Korean J Orthod 2025; 55(1): 26-36 https://doi.org/10.4041/kjod24.136
First Published Date October 11, 2024, Publication Date January 25, 2025
Copyright © The Korean Association of Orthodontists.
Zhizun Wanga , Li Meib
, Zhenxing Tangc
, Dong Wud
, Yue Zhoua
, Ehab A. Abdulghania,e
, Yuan Lia
, Wei Zhengf
, Yu Lia
aState Key Laboratory of Oral Diseases, National Center for Stomatology, National Clinical Research Center for Oral Diseases, Department of Orthodontics, West China Hospital of Stomatology, Sichuan University, Chengdu, China
bDiscipline of Orthodontics, Department of Oral Sciences, University of Otago, Dunedin, New Zealand
cDepartment of Stomatology, Chengdu Seventh People’s Hospital (Affiliated Cancer Hospital of Chengdu Medical College), Chengdu, China
dSection of Preventive and Public Health Dentistry, Division of Oral Health, Growth and Development, Kyushu University, Fukuoka, Japan
eDepartment of Orthodontics and Dentofacial Orthopedics, College of Dentistry, Thamar University, Dhamar, Yemen
fState Key Laboratory of Oral Diseases, National Center for Stomatology, National Clinical Research Center for Oral Diseases, Department of Oral and Maxillofacial Surgery, West China Hospital of Stomatology, Sichuan University, Chengdu, China
Correspondence to:Wei Zheng.
Associate Professor, State Key Laboratory of Oral Diseases, National Center for Stomatology, National Clinical Research Center for Oral Diseases, Department of Oral and Maxillofacial Surgery, West China Hospital of Stomatology, Sichuan University, No.14, the 3rd Section, South Ren Min Rd., Chengdu 610041, China.
Tel +86-28-85503551 e-mail zhengwei81101@163.com
Yu Li.
Professor, State Key Laboratory of Oral Diseases, National Center for Stomatology, National Clinical Research Center for Oral Diseases, Department of Orthodontics, West China Hospital of Stomatology, Sichuan University, No.14, the 3rd Section, South Ren Min Rd., Chengdu 610041, China.
Tel +86-28-85503645 e-mail yuli@scu.edu.cn
How to cite this article: Wang Z, Mei L, Tang Z, Wu D, Zhou Y, Abdulghani EA, Li Y, Zheng W, Li Y. Combined anterior and posterior miniscrews increase apical root resorption of maxillary incisors in protrusion and premolar extraction cases. Korean J Orthod 2025;55(1):26-36. https://doi.org/10.4041/kjod24.136
Objective: Miniscrews are commonly utilized as temporary anchorage devices (TADs) in cases of maxillary protrusion and premolar extraction. This study aimed to investigate the effects and potential side effects of two conventional miniscrew configurations on the maxillary incisors. Methods: Eighty-two adult patients with maxillary dentoalveolar protrusion who had undergone bilateral first premolar extraction were retrospectively divided into three groups: non-TAD, two posterior miniscrews only (P-TADs), and two anterior and two posterior miniscrews combined (AP-TADs). Cone-beam computed tomography was used to evaluate the maxillary central incisors (U1). Results: The AP-TADs group had significantly greater U1 intrusion (1.99 ± 2.37 mm, n = 50) and less retroclination (1.70° ± 8.80°) compared to the P-TADs (–0.07 ± 1.65 mm and 9.45° ± 10.68°, n = 60) and non-TAD group (0.30 ± 1.61 mm and 1.91° ± 9.39°, n = 54). However, the AP-TADs group suffered from significantly greater apical root resorption (ARR) of U1 (2.69 ± 1.38 mm) than the P-TADs (1.63 ± 1.46 mm) and non-TAD group (0.89 ± 0.97 mm). Notably, the incidence of grade IV ARR was 16.6% in the AP-TADs group, significantly higher than the rates observed in the P-TADs (6.7%) and non-TAD (1.9%) groups. Multiple regression analysis revealed that after excluding tooth movement factors, the AP-TADs configuration resulted in an additional 0.5 mm of ARR compared with the P-TADs group. Conclusions: In cases of maxillary protrusion and premolar extraction, the use of combined anterior and posterior miniscrews enhances incisor intrusion and minimizes torque loss of the maxillary incisors. However, this approach results in more severe ARR, likely due to the increased apical movement and composite force exerted.
Keywords: Temporary anchorage device, Apical root resorption, Incisor, Orthodontic extraction
The treatment of maxillary dentoalveolar protrusions generally involves premolar extraction followed by anterior teeth retraction, with temporary anchorage devices (TADs) playing crucial roles. Posterior miniscrews placed in the buccal region reinforce the sagittal anchorage to enhance incisor retraction, while anterior miniscrews positioned in the labial region are used to intrude the maxillary incisors and prevent bite deepening.1 Consequently, the implementation of miniscrews generally falls into two configurations: posterior miniscrews alone or a combination of anterior and posterior miniscrews.2-5
The location of TADs has a biomechanically intuitive impact on tooth movement. In non-extraction therapy, anterior TADs are effective for incisor intrusion and labial crown torquing.6-8 However, in extraction cases, posterior TADs retract the anterior segment with a force directed below the center of resistance, resulting in a significant loss of incisor torque.9 A recent study demonstrated that a single maxillary midline miniscrew can control incisor torque and overbite control in moderate anchorage cases.1 Another study implemented mini-implants at both the anterior and posterior maxilla, resulting in incisor intrusion; however, labiopalatal angulation has not been reported.2 Unlike posterior miniscrews, the effects of anterior miniscrews in protrusion and extraction have primarily been documented in case reports3,10 rather than controlled studies.
Orthodontists are frequently reminded of the dental and periodontal adverse effects of tooth movement. Retraction forces cause moderate apical root resorption (ARR) of approximately 2 mm in maxillary incisors, irrespective of the use of posterior miniscrews.11,12 A recent systematic review concluded that intrusive mechanics results in ARR of 0.72 mm in incisors and 0.41 mm in molars.13 A clinical trial reported that 57% of retracted incisors exhibit root flattening, while 42% demonstrated blunting of the root apex.1 Regarding periodontal outcomes, anterior retraction with maximum anchorage led to a reduction in the palatal alveolar bone thickness at the cervical and middle thirds of the incisor roots.14 In terms of endodontic effects, anterior retraction with moderate anchorage has been associated with a significant reduction in total pulpal volume attributed to tertiary dentin formation.15
Thus, the present study aimed to compare two conventional miniscrew configurations–posterior miniscrews alone versus combined anterior and posterior miniscrews–regarding their effects and side effects on the maxillary incisors in cases of maxillary protrusion and premolar extraction.
This was a retrospective study approved by the Ethics Committee of West China Hospital of Stomatology, Sichuan University (WCHSIRBCT-2023-113). Exemption from obtaining informed consent was granted. A total of 527 patients who underwent fixed orthodontic therapy by the same orthodontist between 2018 and 2023 were screened for eligibility. The inclusion criteria were as follows: adult patients (over 18 years old), maxillary dentoalveolar protrusion, maxillary arch crowding ≤ 4 mm, 0° ≤ ANB ≤ 6°; extraction of bilateral maxillary first premolars, and treatment with no miniscrew (the non-TAD group), two posterior miniscrews only (the P-TADs group), or two anterior and two posterior miniscrews combined (the AP-TADs group) (Figure 1). The posterior miniscrews were placed either mesially or distally to the maxillary first molars to reinforce the posterior anchorage, whereas the anterior miniscrews were positioned either mesially or distally to the maxillary lateral incisors to resolve the deep bite. The exclusion criteria were as follows: periodontitis, prior pulpal treatment or pre-treatment root resorption of the maxillary incisors, and treatment involving other auxiliary devices such as trans-palatal arch, Nance arch, headgear, and torquing springs. Based on the criteria, 82 patients were recruited for the study. According to recent studies on incisor intrusion,16,17 a sample size of 22 per group would provide an 80% probability of detecting a moderate difference in incisor intrusion between labial TADs and conventional methods, with a 95% confidence level, using a priori ANOVA model. Baseline demographic characteristics were comparable across the three groups (Table 1).
Table 1 . Demographics of the Non-TAD, P-TADs, and AP-TADs groups
Non-TAD (n = 27) | P-TADs (n = 30) | AP-TADs (n = 25) | |
---|---|---|---|
Sex | |||
Female | 22 | 27 | 24 |
Male | 5 | 3 | 1 |
Angle classification | |||
Class I | 19 | 18 | 12 |
Class II | 8 | 12 | 13 |
ANB (°) | 2.8 ± 2.6 | 3.8 ± 2.1 | 3.8 ± 2.1 |
Age (yr) | 23.5 ± 5.6 | 23.5 ± 4.2 | 24.4 ± 4.9 |
Treatment duration (mo) | 32.6 ± 11.5 | 36.2 ± 11.9 | 39.4 ± 13.1 |
Values are presented as number only or mean ± standard deviation.
TAD, temporary anchorage device; P-TADs, two posterior miniscrews only; AP-TADs, two anterior and two posterior miniscrews combined.
All patients were treated using a 0.018 × 0.025- inch stainless steel archwire in a 0.022 inch slot metal bracket system (Damon Q, Ormco, Brea, CA, USA). Two proprietary miniscrews (VectorTAS 601-0022, Ormco, Orange, CA, USA; AbsoAnchor AX12-108, Dentos Inc., Daegu, Korea) were used interchangeably based on stock availability. Power chains (G&H Orthodontics, Franklin, IN, USA) were routinely applied for anterior retraction and incisor intrusion. The retraction force was estimated at the chairside to be approximately 180 cN per side, whereas the intrusion force was approximately 100 cN per side.
Cone-beam computed tomography (CBCT) scans were obtained using a 3D Accuitomo (Morita MFG Corp., Kyoto, Japan) according to the manufacturer’s specifications (14 × 10 cm FOV, 85 kV, 4 mA, 360° rotation). The DICOM images were imported into Mimics 21.0 (Materialise, Leuven, Belgium) for 3D landmark registration. The Frankfurt Horizontal plane was defined by the bilateral orbitale and right porion. The mid-sagittal plane (MSP) passes through the ANS and PNS perpendicular to the transverse plane. The coronal plane passes through the PNS perpendicular to the ANS-PNS line, and the palatal plane passes through the ANS and PNS perpendicular to the MSP (Figure 2A). All dental and skeletal landmarks were projected onto MSP (Figure 2B).
The linear and angular measurements are presented in Table 2. Treatment changes, including U1 lingualization, intrusion, and retroclination were then derived. ARR was graded based on Malmgren’s scale18 with a modification from Sharpe.19 Grade I: irregular root contour; Grade II: ARR < 2 mm; Grade III: ARR amounting to 2–4 mm; Grade IV: ARR > 4 mm.
Table 2 . Parameters measured using the cone-beam computed tomography image
Parameter | Definition |
---|---|
U1-PNSVert (mm) | Distance between midpoint of the central incisor edge and the coronal plane through PNS |
U1-PP (mm) | Distance from midpoint of the central incisor edge to PP |
U1-FH (°) | Lower posterior angle between longitudinal axis of the central incisor and FH plane, projected onto MSP |
U1 length (mm) | Distance between midpoint of the central incisor edge to its apex |
U1 pulp area (mm2) | Cross-sectional area of the pulp chamber on its MSP |
U1 palatal dehiscence (mm) | Longest distance from CEJ to palatal alveolar crest on the palatal side |
U1 lingualization (mm) | Pre-tx U1-PNSVert minus post-tx U1-PNSVert |
U1 intrusion (mm) | Pre-tx U1-PP minus post-tx U1-PP |
U1 retroclination (°) | Pre-tx U1-FH° minus post-tx U1-FH° |
U1 ARR (mm) | Pre-tx U1 length minus post-tx U1 length |
U1 pulp chamber shrinkage (mm2) | Pre-tx U1 pulp area minus post-tx U1 pulp area |
PP, palatal plane; FH, Frankfurt Horizontal; MSP, mid-sagittal plane; CEJ, cementoenamel junction; ARR, apical root resorption.
A bone discontinuity observed on three consecutive slices was considered an alveolar defect (Figure 3A). A vertical distance exceeding 2 mm between the alveolar crest and cementoenamel junction (CEJ) was considered pathognomonic for dehiscence.20 The cross-sectional area of the pulp chamber was measured on the mid-sagittal slice (Figure 3B).21 The pulp chamber was demarcated cervically by a line connecting the palatal and labial CEJs (Figure 3C) and its area was quantified using the Mimics software.
To evaluate reproducibility, 20 CBCT images were randomly selected and re-measured by the same investigator at 2-week intervals. The intraclass correlation coefficients consistently exceeded 0.9, indicating a high degree of repeatability.
The Shapiro–Wilk test and Levene’s test were used to assess normality and homogeneity, respectively. ANOVA was employed for intergroup comparisons of baseline and treatment changes, followed by pairwise comparisons with Bonferroni correction when necessary. The proportional difference in the modified Malmgren’s root resorption grade was assessed using Fisher’s exact test. The association between the ARR and predisposing factors was examined in each group using Pearson’s correlation analysis. Subsequently, the P-TAD and AP-TAD groups were combined, and multiple linear regression analysis was performed to determine the potential contribution of tooth movement parameters and different TAD configurations to ARR. The significance level was set at P < 0.05. Bilateral homonymous teeth were analyzed separately.
The U1 baseline values and treatment changes are listed in Tables 3 and 4, respectively.
Table 3 . Baseline comparison among the three groups
Non-TAD | P-TADs | AP-TADs | P value | |
---|---|---|---|---|
U1-PNSVert (mm) | 51.29 ± 3.83a | 54.24 ± 3.25b | 53.39b ± 3.92b | < 0.01 |
U1-PP (mm) | 27.54 ± 2.89a | 27.70 ± 2.95a | 29.27b ± 2.65b | < 0.01 |
U1-FH (°) | 115.06 ± 9.99a | 121.43 ± 8.51b | 117.84 ± 8.47a,b | < 0.01 |
U1 length (mm) | 22.41 ± 1.51 | 22.70 ± 1.60 | 22.83 ± 1.47 | 0.36 |
U1 pulp (sagittal) (mm2) | 5.38 ± 0.90 | 5.36 ± 0.73 | 5.35 ± 1.13 | 0.98 |
Values are presented as mean ± standard deviation. Means not sharing superscripts differ significantly at α = 0.05 level as indicated by Bonferroni’s correction.
TAD, temporary anchorage device; P-TADs, two posterior miniscrews only; AP-TADs, two anterior and two posterior miniscrews combined; PP, palatal plane; FH, Frankfurt Horizontal.
Table 4 . Treatment changes of U1 in the three groups
Non-TAD | P-TADs | AP-TADs | P value | |
---|---|---|---|---|
U1 lingualization (mm) | 3.07 ± 2.87a | 6.53 ± 2.28b | 5.52 ± 2.43b | < 0.01 |
U1 intrusion (mm) | 0.30 ± 1.61a | −0.07 ± 1.65a | 1.99 ± 2.37b | < 0.01 |
U1 retroclination (°) | 1.91 ± 9.39a | 9.45 ± 10.68b | 1.70 ± 8.80a | < 0.01 |
U1 ARR (mm) | 0.89 ± 0.97a | 1.63 ± 1.46b | 2.69 ± 1.38c | < 0.01 |
U1 pulp reduction (mm2) | 0.21 ± 0.46a | 0.31 ± 0.54a,b | 0.50 ± 0.53b | < 0.05 |
U1 palatal dehiscence (mm) | 3.74 ± 1.23a | 5.25 ± 1.57b | 5.13 ± 1.66b | < 0.01 |
Values are presented as mean ± standard deviation. Means not sharing superscripts differ significantly at α = 0.05 level as indicated by Bonferroni’s correction.
TAD, temporary anchorage device; P-TADs, two posterior miniscrews only; AP-TADs, two anterior and two posterior miniscrews combined; ARR, apical root resorption.
U1 lingualization was significantly less in the non-TAD group (3.07 ± 2.87 mm) than that in the P-TADs (6.53 ± 2.28 mm) and AP-TADs (5.52 ± 2.43 mm) groups (P < 0.001). U1 intrusion was significantly greater in the AP-TADs group (1.99 ± 2.37 mm) than in the non-TAD (0.30 ± 1.61 mm) and P-TADs (–0.07 ± 1.65 mm) group (P < 0.001). Collectively, the posterior miniscrews were effective for incisor retraction, and the anterior miniscrews were effective for incisor intrusion. U1 retroclination was significantly greater in the P-TADs group (9.45° ± 10.68°) than in the non-TAD (1.91° ± 9.39°) and P-TADs group (1.70° ± 8.80°) (P < 0.001). This indicates that posterior miniscrews aggravate maxillary incisor torque loss during anterior retraction, which can be alleviated by the incorporation of anterior miniscrews.
U1 ARR was minimal in the non-TAD group (0.89 ± 0.97 mm), moderate in the P-TADs (1.63 ± 1.46 mm), and largest in the AP-TADs (2.69 ± 1.38 mm) group, with significant differences (P < 0.001). When assessed using Malmgren’s scale, Fisher’s exact test yielded highly positive results (P < 0.001), indicating substantial proportional variation across the samples (Figure 4). The incidence of grade IV ARR was significantly higher in the AP-TADs group (16.6%) than in the P-TAD (6.7%) and non-TAD (1.9%) groups. Moreover, the Pearson correlation analysis revealed that the U1 ARR was moderately correlated with intrusion in the non-TAD and AP-TAD groups, whereas it was inversely correlated with retroclination in all three groups (Table 5). Multiple regression analysis for ARR produced the following linear model (F = 18.731, P < 0.001, adjusted R2 = 0.328):
Table 5 . Association between each parameter and the U1 apical root resorption in the three groups according to Pearson’s correlation analysis
Non-TAD | P-TADs | AP-TADs | ||||||
---|---|---|---|---|---|---|---|---|
r | P | r | P | r | P | |||
preU1length | 0.231 | 0.092 | 0.067 | 0.611 | 0.048 | 0.742 | ||
preU1PNSVert | −0.007 | 0.963 | 0.255* | 0.050 | 0.101 | 0.494 | ||
preU1PP | −0.259 | 0.058 | 0.062 | 0.637 | −0.285* | 0.045 | ||
preU1FH° | 0.169 | 0.223 | 0.311* | 0.016 | 0.205 | 0.153 | ||
U1 lingualization | −0.143 | 0.303 | −0.310* | 0.016 | 0.080 | 0.582 | ||
U1 intrusion | 0.497*** | 0.000 | 0.250 | 0.054 | 0.451** | 0.001 | ||
U1 retroclination | −0.493*** | 0.000 | −0.487*** | 0.000 | −0.349* | 0.013 | ||
Duration | −0.257 | 0.061 | −0.160 | 0.221 | −0.326* | 0.021 |
TAD, temporary anchorage device; P-TADs, two posterior miniscrews only; AP-TADs, two anterior and two posterior miniscrews combined; PP, palatal plane; FH, Frankfurt Horizontal.
*The correlation is significant at the 0.05 level (2-tailed).
**Correlation is significant at the 0.01 level (2-tailed).
***Correlation is significant at the 0.001 level (2-tailed).
In this regression equation for U1 ARR, the variable τ assumes the value of 0 for “P-TADs” and a value of 1 for “AP-TADs.” This indicates that in the combined group (P-TADs + AP-TADs), U1 ARR increased with lingualization and the use of AP-TADs, while it decreased with retroclination (lingual crown torquing) (Table 6). Notably, after excluding tooth movement factors, the AP-TADs configuration resulted in an additional 0.5 mm of ARR compared to the P-TAD group.
Table 6 . Multiple linear regression for the U1 apical root resorption in the combination of P-TADs and AP-TADs groups
Variable | Unstandardized B | Coefficient standard error | Standardized β | P |
---|---|---|---|---|
Constant | 1.109 | 0.409 | 0.008 | |
U1 retroclination | −0.101 | 0.017 | −0.707 | < 0.001 |
U1 lingualization | 0.226 | 0.073 | 0.358 | 0.003 |
τ | 0.509 | 0.265 | 0.169 | 0.049 |
P-TADs, two posterior miniscrews only; AP-TADs, two anterior and two posterior miniscrews combined.
A reduction in the U1 pulp chamber dimension was observed across all three groups, which was significantly greater in the AP-TAD group (0.50 ± 0.53 mm2) compared to the non-TAD group (0.21 ± 0.46 mm2) (Table 4). This indicates that the combined use of the anterior and posterior miniscrews may lead to greater pulp chamber shrinkage.
All dehiscent sites in U1 were located on the palatal side. The size of U1 palatal dehiscence was 3.74 ± 1.23 mm in the non-TAD group, 5.25 ± 1.57 mm in the P-TAD group, and 5.13 ± 1.66 mm in the AP-TAD group, with statistical differences between the TAD groups and the non-TAD control (P < 0.001). The incidence rates of U1 palatal dehiscence followed a similar pattern: 16.7% in the non-TAD group, 33.3% in the P-TAD group, and 28.0% in the AP-TAD group. Overall, posterior miniscrews increased U1 palatal dehiscence, but this effect was not exacerbated by the addition of anterior miniscrews.
AP-TAD and P-TAD are two common types of miniscrews used in the management of maxillary protrusion and premolar extraction. This study comparatively evaluated the effects and side effects of these two configurations on maxillary central incisors. Compared to P-TADs, the AP-TADs configuration enhanced U1 intrusion and labial crown torque but increased the severity of ARR as the primary side effect. These findings suggest that the AP-TADs configuration may offer unique advantages in selected cases; however, it should be avoided or used with caution when maxillary incisors have short roots or are susceptible to root resorption.
Clinicians adjust the vertical position of maxillary incisors to correct deep bites and gummy smiles. Ricketts introduced the ‘utility arch’ to intrude mandibular incisors,22 while Burstone identified the adverse effects of the counteracting moment on posterior anchorage teeth, including extrusion, palatal and distal tipping, and steepening of the occlusal plane.23 Anterior TADs help preserve molars from these undesirable outcomes.7,8,24,25 A recent study using 2D cephalograms found that a midline TAD intruded U1 by 1.08 mm, which was less effective than bilateral posterior TADs (1.34 mm).1 In the present study, the effect of maxillary anterior TADs was demonstrated by juxtaposing two TAD groups. The comparison showed a net U1 intrusion of 2 mm, namely 1.99 mm in the AP-TADs versus –0.07 mm in the P-TAD group. Although labial TADs are powerful, clinicians are advised to avoid excessive incisor intrusion, which may result in a flattened smile arc.26 Moreover, excessive intrusion may pose periodontal risks, as supragingival plaque can transform into subgingival plaque in the presence of plaque retentive factors such as calculus or prosthesis marginal misfit, potentially leading to infrabony pockets.27
It has been suggested that true intrusion should be distinguished from relative intrusion through careful landmark selection.28 Previous investigations have used the centroid of the incisor to rule out the effect of labial proclination.29 However, this study employed the incisal edge as a landmark for measuring intrusion for a few reasons. First, the stability of the centroid is unreliable, particularly when root apices experience significant resorption or alveolar defects develop.30 Secondly, the incisal edge tends to underestimate rather than overestimate the intrusion of the centroid in cases of torque loss from en-masse retraction. Thus false detection of intrusion at the incisal edge may be minimized in extraction cases. Most importantly, the vertical position of the incisal edge directly determines the overbite, whereas the clinical relevance of the centroid is less clear.
Various approaches have been developed to control incisor torque during anterior retraction, such as third-order bending in the archwire, long retraction hooks,31 high-torque metal brackets,32 J-retractors,33 etc. In the present study, a 7 mm retraction hook was routinely used to reduce the distance between the force line and the center of resistance of the maxillary incisors. However, U1 was retroclinated by 9.45° in the P-TAD group, far exceeding that in the non-TAD group, likely due to greater U1 lingualization in the former.
The additional use of anterior labial TADs resulted in a net effect of +7.75° on U1 labial crown torque, as U1 retroclination (torque loss) was only 1.70° in the AP-TAD group compared to 9.45° in the P-TAD group. A recent study reported U1 retroclination of 8.06° with a midline TAD and 12.29° with bilateral posterior TADs.1 In the present study, the AP-TADs configuration retracted the incisors almost translatively, indicating that the combined force of posterior retraction and anterior intrusion passes very close to the center of resistance of the maxillary incisors.
Attempts have been made to correlate ARR with treatment-related parameters in cases of premolar extraction.11,34-37 Factors such as positive torque change,34,37 horizontal retractions,35,36 intrusions,34,38 and treatment duration11 have been reported as potential influences. The current study found a consistent correlation between the ARR and palatal root torque in all three groups. Palatal root torque per se increases stress at the root apex, which can be aggravated by the proximity of dense anatomical barriers such as the palatal cortices. Intrusion was moderately correlated with ARR in the non-TAD and AP-TAD groups. However, this trend was not observed for the P-TAD group, probably because U1 extrusion rather than intrusion occurred in this group.
Root encroachment on skeletal structures, such as the palatal cortex and incisive canal,39 during orthodontic tooth movement has been proposed as a contributing factor for ARR during anterior retraction. Although the root trunk impinges upon the dense anatomical barriers before the apex during retraction, it invariably experiences irreversible resorption. Additionally, the surface convexity peaks at the apex and levels off toward the cervical direction. Nano-indentation studies also corroborate that both the hardness and elastic modulus of cementum are lower at the root apex than at the cervical and middle thirds.40 On the other hand, significant ARR has not been observed in cases of pure intrusion without retraction.41 Intrusion, when confined to the spongy part of the alveolar process, seems unlikely to cause severe resorption.13 Thus based on previous studies, resistance to root resorption is higher at cancellous alveolar bone than palatal cortex and at cervical root trunk than root apex.
Notably, the AP-TAD group exhibited a more severe U1 ARR than the P-TAD group. While there was no significant difference in the lingualization of the U1 tip, U1 retroclination was significantly greater in the P-TAD group compared to the AP-TAD group (Table 4). Consequently, the AP-TADs group experienced greater lingual movement of the U1 apex, although this could not be measured directly in this study due to the extent of ARR. A previous study associated the severity of ARR with the amount of horizontal displacement of incisor apices.12 In addition, U1 intrusion was positively correlated with ARR in the AP-TAD group (Table 5), suggesting that it is a contributing factor. Interestingly, previous studies have shown that intrusion alone does not appear to increase incisor ARR, whereas intrusion during anterior retraction does.34,38 Taken together, the palatal and intrusive movements of the U1 apex in the AP-TADs configuration may be associated with increased ARR.
A subsequent question arose: Is the greater severity of ARR in the AP-TAD group relative to the P-TAD group solely attributable to differences in tooth movement? The linear regression model demonstrated that, after excluding tooth movement factors, the AP-TAD configuration continued to exhibit an additional 0.5 mm of ARR compared to the P-TAD group. This can be explained from a mechanical perspective: the combined retraction and intrusion force vectors result in a composite force greater in magnitude than the retraction force alone. This aligns with the ‘parallelogram rule’. Our findings highlight an important but often overlooked clinical recommendation: when posterior and anterior TADs are used in combination, retraction and intrusion forces should be reduced compared to when each is used independently.
A number of pulpal changes have been identified in orthodontically treated teeth, including transient reductions in pulpal blood flow, disruption of the odontoblastic layer, fibrotic tissue formation, and increased expression of Substance P and vascular endothelial growth factor.42 In the current study, a significant reduction in pulp chamber dimension was observed across all three groups, consistent with previous findings.15 Pulpal blood flow was restricted for only a few weeks before fully recovering when performing TAD-assisted incisor intrusion with a heavy force of 120 cN.43 Examination of its cemento-dentine-canal junction shows that only 38.6% of maxillary central incisors have a single main root canal, while 49.1% have lateral branches, 6.7% have apical ramifications, and 5.6% have a combination of both.44 Purely intrusive force may primarily affect the trunk root canal rather than the auxiliary lateral branches. However, when U1 is simultaneously intruded and retracted, as in the AP-TAD group, the arteriovenous system at the apical foramen may find the lateral ‘backup’ branches insufficient to manage ischemic episodes. It is postulated that the excess tertiary dentin formed in the AP-TAD group may have been triggered by the multidirectional and higher force levels applied. Fortunately, pulp chamber shrinkage has largely remained asymptomatic and self-limiting because the widening of the apical constriction by ARR helps to restore pulpal blood supply and venous drainage, thus preserving long-term nourishment.
The post-treatment U1 palatal dehiscence incidence rate was 16.7% in the non-TAD group and approximately doubled in the TAD groups, consistent with a recent study.45 Palatal dehiscence has become less of a concern since reports of self-recovery emerged.46 It has been proposed that palatal dehiscence is associated with a better prognosis, probably due to the naturally ‘thick’ gingival biotype. Recent studies have found evidence supporting self-recovery within 18–24 months. However, the underlying mechanism of spontaneous recovery of exposed root trunks remains unclear. Some studies observed differential anterior drift between the incisal edge and root apex in the dehiscence cohort, with the apex sinking 0.75 mm into the alveolus, whereas the incisal edge advanced 0.27 mm.47 Others reported ‘zero’ relapse, challenging the torque relapse theory.48 Long-term follow-up of such cohort is needed to verify the findings.
This study has some limitations. It is retrospective in nature, and baseline homogeneity cannot be guaranteed since the decision for labial TADs is based on the presence of a deep bite. Additionally, CBCT images were collected immediately after debonding, and a long-term follow-up would better elucidate the prognosis of dental, periodontal, and endodontic side effects.
In light of the increased ARR in the AP-TADs group, a clinical decision tree was tentatively proposed for cases requiring both retraction and intrusion of the maxillary incisors based on pre-treatment root length (Figure 5). When ARR is potentially too severe to be considered clinically insignificant, the ideal incisor position objective should be compromised, involving a reduction in bodily retraction and intrusion of the incisors. Either the AP-TAD configuration must be abandoned, or the magnitude of retractive and intrusive force should be reduced to achieve an optimal combined force. Furthermore, as intermittent forces may mitigate root resorption,49 elastics, rather than power chains, may be recommended for such cases.
Compared to posterior miniscrews alone, combined anterior and posterior miniscrews enhance intrusion and reduce torque loss of the maxillary central incisors, but they also cause more severe ARR in anterior retraction cases, potentially due to increased apical movement and greater composite force exerted. Therefore, the AP-TADs configuration should be used with caution when the roots of the maxillary incisors are short. Further studies are needed to identify strategies for preventing these adverse effects.
Conceptualization: WZ, Yu L. Data curation: ZW, DW. Formal analysis: ZW, DW. Investigation: ZW. Methodology: ZW, ZT, DW. Project administration: ZW, YZ. Resources: ZW, YZ. Software: ZW, ZT, YZ. Supervision: Yu L. Visualization: ZW, LM. Writing–original draft: ZW. Writing–review & editing: LM, ZT, EAA, Yuan L, Yu L.
No potential conflict of interest relevant to this article was reported.
None to declare.
Korean J Orthod 2025; 55(1): 26-36 https://doi.org/10.4041/kjod24.136
First Published Date October 11, 2024, Publication Date January 25, 2025
Copyright © The Korean Association of Orthodontists.
Zhizun Wanga , Li Meib
, Zhenxing Tangc
, Dong Wud
, Yue Zhoua
, Ehab A. Abdulghania,e
, Yuan Lia
, Wei Zhengf
, Yu Lia
aState Key Laboratory of Oral Diseases, National Center for Stomatology, National Clinical Research Center for Oral Diseases, Department of Orthodontics, West China Hospital of Stomatology, Sichuan University, Chengdu, China
bDiscipline of Orthodontics, Department of Oral Sciences, University of Otago, Dunedin, New Zealand
cDepartment of Stomatology, Chengdu Seventh People’s Hospital (Affiliated Cancer Hospital of Chengdu Medical College), Chengdu, China
dSection of Preventive and Public Health Dentistry, Division of Oral Health, Growth and Development, Kyushu University, Fukuoka, Japan
eDepartment of Orthodontics and Dentofacial Orthopedics, College of Dentistry, Thamar University, Dhamar, Yemen
fState Key Laboratory of Oral Diseases, National Center for Stomatology, National Clinical Research Center for Oral Diseases, Department of Oral and Maxillofacial Surgery, West China Hospital of Stomatology, Sichuan University, Chengdu, China
Correspondence to:Wei Zheng.
Associate Professor, State Key Laboratory of Oral Diseases, National Center for Stomatology, National Clinical Research Center for Oral Diseases, Department of Oral and Maxillofacial Surgery, West China Hospital of Stomatology, Sichuan University, No.14, the 3rd Section, South Ren Min Rd., Chengdu 610041, China.
Tel +86-28-85503551 e-mail zhengwei81101@163.com
Yu Li.
Professor, State Key Laboratory of Oral Diseases, National Center for Stomatology, National Clinical Research Center for Oral Diseases, Department of Orthodontics, West China Hospital of Stomatology, Sichuan University, No.14, the 3rd Section, South Ren Min Rd., Chengdu 610041, China.
Tel +86-28-85503645 e-mail yuli@scu.edu.cn
How to cite this article: Wang Z, Mei L, Tang Z, Wu D, Zhou Y, Abdulghani EA, Li Y, Zheng W, Li Y. Combined anterior and posterior miniscrews increase apical root resorption of maxillary incisors in protrusion and premolar extraction cases. Korean J Orthod 2025;55(1):26-36. https://doi.org/10.4041/kjod24.136
This is an Open Access article distributed under the terms of the Creative Commons Attribution Non-Commercial License (http://creativecommons.org/licenses/by-nc/4.0/) which permits unrestricted non-commercial use, distribution, and reproduction in any medium, provided the original work is properly cited.
Objective: Miniscrews are commonly utilized as temporary anchorage devices (TADs) in cases of maxillary protrusion and premolar extraction. This study aimed to investigate the effects and potential side effects of two conventional miniscrew configurations on the maxillary incisors. Methods: Eighty-two adult patients with maxillary dentoalveolar protrusion who had undergone bilateral first premolar extraction were retrospectively divided into three groups: non-TAD, two posterior miniscrews only (P-TADs), and two anterior and two posterior miniscrews combined (AP-TADs). Cone-beam computed tomography was used to evaluate the maxillary central incisors (U1). Results: The AP-TADs group had significantly greater U1 intrusion (1.99 ± 2.37 mm, n = 50) and less retroclination (1.70° ± 8.80°) compared to the P-TADs (–0.07 ± 1.65 mm and 9.45° ± 10.68°, n = 60) and non-TAD group (0.30 ± 1.61 mm and 1.91° ± 9.39°, n = 54). However, the AP-TADs group suffered from significantly greater apical root resorption (ARR) of U1 (2.69 ± 1.38 mm) than the P-TADs (1.63 ± 1.46 mm) and non-TAD group (0.89 ± 0.97 mm). Notably, the incidence of grade IV ARR was 16.6% in the AP-TADs group, significantly higher than the rates observed in the P-TADs (6.7%) and non-TAD (1.9%) groups. Multiple regression analysis revealed that after excluding tooth movement factors, the AP-TADs configuration resulted in an additional 0.5 mm of ARR compared with the P-TADs group. Conclusions: In cases of maxillary protrusion and premolar extraction, the use of combined anterior and posterior miniscrews enhances incisor intrusion and minimizes torque loss of the maxillary incisors. However, this approach results in more severe ARR, likely due to the increased apical movement and composite force exerted.
Keywords: Temporary anchorage device, Apical root resorption, Incisor, Orthodontic extraction
The treatment of maxillary dentoalveolar protrusions generally involves premolar extraction followed by anterior teeth retraction, with temporary anchorage devices (TADs) playing crucial roles. Posterior miniscrews placed in the buccal region reinforce the sagittal anchorage to enhance incisor retraction, while anterior miniscrews positioned in the labial region are used to intrude the maxillary incisors and prevent bite deepening.1 Consequently, the implementation of miniscrews generally falls into two configurations: posterior miniscrews alone or a combination of anterior and posterior miniscrews.2-5
The location of TADs has a biomechanically intuitive impact on tooth movement. In non-extraction therapy, anterior TADs are effective for incisor intrusion and labial crown torquing.6-8 However, in extraction cases, posterior TADs retract the anterior segment with a force directed below the center of resistance, resulting in a significant loss of incisor torque.9 A recent study demonstrated that a single maxillary midline miniscrew can control incisor torque and overbite control in moderate anchorage cases.1 Another study implemented mini-implants at both the anterior and posterior maxilla, resulting in incisor intrusion; however, labiopalatal angulation has not been reported.2 Unlike posterior miniscrews, the effects of anterior miniscrews in protrusion and extraction have primarily been documented in case reports3,10 rather than controlled studies.
Orthodontists are frequently reminded of the dental and periodontal adverse effects of tooth movement. Retraction forces cause moderate apical root resorption (ARR) of approximately 2 mm in maxillary incisors, irrespective of the use of posterior miniscrews.11,12 A recent systematic review concluded that intrusive mechanics results in ARR of 0.72 mm in incisors and 0.41 mm in molars.13 A clinical trial reported that 57% of retracted incisors exhibit root flattening, while 42% demonstrated blunting of the root apex.1 Regarding periodontal outcomes, anterior retraction with maximum anchorage led to a reduction in the palatal alveolar bone thickness at the cervical and middle thirds of the incisor roots.14 In terms of endodontic effects, anterior retraction with moderate anchorage has been associated with a significant reduction in total pulpal volume attributed to tertiary dentin formation.15
Thus, the present study aimed to compare two conventional miniscrew configurations–posterior miniscrews alone versus combined anterior and posterior miniscrews–regarding their effects and side effects on the maxillary incisors in cases of maxillary protrusion and premolar extraction.
This was a retrospective study approved by the Ethics Committee of West China Hospital of Stomatology, Sichuan University (WCHSIRBCT-2023-113). Exemption from obtaining informed consent was granted. A total of 527 patients who underwent fixed orthodontic therapy by the same orthodontist between 2018 and 2023 were screened for eligibility. The inclusion criteria were as follows: adult patients (over 18 years old), maxillary dentoalveolar protrusion, maxillary arch crowding ≤ 4 mm, 0° ≤ ANB ≤ 6°; extraction of bilateral maxillary first premolars, and treatment with no miniscrew (the non-TAD group), two posterior miniscrews only (the P-TADs group), or two anterior and two posterior miniscrews combined (the AP-TADs group) (Figure 1). The posterior miniscrews were placed either mesially or distally to the maxillary first molars to reinforce the posterior anchorage, whereas the anterior miniscrews were positioned either mesially or distally to the maxillary lateral incisors to resolve the deep bite. The exclusion criteria were as follows: periodontitis, prior pulpal treatment or pre-treatment root resorption of the maxillary incisors, and treatment involving other auxiliary devices such as trans-palatal arch, Nance arch, headgear, and torquing springs. Based on the criteria, 82 patients were recruited for the study. According to recent studies on incisor intrusion,16,17 a sample size of 22 per group would provide an 80% probability of detecting a moderate difference in incisor intrusion between labial TADs and conventional methods, with a 95% confidence level, using a priori ANOVA model. Baseline demographic characteristics were comparable across the three groups (Table 1).
Table 1 . Demographics of the Non-TAD, P-TADs, and AP-TADs groups.
Non-TAD (n = 27) | P-TADs (n = 30) | AP-TADs (n = 25) | |
---|---|---|---|
Sex | |||
Female | 22 | 27 | 24 |
Male | 5 | 3 | 1 |
Angle classification | |||
Class I | 19 | 18 | 12 |
Class II | 8 | 12 | 13 |
ANB (°) | 2.8 ± 2.6 | 3.8 ± 2.1 | 3.8 ± 2.1 |
Age (yr) | 23.5 ± 5.6 | 23.5 ± 4.2 | 24.4 ± 4.9 |
Treatment duration (mo) | 32.6 ± 11.5 | 36.2 ± 11.9 | 39.4 ± 13.1 |
Values are presented as number only or mean ± standard deviation..
TAD, temporary anchorage device; P-TADs, two posterior miniscrews only; AP-TADs, two anterior and two posterior miniscrews combined..
All patients were treated using a 0.018 × 0.025- inch stainless steel archwire in a 0.022 inch slot metal bracket system (Damon Q, Ormco, Brea, CA, USA). Two proprietary miniscrews (VectorTAS 601-0022, Ormco, Orange, CA, USA; AbsoAnchor AX12-108, Dentos Inc., Daegu, Korea) were used interchangeably based on stock availability. Power chains (G&H Orthodontics, Franklin, IN, USA) were routinely applied for anterior retraction and incisor intrusion. The retraction force was estimated at the chairside to be approximately 180 cN per side, whereas the intrusion force was approximately 100 cN per side.
Cone-beam computed tomography (CBCT) scans were obtained using a 3D Accuitomo (Morita MFG Corp., Kyoto, Japan) according to the manufacturer’s specifications (14 × 10 cm FOV, 85 kV, 4 mA, 360° rotation). The DICOM images were imported into Mimics 21.0 (Materialise, Leuven, Belgium) for 3D landmark registration. The Frankfurt Horizontal plane was defined by the bilateral orbitale and right porion. The mid-sagittal plane (MSP) passes through the ANS and PNS perpendicular to the transverse plane. The coronal plane passes through the PNS perpendicular to the ANS-PNS line, and the palatal plane passes through the ANS and PNS perpendicular to the MSP (Figure 2A). All dental and skeletal landmarks were projected onto MSP (Figure 2B).
The linear and angular measurements are presented in Table 2. Treatment changes, including U1 lingualization, intrusion, and retroclination were then derived. ARR was graded based on Malmgren’s scale18 with a modification from Sharpe.19 Grade I: irregular root contour; Grade II: ARR < 2 mm; Grade III: ARR amounting to 2–4 mm; Grade IV: ARR > 4 mm.
Table 2 . Parameters measured using the cone-beam computed tomography image.
Parameter | Definition |
---|---|
U1-PNSVert (mm) | Distance between midpoint of the central incisor edge and the coronal plane through PNS |
U1-PP (mm) | Distance from midpoint of the central incisor edge to PP |
U1-FH (°) | Lower posterior angle between longitudinal axis of the central incisor and FH plane, projected onto MSP |
U1 length (mm) | Distance between midpoint of the central incisor edge to its apex |
U1 pulp area (mm2) | Cross-sectional area of the pulp chamber on its MSP |
U1 palatal dehiscence (mm) | Longest distance from CEJ to palatal alveolar crest on the palatal side |
U1 lingualization (mm) | Pre-tx U1-PNSVert minus post-tx U1-PNSVert |
U1 intrusion (mm) | Pre-tx U1-PP minus post-tx U1-PP |
U1 retroclination (°) | Pre-tx U1-FH° minus post-tx U1-FH° |
U1 ARR (mm) | Pre-tx U1 length minus post-tx U1 length |
U1 pulp chamber shrinkage (mm2) | Pre-tx U1 pulp area minus post-tx U1 pulp area |
PP, palatal plane; FH, Frankfurt Horizontal; MSP, mid-sagittal plane; CEJ, cementoenamel junction; ARR, apical root resorption..
A bone discontinuity observed on three consecutive slices was considered an alveolar defect (Figure 3A). A vertical distance exceeding 2 mm between the alveolar crest and cementoenamel junction (CEJ) was considered pathognomonic for dehiscence.20 The cross-sectional area of the pulp chamber was measured on the mid-sagittal slice (Figure 3B).21 The pulp chamber was demarcated cervically by a line connecting the palatal and labial CEJs (Figure 3C) and its area was quantified using the Mimics software.
To evaluate reproducibility, 20 CBCT images were randomly selected and re-measured by the same investigator at 2-week intervals. The intraclass correlation coefficients consistently exceeded 0.9, indicating a high degree of repeatability.
The Shapiro–Wilk test and Levene’s test were used to assess normality and homogeneity, respectively. ANOVA was employed for intergroup comparisons of baseline and treatment changes, followed by pairwise comparisons with Bonferroni correction when necessary. The proportional difference in the modified Malmgren’s root resorption grade was assessed using Fisher’s exact test. The association between the ARR and predisposing factors was examined in each group using Pearson’s correlation analysis. Subsequently, the P-TAD and AP-TAD groups were combined, and multiple linear regression analysis was performed to determine the potential contribution of tooth movement parameters and different TAD configurations to ARR. The significance level was set at P < 0.05. Bilateral homonymous teeth were analyzed separately.
The U1 baseline values and treatment changes are listed in Tables 3 and 4, respectively.
Table 3 . Baseline comparison among the three groups.
Non-TAD | P-TADs | AP-TADs | P value | |
---|---|---|---|---|
U1-PNSVert (mm) | 51.29 ± 3.83a | 54.24 ± 3.25b | 53.39b ± 3.92b | < 0.01 |
U1-PP (mm) | 27.54 ± 2.89a | 27.70 ± 2.95a | 29.27b ± 2.65b | < 0.01 |
U1-FH (°) | 115.06 ± 9.99a | 121.43 ± 8.51b | 117.84 ± 8.47a,b | < 0.01 |
U1 length (mm) | 22.41 ± 1.51 | 22.70 ± 1.60 | 22.83 ± 1.47 | 0.36 |
U1 pulp (sagittal) (mm2) | 5.38 ± 0.90 | 5.36 ± 0.73 | 5.35 ± 1.13 | 0.98 |
Values are presented as mean ± standard deviation. Means not sharing superscripts differ significantly at α = 0.05 level as indicated by Bonferroni’s correction..
TAD, temporary anchorage device; P-TADs, two posterior miniscrews only; AP-TADs, two anterior and two posterior miniscrews combined; PP, palatal plane; FH, Frankfurt Horizontal..
Table 4 . Treatment changes of U1 in the three groups.
Non-TAD | P-TADs | AP-TADs | P value | |
---|---|---|---|---|
U1 lingualization (mm) | 3.07 ± 2.87a | 6.53 ± 2.28b | 5.52 ± 2.43b | < 0.01 |
U1 intrusion (mm) | 0.30 ± 1.61a | −0.07 ± 1.65a | 1.99 ± 2.37b | < 0.01 |
U1 retroclination (°) | 1.91 ± 9.39a | 9.45 ± 10.68b | 1.70 ± 8.80a | < 0.01 |
U1 ARR (mm) | 0.89 ± 0.97a | 1.63 ± 1.46b | 2.69 ± 1.38c | < 0.01 |
U1 pulp reduction (mm2) | 0.21 ± 0.46a | 0.31 ± 0.54a,b | 0.50 ± 0.53b | < 0.05 |
U1 palatal dehiscence (mm) | 3.74 ± 1.23a | 5.25 ± 1.57b | 5.13 ± 1.66b | < 0.01 |
Values are presented as mean ± standard deviation. Means not sharing superscripts differ significantly at α = 0.05 level as indicated by Bonferroni’s correction..
TAD, temporary anchorage device; P-TADs, two posterior miniscrews only; AP-TADs, two anterior and two posterior miniscrews combined; ARR, apical root resorption..
U1 lingualization was significantly less in the non-TAD group (3.07 ± 2.87 mm) than that in the P-TADs (6.53 ± 2.28 mm) and AP-TADs (5.52 ± 2.43 mm) groups (P < 0.001). U1 intrusion was significantly greater in the AP-TADs group (1.99 ± 2.37 mm) than in the non-TAD (0.30 ± 1.61 mm) and P-TADs (–0.07 ± 1.65 mm) group (P < 0.001). Collectively, the posterior miniscrews were effective for incisor retraction, and the anterior miniscrews were effective for incisor intrusion. U1 retroclination was significantly greater in the P-TADs group (9.45° ± 10.68°) than in the non-TAD (1.91° ± 9.39°) and P-TADs group (1.70° ± 8.80°) (P < 0.001). This indicates that posterior miniscrews aggravate maxillary incisor torque loss during anterior retraction, which can be alleviated by the incorporation of anterior miniscrews.
U1 ARR was minimal in the non-TAD group (0.89 ± 0.97 mm), moderate in the P-TADs (1.63 ± 1.46 mm), and largest in the AP-TADs (2.69 ± 1.38 mm) group, with significant differences (P < 0.001). When assessed using Malmgren’s scale, Fisher’s exact test yielded highly positive results (P < 0.001), indicating substantial proportional variation across the samples (Figure 4). The incidence of grade IV ARR was significantly higher in the AP-TADs group (16.6%) than in the P-TAD (6.7%) and non-TAD (1.9%) groups. Moreover, the Pearson correlation analysis revealed that the U1 ARR was moderately correlated with intrusion in the non-TAD and AP-TAD groups, whereas it was inversely correlated with retroclination in all three groups (Table 5). Multiple regression analysis for ARR produced the following linear model (F = 18.731, P < 0.001, adjusted R2 = 0.328):
Table 5 . Association between each parameter and the U1 apical root resorption in the three groups according to Pearson’s correlation analysis.
Non-TAD | P-TADs | AP-TADs | ||||||
---|---|---|---|---|---|---|---|---|
r | P | r | P | r | P | |||
preU1length | 0.231 | 0.092 | 0.067 | 0.611 | 0.048 | 0.742 | ||
preU1PNSVert | −0.007 | 0.963 | 0.255* | 0.050 | 0.101 | 0.494 | ||
preU1PP | −0.259 | 0.058 | 0.062 | 0.637 | −0.285* | 0.045 | ||
preU1FH° | 0.169 | 0.223 | 0.311* | 0.016 | 0.205 | 0.153 | ||
U1 lingualization | −0.143 | 0.303 | −0.310* | 0.016 | 0.080 | 0.582 | ||
U1 intrusion | 0.497*** | 0.000 | 0.250 | 0.054 | 0.451** | 0.001 | ||
U1 retroclination | −0.493*** | 0.000 | −0.487*** | 0.000 | −0.349* | 0.013 | ||
Duration | −0.257 | 0.061 | −0.160 | 0.221 | −0.326* | 0.021 |
TAD, temporary anchorage device; P-TADs, two posterior miniscrews only; AP-TADs, two anterior and two posterior miniscrews combined; PP, palatal plane; FH, Frankfurt Horizontal..
*The correlation is significant at the 0.05 level (2-tailed)..
**Correlation is significant at the 0.01 level (2-tailed)..
***Correlation is significant at the 0.001 level (2-tailed)..
In this regression equation for U1 ARR, the variable τ assumes the value of 0 for “P-TADs” and a value of 1 for “AP-TADs.” This indicates that in the combined group (P-TADs + AP-TADs), U1 ARR increased with lingualization and the use of AP-TADs, while it decreased with retroclination (lingual crown torquing) (Table 6). Notably, after excluding tooth movement factors, the AP-TADs configuration resulted in an additional 0.5 mm of ARR compared to the P-TAD group.
Table 6 . Multiple linear regression for the U1 apical root resorption in the combination of P-TADs and AP-TADs groups.
Variable | Unstandardized B | Coefficient standard error | Standardized β | P |
---|---|---|---|---|
Constant | 1.109 | 0.409 | 0.008 | |
U1 retroclination | −0.101 | 0.017 | −0.707 | < 0.001 |
U1 lingualization | 0.226 | 0.073 | 0.358 | 0.003 |
τ | 0.509 | 0.265 | 0.169 | 0.049 |
P-TADs, two posterior miniscrews only; AP-TADs, two anterior and two posterior miniscrews combined..
A reduction in the U1 pulp chamber dimension was observed across all three groups, which was significantly greater in the AP-TAD group (0.50 ± 0.53 mm2) compared to the non-TAD group (0.21 ± 0.46 mm2) (Table 4). This indicates that the combined use of the anterior and posterior miniscrews may lead to greater pulp chamber shrinkage.
All dehiscent sites in U1 were located on the palatal side. The size of U1 palatal dehiscence was 3.74 ± 1.23 mm in the non-TAD group, 5.25 ± 1.57 mm in the P-TAD group, and 5.13 ± 1.66 mm in the AP-TAD group, with statistical differences between the TAD groups and the non-TAD control (P < 0.001). The incidence rates of U1 palatal dehiscence followed a similar pattern: 16.7% in the non-TAD group, 33.3% in the P-TAD group, and 28.0% in the AP-TAD group. Overall, posterior miniscrews increased U1 palatal dehiscence, but this effect was not exacerbated by the addition of anterior miniscrews.
AP-TAD and P-TAD are two common types of miniscrews used in the management of maxillary protrusion and premolar extraction. This study comparatively evaluated the effects and side effects of these two configurations on maxillary central incisors. Compared to P-TADs, the AP-TADs configuration enhanced U1 intrusion and labial crown torque but increased the severity of ARR as the primary side effect. These findings suggest that the AP-TADs configuration may offer unique advantages in selected cases; however, it should be avoided or used with caution when maxillary incisors have short roots or are susceptible to root resorption.
Clinicians adjust the vertical position of maxillary incisors to correct deep bites and gummy smiles. Ricketts introduced the ‘utility arch’ to intrude mandibular incisors,22 while Burstone identified the adverse effects of the counteracting moment on posterior anchorage teeth, including extrusion, palatal and distal tipping, and steepening of the occlusal plane.23 Anterior TADs help preserve molars from these undesirable outcomes.7,8,24,25 A recent study using 2D cephalograms found that a midline TAD intruded U1 by 1.08 mm, which was less effective than bilateral posterior TADs (1.34 mm).1 In the present study, the effect of maxillary anterior TADs was demonstrated by juxtaposing two TAD groups. The comparison showed a net U1 intrusion of 2 mm, namely 1.99 mm in the AP-TADs versus –0.07 mm in the P-TAD group. Although labial TADs are powerful, clinicians are advised to avoid excessive incisor intrusion, which may result in a flattened smile arc.26 Moreover, excessive intrusion may pose periodontal risks, as supragingival plaque can transform into subgingival plaque in the presence of plaque retentive factors such as calculus or prosthesis marginal misfit, potentially leading to infrabony pockets.27
It has been suggested that true intrusion should be distinguished from relative intrusion through careful landmark selection.28 Previous investigations have used the centroid of the incisor to rule out the effect of labial proclination.29 However, this study employed the incisal edge as a landmark for measuring intrusion for a few reasons. First, the stability of the centroid is unreliable, particularly when root apices experience significant resorption or alveolar defects develop.30 Secondly, the incisal edge tends to underestimate rather than overestimate the intrusion of the centroid in cases of torque loss from en-masse retraction. Thus false detection of intrusion at the incisal edge may be minimized in extraction cases. Most importantly, the vertical position of the incisal edge directly determines the overbite, whereas the clinical relevance of the centroid is less clear.
Various approaches have been developed to control incisor torque during anterior retraction, such as third-order bending in the archwire, long retraction hooks,31 high-torque metal brackets,32 J-retractors,33 etc. In the present study, a 7 mm retraction hook was routinely used to reduce the distance between the force line and the center of resistance of the maxillary incisors. However, U1 was retroclinated by 9.45° in the P-TAD group, far exceeding that in the non-TAD group, likely due to greater U1 lingualization in the former.
The additional use of anterior labial TADs resulted in a net effect of +7.75° on U1 labial crown torque, as U1 retroclination (torque loss) was only 1.70° in the AP-TAD group compared to 9.45° in the P-TAD group. A recent study reported U1 retroclination of 8.06° with a midline TAD and 12.29° with bilateral posterior TADs.1 In the present study, the AP-TADs configuration retracted the incisors almost translatively, indicating that the combined force of posterior retraction and anterior intrusion passes very close to the center of resistance of the maxillary incisors.
Attempts have been made to correlate ARR with treatment-related parameters in cases of premolar extraction.11,34-37 Factors such as positive torque change,34,37 horizontal retractions,35,36 intrusions,34,38 and treatment duration11 have been reported as potential influences. The current study found a consistent correlation between the ARR and palatal root torque in all three groups. Palatal root torque per se increases stress at the root apex, which can be aggravated by the proximity of dense anatomical barriers such as the palatal cortices. Intrusion was moderately correlated with ARR in the non-TAD and AP-TAD groups. However, this trend was not observed for the P-TAD group, probably because U1 extrusion rather than intrusion occurred in this group.
Root encroachment on skeletal structures, such as the palatal cortex and incisive canal,39 during orthodontic tooth movement has been proposed as a contributing factor for ARR during anterior retraction. Although the root trunk impinges upon the dense anatomical barriers before the apex during retraction, it invariably experiences irreversible resorption. Additionally, the surface convexity peaks at the apex and levels off toward the cervical direction. Nano-indentation studies also corroborate that both the hardness and elastic modulus of cementum are lower at the root apex than at the cervical and middle thirds.40 On the other hand, significant ARR has not been observed in cases of pure intrusion without retraction.41 Intrusion, when confined to the spongy part of the alveolar process, seems unlikely to cause severe resorption.13 Thus based on previous studies, resistance to root resorption is higher at cancellous alveolar bone than palatal cortex and at cervical root trunk than root apex.
Notably, the AP-TAD group exhibited a more severe U1 ARR than the P-TAD group. While there was no significant difference in the lingualization of the U1 tip, U1 retroclination was significantly greater in the P-TAD group compared to the AP-TAD group (Table 4). Consequently, the AP-TADs group experienced greater lingual movement of the U1 apex, although this could not be measured directly in this study due to the extent of ARR. A previous study associated the severity of ARR with the amount of horizontal displacement of incisor apices.12 In addition, U1 intrusion was positively correlated with ARR in the AP-TAD group (Table 5), suggesting that it is a contributing factor. Interestingly, previous studies have shown that intrusion alone does not appear to increase incisor ARR, whereas intrusion during anterior retraction does.34,38 Taken together, the palatal and intrusive movements of the U1 apex in the AP-TADs configuration may be associated with increased ARR.
A subsequent question arose: Is the greater severity of ARR in the AP-TAD group relative to the P-TAD group solely attributable to differences in tooth movement? The linear regression model demonstrated that, after excluding tooth movement factors, the AP-TAD configuration continued to exhibit an additional 0.5 mm of ARR compared to the P-TAD group. This can be explained from a mechanical perspective: the combined retraction and intrusion force vectors result in a composite force greater in magnitude than the retraction force alone. This aligns with the ‘parallelogram rule’. Our findings highlight an important but often overlooked clinical recommendation: when posterior and anterior TADs are used in combination, retraction and intrusion forces should be reduced compared to when each is used independently.
A number of pulpal changes have been identified in orthodontically treated teeth, including transient reductions in pulpal blood flow, disruption of the odontoblastic layer, fibrotic tissue formation, and increased expression of Substance P and vascular endothelial growth factor.42 In the current study, a significant reduction in pulp chamber dimension was observed across all three groups, consistent with previous findings.15 Pulpal blood flow was restricted for only a few weeks before fully recovering when performing TAD-assisted incisor intrusion with a heavy force of 120 cN.43 Examination of its cemento-dentine-canal junction shows that only 38.6% of maxillary central incisors have a single main root canal, while 49.1% have lateral branches, 6.7% have apical ramifications, and 5.6% have a combination of both.44 Purely intrusive force may primarily affect the trunk root canal rather than the auxiliary lateral branches. However, when U1 is simultaneously intruded and retracted, as in the AP-TAD group, the arteriovenous system at the apical foramen may find the lateral ‘backup’ branches insufficient to manage ischemic episodes. It is postulated that the excess tertiary dentin formed in the AP-TAD group may have been triggered by the multidirectional and higher force levels applied. Fortunately, pulp chamber shrinkage has largely remained asymptomatic and self-limiting because the widening of the apical constriction by ARR helps to restore pulpal blood supply and venous drainage, thus preserving long-term nourishment.
The post-treatment U1 palatal dehiscence incidence rate was 16.7% in the non-TAD group and approximately doubled in the TAD groups, consistent with a recent study.45 Palatal dehiscence has become less of a concern since reports of self-recovery emerged.46 It has been proposed that palatal dehiscence is associated with a better prognosis, probably due to the naturally ‘thick’ gingival biotype. Recent studies have found evidence supporting self-recovery within 18–24 months. However, the underlying mechanism of spontaneous recovery of exposed root trunks remains unclear. Some studies observed differential anterior drift between the incisal edge and root apex in the dehiscence cohort, with the apex sinking 0.75 mm into the alveolus, whereas the incisal edge advanced 0.27 mm.47 Others reported ‘zero’ relapse, challenging the torque relapse theory.48 Long-term follow-up of such cohort is needed to verify the findings.
This study has some limitations. It is retrospective in nature, and baseline homogeneity cannot be guaranteed since the decision for labial TADs is based on the presence of a deep bite. Additionally, CBCT images were collected immediately after debonding, and a long-term follow-up would better elucidate the prognosis of dental, periodontal, and endodontic side effects.
In light of the increased ARR in the AP-TADs group, a clinical decision tree was tentatively proposed for cases requiring both retraction and intrusion of the maxillary incisors based on pre-treatment root length (Figure 5). When ARR is potentially too severe to be considered clinically insignificant, the ideal incisor position objective should be compromised, involving a reduction in bodily retraction and intrusion of the incisors. Either the AP-TAD configuration must be abandoned, or the magnitude of retractive and intrusive force should be reduced to achieve an optimal combined force. Furthermore, as intermittent forces may mitigate root resorption,49 elastics, rather than power chains, may be recommended for such cases.
Compared to posterior miniscrews alone, combined anterior and posterior miniscrews enhance intrusion and reduce torque loss of the maxillary central incisors, but they also cause more severe ARR in anterior retraction cases, potentially due to increased apical movement and greater composite force exerted. Therefore, the AP-TADs configuration should be used with caution when the roots of the maxillary incisors are short. Further studies are needed to identify strategies for preventing these adverse effects.
Conceptualization: WZ, Yu L. Data curation: ZW, DW. Formal analysis: ZW, DW. Investigation: ZW. Methodology: ZW, ZT, DW. Project administration: ZW, YZ. Resources: ZW, YZ. Software: ZW, ZT, YZ. Supervision: Yu L. Visualization: ZW, LM. Writing–original draft: ZW. Writing–review & editing: LM, ZT, EAA, Yuan L, Yu L.
No potential conflict of interest relevant to this article was reported.
None to declare.
Table 1 . Demographics of the Non-TAD, P-TADs, and AP-TADs groups.
Non-TAD (n = 27) | P-TADs (n = 30) | AP-TADs (n = 25) | |
---|---|---|---|
Sex | |||
Female | 22 | 27 | 24 |
Male | 5 | 3 | 1 |
Angle classification | |||
Class I | 19 | 18 | 12 |
Class II | 8 | 12 | 13 |
ANB (°) | 2.8 ± 2.6 | 3.8 ± 2.1 | 3.8 ± 2.1 |
Age (yr) | 23.5 ± 5.6 | 23.5 ± 4.2 | 24.4 ± 4.9 |
Treatment duration (mo) | 32.6 ± 11.5 | 36.2 ± 11.9 | 39.4 ± 13.1 |
Values are presented as number only or mean ± standard deviation..
TAD, temporary anchorage device; P-TADs, two posterior miniscrews only; AP-TADs, two anterior and two posterior miniscrews combined..
Table 2 . Parameters measured using the cone-beam computed tomography image.
Parameter | Definition |
---|---|
U1-PNSVert (mm) | Distance between midpoint of the central incisor edge and the coronal plane through PNS |
U1-PP (mm) | Distance from midpoint of the central incisor edge to PP |
U1-FH (°) | Lower posterior angle between longitudinal axis of the central incisor and FH plane, projected onto MSP |
U1 length (mm) | Distance between midpoint of the central incisor edge to its apex |
U1 pulp area (mm2) | Cross-sectional area of the pulp chamber on its MSP |
U1 palatal dehiscence (mm) | Longest distance from CEJ to palatal alveolar crest on the palatal side |
U1 lingualization (mm) | Pre-tx U1-PNSVert minus post-tx U1-PNSVert |
U1 intrusion (mm) | Pre-tx U1-PP minus post-tx U1-PP |
U1 retroclination (°) | Pre-tx U1-FH° minus post-tx U1-FH° |
U1 ARR (mm) | Pre-tx U1 length minus post-tx U1 length |
U1 pulp chamber shrinkage (mm2) | Pre-tx U1 pulp area minus post-tx U1 pulp area |
PP, palatal plane; FH, Frankfurt Horizontal; MSP, mid-sagittal plane; CEJ, cementoenamel junction; ARR, apical root resorption..
Table 3 . Baseline comparison among the three groups.
Non-TAD | P-TADs | AP-TADs | P value | |
---|---|---|---|---|
U1-PNSVert (mm) | 51.29 ± 3.83a | 54.24 ± 3.25b | 53.39b ± 3.92b | < 0.01 |
U1-PP (mm) | 27.54 ± 2.89a | 27.70 ± 2.95a | 29.27b ± 2.65b | < 0.01 |
U1-FH (°) | 115.06 ± 9.99a | 121.43 ± 8.51b | 117.84 ± 8.47a,b | < 0.01 |
U1 length (mm) | 22.41 ± 1.51 | 22.70 ± 1.60 | 22.83 ± 1.47 | 0.36 |
U1 pulp (sagittal) (mm2) | 5.38 ± 0.90 | 5.36 ± 0.73 | 5.35 ± 1.13 | 0.98 |
Values are presented as mean ± standard deviation. Means not sharing superscripts differ significantly at α = 0.05 level as indicated by Bonferroni’s correction..
TAD, temporary anchorage device; P-TADs, two posterior miniscrews only; AP-TADs, two anterior and two posterior miniscrews combined; PP, palatal plane; FH, Frankfurt Horizontal..
Table 4 . Treatment changes of U1 in the three groups.
Non-TAD | P-TADs | AP-TADs | P value | |
---|---|---|---|---|
U1 lingualization (mm) | 3.07 ± 2.87a | 6.53 ± 2.28b | 5.52 ± 2.43b | < 0.01 |
U1 intrusion (mm) | 0.30 ± 1.61a | −0.07 ± 1.65a | 1.99 ± 2.37b | < 0.01 |
U1 retroclination (°) | 1.91 ± 9.39a | 9.45 ± 10.68b | 1.70 ± 8.80a | < 0.01 |
U1 ARR (mm) | 0.89 ± 0.97a | 1.63 ± 1.46b | 2.69 ± 1.38c | < 0.01 |
U1 pulp reduction (mm2) | 0.21 ± 0.46a | 0.31 ± 0.54a,b | 0.50 ± 0.53b | < 0.05 |
U1 palatal dehiscence (mm) | 3.74 ± 1.23a | 5.25 ± 1.57b | 5.13 ± 1.66b | < 0.01 |
Values are presented as mean ± standard deviation. Means not sharing superscripts differ significantly at α = 0.05 level as indicated by Bonferroni’s correction..
TAD, temporary anchorage device; P-TADs, two posterior miniscrews only; AP-TADs, two anterior and two posterior miniscrews combined; ARR, apical root resorption..
Table 5 . Association between each parameter and the U1 apical root resorption in the three groups according to Pearson’s correlation analysis.
Non-TAD | P-TADs | AP-TADs | ||||||
---|---|---|---|---|---|---|---|---|
r | P | r | P | r | P | |||
preU1length | 0.231 | 0.092 | 0.067 | 0.611 | 0.048 | 0.742 | ||
preU1PNSVert | −0.007 | 0.963 | 0.255* | 0.050 | 0.101 | 0.494 | ||
preU1PP | −0.259 | 0.058 | 0.062 | 0.637 | −0.285* | 0.045 | ||
preU1FH° | 0.169 | 0.223 | 0.311* | 0.016 | 0.205 | 0.153 | ||
U1 lingualization | −0.143 | 0.303 | −0.310* | 0.016 | 0.080 | 0.582 | ||
U1 intrusion | 0.497*** | 0.000 | 0.250 | 0.054 | 0.451** | 0.001 | ||
U1 retroclination | −0.493*** | 0.000 | −0.487*** | 0.000 | −0.349* | 0.013 | ||
Duration | −0.257 | 0.061 | −0.160 | 0.221 | −0.326* | 0.021 |
TAD, temporary anchorage device; P-TADs, two posterior miniscrews only; AP-TADs, two anterior and two posterior miniscrews combined; PP, palatal plane; FH, Frankfurt Horizontal..
*The correlation is significant at the 0.05 level (2-tailed)..
**Correlation is significant at the 0.01 level (2-tailed)..
***Correlation is significant at the 0.001 level (2-tailed)..
Table 6 . Multiple linear regression for the U1 apical root resorption in the combination of P-TADs and AP-TADs groups.
Variable | Unstandardized B | Coefficient standard error | Standardized β | P |
---|---|---|---|---|
Constant | 1.109 | 0.409 | 0.008 | |
U1 retroclination | −0.101 | 0.017 | −0.707 | < 0.001 |
U1 lingualization | 0.226 | 0.073 | 0.358 | 0.003 |
τ | 0.509 | 0.265 | 0.169 | 0.049 |
P-TADs, two posterior miniscrews only; AP-TADs, two anterior and two posterior miniscrews combined..