Anterior deep overbite is considered a challenging malocclusion associated with periodontal conditions and temporomandibular joint problems.1,2 The orthodontic intrusion of the anterior teeth is a common treatment for resolving anterior deep overbite.3,4 The intrusion arch technique, as a mature method, has been widely used for orthodontic intrusion.5,6

Tooth movement can lead to alveolar bone remodeling and might cause alveolar bone loss in patients undergoing orthodontic treatment.7 More importantly, mandibular incisors are prone to dehiscence and fenestrations due to their buccal and lingual alveolar bone thickness being thinner than that of other sites.8 Moreover, concerns have been raised regarding the intrusion treatment of anterior mandible teeth as alveolar bone loss might occur easily. In addition, various factors may increase the risk of alveolar bone loss during the treatment procedure, including the magnitude of the orthodontic force, treatment duration, direction of tooth movement, tooth position, and craniomaxillofacial anatomy.8 This indicates that alveolar bone loss and its associated risk factors should be considered in the planning and prognosis of orthodontic treatments to minimize or avoid undesirable side effects. However, to the best of our knowledge, no study has evaluated the changes in the alveolar bone of the mandibular incisor in patients undergoing mandibular intrusion arch therapy.

Previous studies used periapical radiographs and lateral cephalograms to evaluate alveolar bone changes, which can only reflect two-dimensional bone dimension changes.9-11 With the development of cone-beam computed tomography (CBCT), three-dimensional visualization of the alveolar bone has become possible, allowing for an accurate quantitative assessment of alveolar bone variation, including alveolar bone height and bone area.

Thus, the primary purpose of this study was to use CBCT to evaluate dental, skeletal, and mandibular incisor-alveolar bone changes in patients undergoing arch intrusion therapy. The secondary purpose of this study was to explore the risk factors of alveolar bone loss to provide evidence for future clinical practice.

This retrospective study was approved by the Ethics Committee of the Affiliated Hospital of Stomatology, Zhejiang University School of Medicine, under protocol number 2021-048. Informed consent was obtained from all participants before the retrospective cohort study began. All participants were selected from patients who underwent mandibular intrusion arch therapy between January 2017 and December 2022 in the Department of Orthodontics at the Affiliated Hospital of Stomatology, Zhejiang University School of Medicine. Moreover, participant selection was based on the treatment time.

The inclusion criteria were (1) aged 18 to 30 years; (2) patients with skeletal Class II (4° ≤ A point-nasion-B point [ANB] ≤ 8°) and normodivergent growth pattern (28° ≤ sella-nasion plane to the mandibular plane angle [MP-SN] ≤ 36°); (3) patients with angle Class II, division 1; (4) mild mandibular anterior crowding (≤ 2 mm) and rotation; (5) patients with a deep curve of Spee (≥ 3 mm), and (6) absence of orthodontic treatment history. The exclusion criteria were as follows: (1) presence of periodontitis at the beginning of orthodontic treatment, (2) medical history related to bone metabolism diseases, (3) previous trauma in the region of the mandibular incisors, (4) cleft palate or other congenital anomalies, and (5) unavailable CBCT or lateral cephalogram data.

Thirty patients treated with mandibular intrusion arch therapy were enrolled in the study based on the detailed inclusion and exclusion criteria. All participants were treated by experienced clinicians. The patients received orthodontic treatment with self-ligating brackets and a 0.022 × 0.025-in slot using a McLaughlin, Bennett, and Trevisi prescription (Quick, Forestadent, Pforzheim, Germany). The archwire sequence was 0.014-in, 0.016-in, 0.016 × 0.022-in, and 0.018 × 0.025-in the nickel-titanium wires. Finally, a 0.019 × 0.025-in stainless steel wire was used as the working wire.

The intrusion arch, fabricated from a 0.016 × 0.022-in stainless-steel wire, was used as an auxiliary wire placed in the mandibular arch during alignment and leveling (Figure 1). The incisal segment of the intrusion arch was ligated to the main wire between the lateral incisors and canines. The molar segment of the wire was placed in the auxiliary tube of the first molar without any torque or backbend. A force between 80 and 100 g was delivered by the intrusion arch, which was checked every 4–6 weeks. Once the curve of Spee was flattened, signifying that the incisors were aligned with the mesiobuccal cusps of the mandibular first molars referring to Aydoğdu and Özsoy,12 the intrusion arch was removed.

Figure 1. Intrusion arch therapy of mandibular incisors. A-C, Pretreatment, D-F, posttreatment.

Basic characteristics (age and sex) and clinical treatment of the patients were collected using an electronic system. Lateral cephalograms were obtained using an Orthoceph device (OP200; Tianyin, Shanghai, China) before treatment (T1) and immediately after intrusion arch removal (T2). The imaging parameters were set at 85 kVp, 13 mA (pulse mode), and a 12 seconds scan time. Additionally, CBCT images were obtained using a NewTom machine (VG, QR srl; Verona, Italy) at T1 and T2. The imaging parameters were set at 110 kVp, 1–20 mA (pulse mode), 20 seconds scan time, 15 × 15 cm field of view, and 0.3 mm isotropic voxel size. The obtained data were exported in Digital Imaging and Communications in Medicine format and imported into the Dolphin Imaging software (version 11.9; Dolphin Imaging and Management Solutions, Chatsworth, CA, USA) for subsequent data processing.

The landmarks used for cephalometric analysis at T1 and T2 are displayed in Figure 2. The following parameters were measured for cephalometric analysis: (1) sella-nasion-point A angle, (2) sella-nasion-point B angle, (3) ANB, (4) MP-SN, the angle between SN plane and the mandibular plane, (5) overbite, the vertical distance between the upper and lower incisor, (6) overjet, the horizontal distance between the front upper and lower teeth, (7) mandibular incisor to mandibular plane angle (IMPA), the angle between the lower incisor axis and mandibular plane, (8) L_CR-MP, the vertical distance between the center of resistance of lower incisor and the mandibular plane.13

Figure 2. Cephalometric measurements. A, Landmarks of lateral cephalometry are used in this study. B, Illustration of the L_CR-MP (representing true intrusion) and IMPA (representing labial tipping).
SN, sella-nasion plane; Me, menton; Go, gonion; SNA, sella-nasion-point A angle; SNB, sella-nasion-point B angle; ANB, point A-nasion-point B angle; MP-SN, sella-nasion plane to the mandibular plane angle; IMPA, mandibular incisor to mandibular plane angle; L1-Cres, center of resistance of lower incisor; L_CR-MP, the vertival distance between L1-Cres and MP.

Vertical alveolar bone level (ABL) and alveolar bone area (ABA) were measured in the sagittal and axial sections of the CBCT images, respectively. The line through the root apex point and the midpoint of the labial and lingual cementoenamel junction (CEJ) was defined as the root axis. Before measurement, the CBCT image was adjusted to maintain the sagittal plane passing through the root axis (Figure 3). The following parameters were measured to evaluate alveolar bone changes in the incisal region: (1) Labial ABL (LaABL), the vertical distance from CEJ to the most coronal point of the alveolar bone crest on the labial side, (2) Lingual ABL (LiABL), the vertical distance from CEJ to the most coronal point of the alveolar bone crest on the lingual side, (3) Labial ABA at 3, 6, 9 mm (LaABA-3, 6, 9), the labial alveolar bone cross-sectional area bounded by the labial alveolar bone margin and the labial root margin at the level of 3, 6, and 9 mm from CEJ, (4) Lingual ABA at 3, 6, 9 mm (LiABA-3, 6, 9), the lingual alveolar bone cross-sectional area bounded by the lingual alveolar bone margin and the lingual root margin at the level of 3, 6, and 9 mm from CEJ (Figure 4).

Figure 3. Standardized orientation of cone-bean computed tomography for measurement.

Figure 4. Measurements of vertical alveolar bone levels (ABLs) and alveolar bone cross-sectional area. A, The yellow points represent the cemento-enamel junction (CEJ), and the green points represent the alveolar bone crest. The labial (La) and lingual (Li) vertical ABLs are defined as LaABL and LiABL, respectively. B, The three levels were separated by 3, 6, and 9 mm from the CEJ. C, LaABA-3 mm (6 mm, 9 mm) and LiABA-3 mm (6 mm, 9 mm) represent the labial and lingual alveolar bone cross-sectional area at 3 mm (6 mm, 9 mm) level, respectively.
ABA, alveolar bone area.

Statistical analysis

All imaging data were measured twice by a trained examiner at intervals of 4 weeks. Intraclass correlation coefficients (ICCs) were used for test-retest reliability and demonstrated acceptable agreement (ICCs for all measurements were > 0.85). Quantitative data are presented as mean ± standard deviation, while qualitative data are presented as percentages. The Shapiro-Wilk test was used to evaluate the normality of the quantitative data, and the results displayed that all quantitative data were normally distributed. A paired t test was used to analyze the differences between T1 and T2 data. Pearson’s and Spearman’s correlations were used to assess the relationships between alveolar bone loss and various factors. Statistical analyses were performed using SPSS Statistics version 23.0 (IBM Corp., Armonk, NY, USA). The significance level was set at P < 0.05. Power analysis conducted using G*Power software (version 3.1.9.7) indicated that a sample size of 30 would provide more than 95% power to detect an effect size of 0.81 (calculated using the change in IMPA from T1 to T2) at a significance level of 0.05.

As is displayed in Table 1, 30 patients with skeletal Class II were finally included in this study, consisting of 11 males (36.7%) and 19 females (63.3%) with a mean age of 23.85 ± 3.98 years. The average duration of intrusion treatment was 6.18 ± 2.31 months. Before the orthodontic procedure, 15 patients (50%) underwent the extraction of mandibular premolars, while the others did not.

Table 1 . Basic characteristics of patients included in this study.

Characteristics	Value
Age (yr)	23.85 ± 3.98
Sex
Male	11 (36.7)
Female	19 (63.3)
Treatment duration (mo)	6.18 ± 2.31
Tooth extraction
Bilateral mandible first premolar teeth extraction	15 (50.0)
Non-extraction	15 (50.0)

Values are presented as mean ± standard deviation or number (%)..

As is displayed in Table 2, IMPA was increased by 5.96° ± 7.35° after intrusion treatment (P < 0.05), representing the mandibular incisor labial proclination. Although not statistically significant, L_CR-MP decreased by 1.57 mm. Furthermore, MP-SN underwent a significant increase of 0.55° ± 0.90° (P < 0.05). A statistically significant decrease was observed in the overjet and overbite (P < 0.05).

Table 2 . Comparison of the dental and skeletal measurements between T1 and T2.

Measurement	T1	T2	T2-T1	P value
SNA (°)	83.18 ± 3.11	83.51 ± 3.08	0.33 ± 0.97	0.073
SNB (°)	78.05 ± 3.08	77.73 ± 2.90	–0.32 ± 0.96	0.078
ANB (°)	5.12 ± 2.33	5.59 ± 2.55	0.47 ± 1.04	0.124
MP-SN (°)	32.24 ± 6.48	32.80 ± 6.43	0.55 ± 0.90	< 0.001***
IMPA (°)	97.59 ± 7.41	103.55 ± 7.80	5.96 ± 7.35	< 0.001***
LCR-MP (mm)	34.27 ± 4.16	32.77 ± 3.87	–1.51 ± 1.05	0.154
Overjet (mm)	5.46 ± 2.22	3.86 ± 1.35	–1.60 ± 2.12	< 0.001***
Overbite (mm)	4.16 ± 1.93	1.13 ± 1.02	–3.03 ± 1.89	< 0.001***

Values are presented as mean ± standard deviation..

SNA, sella-nasion-point A angle; SNB, sella-nasion-point B angle; ANB, point A-nasion-point B angle; MP-SN, sella-nasion plane to the mandibular plane angle; IMPA, mandibular incisor to mandibular plane angle; L_CR-MP, the vertival distance between L1-Cres and MP; L1-Cres, center of resistance of lower incisor..

***P < 0.001..

As no significant difference was identified between the left and right sides, the average value for both sides was used for the analysis of alveolar bone changes. As demonstrated in Table 3, the labial and lingual vertical ABL of the mandibular incisors exhibited a noteworthy change (P < 0.05), indicating significant loss of vertical alveolar bone in the region of mandibular incisors after intrusion treatment. Regarding labial ABA changes, a significant decrease was noted in LaABA-3 in the mandibular incisors from T1 to T2 (P < 0.05), while a significant increase of LaABA-6 and LaABA-9 was observed in the mandibular incisors region (P < 0.05). Regarding lingual ABA changes, LiABA-6 and LiABA-9 levels in the mandibular incisors demonstrated a significant decrease (P < 0.05). Overall, LaABL, LiABL, LaABA-3, LiABA-6, and LiABA-9 represent alveolar bone loss, which could be used to analyze the risk factors associated with bone loss.

Table 3 . Comparison of the alveolar bone dimensions between T1 and T2.

Measurement	T1	T2	T2-T1	P value
Central incisor
LaABL (mm)	1.86 ± 0.84	3.40 ± 2.49	1.54 ± 2.49	< 0.001***
LiABL (mm)	1.96 ± 1.24	3.45 ± 2.65	1.49 ± 2.96	< 0.001***
LaABA-3 (mm2)	2.84 ± 1.19	2.14 ± 1.10	–0.70 ± 1.01	< 0.001***
LaABA-6 (mm2)	1.95 ± 0.92	2.70 ± 1.54	0.74 ± 1.43	< 0.001***
LaABA-9 (mm2)	2.58 ± 1.17	4.06 ± 2.41	1.48 ± 2.21	< 0.001***
LiABA-3 (mm2)	3.63 ± 1.22	3.33 ± 1.77	–0.30 ± 1.52	0.136
LiABA-6 (mm2)	4.24 ± 1.76	3.44 ± 1.98	–0.80 ± 1.51	< 0.001***
LiABA-9 (mm2)	4.29 ± 1.57	3.17 ± 1.91	–1.13 ± 1.56	< 0.001***
Lateral incisor
LaABL (mm)	1.94 ± 1.01	4.32 ± 2.99	2.38 ± 3.15	< 0.001***
LiABL (mm)	1.86 ± 0.51	2.69 ± 1.92	0.82 ± 1.92	< 0.001***
LaABA-3 (mm2)	3.06 ± 1.29	1.98 ± 1.24	–1.10 ± 1.30	< 0.001***
LaABA-6 (mm2)	1.65 ± 0.94	2.09 ± 1.31	0.38 ± 1.45	0.021*
LaABA-9 (mm2)	2.24 ± 0.87	4.07 ± 2.70	1.96 ± 2.44	< 0.001***
LiABA-3 (mm2)	4.40 ± 1.70	5.40 ± 5.17	0.94 ± 5.16	0.137
LiABA-6 (mm2)	5.49 ± 2.03	4.90 ± 2.50	–0.72 ± 1.78	0.011*
LiABA-9 (mm2)	5.20 ± 2.25	4.18 ± 2.33	–1.19 ± 2.11	< 0.001***

Values are presented as mean ± standard deviation..

ABL, alveolar bone level; LaABL, labial ABL; LiABL, lingual ABL; ABA, alveolar bone area; LaABA, labial ABA; LiABA, lingual ABA..

*P < 0.05, ***P < 0.001..

Table 4 displays the results of the correlation analysis for identifying factors influencing IMPA, IMPA change (ΔIMPA), and L_CR-MP change (ΔL_CR-MP), and detecting risk factors affecting alveolar bone loss. Pearson’s correlation tests exhibited a significant positive correlation between the IMPA and ANB (r = 0.381, P < 0.05) and MP-SN (r = 0.502, P < 0.05). However, the tests identified a significant negative correlation between ΔIMPA and ANB (r = –0.466, P < 0.05) and MP-SN (r = –0.373, P < 0.05). Spearman’s correlation tests demonstrated that ΔIMPA was significantly negatively correlated with extraction treatment (r = –0.318, P < 0.05). With the increase in treatment duration, ΔIMPA was also increased during the intrusion treatment (r = 0.459, P < 0.05). In addition, a significant negative correlation between IMPA and ΔIMPA was noted (r = –0.442, P < 0.05).

Table 4 . Correlation analysis between variables and IMPA, ΔIMPA, ΔL_CR-MP, and parameters representing alveolar bone loss.

	Sex			Extraction			Age			Duration			ANB			MP-SN			IMPA			ΔIMPA			ΔLcr-MP
	r	P		r	P		r	P		r	P		r	P		r	P		r	P		r	P		r	P
IMPA	0.159	0.400		0.117	0.349		–0.136	0.475		–0.039	0.838		0.381	0.038*		0.502	0.005**		-	-		–0.442	0.014*		–0.274	0.143
ΔIMPA	0.096	0.615		–0.318	0.038*		0.249	0.184		0.459	0.011*		–0.466	0.009**		–0.373	0.042*		–0.442	0.014*		-	-		0.142	0.455
ΔLCR-MP	0.068	0.720		0.027	0.887		–0.042	0.825		0.360	0.051		0.261	0.164		–0.063	0.741		0.134	0.100		0.142	0.455		-	-
Central incisor
ΔLaABL	0.332	0.072		0.027	0.888		–0.255	0.172		0.135	0.478		0.051	0.791		–0.078	0.680		0.122	0.521		0.003	0.987		0.112	0.557
ΔLiABL	–0.118	0.533		–0.312	0.034*		0.006	0.974		–0.064	0.735		0.236	0.219		0.209	0.118		0.024	0.898		–0.104	0.585		–0.064	0.736
ΔLaABA-3	0.212	0.081		–0.216	0.175		0.124	0.956		–0.150	0.689		0.062	0.404		0.055	0.818		0.126	0.616		–0.137	0.609		–0.282	0.121
ΔLiABA-6	0.318	0.064		0.127	0.053		–0.078	0.681		0.048	0.799		0.142	0.451		0.073	0.700		0.110	0.563		-0.097	0.611		0.116	0.542
ΔLiABA-9	0.387	0.101		0.366	0.041*		–0.325	0.080		–0.071	0.708		0.375	0.041*		0.185	0.328		0.234	0.035*		–0.489	0.006**		–0.283	0.282
Lateral incisor
ΔLaABL	0.332	0.073		–0.166	0.382		–0.154	0.416		0.083	0.663		0.121	0.524		–0.215	0.254		0.055	0.773		0.095	0.617		0.158	0.405
ΔLiABL	0.287	0.124		–0.119	0.530		0.147	0.437		–0.208	0.271		0.224	0.233		0.209	0.267		0.208	0.271		–0.169	0.371		0.063	0.741
ΔLaABA-3	0.215	0.756		0.208	0.371		–0.185	0.282		–0.145	0.582		0.187	0.974		0.245	0.295		0.169	0.757		–0.450	0.013*		–0.159	0.239
ΔLiABA-6	0.196	0.300		0.220	0.244		–0.056	0.768		–0.050	0.793		0.075	0.692		0.068	0.721		0.019	0.922		–0.132	0.485		–0.112	0.556
ΔLiABA-9	0.191	0.311		0.100	0.598		–0.327	0.078		0.003	0.989		–0.061	0.749		0.006	0.974		0.047	0.804		–0.124	0.241		0.013	0.947

ANB, point A-nasion-point B angle; MP-SN, sella-nasion plane to the mandibular plane angle; IMPA, mandibular incisor to mandibular plane angle; ΔIMPA, the change of IMPA between T1 and T2; ΔL_CR-MP, the change of L_cr-MP between T1 and T2; ΔLaABL, the change of labial alveolar bone level between T1 and T2; ΔLiABL, the change of lingual alveolar bone level between T1 and T2; ΔLaABA-3, 6, 9, the change of LaABA-3, 6, 9 between T1 and T2; –, not available..

*P < 0.05, **P < 0.01..

Furthermore, non-extraction treatment was a significant risk factor for alveolar bone loss, including LiABL and LiABA-9 in the central incisors (r = –0.312, P < 0.05; r = 0.366, P < 0.05, respectively). Moreover, ΔIMPA was also identified as a risk factor for alveolar bone loss during the treatment, including ΔLiABA-9 of the central incisor and ΔLaABA-3 of the lateral incisor (r = –0.489, P < 0.05; r = –0.450, P < 0.05, respectively). Additionally, ANB and IMPA were significantly positively associated with ΔLiABA-9 of the central incisor.

This retrospective study evaluated the treatment outcome of the intrusion arch in patients with skeletal Class II and deep overbite, with an emphasis on the changes in the mandibular incisor dental and alveolar bone. To the best of our knowledge, this is the first study to report three-dimensional alveolar bone changes and explore the associated risk factors for alveolar bone loss after intrusion arch therapy.

Intrusion of the incisors is one of the main orthodontic mechanisms that correct deep overbites. According to the movement pattern of incisors, the intrusion of incisors can be divided into true intrusion and labial tipping.14 To accurately quantify the true incisor intrusion, the decrease of perpendicular distance from the center of resistance to the mandibular plane (ΔL_CR-MP) was defined as the true intrusion, as the center of resistance was rarely affected by axis inclination.15 The increase in the angle between the lower incisor axis and mandibular plane (ΔIMPA) represented the labial tipping.

The average correction of overbite following anterior intrusion treatment was 3.03 ± 1.89 mm, signifying the effectiveness of the intrusion arch in addressing deep overbites. However, since the force of the intrusion arch is always transmitted from the labial direction to the center of resistance, the incisor intrusion and the labial tipping may occur simultaneously.16 The results of this study demonstrated that the inclination of the mandibular incisor increased by 5.96°, which was consistent with the inclination observed in previous studies.12,17 In our study, the true incisor intrusion was 1.51 ± 1.05 mm, with no statistical significance. This may be attributed to the fact that the actual tooth displacement patterns were flaring rather than a true intrusion. Previous study has claimed that conventional intrusion arches could achieve incisor intrusion of 1.5 to 2.7 mm.18 Shakti et al.19 reported incisor intrusion of 1.4 mm (0.35 mm/month) in the Connecticut intrusion arch group and 1.66 mm (0.415 mm/month) in Burstone’s three-piece intrusion arch. Varlık et al.20 revealed that a utility arch could achieve a mandibular incisor intrusion of 2.6 ± 1.4 mm. The inconsistency among different studies might be attributed to the inclusion criteria, appliance design, force dimension, and treatment duration.

Furthermore, gaining insight into the periodontal response to orthodontic intrusion therapy is also important. This study demonstrated that the labial and lingual vertical ABLs were significantly decreased, indicating that intrusion treatment resulted in vertical alveolar bone loss. Furthermore, this study demonstrated that the cervical labial ABA decreased significantly after intrusion therapy, suggesting that the intrusion treatment leads to marginal alveolar bone loss. However, the change in the alveolar bone may be a result of flaring rather than intrusion. The bone is normally thin in the labial crestal area and thick in the labial apical area. The above-mentioned results confirm that tooth movement is always accompanied by a change in the supporting alveolar bone. Atik et al.21 evaluated the effect of mini-screws on alveolar bone changes in maxillary incisors and discovered a significant reduction in labial alveolar bone thickness. Bayani et al.22 suggested that molar intrusion induces significant bone resorption. These findings indicate that alveolar bone remodeling should be considered when diagnosing and planning tooth movement, to minimize undesired side effects.

The present study demonstrated a positive correlation between IMPA and ANB (r = 0.381, P < 0.05) and between IMPA and MP-SN (r = 0.502, P < 0.05) at baseline. These findings differed from those reported in previous studies.23-25 Estrella et al.23 reported lower incisor retroinclination in patients with Class III and lower incisor proinclination in patients with Class II. However, a negative correlation was observed between ΔIMPA and ANB (r = –0.466, P < 0.05) and between ΔIMPA and MP-SN (r = –0.373, P < 0.05). This corroborated the findings of a previous study26 which demonstrated a significant increase in IMPA in low-angle individuals (ΔIMPA, 6.58°), compared to the high-angle (ΔIMPA, 0.48°) and normal-angle (ΔIMPA, 1.64°) individuals during the alignment and leveling.

Moreover, the results demonstrated that incisor inclination and changes in the alveolar bone were statistically correlated. With the increase in incisor inclination (ΔIMPA) during the intrusion treatment, the labial alveolar bone loss at the crestal level (ΔLaABA-3) and the lingual alveolar bone loss at the apical level (ΔLiABA-9) were significantly increased. Labial tipping has been acknowledged to increase alveolar bone loss, which is consistent with the results of the present study.27,28 In addition, the decision to extract premolars also affects the remodeling of the alveolar bone during intrusion treatment. In this study, a significantly greater loss of the lingual vertical alveolar bone (ΔLiABL) was observed in the non-extraction group than in the extraction group. As reported by Chung et al.,29 extraction treatment might reduce alveolar bone loss compared to non-extraction treatment. This difference is primarily attributed to the necessity of achieving a curve of Spee leveling in non-extraction patients through incisor labial tipping. As uncontrolled labial tipping of the incisors is not desired, measures such as cinch back, extraction treatment, and lace-back should be employed to reduce the degree of labial inclination, thereby avoiding unexpected alveolar bone loss.30,31

This study also demonstrates a correlation between ANB and alveolar bone changes, which may be attributed to the relationship between IMPA and ANB. Previous studies have reported that the mandibular anterior alveolar bone is thick in patients with skeletal Class II and hypodivergence.32,33 Hoang et al.34 discovered a negative relationship between a decrease in alveolar bone thickness and an increase in SN-MP. Therefore, patients with high-angle undergoing orthodontic treatment are at a high risk of alveolar bone loss.

This study has a few limitations. First, the observation time points were set before treatment and immediately after intrusion arch removal, which does not reflect the long-term effects of intrusion treatment on alveolar bone changes. As per a general agreement, alveolar bone remodeling takes 6 months or longer to stabilize in response to orthodontic treatment. Second, the sample size of this study was limited, which may impact the robustness of the conclusions. Third, the retrospective nature of this study resulted in undesired bias and confounding factors compared to prospective studies, especially in the control of extraction and non-extraction patient selection. Fourth, the lack of a control group prevents an evaluation of the specific impact of the intrusion arch in isolation from other orthodontic interventions. Therefore, a randomized controlled trial with a large sample size and long follow-up period is a future direction for the present study. Moreover, we explored the effects of extraction patterns on intrusion arch therapy.

While the mandibular intrusion arch effectively corrected the deep overbite, it did cause some unwanted incisor labial tipping/flaring. No statistically significant intrusion was observed in the incisors, which could be attributed to the fact that the actual tooth displacement patterns were flaring, rather than true intrusion. During intrusion treatment, patients experienced vertical alveolar bone loss, labial horizontal bone loss at the crestal level, and lingual horizontal bone loss at the apical level, accompanied by an increase in the labial horizontal bone at the apical level. An increase in incisor inclination during intrusion treatment can lead to increased alveolar bone loss at the labial crestal and lingual apical sites. In addition, non-extraction treatment and increased ANB have been identified as risk factors for alveolar bone loss in patients undergoing mandibular intrusion therapy.

This work was supported by the Fundamental Research Funds for the Central Universities (grant number 2021FZZX005-36), National Natural Science Foundation of China (grant number 82271008), and China Oral Health Foundation (grant numbers A2021-090 and A2023-03).

Conceptualization: LL, JS. Data curation: JS. Formal analysis: JS. Investigation: JS, ZW. Methodology: LL, XC, JS. Project administration: LL, XC. Supervision: JS, ZW. Validation: LL. Writing–original draft: JS. Writing–review & editing: all authors.

No potential conflict of interest relevant to this article was reported.