Below is an instruction from Human. Write a response.
    ### Instruction:
    What causes Pigmented villonodular synovitis ?
    ### Response:
    What causes pigmented villonodular synovitis? The exact cause of pigmented villonodular synovitis (PVNS) is unknown.  Some doctors believe that it is similar to arthritis, arising from swelling (inflammation) of the joint tissue. Others believe it develops like a tumor, caused by cells growing and multiplying more quickly than usual.
    