So there was this one time that Professor X turned into a giant space bug and kind of died. Funny thing, really.
Though largely an easily-defeated menace today, once upon a time, the Brood royally jacked the X-Men up, even infecting each one of them with a Queen embryo that would hatch and transform its host into a Brood, killing them in the process. The thing even hatched in both Wolverine and Storm, but other factors (a healing factor and death) prevented the transformation.
Charles Xavier wasn’t so lucky. His first attempt at detecting the thing knocked him into a catatonic stupor, then its influence led him to recruit the original New Mutants. Seriously – go look it up. Finally the X-Men, recently healed from their own infections, made it back to Earth and set out to cure their beloved mentor of his infection.
And they failed.
The Brood embryo hatched and Professor X’s body was irreversibly transformed. Fortunately, the X-Men managed to put the kibosh on the thing before it overtook his mind, and before you can say “space age technology”, the Starjammers had a new Xavier body cloned and ready to go and transferred his mutant brain over. The operation was a complete success, and the neat procedure had two neat little benefits. One, it allowed creators to be able to keep Xavier “ageless” as his new body could be considered any age rather than being stuck with the real-world events of his origin. The same was done with the recovery of Magneto, but that’s a story for another day.
The second and more in-story benefit was that the new body had a fully working set of legs, as the Starjammers didn’t find it necessary to smash his legs so the two would perfectly match. So, with the X-Men and New Mutants watching with bated breaths, Professor X emerged from the medical site and shocked everyone by standing up from his chair for the first time
since his Silver Age psychic braces had been abandoned because they were really stupid.
And then he screamed out in pain and crumpled into a heap. Whoops.
You see, when Xavier initially had his legs smashed by Lucifer, he used his psychic powers as a mental painkiller to block the horrific pain. As he continued over the years, he was always slightly doing such to keep himself sane and preventing his legs from having to be amputated. With the debut of his new body, he finally was able to stop blocking the pain, but his powerful mind was not exactly registering his new limbs and thus still registered the massive pain when he tried to use them.
Think phantom pains, but in reverse.
So thus, Xavier’s fix was not a simple get-’em-and-go, but forced him into months of physical therapy to train his mind to once again use his legs, much like anyone going through rehab to relearn simple tasks upon recovering from injury. It was a fantastic and unexpected move for the character, and is just one more reason why Chris Claremont is awesome.