Causes of Osteoarthritis Pain in a Joint

An elderly person suffering from osteoarthritis

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Osteoarthritis pain is clearly the most common and significant symptom associated with the disease. Because articular cartilage has no blood supply and no nerve endings what causes the pain?

Pain Comes From Changes In the Joint

Osteoarthritis pain likely comes from other structures within the affected joint that undergo change when cartilage becomes worn away. While the cartilage may not have nerve endings, the surrounding structures do. Loss of cartilage, development of bone growth around the edges of the joint, and synovitis inflammation set off pain in the nerves in the bones and muscles around the joint.

For example, structures that change with osteoarthritis include:

Subchondral Bone: This is the layer of bone just below the cartilage. When someone has osteoarthritis, there is increased blood flow and other changes that develop in the subchondral layer. These include these processes that may cause osteoarthritis pain around the joint:

  • Subchondral Sclerosis: this is increased bone density and thickening of this layer. It shows up on x-rays as abnormally dense bone along the joint line.
  • Subchondral Cysts: these are fluid-filled sacs which extrude from the joint, often found in early phases of osteoarthritis.
  • Increased pressure within the bone.

Joint Margin: There can be thickening of the joint capsule and the formation of osteophytes that may cause pain. Osteophytes are also known as bone spurs. They are common in aging joints and may or may not be associated with pain. They develop as the body tries to repair damaged cartilage.

Capsule and Synovium: There can be increased thickness of the synovium and mild inflammation at this site that causes pain. This narrows the joint and increases pressure within the joint.

Tendons and Bursa: Tendinitis and bursitis occurring around a joint may cause pain, decreased range of motion, muscle wasting, and muscle weakness.

In a normal joint, cartilage surrounds the end of each bone. It is hard but slippery and made up of fibrous collagen, mesh-like proteoglycans and the chondrocyte cells that produce and maintain the cartilage. The cartilage along with the proper amount of synovial fluid keep the joint lubricated and moving smoothly within the joint capsule. The ligaments, tendons, and muscles that move the joint are able to do so without snags.

With osteoarthritis, the cartilage has worn away in places and the bone can grow spurs around the edges of the joint. These bony growths prevent the smooth function of the joint. The cartilage that allowed the bones to slip past each other smoothly is damaged or worn away and can't perform this essential function. Synovitis develops, inflammation causes synovial fluid to increase, making the joints swollen and stiff.

With the further breakdown of cartilage, thickening of the ends of the bones and synovium, you have a bone-on-bone rubbing that causes even more pain.

The disease process associated with osteoarthritis is complicated. It's important to treat osteoarthritis pain and to get quick relief when possible, but it's also important to understand the source of the pain.

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Article Sources
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