Modern orthodontic treatment is progressing as a paradigm of soft tissue-based treatment in many areas1. Holdaway published a baseline called the harmony line (H-line) and presented a standard for a harmonious appearance4. Subsequently, in a paper published in 1999, Arnett emphasized the importance of the TVL on the basis of a natural head position and announced the ideal soft tissue position using a method called soft tissue cephalometric diagnosis2.
Camouflage treatment of skeletal Class III malocclusion is the most suitable and realistic treatment method for patients who want nonsurgical treatment15. The traditional method for Class III malocclusion is to extract the mandibular premolars and then retract the mandibular incisors, which has a risk of root exposure or root resorption due to excessive retroclination7–9. A systematic study of soft tissue change after extracted reported that lower lip retraction amounted to 2 mm to 4.5 mm based on “LL to Eline” in adults patients16. A class III research in patients with growing class III patients with lower second premolars extraction indicated that -2.8mm lower incisors retracted resulted in 0.9mm lower lip retraction17. After the development of orthodontic TADs, posterior movement of the mandibular anterior teeth has been widely used as an alternative treatment to mandibular extraction for patients with Class III malocclusion10–13. Obtaining an esthetic look with compensation treatment that is as good as expected with orthognathic surgery may be difficult, but patient satisfaction should be higher if soft tissue changes can be maximized18. It is difficult, however, to predict the position before and after treatment because the soft tissues of the face, including the lips, are viscoelastic19.
This study aimed to determine the factors affecting the ratio of the lower labial protrusion and the lower lip change with compensation treatment of Class III malocclusions and to quantify the expected retraction amount of the lower lip. The CKA, CKH angle, TVL lower lip, TVL soft tissue B, B’ to Pog’, and LLA to Pog’ showed statistically significant differences when the measured values before and after the treatment of patients who received the Class III malocclusion compensation treatment were compared (Table 1). The rearward movement of the mandibular anterior incisor, which is currently mainly used for the compensation treatment of Class III malocclusion, was strongly correlated with the amount of lower lip retraction, CKA (Table 2).
An additional regression analysis was performed on the CKA, which is statistically correlated with L1 to NB, indicating the position of the mandibular incisors. An interpretation of the result suggested that the lower lip position, which changes according to the movement of the mandibular incisors, can be quantitatively predicted through the CKA. In other words, when the mandibular anterior teeth move 1 mm posteriorly, the lower lip can retrovert about 79% compared to the facial plane (N’-Pog’). This can be expected even with posterior movement of the mandibular incisors through non-extraction, suggesting that the mandibular full arch disatalization treatment will be highly effective in soft tissue changes.
In multiple regression analysis, ΔCKA = 0.5(ΔL1 to N B) − 0.42(LLtoE) −1.48. was formulated. The result explained that the more protrusive lip retracted further while the lower incisors moved backward, and the retraction ratio was 42%. When the initial lower lip protrusion became a constant, lower incisor 1 mm retraction resulted in 0.5°retraction of the lower lip (CKA). Because the initial lower incisor position affected the initial lower lip position, the effect of lower incisor movement decreased when the lower lip became a constant.
A limitation of this study is that the study was conducted only on patients who had undergone treatment for posterior movement of the anterior teeth of the mandible; that is, non-extraction treatment. Although 3.7 mm retraction on average of the lower incisors presented significant lower lip retraction (2.1mm), which was comparable to reported amount with extraction16,17, a comparison study with extraction or mandibular ASO will provide further insight into the differences and limitations between extraction and nonextraction treatments.