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Doctor Who Unleashed: Russell T Davies speaks about Davros
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A good old fashioned Doctor Who Community
The next episode has the wonderful ruth madeley, who is a wheelchair user helping the doctor. She also played a different character as a companion to the 6th in big finish. I'm sure we'll see others too. Because of the number of evil wheelchair users in media over the years it's simply better to stay well away from this trope from now on.
It certainly invites unwanted conversations when a companion this season uses a wheelchair. If that wasn't the case, I would bet this would never have even occurred to anyone. It's probably a good thing to have the conversation.
As far as the in universe explanation, Davros is a mad scientist. He made new legs.