From frictionless knees to fused skull sutures, joints come in many designs
A joint is any place where bones or other hard parts meet to make a working skeleton. Some, like the knee and shoulder, lubricate themselves and glide almost without friction under heavy loads; others, like the sutures of the skull, barely move at all except during birth.
Joints let us bend, stretch, run and jump, but they also hold the skeleton steady, supporting posture, balance and body weight. How many a person has depends on the definition used, on age, and on whether the small sesamoid bones are counted, although the number of sesamoids rarely varies between people. Anatomists sort joints in two overlapping ways: by the tissue that binds the bones and by how much movement they allow.
By binding tissue there are fibrous joints held by collagen-rich connective tissue, cartilaginous joints joined by hyaline cartilage or fibrocartilage, synovial joints whose bones sit in a fluid-filled cavity wrapped by a capsule and ligaments, and the facet joints between neighbouring vertebrae. By movement, a synarthrosis such as a skull suture is nearly rigid, which protects the brain; an amphiarthrosis such as an intervertebral disc allows a little give, and together the discs let the spine twist and bend; and a freely moving synovial joint may be a plane, hinge, pivot, condyloid, saddle or ball-and-socket joint.
Other schemes count axes of motion or degrees of freedom, from one to three, or count surfaces. The shoulder and hip are simple joints with two surfaces, the wrist’s radiocarpal joint is compound with three or more, and the knee is complex because it adds a meniscus. Doctors also count affected joints: two to four makes a condition oligoarticular, five or more polyarticular.
Joint disease is common. Osteoarthritis wears away cartilage and brings pain and stiffness, rheumatoid arthritis is an autoimmune inflammation that can deform joints, and gout comes from uric acid crystals. Painful, swollen joints can also point to infection, inflammation or metabolic disorders elsewhere in the body, and treatment may combine physical therapy, drugs, lifestyle changes and sometimes surgery.
Source: Joint
