Knee Swelling can be from inside the knee joint or from the tissues surrounding the knee joint.
If you injure your knee by falling over or twisting your knee you may create some bleeding in the joint. The swelling usually comes on quite rapidly. Typically the swelling indicates that there has been an internal derangement to the joint. This might be a meniscal tear, ACL rupture, joint cartilage injury or perhaps even a fracture of the bone. Hearing a pop or crack may indicate an ACL tear but in this situation it usually takes several hours for the knee to swell even though it is usually immediately painful. A careful clinical examination and an x-ray are usually required to determine what caused the knee to swell.
If you fall directly onto the knee the tissues outside the joint may be bruised and swollen. The joint itself is usually not swollen and there are usually no long lasting injuries if the bones are not broken. Ice, physiotherapy and soft tissue massage can be used to successfully treat this injury.
Chronic or recurrent swelling which occurs without a specific new injury is sometimes due to arthritis or other cause of joint lining irritation. This includes gout, pseudo-gout, psoriatic arthritis, rheumatoid arthritis, osteoarthritis and pigmented villonodular synovitis. It is unusual for an isolated meniscus tear to create swelling so if the knee is swelling it usually means that the joint lining cartilage is damaged also.
There are several fluid filled sacs around the knee than can swell when they are inflamed. Rest, Ice and ant inflammatory tablets are usually used to treat these. This includes pre-patella bursitis (housemaids knee) and retropatella bursitis.
Chronic swelling of the joint is generally not good for the joint lining surfaces so I always try to identify a cause for the swelling and treat it.