Q. I'm 72 years old. I've had left knee pain on and off for several months. My doctor sent me for an MRI of both knees. It showed "mild to moderate osteoarthritis" in both knees and a torn meniscus in my right knee, which feels fine. Do I need to do anything about the torn meniscus in my "good" knee?
A. You ask an interesting question. Let's start with some background information. The meniscus is a crescent-shaped disk of fibrous tissue and cartilage. Each knee has two menisci located between the thighbone (femur) and the lower leg bone (tibia).
The menisci act as shock absorbers. They protect the other cartilage tissue that covers the end portions of the femur and tibia. The kind of injury most likely to tear a meniscus is a twisting injury of the knee. Often this occurs during sports.
In older people, however, the meniscus becomes more prone to injury; it can tear for no apparent reason. Meniscal tears are more common with increasing age.
When a meniscus tears, you sometimes know it: There's a "pop" or a twinge of discomfort in the knee. But sometimes there are no symptoms when a meniscus gets torn.
After a meniscus tears, you can have different symptoms. Some of my patients with a torn meniscus suddenly have trouble kneeling or squatting: It hurts when they do it, and it really hurts when they try to stand up.
Other patients report that the knee sometimes "locks" or "catches" just during walking. Sometimes they say the knee will just give out on them. Sometimes it's even more subtle: They just have the impression that something is wrong in the knee, that it's not operating properly.
If a meniscus gets badly torn, it provides less protection to the ends of the bones. This can lead to osteoarthritis, as was diagnosed on your magnetic resonance imaging scan (MRI). Osteoarthritis is the most common form of arthritis.
Should you have treatment on your "good" knee? Surprisingly, meniscal tears are nearly as common in people who have no knee pain as in those with pain. MRIs are so sensitive that they often detect abnormalities that are not causing any problems to the patient.
Doctors should always treat patients, not X-rays or MRIs. Since your right knee feels fine, there is no need to treat the meniscal tear.
If you had pain -- or if you develop pain -- the decision would be more difficult. It would be hard to know if your pain was caused by the arthritis, the tear or both. If your symptoms were mild, it would probably be best to treat the arthritis first. If your symptoms got worse, you could consider meniscus surgery, which can help relieve the pain.
That's the long answer to your question. Here's the short version: If it ain't broke, but it ain't hurting, don't fix it.
• Dr. Komaroff is a physician and professor at Harvard Medical School. Send questions to AskDoctorK.com.