Wacky Windows is a fun website that aims to provide fun and engaging learning actvities around the National Curriculum.
The site is aimed at students from 3- 7 yrs, although would be appropriate for older students who have Special Needs.
They have a good clear information page relating to arthritis.
While you are there you could look at some of the fun games too.
http://www.wackywindows.com/juvenilerheumatoidarthritis.html