Q.How do our bone joints function without grinding noise and pain?

Not all the places where two bones meet allow the same freedom of movement — some are locked solid, some give a little, and some swing freely. Joints where the bones can move most freely are the synovial joints, and it is these that make locomotion and most large body movements possible. A joint, remember, is a point of contact between two bones, or between a bone and a cartilage, and it is where the force made by muscles is actually applied, the joint acting as a fulcrum about which the bony lever turns.

What sets a synovial joint apart is a fluid-filled synovial cavity lying between the two articulating bone surfaces. This fluid-filled space is exactly what permits considerable movement, unlike the fibrous joints (which allow no movement) or the cartilaginous joints (which allow only limited movement). Because synovial joints can move so freely, they come in several kinds, each named for the way its surfaces fit together and move:

• Ball and socket joint — between the humerus and the pectoral girdle (the shoulder). One rounded head sits in a cup, allowing movement in many directions.
• Hinge joint — the knee joint. It opens and closes in one plane, like a door hinge.
• Pivot joint — between the atlas and the axis, the first two vertebrae. It allows a rotating or turning movement. …