## 1 Introduction  
The painful and uncomfortable experiences described by orthodontic patients are common side effects related to dental therapy involving either fixed multibrackets braces or plastic aligners. The maximum peak of pain described by patients was observed within the first 24 h after starting treatment and gradually decreased from the second day and onward [1]. Pain is a subjective experience that is influenced by a patient's age, sex, or emotional factors. It has a negative effect on patient oral hygiene and compliance that may be indicated during orthodontic treatment. At times, specialists have underestimated the level of pain perceived by patients during treatment [2].  
Published studies on pain during orthodontic treatment have concluded that the majority of patients, between 91% and 95%, have described significant discomfort and pain during orthodontic treatment, regardless of the techniques used [3,4].  
For tooth movement to occur during orthodontic treatment, it is essential that the periodontal tissue and the alveolar bone tissue be remodeled. When a gentle force is applied over a period of time, inflammation of the periodontal tissue occurs, which then leads to bone resorption. During bone resorption, which is essential for tooth movement, various biochemical mediators such as prostaglandin E2, tumor necrosis factor alpha, and epidermal growth factor are released [5, 6, 7]. The forces applied during the first phases of orthodontic treatment produce an increase in various biochemical mediators. This increase in the concentration of biochemical factors reaches its maximum value in the first twenty-four h after the start of orthodontic treatment, and from this initial twenty-four h onwards, the levels decrease [8]. The intensity of the applied force is related to the induced pain. The interleukin-1 beta levels that increase within the first twenty-four hours after the start of orthodontic treatment were correlated with increased pain intensity [9].  
Among the scientific literature, studies evaluating pain during orthodontic treatment have previously been published. In orthodontic treatments using fixed multibrackets, we observed that patients treated with a conventional bracket system described a higher level of pain compared to patients treated with self-ligating brackets [10, 11, 12]. However, there are other authors who have concluded that there were no statistically significant differences between patients' pain scores when comparing both bracket systems [13, 14, 15].  
In recent years, orthodontic treatments with transparent aligners such as Invisalign (Align Technology, Santa Clara, CA, USA) have become one of the most demanded treatment options by patients as a therapeutic alternative to fixed-bracket treatments. The main advantages of aligner treatments, compared to fixed brackets, are superior aesthetics, improved oral hygiene, and greater comfort [16, 17, 18, 19].  
A systematic review by [PERSON] et al. found that patients undergoing treatment with aligners (e.g., Invisalign) described a lower level of pain, especially during the first phases of treatment than those with fixed-multibracket appliances. In more advanced stages, however, no statistically significant differences in pain levels were observed between the two treatment techniques [20].  
A study by [PERSON] et al. in 2017 concluded that patients undergoing orthodontic treatment with aligners consume fewer analgesic drugs during treatment compared to patients undergoing treatment with fixed multibrackets [16].  
The objective of this longitudinal, clinical study was to evaluate the levels of periodontal pain in patients treated with two different orthodontic techniques, fixed-multibracket appliances, and removable aligners during the first 12 months of treatment. The null hypothesis of this study was that there were no statistically significant differences in pain levels as described by orthodontic patients undergoing treatment with fixed multibrackets and aligners.