Hi Jill,
I am sorry to hear that you have a teenager with a boo-boo (can you tell that I have younger kids?). You'd be amazed at how many people travel to Disney with casts. I am not a doctor, nor do I play one on TV but the first thing you should do is to check with the orthopedist and see what restrictions and precautions they have set.
If the cast or walking boot would prevent your teenager from sitting in a ride vehicle in the correct position, they will not be able to ride. Also think about how well your teenager can maneuver in the cast. Are they able to board and unload from the ride vehicle in a safe manner? Guests have to step down in order to board some attractions like Space Mountain and Rock 'n' Roller Coaster. Cast Members are not allowed to physically lift guests so keep that in mind. If your teenager doesn't have any problems with any of these, then they should be able to ride. Cast Members do have the final decision in deciding so let your teen know that just in case. It can be super disappointing if someone isn't allowed to board so just make sure they understand that.
Also keep in mind that some rides involve water so unless your teenager has a water proof cast, you should avoid those rides.
I am familiar with knee scooters for people with lower leg injuries but not sure about a Roll-A-Round. I have never seen a guest with a knee scooter in the parks. They don't allow regular scooters in the parks so they may not allow knee scooters in for safety reasons. Disney has wheelchairs and ECVs for rent in the parks. For more information on wheelchairs and ECVs, click
here.
Please feel free to visit again with any other questions. Its our pleasure to assist you!
Wishing you minnie magical memories :-) Allison