Joints and disorders

A joint is where two bones (or a bone and a cartilage) meet. All movement needs joints: the joint acts as the pivot (fulcrum) for the force of the muscles. There are three main kinds:

• Fibrous joints allow no movement. The flat bones of the skull are fused by dense fibrous tissue in wavy lines called sutures, forming the cranium.
• Cartilaginous joints allow limited movement. The vertebrae of the backbone are joined this way, with cartilage between them.
• Synovial joints have a space filled with fluid (the synovial cavity) between the bones, and allow a lot of movement.

Kinds of synovial joint, with examples:

• Ball and socket: between the humerus and the pectoral girdle (the shoulder). It turns in all directions.
• Hinge: the knee. It bends one way, like a door hinge.
• Pivot: between the atlas and axis (the top two vertebrae), letting you turn your head.
• Gliding: between the carpals of the wrist.
• Saddle: between the carpal and metacarpal of the thumb.

Disorders of muscles:

• Myasthenia gravis: an auto-immune disorder (the body attacks itself) affecting the neuromuscular junction, causing fatigue, weakness and paralysis of skeletal muscles.
• Muscular dystrophy: the skeletal muscles slowly break down, mostly because of a genetic disorder.
• Tetany: rapid, uncontrolled spasms of the muscles, caused by low Ca²⁺ in the body fluid.

Disorders of bones and joints:

• Arthritis: inflammation of the joints.
• Osteoporosis: with age, bones lose mass and become weak, so they break easily. A fall in the hormone oestrogen is a common cause.
• Gout: inflammation of joints caused by a build-up of uric acid crystals.