To ensure objective comparability of the MB and Aligner subject groups, standardized measurements and data collection in both comparison groups were done identically. The presence of WSL was measured at two time points (Tab 5-8). The comparatively small size of the aligner sample should be mentioned as a limitation. Thus, the 840 teeth in the aligner sample comprise only just under one third of the teeth from the MB sample. The subject sample size of the group undergoing aligner therapy was similar to that of subject sample sizes in literature (Tab 3). In future research projects with aligner therapy, multicenter studies should be preferred in order to include more subjects.
The evaluation was performed with three images per tooth, these being, a right-lateral, left-lateral and front view photo. Identical exclusion criteria were used for both studies. Subjective evaluation criteria was not an influencing factor on the measured values in the aligner and MB study since the examination was performed by the same examiner during orthodontic treatment. The ratio between female and male subjects was largely balanced in both subject groups. The anterior teeth, canine teeth, and the premolars were included in the photo evaluation. The labial tooth surfaces proved to be the most accessible for photo diagnostics and visual examination when screening for WSL, and in turn made them the easiest to evaluate. Furthermore, only subjects with labial conventional MB appliances were included in this study. In addition to the evaluation of the labial surfaces, the vestibular tooth surfaces were also included in the study with aligner subjects. This was due to the investigation on the effect of attachment-fitted tooth surfaces on WSL.
Retrospective evaluation of digital photographs represents the methodological investigation in this study. Photographs required for photo diagnostics are usually taken before, during, and at the end of orthodontic therapy. This means that photo diagnostics provide easily accessible information that can be viewed at any time, and allow for re-evaluation of the pictured tooth surfaces. Since the evaluation is performed by an examiner and not by a computer program, the diagnostic procedure with photographs is not an objective examination. QLF is an alternative highly sensitive diagnostic approach, but is routinely impractical due to its long application time.
Plaque, discoloration and calculus cause fluorescence loss and are thus all complicating factors that can impair examination. Furthermore, the localization of the lesions, the incident light and the salivation and dehydration of the tooth surfaces are key factors that influence the evaluation. Each study represents higher results in diagnosing incisive WSL with the QLF method than the last. The study by Ifland and Heinrich-Weltzien used the QLF method to measure occlusal enamel lesions. This method displayed a high sensitivity to the detection of carious lesions, with values ranging from 84 to 91%.
The intensity of the white spot lesions that occurred in this study, were assessed using the Enamel Decalcification Index (EDI) according to Banks & Richmond (1994). The EDI evaluated the severity of WSL and organized them in grades of 0 to 3 (Fig. 1), while an assessment of the extent of WSL was performed separately. Thus, though WSL could be similar in grade, the extent could always vary. The extent index analyzes the extent of a lesion in 20% steps using a specially designed digital scaled graticule with concentric circles as an evaluation aid (Fig. 2). The EDI is not used often in empirical studies, which makes it difficult to compare results . However, the 1982 index by Gorelick et al.  was not used in this study because the assessment of vestibular tooth surfaces does not divide the vestibular tooth surfaces into four areas, and areas with bonded brackets or attachments are not taken into consideration. No differentiation is made regarding the area in which WSL occurs on the vestibular tooth surface, and the intensity of the WSL is assessed without considering the extent. The index of Gorelick et al. underwent modification by Artun and Brobakken in 1986 . In 1994, modification was made by Banks and Richmond  and further development to EDI with the more precise evaluation by dividing the vestibular area around the bracket into four areas. Fehr's (1961) index  assesses the surface texture of the tooth surfaces, as well as their opacity in order to evaluate the severity of lesions. Though this index was not used in this study to assess and divide WSL.
A total of 17% of subjects had at least one WSL prior to aligner therapy in this study. The prevalence of WSL before aligner therapy was much lower than for MB therapy. Before MB therapy, 69% of subjects were diagnosed with at least one WSL. This unexpectedly large discrepancy between the two groups makes it difficult to compare the incidence of new WSL, since more than half of the subjects in the MB group were already affected by a WSL before treatment begin. The probability that a subject with prior WSL will develop more WSL during treatment is much higher than that of a subject who begins treatment with no WSL . The large range of prevalence at time T0 for aligner therapy can be credited to aligner therapy being an aesthetic treatment method, and is not only used for functionally recommended treatments. It can be concluded that the subjects of aligner therapy pay more attention to the aesthetics and dental care of their teeth than the MB therapy subjects.
The increase in prevalence between T0 and T1 during MB therapy was 29%. After aligner therapy, a total of 52% of subjects had detectable WSL, which is a 35% increase. This increase is slightly greater than the prevalence increase between T0 and T1 during MB therapy. Pre-existing WSL prior to orthodontic therapy may indicate poorer oral hygiene in the subject, which in turn increases the likelihood that the subject will develop further WSL during orthodontic therapy. This is evident in the study, which concluded that subjects with poor oral hygiene had significantly more WSL at the start of treatment than subjects with good oral hygiene . This shows that dental hygiene before the start of orthodontic treatment, is also an influencing factor for the development of WSL. In view of this, Azeem et al  ensured that patients with existing WSL prior to the start of treatment were excluded from their study.
The study by Buschang et al. (2019)  also draws a comparison between aligner and MB therapy. Photographs are also used for evaluation, which in the study by Buschang et al. are evaluated by two investigators. There are 244 subjects in the aligner group and 206 subjects in the MB group. The mean age in the aligner subject group is 30.4±14 years and 29.2±11.5 years in the MB group. The incidence in the aligner proband group is 1.2% of patients and 3 new WSL occurred during the study period. The incidence is 26% of patients in the MB proband group, and 174 new WSL occurred. Thus, the study confirms the study by Buschang et al. and shows that subjects have a lower risk of developing WSL when treated with aligner therapy. Likewise, Buschang et al. also saw a higher incidence of WSL in the maxilla compared to the mandible. The prevalence of WSL in MB therapy has literature values ranging from 2–96% [5, 6], which can be explained by the varying amount of teeth examined in each study. Other variables were controlled, for the study included treatment duration, age at treatment initiation, and whether additional fluoride applications and standardized oral hygiene instructions were provided. The comparability of the studies is further hampered by different sized subject groups and the lack of control groups. In addition, most studies differ in terms of diagnosis and methodological evaluation.
After MB treatment, Lovrov et al  found a WSL prevalence of 26% by examining photographs. Similar results were obtained by Geiger et al  with 34% and Lucchese  with 40-43% WSL prevalence. Lucchese et al. examined subjects after 12 months and used newer composite materials to secure the brackets. The advanced technology and the use of different bonding materials may have resulted in a lower prevalence . Tufekci et al  reviewed subjects as early as six months into treatment and found a prevalence of 38%, and a prevalence of 46% at 12 months. Boersma et al. 2005  found prevalence’s of 97% after completion of MB treatment, where the subjects had one or more lesions. The reason for the higher prevalence may be due to the quantitative light-induced fluorescence method for detecting WSL. This is a more sensitive method than visual assessment. Furthermore, the study period of 24 months is longer than in the study of Lucchese et al. The results of Ogaard et al  showed similarly high results of 96%. These results are consistent with those of Mizrahi et al.  with 84%. Akin et al  showed a prevalence in their subject group of 65%. However, the study does not include information on whether regular follow-up appointments took place, the socioeconomic background and ethnicity of the subjects, or whether prophylactic fluoride therapy was concomitant. If explanations for the different prevalence values are sought, then the different study design and differences in the analytical procedure should be taken into account.
The selection of teeth that were chosen for examination were different in each study examined. Some studies investigated only the anterior teeth or only the anterior teeth in the maxilla . This study showed a higher prevalence and incidence for WSL in the maxilla with aligner therapy. Thus, studying the maxilla and mandible separately certainly has an impact on the results, compared to evaluating both jaws together. Of the subjects in this study, 43% of the adolescents acquired new WSL under aligner therapy, whereas only 29% of the adolescents in the MB therapy comparison group had new WSL. Since all subjects from both groups were from the patient population of the Department of Orthodontics of the University Medical Center Mainz, the percentages seem comparable at first glance and would seem to be able to assure a better outcome of aligner therapy in terms of WSL incidence. However, with regard to the relatively low number of subjects in the aligner group, it seems reasonable to compare the percentages with the incidence rates reported in the literature. Compared with the study by Azeem et al. , who found the occurrence of at least one WSL in only 28% of subjects after aligner therapy, the WSL incidence of 43% measured in this study is much higher. In this study, 99% of the tooth surfaces examined showed no WSL before the start of aligner therapy, while 91% of the tooth surfaces were still free of WSL after the completion of aligner therapy.
A comparison of the two studies yields the following figures for the recurrence of WSL in each case, shown as a percentage of the subjects and as a percentage of the tooth surfaces. The incidence in terms of tooth surfaces by Azeem et al  is 3%, and 8% in this study. This shows a relatively small difference in tooth surface incidence. The patients who had WSL for the first time after MB therapy represented 74% of their collective in Banks et al . Similar high results were obtained by Enaia et al.  with 61%, they examined only four anterior teeth, and Hadler-Olsen et al.  found an incidence 60%. Richter et al  showed an incidence of 73%. They also examined teeth 16 to 46, and Gorelick et al.  diagnosed WSL in 50% of cases, using the same index and comparing orthodontically treated patients with untreated controls. Akin et al  recorded an incidence of 55%. They studied 150 patients at two different time points. The incidence and prevalence results are comparable to those of Enaia  and Gorelick . The variance in WSL incidence under MB therapy could be attributed to the different methods used and lack of standardization of the studies. The studies do not uniformly address whether oral hygiene instructions and fluoride applications were suggested to the subjects. Similarly, the number of teeth examined, the age of the patients at the start of treatment, the duration of treatment, oral hygiene at the start of treatment, and whether the subjects used fluoridated drinking water all influence the available results. Richter et al  showed that the geographic and socioeconomic status of the subjects and whether the study was conducted in a private practice or in a dental clinic, could also contribute to the variance in incidence. The prevalence and incidence in the study on gender distribution in MB therapy showed significantly higher percentages in male adolescents. Before MB therapy, there was a higher rate of disease in the female subjects. One explanation for this observation was that the permanent teeth eruption occurs earlier in girls than in boys, meaning that permanent teeth have a longer residence time in the mouth of girls than in boys. This results in a longer exposure period in the oral cavity where demineralization can occur.
Compared to the results with aligner therapy, there was no significant difference in the gender distribution of WSL in both prevalence and incidence. Similarly, Oogard. et al showed no significant difference in gender distribution under MB therapy. This study showed significantly higher prevalence and incidence of WSL for maxillary teeth under aligner therapy (Tab. 4). Evaluation and comparison of the tooth surfaces affected by WSL in this study showed a greater burden of WSL in the premolars and canines compared to the anterior teeth in the University Medicine patient population (Tab.7). Boersma et al  found a lower prevalence of caries on the incisors and canines compared to the molars and premolars, supporting the findings of this study. This study showed that age at treatment initiation had no effect on prevalence and incidence in this study. Akin et al  showed a significant association between the age of patients at the start of treatment for MB therapy and the increased incidence of WSL. Richter et al  confirmed that 11- to 14-year-old patients under orthodontic therapy had a higher risk of WSL occurrence. Levine et al.  also associated prepubertal age (9-12 years) with a higher risk of WSL. This point may play a role in the decision around when to start treatment.
There are currently no known comparative studies that have examined the influence of different attachment types on the development of WSL. This study demonstrated that tooth surfaces with rectangular attachments were more likely to be affected by WSL compared to tooth surfaces without attachments. The shape and size of the attachment played a minor role. A possible explanation for this observation is the creation of plaque retention sites due to the applied attachments. This is consistently communicated in the literature .