Considering the study findings, the null hypothesis could not be confirmed. NCCLs increased only 8.2% at the end of the orthodontic treatment. Premolars were the most affected teeth and the age factor influenced the prevalence of NCCLs (Fig. 1).
In recent decades, an increase in the number of adults seeking correction of malocclusion has been reported and is attributed to the growing esthetic demand in the society, comfort of orthodontic appliances, and greater access to information and oral health care by the population .
Concurrently, several hypotheses have emerged in the literature to explain the etiology of NCCLs, with a consensus of it being multifactorial and involving the association of friction, corrosion, and occlusal stress factors [4, 5, 12, 19].
One of these hypotheses also states that malocclusion and occlusal trauma/interferences, which possibly increase the stress concentration in the cervical region, contribute to the weakening of the dental structures along with other etiological factors [4, 9,10,11, 20,21,22]. The increased stress in this region of the dental crown would vary in intensity depending on the presence of different factors, such as age, type and severity of malocclusion, craniofacial pattern, type of orthodontic movement, treatment duration, root length, amount of bone loss, and orthodontic treatment .
In the present study, the prevalence of NCCLs increased from 22.71% to 30.91% after orthodontic treatment. NCCLs were equally distributed in both the upper and lower dental arches, and the most susceptible teeth were the premolars, followed by the first molars, canines, and incisors (Fig. 1). The premolars have lesser crown volume and a considerably thinner buccal bone plate compared to other teeth. Furthermore, they are subjected to excessive non-axial loads during mandibular excursive movements. These factors may lead to higher flexion of teeth and increased stress concentration in the cervical region, thus explaining the increased prevalence of NCCLs (Fig. 1) [22,23,24]. Furthermore, these findings are consistent with those of previous studies [23, 25,26,27].
Few variables, such as sex, the type of malocclusion, and especially the craniofacial pattern were expected to influence the prevalence of NCCLs. Different occlusal disorders combined with distinct craniofacial patterns, were expected to generate stress in the cervical vestibular regions of the teeth, and were capable of generating new NCCLs. However, none of these variables were related to the emergence of new NCCLs during orthodontic treatment (Table 2).
In contrast, one of the patient-related characteristics that influenced the prevalence of NCCLs was age. Participants aged 21–35 years and > 35 years were 1.14 and 1.19 times more likely to have NCCLs than those aged < 20, respectively (Table 2). This increase in the prevalence of NCCLs may be due to the extended exposure to etiological factors of NCCLs among older adults and makes them more susceptible to the development of NCCLs during orthodontic treatment .
Regarding the characteristics of orthodontic treatment, patients who underwent 6–18 activation sessions showed no statistically significant difference in the prevalence ratio of NCCLs compared to those who underwent 19–36 or > 36 activation sessions (Table 2). According to few studies , the number of orthodontic activations depends on the patient compliance, missed appointments and problems/breakage of devices, inadequate oral hygiene, initial malocclusion severity, and age of the individual, thereby significantly affecting the treatment time. As the individual ages, their metabolism tends to slow down. Consequently, the same orthodontic procedure will probably require greater activation and longer time to achieve the desired results, thus prolonging the duration of the treatment . However, despite the longer duration of treatment resulting in extended duration of orthodontic forces acting on the teeth and a more significant number of activations, this variable did not influence the prevalence of NCCLs.
Similarly, individuals who underwent compensatory orthodontic treatment were unlikely to have an increase in the prevalence of NCCLs. However, this result contradicted expectations because changes in the positioning of a group of teeth to compensate for an existing anteroposterior, vertical, and/or transverse skeletal discrepancy modified the axial inclinations of the teeth involved. Changes in the axial inclinations of these teeth could modify the direction of the masticatory forces that act on them, which can generate an increase in the concentration of tension in the cervical region, favoring the development of NCCLs . However, in this study, this finding was not elicited.
In specific situations, such as incorrect diagnoses and planning, poor quality results, muscle imbalance, unfavorable residual craniofacial growth, genetic factors, or inadequate retention protocols, orthodontic retreatment may be necessary . In this study, patients who underwent orthodontic retreatment were no longer susceptible to NCCLs. Although they were subjected to new force applications and respective tooth movements at a slightly older age, these interventions were not sufficient to impact a statistically significant increase in the prevalence of NCCLs.
The lack of statistically significant differences between the prevalence of NCCLs and the number of activations, compensatory treatment, and orthodontic retreatment should not be seen as a total lack of participation of these variables in the development of NCCLs. Their association was investigated only with respect to the ability of each variable in this study to trigger new NCCLs and not their capacity to promote dimensional increases in the pre-existing NCCLs.
This study aimed to analyze patients exclusively by an experienced professional with more than 30 years of experience. The idea of a single professional was adopted to eliminate biases arising from different techniques or levels of training and experience. A limitation of this study was the lack of a control group. Moreover, this was a retrospective study with limited control over the collection of sample variables, which may affect the possible inferences of the association between the etiological factors for NCCL and orthodontic treatment. Therefore, we suggest that further cohort and case–control studies are needed on this topic using more precise methodologies that assess the presence or absence of NCCLs and the possible dimensional changes based on potential etiological factors.