Why do some people get hip and knee arthritis and others don't? I'm in my 70s and most of my friends still alive have a hip or knee replacement. I don't have a single ounce of pain or problems in any joint!

You are, indeed, most fortunate. Doctors are trying to understand why some people are at increased risk for osteoarthritis (OA). There are many theories based on current research. Dr. David Felson from Boston University School of Medicine offers some ideas.

Older age and female gender are top on the list as risk factors for OA. Aging by itself isn't enough as you have pointed out. Changes in the joint and loss of muscle strength are usually a part of the picture. As adults reach age 75 they have less joint pain than at younger ages. This may be because they are less active.

Ligaments tend to loosen up with age and the cartilage thins out. Some joints seem more susceptible to this from load and stress on the joint. Perhaps it's a genetic-based event.

It's not clear why women have more OA than men. Studies haven't shown a hormonal link yet. Diet and lifestyle may be key factors. More study is needed to compare men and women before an answer will be known.