WHAT???!!!!! What do you mean you assist feed? Unless that thing is a hatchling that has never eaten and is near death, or it is a WC situation and it hasn't eaten in well over a year, what are you doing???!!!!!!
There is something very wrong here if you have to assist feed! Please explain in detail how exactly you assist feed. Only way I know of is holding the snake behind the head and forcing food into its mouth....
Bottom line, an injury like that does not just "happen" when feeding a frozen / thawed mouse in a seperate container by itself!!!! What really happened here?