## 4 Discussion  
The primary advantage of 3D printing is its ability to create almost any complex shape or geometric feature, which has allowed for its widespread application in dentistry. In orthodontics, 3D printing has been used to fabricate customized brackets [8; 9] and transparent aligners [12]; moreover, there has been a gradual increase in its use for fabricating orthodontic accessories, such as digital titanium, Herbst [13], and 3D-printed hyrax devices [14]. In this study, we further expanded the application of 3D printing in orthodontics and explored its benefits as a method that simplifies the treatment process, enhances the treatment outcomes, and reduces patient discomfort. TPA is commonly used in orthodontics because it has many functions relevant to orthodontic treatments [16; 17; 18; 19; 20; 21; 22; 23; 24; 25; 26; 27] _ENREF_19. Unfortunately, traditional TPA has some disadvantages and contraindications. Therefore, efforts have been made to use the advantages of 3D printing to refine TPA and overcome these shortcomings. It is well known that only reliable retention of appliances can ensure favorable expressions of orthodontic force. However, a traditional TPA may sometimes become detached from the teeth due to the limitations of its structure. Usually, traditional TPA is composed of metal bands and bars. The metal band is prefabricated rather than customized, which is not suitable for some situations, such as when the patient presents with a deep undercut, a large cusp of Carabelli as shown in the second case with a short crown. Sometimes, the band cannot fit the tooth surface very well, which may result in a big gap between the band and the tooth; in this instance, when bonding the TPA, the gap is filled with a large number of adhesives. In addition to this, the inner surface of the band is smooth. Altogether, these disadvantages can potentially compromise the final bonding effect. To solve these problems, a customized lingual plate with an undercut-modified adhesive surface (see Figure 2) was designed. Because the plate was personalized, it fitted the surface of the tooth very well regardless of the anomalous shape of the teeth, which ensured that only a thin layer of the adhesive was required between the teeth and plate. The undercut enhanced the connection between the adhesive and the plate. Therefore, the 3D-printed plate might result in a better bonding effect than a traditional metal band. It is worth noting that, the enhanced adhesive strength remains a subjective assumption; as such, extensive experiments on adhesive strength and clinical trials on appliance loosening are necessary to verify the bonding effect of 3D TPAs. In addition to potentially improving the reliability of bonding and retention, 3D TPA might also be healthier and more comfortable. To ensure good retention, traditional TPAs are sometimes inserted close to the gingival margin or deep into the gingival sulcus, which might be harmful to gingival health. In contrast, 3D TPA can be inserted at least 1.5 mm away from the gingiva, which might make it easier for clinicians to thoroughly remove excess adhesive when bonding the 3D TPA; it, therefore, represents a more hygienic alternative to traditional TPAs. Additionally, traditional TPAs encompass four surfaces, including the buccal surface, the lingual surface, and two proximal surfaces, which can lead to gaps between teeth after debonding. As such, patients might sometimes complain of food impactions after removing their traditional TPAs. In contrast, 3D TPA only attaches to the lingual surface and not the proximal surface, which might reduce the possibility of proximal gaps and food impactions. Overall, in theory, 3D TPA has the potential to reduce periodontal complications and enhance patient comfort. However, further clinical trials should be conducted, focusing on periodontal indices such as the gingival index and sulcus bleeding index, and on patient feelings.