The basic appliances are separate upper and lower plates. The upper plate includes an expansion screw for lateral development and occlusal pads that cover the molars. The lower plate includes occlusal pads to cover the lower bicuspids. These plates interface at 70º angles and posture the mandible in a forward position.
The Twin Block Appliance was developed by Dr. William Clark during the early 1980s. The appliances are separated upper and lower appliances consisting of two bite blocks. The blocks are designed to interlock at 70 degrees such that the mandible is held in a protrusive position. Which in time eliminates the over jet. When acrylic is removed from the upper blocks, eruption of the lower first molars occurs eliminating deep bite and overbite.