A woman arrived at the emergency room with pain and swelling around her knee replacement, which had been fitted five years previously. Doctors eventually identified that her tooth flossing was the likely, albeit unusual, culprit. 

When she arrived at the hospital with chills and in pain, doctors decided to take a fluid sample from around the prosthetic. It turned out that her knee was infected with Streptococcus gordonii bacteria. This was perplexing since this bacterium is usually found in the mouth. How could this species suddenly wind up around the woman's prosthetic knee after five years with no problems?

It turns out that the answer was a sudden change in the woman's dental hygiene routine. She had started a vigorous dental flossing regimen that had caused her gums to bleed.

Her doctors said in their report that it is likely that her vigorous flossing introduced the bacteria into her bloodstream through tiny cuts in the gums. The bacteria then made their way through the bloodstream and settled onto her knee replacement. You can see their report in the BMJ Case Reports.

It was easy for the bacteria to hang out on the prosthesis, as opposed to elsewhere in the body, because the prosthetic joint lacks a line of defense that the rest of the body has: an immune system. With an inability to fend off potentially problematic microbes, organisms happily grow on artificial joints. 

Any infections like this need to be treated quickly. If left too long, the bacteria can create a biofilm barrier around the prosthesis. If the bacteria have time to build up, the prosthesis becomes incredibly difficult to remove. 

To treat the infection, the doctors opened up the woman's knee and washed out as much of the bacteria as they could. They then prescribed a long-term course of antibiotics. Fortunately, the woman was able to keep her original knee replacement. However, she will have to take antibiotics for as long as she has the prosthesis.

It must be emphasized that this is just one case and that the majority of people floss without issue. Even if you have a replacement joint, there is no reason to stop flossing. It is not advised to floss until your gums bleed, though.

Knee joint infections after a knee replacement occur in fewer than 2% of patients. This number is already very low, and the percent that suffer from this specific condition is even lower.

There have been other cases of a knee joint replacement being infected with S. gordonii but without the history of vigorous dental care. A 78-year-old woman had a relatively similar case in Belgium. She too had swelling around her prosthetic knee from S. gordonii bacteria, but she also had swelling in her aorta, an artery in the heart. This patient had her knee prosthesis replaced as well as a valve replacement in her heart. You can read the report in BMC Infectious Diseases.

[H/T: Live Science]