A study released by a team of investigators from the Centers for Disease Control and Prevention (CDC) in Atlanta and the University of North Carolina at Chapel Hill published in Arthritis Care & Research shows that the risk of knee osteoarthritis is far greater than experts had known. We interviewed knee study authors Louise Murphy, Ph.D., an epidemiologist at the CDC’s Arthritis Program, and Cpt. Chad Helmick, M.D., a medical epidemiologist in the commissioned corps of the Public Health Service.
AT: Tell us a little bit about the significance of this knee study.
Dr. Helmick: We’ve had prevalence estimates before, but this study is very different. I don’t think we’ve had any lifetime risk estimates before in the field of rheumatology. As far as I know, these are the first ones looking at knee osteoarthritis.
AT: Can you explain what having a 1 in 2 risk of knee osteoarthritis means?
Dr. Helmick: It’s a number that’s a way of communicating the risk of developing osteoarthritis of the knee over the course of your lifetime. The risk rises with age, so the older you get, the more likely it is that you will have osteoarthritis in at least one knee. We’re working on getting the risk numbers for more specific age groups, from age 40-49, for example, and from age 50 to 59 and so on.
This number is also an important tool for public policy makers. The breast cancer number [1 in 8] everyone knows. This is even higher than it is for breast cancer. Hopefully the government will put more resources into arthritis when people realize its true impact.
AT: Your definition of osteoarthritis included people with mild osteoarthritis scores on radiographs [a kind of X-ray]. Other researchers have concentrated counting cases that score in the moderate to severe range on radiographs. Do you think including mild cases in this knee study could have exaggerated the risk?
Dr. Helmick: We concentrated on symptomatic knee osteoarthritis in our study. There’s a large movement in the field of rheumatology away from using radiographic measures of disease, because how a knee looks on a radiograph doesn’t seem to correlate to the kinds of symptoms people have. Some people have radiographs that show a lot of damage, but have almost no symptoms, while other people have almost no damage but are in excruciating pain.
AT: Critics have questioned whether results from the knee study of residents in rural farming communities can really be generalized to the rest of the country. Some studies have found, for example, that farmers are at higher risk for getting osteoarthritis of the hip, though researchers aren’t sure if that’s because of their occupation, which requires heavy physical labor. Do you think that results from Johnston County can be applied to people in the rest of the country?
Dr. Helmick: That’s a good question. There are always going to be questions, when you’re studying one population, about how it can be applied around the rest of the country.
AT: What other attributes of this population may influence whether or not the results can be applied broadly?
Dr. Helmick: Participants in this study were a little bit less educated, a little bit lower income, and they’re in an area that’s transitioning from rural to suburban. But we had a significant number of African-Americans in our study, which is one of its great strengths. If you look at Framingham [another population-based study in Framingham, Mass.], that’s pretty much an all-white community. In Johnston County, we’ve at least covered that.
AT: This study suggests that people should watch their weight and try to prevent injuries if they want to lower their risk of getting arthritis in their knees. Do you agree with that?
Dr. Helmick: We study authors are a little bit nervous about pushing prevention too much. Based on these results it would seem like keeping your weight down and preventing injuries would prevent arthritis, but so many times in science, when you go and test these logical assumptions, they just aren’t true. What we can say is that preventing injuries and maintaining a healthy weight is certainly prudent and won’t cause any harm. We’re just not sure what prevents arthritis yet, so we can’t say for sure that these measures help.
Dr. Murphy: We certainly endorse participation in regular physical activity, which seems to prevent and improve the symptoms of arthritis.
AT: What effects might this knee study have?
Dr. Murphy: I hope our study will raise awareness that this is a big problem, and it’s a problem across all age groups. It’s not just a big problem when you get to be age 85.