As evidenced by the recent surge of

As evidenced by the recent surge of media attention, it is generally agreed that for the most part Americans are overweight. In fact, hundreds of thousands of Americans are morbidly obese. Doctors use the body mass index, or BMI to measure if a person is obese. (Click here for BMI calculations)

Obesity has many wide-reaching effects on health, but one that is not discussed as much is the effect weight has on your joints. While there is some controversy as to whether being overweight or obese is a direct cause of arthritis, it is generally agreed that excess weight can affect joint health. For example, osteoarthritis, a common type of joint arthritis, occurs when the cartilage on the ends of your bones wears away with use. As this cushion degenerates, the bones rub against each other, causing pain (Fig.1). Since excess weight puts more force on your joints, it seems logical that you want to avoid that extra force.

The field of biomechanics (the relationship between forces and motion in the body) gives us some interesting information about loads that our weightbearing joints undergo. In the hip and the knee, two joints that are commonly affected by arthritis, joint forces are approximately 1.5 times body weight when walking on level ground. This means that when a person weighing 200 pounds is walking along a level sidewalk, the forces on the joint are the same as for a 300-pound person standing still. Other activities place even greater forces on the joints. Getting up out of a chair increases the force to approximately 2 to 3 times body weight. The same is true of going up and down stairs. With more vigorous activities, such as running or jumping, the forces on the joints can approach 4 to 5 times body weight. For patients who are overweight, this means dramatically increased force on the joints (Fig. 2).

Unfortunately, obesity accompanied by arthritis creates a vicious circle of inactivity and pain that prevents exercise. Patients with arthritic joints have a tendency to be more sedentary, which leads to weight gain. The weight gain in turn can make exercise more difficult, so the patient becomes more sedentary, creating the full effect of a negative spiral.

When joints become too worn and painful, joint replacement may be necessary. However, since obesity increases other health problems, it can be a factor when patients are considering joint replacement. Being morbidly obese increases your risk of complications after surgery. At one time it was thought that obesity increased the risk of failure of joint replacements, but this has not been proved. In fact, obese patients do quite well with joint replacement once they are over the initial operation, rehabilitation, and the healing process.

Losing weight is the best way to help your joints. But if your joint pain is so severe that it affects your daily living, then being obese does not prevent you from having a joint replacement operation.