What holds a joint together and stops bone grinding on bone?

Three kinds of connective tissue do the work. Cartilage covers the ends of the bones inside a joint, so they glide instead of grind. Ligaments tie bone to bone, so the joint holds together. Tendons tie muscle to bone, so muscle force can move it.

Start with cartilage. It is resilient and smooth, and at the joints it covers and protects the ends of long bones as articular cartilage. It is not as hard as bone, but it is much stiffer than muscle or tendon. Joints like the knee, elbow and shoulder are self-lubricating, almost frictionless, and manage to carry heavy loads while moving smoothly. Part of the secret is a slippery protein called lubricin, found in cartilage and in the joint's fluid.

Cartilage has an odd weakness. It contains no blood vessels or nerves. That makes it insensitive, but it also means it repairs at only a very slow rate. Its cells are stuck in small cavities and cannot travel to damage, so cartilage damage is difficult to heal. Osteoarthritis is a joint disease in which cartilage breaks down, leading to pain, stiffness and reduced mobility.

Then there are the cords. Ligaments connect bones to other bones, and some limit movement or block certain movements entirely. Tendons connect muscle to bone and pass on the pull of muscle contraction to the skeleton. A good way to remember it: ligament, link between bones; tendon, tie between muscle and bone.

Both are made of collagen, a tough fibrous protein, and there are about 4,000 tendons in an adult's body.