Joint effusion is an excessive accumulation of fluid in the joint and clinically results in an overall swollen knee. This fluid can be of several kinds.
Synovial fluid
Naturally, each joint contains a film of synovial fluid, which allows the joint surfaces to slide and allow smooth motion; in the case of joint inflammation due to a meniscal injury, for example, or due to rheumatic diseases, the synovial membrane of the knee (inner skin of the joint) secretes more synovial fluid and the knee swells up. Only an aspiration of this fluid will make it possible to differentiate its precise nature (mechanical versus inflammatory). However, joint aspiration is rarely necessary in daily practice because the anamnesis, the clinical examination and complementary examinations such as X-rays or MRIs are most often sufficient to make a diagnosis and thus explain the cause of the effusion.
Blood (hemarthrosis)
Typically found in the case of a torn anterior cruciate ligament, for example.
Blood WITH FAT DROPLETS (LIPO-hemarthrosis)
Blood with fat droplets (lipo-hemarthrosis) is found in the knee in the case of intra-articular fractures, such as tibial plateau fractures, because some blood and bone-marrow fat leaks into the joint.
pus
In case of joint infection (septic arthritis), some pus is found in the knee joint. The clinical picture of septic arthritis is usually obvious (fever, swollen and hot knee, sometimes red) and the aspiration of the joint fluid will confirm the joint infection (cloudy fluid).