What is bone joint?
A bone joint is a point where two or more bones meet. The bones at a joint are held by strong cords called ligaments. There is great variation in the shape and mobility of bone joints to suit their particular functions, for example, the joints in the adult skull are usually fused together so that the bones of the skull form a solid casing to protect the brain. In the limbs, where a great deal of mobility is required, the bones are separated by joint cavities. According to the mobility of bone joints, they are usually classified into three categories, those are:
(i) Immovable bone joint
(ii) Slightly movable bone joint
(iii) Freely movable bone joint.
Immovable joints hold the bones tightly together with tough fibrous tissue, as in skull. Slightly movable joints hold the bones together by a disc of cartilage, as in spine. On the other hand, the freely movable joints hold the bones together by a fibrous capsule containing a lubricating fluid, as in elbows and knees.
Immovable bone joints
These joints are fixed, fibrous or suture. As the name implies, these joints do not permit movement of the bones they connect; the articulating bones are decisively held together by dense bands of tough, inextensible white fibrous tissue. They are usually found in the skull and the pelvic girdle, where movement of bones is not desirable. Such joints provide strength and support to the body, or protection to delicate structures which cannot withstand any kind of deformation.
Slightly movable bone joints
These joints are cartilaginous in type. These joints allow a limited degree of movement; and the bones are separated from each other by cartilaginous pads. Joints between vertebrae, and wrist and ankle bones which are gliding joint and joint between atlas and axis vertebrae i.e. pivot joint are the best examples of these joints. In the gliding joint, bones glide over each other to a limited extent. Collectively, they provide a wide range of movement and confer strength to the limb, whereas the pivot joint permits shaking of head from side to side.
Freely movable bone joints
These are called the synovial joints. These bone joints allow greater freedom of movement. In a typical movable joint, the ends of the articulating bone surfaces are covered with cartilage and remain separate from each other by a synovial cavity containing a viscous slippery synovial fluid, This fluid lubricates the joint for easy bone movements. According to the kind of movement permitted by the movable joints, they are divided into the following four categories.
Ball and socket bone joints
They are the most mobile of all synovial joints. The spheroidal ball-like end of one bone articulates with the cup-shaped depression of another bone. This allows the bone with the ball head to be moved freely in all planes. Shoulder and hip joints are the best examples of these joints.
These joints allow restricted movements in one plane only. Elbow, knee and finger joint are examples of hinge joints. They are capable of bearing heavy loads.
These bone joints allow rotation about an axis (the pivot). The upper ends of the forearm bones articulate with each other in a pivot joint.
In these types of bone joints, the surfaces of the articulating bones glide over one another and thereby permit a gliding movement. Such joints include those between some of the bones in the palm or in the sole of foot.