Your knee is like a hinge. Cartilage covers the ends of the bones, to allow smooth movement when you bend or straighten it.
Osteoarthritis is when the cartilage is gradually worn away with age or after injury, mainly at the points of greatest pressure. The two surfaces rub against each other - sometimes you can hear or feel it. This can lead to pain, stiffness, loss of movement, swelling and deformity.
It can lead to your knees giving way because the muscles around the joint can become weak.
For more information about osteoarthritis in the knee, download the patient advice leaflet on this page.