If I go through the airport with her, there is no guarantee the agents will let me stay with her and explain the situation. She can't remove the leg brace on her own for them to make sure it's not a weapon, and TSA agents can't do it properly.
So, basically, taking her to the airport is a recipe for having her verbally abused (treated like a mentally handicapped or non-cooperative person), and then physically abused (TSA agents going under her clothes to determine that she's just wearing a leg brace).
Maybe there's some special procedure for dealing with this, but it's easier to just not travel than risk it.
The TSA is a direct, overt violation of the 4th amendment (search and seizure). Thus, the US federal government is not legitimate under the US Constitution. (No, I am not advocating rebellion, violent or otherwise.)
If we simply let the airlines handle their own security, which is not a violation of the 4th amendment, none of this would be an issue. Moreover, it would make air travel and security screening a voluntary relationship between consenting parties (travellers and airlines), instead of a coercive relationship between two consenting parties and government agents.