I will give you my take on things.
I think MS and arthritis are related, are auto immune diseases, caused by the herpes family.
In MS, the herpes virus anchors in permissible cells, that is the OPCs, in time of immune deficiency establishing transgenic cells, without immune memory established. I think inflammation of the sinus cartilage is an collateral issue. Later on, if infected again - may be a chronic EBV infection - immune complexes cross react with transgeneic cells causing autoimmunity i.e the reaction against the self.
In arthritis, the same process takes place. The herpes virus archors in the cartilage between the joints in times of immune deficiency without immune memory established making cells transgenic. Later on, this invokes autoimmunity of the joints in the same way.
The common thing in MS and arthritis is the virus.
See also the last few pages of general-discussion-f1/topic15188-585.html
You don't have to believe me, but at least read the pages and think it over.