###### Abstract  
This retrospective real-world study aimed to evaluate the outcomes of clear aligner treatment (CAT) and patient demographics of patients seeking orthodontic re-treatment who had previously received orthodontic treatment in childhood or adolescence. The study was conducted using anonymized data from DrSmile (Berlin, Germany), a CAT provider in Europe. Data were collected from a total of 2080 patients in Germany (71.5% female, 28.5% male, ranging from 18-70 years, median 29.0 years), who completed their CAT between 1 June and 30 June 2022. Patients were divided into two groups: adult first-treatment patients (\(n=1587\)) and adult re-treatment patients (\(n=493\)). Results showed that re-treatment patients had a shorter treatment duration (\(p<0.05\)), required fewer interpersonal reductions (IPR) (\(p<0.001\)), and had fewer clinical visits (mean 2.96 versus 4.14 visits) than first-treatment patients (\(p<0.001\)). Data collected from adult orthodontic re-treatment and first-treatment patients indicated that the demographic characteristics of these two groups were very similar. No significant differences were found between re-treatment patients and first-treatment patients in terms of motives for orthodontic treatment (\(p=0.068\)), types of insurance (\(p=0.615\)), and treatment satisfaction (\(p=0.673\)). The study highlights the need for orthodontists and dentists to provide adequate information to patients about long-term retention and decision-making for orthodontic treatment, especially for re-treatment.  
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increases, many orthodontic patients are likely to have already undergone orthodontic treatment as adolescents and are commonly referred to as re-treatment patients [9].  
Lack of cooperation during the first orthodontic treatment leading to an unsatisfactory outcome, and increasing patient perception of dental aesthetics, are possible reasons for re-treatment [10; 11]. Patients with previous orthodontic treatment show greater dental awareness, strong motivation, and objective treatment needs, and are more likely to request orthodontic re-treatment [12; 13]. However, patients with a history of orthodontic treatment have been shown to be less likely to adhere to orthodontic wear protocols than first-time patients [14].  
There is limited data available on adult orthodontic re-treatment, particularly on what motivates patients to seek re-treatment, how previous orthodontic treatment and retention protocols affect CAT outcomes, and how orthodontic re-treatment differs from first treatment in terms of required interproximal reduction (IPR), number of clinical visits, and treatment duration. This lack of knowledge about the differences between re-treatment and first treatment could lead to inadequate patient care and decision-making for orthodontic treatment.  
This retrospective cohort study aimed to evaluate the outcomes of clear aligner treatment (CAT) and patient demographics of patients seeking orthodontic re-treatment who had previously received orthodontic treatment in childhood or adolescence. The primary outcomes of this study included a comparison of treatment procedures and associated socio-demographic or other covariates of first-treatment, and re-treatment, patients. In addition, this study examined how previous orthodontic treatment and retention protocols affect CAT treatment duration, including treatment progression in terms of IPR, patient satisfaction, and clinical visits. By using real-world data and a retrospective study design, this study aimed to provide insight into real-world scenarios and may be able to provide short-term, large-sample results and support clinical decision-making.