The Standard Twin Block is a removable functional appliance consisting of two (twin) bite blocks (upper and lower) that are designed to interlock at 70 degrees in such a manner that the mandible is held in a more protrusive position. Over time, the repositioning of the mandible forward eliminates the overjet. When acrylic is removed from the upper block, eruption of the lower first molars occurs -- thus eliminating the overbite. When treatment with the Twin Block is complete, the first molars will be in contact and the maxillary and mandibular incisors will be nicely coupled. In the mixed dentition, the lower molars will usually erupt passively. In the permanent dentition, the lower molars often need to be erupted actively (with vertical elastics). To ensure that the patient does not end up with a dual bite, the appliance must be worn a minimum of 7 to 9 months. When the patient’s bite is stable, retrusion of the mandible should not be possible without causing patient discomfort. Once the first treatment phase is completed and the case is at the desired vertical and AP position, it is necessary to place a Phase 2 or “Support Appliance.”

The Standard Twin Block has an upper block that covers the second primary molar or bicuspid, first molar and second molar. The lower block covers the first primary molar and two thirds of the second primary molar. It is vital to the success of the Twin Block treatment that the lower block be held ahead of the upper block at all times. To accomplish this, the blocks must be at least 5 to 6 mm thick. If they are not, adjusting them (to allow for the eruption of the lower first molars) will destroy the interlocking effect of the two blocks. Appliance retention is also critical to the success of this appliance. If you do not have first primary molars that are going to be present for 7 to 9 months --or first bicuspids that are sufficiently erupted -- it is better to choose another appliance...or delay treatment until the first bicuspids have erupted and adequate retention is available. Note: The expansion screw added to this design can be used at the same time to initiate lateral arch development.