The accuracy of prediction in the changes of the soft tissue chin area after orthodontic treatment using cephalogram is still poorly understood. The relationship between incisor movement and soft tissue change is still controversial. This may be because the soft tissue changes can be affected by not only incisor movement but also many other factors, such as dentofacial morphology, age, sex, ethnicity, soft tissue thickness and tension, and the technologies used for estimation [17, 18]. To minimize the influence of sex on soft tissue changes following the incisor movement, only female patients were included in this study.
Some studies have reported that the ratio between lip change and incisor retraction ranged from 1:0.45 to 1.25 for the upper lip, and from 1:1.2 to 1:6.2 for the lower lip in nongrowing patients with bimaxillary protrusion [19, 20]. In addition to the changes of lip position, the incisor retraction could also induce soft tissue thickness changes [19, 21]. In the current study, after incisor retraction (5.35 ± 1.79 mm and 4.42 ± 1.62 mm for the upper and lower, respectively), the soft tissue thickness of L1c-LL, B-B′ and Pog-Pog’ increased 0.64 ± 1.67 mm, − 1.21 ± 1.34 mm and 0.44 ± 1.10 mm, respectively. This may be due to the reduced tension and deformation of the muscles around the lower lip, such as orbicularis oris and mentalis . Another reason may be the thickness measured in the study, in fact, included the real thickness of the lower lip as well as the labial vestibule, which may also re-arrange after incisor retraction . The rotation of the mandible could also influence the tension of soft tissues around the chin area. However, no statistically significant change existed in SN-MP and Stimi-Me’ before and after treatments. Moreover, although ANB decreased 0.45 ± 1.11°after treatments, no correlation was found between the change of ANB and the change of soft tissue thicknesses. One possible explanation is that the change of ANB was clinically inconspicuous in our study.
As for the thickness change of the lower lip, Kuhn found that the lower lip thickness decreased about 2.5 mm in patients with extraction treatments . Some studies, on the other hand, found that the thickness of the lower lip increased . They attributed these lip thickness changes to the muscular tension and deformation of lips, as well [17, 24, 25]. Nevertheless, many studies found no significant change in the lower lip thickness after incisor retractions [22, 26]. The multiple regression analysis in our study showed that the thickness of L1c-LL was affected by the retraction of upper and lower incisors at the same time. Many scholars detected that the upper incisors had effects on the shape and position of the lower lip, probably because the lower lip often covers the upper incisor by a third .
The majority of studies found the thickness of B-B′ had decreased [17, 24, 27], which are consistent with our study. The Pearson coefficient showed a negative correlation between the soft tissue thickness change of B-B′ and the retraction of the lower incisor. The average change of soft tissue thickness of B-B′ was less than zero. Thus, the more the retraction, the larger the thickness reduction of B-B′ within limits. Unfortunately, the changes of soft tissue thickness around the chin area following incisor retraction are still inconclusive, especially for the change of Pog-Pog’ which was found to increase, decrease or stay the same after tooth extraction .
Most of the previous studies on soft tissue changes were focused on linear or/and angular measurements using conventional two-dimensional cephalograms; a few studies investigated the soft tissue area changes [8, 28]. The changes of soft tissue are complicated; therefore, we need various kinds of data to get a more consummate prediction. The thickness and volume changes of the soft tissue could give us a rounded analysis of a certain area that couldn’t be provided by the changes of merely linear or/and angular measurements. For example, Dai detected the buccal facial depth decreased in adult female patients undergoing extraction by using a three-dimensional structured light scanning system . To measure the volume changes, CBCT scanning is preferable. However, this is not quite applicable so far due to ethical and technique reasons. Thus, measurements of the area on 2D cephalogram remain a practical approach. In this study, besides the linear and angular measurements on the cephalography, incisor retractions, and soft-tissue thicknesses, the areas of soft tissue chin and lower lip were also measured using a digital planimeter . It was found that the muscle tension around the chin region decreased with the degree of maxillary incisor retraction, which might increase the area of the soft tissues around the chin . However, no significant change was found in the areas of the soft tissue chin and lower lip in the study. This may because the soft tissues around the chin region are relatively attached to the basal bone with less mobility. According to this finding, the areas of soft tissue chin and lower lip should be set as invariants in software programming. Based on this rule, a more accurate prediction could be made for the morphological changes of the soft tissue chin combined with the changes of other anatomical landmarks after incisor retractions. Also, if the area of soft tissue chin was found increased after treatments, we would highly suspect that filling material existed in the soft tissue chin.
Apart from the fact that the measurements in the study were two-dimensional and performed on lateral cephalometric radiographs, other limitations of the study may also restrict the generalization of the results, for example, samples were all young female adults and of Asian ethnicity. Studies in the future could include both men and women with a wide range of age and ethnicity, and consider using three-dimensional techniques, such as CT and stereophotogrammetry , in order to get a more accurate prediction of the soft tissue changes following different types of tooth movements.