The medical term for fluid in the knee is called effusion. It describes the presence of, usually, an abnormally large amount of fluid within a joint, typically, but not limited to, the knee. Joints in the body are nourished to a large extent by this fluid, and only require a small amount to lubricate them. When conditions, injuries or diseases occur that cause irritation, inflammation or disruption to the normal smooth surfaces of the joint, the irritation stimulates the body to produce excess fluid.
Torn Cartilage
There are two types of cartilage in the knee joint. The type that covers the surfaces of the ends of the bones that form the joint is called hyaline articular cartilage. It is soft and smooth and provides a cushion of virtually friction-less motion between the two bone ends that form the joint. The other cartilage type is called fibrocartilage, which make up the meniscus. The meniscus, or menisci for plural, is a tough gristle-like material that serves as shock absorbers for the knee. There are two menisci in each knee, which also serve to help distribute weight through the knee joint.
When a meniscus becomes torn through injury, the resulting irregularity and fragmented edges become a mechanical irritation to the knee. In response to this irritation, the knee produces excess fluid in hopes of battling the resultant inflammation.
Infection
Infection in a knee joint can be a serious problem. Fluid build up as a result of infection can be of two types: excessive joint fluid from over-production due to inflammation caused by the infection and pus; or a thick material, called an exudate that is created by different microorganisms that can be responsible for the infection.
Arthritis
Arthritis typically begins as a mild ache or soreness, and gradually advances in severity to a state of severe degeneration of the soft joint cartilage, which leaves the joint rough and irregular causing substantial irritation, pain and joint fluid production.
Gout
Gout is a form of arthritis that involves the body's over-production, under excretion, or passage of uric acid crystals in the blood stream. Gout presents problems with arthritis, mainly due to the production of uric acid crystals, which circulate in the blood and frequently exit and deposit themselves inside joints. These crystals cause considerable irritation and inflammation, which in turn, turn on the switch for excess fluid production to combat the inflammation.
Fracture
When fractures occur in the knee area and extend into the joint, they can cause significant disruption of the normal smooth surfaces. Fracture fragments and bone edges can protrude up into the joint, or be of a depressed nature and literally sink away from the joint, causing abnormal low spots within the joint cartilage. These irregularities can, and do, cause excessive grinding of the joint surfaces, damaging these surfaces and stimulating excessive fluid production.
