The Effects of Obesity on the Knee & Hip

According to the American Academy of Orthopaedic Surgeons (AAOS) obese individuals are 20x more likely to need a knee replacement.¹ Further, up to 97% of total knee replacements in 2004 can be attributed to osteoarthritis (OA).² We know that OA of the hip is also accelerated in obese patients. Knowing recent obesity trends, one can assume that the numbers have not improved. This statistic not only shows the damage that excess weight causes, but how preventable knee surgery may be if only weight issues were addressed earlier.

We have learned a lot more about obesity and its effects on the knee and hip. It stands to reason that excess weight would add additional pressure, especially on the knee, but the pathogenesis of obesity-related knee osteoarthritis and the metabolic and inflammatory causes has become clearer.

The primary driver of this research is a significant increase in obesity across the entire US population, and around the world, which has led to an increase in knee and hip osteoarthritis, amongst other bone and joint problems.

The Old Thinking on the Osteoarthritis

Just a few decades ago we believed that osteoarthritis of the knee and hip was solely due to the wear and tear caused by the excess pressure and tension on the knee. The bones in the knee and hip are separated by a thin layer which allows the joint to glide without causing pain, called smooth cartilage. But this cartilage is soft and can be damaged with too much pressure. We know that for every excess pound of body weight, approximately four-to-seven extra pounds of pressure are placed on the knees and hips. As obesity has accelerated, the degree of joint degeneration has as well, and more and more patients are visiting their orthopedic surgeons at younger ages.

While we have realized that osteoarthritis is certainly, in part, caused by wear and tear, there are actually other, more insidious issues at play that go far beyond extra pressure.

Visceral fat is the white or “bad” body fat that accumulates around the abdomen and is directly related to many health problems associated with obesity. This fat actually creates a hormonal and chemical imbalance in the body that allows pro-inflammatory cytokines to run rampant. This can actually cause and accelerate joint degeneration due to local inflammatory response.²

It’s also very interesting to note the results of a 2010 study, which argues that all forms of obesity are not equal.² In other words, two people with the same excess body weight, may not actually have the same inflammatory response, pound-for-pound. Rather, it seems that both blood sugar and blood cholesterol play a role in increasing joint degradation due to obesity.

Young, overweight or obese people are also susceptible to the problems associated with their weight. Adolescents may suffer from Slipped Capital Femoral Epiphysis, where the ball of the hip joint slips backward as a result of the added stress to the growth plate, often due to hormonal changes caused by obesity. Blount’s disease, or bowing of the legs, fractures and flat feet are all more common in children and adolescents with a weight problem.

The bottom line is that newer research has uncovered the detrimental effects of obesity and how excess visceral fat tissue poses a concern far beyond additional weight pressures on joints. The wear and tear, combined with pro-inflammatory response, creates a perfect storm of joint degradation that has not only accelerated the number of our osteoarthritic knee problems that lead to surgery, but also makes the treatment and outcomes of such procedures less predictable.

References:

• ¹ A. (2015, March). The Impact of Obesity on Bone and Joint Health [PDF]. Chicago, Illinois: American Academy of Orthopaedic Surgeons.
• ² Sowers MR, Karvonen-Gutierrez CA. The evolving role of obesity in knee osteoarthritis. Curr Opin Rheumatol. 2010;22(5):533-537. doi:10.1097/BOR.0b013e32833b4682