When permanent maxillary molars move anteriorly, they rotate mesiolingually around the large lingual root. The space between the buccal and lingual cortical plates becomes narrow anterior to the fi rst molar roots, preventing the molar from advancing directly and limiting its movement to a rotation. The large lingual root contacts the lingual plate and acts as a pivot, allowing the 2 buccal roots to rotate mesiolingually. The TPA reduces anterior molar movement by coupling the right and left permanent molars together and, thus, preventing any possibility of rotations.