Orthodontically Induced Inflammatory Root Resorption (OIIRR) remains one of the most significant iatrogenic consequences of orthodontic treatment, characterized by the complex morphological and chemical degradation of root cementum [1-3]. When this resorptive process penetrates the cementum and involves the underlying dentin, the reparative capacity of the tissue is significantly compromised. This imbalance between clastic activity and tissue repair often results in a permanent and clinically relevant loss of root structure [4, 5].

The etiology of external root resorption is multifactorial, involving a complex interplay of patient-related variables such as genetic predisposition, age, sex, and systemic health [4, 6, 7], as well as local factors including dental morphology, previous trauma [8, 9], and periodontal status [10]. Furthermore, occlusal factors and dentoalveolar morphology play a critical role in individual susceptibility [11, 12]. Beyond these biological traits, attention must be directed toward appliance-related variables, specifically the magnitude, duration, and direction of the applied orthodontic forces [5, 13, 14]. Studying these factors through contemporary evidence allows for a clearer understanding of how modern orthodontic appliances influence the tissues and biology of the supporting periodontium [1-3].

Self-ligating brackets represent a major advancement in contemporary orthodontics in terms of design, biomechanics, and clinical efficiency. These systems employ a clip or sliding mechanism to secure the archwire in place [15, 16]. Passive self-ligating brackets act as rigid tubes that retain the archwire, theoretically allowing tooth movement through light, physiologic forces [17]. In contrast, active self-ligating brackets use an elastic clip that exerts pressure on the archwire, storing and releasing energy during elastic deformation [16-20].

A total of 20 maxillary first premolars from 10 patients (7 females and 3 males; mean chronological age: 15.4 years) requiring bilateral extractions as part of their orthodontic treatment were included in this study. The unequal distribution between female and male participants resulted from the strict inclusion criteria and the limited number of patients who met the requirement for bilateral premolar extraction during the recruitment period. Because the study followed a split-mouth design, each patient simultaneously received both bracket systems and therefore served as their own biological control, minimizing potential confounding effects related to sex. Current evidence indicates that sex-related variations in short-term Orthodontically Induced Inflammatory Root Resorption (OIIRR) are generally small and inconsistent, and thus unlikely to influence the primary outcome of this study.

This prospective observational split-mouth study included 10 patients. The sample size was determined based on previous studies evaluating root resorption associated with self-ligating brackets, aiming to detect a clinically significant difference between groups with a statistical power of 80% at a significance level of 5%. The number of patients was considered adequate to address the primary outcome of the study. The study was conducted between 2011 and 2016.

Ethical approval was granted by the Institutional Health Research Ethics Committee (CIEIS) of the School of Dentistry, National University of Córdoba, and authorization was obtained from the Council of Ethical Evaluation of Research (CoEIS), Ministry of Health of Córdoba Province, Argentina (Approval Number: V.1.2011). All patients and their parents/guardians received both verbal and written explanations of the study and provided informed consent prior to participation. All procedures performed in this study involving human participants complied with the ethical standards of the institutional and/or national research committee and with the 1964 Helsinki Declaration and its later amendments or comparable ethical standards.

From the original 284 patients screened, 10 met the inclusion criteria. A total of 20 extracted premolars (10 right and 10 left) were analyzed (Fig. 4).

In total, 85 resorption sites were identified in the active bracket group and 83 sites in the passive bracket group.

Comparative analysis revealed no statistically significant differences between active and passive self-ligating systems in total external root resorption for surface area (all p > 0.05; Fig. 5; Tables 1 and 2), volume (all p > 0.05; Fig. 6; Tables 3 and 4), or overall depth (all p > 0.05; Fig. 7; Tables 5 and 6).

A statistically significant difference was observed only in resorption depth within the middle root third, where the passive bracket group showed slightly greater resorption depth (P < 0.05).

In both groups, resorption extended beyond the cementum and involved the dentin.

In the active bracket group, 20% of resorption sites did not extend into the dentin, compared with 16% in the passive bracket group.

The present study evaluated early Orthodontically Induced Inflammatory Root Resorption (OIIRR) in maxillary first premolars subjected simultaneously to active and passive self-ligating brackets. Quantitative analysis demonstrated that both systems produced comparable levels of cementum and dentin resorption, with no significant differences in crater surface area, volume, or depth. These findings reinforce the growing evidence that bracket clip design alone does not substantially modify the early biological response of the periodontal ligament when comparable mechanical conditions are applied.

Consistent with our results, previous CBCT studies have reported no significant differences in OIIRR between self-ligating and conventional systems [21, 22]. More recently, Villaman-Santacruz et al. reported that variations in root resorption during dental movement were more strongly associated with the type of mechanical activation than with bracket design [23]. At the same time, the CBCT-based analysis of Lin et al. emphasized patient-specific biological factors as the main determinants of OIIRR risk [24]. Collectively, these findings align closely with the outcomes of the present investigation.

A major strength of this study lies in its intra-individual split-mouth randomized design, which inherently controls for inter-patient variability in root morphology, bone turnover, genetic susceptibility, and periodontal condition. This design enhances internal validity and allows a more accurate assessment of the mechanical differences between bracket systems. Regarding imaging methodology, conventional radiographs and CBCT studies [21, 22, 24, 25] often lack the spatial resolution required to detect shallow cementum-level defects. While scanning electron microscopy provides excellent surface detail [26, 27], it does not always allow for standardized volumetric quantification. In this study, high-resolution 3D confocal laser scanning microscopy was employed, offering three-dimensional accuracy comparable to modern micro-CT investigations [28-32] and enabling the reliable quantification of surface area, volume, and lesion depth.

In agreement with recent meta-analyses and clinical trials [16, 17, 18], although passive self-ligating brackets are designed for low-friction force delivery, the resulting biological response often remains equivalent to that of active systems. Our findings confirm the multifactorial nature of OIIRR, where factors such as force duration, individual molecular susceptibility, and complex hyalinization patterns play a more decisive role than the specific bracket clip design [1-3, 5]. The minor variations observed in the middle third of the root in our study likely reflect individual biological variability rather than a definitive mechanical disadvantage of either system.

From a clinical perspective, the fact that most resorption lacunae in both groups extended into the dentin is highly significant, as reparative potential at this depth is limited [25, 27]. This underscores the necessity for rigorous biomechanical control during the initial stages of alignment, irrespective of the bracket system used. Furthermore, contemporary evidence suggests that clinicians should implement personalized monitoring protocols, especially in patients identified with morphological or genetic risk factors [6-8, 24].

The study also offers methodological strengths, particularly the split-mouth randomized design, which effectively minimizes inter-patient biological variability, and the use of high-resolution confocal microscopy, which enhances the precision and reproducibility of quantitative measurements compared with conventional radiographic tools. However, certain limitations should be acknowledged. The sample size was constrained by strict inclusion criteria requiring bilateral premolar extractions, and the in vitro conditions do not replicate dynamic in vivo responses. Furthermore, the standardized observation period may not reflect long-term remodeling behavior. Larger clinical studies with extended follow-up intervals are warranted to validate and expand upon these findings.

While this study provides valuable quantitative insights into Orthodontically Induced Inflammatory Root Resorption (OIIRR) associated with self-ligating systems, certain limitations should be considered. The sample size was moderate and derived from a single institution, which may limit the extrapolation of results to broader populations. Nonetheless, the split-mouth design minimized inter-individual variability and strengthened internal validity. The use of an in vitro confocal microscopy model, although not replicating the full biological environment, allowed for highly precise and reproducible 3D evaluation of resorption craters. In addition, the observation period was intentionally standardized to control for the influence of treatment duration, though longer follow-up studies would further clarify cumulative biological effects. Future investigations, including larger and more diverse samples, as well as in vivo longitudinal analyses, are encouraged to expand upon the present findings.

The extent of orthodontically induced root resorption does not depend on the type of self-ligating bracket (active or passive). All extracted premolars exhibited resorption extending into the dentin, indicating irreversible cementum–dentin loss.

To minimize iatrogenic effects, orthodontic forces should always be maintained at the lowest biologically acceptable levels, consistent with tissue physiology and principles of tooth preservation.