Abstract
Metallic implants are used frequently in the operative repair of joints and fractures in orthopaedic surgery. Metal infection is a catastrophic complication of the surgery with patients loosing their newfound mobility and independence, associated morbidity and mortality is high. Orthopaedic implant infection is chronic and biofilm based. Present treatment focuses on removing the infective substratum and implant surgically as well as prolonged anti-microbial therapy. Biofilms are 500 times more resistant than planktonic strains of bacterial flora to antibiotics, and with evolving resistant strains this form of therapy is loosing ground. Silver coatings on polymers and nylon (catheters, heart valve cuffs, burn dressings) have shown inhibition of this biofilm formation in its adhesion stage. Our aim was to deposit effective, minute, biocompatible, anti-bacterial layers of silver on orthopaedic stainless steel K-wires.
Combining magnetron sputtering with a neutral atom beam (Saddle Field) plasma source at 10−4 mbar in argon gas at temperatures of 60°C, a silver coating of 99.9% purity was deposited onto stainless steel orthopaedic K-wires. Coating thickness measurements were obtained using glancing angle x-ray diffraction of glass slides coated adjacent to wires. Magnetron parameters were modified to produce varying thickness of silver. Adhesiveness was examined using Rockwell punch tests and tape tests. Silver leaching experiments were carried out in phosphate buffered saline at 37°C for 48hrs and using inductive coupled plasma spectrometry to assess leached silver ions. Surface microscopy visualised physical changes in the coatings. Biofilm adhesion was determined by exposing wires to Staphylococcus aureus ATCC 29213 -NCTC 12973 for 15 min to allow biofilm adhesion and initiation. Wires were then cultured for 24h at 37°C in RPMI. Subsequently wires were sonicated at 50Hz in ringer’s solution and gently vortexed to dislodge biofilm. Sonicate was plated by the log dilution method on blood agar plates. Bacterial colonies were then counted and changes expressed in log factors. Surface biofilms were visualised using scanning electron microscopy. Cytotoxicity was assessed using fibroblast cell cultures lines.
K-wires were coated with 5 to 50 nm of silver by running the magnetron sputtering at low currents. These coatings showed excellent adhesive properties within the 48hr exposed with only 5% of silver leaching in buffered saline. The silver coated wires showed a log 3–4 fold reduction in biofilm formation as compared to control wires. The coatings showed no cytotoxic effects.
Silver coating of medical implants has been shown in urological catheters to reduce biofilm infection. We have perfected a method of depositing thin layers of anti-bacterial silver onto stainless steel, which is both anti-infective and biocompatible. This coating could potentially add to the armourary of anti-infective agents in the elimination of infection related orthopaedic implant failure.
Abstracts prepared by Dr P E Watkins, Hodgkin Building, Guys Campus, King’s College London.
