Siderotic synovitis unknown origin, subtotal synovectomy 8/2013, rt knee, good rom but always stiff, often sore down stairs, no arthritis, normal??

This is due to bleed: Since not all of the synovium was removed there may be persistent synovitis. The question also is do you still have bleeding into the joint. Have you had a post operative MRI to see if synovitis persists? You of course also need persistent exercise to make certain the surrounding musculature is good and stays strong to support the knee. You need to discuss with your doctor.