Fighting artificial joint infections


Share with others:


Print Email Read Later

For several years, Elton Petersen has lived with two artificial hips.

But that's not his biggest challenge.

The problem that sometimes tests the 83-year-old Mt. Lebanon man's natural ebullience is a persistent infection in his left hip implant.

And by persistent, we mean an infection that has lasted for eight solid years and has forced three subsequent surgeries to try to correct the problem.

Bacterial infections in artificial hips and knees are relatively rare -- about 1 to 4 percent nationally -- but when they occur, they can be devilishly hard to get rid of, say Nicholas Sotereanos, Mr. Petersen's orthopedic surgeon, and William Costerton, a bacteria research pioneer, both based at Allegheny General Hospital.

Bacterial infections in artificial joints form slimy colonies called biofilms that are largely impervious to standard antibiotics, Dr. Costerton said. Making matters worse for Mr. Petersen is the fact that his infection is MRSA -- methicillin-resistant staphylococcus aureus -- that requires newer, more powerful antibiotics.

As Dr. Sotereanos put it, though, even newer antibiotics like vancomycin "can't penetrate the slime layer of the mothership" colony in Mr. Petersen's hip, "and so that mothership will remain in Elton forever."

Dr. Costerton and his colleagues at the Center for Genomic Sciences at the Allegheny-Singer Research Institute are on a quest to prevent or eradicate these artificial joint infections.

One major difficulty they face is that it's very hard to identify bacteria from biofilms in standard laboratory petri dishes. "It's a sludgy mess, it's all bonded together, and it doesn't grow well on the culture medium," he said. Growing bacteria in the lab "is a technique invented 150 years ago, and there aren't many 150-year-old techniques that are still being used."

That is one reason Dr. Costerton is holding out high hopes for the Ibis T5000, a complex diagnostic machine made by Ibis Biosciences, a division of Abbott Laboratories.

The Ibis biosensor system was invented less than a decade ago in California by David Ecker, the company's chief scientific officer, partly to develop a rapid diagnostic tool for bioterrorism attacks.

It can amplify the genetic material in a bacterial colony and then figure out the identity of the bugs by weighing their nucleic acids with a mass spectrometer.

Dr. Ecker compared the process recently to figuring out what coins are in your pocket by weighing them on a scale. "If you took out two coins and put them on a pan balance, and if you just looked at the dials and never looked at the coins, you'd say there's only one way to get that particular weight, and that's because it's two dimes."

In the same way, the Ibis device can weigh all the nucleic acids in a biological sample and come up with a highly accurate estimate of what organisms it contains, he said, all in about five to six hours, compared with several days to grow them in culture.

The Ibis T5000 is not yet approved for diagnostic use by the federal Food and Drug Administration, but Dr. Costerton hopes tests he and others are doing will lead to that.

Allegheny General is using its prototype Ibis device to diagnose infections in up to 100 patients with artificial joints, fractures that won't heal and other joint injuries.

Dr. Ecker said the Ibis may be especially valuable with joint implant infections. "This is what I call a very high-value problem. You're going to make a decision -- do I take someone's artificial knee out? Do I take away more of the patient's bone and go further up before putting a new one in? These are life-changing decisions, and there's really no good way to get those answers without our technology."

Even with good detection, though, biofilm infections can't always be wiped out.

When Mr. Petersen began to feel "washed out" about a month after his left hip implant in 2002, it didn't take Dr. Sotereanos long to figure out it was a MRSA infection. Getting rid of it was another matter.

The first step was removing the artificial joint and giving Mr. Petersen a cocktail of antibiotics before redoing the hip implant. That didn't obliterate the infection, so Dr. Sotereanos next did a minimally invasive procedure to remove pus and try to get more of the infection out.

Finally, he put Mr. Petersen on an antibiotic called doxycycline, which can't eradicate the infection, but "will suppress it so it doesn't [spread] into the femur," Mr. Petersen said.

In the meantime, the infection loosened the cup that attached to his left pelvis, and so Dr. Sotereanos put in a larger cup with more screws five years ago.

Mr. Petersen gets around his apartment now with a special walker, but lately, even his new implant cup has started to wiggle.

"I told my friends that the doctors have found the problem, they found a screw loose, and they all said, 'We could have told you that,' " Mr. Petersen said with a laugh.

"Our main goal now," Dr. Sotereanos said, "is to keep Elton walking and not having to go into a nursing home where he'd need help to get to the bathroom."

He said he would much rather prevent such infections than try to eliminate them.

The doctor now takes nose swabs of every patient who comes into his orthopedic office to see if they are carriers of MRSA, so they can be treated prophylactically before surgery.

He has found that 6 percent of his patients carry the resistant germs, and he estimates some other populations, like high school athletes, probably have much higher rates.

MRSA infections from cuts or abrasions often can be treated successfully, he said, but they are much more tenacious in artificial joint patients because they attach to the metal surfaces of the implants themselves.

One advantage of the Ibis technology, Dr. Costerton said, is that it not only can figure out which pathogens have infected someone, but can determine whether they have genes for antibiotic resistance.

While Ibis may be the hope for improving future treatment, the best remedy now is the biomedical equivalent of getting down on hands and knees to scrub the kitchen floor.

"That's our current mantra to surgeons now," he said. "Just clean out whatever you can, and try to get it all."


Mark Roth: mroth@post-gazette.com or 412-263-1130. First Published February 24, 2010 5:00 AM


Advertisement
Advertisement
Advertisement

You have 2 remaining free articles this month

Try unlimited digital access

If you are an existing subscriber,
link your account for free access. Start here

You’ve reached the limit of free articles this month.

To continue unlimited reading

If you are an existing subscriber,
link your account for free access. Start here