Quantifying the relationship between biofilm reduction and thermal tissue damage on metal implants exposed to alternating magnetic fields.

AIM: Metal implant infections are a devastating problem due to the formation of biofilm which impairs the effectiveness of antibiotics and leads to surgical replacement as definitive treatment. Biofilm on metal implants can be reduced using heat generated by alternating magnetic fields (AMF). In this study, the relationship between implant surface biofilm reduction and surrounding tissue thermal damage during AMF exposure is investigated through numerical simulations. METHODS: Mathematical models of biofilm reduction with heat were created based on in vitro experiments. Simulations were performed to predict the spatial and temporal heating on the implant surface and surrounding tissue when exposed to AMF. RESULTS: The modeling results show that intermittent and slow heating can achieve biofilm reduction with a narrow zone of tissue damage around an implant of less than 3 mm. The results also emphasize that uniformity of implant heating is an extremely important factor impacting the effectiveness of biofilm reduction. For a knee implant, using a target temperature of 75 °C, an intermittent treatment strategy of 15 exposures (10 s to target temperature followed by cooldown) achieved a bacterial CFU reduction of 6-log10 across 25% of the implant surface with less than 3 mm of tissue damage. Alternatively, a single 60 s heating exposure to same temperature achieved a bacterial reduction of 6-log10 across 85% of the implant surface, but with 4 mm of tissue damage. CONCLUSION: Overall, this study demonstrates that with uniform heating to temperatures above 70 °C, an implant surface can be largely reduced of biofilm, with only a few mm of surrounding tissue damage.

Feasibility of heating metal implants with alternating magnetic fields (AMF) in scaled up models.

Aim: Treatment of infected orthopedic implants remains a major medical challenge, involving prolonged antibiotic therapy and revision surgery, and adding a >$1 billion annual burden to the health care system in the US alone. Exposure of metallic implants to alternating magnetic fields (AMF) generates heat that can provide a noninvasive means to target biofilm adhered to the surface. In this study, an AMF system with a solenoid coil was constructed for targeting a metal plate surgically implanted in a sheep model.Methods: A tissue-mimicking phantom of the sheep leg was developed along with simulation model of phantom and the live sheep leg. This was used evaluate heating with the AMF system and to compare experimental results with numerical simulations. Comparative AMF exposures were performed/simulated in these model for feasibility of design, verification, and validation of simulations.Results: The system produced magnetic field strengths up to 12mT and achieved plate temperatures of 65-80 °C within 10-14 s. Single and intermittent AMF exposures of a tissue-mimicking phantom agreed with numerical simulations within 5 °C. Similar agreement between experimental measurements and simulations was also observed in the live sheep metal implant model. The simulations also predicted 2-3 mm of tissue damage using a CEM43 thermal dose model for 1-h AMF exposures targeting 65 °C for pulse delays of 2.5 and 5 mins.Conclusion: This study confirmed that AMF technology can be scaled up to treat implants in a large animal model with the same rates of heating and peak temperatures achieved in prior in vitro studies. Further, numerical simulations provided accurate predictions of the heating produced by AMF on metal implants and surrounding tissues, and can be used to design AMF coils for treating human prosthetic joint implants with more complex geometrical shapes.