Joint
A joint or articulation (or articular surface) is the connection made between bones in the body which link the skeletal system into a functional whole. They are constructed to allow for different degrees and types of movement. Some joints, such as the knee, elbow, and shoulder, are self-lubricating, almost frictionless, and are able to withstand compression and maintain heavy loads while still executing smooth and precise movements. Other joints such as sutures between the bones of the skull permit very little movement (only during birth) in order to protect the brain and the sense organs. The connection between  a tooth and the jawbone is also called a joint, and is described as a fibrous joint known as a gomphosis. Joints are classified both structurally and functionally. Classification. The number of joints depends on if sesamoids are included, age of the human and the definition of joints. However, the number of sesamoids is the same in most people with variations being rare. There are 360 joints in the adult human body. Joints are mainly classified structurally and functionally. Structural classification is determined by how the bones connect to each other, while functional classification is determined by the degree of  movement between the articulating bones. In practice, there is significant overlap between the two types of classifications. Structural classification (binding tissue). Structural classification names and divides joints according to the type of binding tissue that connects the bones to each other. There are four structural classifications of joints: Functional classification (movement). Joints can also be classified functionally according to the type and degree of movement they allow: Joint movements are described with reference to the basic anatomical planes. Joints can also be classified, according to the number of axes of movement they allow, into nonaxial (gliding, as between the proximal  ends of the ulna and radius), monoaxial (uniaxial), biaxial and multiaxial. Another classification is according to the degrees of freedom allowed, and distinguished between joints with one, two or three degrees of freedom. A further classification is according to the number and shapes of the articular surfaces: flat, concave and convex surfaces. Types of articular surfaces include trochlear surfaces. Biomechanical classification. Joints can also be classified based on their anatomy or on their biomechanical properties. According to the anatomic classification, joints are subdivided into "simple" and "compound", depending on the number of bones involved, and into "complex" and "combination" joints : Anatomical. The joints may be classified anatomically into the following groups: Unmyelinated nerve fibers are abundant in joint capsules and ligaments as well as in the outer part of intraarticular menisci. These nerve fibers are responsible for pain perception when a joint is strained. Clinical significance. Damaging the cartilage of joints (articular cartilage) or the bones and muscles that stabilize the joints can lead to joint dislocations and osteoarthritis. Swimming is a great way to exercise the joints with minimal damage. A joint disorder is termed arthropathy, and when involving inflammation of one or more joints the disorder is called  arthritis. Most joint disorders involve arthritis, but joint damage by external physical trauma is typically not termed arthritis. Arthropathies are called "polyarticular" (multiarticular) when involving many joints and "monoarticular" when involving only a single joint. Arthritis is the leading cause of disability in people over the age of 55. There are many different forms of arthritis, each of which has a different cause. The most common form of arthritis, osteoarthritis (also known as degenerative joint disease), occurs following trauma to the joint, following an infection of the joint or simply as a result of aging and the deterioration of articular  cartilage. Furthermore, there is emerging evidence that abnormal anatomy may contribute to early development of osteoarthritis. Other forms of arthritis are rheumatoid arthritis and psoriatic arthritis, which are autoimmune diseases in which the body is attacking itself. Septic arthritis is caused by joint infection. Gouty arthritis is caused by deposition of uric acid crystals in the joint that results in subsequent inflammation. Additionally, there is a less common form of gout that is caused by the formation of rhomboidal-shaped crystals of calcium pyrophosphate. This form of gout is known as pseudogout. Temporomandibular joint syndrome (TMJ) involves the jaw joints  and can cause facial pain, clicking sounds in the jaw, or limitation of jaw movement, to name a few symptoms. It is caused by psychological tension and misalignment of the jaw (malocclusion), and may be affecting as many as 75 million Americans. History. Etymology. The English word "joint" is a past participle of the verb "join", and can be read as "joined". Joint is derived from Latin "iunctus", past participle of the Latin verb "iungere", join, unite, connect, attach. The English term "articulation" is derived from Latin "articulatio". Humans have also developed lighter, more fragile joint bones over time due  to the decrease in physical activity compared to thousands of years ago.