The articulating surfaces of the bones are covered by a thin layer of very smooth hyaline cartilage (articular cartilage) and lubricated by a special fluid, the synovial fluid secreted by the synovial membrane which lines the cavity. This fluid is composed of mucopolysaccharides, is highly viscous and slippery and reduces friction.

The joint cavity is enclosed by a double layered membrane: the articular capsule.

The outer layer is a tough membrane of collagen fibers (dense irregular connective tissue proper) which is firmly attached to the surface of the bones on either side of the joint. It is continuous with the periosteum.

The internal layer is the synovial membrane (loose connective tissue proper) which covers all internal joint surfaces that are not hyaline cartilage.

Synovial joints are reinforced by a number of ligaments. Ligaments are bands of dense regular connective tissue proper that connect bones to other bones. (Please do NOT mistake them for TENDONS: bands of dense regular connective tissue proper that connect muscles to bones.) The ligaments may be part of the fibrous capsule (intrinsic or capsular ligaments), or may be distinct from the fibrous capsule and found outside the capsule (extracapsular ligaments)or deep to it (intracapsular ligaments). Since intracapsular ligaments are covered with synovial membrane they do not actually lie within the joint cavity.

In some joints such as the knee, complete or partial discs (menisci) of fibrocartilage occur within the synovial cavity. They do not function in weight bearing, but act as swabs to spread synovial fluid into the joint, and help to stabilize the joint. These discs (often called `knee cartilages') are frequently torn or displaced in body contact sports .

Bursae (singular = bursa) are closed, partially collapsed balloon containing synovial fluid and lined with synovial membrane on the inside and a fibrous membrane on the outside. They are found in the vicinity of joints where movement between two adjacent tissues might otherwise result in excessive friction. They are located between any two of bone, tendon, muscle or skin and they prevent these organs to rub against each other: like the joint cavity, with which they frequently connect, they serve to reduce friction.

Tendon sheaths are similar to bursae, but differ in shape. They look like sausage-shaped ballons that wrap around long tendons subjected to friction.

Three factors determine the strength or stability of the synovial joint, and the range of movement permitted by it. These are:

1. The shape of the articular surfaces of the bones

2. The ligaments: strong bands of dense fibrous connective tissue which bind the adjacent bones together,

3. Muscles which extend between the two bones comprising the joint.

Synovial joints are classified according to the shape of the articulating surfaces which, in turn, determines the range of movement permitted. They can be classified into six major categories:

1. Plane (= gliding)

Opposite bone surfaces are flat or slightly curved.

Only sliding motion in all directions are allowed. Since there is no bone movement around an axis, the joints are nonaxial.

2. Hinge

Convex surface of one bone fits smoothly into concave surface of the second bone

The movements allowed are similar to those allowed by a mecanical door hinge. Since the movements (flexion/extension) are all in one plane and around one axis, the joints are uniaxial.

3. Pivot

A rounded, pinted or conical surface of one bone is inserted into a ring made partly of another bone and partly of a ligament.

Since the only movement allowed is the rotation of one bone around its own axis, the Joints are uniaxial.

4. Ellipsoidal (= condyloid)

Oval-shaped surface fits into an oval-shaped cavity (ellipse means oval).

The movements allowed are flexion/extension, adduction/abduction and circumduction but NO ROTATION. Since bones can move in both planes: side to side and back and forth movements the joints are biaxial.

5. Saddle

First bone's articular surface is concave in one direction and convex in the other while the second bone is just the opposite (or if you prefer, one bone is shaped like a saddle, and the other is shaped like its rider).

The saddle joint is similar to the Ellipsoidal Joint but the movements are freer. The movements allowed are flexion/extension, adduction/abduction and circumduction but NO ROTATION. Since bones can move in both planes: side to side and back and forth movements the joints are biaxial.

6. Ball and socket

Ball-shaped head fits into a cup-shaped depression

These joints are the most freely moving of all synovial joints. The movements are allowed in all axes and planes: flexion/extension, adduction/abduction, circumduction and rotation. These joints are multiaxial.