Methods of Anterior Torque Control during Retraction: A Systematic Review

In vitro biomechanics of divot use, and their placement, in orthodontic aligner therapy

INTRODUCTION

To evaluate biomechanics of an aligner utilizing divots and the effect of their vertical placement on the right maxillary central incisor.

METHODS

An in vitro Orthodontic SIMulator (OSIM) was used to test forces and moments generated by aligners incorporating divots. The OSIM arch was scanned to generate a. STL version that was modified to create four models by placing divots on different positions of the right central maxillary incisor: GI - divots on gingival-third of lingual surface and incisal-third of labial surface; GM - divots on gingival-third of lingual surface and middle-third of labial surface; MI - divots on middle-third of lingual surface and incisal-third of labial surface; MM - divots on middle-third of lingual surface and middle-third of labial surface. Aligners (n = 30/model) were fabricated using a 0.75 mm thick polyethylene terephthalate material and Biostar® machine following the manufacturer's recommendations. A one-way MANOVA followed by one-way ANOVA (α = 0.05) was utilized to test effect of models on buccolingual force (Fy) and mesiodistal moment (Mx) at 0.20 mm of lingual displacement of the right maxillary central incisor.

RESULTS

Mean Mx for GI (-5.68 ± 7.38 Nmm), GM (3.75 ± 5.54 Nmm), MI (-4.27 ± 1.48 Nmm) and MM (1.96 ± 0.99 Nmm) models showed statistical differences between GI and GM, GI and MM, GM and MI and MI and MM. GI exerted the largest Fy (1.87 ± 0.75 N) followed by GM (1.10 ± 0.47 N), MI (0.70 ± 0.23 N) and MM (0.28 ± 0.08 N) with significant differences between GI and GM, GI and MI, GI and MM and GM and MM models.

CONCLUSIONS

Vertical divot placement on a right central incisor had a significant effect on aligner biomechanics. Buccolingual forces exerted by models GI, GM and MI were within the range suggested by literature for bodily tooth movement without major root tipping for GM and MI models.

Efficacy of planned moderate to severe torque changes in maxillary central incisors with the Invisalign appliance: A retrospective investigation

INTRODUCTION

This study aimed to investigate whether there was a difference between the planned and achieved lingual root torque of the maxillary central incisors in patients treated with an initial series of Invisalign (Align Technology, Santa Clara, Calif) aligners in which >10° change in the inclination of these teeth was prescribed via the ClinCheck facility.

METHODS

The pretreatment, planned, and the digital models after wear of the initial series of aligners regarding adult patients who satisfied selection criteria and were treated using the Invisalign appliance were measured using the Geomagic Control X software (version 2017.0.3; 3D systems, Rock Hill, SC).

RESULTS

A sample of 63 patients with 126 maxillary central incisors satisfied the inclusion criteria. The mean accuracy of the planned torque change of the maxillary central incisors that was achieved was 41.9%. A clinically significant shortfall (≥5°) was detected in 78.6% of the incisors, with the majority showing an underexpression between 10°-15°. Linear regression analyses indicated that weekly or biweekly wear protocols or the presence or absence of power ridges did not influence the accuracy of planned torque expression (P >0.05).

CONCLUSIONS

The changes in torque expression with an initial series of Invisalign aligners were less than half of what was planned in patients in which at least a 10° change in lingual root torque of these teeth was prescribed. The presence of power ridges and the aligner change protocol did not appear to significantly affect the accuracy of maxillary central incisor torque expression.

En masse retraction of anterior teeth through rapid periodontal distraction by a retraction screw: A randomized control trial

BACKGROUND

Rapid distraction of the periodontal ligament is an effective method to shorten the orthodontic treatment time. The objectives of the present study were to assess the effects of an HYCON device (Adenta GmbH, Germany) on the rate of en masse retraction of the anterior teeth, duration of retraction, anchorage loss, root resorption, and soft tissue changes.

METHODS

This study was conducted on 60 female patients aged >18 years, divided randomly into two equal groups: Group 1 comprised 30 patients with HYCON, and group 2 comprised 30 patients with nickel-titanium closed coil springs. Skeletal, dental, and soft tissue changes were evaluated on pre- and post-retraction lateral cephalograms, and the rates of anterior tooth movement and anchorage loss were assessed monthly on the dental casts of the patients. Root resorption was assessed using intraoral periapical radiograph. Student's t test was used for the analysis of parametric data, and the Mann-Whitney U test was used for nonparametric data.

RESULTS

HYCON significantly shortened the retraction duration by 3 months. The rate of anterior teeth retraction was two times faster in group 1, compared with group 2. There was a significant difference in the anchorage loss between the groups in only first 2 months of treatment. Group 2 showed significantly more root resorption and soft tissue changes than group 1 (P < 0.05).

CONCLUSIONS

HYCON is an effective device for significantly shortening the duration of retraction with anchorage loss of 2 to 2.5 mm. However, careful monitoring for possible root resorption should be performed.

The Hydrostatic Pressure Distribution in the Periodontal Ligament and the Risk of Root Resorption-A Finite Element Method (FEM) Study on the Nonlinear Innovative Model

Excessive orthodontic force can induce inflammatory tooth root resorption due to sustained high stresses within the periodontal ligament (PDL). This study aimed to analyze the PDL pressures during upper incisor retraction using the en masse method with TISAD. The finite element method (FEM) ensured consistent conditions across cases. The models included bone geometry, adjacent teeth, PDL, and orthodontic hardware, analyzed with LS-Dyna. The pressure ranged from 0.37 to 2.5 kPa across the dental arch, with the central incisors bearing 55% of the load. The pressure distribution remained consistent regardless of the force or hook height. The critical pressure (4.7 kPa) was exceeded at 600-650 g force, with notable pressure (3.88 kPa) on the palatal root wall of the right central incisor. Utilizing 0.017 × 0.025 SS archwires in MBT 0.018 brackets provided good torque control and reduced the root resorption risk when forces of 180-200 g per side were applied, maintaining light to moderate stress. Triple forces may initiate resorption, highlighting the importance of nonlinear finite element analysis (FEA) for accurate oral cavity simulations.

Biomechanical Simulation of Orthodontic En-Bloc Retraction Comparing Compound Technique and Sliding Mechanics Using a HOSEA Robotic Device

En-bloc retraction is a common procedure in orthodontic therapy. The application of palatal root torque moments is required to control incisor inclination during retraction, yet studies comparing forces and moments with respect to different mechanics are lacking. This study aimed to investigate the forces and moments during orthodontic en-bloc retraction using a robotic biomechanical simulation system, comparing two distinct approaches: (I) compound technique [stainless steel (SS) combined with nickel-titanium (NiTi)] using industrially pretorqued retraction-torque-archwires (RTA) in combination with NiTi closed coil springs; (II) conventional sliding mechanics using SS archwires with manually applied anterior twist bends in combination with elastic chains. Two dimensions (0.017" × 0.025" and 0.018" × 0.025") and ten archwires per group were investigated using 0.022" slot self-ligating brackets. Kruskal-Wallis tests with a significance level of α = 0.05 were conducted. The biomechanical simulation showed that en-bloc retraction was characterized by a series of tipping and uprighting movements, differing significantly regarding the examined mechanics. Collateral forces and moments occurred in all groups. Notably, RTA exhibited fewer extrusive forces. The most bodily movement was achieved with the compound technique and the 0.018" × 0.025" RTA. Sliding mechanics exhibited maximum palatal root torque moments of more than 20 Nmm, exceeding recommended values.

Efficacy of a four-curvature auxiliary arch at preventing maxillary central incisor linguoclination during orthodontic treatment: a finite element analysis

BACKGROUND

Correct torque of the incisors is beneficial in the assessment of the effects of orthodontic treatment. However, evaluating this process effectively remains a challenge. Improper anterior teeth torque angle can cause bone fenestrations and exposure of the root surface.

METHODS

A three-dimensional finite element model of the maxillary incisor torque controlled by a homemade four-curvature auxiliary arch was established. The four-curvature auxiliary arch placed on the maxillary incisors was divided into four different state groups, among which 2 groups had tooth extraction space retracted traction force set to 1.15 N. Initial displacements and pressure stresses of the periodontal tissue in the maxillary incisors and molars were calculated after torque forces (0.5, 1, 1.5, and 2 N) were applied to the teeth at different stable states.

RESULTS

The effect of using the four-curvature auxiliary arch on the incisors was significant but did not affect the position of the molars. Given the absence of tooth extraction space, when the four-curvature auxiliary arch was used in conjunction with absolute anchorage, the recommended force value was < 1.5 N. In the other 3 groups (i.e., molar ligation, molar retraction, and microimplant retraction groups), the recommended force value was < 1 N. The application of a four-curvature auxiliary arch did not influence the molar periodontal and displacement.

CONCLUSION

A four-curvature auxiliary arch may treat severely upright anterior teeth and correct cortical fenestrations of the bone and root surface exposure.