Structures connecting two or more bones, joints enable your skeleton to move. Synovial joints contain fluid, while the joints between the vertebrae in your back incorporate a disk. There are some immovable joints in your body, including the fontanelle or ‘soft spot’ on the top of a baby’s skull, but most are designed for movement. The bones of a joint are connected by ligaments, and these are kept in place by muscles.
Your body has three types of joint:
Two or more bony surfaces are directly connected to each other with fibrous tissue, and allow for little or no movement. The roots of your teeth are an example of this type of joint.
Bones are directly connected to each other with cartilage, which only allows a small amount of movement. An example of this is the connection of theh two pubic bones in the pelvis.
This type has the greatest range of movement. Two surfaces slide against each other, and can also roll to varying degrees. The surfaces that make contact are covered with articular cartilage, a very durable sort of soft tissue that acts as a shock absorber and provides lubrication to the joint. The whole joint is also surrounded by a capsule full of synovial fluid, to provide nutrients to the joint tissue, and additional lubrication. Muscles attached to the bone with tendons enable movement, while strong and flexible ligaments connect the two bones and provide stability, although they can also restrict movement. Different ranges of motion are enabled by a variety of diarthrordial systems: pivot, gliding, hinge, saddle, condyloid (as in the wrist), and ball-and-socket.