What is osteoarthritis?
Osteoarthritis, also known as degenerative joint disease (DJD), is the most common type of arthritis. Osteoarthritis is more likely to develop as people age. The changes in osteoarthritis usually occur slowly over many years, though there are occasional exceptions. Inflammation and injury to the joint cause bony changes, deterioration of tendons and ligaments and a breakdown of cartilage, resulting in pain, swelling, and deformity of the joint.
There are two main types of osteoarthritis:
- Primary: Most common, generalized, primarily affects the fingers, thumbs, spine, hips, knees, and the great (big) toes.
- Secondary: Occurs with a pre-existing joint abnormality, including injury or trauma, such as repetitive or sports-related; inflammatory arthritis, such as rheumatoid, psoriatic, or gout; infectious arthritis; genetic joint disorders, such as Ehlers-Danlos (also known as hypermobility or "double-jointed; congenital joint disorders; or metabolic joint disorders.
What is cartilage?
Cartilage is a firm, rubbery, flexible connective tissue covering the ends of bones in normal joints. It is primarily made up of water and proteins whose primary function is to reduce friction in the joints and serve as a "shock absorber." The shock-absorbing quality of normal cartilage comes from its ability to change shape when compressed, because of its high water content. Although cartilage may undergo some repair when damaged, the body does not grow new cartilage after injury. Cartilage is avascular, meaning there are no blood vessels in it. Therefore, healing is a slow process.
Cartilage is made up of two main elements: cells within it known as chondrocytes and a gel-like substance called matrix, composed mostly of water and two types of proteins (collagen and proteoglycans).
- Chondrocytes, and the precursor form chondroblasts, are highly complex multifunctional cartilage cells. Functions include synthesizing and maintaining the extracellular matrix comprised of collagen and proteoglycans that help healthy cartilage grow and heal.
- Collagen is a structural protein found in many tissues such as skin, tendons and bone and is a key structural component of cartilage. Collagen provides cartilage with its strength and creates a framework for the other components.
- Proteoglycans are complex molecules composed of protein and sugar combinations that are interwoven in the matrix of cartilage. Their function is to trap large amounts of water in cartilage, which allows it to change shape when compressed thus acting as a shock absorber. At the same time, proteoglycans repel each other, allowing cartilage the ability to maintain its shape and resilience.
Who is affected by osteoarthritis?
Approximately 80% of older adults, ages 55 years and older, have evidence of osteoarthritis on X-ray. Of these, an estimated 60% experience symptoms. It is estimated that 240 million adults worldwide have symptomatic osteoarthritis, including more than 30 million U.S. adults. Post-menopausal women have an increased incidence of knee osteoarthritis compared to men.
Are there other risk factors for osteoarthritis?
In addition to age and secondary causes such as inflammatory arthritis and prior injury/ trauma, several other risk factors increase the chance of developing osteoarthritis including obesity, diabetes, elevated cholesterol, sex, and genetics.
- Obesity is a risk factor for osteoarthritis, particularly of the knee. In addition to overloading the weight-bearing mechanisms of the body, the metabolic and pro-inflammatory effects of obesity have been studied as contributory to osteoarthritis. Maintaining ideal body weight or losing extra weight is important for those at risk.
- Both diabetes and hyperlipidemia (elevated lipids/cholesterol) contribute to the inflammatory response within the body, increasing the risk of osteoarthritis. Oxidation of lipids can also create deposits in cartilage which affects affecting blood flow of subchondral bone in the same way that blood vessels are affected by atherosclerosis. Elevated blood sugars, as well as elevated cholesterol/lipids, increase free radicals within the body, this oxidative stress exceeds the resilience of cartilage on the cellular level. Controlling diabetes and hyperlipidemia is important for bone health in addition to general health.
- Decreased estrogen as experienced by post-menopausal women increases the risk of knee osteoarthritis as estrogen is protective of bone health specifically reducing oxidative stress to the cartilage.
- Heredity can play a role in osteoarthritis, as individuals born with other bone diseases or genetic traits may be more likely to develop osteoarthritis. For example, Ehlers-Danlos, which is characterized by joint laxity or hypermobility, can contribute to osteoarthritis.
What causes osteoarthritis?
Primary osteoarthritis is a heterogeneous disease meaning it has many different causes, it is not only “wear and tear” arthritis. Some contributing factors to OA are modifiable (can be changed) and others are non-modifiable (cannot be changed such as born with it or now permanent). Age is a contributing factor, although not all older adults develop osteoarthritis and for those who do, not all develop associated pain. As discussed above, there can also be inflammatory and metabolic risks that can increase the incidence of osteoarthritis, particularly in the setting of diabetes and/or elevated cholesterol.
Osteoarthritis can be genetic both as primary such as nodular OA of the hands as well as secondary related to other genetic disorders, such as hypermobility of joints. Inflammatory and infectious arthritis can contribute to the development of secondary osteoarthritis due to chronic inflammation and joint destruction. Previous injuries or traumas including sports-related and repetitive motions can also contribute to osteoarthritis.
Although the exact mechanisms of cartilage loss and bone changes are unknown, advancements have been made in recent years. It is suspected that complex signaling processes, during joint inflammation and defective repair mechanisms in response to injury, gradually wear down cartilage within the joints. Other changes cause the joint to lose mobility and function, resulting in joint pain with activity.