after open fractures fixed by an IM nail (Khodaparast et al, 2003). More recently, a large animal model was used in the evaluation of antimicrobial implant surfaces as a prophylaxis against infection in a sheep model of plate osteosynthesis Stewart et al, 2012), resembling other large animal models for prevention of infection using bone cement beads and spacers (Petty et al, 1988;Wenke et al, 2006).…”

“…For example, very poor outcomes are observed in patients with a prior failed debridement and exchange procedure [reinfection rate of 34 % (Sherrell et al, 2011)], those infected with antibiotic-resistant pathogens [e.g. MRSA reinfection rate of > 42 % (Kilgus et al, 2002;Mortazavi et al, 2011)] or patients with culture-negative infection (Mortazavi et al, 2011). Equivalent large-scale data are not available for FRI treated with staged renailing.…”

“…Whilst a range of animal models of device-related bone infection have been described in the literature (see reviews: Calabro et al, 2013;Reizner et al, 2014), there are comparatively few models involving the treatment of device infection (e.g. Brady et al, 2011;Gatin et al, 2014;Inzana et al, 2015;Moskowitz et al, 2010). Furthermore, there are no models of staged hardware exchange due to infection available, to the best of our knowledge.…”

“…The incidence of orthopaedic device-related infection (ODRI) following elective orthopaedic surgery is in the range of 0.7 % to 4.2 % (Kurtz et al, 2008;Sugarman and Young, 1989;Zimmerli et al, 2004), while the incidence in non-elective trauma cases ranges from approximately 1 %, after operative fixation of closed low-energy fractures, to more than 30 % in complex open tibia fractures (Boxma et al, 1996;Patzakis and Wilkins, 1989). Approximately 35 % of these infections are caused by Staphylococcus aureus (S. aureus) (Trampuz and Zimmerli, 2006) and, of these, up to 70 % may be caused by antibioticresistant strains (e.g.…”