High reliability relies on repeated measurements by the same or different examiners yielding consistent results [18]. In this study, we were concerned with the consistency of different orthodontists in IPO devising. As seen from the result of inter-examiner reliability, the senior and junior orthodontists were more likely to achieve an agreement on the lower incisor extrusion/intrusion movement. However, there was more variation in the devising of other types of incisor movement, which might be influenced by the orthodontists’ clinical experience.

Interestingly, during each time’s IPO devising, the senior orthodontists had high agreement and low mean absolute differences in upper incisor position objectives devising. It seemed that before IPO devising began, the senior orthodontists had a unified concept (ICC*: p < 0.05, which is defined as “there are no significant difference between the incisor positions made by orthodontists in the same group”, in other words, their concept and the results of IPO have kept step) in the upper incisor position objective devising. After the first IPO devising, this concept underwent some changes but remained unified. However, such a phenomenon did not appear in the junior orthodontists. They neither formed a unified concept before the first IPO devising nor after the first IPO devising. It can be inferred that with an increase in clinical experience, orthodontists were more likely to form a unified concept when devising the upper incisor position objectives, and this concept may change with the practice of IPO devising.

Although the study only included lip-protrusive patients, it is the most typical cases requiring IPO devising. As for clinical practice, orthodontists should combine other diagnostic materials, such as CBCT, panorama radiograph and study casts, to devise IPO more comprehensively [19, 20]. We suggest that the IPO devising method proposed in the present study be used as an auxiliary tool in orthodontic treatment planning, especially in digital teeth arrangement for clear aligner, customized labial and lingual orthodontic appliance treatment. The technician from the company can take the data of IPO, which devised and submitted by the orthodontists, as reference for incisors and other teeth arrangement. It is expected to improve the accuracy and efficiency of digital teeth arrangement.

The significance of this article, in addition to comparing the design reliability of the incisor target position of doctors in two ages, could also be concluded as follows. First, this model can be further promoted. This study can show that for young doctors, the superior doctors need to pay attention to the incisor position design and other problems during supervision. Of course, older doctors may prefer more adduction and more lingual inclination, which could provide valuable reference for increasing the accuracy of the IPO design. Second, it can be further studied whether there are differences in the aesthetics of doctors in the two ages, such as the era and environment of contact, education, etc., whether the aesthetics [21] formed in this way will be one of the factors affecting the design of IPO? References can be found on aesthetic among orthodontists, or similar age-related design factors [21]. It is equivalent to that our research can provide reference ideas for subsequent research, so the significance of this research can be imagined as a relatively important “key”. Third, this idea of testing inter- and intra-examiner reliability via a software was novel and convenient, for that deciding the IPO on a 2D model was more visualized. Herein, this research could also provide reference for the future IPO design among orthodontists of different genders, districts or with education backgrounds.

While the study focused on lip-protrusive patients, the IPO devising method can be a valuable addition to orthodontic treatment planning when combined with other diagnostic materials, and it has the potential to improve the precision and effectiveness of digital teeth arrangement.