Pain in a joint may develop in the joint capsule, the lining of the bone (periosteum), the bone, or the lining of the joint (synovium). The cartilage within the joints that cushion movement and stress have no nerves and cannot cause pain by themselves. Joint pain does not necessarily arise within the joint. The pain may actually be due to a problem in a structure nearby, such as a bursa, ligament, tendon, bone, or muscle. It can even be referred pain from an internal organ or a distant part of a nerve, which feels like it is coming from the joint.
There are several disease mechanisms which can cause pain within the joint. Disease and degeneration can develop as a result of inflammation, infection, wear and tear over time, acute injury, or the accumulation of crystals in the soft tissues of the joints. Here are some examples of these pathological processes.
Inflammatory diseases include the auto-immune disorders such as rheumatoid arthritis and lupus. The reaction generally involves the synovial membrane, which lines the inside of the joint. Lymphocytes and plasma cells have been converted by the disease from protectors to attackers, infiltrating the synovium. The membrane swells and becomes ragged. A process called neovascularization creates erratic, deviant blood vessels. The swollen joint lining can cause erosion of the bone and cartilage within the joint, which becomes red, swollen, and painful.
Infection can develop within the closed space of a joint, usually from bacteria, viruses, or even fungi brought in by blood from an infection elsewhere. Occasionally the disease occurs after a puncture of the joint with subsequent contamination. The microbes, generally a bacteria called Staphylococcus aureus, lodge in the synovium where the devastating pathological changes occur rapidly. The membrane is destroyed in places, replaced by fibrous scar tissue. The infection usually involves just one joint, generally the knee. It becomes very red, hot, and swollen with associated systemic symptoms like fever, chills, and body aches. The source of the infection may be an abscess far removed from the knee or the source may never be found. Treatment needs to be started quickly in order to prevent permanent damage to the joint.
Degeneration of the joint develops in almost everyone who leads an active life. This is usually called osteoarthritis and is the commonest form of arthritis. Millions of people experience the pain of osteoarthritis every day. The major problem is the wear and tear on the cartilages, usually in the knees, hands, spine, and hips. The cartilages erode away over the years, especially in athletes, farm workers, or anyone who spends a lot of time walking, running, and working with their hands. The damage cannot be reversed, producing pain and tenderness, swelling, stiffness, and decreased range of motion in the joint. The joint can also be noisy with crackling and crunching sounds. Treatment targets symptoms. In more serious cases, joint replacement may be necessary.
Trauma can also cause devastating injuries to joints. Damage is often extensive, requiring surgical repair. Minor trauma, such as a sprain or contusion, is usually self-healing. Repetitive injuries, as seen in athletes, like football and soccer players, accumulate damage, leading to early development of degenerative arthritis. Some trauma, especially associated with lacerations, even if there does not appear to be penetration of the joint, are treated with antibiotics.
Crystal deposits in the soft tissue, as in gout and pseudogout, produce an extremely red, swollen, angry appearing joint. Gout occurs when uric acid levels in the blood get too high. It usually manifests in the MCP joint of the big toe. Causes include kidney disease, excessive alcohol intake, a diet high in purine containing foods, and genetic disorders. Pseudogout, which is caused by crystals, generally presents with a red, hot, swollen knee. These crystals are found in the knees of over 50% of elderly people, yet only a small percentage develop the disease.
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