Final answer:
Most joints in the body are covered with a tough, stiff material called cartilage, specifically articular cartilage, which reduces friction and acts as a shock absorber at synovial joints.
Step-by-step explanation:
The tough, stiff material that covers the ends of bones at joints is called cartilage. This articular cartilage is a type of dense connective tissue composed of tough protein fibers. It serves a vital role in the function of joints, offering a smooth, flexible surface that allows bones to move effortlessly over one another. Along with reducing friction, cartilage acts as a shock absorber, protecting the ends of bones from the stresses of movement.
Joints may be categorized by their structure or function. Among the most common types is the synovial joint, like the knee or hip, which is encapsulated in a joint cavity filled with synovial fluid serving as a lubricant to further minimize friction and wear within the joint. This biological design enables our joints to have coefficients of friction significantly lower than many everyday surfaces, such as ice.
When cartilage wears down or is damaged, joint function is compromised, which can lead to arthritis or other joint-related issues. In severe cases, the native joint may be replaced with an artificial joint made from low-friction materials such as stainless steel, titanium, or plastic polymers like polyethylene. These materials can mimic the smooth, almost glassy surface of healthy cartilage and restore movement for those with joint damage.