We've had hospitalized patients with external fixators and I can't imagine going to WDW with one. There are many attractions which are difficult to get in and out of because of steps in, then down. Follow the link in my signature (Or look near the top of the board to the disABILITIES FAQs thread.
Ther is quite a bit of helpful information, and one particular post on page 2 has boarding information.
If you have to talked to your doctor yet, you need to. They may not want you to travel so far away from home because the risk of infection, risk of DVTs (clots) due to the long flight an/or of anything happening to the fixator. Even if they say going is OK, you will want to have a list of possible issues to watch out for and who to contact/what to do. If the doctor feels you should not go, they may be able to write letters/help provide documentation to get a refund or reschedule the trip.
If the doctor says you can go, I would suggest a wheelchair rather than an
ECV. There are some lines that are wheelchair accessible, but not ECV accessible;for those, guests using
ECVs need to transfer to a wheelchair. There are also some wheelchair accessible ride cars that are not ECV accessible.
So, using a wheelchair would mean less transfers. I would not go on anything where you can't stay in the wheelchair for the entire attraction - so shows and the rides with accessible ridd cars.
For readers who don't know what an external fixator is or how it looks, you can check out this weblinks. Warning: the pictures are pretty shocking, so you may not want to look
http://www.ncbi.nlm.nih.gov/pmc/articles/PMC2504087/#!po=1.47059