# Open-Bite Correction Using a Mid-Palatal TAD with an Intrusion TPA: Anteroposterior Position of TAD
## 1 Introduction  
Attempts to treat an open bite with molar intrusion using a temporary anchorage device (TAD) have been reported since the early 2000s [1; 2; 3; 4]. Figure 1 shows an open-bite case treated by intruding maxillary molars with buccal screws between the roots of the maxillary first and second molars. Molar intrusion resulted in counterclockwise rotation of the mandible, a lower anterior facial height decrease, and resolution of the retrusive chin (Figure 2). This method made it possible to get a maxillary impaction effect orthodontically without orthognathic surgery. Screw-type TADs, which are convenient for placement and removal, became popular [3; 4]. Buccal TADs were placed between the maxillary first and second molar roots. Molar intrusion using buccal TADs showed buccal tipping of molars as a side-effect, which was controlled by the palatal inter-radicular TADs (Figure 1G,H and Figure 3A), a mid-palatal TAD [5], or a transpalatal arch (TPA) (Figure 3B) [6]. However, there were some disadvantages. The inter-radicular space between the maxillary first and second molar is usually narrow, making it difficult to place screws. Moreover, it is not uncommon for buccal inter-radicular TADs to fall out during posterior intrusion.  
Molar intrusion using plates was also introduced in the early 2000s [1; 2]. It has the advantage that the failure rate is lower than the buccal TADs. However, the molar intrusion method using plates was limited to exceptional cases due to the inconvenience of implantation and removal surgery [7; 8; 9], although they had a higher survival rate than the TADs.  
The buccal screw-type TADs (Figure 3A,B) are more convenient than plates which need surgery for placement or removal [7; 8; 9]. Nevertheless, it is hard to find good indications because buccal inter-radicular spaces between the first and second molars are usually too narrow to place them safely without root trauma. The other disadvantage is that they frequently fail as the posterior teeth intrude. As the posterior teeth intrude, the screws become closer to the alveolar crests, whose areas are easily bent or traumatized by the occluding forces.  
Figure 1: _Case 1_. Maximly molars were intruded with buccal and palatal interradicular TADs. (**A–D**) Initial photos. Only four maxillary molars were occluding. (**E**,**F**), Buccal view. (**G**,**H**), Palatal view. TADs were placed between the first and second molars. These teeth were intruded with power chains. (**I–L**), After 4 months, as the maxillary four molars were intruded, both overbite and overjet decreased.  
Figure 2: _Case 1_. The black line is before, and the red line is after the intrusion of the maxillary first and second molars. As only four upper molars were intruded, the mandible was rotated counterclockwise, and the pogonion protruded. In the early 2000s, it was good news that these skeletal changes were possible only through the intrusion of maxillary molars using TADs without maxillary impaction surgery.  
At first, [PERSON] et al. used a mid-palatal TAD to intrude the maxillary teeth in a mouth protrusion case accompanying the maxillary vertical excess without an open bite [10,11]. They used a TPA to transfer the intruding force to the first molars and to prevent lingual tipping of intruding molars. This method was modified by [PERSON] to treat open-bite patients [12], which became extensively used in various open-bite cases because of its safety, simplicity, and stability [13]. This article aims to introduce open-bite correction with a mid-palatal TAD combined with an intrusion TPA and to explain its biomechanics, especially the different effects of the anteroposterior position of TAD.