Hi Carol!
A cast is NO FUN! Neither are broken bones! I'm so sorry to hear that you (or someone in your travel party) has experienced a recent injury.
First of all, speak with your Doctor, and make sure that you've got their ok to hit the parks. They'll know the specific details of the medical condition, and any restrictions the patient would be under.
Second, for the most part, you should be just fine. Some casts might interfere with the position of the safety restraints that are available, but you would be able to simply ask a Cast Member at the attraction queue about those limitations. Two attractions that might be a bit tricky to maneuver (depending on the type of cast) would be
Rock 'n' Roller Coaster® Starring Aerosmith and
Mission: SPACE®. The safety restraints are over the head and shoulders, and it might prove difficult (but not impossible) to maneuver the cast. As safety is paramount, I'd make sure to check with Cast Members prior to boarding.
Have a magical vacation!
Amanda