The underlying reason for the appliance is that the jaw closing muscles are far stronger than the jaw opening muscles. Muscles work in groups, so when the mouth wants to open, the closing muscles relax and the open muscles contract and pull open the lower jaw. However the closing muscles are stretched and puts tension on the fibres. These muscles ideal resting position is when the mouth is slightly open (this is achieved by saying the letter ‘M’). 

The closing muscles slowly tense up and the smaller opening muscles get tired from opening and fighting the tensing closing muscle and the person starts to aches mainly in the closing muscles which want to close the jaw.   

There are two common names for this appliance that keeps the mouth open- mouth prop or bite block. However the two names highlight subtle differences in the use of the appliance.

Let’s take the bite block use.

If there is a long procedure that requires continual opening of the mouth the jaw closing muscles will always win! However the bite block enables the person to activate the closing muscle by biting on the block. The muscles feel like have closed the jaw. The ache of the wanting to close is gone. The opening muscles are not need as much so they can rest. Patient and dentist are happy! 

Sometimes it may not be a long procedure but when the closing muscles are overworked due to clenching or grinding of the teeth, they want to rest very quickly, so the bite is need even for short treatments.

Injury to the muscle called trismus due to surgical procedure such as removing a wisdom tooth or trauma causing bruising may cause the closing muscles to cramp. This means the closing muscle will not relax to allow the jaw to be opened. Any bleed into the muscle is an irritant and the muscle fibres tighten.

An example of a long procedure is in chair bleaching which is usually an hour. The other reason for the bite block is to keep jaws apart so that the tongue mouth is kept away from the bleaching chemicals. In other words we do not the person to do a full swallow on the bleach work.