dental gag

There are several different gags that dentists use in their work to keep a patient's mouth open and teeth apart, especially during surgeries. What is common to them all is that they do not block the entrance to the mouth, unlike ball gag for example.

A common type of dental gag is the Whitehead gag, which has a ratchet mechanism allowing for opening the teeth wider, but not returning back.

https://en.wikipedia.org/wiki/Gag_(medical_device)