What about your ankles? Are you sure it is caused my the MTX?
Did you talk about this with your doc?

There are risks, but not more than any other immune suppressant has. I had the unfortunate luck it caused irreversible damage in only a few months. But there are a lot of people who use it without any problems whatsoever. For years!
You just have to be very careful and check your blood regularly and take folic acid.

Anyway, if you do not have severe MG problems without it, it's up to you.