Swollen knee secondary to haemarthrosis.
The blood is contained within the knee capsule, giving the haemarthrosis a balloon-like quality.
Bleeding inside the knee joint
Haemarthrosis may be a result of trauma, such as a torn cruciate ligament, or it may be a complication of knee surgery, such as a lateral release, where the blood vessels have not been sealed prior to release of the tourniquet.