This study is the first to evaluate the quality of the occlusal outcome with total arch distalization using ABO MGS scores. Previous studies of mini-screw-supported maxillary distalization have primarily used cephalograms or digital 3D casts to evaluate the amount of distalization in the maxillary first molar region, along with any first molar tipping [21, 22]. Numerous papers on mini-screw-supported maxillary distalization include in their analysis the first treatment stage only, meaning the distalization of the upper molars [3, 4, 23,24,25,26,27,28,29,30,31,32,33]. The outcome of the subsequent retraction of the anterior segment that such an approach requires to achieve a Class I canine relationship, was not included in many cases. Beyling et al. were the first to describe the MTAD results in relation to the lower jaw based on the canine relationship and overjet corrections and to compare them to the intended outcome as defined by the individual target set-up [10]. In the present study, the dentoalveolar correction of the Class II from the upper jaw was also achieved by using a CCLA combined with interradicular MSs according to the method described by Beyling et al. [10]. For more in-depth assessment of the treatment outcome, the Class II patients were compared to a Class I control group matched for age and gender. The null hypothesis was rejected: There was no significant difference in the quality of the treatment outcome between a group of patients with a Class II malocclusion treated with MSs for maxillary total arch distalization and a Class I group.

The average sagittal correction achieved in the area of the first molars amounted to 4.5 mm, representing an outcome of 99% of what had been intended (T2A). Earlier studies have shown that when lingual appliances are used in patients exhibiting Class II malocclusion, the clockwise rotation of the lower jaw immediately after indirect bonding will result in a sagittal relationship that is worse by 1 mm on average (Figs. 2 and 3) [10, 34, 35]. Considering the intended overcorrection of 1 mm, an average total correction of more than 6 mm can be assumed at the end of the MTAD phase. In this regard, the success of the sagittal correction has been demonstrated not to depend on the initial severity of the class II relationship (Fig. 5). This emphasises the effectiveness and efficiency of the selected method compared to alternative MTAD approaches [23, 24]. One essential precondition for successful sagittal correction is the comprehensive levelling of the lower curve of Spee and the overbite correction associated. There is ample evidence that CCLAs are very effective for this purpose [10, 34,35,36]. No statistically significant difference was found at the end of treatment (T2B) between both groups, while the bite raising (T2A) in the Class II group even exceeded what had been intended by 0.2 mm on average (Table 6). The mean correction of the overbite in Group 1 was 0.89 mm. This is because both open and deep bites were included. Nevertheless, the overbite could be fully corrected as planned in the set-up. The fact that there was a mathematical overcorrection of the overbite 113% in Group 1 is somewhat diminished clinically, as open and deep bites were included, and the range was large.

With a comparable approach and method, Patterson et al. and Leavitt described the outcome of aligner treatment in adult patients with Class II malocclusion and compared the post-treatment results to a matched group of Class I patients [11, 12]. As opposed to the outcomes in this study, the adequate outcome represented by the Class I patients could not be achieved in the preselected Class II patients despite good compliance in using intermaxillary elastics, neither with the first set of aligners nor after more than 3.5 refinements on average [11, 12]. After the first set of aligners, not only unsuccessful sagittal correction was found, but both groups also had significantly worse scores for occlusal contacts (> 10 penalty points) [11]. The evaluation of the same component in this study, on the contrary, yielded a particularly good result with less than 1.5 penalty points in both groups at T2B.

Looking at the MTAD per se, the most astounding aspect is the simplicity of the distalization mechanics placed in this study and its convincing efficiency. Interradicular insertion allows direct use of the anchorage and eliminates the need for a supra-construction. The screw loss rate in this study of less than 3% is also a consequence of the operator learning curve already described by Berens et al. for the insertion of the interradicular MSs in the relevant areas [37]. These last two points add up to a practical and pragmatic approach.

The retrospective nature of this study is the result of its innovative approach to MTAD. Strict inclusion and exclusion criteria helped to minimize the risk of bias. No patient was excluded from this retrospective analysis for any reason other than the defined exclusion criteria, i.e., no exclusion due to missed appointments, lack of compliance, or missing records, as is occasionally seen in sample compositions of retrospective studies.

The evaluation of the quality of the occlusal outcome after orthodontic treatment using the criteria by the American Board of Orthodontics is one of the few methods, along with the PAR index, for assessing the occlusion after orthodontic treatment as objectively as possible. Despite individual rater calibration, a subjective component in the evaluation, albeit small, cannot be ruled out. In previous studies on the quality of the occlusion after CCLA treatment, different raters, who had also been calibrated, found above-average outcomes [19, 34, 38, 39]. The results of this study confirm that the treatment standard is above average.

In both Groups a statistically significant difference in total ABO scores between T2A and T2B became obvious. As previously defined, T2A represents the patient specific ideal occlusion defined by the individual target set-up. Theoretically, an ideal appliance would correct the occlusion comparable to a mathematical “function of a limit” more and more towards the individual ideal situation, making the differences between both of them smaller and smaller over time. The high quality of the final occlusions achieved in this investigation underlines the thoughtful definition of the endpoint of active lingual fixed appliance therapy despite a statistically significant difference in the scores at T2A and T2B.

Many previous studies into MTAD assessed the outcome quality based exclusively on the amount of distalization achieved in the maxilla, not considering the posttreatment occlusion [21, 22]. They relied on the analysis of superimposed lateral headfilms and/or digital casts of the upper jaw. The question this raises, as to the occlusal quality of the treatment outcome achievable by a distalization of this kind, could be answered in this study.

The study was conducted in a single orthodontic practice in Bad Essen, Germany. The results may therefore not be fully generalizable to other orthodontic settings. In addition, all mini-screws were placed by a single experienced operator (FB), which could affect the reproducibility of results.