Joints

A joint is the part of the body where two or more bones meet to allow movement. Every bone in the body – except for the hyoid bone in the throat – meets up with at least one other bone at a joint. The shape of a joint depends on its function. A joint is also known as an articulation.

Generally speaking, the more movement that is possible through a joint, the higher the risk of injury. This is because greater range of movement reduces the strength of the joint.

Types of joints

Joints are described by how much movement they allow. The three broad classes of joints include:

• Immovable – the two or more bones are in close contact, but no movement can occur – for example, the bones of the skull. The joints of the skull are called sutures.
• Slightly movable – two or more bones are held together so tightly that only limited movement is permitted – for example, the vertebrae of the spine.
• Freely movable – most joints within the human body are this type. Motion is the purpose of the joint.

Freely moving joints

The six types of freely movable joint include:

• Ball and socket joint – the rounded head of one bone sits within the cup of another, such as the hip joint or shoulder joint. Movement in all directions is allowed.
• Saddle joint – permits movement back and forth and from side to side, but does not allow rotation, such as the joint at the base of the thumb.
• Hinge joint – the two bones open and close in one direction only (along one plane) like a door, such as the knee joint.
• Condyloid joint – permits movement without rotation, such as the jaw or finger joints.
• Pivot joint – one bone swivels around the ring formed by another bone, such as the elbow joint that allows the forearm to turn from side to side.
• Gliding joint – or plane joint. Smooth surfaces slip over one another, allowing limited movement, such as the wrist joints.

Types of movement

To achieve movement, the joint may:

• Slide one broad and flat surface across another – examples include the bones in the wrist or ankle.
• Increase or decrease the angle between the two bones – this only occurs in the long bones of the body (arms and legs): for example, when the arm is bent or extended.
• Allow a circular movement – this is how ball and socket joints work: for example, the shoulder.
• Allow rotation without displacing the bones: for example, the head as it turns from side to side swivels the cervical vertebrae on top of one another.

Structure of a joint

Joints are held together and supported by tough bands of connective tissue called ligaments. Smooth cartilage prevents friction as the bones move against one another. In freely movable joints, the entire joint is enclosed inside a membrane filled with lubricating synovial fluid, which helps to provide extra cushioning against impact.

Muscles are attached to bones with thick, tough bands of connective tissue called tendons. Where tendons lie close to bone, tiny sacs called bursae sit between the tendon and the bone to reduce friction. A bursa is filled with synovial fluid.

Joint conditions

Common causes of joint pain include:

• Arthritis – inflammation that causes stiffness and pain in the joints (rheumatoid arthritis or gout) or degeneration (osteoarthritis)
• Bursitis – inflammation of the bursae (fluid-filled sacs that cushion and pad bones)
• Infection – within the joint
• Tendonitis – inflammation, irritation and swelling of a tendon that is attached to the joint.
• Injury – including sprain or strain of a ligament or nearby tendon or muscle, or bone fracture.

Where to get help

• Your doctor
• Physiotherapist
• Exercise physiologist ESSA Exercise & Sports Science Australia
• Osteopath
• NURSE-ON-CALL Tel. 1300 60 60 24 – for expert health information and advice 24 hours, 7 days

Things to remember

• A joint is the part of the body where two or more bones meet to allow movement.
• Generally speaking, the greater the range of movement, the higher the risk of injury because the strength of the joint is reduced.
• The six types of freely movable joint include ball and socket, saddle, hinge, condyloid, pivot and gliding.
• Common causes of joint pain include inflammation (pain and swelling), infection and injury.

You might also be interested in:

• Ankle sprains.
  
• Ankylosing spondylitis.
  
• Arthritis.
  
• Arthroscopy.
  
• Bursitis.
  
• Developmental hip dysplasia.
  
• Elbow - common causes of pain.
  
• Hip disorders.
  
• Knee injuries.
  
• Knee replacement surgery.
  
• Neck and shoulder pain.
  
• Osgood Schlatter syndrome.
  
• Perthes' disease.
  
• Reactive arthritis.
  
• Sports injuries.
  
• Sprains and strains.
  

Want to know more?

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The Better Health Channel

Last reviewed: March 2012

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<a href="http://www.betterhealth.vic.gov.au/bhcv2/bhcarticles.nsf/pages/Joints?open">Joints - Better Health Channel</a><br/>
A joint is the part of the body where two or more bones meet to allow movement. Types of freely moving joints include ball and socket, saddle, hinge, pivot, condyloid and gliding joints. Joints that allow greater range of movement have a higher risk of injury. A joint is also known as an articulation.

Content on this website is provided for education and information purposes only. Information about a therapy, service, product or treatment does not imply endorsement and is not intended to replace advice from your qualified health professional. Content has been prepared for Victorian residence and wider Australian audiences, and was accurate at the time of publication. Readers should note that over time currency and completeness of the information may change. All users are urged to always seek advice from a qualified health care professional for diagnosis and answers to their medical questions.

For the latest updates and more information, visit www.betterhealth.vic.gov.au

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