It is important to know the basic anatomy of a joint and how it works in order to understand some of the disease processes that can affect joints. Most actual joint diseases (as well as joint pain) are types of arthritis. There are several different kinds of arthritis, and their actions are different even if the end result is the same.
Basic Anatomy
A joint is where two bones meet up. The bones have cartilage on the ends that act as shock absorbers and allow for smooth movement. The two bones are encased in ligamentous tissue (a tough fibrous tissue) called the joint capsule, inside of which there is a layer of tissue called synovial tissue that produces synovial fluid. This fluid is the "grease" of the joint and allows for some shock absorption as well as smooth movement within the joint.
Autoimmune
This is when the immune system no longer recognizes its own body tissues. The body will start creating antibodies to the cartilage or other tissues within the joints. Rheumatoid arthritis is a type of autoimmune joint disease. The body will attack the cartilage tissues and the cartilage will start to lose its smooth texture and become rough. When no longer smooth, the joint itself will start to alter in function and movement. Bony spurs will grow under the cartilage as an attempt to help the joint heal. Eventually the cartilage can wear down completely.
Inflammatory
This process is either the result of poor diet choices, injury or infection. Many of the auto immune diseases can become inflammatory diseases as well. This type of joint disease is any process that causes the joint to be red, painful and swollen; it can cause the joint to swell to the point where fluid has to be removed (e.g., water on the knee). Gout is a type of inflammatory disease in the joint. It can cause severe pain and swelling. Inflammatory joint disease can be a primary cause on its own or secondary to another disease process like Crohn's disease (enteropathic arthritis), psoriasis or something like Reiter's syndrome (a triad of symptoms that often appear after contracting a venereal disease). The inflammatory joint diseases will often have periods of exacerbation and remission.
Degenerative
This is the most "simple" of the joint diseases in that it is directly related to trauma, injury or damage to structures or tissues within the joint. If the joint loses the smoothness to the cartilage, or there is damage to the tissues that make the joint fluid, the joint will start to catch and not move as well. Pain may or may not initially be associated with this process. If the joint cannot heal itself, the altered motion will cause wear and tear to the cartilage, eventually thinning and causing the bone to spur.
Infectious
The joint can become inflamed due to an infection in the body or within the joint. This is called septic arthritis, and is usually caused by a pathogen in the blood or direct transfer into the joint through trauma. This process can be very fast, quickly disintegrating the joint. If left untreated, this process is very likely to cause bony fusion within the joint.
References
- "Essentials of Skeletal Radiology, second edition;" Terry Yochum and Lindsay Rowe; 1996
