Sample selection and study design
The specific criterion for sample selection in this study was that the patients presented an open bite of at least 1 mm, which is similar to other investigations [6,7,8, 13, 31]. No effort was made to include only open bites with predominant skeletal features. Therefore, the mean characteristics of the groups represent the predominant type of patient in an orthodontic practice, and the results are mostly adequate for these patients.
The random errors ranged from 0.16 (over-bite) to 4.29 (Mx7.Md7). The random errors do not significantly compromise the validity of this work, since four landmarks are needed to obtain these angles and some of these landmarks are subjective [32,33,34]. Therefore, a few random errors of greater magnitude were expected. Similar studies do not mention the random error magnitude [16, 24]. Therefore, probably, they would be similar for the mentioned reasons.
The decision to use non-extraction and extraction patients was because there are some differences in treatment mechanics between these groups [14,15,16, 24, 31]. Basically, when treating non-extraction, the open bite is closed only consequent to the anterior vertical intermaxillary elastics [13, 14, 16, 35, 36]. In the extraction group, besides the elastics, the open bite is also closed through the drawbridge principle and by some mesialization of the posterior teeth [35, 37]. Therefore, there was a need to know whether these procedures would influence the angulation of the posterior teeth.
Posterior teeth angulation
In the non-extraction group, the treatment changes caused a significant mesial angulation of the maxillary second molar in relation to BOP (Table 2). This is contrary to what would be expected [13, 14]. The explanation may be based on its eruption orientation. Before eruption, the second molar crown is usually distally angulated, and after eruption, as the teeth occlude, the apex moves distally, uprighting it [38, 39]. The patients began treatment at a mean age of 15.26 years, in which the second molars are still more distally angulated than their final position [38, 39]. As the teeth are leveled and aligned, it was uprighted by these procedures [15, 16, 24].
In the mandibular arch, all posterior teeth were significantly distally angulated with treatment (Table 2). This shows that even without intentionally trying to distally tip these teeth with tip-back bends in the archwires or by mesially angulating the posterior teeth accessories, it is possible to upright, at least, the mandibular posterior teeth. If uprighting the posterior teeth actually leads to more stability, as speculated [13, 14, 16, 17, 24], this may have contributed to it in an earlier study with some of these patients . Therefore, the principle of uprighting the posterior teeth can be partially accomplished even without the mentioned procedures [13,14,15,16, 24]. This uprighting may also be seen as a necessary dentoalveolar change to compensate for the divergent palatal and mandibular planes that are usually associated with open-bite characteristics .
The mandibular premolars distal tipping significantly increased the interpremolar and interfirst molar angles (Table 2). However, the mandibular second molars distal tipping was not enough to significantly increase the intersecond molar angles.
In the extraction group, the treatment changes caused significant mesial angulation of all the maxillary teeth (Table 3). This is most likely due to the extraction mechanics that may have allowed some mesialization of these teeth, through mesial tipping . Additionally, for the second molars, the eruption orientation may have also contributed to a certain amount, as explained for the non-extraction group.
In the mandibular arch, all posterior teeth were significantly distally angulated with treatment (Table 3). The same explanation given to the non-extraction group applies here. Additionally, it has to be emphasized that despite some tendency that the posterior teeth have to mesially tip during space closure , the mechanics with vertical elastics was able to overcome it and distally tip them.
The mandibular second premolar and molar distal tipping was able to overcome the maxillary second premolar and molar mesial tipping and significantly increased the intersecond premolar and intersecond molar angles, respectively (Table 3). However, the mandibular first molars distal tipping was not enough to overcome the maxillary first molars mesial tipping and significantly increase the intermolar angles.
This study showed that posterior teeth uprighting is usually obtained when using continues archwires and anterior vertical elastics to close anterior open bite, especially in the mandibular arch. However, uprighting of the posterior maxillary teeth is more difficult, primarily in extraction cases. Therefore, mechanical procedures such as the multiloop edgewise archwire technique [13, 14] or mesially angulating the posterior teeth accessories with continuous archwires [15,16,17, 24] are recommended to increase the possibility of actually uprighting the posterior teeth. If, in fact, this mesial angulation increases treatment stability, this will be a contributing factor. Nevertheless, despite some clinical evidence that uprighting the posterior teeth will increase treatment stability, this has yet to be investigated.