DETERMINING OPTIMAL CURRENT INTENSITY AND DURATION FOR ELECTRICALLY ACTIVATED SILVER-BASED PROPHYLACTIC HIP IMPLANT PROTOTYPE DESIGN

摘要

Infections associated with medical prostheses result in notable morbidity, and traditional osteomyelitis treatments are often accompanied by high risk and cost. The probability of prosthetic joint infections is 1-2.5%for primary hip or knee replacements and 2.1-5.8%for revision surgeries, and the cost of treating such an infection is estimated to be over $50,000 per episode. [1] While the potential benefits of silver surfaces stimulated by low intensity direct current (LIDC) have been discussed in literature, we have recently utilized that concept in the actual design of prophylactic indwelling residual hardware prostheses for the very first time. [2-4] A modular titanium hip stem coated with silver at the anode (and titanium as the cathode) and activated by a watch battery encapsulated within the two electrode modules (Figure 1) will result in oligodynamic iontophoresis (OI) in the soft tissue surrounding the implant which is prone to infections. Preliminary in vitro and in vivo results have demonstrated the potency of silver-based OI as an effective local antibacterial therapy in osteomyelitis treatment with advantages over various antibiotics. However, the main challenge here is achieving the antibacterial potency while minimizing any potential toxic effects on local tissues. [4] In order to evaluate the antimicrobial efficacy that can be achieved without compromising on non-toxic/safe silver dosages, this paper proposes an in vitro model of the LIDC-activated silver-titanium implant configuration (scaled-down) to determine the influence of two major factors: current intensity and duration. According to Faraday's law of electrolysis, the mass of a substance altered at an electrode (in ionic form) is directly proportional to the quantity of electricity transferred at that electrode. Intuitively, the antimicrobial efficacy of such an implant configuration would be directly correlated to the product of intensity and duration of the LIDC. However, the experimental results reveal a non-linear interaction between these two parameters. An optimal combination of current and duration is therefore proposed in this study. We have also tested the endurance effectiveness of silver anodes which were exposed to a protein-rich environment for 24 hr prior to the antimicrobial testing. An electrical circuit was configured to provide a controlled current so that the theoretical daily ion amount was set to be within the range from 1/60 to 4 times of upper limit of FDA-approved silver dosage - 0.11 mg.