Healthy knee

The bones in your knee joint work like a hinge. They are covered by cartilageCartilage
A usually translucent somewhat elastic tissue that composes most of the skeleton to keep them from rubbing together.

The space between the bones is filled with a thick, slippery liquid (joint fluid). Joint fluid acts like a shock absorber to cushion and lubricate your knee while walking, working, or playing sports.

Mild-to-moderate osteoarthritis

Your body’s joint fluid becomes less healthy and the cartilage begins to break down, leading to irritation and discomfort in the knee. Without healthy joint fluid, your knees become sore and swollen.

Progression to severe osteoarthritis

As time passes, larger pieces of bone and cartilage can break off and float in the joint, causing pain and swelling. Large areas of cartilage can be completely worn away. Growths (bone spurs) may appear on the bone, leading to more irritation. In severe cases of osteoarthritis, your cartilage can be completely worn away. Your bones may actually rub together, causing pain and swelling.