Why do obese people have a higher incidence of osteoarthritis compared to the non-obese population?
The thought has been that the increased weight causes mechanical “wear and tear” on the articular cartilage, leading to arthritis. Research is now suggesting different reasons. An outstanding review article was published in the March, 2013 edition of the American Journal of Orthopaedic Surgeons, authored by Dr. Ryan Koonce and Dr. Jonathan Bravman discuss the scientific link between obesity and osteoarthritis including knee osteoarthritis.
The definition of obesity is based in body mass index (BMI) not total body weight. BMI is calculated by taking the weight of the person in kilograms divide by their height in meters squared. Obesity is defined as a BMI of greater than 30 whereas over weight is defined as a BMI between 25 and 29. In the US 68% of the population is overweight and 34% are obese. These numbers have double in the last twenty years. Obesity is associated with a variety of medical conditions such as coronary artery disease, diabetes, asthma and some types of cancer.
Quality adjusted life years (QALY) lost is a way to calculate the combination of premature mortality and quality of life lost because of the disease. Obesity resulted in an increase of quality life years lost of 127% from 1993 to 2008. Obesity results in more health problems in relation to quality of life and health care dollars spent than cigarette smoking or alcohol consumption.
Up to now it has been assumed that obesity plays a role in development of osteoarthritis (OA) including knee osteoarthritis because of an increased joint reaction force, also known as wear and tear. Increased weight results in increased force on the joint causing the breakdown of the articular cartilage resulting in OA including knee osteoarthritis. There is some scientific data to support this hypothesis but other areas of research are revealing that other mechanisms also exist.
New information now relates a complex interplay between the chondrocytes and mechanical pressure. Chondrocytes are the cells that make up cartilage in our joints. Mechanoreceptors have been identified in the chondrocytes that are located in the articular cartilage in the knee joint. There are 3 different receptors that have been identified: stretch-activated channels, alpha-5 beta-1 integrin and CD 44. If any of the three mechanoreceptors are activated they release enzymes that will lead to the expression of cytokines. The cytokines can effect cartilage causes it to degenerate.
There is also an inflammatory component that causes osteoarthritis like knee osteoarthritis. We know that non weight bearing joints also have a higher rate of OA in obesity patients. Joints that are located in the hands are an example of non-weight bearing joints. The increase of pro-inflammatory markers and cytokines are caused from the excess adipose tissue. White adipose tissue is vast in obese adults. It is this tissue that secretes pro inflammatory and cytokine enzymes. The amount of pro inflammatory and cytokine enzymes that are produced are directly related amount of excess white adipose tissue. Examples of pro inflammatory enzymes are: sed rates, interleukin-6 , tumor necrosis factor and high sensitive CRP.
Cytokines produced by white adipose tissue are called adipocytokines and are found in high concentration in plasma and synovial fluid in obese patients. Leptin is a common adipocytokine that also is involved with regulation of metabolism and appetite and is currently being intensively studied. Research is being done at this time to figure out a way to regulate these powerful hormones which may help prevent knee osteoarthritis in the future.
Obesity is also associated with chronic musculoskeletal pain and weight gain early in life is associated with higher rates of joint replacement surgery. Weight lost is very important. It is estimated that reversing the obesity rates to what they were 10 years ago would have reduced the amount of knee osteoarthritis by 50%, averting close to 900 thousand cases of diabetes and reduced coronary artery disease by 178 thousand cases.
Bariatric surgery is an option but not the best way to lose weight. Bariatric surgery has been associated with reduced calcium reabsorption which can lead to increased fracture rates in addition to the risks associated with undergoing a surgical procedure. Surgical complication rates increase with obesity.
Exercise and reduced caloric intake are the safest ways to lose weight. One of the most successful weight loss programs is through Weight Watchers, which can be done online or at local meetings. Another excellent resource is thought the National Institute of Diabetes, Digestive and Kidney Diseases. Their online resource is “Choosing a Safe and Successful Weight Loss Program.”
In summary, obesity results in OA in three different mechanisms:
- Joint reaction forces aka wear and tear
- Through the secretion of enzymes secondary to mechanoreceptors located in chondrocytes, and
- Through pro inflammatory / cytokine enzyme secretion located white adipose tissue.
Weight reduction helps to reduce all three mechanisms best achieved through caloric intake reduction and exercise. For more information on treatment for knee osteoarthritis and arthritis, contact Louisville Orthopedic Dr. Stacie Grossfeld at 502-212-2663.
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