Your joints

Arthritis is a progressive disease which causes inflammation and pain in the joints leading to stiffness and restrictive movements. This has a major impact on a person’s quality of life.

In order to help you learn more about arthritis we have included a section about the joints of the human body; what they are; how they work and what can go wrong.

This is useful information to know and can be helpful when discussing your condition with your GP, orthopaedic surgeon or another specialist connected with your treatment.

This section is arranged as follows:

• What are joints?
• Three types of joints
• Movement of the joints
• Biomechanics of the joints

What are joints?

A joint is a connection between two or more bones. It acts as a meeting point for these and holds the bones together, enabling them to move (articulate) as well as supporting the entire skeleton.

Your joints include knee joints, hip joints, shoulder joints, elbow joints and wrist joints.

Three types of joints

Your joints are categorised into the following three classes:

• Cartilaginous
• Fibrous
• Synovial

These classes describe the way that the bones within these are joined together.

Cartilaginous

A cartilaginous joint is one in which the area which connects bones to each other is comprised of cartilage –hence the name ‘cartilaginous’.

This is a partially moveable type of joint which has more movement than a fibrous joint but less than a synovial joint. An example of a cartilaginous joint is the spinal vertebrae which are joined to each other via cartilage. These discs allow a small amount of movement in the vertebrae.

Fibrous

This is a type of joint, connected together by connective tissue which is composed of collagen fibres. Collagen is a type of fibrous protein which is extremely strong and acts as a support mechanism for most areas of the body.

A fibrous joint is the most rigid out of the three. Examples of fibrous joints include the bones of the skull, the sockets which hold the teeth and the shinbone (tibia).

Synovial

Synovial joints are the most flexible of these three joints. There are seven types of synovial joint, all of which are capable of a wide range of movement.

These seven joints vary according to their degree of movement: some are more flexible than others but are more prone to disease or injury. They include

• Ball and socket joints, e.g. shoulder joint
• Compound joints, e.g. knee joint
• Hinge joints, e.g. elbow joint
• Gliding joints, e.g. wrist joint
• Pivot joints, e.g. neck joint
• Saddle joints, e.g. thumb joint
• Condyloid joint, e.g. wrist joint

Ball and socket joint

A ball and socket joint is as the name says. It consists of one bone shaped like a sphere which fits neatly into a cup shaped bone. This allows a 360 degree range of movement.

The shoulder joint is a good example of this. It is known by its medical name - ‘glenohumeral’joint - and is a connection between the humerus (upper arm bone) and the scapula (shoulder blade).

The head of the humerus (ball) fits neatly into the scapula (socket).

The hip joint is another example of a ball and socket joint. This is where the head of the femur (thigh bone) acts as a ‘ball’ joint and slots into the cup shaped ‘socket’ of the pelvis.

Compound joint

This is a complex type of joint which enables a flexible range of movement such as flexing and extending, and also supports the entire weight of the body.

The knee joint is the largest joint in the body.

It is prone to injury or conditions such as osteoarthritis.

Hinge joint

This type of joint behaves in a similar way to a door hinge in that it allows only a forward and backward type of movement. It does allow a slight range of movement to one side but is usually known for its movement in a straight line.

The elbow joint is an example of a hinge joint. It acts as a hinge between the humerus (upper arm bone) and the ulna (one of two long bones in the forearm).

Gliding joint

This joint is characterised by a series of sliding movement. It is the join between two flat bones which enable only a minimal amount of movement due to tight articular (joint) capsules.

The bones of the wrist are a good example of a gliding joint. They allow small, frequent movements to be made in a range of directions.

Pivot joint

A pivot joint is defined as a joint which enables one bone to rotate or ‘pivot’around another. It moves in a similar way to a door hinge but is characterised by a circular type of motion.

Examples of pivot joints include the bones in the skull and two small bones in the neck or the ‘cervical vertebrae’. These two tiny bones are known as ‘atlas’(C1) and ‘axis’(C2) bones which are situated at the top of the vertebrae.

Saddle joint

A saddle joint does indeed look like a riding saddle. It is the join between two bones which allows two type of movement or ‘axes’which are performed at right angles to each other.

To refer back to the riding saddle metaphor: imagine one bone as the ‘saddle’and the other bone as the ‘rider’. The ‘rider’bone straddles the saddle bone.

The joint of the thumb is a good example of a saddle joint.

Condyloid joint

Another name for this is an ‘ellipsoidal joint’. This type of joint consists of an oval shaped bone which fits into a concave cavity. This allows a range of movement which includes flexing and extending, plus movements towards and away from the body.

The wrist joint is a good illustration of this. The small bones which make up the wrist enable a range of movement in all directions.

This completes the section about the structure of the joints. The next section talks about their range of movement.

Movement of the joints

There are three basic sets of movements which are described in terms of surfaces or ‘planes’ These are:

• Frontal plane
• Transverse plane
• Sagittal plane

Frontal plane

This is used to describe the front and back of the body which perform sideways movements.

Transverse plane

This splits the body into two: the top half and the bottom half. Movements carried out along the transverse plane involve rotation, for example pronation of the foot in which it rolls inwards during walking or running.

Sagittal plane

This divides the body into left and right halves. Any movements performed along this plane are up and down motions. An example of this is flexing and extending an arm.

Biomechanics of the joints

The term ‘biomechanics’refers to movements of the human body which involve the muscles, tendons, ligaments and joints. It looks at how these work in conjunction with each other to enable the body to move in a variety of different ways.

Another term for the joints is articulations.

Your joints are categorised as ‘simple’, ‘compound’and ‘complex’.

Simple joints are those which are composed of a simple ball and socket mechanism. These include the hip and shoulder joints.

A compound joint consists of three articulations (joints), for example the wrist joint.

Complex joints consist of two areas where the bones join (articulations) and a cartilage based disc which enables a range of separate movements.