The spine is almost always under pressure when upright, and therefore prone to the wear-and-tear that leads to osteoarthritis.
This degenerative condition can cause minor to debilitating pain. Understanding how osteoarthritis causes back pain can help patients stop or slow the disease's progression and also reduce pain.
Osteoarthritic Changes in Vertebrae
Facet joints connect the vertebral bone. On the back of each vertebra there are two upper and two lower facet joints. Facets are small, boney projections with smooth, flat surfaces that are normally covered in protective articular cartilage. The facet joint, where two facets meet, is wrapped a fluid-filled sac called a facet capsule.
Each facet joint is an integral part of a larger spinal segment, made up of two vertebrae that are attached by ligaments and separated by a vertebral disc. Each segment bears a portion of the total pressure, or load, put on the entire spine. Within the segment, the vertebra below is responsible for preventing the vertebra above from slipping forward, out of alignment.
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When spinal osteoarthritis occurs, the facets' cartilage is thinned, damaged, or missing. Without adequate cartilage to serve as a buffer, the vertebrae's facets rub against each other. This friction can cause the following changes to take place:
- New cartilage may be produced as the body tries to recover from the loss of cartilage, but this cartilage growth is limited and inadequate, growing in irregular, bumpy patterns.
- The joint space narrows, i.e. the space between the bones becomes smaller. In turn, the smaller spaces between the bones may impinge on a nerve.
- To compensate for the deteriorated or missing cartilage, the vertebrae may produce small scalloped growths called bone spurs, or osteophytes. These boney growths can:
- Create even more bone-on-bone friction in the spine
- Impinge nerves, ligaments or other nearby soft tissue
- The bone may erode and become less dense.
- Synovial cysts may form in the joint. These fluid-filled sacs are not harmful unless they impinge on spinal canal or nerve roots. Synovial cysts are common in lumbar osteoarthritis but are rare in cervical osteoarthritis.
- The joint may swell and become stiff.
- Degeneration may lead to the wearing out of the facet capsule as well as abnormal joint motion.
Because cartilage does not contain nerves, damaged cartilage is not a source of pain. Likewise, bone spurs are normal signs of aging, and their presence alone is not a cause for concern.
It is the bone friction, nerve impingement, inflammation and other resulting abnormalities in the spine that can cause symptoms. The facet capsule does have innervation. Inflammation or abnormal movement can trigger pain in this tissue.