Types of Joints

A point where two or more bones get articulated is called a joint, articulation, or arthrosis; the study of joints is called arthrology. Though bones themselves are rigid, the ligaments that bind them at a joint give the joint some flexibility. Joints are classified, based on the degree of flexibility or movement they permit, into three types: fibrous joints (also called synarthroses, or immovable joints), cartilaginous joints (also called amphiarthroses, or slightly movable joints), and synovial joints (also called diarthroses, or freely movable joints).

A. Fibrous joints (synarthroses): The articulating bones are held together directly by fibrous connective tissue, and show no real movement. They are further classified into:

• Sutures — composed of a thin layer of dense fibrous connective tissue. Sutures are sites of skull growth and remain open (permitting some moulding of the skull during childhood) until growth is complete, when they tend to ossify. By the shape of the joint line, sutures are further classified as butt joints (square-edged, e.g. between the two nasal bones), scarf joints (tapering, various skull bones), lap joints (overlapping, e.g. the temporal and parietal bones), and serrate joints (irregular/interlocking, various skull bones).
• Syndesmoses — present where there is a greater distance between the articulating bones; at such locations, the fibrous connective tissue is arranged as a sheet or bundle, e.g. the distal tibiofibular ligament, and the interosseous membranes between the tibia and fibula and between the radius and ulna.
• Gomphoses — in this type of joint, a cone-shaped structure of one bone fits into a socket provided by another bone, e.g. a tooth fitting into its socket in the jaw bone, held by the periodontal ligament.

B. Cartilaginous or slightly movable joints (amphiarthroses): These joints are neither fixed nor freely movable; the articulating bones are held together by hyaline or fibrocartilage. They are further classified as:

a. Synchondroses — the two bones are held together by hyaline cartilage, and are typically sites meant for growth; on completion of growth, the joint gets ossified. Examples: the epiphyseal plate found between the epiphysis and diaphysis of a long bone, and the rib-sternum junction.

b. Symphysis — a broad, flat disc of fibrocartilage connects two bones; these occur in the midline of the body. Example: the intervertebral discs (and, as described earlier, the pubic symphysis).

C. Synovial joints (diarthroses), or freely movable joints: These are characterised by the presence of a space called the synovial cavity between the articulating bones, which allows free movement. The articulating surfaces of the bones at a synovial joint are covered by a layer of hyaline cartilage, so the bones themselves never touch directly — this reduces friction during movement and helps absorb shock. The synovial cavity is lined by a synovial membrane, which forms the synovial capsule and secretes synovial fluid — a clear, viscous, straw-coloured fluid, similar to lymph, that is made viscous by hyaluronic acid and also contains nutrients, mucous, and phagocytic cells that remove microbes. Synovial fluid lubricates the joint, absorbs shocks, nourishes the (otherwise avascular) hyaline cartilage, and removes waste from the cartilage cells and debris from wear and tear; if the joint has been immobile for a while the fluid becomes more viscous, and it becomes less viscous once joint movement starts. The joint is further reinforced by a capsular ligament and numerous accessory ligaments; the fibrous capsule is attached to the periosteum of the articulating bones, and the ligaments help prevent dislocation of the joint. Every synovial joint shows the components described above; they differ chiefly in the shape of the articulating surfaces and the movements this shape permits:

• Pivot joint — the rounded or pointed surface of one bone articulates with a ring formed partly by another bone and partly by a ligament, permitting rotation only about the bone's own longitudinal axis. Example: the atlas-axis joint, where the head turns sideways, forming the 'no' movement.
• Ball-and-socket joint — a ball-like surface of one bone fits into a cup-like depression of another bone, allowing multiaxial movement along all three axes and in all directions. Example: the shoulder and hip joints.
• Hinge joint — the convex surface of one bone fits into the concave surface of another; in most hinge joints one bone remains stationary while the other moves. Only monoaxial movement is possible — angular opening-and-closing motion, like flexion and extension. Example: the elbow and knee joints. …