A joint is a connection between two bones. The ends of bones are covered with cartilage, which create a smooth surface for the bones. Several structures of the joint are important for providing stability. An articular capsule surrounds the joint. This capsule secures the joint and contains synovial fluid (a lubricant providing nutrition to the cartilage). Muscles and ligaments surround the joint and provide movement and stability.
The cartilage surface is smooth and lets bones slide easily when moving. Cartilage is solid, but flexible. It absorbs shock and spreads loads over its surface. As far as we know, cartilage has no pain sensors and therefore pain can’t be felt if it is injured. The cartilage has no blood vessels. Nutrients are brought to the cartilage by the synovial fluid.
Think of cartilage as a wet sponge; when loads are applied, fluids are pressed out of the sponge. When loads are removed, the sponge sucks the fluids back in. This is what happens with the fluid in and around our cartilage. When we walk for example, loads press down on our cartilage. The cartilage absorbs the shock and fluids squeeze out into the articular capsule. Once loads are removed, the cartilage sucks the fluid back in from the the surrounding area. This is why loading the joint is needed for cartilage to be healthy. In a healthy joint, there is a balance between degeneration and regeneration of cartilage cells.