Swelling within the knee can be due to many sources. With a history of an injury or trauma, it is most commonly due to blood within the joint. The most common injury found in patients that have an injury with blood within the joint (an acute hemarthrosis) is an anterior cruciate ligament tear. This injury is found in almost 3 out of 4 patients who have an acute hemarthrosis. Other common injuries with blood in the joint due to injury include patellar dislocations, meniscal tears, articular cartilage injuries, or other associated ligament injuries.
Fluid within the joint is a very common sign in patients who have osteoarthritis. In patients who do have underlying degenerative arthritis, an increase in activities can sometimes result in significant joint swelling.
Generalized deconditioning of the quadriceps muscles can also sometimes result in swelling within the knee. This can occur with or without any arthritic changes. This is most commonly seen in patients after surgery who may have significant atrophy of their quadriceps mechanism, along with tight hamstrings, which can be due to quadriceps deconditioning due to injury and/or surgical procedure.
Other causes of knee swelling include gout and pseudogout. Both of these are caused by crystalline deposits within the knee which will cause irritation of the tissues and result in swelling. Gout is caused by sodium urate crystals, while psuedogout is caused by CPPD crystals. Unless a patient has a known history of this crystalline abnormality in the joint, this diagnosis must be made by an analysis of their joint fluid to see if they have any of these crystals present in the fluid which may be causing the joint irritation and swelling.
Other causes of knee swelling include joint effusions and a lot of quadriceps atrophy with or without any associated arthritis. This quadriceps atrophy can be due to patellofemoral dysfunction or due to the effects of immobilization, injuries, or surgical procedures. Treatment of the effusions is best centered around strengthening of the quadriceps musculature and stretching out the hamstrings so as not to put extra pressure on the anterior aspect of the knee.
Other causes of knee swelling include benign synovial tumors of the knee (lipoma arborescens, osteochondromatosis, and pigmented villonodular synovitis. Other causes of knee swelling include bleeding problems (such as hemophilia), rheumatoid arthritis, or infection.