## 4 Discussion
Moreover, the application of the 3D printing technique might reduce clinicians' and technicians' workloads because the workflow of 3D TPA is simpler than that of traditional TPA. In summary, to fabricate a traditional TPA, the teeth are routinely separated, and the bands are tied in, then an impression is made. Before pouring casts, it is necessary to check whether the bands were correctly localized in the impression. Sometimes, the bands are very tight on the teeth but not in the impression; in these cases, the band needs to be repositioned in the impression, which might compromise accuracy. In contrast, the process of acquiring digital models to design and fabricate 3D TPAs might be simpler than using-plaster casts. During the design and manufacture process, traditional TPAs require a bar to be manually bent; then, the bars are positioned manually and soldered to the bands. It has been reported that there is a great likelihood of inaccurate placement and soldering defects when this technique is used [13]. In contrast, the plates and the bars of 3D TPAs are designed virtually by technicians using computers; then, the digital TPAs are sent to the printers for manufacture. The 3D TPAs might provide better stability than traditional TPAs because of their exact fit, discrepancies between the finished product and the virtual design are in the order of 10-30 microns, and no soldering defects would appear during the printing phase. In addition, 3D TPA has some advantages over traditional TPA during the bonding phase. When bonding traditional TPA, it might be difficult to precisely locate the band to ensure the correct positioning of the buccal tube. Sometimes, an auxiliary appliance is necessary to press the band in place; this pressure might distort the band, further compromising the accuracy of the buccal tube. In contrast, the process of bonding 3D TPAs is easier and more labor-saving and time-saving. In brief, the lingual surface is etched according to the designed bonding area, then, the TPA is bonded with light-cure adhesive. Moreover, the adhesive plates are attached to the lingual side of the teeth and do not implicate the buccal side. Therefore, it is easier to precisely locate the buccal tube or the brackets on the buccal side, and their positions are not affected by the 3D TPA, as shown in cases 3-6. Although the manufacturing and clinical application of 3D TPAs seem to require fewer steps than that of traditional TPAs, further investigations should be conducted to examine technicians' and doctors' responses to this technique. The greatest advantage of 3D TPA might be that the design of appliances can be adapted according to the clinician's preferences. The CAD technology allows the clinicians' ideas to be precisely specified during the design phase, taking into account factors such as the number of teeth included, the distance from the bar to the palatal mucosa, the need for affiliated hooks, and so on. Moreover, the treatment order is very flexible. The matter of when 3D TPAs should be worn depends on the phase of treatment and the preferences of doctors. For example, if the target teeth were irregular and not suitable for TPA bonding, the buccal tube or bracket could first be bonded to align the teeth. After the target teeth were aligned, the digital model can be acquired to manufacture the 3D TPA. It is worth mentioning that, in order to expand the application of 3D TPAs, more highly standardized and efficient designing, manufacturing and application processes should be established. Since 3D TPA has advantages over traditional TPA in terms of bonding strength, patient health and comforts, workflow and design flexibility, the use of 3D TPA represents a viable alternative technique. At the same time, however, 3D TPA has some limitations that should not be ignored.