## 5 Limitations and Future Directions  
The selected studies exhibited considerable methodological heterogeneity. This variability limits the comparability of the findings and restricts the generalizability of the results across different populations and clinical settings. Variations in study design, including cross-sectional versus longitudinal approaches, differences in sample sizes, and the timing of outcome assessments, may contribute to inconsistent results. For instance, differences in the use or calibration of measurement tools (such as variations in the VAS scale for pain perception and in the scales used to assess treatment attitudes) likely contributed to inconsistent results. Additionally, cultural and socioeconomic differences across study samples may have influenced how personality traits were expressed and how patients responded to orthodontic treatment, increasing heterogeneity. This can explain why certain studies did not find significant associations between personality dimensions and outcomes such as patient cooperation (e.g., [PERSON] et al. [61]) or treatment attitudes (e.g., [PERSON] et al. [57]), whereas other investigations reported clear links between these variables. Moreover, the lack of standardized assessments across studies further complicates direct comparisons. Some investigations that reported null findings may have been limited by low statistical power or may have measured outcomes in less sensitive ways to the subtle influences of personality. These discrepancies underscore the need for future research that employs more consistent methodologies, larger and more culturally diverse samples, and standardized instruments. Moreover, it is important to note that the variability in sample characteristics, including age ranges, could partially account for the heterogeneity observed in the associations between personality traits and treatment-related outcomes such as compliance, pain perception, and satisfaction. This suggests that age might be a potential moderating factor worth exploring in future research. Not all studies systematically measured or controlled for potential confounding variables, such as gender. Although some studies reported gender differences, these were not the review's primary focus and were not consistently assessed using a standardized framework. This gap suggests that future research should explicitly integrate these factors into study designs to understand better their moderating effects on the relationship between personality traits and orthodontic outcomes. Another important limitation is the reliance on self-reported measures for treatment outcomes that may also introduce reporting bias. Future research should prioritize longitudinal studies with standardized methodologies that allow for more robust causality analyses. The current literature indicates that patient cooperation is closely related to the type of orthodontic appliance used. Traditional fixed appliances and removable devices, such as clear aligners, each influence treatment compliance and quality of life in distinct ways. In recent years, especially among adults, clear aligners have gained popularity due to their aesthetic and comfort advantages, which have led to improved patient engagement and adherence. However, the studies included have primarily focused on fixed appliances and conventional removable devices, offering limited insight into how different orthodontic techniques specifically affect quality-of-life outcomes. A more detailed examination of various orthodontic methods, particularly the rising use of aligners, could elucidate the benefits and challenges of each modality, enabling clinicians to tailor treatment plans to enhance patient satisfaction and compliance. Finally, the GRADE evaluation highlighted a very low certainty of evidence for all outcomes analyzed (pain perception, attitude toward treatment, compliance, satisfaction, and OHRQoL). Downgrades were mainly due to serious issues with imprecision, inconsistency, and potential residual confounding across studies.