Bone-on-bone is a common way to describe a joint with severe arthritis, but it is not exactly true if the joint can still move. Joints are covered with a special smooth and slippery cartilage that acts like a cushion and helps them move without friction. Even if this cartilage gets very thin, a thin layer or fluid still separates the bones, stopping them from rubbing directly against each other. True bone-on-bone contact only happens if the joint completely loses all movement, often when bones fuse together.

This means that when a doctor says "bone on bone," it usually means there's very little space in the joint, but some cushioning and joint movement often remain. Understanding this helps explain why joints can still move and why treatments aim to protect or improve that joint space and cartilage.

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