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INTRODUCTION
Bimaxillary protrusion, which is prevalent in Asian and African populations, is frequently observed in orthodontic practice.1,2 The advent of temporary anchorage devices (TADs) has expanded the spectrum of orthodontic treatments by enhancing anchorage capabilities. Nevertheless, anterior segmental osteotomy (ASO) remains the most viable option for patients desiring short treatment duration.
A systematic analysis conducted by Jayaratne et al.2 reported an increase in the nasolabial angle (NLA) from 8.9° to 18.0° following ASO. Furthermore, the labial superius underwent posterior displacement within a span of 0.90–7.25 mm. The horizontal displacement of the maxillary incisors owing to anterior movement in the maxilla varied from 33% to 67%, whereas that of the mandibular incisors varied from 67% to 89% owing to anterior mandibular adjustments. As reported in various studies, the variability in soft tissue alterations following ASO could be attributed to ethnic variations within the studied populations.2,3 In a comparative analysis of ASO and corticotomy-assisted treatment with skeletal anchorage, Lee et al.4 observed that the group undergoing ASO exhibited greater posterior displacement of the upper incisors and more significant alterations in the NLA. Although extensive research has been conducted on soft tissue modifications following ASO, comparative studies between ASO surgical correction and nonsurgical tooth extraction orthodontic treatment (non-ASO [NASO]) are lacking. Treatments previously considered achievable only through surgical intervention, such as precise tooth movements, anterior-posterior and vertical adjustments of the anterior teeth, correction of asymmetries, and independent movement of impacted teeth, are now considered feasible with the use of TADs.5-7 However, the extent to which these technological advancements can replace traditional surgical approaches remains unclear. This study posits a null hypothesis that there is no difference in the soft tissue responses between ASO and NASO. Our study aims to provide guidelines for treatment selection accounting for differences in the soft tissue responses between the two methods.
This study aimed to evaluate soft tissue changes following ASO versus NASO using TADs with premolar extraction. By comparing the outcomes of treatments using ASO and NASO, this study aimed to facilitate the formulation of tailored treatment plans optimized for individual patient needs and decision-making between the two plans.
MATERIALS AND METHODS
Study sample
The study sample size was determined using the G*Power software (version 3.1; Heinrich-Heine University, Düsseldorf, Germany). This involved conducting two independent sample t tests, assuming a two-tailed distribution, effect size of 0.5 (indicative of a medium effect), significance level of 0.05, and power of 0.8. Under these conditions, each group required a minimum sample size of 26 participants for performing an independent t test. A minimum of 29 participants per group were required to account for a 10% dropout rate.
This retrospective study was conducted between September 1, 2016, and September 1, 2021; among 562 patients diagnosed with skeletal Class II malocclusion, 67 (ASO, 31; NASO, 36) aged 19–40 years who visited the Department of Orthodontics at Ajou University Dental Hospital were selected based on the study’s inclusion and exclusion criteria. Pre- and post-treatment lateral cephalometric radiographs were obtained and analyzed.
The inclusion criteria were as follows: 1) diagnosis of skeletal Class II malocclusion (ANB > 3°), 2) adults aged ≥ 18 years, 3) individuals who received orthodontic treatment, 4) patients with extracted maxillary and mandibular premolars accompanying the mandibular ASO in the ASO group, and 5) patients seeking treatment for lip protrusion. The exclusion criteria were: 1) congenital dental anomalies; 2) congenital conditions, including craniosynostosis and cleft palate; 3) systemic diseases affecting bone metabolism; 4) severe facial asymmetry (chin displacement < 4 mm relative to the facial midline); and 5) discontinuation of orthodontic treatment.
Demographic data, including age, sex, extraction patterns, and initial measurements for the ASO and NASO groups are shown in Supplementary Table 1.
All the human research procedures were conducted in accordance with the ethical standards of the committee responsible for human experimentation (institutional and national) and the Declaration of Helsinki of 1975, as revised in 2013. This study was conducted in accordance with the approval and guidelines of the Institutional Review Board of Ajou University (AJOUIRB-DB-2022-354). The IRB Committee waived the requirement for obtaining patient consent. The reason is that there is no justifiable ground to presume refusal of consent from the subjects, and the risk to the research subjects is extremely low even if consent is waived.
Cephalometric measurement
Lateral cephalometric radiographs were obtained with the participants in a natural head posture, and the patients were instructed to maintain their lips at rest to accurately assess lip incompetence, as outlined by Park and Burstone.8 The cephalometric measurements and analyses were conducted by a single examiner (PH) using the V-ceph 8.4 software (Osstem, Seoul, Korea). Cephalometric analyses of 10 randomly chosen patients were repeated at 4-week intervals to assess the precision of the examiner’s measurements. The reproducibility of these measurements was confirmed by calculating the intraclass correlation coefficient, which exceeded 0.9, indicating a high level of measurement reliability.
Eighteen specific measurements (Supplementary Table 2) were used to assess the changes in the facial profile before and after treatment. These measurements were analyzed on the X-axis, defined by the Frankfort horizontal line, and on the Y-axis, determined by the line perpendicular to point N (nasion-perpendicular line [N-perp]), facilitating both vertical and horizontal comparisons (Figure 1).
To evaluate the morphological alterations of the lips, measurements such as the soft tissue angle to the line (SAL) depth, palatal to the line (PBL) depth, SAL angle, and PBL angle were employed.9 Specifically, the SAL and PBL depth refer to the distances from A’ to Sn-LS and from B’ to Li-Pg’, respectively, and the SAL and PBL angles denote the angles formed by Sn-A’-LS and Li-B’-Pg’, respectively (Figure 2). Furthermore, to examine the nuances of the upper and lower labial angles, the CK, H, and angle between the facial plane and CK line (CKHA, which equals the H-angle minus the CKA) were used (Figure 2).10,11
ASO was performed exclusively by a single surgeon (YJC), whereas all the orthodontic treatments were administered by a single ABO-certified orthodontist (HSC) using a 0.022 slot bracket system with an MBT prescription. In the ASO treatment group, a range of minimum to maximum anchorage was applied to both the maxilla and mandible for closing the spaces remaining after surgery. Patients in whom posterior TADs were used for further anterior teeth and lip retraction following ASO were excluded from this study. In the NASO group, maximum anchorage was achieved using TADs in the maxilla and a moderate-to-maximum anchorage approach was adopted in the mandible.
Surgical procedures
ASO and/or genioplasty were performed under general anesthesia. The first premolars were extracted during surgery to facilitate posterior translation of the anterior segment. Owing to the superior blood supply to the mandible compared to that to the maxilla, mandibular osteotomy was prioritized to minimize the duration of ischemia in the maxillary region. Osteotomy was performed using a 4-mm diameter round burr and a reciprocating saw. For the mandible, an incision was made below the mucogingival junction, extending from canine to canine, to ensure protection of the bilateral mental nerves. Following repositioning of the maxillary and mandibular segments to their intended locations, they were secured using miniplates and screws. An additional genioplasty guided by intraoperative evaluation was also performed.
Statistical analysis
The data obtained were subjected to statistical analyses using the SPSS software (version 29.0; IBM, Armonk, NY, USA). The Shapiro–Wilk test, for both pre- and post-treatment measurements, was used to evaluate the data distribution for normality. An independent t test was used to compare the measurements obtained before treatment, changes observed before and after treatment, and measurements recorded after treatment between the ASO and NASO groups. Categorical data comparisons between the groups were conducted using the chi-square test. In the statistical analysis, Pearson correlation analyses were performed to assess the relationships between changes in soft tissue measurements, including the interlabial gap, CKA, and CKHA. Subgroup-specific correlations were also examined to identify treatment-specific patterns. Statistical significance was set at P < 0.05 indicated statistical significance.
RESULTS
Pre-treatment comparison between ASO and NASO
Before treatment initiation, significant differences were observed across several parameters between patients who underwent ASO and those who underwent NASO. Notably, the interlabial gap, UL-E (upper lip anterior to E-line), LL-E (lower lip anterior to E-line), PBL depth, H-angle, distance from B’ to the N-perp, UL position to N-perp, pogonion (Pog’) to N-perp, and LL thickness exhibited statistically significant differences (P < 0.05) between the ASO and NASO groups (Table 1). The interlabial gap, a potential indicator of lip incompetence, was 2.05 mm larger in the ASO group than that in the NASO group before treatment. The UL and LL protrusion relative to the E-line was approximately 2 mm more pronounced in the ASO group than in the NASO group. Regarding the N-perp, soft tissue points A and B were 1.74 mm more anterior and 4.83 mm more posterior in the ASO group than in the NASO group, respectively. The position of the UL was 2.83 mm further anterior in the ASO group, and the soft tissue Pog’ was 9.65 mm more posterior than in the NASO cohort. The SAL angle, SAL depth, PBL angle, and PBL depth demonstrated significant disparities, with the PBL depth being notably greater in the ASO group by –1.97 mm than in the NASO group. Although no significant difference was noted in the UL thickness between the two groups, the LL thickness was significantly greater in the ASO group (1.46 mm than in the NASO group (Table 1).
Comparison of post-treatment outcomes between ASO and NASO
Following the treatment phase, statistically significant differences were observed in several parameters, including PBL angle, PBL depth, NLA, B’ to N-perpendicular, Pog’ to N-perpendicular, and LL thickness when comparing outcomes between patients undergoing ASO and NASO. Specifically, in the ASO group, the PBL angle decreased by an average of 18.78° compared to the NASO group. Additionally, the LL thickness in the ASO group was 1.61 mm thicker than that in the NASO group. Regarding the positional changes of the anatomical points, the B’ point in the ASO group was positioned 5.90 mm more posteriorly and the Pog’ point was positioned 8.76 mm further backward than in the NASO group. Finally, NLA was 5.39° larger in the ASO group than in the NASO group (Table 2).
Analysis of pre- and post-treatment modifications in ASO versus NASO
Significant variations were identified in the alterations observed before and after treatment across several measurements between patients treated with anterior ASO and NASO. These measurements included changes in the interlabial gap, UL-E, LL-E, PBL depth, CKA, H-angle, NLA, UL to N-perpendicular, and LL to N-perpendicular (Table 3). A notable difference was also observed in the change from B’ to N-perpendicular between the two groups (P < 0.01). Specifically, the ASO group reduced the interlabial gap by –1.54 mm, whereas the NASO group showed no significant change. Correlation analyses of soft tissue measurements revealed significant relationships between the change in the interlabial gap and the change in the CKA (P < 0.05), as well as between the interlabial gap and the CKHA (P < 0.05) across all patients. Within the ASO group, a significant correlation was found only between the change in the interlabial gap and the CKHA (P < 0.05), indicating specific patterns of facial profile adjustment in response to the respective treatments. Ricketts’s E-line pre- and post-treatment changes were more posterior in the ASO group than in the NASO group by –2.32 mm in the upper and –2.58 mm in the LLs. The change from pre- to post-treatment in the upper and LLs to N-perpendicular was more posterior in the ASO group than in the NASO group by –2.42 and –2.58 mm in the UL and LL, respectively. Changes in the SAL angle, SAL depth, PBL angle, and PBL depth were analyzed to assess the lip curvature. Only the change in ΔPBL depth was notably greater in the NASO group than the ASO group by –1.30 mm, indicating soft tissue B point retraction, which was unfavorable. For the CKA, which is indicative of LL protrusion, the ASO group exhibited a more significant reduction of –4.28° than the NASO group. The H-angle, assessing UL protrusion, demonstrated a greater retraction of –2.21° in the ASO group. The combined change in the CKHA was 2.07° more substantial in the ASO group, reflecting different degrees of facial profile correction between the groups. The NLA change was significantly larger in the ASO group (6.96°) than in the NASO group, highlighting the impact of surgical intervention on the esthetic angle between the nose and UL. The impact of genioplasty on the outcomes was not statistically significant (Table 3).
Comparison of the borderline subgroup cases
To investigate the borderline cases, borderline subgroups were categorized based on the condition (1 < UL-E < 2.98, where the UL-E values in the NASO group were above the group mean and those in the ASO group were below the group mean). Therefore, nine patients were selected from each group (ASO and NASO). Before treatment, no significant differences were observed in the UL-E or LL-E levels between the two groups (Supplementary Table 3).
Before treatment, significant differences were observed in the NLA (P = 0.01). The B’ to N-perpendicular (P < 0.00) and Pog’ to N-perp distances (P < 0.01) were more protrusive in the NASO group and significantly lower PBL angle (P < 0.00) were observed in the ASO group, indicating a more pronounced mandibular soft tissue protrusion in the NASO group before treatment. The LL was thicker in the ASO group (P = 0.02; Supplementary Table 3). After treatment, the interlabial gap remained larger in the ASO group (P = 0.01) than in the NASO group. The NLA was significantly larger in the ASO group than in the NASO group (P = 0.02). The LL was still thicker in the ASO group (P = 0.02). The initial discrepancy in the mandibular soft tissue component, B’ to N-perpendicular (P < 0.00) and Pog’ to N-perp distance (P < 0.00), persisted. The LL to N-perpendicular measurement was also significantly larger in the NASO group than in the ASO group (P = 0.02; Supplementary Table 4). The changes from pre- to post-treatment were investigated, and three variables demonstrated significant differences. The ASO group exhibited a significantly greater reduction in the UL prominence relative to the E-line (–1.32 mm, P = 0.01) and N-perpendicular (–2.21 mm, P < 0.00) than the NASO group, whereas LL retraction displayed no significant difference (Supplementary Table 5).
DISCUSSION
This study evaluated the effectiveness of TADs in achieving maximum anchorage for UL and LL retraction in extraction cases (NASO group) compared to the ASO group, with the aim of establishing evidence-based clinical guidelines. Chang et al.12 recognized the esthetic interplay and balance between the UL and LL as crucial elements for defining facial attractiveness. Specifically, the decision-making process must be approached with caution when considering surgical interventions owing to the potential for irreversible effects on facial esthetics. The analysis of pre- and post-treatment changes relative to Ricketts’s E-line revealed that the UL moved 4.10 mm posteriorly in the ASO group, compared to 1.77 mm in the NASO group. Similarly, the LL was retracted by 4.91 mm in the ASO group and 2.23 mm in the NASO group (Table 3). Based on these findings, the ASO approach is recommended for achieving effective lip retraction of approximately –4.0 mm to –5.0 mm. Conversely, the NASO approach was considered more suitable for smaller retractions of approximately –2.0 mm.
The inclusion of borderline cases provided a more comprehensive comparison of the soft tissue responses between the ASO and NASO treatment modalities. The results highlighted that, among patients with borderline mild-to-moderate lip protrusion, ASO resulted in significantly greater retraction of the UL, whereas no significant differences were observed in LL retraction between the two approaches. Based on the findings presented in Supplementary Tables 3–5, if B’ and Pog’ are not retrusive, then for patients with borderline lip protrusion (1 mm < UL < 2.98 mm) who desire moderate lip retraction (~2–3 mm), NASO using TADs may serve as a viable non-surgical alternative. However, for cases requiring more pronounced lip retraction (~4–5 mm), ASO remains the preferred treatment option owing to its ability to achieve greater soft tissue changes, which is consistent with the evaluation of all cases.
This study demonstrated that the ASO group experienced a more significant reduction in the interlabial gap than the NASO group. Initially, the interlabial gap measured 4.54 mm in the ASO and 2.49 mm in the NASO group, which was within the normal limit, aligning with the findings by Kim et al.9 However, the borderline subgroup cases were not significantly different (Supplementary Table 5).
Lee et al.4 further elucidated that, in patients with Class II malocclusion, the forward protrusion of the LL, despite a retrusive mandible, could often be attributed to lip incompetence or the forward pressure exerted by the protruding maxillary anterior teeth, which in turn displaced the LL downwards and forward. Therefore, for patients with Class II protrusion, a treatment approach that entails greater movement of the maxilla and minimal movement of the mandible may represent an optimal strategy for resolving maxillary and mandibular protrusions. In Class II skeletal patterns, the mandible is typically smaller than the maxilla. Consequently, the decision to perform ASO on the mandible warrants careful consideration as a reduction in the hard tissue volume can contribute to a corresponding reduction in the soft tissue volume, resulting in soft tissue B retrusion (Figure 3A and 3B). Thus, the initial angulation of the anterior mandibular teeth and projected extent of movement within the treatment plan are critical considerations. Thus, deliberations regarding the need of mandibular surgery should be approached with caution. Although not explicitly analyzed in this study, an ideal esthetic treatment plan for patients with Class II malocclusion may involve maxillary ASO and mandibular full-arch distalization (Figure 3).
A notable limitation of this study is the discrepancy in the pretreatment conditions between the ASO and NASO groups. Specifically, the ASO group exhibited more pronounced Class II characteristics at the outset. One confounding variable was the inclusion of both U4 and U5 extraction cases in the NASO group, with 24 U4s and 12 U5 extractions combined (Supplementary Table 1). To assess the impact of the extraction pattern on treatment outcomes, the NASO group was further divided into two subtypes, which were analyzed separately (Supplementary Table 6). Pre- and post-treatment comparisons between the U4 and U5 extraction subgroups within the NASO group revealed differences between the two variables. UL-E demonstrated a 1.49 mm greater retraction in the U5 extraction subgroup (P < 0.001), indicating greater UL retraction. Although B’ to N-perp exhibited 0.79 mm more retraction in the U5 extraction subgroup than that of the U4s subgroup (P = 0.04), which was clinically insignificant. The decision to extract the U5s was based on several factors, including the initial root angulation of the canines and premolars, tooth shape and condition, and clinician preference for preserving the initial root inclination of the maxillary incisors. These results suggest that TAD-aided absolute anchorage enabled sufficient UL retraction regardless of U4 or U5 extractions. Another limitation is that the data were derived from two-dimensional (2D) analyses. Recent advancements in the cone-beam computed tomography (CBCT)-based three-dimensional (3D) analysis have addressed limitations such as the less-defined soft tissue imaging noted by Lim et al.13 Additionally, recent reports have highlighted the use of facial scanners to acquire and analyze more detailed 3D digital representations of soft tissues.14-16 However, aside from volume measurements obtained through 3D scanners, most studies predominantly rely on 2D linear and angular measurements, limiting the comprehensive application of 3D data for detailed profile analysis. Further studies employing CBCT with long-term follow-up are recommended to directly compare the adverse outcomes associated with ASO and NASO. Further research is warranted to clarify and delineate the differences in the treatment effects and potential sequelae between these approaches, thus providing a more comprehensive understanding of their respective risks, benefits, and long-term stability.
CONCLUSIONS
The current study revealed significant differences in the soft tissue measurements before and after treatment between the ASO and NASO groups. Notable differences related to the LL change were noted in the LL-E, PBL depth, CKA, LL to N-perp, and B’ to N-perp. Based on these findings, we suggest using the required movement as a guideline for choice between ASO and NASO. If the required retraction for the UL and LL is approximately –4.0 mm and –5.0 mm, respectively, ASO is advised owing to its effectiveness in achieving significant profile improvements. For modifications of approximately –2.0 mm, NASO is preferred.
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