You could also try adding another lower sided litter box in a spot that may be easier for her to access ie on the floor itself not a place she needs to climb up. It's possible her hips could be bothering her as she may be over compensating for the shoulder injury.
Avoid biting when a simple growl will do
The Spirit Lives As Long As Someone Who Lives Remembers You - Navaho Saying
Vindication ~ For all those pets who became sick or lost their lives from tainted pet food