Gum disease is common among people with arthritis
People who have both gum disease and rheumatoid arthritis can relieve both conditions by treating their mouth infection, US researchers have found.
Patients who had treatments such as scaling and improved oral hygiene also saw their arthritis symptoms lessened.
Gum care plus arthritis drugs was the best combination, the Journal of Periodontology study found.
Dental experts said the work supported previous research which found removing teeth could relieve arthritic pain.
Gum disease is prevalent in people with rheumatoid arthritis - and vice versa. In both conditions, soft and hard tissues are destroyed due to inflammation caused by toxins from bacterial infection.
Rheumatoid arthritis is an incurable disease affecting mainly the small joints such as hands and feet, and affects around half a million people in the UK.
The researchers from Case Western Reserve University School of Dental Medicine and University Hospitals of Cleveland studied 40 patients who had both moderate to severe periodontal disease and a severe form of rheumatoid arthritis.
One protein, called tumour necrosis factor (TNF), is present in the blood when there is inflammation.
The study's participants were divided into four groups.
Two groups received anti-TNF arthritis drugs.
One of these groups also received standard non-surgical dental treatment to clean and remove the infection from the bones and tissues in the gum areas. The other did not.
A third group was given dental treatment alone and the fourth was given nothing.
Those who were given the dental treatment saw an improvement in arthritis symptoms, such as swollen joints and pain, but those who were given both dental treatment and anti-TNF drugs saw the biggest improvement.
Nabil Bissada, head of the department of periodontics at the dental school, said: "It was exciting to find that if we eliminated the infection and inflammation in the gums, then patients with a severe kind of active rheumatoid arthritis reported improvement on the signs and symptoms of that disease.
"It gives us a new intervention."
The scientists said rheumatologists should encourage their patients to be aware of the link between gum disease and arthritis.
Dr Nigel Carter, chief executive of the British Dental Foundation, said: "This research supports existing evidence which found that extracting painful teeth had a positive impact on arthritic pain.
"Visiting the dentist is an important part of our overall health routine - especially as research potentially links gum disease to not only arthritis, but heart disease, strokes, diabetes and premature births.
"Brushing twice a day with fluoride toothpaste, cleaning between teeth daily with floss or an inter-dental brush and visiting the dentist regularly, as often as they recommend, will all help you to look after yourself."
Professor Rob Moots from the Arthritis Research Campaign said: "Rheumatoid arthritis is a disease of inflammation caused by dysfunction in the immune system.
"As yet we don't understand what triggers it, but it seems sensible to think that infection might trigger something in the immune system. It therefore follows that chronic infection aggravates rheumatoid arthritis and dealing with that infection may help rheumatoid arthritis too.
"This study confirms that people with rheumatoid arthritis should look after their whole body, including their gums and teeth - and not just their joints."