JOINTS

We have already learnt that the human skeleton consists of more than 200 bones. The individual bones are attached in such a way that a large variety of co-ordinated movements are made possible in different parts of the body. These movements are made possible by skeletal muscles, the fact that the bones act as levers, cartilage which reduces fricton and ligaments which prevent dislocation and the presence of movable joints. The site or place where 2 or more bones of the skeleton are attached to each other is called a joint or place of articulation.

A joint can be defined as follows:

A joint or place of articulation is formed where 2 or more bones come inclose contact in the body and are attached to each other by ligaments or cartilage.

Types of Joints

Joints can be classified according to the degree and type of movement they allow. The following types of joints can be recognized:

Fibrous (or Immovable) Joints

These joints are firmly held together by a thin layer of strong connectice tissue. There is no movement between the bones such as the sutures of the skull and the teeth in their sockets.

Fibrous joints (sultures in the skull)

Cartilagenous Joints

Cartilagenous joints are joints where the articular surfaces of the bones forming the joints are attached to each other by means of white fibrocartilaginous discs and ligaments which allow only a limited degree of movement. Examples are the cartilaginous between the vertebrae, the cartilage in the symphysis which binds the pubic bones together at the front of the pelvic girdle and the cartilage in the joint between the sacrum and the hip bone.

Cartilaginous joints between the thoracic vertebrae

Synovial Joints

These are freely movable joints. Most of the joints in the body are of the synovial type. The following are the main characteristics of a synovial joint:

The ends of the bones are covered with a layer of smooth hyaline cartilage, called articular cartilage in the joint regions. This reduces fricton at the point.
The joint is completely enclosed by a bag-like capsular ligament which holds the joint together and helps to contain the synovial fluid.
The capsular ligament is lined with a synovial membrane. This membrane secretes synovial fluid into the synovial cavity and acts as a seal, waterproofing the joint. The synovial fluid lubricates the joint.
In addition to the capsule, the bones are also attached and held together by strong, tough ligaments made of dense connective tissue. These ligaments prevent dislocation during normal movement.
The articulating surfaces of adjacent bones are reciprocally shaped.

A generalized synovial joint

Synovial joints can be subdivided into the following groups according to the type of movement they carry out:

Ball-and-Socket Joints

These joints are formed where the rounded head of one bone fits into the hollow, cup-shaped socket of another bone such as the shoulder joint and the hip joint. Such joints allow freedom of movement in all directions.

The Hip Joint - ball-and-socket joint

Hinge Joints

These joints occur where the convex surface of one bone fits into the concave surface of another bone, so making movement possible in one plane only. Examples of these joints are the knee and the elbow joints. Hinge joints have ligaments mainly at the sides of the joints.

A hinge joint (elbow region)

Gliding Joints

This type of joint allows for gliding movements between flat surfaces as the surfaces slide over one another. Only a limited amount of movement is allowed such as the joints between the carpal bones, the joints between the tarsal bones and those between the articular processes (zygapophyses) of successive vertebrae.

Gliding joints in the wrist region

Pivot Joints

These joints occur where:

a bony ring rotates round the pivot (axis) of another bone such as the ring-like atlas rotating around the odontoid process of the axis, allowing the head to turn from side to side.
the end of one one bone rotates round the axis of another bone such as the end of the radius rotating around the ulna as the palm of the hand is turned inwards or outwards.

A pivot joint between the radius and ulna

Compound Joints

These joints are made up of several joints between a number of different bones. The bones articulate with one another in different ways, allowing for a variety of movements such as the set of joints which operate the movement of the skull on the vertebral column. The condyles at the base of the skull fit into the facets of the atlas, allowing for the nodding movement of the head. While one moves one's head, the atlas is able to rotate round the odontoid process of the axis, allowing the head to turn from side to side. There are also other articulating surfaces, where the atlas and axis meet. All these joints together make a compound joint with its many possible movements in the neck region.

Model showing compound joints
in the neck region