Also known as Splint is a type of mouth guard which is used to protect the teeth and jaw joint against the pressure of grinding or clenching in sleep. Traditionally it is made of a hard clear plastic which sits on upper teeth. These days we can have it made of soft plastic or combination of inner layer soft and outer layer hard plastics.
Depending on the case it can be made in many different shapes. It may cover all upper teeth or just the front ones (NTI Splint). It may prevents the back teeth to touch the splint or not. What Splint does is, it keeps all the upper teeth together to withstand the grinding pressure better. Also prevents direct contact between upper and lower teeth So if there is any grinding, it won’t damage the teeth.
There is a type which is called Canine rise or Michigan Splint is made in a way that as soon as patient starts to move the lower jaw to the side only lower canine stays in contact with Splint and separates all the back teeth. Usually the less contact between lower teeth and Splint, the less pressure muscles of face can produce and this means better protection of teeth and jaw joint.