Pigmented villonodular synovitis
is a disease in which the tissue lining a joint in the body begins to grow abnormally. There are two types of pigmented villonodular synovitis: the nodular form (where the abnormal tissue growth forms a single bump) and the diffuse form (where the entire lining of the joint grows unusually). The additional tissue in the joint can cause pain, limit movements or cause the joint to lock, and in some cases, can destroy the normal joint structure. The knee is most commonly affected by this condition, though it can occur in other joints such as the hip, shoulder, elbow, ankle, or wrist. The average age of diagnosis for this condition is 35 years. Source: Genetic and Rare Diseases Information Center (GARD), supported by ORDR-NCATS and NHGRI.