Osteoarthritis (OA) is the most common joint disease. It affects approximately 28 million Americans and tends to become more common with increasing age. It is a universal condition in people past the age of 70 although not everyone is symptomatic. It is a disease that affects articular cartilage, the gristle that caps the ends of long bones.
There are risk factors for the disease. Increasing age, as mentioned earlier is one. In addition, female gender, genetic predisposition, obesity, and trauma are the other important items.
The most frequent targets for OA to strike are the neck, low back, fingers, base of the thumb, knees, and hips.
Less commonly the ankles, shoulders, and elbows are involved. In these areas, antecedent trauma appears to be the primary cause.
OA is what is called, a focal disease. What that means is that it doesn’t affect the whole joint. It preferentially attacks certain areas within the joint. An example would be the knee where the medial (inside) part of the joint is affected far more often than the lateral (outside) part.
In the hip, the top part of the joint tends to become involved while the rest of the joint is relatively spared. The same is true for other joints affected by OA.
Symptoms of the condition consist of pain that is aggravated by use and relieved by rest. Also, there is short term stiffness with inactivity. This is called “gelling.” Night time pain is also a common feature. Another common lament is crunching of the joint with movement. This is called “crepitus.”
On exam the rheumatologist will often spot swelling due to the formation of bone spurs, called “osteophytes.” There can be tenderness of the joint, pain with movement, swelling due to fluid accumulation, and muscle wasting around the joint.
In advanced cases, there is clear deformity and sometimes evidence of instability.
Laboratory tests are usually normal. Imaging studies can help with the diagnosis. Magnetic resonance imaging will pick up early changes. X-ray findings also can help with the diagnosis. The problem is that if x-ray changes are evident, then the disease has progressed substantially. The primary changes seen on x-ray are narrowing of the joint, bone spurs, and changes in the bone underlying the cartilage.
Joint fluid, if present, should be aspirated. The joint fluid is typically viscous, translucent, and has fewer than 200 white blood cells per cubic milliliter. Occasionally the white blood cell count will be higher if a patient has a particularly inflammatory form of osteoarthritis. This is the conundrum of OA. While the old thinking was that the condition was primarily a mechanical disease, it has become quite clear that OA has a significant inflammatory component as well.
What has been a perplexing question is, “What causes pain in OA of the knee?” Cartilage has no blood vessels nor does it have nerves. So the topic of pain mechanism in osteoarthritis has been the subject of intense interest.
There are a number of potential suspects. For example, when osteophytes (bone spurs) develop, they can lift the periosteum (the thin top layer of the bone). Periosteum is rich in nerve fibers and certainly can be a source of pain.
It has been noted that blood vessels in bone underlying osteoarthritic cartilage can become engorged and this may elevate the pressure inside the bone which could also, theoretically, cause pain.
The lining of the joint (synovium) becomes inflamed in OA. Pain fibers are located within the joint capsule and these inflammatory processes could irritate them.
The joint capsule can contract or shrink leading to irritation of nerve fibers located within the capsule.
By the same token, if fluid builds up within the joint, it can stretch the joint capsule again leading to stimulation of pain fibers.
As mentioned earlier, there are two small pieces of fibrocartilage located within the knee. These pieces of fibrocartilage (menisci) have a rich blood and nerve supply where they attach to the joint capsule. OA often leads to tearing of these menisci. This can cause damage to the capsular attachment leading to pain.
Spasm of the muscles surrounding the knee can also lead to pain.
Finally, there is increasing interest in the role of the central nervous system- the brain- in causing the pain of knee OA. Recent studies showing the effectiveness of drugs like Cymbalta, a drug originally prescribed for depression, but also showing beneficial effects on pain relief in patients with OA, led to FDA approval for this indication in 2011.
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