Which one of the following is not a characteristic feature of rheumatoid ahritis?

In 80%of Rheumatoid ahritis have serum IgM autoantibodies that bind to fc poion of the IgG. These autoantibodies are called rheumatoid factor. Ahritis and metacarpophalangeal joints involvement is surely present in rheumatoid ahritis. But sacrolitis is not at all related to rheumatoid ahritis. It is a condition in which one or both sacroiliac bones are affected with inflammation. In this condition there is pain in buttock or lower back. Ref:Robbins basic pathology 9th edition page no 784 785,786