What is osteoarthritis?

Osteoarthritis, also known as degenerative joint disease or age-related arthritis, 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 a breaking down of cartilage tissues, resulting in pain, swelling, and deformity of the joint.

There are two main types of osteoarthritis:

  • Primary: more generalized osteoarthritis that affects the fingers, thumbs, spine, hips, and knees
  • Secondary: osteoarthritis that occurs after injury, such as repetitive or sports-related injury, or inflammation in a joint

What is cartilage?

Cartilage is a firm, rubbery material 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 made up of two main elements: a gel-like substance called matrix, composed mostly of water and two types of proteins (collagen and proteoglycans), and cells known as chondrocytes that are contained within the matrix.

  • 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 of cartilage.
  • 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 and thus act as shock absorber.
  • Water makes up a large part of healthy cartilage. Water helps to form a "gel-like" matrix with the other components outlined, allowing it to function as the "shock absorber" for the joint. In diseases like osteoarthritis, cartilage loses a lot of its water content. This water loss reduces protection of the joint. Water content may also diminish with age, as older individuals are more prone to having osteoarthritis.
  • Chondrocytes are highly complex multifunctional cartilage cells. Functions include production of collagen, proteoglycans, and enzymes, all of which help healthy cartilage grow. As cartilage forms, these cells move through the different layers of cartilage, changing as they do so by mechanisms that are not fully understood.

Who is affected by osteoarthritis?

Although most older individuals (over 60 years old) have evidence of osteoarthritis by X-ray, most people have no symptoms. More than 27 million U.S. adults have symptomatic osteoarthritis. Women are three times more likely than men to develop primary generalized osteoarthritis. Because age is a risk factor, people are more likely to develop this form of arthritis as they get older.

What causes osteoarthritis?

Primary generalized osteoarthritis affects women more than men, often occurring shortly after menopause. Most people with primary osteoarthritis have other family members who are affected by the same problem, and usually have swelling and or pain of the finger joints, base of the thumbs, and knees or hips. Secondary osteoarthritis may occur in a joint that was previously injured from trauma, or after the joint has been damaged by some other cause, such as infection or rheumatoid arthritis.

Although the exact mechanisms of cartilage loss and bone changes are unknown, great advancements have been made in recent years. It is likely 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.

Are there other risk factors for osteoarthritis?

In addition to hereditary factors (genes) and other forms of arthritis, several other risk factors increase the risk for developing osteoarthritis, including other hereditary disorders, obesity, and injuries to or around the joint.

  • Heredity plays a role in osteoarthritis, as individuals born with other diseases are more likely to develop osteoarthritis. Examples include conditions associated with increased joint laxity (being "double-jointed"), some dysplasias (abnormal growths of bone and cartilage), and Paget's disease (a type of inflammation in bone that occurs in older people).
  • Obesity is a risk factor for osteoarthritis of the lower extremity and spine. Being overweight also may speed the rate of development of osteoarthritis. Maintaining ideal body weight or losing extra weight is important for those at risk.
  • Injuries (such as those occurring in many athletes) contribute to the development of osteoarthritis. An injury that results in abnormal stress on a joint is another important risk factor for osteoarthritis. Overuse injuries are more controversial, as many people who have similar lifestyles do not develop osteoarthritis. However, repetitive trauma – which may result from overuse – does increase that risk.
  • Other conditions that may lead to the development of osteoarthritis include peripheral neuropathies (diseases of the nervous system) and neuromuscular disorders that put abnormal stress on the joint.

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