If they don't fit in the overhead or in the closet, then another option (which I didn't like when I was on crutches and did not like when I was an airline manager and crew would suggest this....) is place on the floor parallel to the side of the plane, along the window. They won't/shouldn't try to check them... there are no limits of liability on assistive devices so any scratch and they have to provide new, but that does you zero good sitting in a wheelchair in a jetway with a crutch bent in half.

On the CRJ-550 they might fit between cabins in the baggage cabinet on top of other items at an angle.