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A twin block appliance is a type of plate that is called a functional appliance. Functional appliances are used to encourage growth of the lower jaw and reduce large protrusions of upper front teeth. On average, the upper teeth protrude by 2 – 3 mm ahead of the lower teeth. Functional appliances are suitable for cases where the teeth protrude by 6 – 8 mm of more. Extreme protrusions of 12 – 14 mm can also be successfully treated with a twin block appliance.

The twin block consists of two plates, one for the upper arch and one for the lower arch. The upper plate has blocks towards the back and the lower plate has blocks towards the front. When the teeth bite together the lower jaw is guided forward so that the blocks will lock together. The action of the muscles provides gentle functional force to encourage forward growth of the lower jaw and change the position of the front teeth.

A twin block should only be used around the pubertal growth spurt. In a girl this is usually between 11 and 12 years of age and in a boy this is usually between 12 and 13 years of age. Starting functional appliance treatment too early or too late will produce reduced results because the patient will not be growing at the maximum rate.

Twin blocks are usually worn for about 12 months. Sometimes the corrections will occur relatively quickly but the plates are still worn to allow growth to catch up with the dental changes that have occurred.