Synovial Joints

Synovial joints are the most common type of joint in the body.

Synovial joints allow for smooth movements between the adjacent bones.

A synovial joint, also known as a diarthrosis.

Generally, the more movement a joint has, the less stability it is.

Several factors affect stability around the joints including:

the shape of the bones and whether they interlock with each other or not
the area over which the bones are in contact
the flexibility of the ligaments
the influence of other soft tissue structures (muscles, tendons, joint capsules etc.)

SYNOVIAL JOINT ANATOMY

The space between the bones is called the joint cavity.
A smooth, white layer of articular cartilage covers the articulating surfaces of the bones. This is usually hyaline cartilage. The main functions of cartilage are to reduce friction, absorb shock and protect the bones.
A sleeve-like structure called an articular capsule surrounds the joint. This capsule is flexible enough to allow joint movements to take place while the tensile strength protects the joint from dislocation.
The inside of the capsule is lined by a synovial membrane that secretes synovial fluid. This fluid has the consistency and appearance of uncooked egg whites which makes it quite viscous, however it does become more fluid with movement. It lubricates the joint cavity, reduces friction and provides nutrients to the cartilage.
Menisci are semi-lunar discs of fibrocartilage that are found between some articulating bones, for example between the femur and tibia in the knee joint. This provides greater cushioning and stability to the joint.
Bursae are small fluid-filled sacs found where two structures rub against each other, for example between ligament and bone, between tendon and bone, or between skin and bone. As bursae are lined by a synovial membrane, they provide lubrication to the structures and therefore reduce friction. Bursae are found in areas of high stress all over the body.

SYNOVIAL JOINTS

Watch this Video for Animation of Synovial Joints

Gliding joints are found between the tarsal bones and between the carpal bones. They have the least amount of movement. The surfaces of the bones are flat or slightly curved and they glide back and forth and from side to side across each other.

Hinge joints bend (flex) or straighten (extend) in one direction only, for example the elbow joint. Similar movements take place at the knees and in the small joints between the phalanges of the fingers and toes. Usually the surface of one bone is convex and it fits into a reciprocally shaped concave surface.

Pivot joint, one bone forms a ring in which the other bone rolls or pivots allowing rotation of the joint, for example the radius rotates around the ulna at the radioulnar joint.

Condyloid joints are formed where an oval or egg-shaped convex surface fits into a reciprocally shaped concave surface. For example, they are found between the radius and carpal bones of the wrist and allow movement in two directions (you can move your wrist both up-and-down and from side-to-side).

Saddle joint, the bones are shaped like a saddle and a rider sitting in the saddle. An example of a saddle joint is found between the metacarpal of the thumb and the carpal bone next to it. These joints can move in two directions (side-to-side and up-and-down).

Ball and Socket joints can move in all directions and therefore have the greatest amount of movement. They are formed when the sphere shaped head of one bone fits into a rounded cavity on the other bone, for example the shoulder and hip joints.