Anterior segmental osteomy versus orthodontic camouflage treatment decision: The importance of incisor mandibular plane angle

doi:10.21203/rs.3.rs-4912484/v1

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Research Article

Anterior segmental osteomy versus orthodontic camouflage treatment decision: The importance of incisor mandibular plane angle

https://doi.org/10.21203/rs.3.rs-4912484/v1

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Objectives

Temporary anchorage devices (TADs) have reduced the need for anterior segmental ostomy (ASO) in Class I patients; however, ASO remains relevant in Class II patients. As previous studies were conducted before the usage of TADs, updated guidelines are required to determine the relationship between surgery and orthodontic treatment. This study aimed to evaluate the hard tissue changes following ASO versus non-ASO (NASO) orthodontic treatment utilizing TADs with premolar extraction.

Materials and Methods

This retrospective clinical study involved 67 patients (18 male, 49 female; mean age: 33.9 years) diagnosed with skeletal Class II malocclusion. The patients were categorized into the ASO (n = 31) and NASO (n = 36) groups. Cephalometric analyses were conducted to compare initial and final records to assess the treatment effects

Results

Pre-and post-treatment changes in the ASO and NASO groups exhibited significant differences in ΔSNA, ΔSNB, ΔANB, ΔA to N-perp, ΔB to N-perp, ΔPog to N-perp, and ΔA to FH (P < 0.05). While the reduction in ANB in the ASO group compared with that in the NASO group was clinically significant at 1.77, both groups demonstrated normalized ANB, indicating significant skeletal improvement in Class II malocclusion. Specifically, the change in FMIA (ΔL1 to FH [°]) was 4.98° greater in the NASO group than in the ASO group (P < 0.05).

Conclusions

Our findings revealed the importance of the incisor mandibular plane angle (IMPA) in treatment decision-making between ASO and NASO treatment.

Clinical Relevance:

This study highlights IMPA's importance in treatment planning and aids in selecting ASO and NASO treatments.

Anterior Segmental Osteotomy

Extraction

Skeletal Class II malocclusion

IMPA

Bimaxillary protrusion is prevalent among individuals of African and Asian descent, prompting patients to seek orthodontic treatment [1, 2]. The introduction of orthodontic temporary anchorage devices (TADs) has broadened the scope of non-extraction tooth alignment by improving anchorage. However, these orthodontic treatment methods may have limitations in terms of enhancing facial esthetics due to risks of root resorption, causing excessive posterior inclination of the anterior teeth and reduction of the alveolar bone in the palatal region [3].

Anterior segmental osteotomy (ASO) with premolar extraction was developed to effectively address these side effects while efficiently correcting protrusions [4]. However, despite its ability to overcome several limitations of orthodontic treatment, ASO may still lead to various postoperative complications and impose economic and psychological burdens on patients. In a comparative study by Lee et al [5] between ASO and nonsurgical extraction orthodontics (NASO), the ASO group exhibited a reduction in the Orthodontic Discrepancy Index compared with that of the NASO group. Additionally, the ASO group showed the posterior movement of the maxilla (SNA, A to N-perpendicular) and a more significant reduction in the ANB angle. Regarding dentition, prior research indicated that changes in the angle of the maxillary anterior teeth were less pronounced in the ASO group than in the NASO group [5]. Moreover, Hwang et al [6] reported significant differences in incisal angle changes between the ASO and NASO groups, with values of 7.18° and 11.93°, respectively. Furthermore, recent comparative studies by Yong et al [7] demonstrated that patients who underwent ASO were more likely to achieve an interincisal angle of 135 ± 5° after treatment compared to those who underwent NASO, and long-term follow-up of patients who underwent ASO revealed that side effects were rare. However, so far, no studies have comprehensively compared the treatment outcomes between Class II ASO and NASO.

Therefore, this study aimed to analyze and compare the skeletal and dental changes resulting from ASO and treatment with TADs in NASO. We hypothesized that the clinical significance of this research lies in its potential to assist in determining the most effective treatment outcome based on a patient's initial characteristics and concerns, thereby aiding the formulation of an appropriate treatment plan.

Study sample (Table 1)

Table 1

Demographic data of the participants
Demographic data		ASO	Non-ASO	P-value
Count	Male	9	9	-
	Female	22	27	-
Mean	Age (y)	36.80	31.43	0.04*
	ANB (°)	5.48	3.77	0.00**
	FMA (°)	29.51	29.22	0.82
	U1 to FH (°)	114.81	116.51	0.29
	IMPA (°)	104.81	98.94	0.00**
Independent t-tests were performed to compare variables between the two groups before treatment P < 0.05, *P < 0.01.
IMPA, incisor mandibular plane angle; ASO, anterior segmental osteotomy

The study sample size was determined using G*Power (version 3.1; Heinrich-Heine University, Düsseldorf, Germany). Briefly, this involved conducting two independent sample t-tests, assuming a two-tailed distribution, an effect size of 0.5 (indicating a medium effect), a significance level of 0.05, and a power of 0.8. The calculation indicated that a minimum of 63 patients were necessary for a comparative analysis between the two groups.

Consequently, we enrolled 67 patients aged 19–40 years who visited the Department of Orthodontics at Aju University Dental Hospital between September 1, 2016 and September 1, 2021 and met the study's eligibility criteria. Subsequently, their pre- and post-treatment lateral cephalometric radiographs were obtained and analyzed.

The inclusion criteria were as follows:

A diagnosis of skeletal class II malocclusion (ANB > 2°);

Adults aged ≥ 18 years;

Patients who have undergone orthodontic treatment;

Those with premolars extracted;

Those seeking treatment for lip protrusion.

The exclusion criteria encompassed:

Patients with congenital dental anomalies;

Those with congenital conditions such as craniosynostosis and cleft palate;

Those with systemic diseases affecting bone metabolism;

Those with severe facial asymmetry (chin displacement < 4 mm relative to the facial midline);

Those who have discontinued orthodontic treatment.

This study was conducted following the approval and guidelines of the Institutional Review Board of Ajou University Hospital (IRB No: AJOUIRB-DB-2022-354).

Variables

Cephalometric measurement (Table 2)

Table 2

Skeletal, dental, and soft tissue measurements to compare ASO and NASO
Measurement		Explanation
Soft tissue measurements
	A'_N-perp (mm)	Distance between soft tissue A point and N-perpendicular
	B'_N-perp (mm)	Distance between soft tissue B point and N-perpendicular
	UL_N-perp (mm)	Distance between the upper lip and N-perpendicular
	LL_N-perp (mm)	Distance between the lower lip and N-perpendicular
	Pog’_N-perp (mm)	Distance between soft tissue pogonion point and N-perpendicular
	Interlabial gap	Distance from the upper to lower stomion perpendicular to line (SN-7)
	UL-E (mm)	Distance between the upper lip and E line
	LL-E (mm)	Distance between the lower lip and E line
	UL thickness (mm)	Distance from the labial surface of the upper incisors to the vermilion border of the upper lip (Holdaway, 1983)
	LL thickness (mm)	Distance from the labial surface of the lower incisors to the vermilion border of the lower lip (Holdaway, 1983)
	H angle	Angle of H line (line tangent to the soft tissue chin and upper lip) to Na'-Pog'(soft tissue facial plane)
	CKA	Angle of CK line (line tangent to the soft tissue chin and lower lip) to Na'-Pog' (soft tissue facial plane)
	CKHA	Angle of H line to CK line
	SAL depth	Distance between A' and Sn-Ls
	SAL angle	Angle of Sn-A' to A'-Ls
	PBL depth	Distance between B' and Li-Pg
	PBL angle	Angle of Li-B' to B'-Pg
	Nasolabial angle	Angle of Columella-Subnasale to Subnasale-Labiale superius
ASO, anterior segmental osteotomy; NASO, non-ASO

Lateral cephalometric radiographs were obtained with the patients positioned in their natural head posture, with instructions to keep their lips close to ensure accurate assessments of lip incompetence per Burstone’s guidelines [8]. Cephalometric measurements and analyses were performed by a single examiner (PR Hong) using the V-ceph 8.4 software (Osstem, Seoul, Korea). To assess measurement precision, cephalometric analyses of ten randomly selected patients were repeated at 4-week intervals’. The reproducibility of these measurements was confirmed by calculating the intraclass correlation coefficient, which exceeded 0.9, indicating a high level of measurement reliability.

Forty-one specific measurements were utilized to assess the cephalometric changes before and after treatment. These measurements were analyzed on the X-axis, defined by the Frankfort Horizontal (FH) line, and on the Y-axis, determined by the line perpendicular to point N (N-perpendicular line), enabling both vertical and horizontal comparisons (Fig. 1).

ASO procedures were exclusively performed by a single surgeon (YJ Choi), while all orthodontic treatments were administered by a single American Board of Orthodontics-certified orthodontist (HS Chae) using a 022 slot bracket system with MBT prescription. In the ASO treatment group, a range of minimum to maximum anchorage was applied to both the maxilla and mandible to close the spaces remaining after surgery. Particularly, in the NASO group, maximum anchorage was achieved using TAD in the maxilla, with a moderate-to-maximum anchorage approach adopted in the mandible.

Data collection techniques

Surgical procedures

ASO and/or genioplasty were conducted under general anesthesia. The first premolars were extracted during surgery to facilitate the posterior translation of the anterior segment. Owing to the superior blood supply to the mandible compared with that to the maxilla, mandibular osteotomy was prioritized to minimize the ischemic duration for the maxillary region. Briefly, osteotomy was executed 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 the protection of the bilateral mental nerves. In the maxilla, a minor Le Fort I incision spanning from canine to canine was made on the labial side. The palatal incision involved a central stab incision coupled with a crevicular incision extending from the canine to the second premolar. These specific maxillary incisions were designed to preserve blood supply to the palatal area. After repositioning the maxillary and mandibular segments to their intended locations, they were secured using miniplates and screws. Additionally, a genioplasty guided by intraoperative evaluation was performed.

Data analyses

All statistical analyses were conducted using the SPSS software (IBM, Armonk, New York, United States, version 29.0). The Shapiro–Wilk test was employed to assess the normality of data distribution for both pre- and post-treatment measurements. An independent t-test was utilized to compare measurements obtained before treatment, changes observed from before to after treatment, and measurements recorded after treatment between the ASO and NASO groups.

Pretreatment comparison between the ASO and NASO groups (Table 3)

Table 3

Comparison of skeletal and dental measurements between the ASO and NASO groups before treatment
	ASO group		Non-ASO group
Measurements	Mean	SD	Mean	SD	Difference	P
SNA	82.17	4.16	79.85	3.07	2.31	0.01**
SNB	76.69	4.01	76.08	3.12	0.60	0.49
ANB	5.48	1.13	3.77	1.41	1.71	0.00**
FMA	29.51	6.03	29.22	4.46	0.29	0.82
SN-GoMe	38.13	6.47	39.47	4.84	-1.34	0.35
A to N-perp	0.87	3.13	0.11	2.91	0.76	0.31
A to FH	35.77	3.35	34.65	4.22	1.11	0.24
B to N-perp	-9.35	5.78	-6.67	4.89	-2.68	0.04**
B to FH	82.85	6.58	73.16	6.47	9.69	0.00**
Pog to N-perp	-10.41	6.80	-8.84	5.94	-1.56	0.32
FABA	77.79	3.64	80.13	3.80	-2.33	0.01**
U1 to FH(°)	114.81	6.50	116.51	6.58	-1.70	0.29
U1 to FH (mm)	63.50	4.17	57.50	5.87	6.00	0.00**
L1 to FH(°)	45.68	4.84	51.83	7.23	-6.15	0.00**
L1 to FH (mm)	61.31	4.11	55.89	5.36	5.42	0.00**
U1 C to N-perp	8.52	4.95	6.24	4.24	2.28	0.046**
U1 R to N-perp	-2.99	3.99	-4.19	2.93	1.20	0.16
L1 C to N-perp	5.16	4.89	2.87	4.17	2.29	0.04**
L1 R to N-perp	-10.77	5.81	-9.49	4.31	-1.27	0.31
Interincisal angle	110.87	7.70	115.32	10.70	-4.45	0.053*
BOP	11.19	3.04	10.30	3.58	0.89	0.28
FOP	11.74	3.50	11.43	4.07	0.31	0.74
IMPA	104.81	6.95	98.94	7.46	5.87	0.00**
Independent t-tests were performed to compare variables between the two groups before treatment P < 0.1, *P < 0.05.
IMPA, incisor mandibular plane angle; ASO, anterior segmental osteotomy; SD, standard deviation; FH, Frankfort Horizontal

Skeletal differences before treatment

Significant differences were observed before treatment between the ASO and NASO groups in the following measurements: SNA, ANB, B-to-N-perp, B-to-FH, and FH to AB plane angle (FABA) (P < 0.05). Notably, the ASO group showed a higher tendency for skeletal Class II malocclusion compared to that in the NASO group, and the SNA was 2.31° higher in the ASO group than in the NASO group, indicating a more protrusive maxilla. However, no significant difference was noted between the two groups in the absolute evaluation of the anterior-posterior position of the maxilla measured using A to N-perp.

Dental differences before treatment

Significant differences were also present before treatment between the ASO and NASO groups in measurements such as U1 to FH (mm), L1 to FH (°), L1 to FH (mm), U1 C to N-perp, L1 C to N-perp, and incisor mandibular plane angle (IMPA) (P < 0.05). While there were no significant differences in U1 to FH (°) between the two groups, the ASO group exhibited a significantly larger IMPA (5.87°) than that of the NASO group. Additionally, the horizontal positions of the upper and lower incisors (U1 C to N-perp, L1 C to N-perp) were significantly different, with the ASO group having them positioned 2.28 mm and 2.29 mm more forward, respectively, compared to the NASO group.

Post-treatment comparison between the ASO and NASO groups (Table 4)

Table 4

Comparison of skeletal and dental measurements between the ASO and NASO groups after treatment
	ASO group		Non-ASO group
Measurements	Mean	SD	Mean	SD	Difference	P
SNA	78.04	4.14	79.15	3.32	-1.11	0.23
SNB	74.82	3.92	75.88	2.98	-1.05	0.22
ANB	3.22	1.22	3.28	1.37	-0.06	0.85
FMA	28.79	3.84	28.46	4.42	0.33	0.75
SN-GoMe	37.33	4.65	38.67	4.95	-1.34	0.26
A to N-perp	-3.86	2.76	-0.72	2.78	-3.13	0.00**
A to FH	34.22	3.47	35.30	4.16	-1.09	0.13
B to N-perp	-13.10	5.29	-7.05	4.36	-6.06	0.00**
B to FH	82.57	5.94	72.98	6.80	9.59	0.00**
Pog to N-perp	-11.13	5.80	-8.16	5.52	-2.97	0.04**
FABA	79.15	3.67	80.50	3.60	-1.34	0.14
U1 to FH(°)	110.31	6.06	110.00	7.53	0.31	0.86
U1 to FH (mm)	62.90	4.42	57.26	5.60	5.65	0.00**
L1 to FH(°)	51.86	6.34	62.99	7.97	-11.13	0.00**
L1 to FH (mm)	60.23	4.34	55.08	5.67	5.14	0.00**
U1 C to N-perp	1.95	4.66	2.13	3.86	-0.18	0.86
U1 R to N-perp	-7.58	3.81	-5.64	3.00	-1.93	0.02**
L1 C to N-perp	-1.16	4.75	-1.00	3.75	-0.16	0.88
L1 R to N-perp	-15.23	5.36	-9.71	4.27	-5.52	0.00**
Interincisal angle	121.56	9.49	132.99	11.88	-11.44	0.00**
BOP	12.17	3.69	10.88	4.09	1.29	0.18
FOP	12.29	3.87	10.68	4.67	1.61	0.13
IMPA	99.34	7.39	88.54	7.54	10.80	0.00**
Independent t-tests were performed to compare variables between the two groups after treatment P < 0.1, *P < 0.05.
IMPA, incisor mandibular plane angle; ASO, anterior segmental osteotomy; SD, standard deviation; FH, Frankfort Horizontal

Skeletal differences post-treatment

Following treatment, significant skeletal differences were observed between the ASO and NASO groups in measurements such as A to N-perp, B to N-perp, B to FH, and Pog to N-perp (P < 0.05). Specifically, point A of the ASO group was positioned 3.13 mm further back than that of the NASO group after treatment (A to N-perp). Similarly, point B of the ASO group was positioned 6.06 mm further back than that of the NASO group after treatment (B to N-perp). Additionally, Pog to N-perp in the ASO group was positioned 2.97 mm further back than that in the NASO group after treatment. However, there were no significant differences between SNA, SNB, and ANB post-treatment. The ANB values of 3.22 and 3.28 in the ASO and NASO groups, respectively, implied that the proportion between the maxilla and mandible in the sagittal dimension was normalized in each group.

Dental differences post-treatment

After treatment, significant dental differences were noted between the ASO and NASO groups in measurements such as U1 to FH (mm), L1 to FH (°), L1 to FH (mm), U1 R to N-perp, L1 R to N-perp, interincisal angle, and IMPA (P < 0.05). After treatment, U1 R to N-perp and L1 R to N-perp in the ASO group were positioned 1.93 mm and 5.52 mm further back, respectively, compared to those in the NASO group. The interincisal angle was 11.44° smaller in the ASO group than in the NASO group after treatment. Furthermore, the IMPA was 10.80° greater in the ASO group than in the NASO group after treatment.

Comparison of pre- and post-treatment changes in the ASO and NASO groups (Table 5)

Table 5

Comparison of pre- and post-treatment changes between the ASO and NASO groups
	(T1 – T0)
Measurements	ΔASO Mean	Δ MI non-surgery Mean	Mean difference	SD difference	P
SNA diff	-4.13	-0.70	-3.43	0.34	0.00**
SNB diff	-1.87	-0.21	-1.66	0.23	0.00**
ANB diff	-2.26	-0.49	-1.77	0.35	0.00**
FMA diff	-0.72	-0.76	0.04	0.94	0.48
SN-GoMe diff	-0.80	-0.80	0.00	0.91	0.50
A to N-perp diff	-4.73	-0.84	-3.89	0.48	0.00**
A to FH diff	-1.55	0.65	-2.20	0.46	0.00**
B to N-perp diff	-3.76	-0.38	-3.38	0.62	0.00**
B to FH diff	-0.29	-0.18	-0.11	0.64	0.43
Pog to N-perp diff	-0.73	0.68	-1.41	0.83	0.048**
FABA diff	1.36	0.37	0.99	0.66	0.07
U1 to FH(°) diff	-4.50	-6.51	2.01	1.49	0.09*
U1 to FH (mm) diff	-0.60	-0.24	-0.36	0.56	0.27
L1 to FH(°) diff	6.18	11.16	-4.98	1.83	0.00**
L1 to FH (mm) diff	-1.08	-0.81	-0.27	0.55	0.31
U1 C to N-perp diff	-6.57	-4.11	-2.46	0.79	0.00**
U1 R to N-perp diff	-4.58	-1.45	-3.14	0.53	0.00**
L1 C to N-perp diff	-6.31	-3.87	-2.44	0.75	0.00**
L1 R to N-perp diff	-4.47	-0.22	-4.25	0.60	0.00**
Interincisal angle diff	10.69	17.67	-6.98	2.71	0.01**
BOP diff	0.98	0.58	0.40	0.58	0.25
FOP diff	0.54	-0.76	1.30	0.82	0.06*
U1 movement diff	6.76	5.80	0.97	0.62	0.06*
L1 movement diff	6.59	5.55	1.04	0.60	0.04**
IMPA diff	-5.46	-10.40	4.93	2.12	0.01**
Independent t-tests were performed to compare variables between Δ ASO and Δ MI non-surgery P < 0.1, *P < 0.05.
IMPA, incisor mandibular plane angle; ASO, anterior segmental osteotomy; SD, standard deviation; MI

To account for differences between the initial conditions of the ASO and NASO groups, we examined the pre- and post-treatment changes within each group. This analysis allowed us to evaluate the skeletal and dental effects based on the treatment method.

Skeletal changes pre- and post-treatment

The pre- and post-treatment changes in the ASO and NASO groups revealed significant differences in ΔSNA, ΔSNB, ΔANB, ΔA to N-perp, ΔB to N-perp, ΔPog to N-perp, and ΔA to FH (P < 0.05). While the reduction in ANB in the ASO group compared with that in the NASO group was clinically significant at 1.77, both groups exhibited normalized ANB values, indicating substantial skeletal improvement in Class II malocclusion. Similarly, there were significant differences between the two groups in the sagittal changes of points A and B, as measured by ΔA to N-perp and ΔB to N-perp (P < 0.05). The A-point moved 3.89 mm and the B-point moved 3.38 mm further back in the ASO group compared to that in the NASO group, suggesting that selective surgical retraction considering skeletal Class II was not achieved. Additionally, the difference in Pog to N-perp change between the ASO and NASO groups was − 1.41 mm.

Dental changes pre- and post-treatment

The pre- and post-treatment changes in the ASO and NASO groups exhibited significant differences in ΔL1 to FH (°), ΔU1 C to N-perp, ΔU1 R to N-perp, ΔL1 C to N-perp, ΔL1 R to N-perp, Δinterincisal angle, ΔL1 movement, and ΔIMPA (P < 0.05). Specifically, the change in the Frankfort Mandibular Incisor angle (FMIA) (ΔL1 to FH [°]) was 4.98° greater in the NASO group than in the ASO group (P < 0.05), and the change in IMPA was 4.93° greater in the NASO group than in the ASO group (P < 0.05). Likewise, the change in U1 to FH (°) was 2.01° greater in the NASO group than in the ASO group, although not statistically significant. There were significant differences between the two groups in the sagittal changes of the maxillary and mandibular incisors, as measured by ΔU1 C to N-perp, ΔU1 R to N-perp, ΔL1 C to N-perp, and ΔL1 R to N-perp (P < 0.01). In the ASO group, the crown of the maxillary incisors moved 2.46 mm and the root moved 3.14 mm further back compared to that in the NASO group. Additionally, the crown of the mandibular incisors moved 2.44 mm, and the root moved 4.25 mm further back. The change in the interincisal angle was 6.98° less in the ASO group than in the NASO group.

Tweed [9] emphasized the importance of the position of mandibular incisors in determining orthodontic treatment esthetics. However, despite being rooted in hard tissue analysis, such as teeth and alveolar bone, Tweed's diagnostic approach has been overshadowed by modern soft tissue-focused diagnostics. The present study investigated the importance of mandibular incisor position and inclination in Class II orthodontic patients' satisfaction with facial esthetics and occlusion, particularly after ASO and NASO treatments resulting in normalized ANB. Our findings revealed that the ASO group exhibited significantly greater IMPA values than those of the NASO group both before and after treatment. Additionally, mandibular incisor movement and IMPA changes were smaller in the ASO group than in the NASO group, suggesting that mandibular incisor inclination, camouflaged by severe skeletal disharmony pretreatment, persisted post-treatment despite surgical intervention. However, unlike two-jaw surgeries, ASO still preserves basal bone integrity, limiting the dentition’s camouflaged potential, as observed in this study. Table 3 shows that the pretreatment FMIA values were 45.68° for the ASO group and 51.84° for the NASO group. After treatment, while the ASO group’s FMIA was 51.87° (Δ6.19°), the NASO group’s FMIA was 63° (Δ11.16°). This suggests that if mandibular incisor inclination falls within the range where normal FMIA values can be achieved through tooth movement and inclination adjustments (Δ -10.40°) with extraction orthodontics, NASO may yield esthetically pleasing and stable treatment outcomes, obviating the need for ASO. Consequently, reevaluating the importance of the mandibular incisor inclination improvement in patients with Class II skeletal malocclusion while considering Tweed's diagnostic method underscores the significance of both initial mandibular incisor inclination and achievable improvement in labioversion of the mandibular incisors.

In a comparative study by Baek and Kim [3], orthodontic treatment was observed to be more effective than ASO, especially in cases where the upper teeth were inclined forward, the occlusal plane was less steep, the interlabial gap was large, and there was an acute lower nasolabial angle. Conversely, Lee et al [5] recommended ASO for cases with normal-range maxillary incisor inclination, relatively deficient chin growth, and a gummy smile. They also suggested orthodontic treatment rather than ASO in patients with a skeletal Class I relationship and proclined upper and lower incisors. These recommendations were based on the assumption that orthodontic treatment with extraction is preferable for cases with proclined maxillary or both maxillary and mandibular incisors, whereas ASO is suitable for cases with skeletal issues or normal-range maxillary incisor inclination. However, our results revealed no significant difference in maxillary incisor inclination between the ASO and NASO groups before treatment. Specifically, it was noted that during orthodontic treatment, maxillary incisors can move more easily within the alveolar bone compared to mandibular incisors due to the thicker medullary space of the palatal bone. Furthermore, advancements in orthodontic devices, such as TADs, have increased the scope of tooth movement in contemporary orthodontics.

While limited tooth movement has been attributed to NASO, Son et al [10] reported the high risk of palatal bone resorption in high-angle Class II patients with the normal inclination of the incisors and small medullary space in the palatal bone. On the other hand, Hong et al [11] cautioned against potential esthetic and bone thickness-related issues in the labial side during extraction orthodontic treatment. These studies highlighted the importance of performing extraction orthodontic treatment (NASO) within a range that avoids functional problems with the dentition and maxillary alveolar bone, emphasizing that orthodontic treatment alone should not be pushed beyond limits to achieve a desirable outcome. Table 5 indicates that maxillary incisor retraction, measured by the U1 C to N-perp value, was Δ -6.57 for ASO and Δ -4.11 for NASO. Similarly, for U1 R to N-perp, it was Δ -4.58 for the ASO group and Δ -1.45 for the NASO group. Thus, these findings suggest that NASO alone can provide sufficient movement with a root movement of Δ -1.45, achieving satisfactory results through bodily movement to controlled tipping of the body.

In contrast, previous studies have often overlooked the importance of considering the inclination of the mandibular incisors in ASO or NASO diagnosis and treatment planning. This might probably be due to the varying thickness of the symphysis in the mandibular incisor area, with generally less bone present in most cases compared to that in the maxillary incisor area, limiting root movement. Consequently, moving mandibular incisors with bodily movements is challenging, leading mostly to tipping movements [12]. Nonetheless, the advantage of mandibular ASO lies in its ability to move the mandibular incisors, including the roots, which is beneficial in Class III patients requiring mandibular incisor retraction. However, in Class II patients, where roots are already posteriorly positioned, there are fewer indications for moving the root backwards through ASO.

According to Son et al [10], even though the mandible is smaller in the anteroposterior dimension in Class II patients, the lower lip protrusion can occur due to lip incompetence or protruding maxillary incisors pushing the lower lip downward and forward. In such cases, reducing maxillary protrusion alone may be sufficient to secure lower lip retraction. However, mandibular extraction orthodontics or posterior movement of point B through ASO in Class II patients may lead to excessive retrusion of the lower lip, resulting in an unesthetic outcome [13]. Mandibular ASO may be indicated in cases where the IMPA is not large, but the mandibular incisors exhibit severe extrusion despite being upright, requiring bodily movement and intrusion, or when there are concerns about gingival recession. However, in mandibular ASO for Class II patients with a large IMPA, counterclockwise rotation of point B is often necessary, which can make surgical technique difficult and potentially lead to unsatisfactory outcomes [13]. The current study findings suggest that although ANB was normalized, increased selective retraction of point A than of point B was not observed in the ASO group, primarily due to technical difficulties in B-point-oriented counterclockwise rotation of the mandibular anterior segment.

Therefore, in Class II patients with a normal Spee curve in the mandibular incisors, where sufficient movement of the maxillary incisors through the ASO is expected to enhance facial esthetics, it is advisable to consider orthodontic treatment in the mandible to improve IMPA while maximizing the preservation of point B, rather than opting for mandibular ASO.

While this study did not include cases of ASO in the maxilla and lower full arch distalization, if the treatment plan requires retracting the maxillary incisor roots by approximately 4.6 mm (average in this study) and retroclining the mandibular incisors by approximately Δ -10.40° (average in this study), a suitable approach could involve ASO in the maxilla and total arch distalization using TADs in the mandible (Online Resource Fig. 1).

Despite its strength, one notable limitation of this study is that the pretreatment conditions of patients undergoing ASO and NASO did not completely align in terms of SNA, ANB, and IMPA. The ASO group exhibited a more pronounced skeletal Class II tendency in the pretreatment measurements. Therefore, we posit that with an increase in sample size and the inclusion of more borderline cases, a more comprehensive analysis of hard tissue changes in patients undergoing ASO compared to those undergoing NASO can be achieved.

Our findings revealed that the importance of the IMPA, as advocated by Tweed, should be considered in treatment planning. Notably, while advancements in orthodontic devices, such as TADs, have enabled significant movements of the maxillary incisors, limitations exist in moving the mandibular incisors due to anatomical constraints. Therefore, careful consideration is needed when opting to use ASO in Class II cases, given the posterior movement of point B, thus highlighting the significance of IMPA in treatment decision-making. In addition, even though the mandibular incisors typically require less movement, ASO on the maxilla combined with total arch distalization using TADs on the mandible should be considered.

Funding

The authors declare that they have no funding for this research article.

Conflict of Interest

The authors declare that they have no conflict of interest.

Research involving Human Participants and/or Animals

All human research procedures were followed in accordance with the ethical standards of the committee responsible for human experimentation (institutional and national), and with the Helsinki Declaration of 1975, as revised in 2013. This study was reviewed and approved by the Institutional Review Board (IRB) of Ajou University Hospital (IRB no.: AJIRB-MED-MDB-2022-354).

Informed Consent

Informed consent was obtained from all individual participants and/or their legal guardian(s).

Consent for publication

Not applicable

Data availability statements

The datasets generated during and/or analyzed during the current study are available from the corresponding author on reasonable request.

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No competing interests reported.

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Anterior segmental osteomy versus orthodontic camouflage treatment decision: The importance of incisor mandibular plane angle

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Abstract

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Materials and Methods

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Introduction

Materials and Methods

Study sample (Table 1)

Variables

Cephalometric measurement (Table 2)

Data collection techniques

Surgical procedures

Data analyses

Results

Pretreatment comparison between the ASO and NASO groups (Table 3)

Skeletal differences before treatment

Dental differences before treatment

Post-treatment comparison between the ASO and NASO groups (Table 4)

Skeletal differences post-treatment

Dental differences post-treatment

Comparison of pre- and post-treatment changes in the ASO and NASO groups (Table 5)

Skeletal changes pre- and post-treatment

Dental changes pre- and post-treatment

Discussion

Conclusions

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