Three-dimensional analysis of hard and soft tissue changes in skeletal class II patients with high mandibular plane angle undergoing surgery

This study aimed to study 3-dimensional (3D) changes of hard and soft tissues of skeletal class II patients after 2-jaw surgery and genioplasty. 32 adult patients diagnosed with mandibular hypoplasia who underwent 2-jaw surgery of maxillary impaction, mandibular advancement and genioplasty were enrolled. Cone-beam computed tomography and 3D stereophotogrammetry was conducted 1 week before and 6 months after surgery. Dolphin imaging software was used to establish a 3D digitizing model and 3D measurement system. Paired t-test was performed to compare the values before and after surgery. Pearson’s correlation test assessed the degree of correlations between hard and soft tissue change. The mean impaction of the maxilla was 2.600 ± 3.088 mm at A. The mean advancement of the mandible was 7.806 ± 2.647 mm at B. There was a significant upward and forward movement for most landmarks of the nose and lip, while a significant decrease in nasal tip height (lateral view), upper lip height, and upper and lower vermilion height. The nose's width was significantly increased. For maxillary, Sn, Ac-r, Ac-l, and Ls demonstrated a significant correlation with A and U1 in the anteroposterior axis. However, there were no significant correlations among them in the vertical axis. For mandibular, Li demonstrated a significant correlation with L1 in the anteroposterior axis specifically for the mandible. Notably, correlations between the landmarks of the chin's hard and soft tissues were observed across all axes. The utilization of 3-D analysis facilitated a quantitative comprehension of both hard and soft tissues, thereby furnishing valuable insights for the strategic formulation of orthognathic treatment plans targeting patients with skeletal class II conditions.

meaningful to study the changes and ratios of soft-to-hard tissue from three dimensions for the patients treated with a LeFort I osteotomy, a bilateral sagittal split osteotomy (BSSO), and a genioplasty.
Cone-beam computed tomography (CBCT) provides three-dimensional (3D) information about deep skeletal structures and superficial skin but has disadvantages in soft tissue evaluation due to a low resolution with large slice gaps and deformations of the soft tissues of the chin due to the device's chin holder 10,11 .Additionally, by using 3D stereophotogrammetry, texture and color information of the face can be readily obtained in high resolution without additional radiation hazards, along with advantages such as a short scan time, no risk to the naked eye, and easy operability 12 .Therefore, CBCT and 3D stereophotogrammetry have been combined to evaluate the relationship between hard and soft tissue changes 13,14 .However, most 3-D studies of hard and soft tissue changes after orthognathic surgery focused on skeletal class III patients 13,14 .A few researchers have focused on skeletal class II patients, but only mandibular advancement was performed 6,15 .
In this study, we aimed to study the 3D changes and correlation between hard and soft tissue movements in patients with skeletal Class II malocclusion with a severe retruded mandible and a vertical growth pattern which had received LeFort I osteotomy for maxillary impaction, a BSSO for mandibular advancement and rotation, and a genioplasty for correcting the skeletal deformities using CBCT and 3D stereophotogrammetry.

Subjects
Skeletal Class II patients who had received LeFort I osteotomy, BSSO, and genioplasty only by the same surgeon between 2017 and 2022 were screened.Similar surgical techniques were used in all cases, and rigid internal fixation was applied, according to Tulasne 16 .The following criteria were considered for this study: (1) Patients must have complete pre-and postoperative stereophotogrammetry records (postoperative records must be at least 6 months after surgery after debonding).( 2) There was no presence of a congenital anomaly or craniofacial deformity, no history of accidents or trauma, and no presence of severe asymmetry at Me (< 5 mm to the midsagittal plane).(3) Maxillary impaction surgery with one piece Le Fort I osteotomy and mandibular advancement bilateral sagittal split ramus osteotomy with genioplasty was performed on all patients during a correction of skeletal class II malocclusion.(4) Patients underwent surgery at the same center (Nanjing Stomatological Hospital, Medical School of Nanjing University, Department of Oral and Maxillofacial Surgery).( 5) Similar rigid fixation methods were performed for stabilization after the surgery.Exclusion criteria were as follows: (1) history of accidents or trauma, (2) presence of severe asymmetry at Me (> 5 mm to the midsagittal plane), (2) subjects with a body mass index greater than 30 kg/m 2 , and (3) subjects with increased or decreased body weight more than 5 kg before and after surgery.Thus, 32 adult Chinese subjects (15 men and 17 women; mean age, 21.73 ± 3.87 years at the start of treatment) (the power calculated by PASS 15.0 software was more than 90%) were included in this study.This retrospective study was registered and approved by the Ethics Committee of Nanjing Stomatological Hospital.Furthermore, all methods were performed following the approved guidelines and regulations, all subjects had signed the informed consent form for both study participation and publication of identifying information/images.

Data acquisition
CBCT and 3D stereophotogrammetry were taken 1 week before and 6 months after surgery (without braces debonding) (T0 and T1, respectively).The CBCT (KaVo 3D eXam, USA) image data was obtained within 1 week before surgery.The CBCT imaging conditions were as follows: voxel size: 0. 25 mm; field of view: 16 × 13 cm; tube voltage: 120 kVp, and tube current: 5.0 mA.Digital Imaging and Communications in Medicine (DICOM) data from multislice CT images were reconstructed and analyzed using image-processing software Dolphin (version 11.8; Dolphin Imaging and Management Solutions, Chatsworth, CA, USA).Additionally, 3D stereophotogrammetry images were obtained using the 3dMDface system (3dMDface LLC, Atlanta, GA, USA), with a natural head position, relaxed facial expression, and eyes looking straight ahead 17 .The data of 3dMD was saved in the BMP format.Both formats were transformed into Dolphin software and integrated into a complete 3D digital image before measuring.Five landmark points (bilateral outer canthus, the tip of the nose, and bilateral angulus oris) in the soft face were selected to manually match the 3dMD images and CBCT scans.The final images were presented for measurement.

Landmarks, planes, and measurements
A 3D coordinate system was established in CBCT images as follows: the X-direction represents the horizontal plane(x), passing through the left and right orbitale (the lowest point on the inferior margin of the orbit) and the right porion (the midpoint of the upper contour of the external auditory canal); the Y-direction which represents the anteroposterior plane(y) is vertical to the X-direction, passing through the right porion; and the Z-direction is the vertical direction passing through the nasion (Fig. 1).
A total of 10 skeletal and 24 soft tissue landmarks were identifiedand measured in the 3D coordinate values before and after surgery (Figs. 2 and 3, Tables 1 and 2).A positive ( +) sign indicated the point deviated to the left, anterior, and upper side to N of the subject.A negative ( −) sign indicated the opposite.The changes in the landmarks, correlation coefficients (p), and soft-to-hard tissue movement ratios were evaluated in x, y and z axis.Furthermore, 14 linear measurements of soft tissue were assessed (Table 3).
The datasets used during the current study available from the corresponding author on reasonable request.

Statistical analysis
All measurements were conducted by one investigator and repeated after 2 weeks, and there was no difference between the two assessments revealed by a paired t-test (p > 0.05).The second set of measurements was used for the following statistical analysis.A paired t-test was conducted to compare the values between T1 and T2.And a Pearson's correlation test was performed to assess the degree of correlations between hard and soft tissue changes after orthognathic surgery.Also, soft-to-hard tissue movement ratios were defined.p < 0.05 was defined as statistically significant.

Changes in hard tissue landmarks
In the transverse axis, since no severe asymmetry was presented at Me in our patients, there were no significant changes in all hard tissue landmarks; (Table 4).
Since the patients in our study were diagnosed with skeletal Class II malocclusion with a severe retruded mandible and a vertical growth pattern and received LeFort I osteotomy for maxillary impaction, there were no significant changes for maxillary landmarks in the anteroposterior axis.In contrast, the changes were significant for most skeletal mandibular landmarks in the anteroposterior axis (Table 4).There was a significant advancement in lower incisors (ΔL1, 6.166 ± 2.008 mm, p < 0.01).The average antedisplacement of anteroposterior correction was 7.806 ± 2.674 mm at B (p < 0.01), 13.228 ± 4.020 mm at Pog (p < 0.01), 13.213 ± 4.282 mm at Gn (p < 0.01), and 12.747 ± 4.123 mm at Me (p < 0.01).However, there was no significant setback in the maxillary (point A, p > 0.05).
In the vertical axis, there was a significant upward movement both in the maxillary and mandible, and the changes in the maxillary were more evident than the ones in the mandible.In maxillary, the average upward correction was 2.600 ± 3.088 mm, p < 0.01 at point A, 3.806 ± 2.491 mm, p < 0.01 at ΔU1L and 3.994 ± 2.415 mm, p < 0.01 at ΔU1.Additionally, the average upward correction in the mandible was 2.825 ± 2.148 mm, p < 0.01   1 for the description of 3D CBCT landmarks.at ΔL1 and 2.191 ± 1.879 mm, p < 0.05 at point B. However, none of the landmarks related to the chin showed statistically significant changes (p > 0.05).

Changes in soft tissue landmarks and measurement
Table 5 shows the changes in soft tissue landmarks after orthognathic surgery in facial scan images' transverse, anteroposterior, and vertical axes.In the transverse axis, only the landmarks related to the nose were widened (Al, p < 0.01; Ac, p < 0.01).There was a significant increase in the width of the nose (ΔWidth of Alare, 2.678 ± 1.671 mm, p < 0.01) (Table 6).No significant changes were observed for other points (p > 0.05).
In the anteroposterior direction, most nose, lip, and chin landmarks moved forward significantly (p < 0.01).The changes in the chin were the most, the lips second, and the nose least.There was a significant decrease in the height of the nasal tip (lateral view) (− 0.703 ± 0.772 mm, p < 0.01).The eyes and cheek landmarks exhibited no significant changes (p > 0.05).
Additionally, in the vertical axis, there were significant upward movement in the landmarks related to the nose, lips, and cheek (p < 0.01) but not in the eyes and chin.And the upward shift of the lower lip was the most, the upper lip second, and the nose the least.There was a significant decrease in upper lip height (− 0.813 ± 1.446 mm, p < 0.05) and in lower vermilion height (lower, − 1.578 ± 2.653 mm, p < 0.01).

Correlations and ratio between changes in the hard and soft tissues
Since there was no significant change in the transverse axis, our study did not discuss the correlations and ratios between corresponding hard and soft tissue landmarks.
In the anteroposterior axis, soft tissue landmarks related to the nose (Sn, Prn, Ac-l) and upper lip (Ls) demonstrated a significant correlation with hard tissue landmarks (A and U1) in the maxilla.And the ratios of maxillary were more than 1(Sn/A 1.431, and Ls/U1 2.930).However, soft tissue landmarks related to the lower lip (Li) in the mandible showed a significant correlation with lower incisors (L1).Correlations between all of the soft tissue and underlying corresponding hard tissue in chin-related landmarks were significant (p < 0.01) (Tables 7 and 8, Fig. 4a-m).Furthermore, there was an increasing gradient of ratios from Li/L1 to Me′/Me (Li/ L1 0.921, B′/B 0.974, Pog′/Pog 1.103, Gn′/Gn 1.008, Me′/Me 1.169), which meant that the closer the points to the chin, the closer the ratio to 1 (Fig. 4n-p).www.nature.com/scientificreports/ In the vertical axis, there were no significant correlations among maxillary hard tissue landmarks.In the mandible, only the soft tissue and underlying corresponding hard tissue landmarks in the chin showed significant correlations (p < 0.01), and the ratios were approximately 1 (Li/L1 1.145, Gn′/Gn 0.922, Me′/Me 0.287) (Table 7  and 8).

Discussion
This study used CBCT and 3D stereophotogrammetry to study the changes and correlations between hard and soft tissues for skeletal Class II patients after orthognathic surgery.This method has the advantage of three dimensions change measurement by using an identical 3D coordinate system, accurate and precise measurements because of rotating the images in any direction, and high reproducibility.However, there are disadvantages of high cost, the impossibility of acquiring dynamic images, the need to calibrate the system frequently.The accuracy of superimposing the surface image of CBCT and the facial scan has been evaluated in previous studies, www.nature.com/scientificreports/which reported that the image fusion was acceptable with a minimum error of less than 1 mm 18,19 .According to the literature, postoperative swelling is resolved by almost 80% after 6 months 20 .Additionally, we analyzed the images taken 6 months after surgery without debonding to minimize any unwanted effects, such as postsurgery swelling and ensure sufficient stabilization of the soft facial tissues.
For hard tissue landmarks, we found significant changes in landmarks only related to the incisors in the transverse axes.These changes might be due to no significant asymmetry observed in our patients.In anteroposterior axes, there was a significant advancement for the landmarks of the mandibular and chin, as reported in other articles about surgical treatment of skeletal Class II patients, who had their mandibular and chin positioned anteriorly after mandibular advancement surgery 4,21 .However, we did not noticed a significant significant setback in the maxillary, and this might be due to the selection criteria included in this study.In our study, most patients were diagnosed with mandible hypoplasia with a severe retruded mandible and a vertical growth pattern, one piece Le Fort I osteotomy was conducted mainly for maxillary impaction.There was a counter clock rotation and an upward shift of the forepart of the maxillary other than setback movement for these patients.In our study, many patients had the advancement of point A in the anteroposterior axis.Notably, this was correlated with the upward movement of the landmarks of the incisors (U1L, U1, and L1) and point A and point B in the vertical axes.However, no landmarks related to the chin showed statistically significant changes.This might be because of the hard-tissue vertical reduction at the chin after genioplasty, which was consistent with the results of Sylvain Chamberland, who reported that vertical soft-tissue change of chin points was less predictable after isolated functional genioplasty 5 .The changes in soft tissue landmarks were correlated with the changes in inferior hard tissue.There was a significant forward and upward movement of the landmarks of the nose and lip.Notably, these might be due to the counter clock rotation and upward shift of the forepart other than the setback movement of the maxillary.The significant decrease in nasal tip height (lateral view) meant that Sn's forward movement was more than that of Prn.Likewise, the decrease in upper lip height meant a lesser upward shift of Sn than Ls.This may be because the Sn point was closer to the osteotomy line, and the soft tissues underlying these points were thin.A significant decrease in upper and lower vermilion height could be explained by a decrease in soft tissue tension after orthognathic surgery because stretching of the lip area's soft tissue can be reduced as the changes after retraction 22 .Our results showed a significant increase in nose width, usually noticed after maxillary advancement surgery 14,23 but seldom reported in previous studies about skeletal Class II patients.This finding suggested that the counter clock rotation and upward shift of the forepart, other than the setback movement of the maxillary, would push forward the nasal base, contributing to the uplift of the soft tissue around the nose and upper lip, leading to the broadening of nasal width.Another potential factor contributing to the widening of the alar bases was soft tissue dissection.Periosteal elevation will sever necessary muscular attachments leading to muscular retraction, alar flaring and shortening, and flattening and thinning of the upper lip 24 .In some cases, a combined alar cinch suture was recommended to control alar flaring at the time of surgery 25 .
For the correlations between changes in the hard and soft tissues, a significant correlation was observed between soft-tissue and hard-tissue changes only in the anteroposterior axis for maxillary.The ratios of maxillary were more than 1(Sn/A 1.431, Ls/U1 2.930) in our study.However, the respective measurements in the study by Suyun Seon and Hyun-Woo Lee were 0.298(ΔSN/ΔA) 26 and − 0.23 in the study conducted by Zhou Zhijie 27 .The big difference might be due to a counter clock rotation and an upward shift of the forepart of the maxillary other than setback movement for half of the patients in this study.The mean maxillary advancement and impaction amounts were − 0.675 and 2.600 mm in this study, respectively, while they were 1.84-1.54mm in the study by Zhou Zhijie 27 .The upward shift of the maxillary in this study caused the forward movement of Sn and Ls.Furthermore, in the vertical axis, the ratio of Prn and point A (0.455) was consistent with the results of Suyun Seon (0.647) 26 , although there was no significant relationship between them.
For the mandible, there was an increasing gradient of ratios from Li/L1 to Me′/Me in the anteroposterior axis, which was consistent with the studies of A.S. Storms, who found that the lower lip followed the lower incisor to a lesser extent than Pog′:Pog, B′:B, and Me′:Me (which approached a 1:1 relationship) 26 .These findings suggest that the lower lip could be under the influence of the muscle rather than the bone 29,30 , and this might be related to the inherent differences in the soft tissue between the lip and chin.Additionally, the ratios of the chin were also consistent with the results of Alexander Bral, who found an sPg:Pg ratio of 87% for movement in the anterior direction among the patients who underwent BSSO and a higher ratio of 102% among patients who underwent bimaxillary surgery 4 .A systematic review also had ratios ranging from 71 to 110% of included studies, and the average ratio was 100% when BSSO was performed to produce anterior displacement 31 .The small differences could be due to differences in patient populations or errors in measurement.In the vertical axis, the ratios were approximately 1.However, a systematic review studying the hard and soft tissue response following isolated genioplasty reported that the ratio of soft and hard tissue changes after genioplasty ranged from 0.09 to 0.7 among the studies in the vertical plane 32 .Furthermore, Sylvain Chamberland revealed that chin soft-tissue changes could be predicted horizontally more precisely than vertically 5 .Therefore, more studies were needed to analyze the relationship between the soft and hard tissues of the chin in the vertical axis.This study showed the 3D soft and hard tissue changes after bimaxillary surgery in Class II patients using CBCT and facial scanning.The results of this study would contribute clinicians to predict the effect of orthognathic surgery for soft tissue.It could guide surgical planning to improve patient outcomes.As machine learning and artificial intelligence participated more in the guidance of therapeutic approaches [33][34][35] , We hoped the results of this study would contribute to the database of hard and soft tissue changes in skeletal class II patients with high mandibular plane angle undergoing orthognathic surgery, which could further facilitate the precise prediction of the effect of orthognathic surgery for soft tissue.
However, there are some limitations in this retrospective study.A double-blinded, randomized controlled trial is the ideal experimental design for a comparative study.However, because orthognathic surgery is performed according to the patients' profile and occlusion, randomization of the indications for orthognathic surgery is

Figure 1 .
Figure 1.Superimposed 3D image of CBCT and facial scan and 3-dimensional coordinate system.
Abb. DefinitionNasionN The anterior point of the intersection between the nasal and frontal bones Anterior nasal spine ANS The tip of the anterior nasal spine (sometimes modified as the point on the upper or lower contour of the spine where it is 3 mm thick; see Harvold analysis) A Point A The innermost point on the contour of the premaxilla between anterior nasal spine and the incisor tooth U1 Labial Gingival Border U1L Labial cemento-enamel junction of the upper central incisor U1 Tip U1 Incisor tip of the upper central incisor L1 Tip L1 Tip of the lower central incisor B Point B The innermost point on the contour of the mandible between the incisor tooth and the bony chin Pogonion Pog The most anterior point on the contour of the chin Anatomical Gnathion Gn The midpoint between the most anterior and inferior point on the bony chin Menton MeThe most inferior point on the mandibular symphysis (i.e., the bottom of the chin)

Table 3 .
Abb. DefinitionExocanthion-right Ex-r The soft tissue point located at the outer commissure of the right eye fissure Exocanthion-left Ex-l The soft tissue point located at the outer commissure of the left eye fissure Endocanthion-right En-r The soft tissue point located at the inner commissure of the right eye fissure Endocanthion-left En-l The soft tissue point located at the inner commissure of the left eye fissure Nasion′ N′ The point in the midline of both the nasal root and the frontonasal suture Pronasale Prn The most prominent point on the nasal tip Subnasale Sn The midpoint on the nasolabial soft tissue contour between the columella crest and the upper lip Alare-right Al-r The most lateral point on the right alar contour Alare-left Al-l The most lateral point on the left alar contour Alare curvature-right Ac-r The point located at the facial insertion of the right alar base Alare curvature-left Ac-l The point located at the facial insertion of the left alar base Subalare-right Sbal-r The point of the end of the alare extends to the facial skin Subalare-left Sbal-l The point of the end of the alare extends to the facial skin Labrale superior Ls The midpoint of the vermilion line of the upper lip Labrale inferior Li The midpoint of the vermilion line of the lower lip Cheilion-right Ch-r The point located at the right labial commissure Cheilion-left Ch-l The point located at the left labial commissure Stomion Sto The midpoint between the lower border of the upper lip and upper border of the lower lip Cheek point-right Ck-r The point where a vertical line from exocanthion and a horizontal line from cheilion meet on the right side Cheek point-left Ck-l The point where a vertical line from exocanthion and a horizontal line from cheilion meet on the left side B′ point B′ The most deepest point from lateral view, on the facial midline, between the lower lip and chin Pogonion′ Pog′ The most anterior midpoint of the chin Gnathion′ Gn′ The midpoint between the most anterior and inferior point of the soft tissue chin Menton′ Me′ The lowest point on the midline of the chin

Figure 4 .
Figure 4. Figures for Pearson correlation.Correlation between soft tissue landmarks (Sn, Prn, Ac-l and Ls) and hard tissue landmarks U1 (A-D) and A (E-H) in the anteroposterior axis.Correlation between B′ and B (I), Pog′ and Pog(J), Gn′ and Gn(K), Me′ and Me(L), Li and L1(M) in the anteroposterior axis.Correlation between Li and L1(N), Gn′ and Gn(O), Me′ and Me(P) in the vertical axis.

Table 2
for the description of 3D soft tissue landmarks.

Table 1 .
Definitions of hard tissue landmarks.

Table 2 .
Definitions of soft tissue landmarks.

Table 4 .
Changes in hard tissue landmarks on CBCT images.

Table 5 .
Changes in soft tissue landmarks on 3D stereophotogrammetry images.

Table 6 .
Linear changes in soft tissue measurements after orthognathic surgery.

Table 7 .
Correlation coefficients (p) and ratio of soft-to-hard tissue movement (S/H) in maxillary.

Table 8 .
Correlation coefficients (p) and ratio of soft-to-hard tissue movement (S/H) in mandible.