In the past few years, manufacturers have boasted about the antimicrobial coatings of devices that come in contact with the surgical site:
- wound dressings coated with a layer of silver;
- suture coated with an anti-bacterial ingredient;
- surgical drapes filled with iodine; and
- catheters and orthopedic implants coated with antibiotics.
The CDC says that more than 60 percent of surgical site infections occur at the incision site, but do these devices (see "Antimicrobial Products Available for Your Facility" on page 42) deliver on their promise to curb SSIs? Here's what a few experts have to say.
Science to the substances
Marek Urban, PhD, professor of polymer science at the University of Southern Mississippi in Hattiesburg, is working to prevent infections. His research involves developing coatings for medical devices that can hold antibiotics, such as implants. Having lost a family member to infection-related causes, he says he is alarmed by the number of people who die each year from infections and the costs associated with treating them.
"The main benefit of antibiotic coatings is that you can use relatively minute quantities of antibiotics to keep microbials under control and minimize the risk," he says. "If you have a surgical tool or something that is in contact with the blood in a surgical environment, chances are microbials will form on it quite quickly, and they do based on the experience of a number of physicians. If you have a surface on these objects that inhibits the formation, you can also inhibit infections."
This is especially true for implants, says Dr. Urban. "If you look at the typical operating conditions when a surgeon implants a plastic device, it takes very little for microbials to form on that surface," he says. "Having a coating will help alleviate that process by sterilizing the device from microbials in the patient's body."
The coated device would be safe, says Dr. Urban, even for patients with allergies to common antibiotics. "The patient has to take a large amount of a drug to have an allergic reaction, significantly more than you would have on a catheter or other device," he adds.
But antibiotic coatings have their limitations, says Robert Burrell, PhD, professor and chair of the department of biomedical engineering at the University of Alberta in Edmonton, Canada. "It's an interesting approach, but the problem with antibiotics is that they tend to have a narrow spectrum," he says. "If you have a catheter and you know that a patient has a certain organism, you can select the appropriate antibiotic. But unless you know what you're dealing with, picking a random agent isn't very helpful."
Another consideration, says Dr. Burrell, is that using an antibiotic as a preventive measure without knowing what you want to treat can contribute to the organisms becoming antibiotic-resistant. "You have to be judicious in how you pick the antibiotics," he says.
The silver standard
The limitations for antibiotics may not be an issue for devices coated with a shinier material. A layer of silver can keep a device from becoming a conduit for infectious microorganisms at the surgical site, says Jay Singh, MD, director of surgical residency at Piedmont Colorectal Associates and associate professor of surgery at Emory University in Atlanta. "Silver kills gram-positive and gram-negative bacteria and fungi," he says. "These classes of microorganisms include those frequently associated with nosocomial or hospital-acquired infections."
Dr. Singh says silver disrupts bacteria's cell membranes and their adenosine triphosphate production, which is important for the cells' energy. Silver also impairs the cells' ability to reproduce and perform other internal processes. "Silver attacks bacteria from up to 10 different sites, which kills it or prevents it from reproducing," he says. "Antibiotics typically attack the organism from a single aspect, which can lead to resistance, but the multi-pronged attack from silver ions also prevents the development of resistance."
Another advantage, says Dr. Urban, is that silver coatings last on devices for a long period of time. "If a catheter has a coating that will last for several days before it has to be serviced, that's a tremendous savings not just from a sales perspective but also from an infection prevention standpoint," he says.
But quantity doesn't always relate to quality. It's not a question of how much silver a catheter or wound dressing has in it but rather how much reaches the patient, says Dr. Urban. "It's questionable how effective silver is if it's embedded into the body of a device when the more important aspect is how much is on the surface that is making contact with the surgical site," he says.
To illustrate that point, Dr. Burrell uses the example of a woman who had her ears pierced with silver studs and still got an infection. "The question was never how much silver was there, because the amount of silver in a stud is huge compared to the lobe," he says. "It's not the total amount of silver that's important, it's the amount of available silver."
The form of silver used in a device can also determine its effectiveness, says Dr. Burrell. "Fundamentally, if your device uses silver, you have to be aware that the silver ion reacts with chloride in the body and creates silver chloride, which precipitates out and is essentially inactive because it's not available to the patient's body," he says.
This, he says, is the reason so many of the early experiments with high concentrations of silver nitrates and silver sulfadiazine called for frequent doses of 12 times a day and twice a day, respectively. It's also a reason why you may want to be skeptical about silver-coated devices on the market if the manufacturers don't explain their coatings' active form of silver.
"If you look at many of the antimicrobial products out there today, they have far less silver in them and they're expected to last much longer," says Dr. Burrell. "But if you've done nothing to the silver ion, how do you get that to work? If you put a quarter the amount of silver in a daily dose of silver sulfadiazine on a device and tell me it'll last for 14 days, you better have a good technological explanation for how that's possible."
Determining the efficacy of a specific coating formulation may come down to checking current data or conducting your own experiments. For example, if you want to see if a silver-coated wound dressing that claims to be effective for a week actually is, you may want to try a quick experiment to first see that it can work for one day. To do that, Dr. Burrell says you'd apply bacteria to the dressing, let it sit for about half an hour, put the dressing in saline to neutralize the silver's properties and then put parts of the dressings into Petri dishes. If the form of silver on the dressing was effective, most of the dishes will have so few colonies you'll be able to count them without a microscope. But if it was ineffective, the dish will be filled with bacteria. Even though the bacteria on the dressing had less than an hour to incubate, Dr. Burrell says this simple test will show how effective the silver will be in killing bacteria in a wound. "A wound is a dynamic environment, so letting the dressing sit for 24 hours is not good because that's a static environment," he says. "For this experiment, you're determining the speed that the silver kills bacteria."
The data supporting antimicrobial and silver coatings are favorable. Some noteworthy studies include:
- A systematic review of randomized studies found that antimicrobial catheters could prevent bacteria during short-term catheterization (Ann Intern Med. 2006;2:116-126).
- Vancomycin bonded with the titanium surface of orthopedic implants effectively reduces Staphylococcus aureus colonies and retains its antibacterial activity over time (Chemistry and Biology. 2005;12:958-959).
- A research team led by Dr. Urban found that ePTFE, a polymer used in cosmetic surgery, can be coated with penicillin and remains highly effective against gram-positive aureus bacteria (Biomacromolecules. 2007;2:713 -718).
- A comparative study found that silver nitrate liquid, silver sulfadiazine cream and silver-coated dressings all reduced the number of viable bacteria. Of these, the dressing was the most effective and the liquid was the least effective (Am J Infect Control. 1998;26:572-7).
- Research comparing the On-Q PainBuster with silver-coated catheters to systemic narcotics for patients following colorectal surgery found that patients who received silver catheters were 55 percent less likely to develop surgical site infections, according to I-Flow Corporation.
Better coatings in the pipeline?
"Although a surgical site infection rate of zero may not be achievable, continuing investigation of the biology of infection at the surgical site and methods of prevention may allow us to further reduce the frequency, cost and morbidity associated with surgical site infections," says Dr. Singh.
Until more definitive research becomes available, Dr. Singh suggests looking at current data to be sure the device and the technique used to coat it are backed by science. "Whether a coating is beneficial depends on the type of coating, the device, the amount that is applied and whether there is any data to support it," he says.
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