Water on the Knee

Patients often present or are diagnosed by their GP or physiotherapist with water on the knee. This is not a diagnosis but a symptom. Water on the knee means that there is an increased amount of fluid inside the knee and this has many causes. When the doctor examines your knee and finds the fluid, they will call it an effusion of the knee.
The main causes are:

  • Blood (Haemarthosis)

This may be caused by trauma or an injury to the knee. Inta-articular fractures, ACL tears and patella (kneecap) dislocations present in this way. In 50 to 70% of cases with an acute haemarthrosis, an ACL tear has occurred.

  • Inflammatory

An acute flare up of an underlying rheumatological disorder such as gout, rheumatoid arthritis.

  • Reactive inflammatory

This is usually caused by a meniscal tear or loose bits of articular cartilage that have broken free. These 2 conditions irritate the synovium (lining of the joint) and cause inflammation resulting in fluid.
In general, an effusion of the knee is a very important finding and means that something is wrong in the knee. When it is present, not only may there pain associated with it but the patient feels that the knee is stiff and may battle to gain full range of motion.
If there is water on the knee, then the best option is to have it seen to by a knee specialist who can guide you on the best way forward.


The diagnosis may be gained from a thorough history and examination. Meniscus and ligament testing are important in this regard. Sometimes an x-ray may be ordered to rule out a fracture or infection. In certain circumstances an MRI will be asked for as this investigation gives a better indication of the soft tissue (i.e. meniscus, cartilage and ligaments).
When these investigations do not reveal anything significant, bloods may be ordered to check for signs of inflammation and the joint may be aspirated and the fluid sent to the laboratory for investigations.


Treatment depends on the cause for the effusion. In the case of a fracture, this may entail plaster of Paris or surgery. Surgery may also be needed in the case of an ACL tear, or when there is meniscal damage or a cartilage lesion. Anti-inflammatories are the main stay of treatment for inflammatory conditions such as gout.
Sometimes the treatment may entail aspiration of the joint especially to relieve a tense effusion or haematoma. At the same time, cortisone to relieve the inflammation may be injected into the joint.