Knee joint may become firm and swollen due to accumulation of the excessive fluid in or around the joint. In general terms it is also called as "water on the knee."
This mainly occurs due to trauma, overuse injuries, infection or an underlying disease or condition like osteoarthritis. The exact cause of the fluid accumulation can be evaluated after taking a sample of the fluid for diagnostic evaluation.
Swelling of the knee joint may cause puffing up of the kneecap that is easily noticeable and comparable with the normal knee. This is generally accompanied with stiffness or inability to bend or straighten the leg. In some cases, the condition can be extremely painful to an extent that it's difficult or impossible to bear weight on it.