The Effects of Antibiotic-Impregnated Spacers on Bone Healing in an Animal Model of the Induced Membrane Technique

Background: Surgeons performing the induced membrane technique (IMT) often incorporate antibiotics into the spacer at the first stage of the surgical procedure to prevent or treat infection. However, the effect of antibiotic use on subsequent bone healing is not clear. This study aimed to investigate if antibiotic-impregnated spacers impact subsequent bone healing in a rat model of the IMT. Methods: Inbred male rats (Fischer 344) were randomly divided into 3 groups according to the antibiotic dose in the spacer: (1) control (no antibiotics), (2) low-dose (1.2 g tobramycin and 1.0 g vancomycin per 40 g of polymethylmethacrylate [PMMA]), and (3) high-dose (3.6 g tobramycin and 3.0 g vancomycin per 40 g of PMMA). We created a 5-mm segmental defect in the right femoral diaphysis. The bone was stabilized with a plate and screws, and the assigned spacer was inserted into the defect. Four weeks later, the spacer was removed and bone graft was placed within the defect. Radiographs made 12 weeks after grafting were scored according to union status and degree of bone healing. Micro-computed tomographic (CT) analysis and biomechanical testing were also performed at 12 weeks. Results: Full radiographic union was achieved in 10 (83%) of 12 control animals, 13 (100%) of 13 low-dose animals, and 8 (62%) of 13 high-dose animals (high-dose compared with low-dose: risk ratio, 11.0; p = 0.039). The control group demonstrated higher bone volume compared with the high-dose group (mean difference, 9.0 mm 3 ; p = 0.039), and there was a trend toward higher bone volume in the low-dose group compared with the high-dose group (mean difference, 8.1 mm 3 ; p = 0.06). The biomechanical results demonstrated that maximum stiffness was significantly higher in the low-dose group compared with the high-dose group (mean difference, 14.1 N*mm/degree; p = 0.009). Conclusions: Our results demonstrated that low doses of antibiotics in PMMA spacers used for the IMT did not impair bone healing. However, high doses of antibiotics demonstrated inferior bone healing. Clinical Relevance: The addition of high-dose antibiotics to the PMMA spacers used for the IMT may result in impaired bone healing and should be used with caution.