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Joint

Structural classification

Structurally, joints are classified as:

fibrous - bones are connected by fibrous connective tissue.
cartilaginous - bones are connected by cartilage.
synovial - there is a space (synovial cavity) between the articulating bones.

Fibrous joints

In fibrous joints bones are joined by tight and inflexible layers of dense connective tissue, consisting mainly of collagen fibers. In adults, these are not designed to allow any movement; however, in children, fibrous joints have not solidified and are movable. Examples of fibrous joints are:

Cranial sutures, joining the bones of the cranium.
Gomphoses, the joints between the roots of the teeth and their sockets (or alveoli) in maxilla and mandible.

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Cartilaginous joints

In cartilaginous joints (also known as synchondroses) bones are connected entirely by cartilage. In comparison to synovial joints, cartilaginous joints allow only slight movement. Examples of cartilaginous joints are the pubic symphysis, the joints between the ribs and the sternum, and the cartilage connecting the growth regions of immature long bones. Another example is in the spinal column - the cartilaginous region between adjacent vertebrae.

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Synovial joints

The term "Synovial joint" and "Diarthrosis joint" are often used interchangably, although the first term refers to the structure and the second one to the function. For more details, see "Diarthrosis joints" below.

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Functional classification

Functionally, they can be classified as:

synarthrosis - permit no movement.
amphiarthrosis - permit little movement.
diarthrosis - permit a variety of movements (e.g. flexion, adduction, pronation). Only synovial joints are diarthrosis.

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Synarthrosis joints

Synarthroses are joints with very little (if any) mobility. They can be categorized by how the two bones are joined together:

Syndesmoses are joints where the two bones are joined by one of more ligaments.
Synchondroses are joints where the two bones are joined by a piece of cartilage.
Synostoses are the fusion of two bones, to the point that they are practically one bone. In humans, the plates of the cranium, initially separate, fuse together as the child approaches adulthood. Children whose craniums fuse too early may suffer deformities and brain damage, as the skull does not expand properly to accommodate the growing brain - a condition known as craniostenosis.
Amphiarthroses are slightly moveable joints where the two bone surfaces at the joint - both covered in hyaline cartilage - are joined by strands of fibrocartilage.

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Amphiarthrosis joints

Most amphiarthrosis joints are cartilaginous. See above for more details.

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Diarthrosis joints

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Joints:
1-ball & socket; 2-ellipsoid; 3-saddle; 4-hinge; 5-pivot;

Diarthroses (sometimes called synovial joints and also diarthroidal joints) are the most common and most moveable type of joint in the body.

The whole of a diarthrosis is contained by a ligamentous sac called the articular capsule.

The surfaces of the two bones at the joint are covered in cartilage. The thickness of the cartilage varies with each joint, and sometimes may be of uneven thickness. Articular cartilage is multi-layered. A thin superficial layer provides a smooth surface for the two bones to slide against each other. Of all the layers, it has the highest concentration of collagen and the lowest concentration of proteoglycans, making it very resistant to shear stresses. Deeper than that is an intermediate layer, which is mechanically designed to absorb shocks and distribute the load efficiently. The deepest layer is highly calcified, and anchors the articular cartilage to the bone.

In joints where the two surfaces do not fit snugly together, a meniscus or multiple folds of fibro-cartilage within the joint correct the fit, ensuring stability and the optimal distribution of load forces.

The synovium is a membrane that covers all the non-cartilaginous surfaces within the articular capsule. It secretes synovial fluid into the joint, which nourishes and lubricates the articular cartilage. The synovium is separated from the capsule by a layer of celluar tissue that contains blood vessels and nerves.

Synovial joints can be further grouped by their shape, which controls the movement they allow:

Gliding joints, such as in the carpals of the wrist. These joints allow a wide variety of movement, but not much distance.
Hinge joints, such as the elbow (between the humerus and the ulna). These joints act like a door hinge, allowing flexion and extension in just one plane.
Pivot joints, such as the elbow (between the radius and the ulna). This is where one bone rotates about another.
Condyloid (ellipsoid) joints, such as the knee. When the knee is extended there is no rotation, when it is flexed some rotation is possible. A condyloid joint is where two bones fit together with an odd shape (e.g. an ellipse), and one bone is concave, the other convex. Some classifications make a distinction between condyloid and ellipsoid joints.
Saddle joints, such as at the thumb (between the metacarpal and carpal). Saddle joints, which resemble a saddle, permit the same movements as the condyloid joints.
Ball and socket joints, such as the hip joint. These allow a wide arrange of movement.