Knee effusion or swelling of the knee (colloquially known as water on the knee) occurs when excess fluid accumulates in or around the knee joint. There are many common causes for the swelling, including arthritis

Arthritis

Arthritis is a form of joint disorder that involves inflammation of one or more joints....

, injury to the ligaments or meniscus

Meniscus (anatomy)

In anatomy, a meniscus is a crescent-shaped fibrocartilaginous structure that, in contrast to articular disks, only partly divides a joint cavity. In humans it is present in the knee, acromioclavicular, sternoclavicular, and temporomandibular joints; in other organisms they may be present in other...

, or when fluid collects in the bursa

Bursa (anatomy)

A bursa is a small fluid-filled sac lined by synovial membrane with an inner capillary layer of slimy fluid . It provides a cushion between bones and tendons and/or muscles around a joint. This helps to reduce friction between the bones and allows free movement...

. This condition is known as prepatellar bursitis

Prepatellar bursitis

Prepatellar bursitis, also known as Housemaid's knee, is a common cause of swelling and pain above the patella , and is due to inflammation of the prepatellar bursa. This structure is a superficial bursa with a thin synovial lining located between the skin and the patella...

.

Signs and symptoms

Signs and symptoms of water on the knee depend on the cause of excess fluid build-up in the knee joint. These may include:

Pain

Osteoarthritis knee pain usually occurs while the joint is bearing weight, so the pain typically subsides with rest; some patients suffer severe pain, while others report no discomfort.

Swelling

One knee may appear larger than the other. Puffiness around the bony parts of the knee appear prominent when compared with the other knee.

Stiffness

When the knee joint contains excess fluid, it may become difficult to bend or straighten the knee in certain cases. It may become painful to bend in some areas. Fluid may show under the knee when straightened as well. Icing may help to decrease swelling. Heat may help relax the muscles of the knee.

Bruising

If an individual has injured his or her knee, he or she may note bruising on the front, sides or rear of the knee. Bearing weight on the knee joint may be impossible and the pain unbearable.

Causes

Causes of the swelling can include arthritis, injury to the ligaments of the knee or an accident after which the body's natural reaction is to surround the knee with a protective fluid. There could also be an underlying disease or condition. The type of fluid that accumulates around the knee depends on the underlying disease, condition or type of traumatic injury that caused the excess fluid. The swelling can, in most cases, be easily cured.

Underlying diseases may include

• Osteoarthritis
  Osteoarthritis
  Osteoarthritis also known as degenerative arthritis or degenerative joint disease, is a group of mechanical abnormalities involving degradation of joints, including articular cartilage and subchondral bone. Symptoms may include joint pain, tenderness, stiffness, locking, and sometimes an effusion...
  
  
• Rheumatoid Arthritis
  Rheumatoid arthritis
  Rheumatoid arthritis is a chronic, systemic inflammatory disorder that may affect many tissues and organs, but principally attacks synovial joints. The process produces an inflammatory response of the synovium secondary to hyperplasia of synovial cells, excess synovial fluid, and the development...
  
  
• Infection
  Infection
  An infection is the colonization of a host organism by parasite species. Infecting parasites seek to use the host's resources to reproduce, often resulting in disease...
  
  
• Gout
  Gout
  Gout is a medical condition usually characterized by recurrent attacks of acute inflammatory arthritis—a red, tender, hot, swollen joint. The metatarsal-phalangeal joint at the base of the big toe is the most commonly affected . However, it may also present as tophi, kidney stones, or urate...
  
  
• Pseudogout
• Kneecap Bursitis
  Prepatellar bursitis
  Prepatellar bursitis, also known as Housemaid's knee, is a common cause of swelling and pain above the patella , and is due to inflammation of the prepatellar bursa. This structure is a superficial bursa with a thin synovial lining located between the skin and the patella...
  
  
• Cysts
• Tumours
• Overuse

Having osteoarthritis or engaging in high-risk sports that involve rapid cut-and-run movements of the knee — football or tennis, for example — means an individual is more likely to develop water on the knee.

In overweight

Overweight

Overweight is generally defined as having more body fat than is optimally healthy. Being overweight is a common condition, especially where food supplies are plentiful and lifestyles are sedentary...

 or obese individuals the body places more weight on the knee joint. This causes more wear in the joint. Over time, the body will produce excess joint fluid.

Joint aspiration

Also known as arthrocentesis

Arthrocentesis

Arthrocentesis is the clinical procedure of using a syringe to collect synovial fluid from a joint capsule. It is also known as joint aspiration. Arthrocentesis is used in the diagnosis of gout, arthritis, and synovial infections....

, this procedure includes withdrawal of fluid from inside the knee for analysis such as cell count, culture for bacteria, and examination for crystals, such as uric acid

Uric acid

Uric acid is a heterocyclic compound of carbon, nitrogen, oxygen, and hydrogen with the formula C5H4N4O3. It forms ions and salts known as urates and acid urates such as ammonium acid urate. Uric acid is created when the body breaks down purine nucleotides. High blood concentrations of uric acid...

 or calcium pyrophosphate dihydrate crystals found in gout or pseudogout.

Imaging

An X-ray

X-ray

X-radiation is a form of electromagnetic radiation. X-rays have a wavelength in the range of 0.01 to 10 nanometers, corresponding to frequencies in the range 30 petahertz to 30 exahertz and energies in the range 120 eV to 120 keV. They are shorter in wavelength than UV rays and longer than gamma...

 is useful to verify that there is no break or dislocation when there is a history of trauma. May show signs of osteoarthritis.

MRI

Magnetic Resonance Imaging detects abnormalities of the bone or knee joint, such as a tear in the ligaments, tendons or cartilage.

Blood tests

If the knee is swollen, red and warm to the touch when compared to the other knee, a doctor may be concerned about inflammation due to rheumatoid arthritis or a crystalline arthritis, such as gout or pseudogout, or joint infection. Besides sending the joint fluid to a laboratory for analysis, he or she may request blood tests to determine a white blood cell count, erythrocyte sedimentation rate, and perhaps the level of C-reactive protein

C-reactive protein

C-reactive protein is a protein found in the blood, the levels of which rise in response to inflammation...

 or uric acid. If blood tests reveal Lyme's disease antibodies forming, the condition may be attributed to it.

Treatment

Treatment of fluid in the knee depends on the underlying cause of the swelling. General measures such as rest, ice, and analgesics such as acetaminophen and NSAIDS are often recommended.