THE JOINTS

A bone joint or articulation may be defined as the junction of two bones. The study of such joints is known as arthrology.

There are three principal types of bone joints.

(i) Fibrous joints or immovable or fixed joints/synarthrosis

(ii) Cartilaginous joints/Amphiarthrosis

(iii) Synovial joints/Diarthrosis

(i) Fibrous Joints.

These joints are immovable or fixed.

They do not show any movement due to the presence of strong and tough white collagenous fibres and there is no joint cavity.

These joints include:

(a) Sutures: Found between skull bones, articulating bones are held together by white fibrous tissue.

(b) Gomphoses : Teeth in mandibles and in maxillary bones.

(c) Shindylases : One bone fits into slit in another e.g., ethmoid bone in vomer.

(ii) Cartilaginous Joints.

They are slightly movable joints.

Discs of white fibrocartilage, strong but more elastic and compressible than the white fibrous tissue, hold the bones together at the joints between the bodies of the vertebrae, at the symphysis pubis, and between the sternum and ribs.

The bones make some movements at such joints through compression of the discs of the cartilage.

(iii)Synovial Joints.

Synovial joints are of different types depending upon the nature of articulation and degree of movement.

Synovial joints are of the following types:

(a) Ball and socket joints ( = Enarthroses) : The 'head' of one bone fitting with the 'socket' of the other bone and allowing free movement in all planes; e.g. shoulder joint and hip joint.

(b) Hinge joints ( = Ginglymi) : The perfect joints which allow the movements only in a single plane. e.g. elbow joint, knee joint and ankle joint

(c) Pivotal joints (=Rotary joints or rotaria) : One of the two bones is fixed in its place and bears a peg like process over which rotates the other bone; e.g. atlas along with the skull rotating over the odontoid process of axis vertebra in mammals.

(d) Saddle joints : It is similar to ball and socket joints but are poorly developed and movements are comparatively less free, e.g. the joint between the metacarpal of thumb with the carpals below.

(e) Gliding joints: The joints which permit sliding of the articulating bones on each other; e.g. joint between the zygapophyses of successive vertebrae, and between sternum and clavicle.

(f)  Angular joints (= or ellipsoid or condyloid) : These joints allow the movements in two directions, i.e., side to side and back and forth; e.g., metacarpophalangeal joints.