Definition of joints
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A joint is a place where two or more bones meet, and is classified according to the amount of movement there is.
Classification of joints
- Fixed joints are very stable and do not allow any movement. Examples are the sutures of the skull bones and the pelvic bones.
- Slightly moveable joints help provide stability and only allow limited movement. The individual bones are joined by cartilage. For example, when the back bends, the joint between two vertebrae moves only a small amount. The disc between the vertebrae is compressed on one side.
- Freely moveable (synovial) joints allow free movement. Examples include the knee, wrist and shoulder joints.
There are many common characteristics between the different types of synovial joints. We will use the knee as an example:
- Hyaline cartilage – the articular surfaces of the bones (the ends of the bones that meet in the joints) are covered with hyaline (articular) cartilage, which forms a smooth, white, shiny mass on the surface. This protects the bone tissue, and helps to reduce friction between the bones.
- Joint/articular capsule – this is a strong, fibrous tissue envelope surrounding the joint, and attached to the bones near the edge of the articular surfaces. The capsule adds stability to a joint, and stops unwanted material from entering the joint. Capsules are reinforced with ligaments. Surrounding muscles also help to add stability to joints.
- Synovial membrane – this is a fine membrane which lines the inside of the joint capsule, but does not cover the hyaline cartilage. Its role is to produce synovial fluid.
- Synovial fluid is a yellowish oily fluid that lubricates the articulating surfaces, forms a fluid cushion between surfaces, provides nutrients for the hyaline cartilage, and absorbs debris produced by friction between joint surfaces. Synovial fluid becomes thicker in cold weather. If the synovial membrane becomes inflamed (synovitis) because of injury or illness, the feeding capillaries become more permeable and fluid rushes into the joint – commonly known as water on the knee.
- Ligaments are strong fibrous bands, such as the cruciate and collateral ligaments of the knee, which join articular surfaces. They help control movement and add stability to joints.
- Articular discs (menisci) lie between the articular surfaces and are attached to the capsule at the outer edge of the joint. Their function is to absorb shock, maintain joint stability and protect the bone surfaces.
- Pads of fat fill the crevices in and around the joints, and form protective cushions for vital joint structures.
- Bursae are closed sacs filled with synovial fluid, and are located wherever friction may develop, such as between the tendons and the bones.
Freely moveable, or synovial joints, allow movement to take place. There are six different types of synovial joint which differ in shape and movement range:
- Ball and socket: the ball shaped head of one bone fits into the cup-shaped socket of another. Examples include the hip and shoulder joints. It allows movement in all directions.
- Hinge: examples include the ankle, elbow and knee joints. It allows only back-and-forth movements.
- Pivot: a ring on one bone fits over a peg on another. An example is the atlas and axis (the top two vertebrae). It allows rotation (twisting) only.
- Saddle: a bone fits into a saddle-shaped surface on another bone. An example is the thumb. It allows side-to-side and back-and-forth movements.
- Gliding: two relatively flat surfaces slide over one another. An example occurs at the articular processes of the vertebrae (although the spine as a whole is classified as a slightly moveable joint). It allows side-to-side and back-and-forth movements. It also occurs between the carpals and between the tarsals.
- Condyloid: the full convex shape of one bone end fits into the full concave shape of an adjoining bone. An example is the wrist joint. It allows back-and-forth and side-to-side movement, with ligaments preventing rotation.