Osteoarthritis in the knee means wear and damage of the cartilage covering the ends of the bones.
For various reasons cartilage may become thinner and of poorer quality. Many known and unknown risk factors for osteoarthritis exist - extrinsic factors such as such as meniscal tears, cruciate ligament tears, cartilage lesions, bowed legs and relevant fractures, as well as more intrinsic factors such as heritage and nutrition (D-vitamin).
In my opinion this can be explained by looking at the synovial tissue.
The synovium is the material that lines the cavity of the knee joint ( click this link to see image ) and which is responsible for lubricating it and nourishing those tissues in the knee that do not have their own good blood supply. The synovium contains nerve endings which trigger the tissue to respond to changes in circumstances inside the joint, and it will become aware of microscopic cartilage particles needing to reabsorbed from the joint fluid. For a small proportion of people, the synovial tissue will get swollen and painful early when coming into contact with even a small amount of tiny cartilage particles, and for those people the symptoms of osteoarthritis will start even before the damage becomes obvious on X-rays and MRI.
For most people, though, the symptoms of osteoarthritis will first appear when the wear is more pronounced.
The subchondral bone
With more severe osteoarthritis some of the pain seems to originate from the subchondral bone with increased pressure in the bone itself. I think that the synovitis part of the pain is more in the initial phase of the disease when pain is sharper and swelling more acute. I think that night pain is also a later feature but I am unsure of that.
Prevention of cartilage damage in the knee
How can you prevent further degenerative changes in the knee?
Top of the list is to live a healthy life and keep your weight down. You should avoid activities that put a lot of stress on the same part of knee, such as walking long distances or running, when the forces are applied to the same small spot in the knee for every step. Contrary to this are activities like bicycling, rowing and swimming that are much more healthy for the knee. Actually activity is of importance for bringing fresh joint fluid to the cartilage and for this is bicycling very healthy.
In cases where there is malalignment - meaning that something is loaded in the wrong way - it can be preventive to have surgical correction of the deformity. This is the situation when you are bowlegged or when you kneecap is tracking crooked or irregularly.