Knee pain can be caused by a variety of factors. Aging leads to wear and tear in the joint. Knee pain can also be caused by undue stress (such playing rugby).
Sometimes the cartilage beneath the kneecap (the patella) begins to soften resulting in small areas of erosion causing pain in the knee. Instead of a smooth, easy glide over cartilage, the kneecap rubs directly against the thighbone (the femur). These changes can be mild or lead to complete erosion of the cartilage.
Over-pronation of the feet is another cause of knee pain. The knee acts as a hinge between the upper and lower leg. The knee is only supposed to flex and extend (pull up and pull down) the lower leg. With rotation, the knee is moved in an unnatural direction causing pain. The knee is not supposed to rotate.
When the foot over-pronates and rolls inwards, the lower leg has no choice but to rotate. This causes poor knee function that, over time, leads to hard wearing on the knee joint with long-term injury and pain.