INTRODUCTION

Clear aligner therapy (CAT) is an integral part of modern orthodontics.^1–3 The Invisalign (Align Technology, San Jose, Calif) appliance was the first CAT appliance to use transparent, thermoplastic polymeric materials to fabricate a series of removable aligners using computer-aided design and computer-aided manufacturing (CAD-CAM) technology.¹

The proprietary online software ClinCheck is the communication interface between the treating clinician and Align Technology.⁴ It provides a virtual representation of the treatment plan, progression of tooth movement stages, and a variety of digital data.⁵ When the treatment plan is approved by the clinician, a series of aligners is manufactured to enable the prescribed occlusal result to be achieved. One or more additional series of aligners is commonly required to achieve desired occlusal outcomes.^2,4

Root torque is defined as the buccolingual inclination of the tooth root.^6,7 It was described by Andrews⁷ as the “third key of occlusion” and determines the position of the tooth crown. Root torque plays a critical role in orthodontic treatment outcome and posttreatment stability.^7–9 Crown and root movement have also been considered in relation to the center of rotation (Crot) and center of resistance (Cres) of the tooth. Four movement types have been described. Controlled tipping takes place when the Crot is within 2 mm of the root apex. Uncontrolled tipping occurs when the Crot is within 2 mm of the Cres. Torque happens when the Crot is within 2 mm of the incisal edge. Translation occurs when the Crot is more than six times the alveolar bone height of the original Cres position.

In fixed appliance therapy, changes in root torque principally result from the mechanical interaction between the arch wire and bracket geometries.^6,10 Due to the complex force delivery system of the aligner, however, the ability to achieve the prescribed amount of root torque and, consequently, incisor inclination, has proven to be challenging.^1,11,12

To overcome the purported biomechanical limitations of CAT in this regard, modifications to aligner characteristics have been introduced.¹³ One such modification is the power ridge (PR), which is a localized indentation integrated within the appliance with the aim of enabling greater control of root movement.^1,3

Despite the popularity of CAT, high-quality evidence is limited regarding its efficacy in achieving planned tooth movements.^14,15 Although several studies^11,16–19 have examined the efficacy of planned changes in maxillary central incisor root torque using CAT, only two^12,20 appear to have focused on planned lower incisor torque changes. While these provided some information regarding lower incisor root movement, uncertainty over the definition of torque and sample sizes reported in the studies has prompted the need for further research.

Therefore, the aim of this study was to determine whether the achieved mandibular central incisor lingual root torque (LRT) changes matched the changes planned via ClinCheck after treatment with an initial series of Invisalign aligners when ≥10° of torque change was planned.

The null hypothesis was that no difference between the planned and the achieved changes in LRT would be found.

MATERIALS AND METHODS

Ethical approval was obtained from the University of Adelaide Human Research Ethics Committee. The sample for this retrospective study was obtained from an independent database of approximately 12,000 patients treated with the Invisalign appliance between 2013 and 2021 by 17 experienced Invisalign provider orthodontists. All patients provided informed consent for their information to be used for research purposes. The entire database was screened to determine the number of patients that satisfied the selection criteria (Table 1).

Table 1. Inclusion and Exclusion Criteria

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A planned torque change ≥10° was determined to be moderate to severe, as indicated in a recent similar investigation.¹⁹ Based on previous studies,^21,22 it was ascertained that a sample size of 44 mandibular central incisors was required to identify clinically relevant root movement ≥5°.

The patients that met the inclusion criteria were de-identified, and information regarding their age at the beginning of treatment, sex, number of aligners prescribed in the initial series, aligner wear protocol (WP; either 1 or 2 weeks) and the presence or absence of PRs were recorded on an Excel (Microsoft, Redmond, Wash) spreadsheet.

Digital models representing three time points (pretreatment [T0], the planned treatment outcome [T1], and the achieved outcome at the end of the initial series of aligners [T2]) were imported into the Geomagic Control × (Geomagic US, Research, Triangle Park, NC) metrology software facility in 3D stereolithographic (.STL) file format. Angular and linear measurements were recorded on the models at the different time points.

Superimposition of the three models was performed using the best-fit surface registration with global and fine best fit with a 50-iteration count and sampling ratio of 80%, which was validated in a recent study.²³ A transverse reference plane, as used by Gaddam et al.,¹² was generated on the T0 model (Figure 1A). To determine the amount of LRT, angular measurements were performed between the reference plane and a vertical vector, which was autogenerated by the flood selection tool as per similar studies.^12,19,21,24 The vertical vector representing the virtual long axis of the mandibular central incisors was determined for all three models (Figure 1B).

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Orthodontic tooth movement (OTM) can be defined by assessing the position of the Crot of the tooth relative to the Cres and the incisal edge.^21,25 Jiang et al.²⁵ determined that root torque OTM resulted if the Crot was within 2 mm of the incisal edge. To investigate the accuracy and type of OTM, the position of the Crot relative to the incisal edge, root apex, or the Cres was obtained and categorized according to the criteria described by Smith et al.²¹

RESULTS

A total of 35 patients with 70 mandibular central incisors satisfied selection criteria with a mean ± SD patient age of 32.3 ± 14.5 years. Table 2 shows that most patients were female (N = 48; 68.6%). Most central incisors (N = 38; 54.3%) had PRs integrated into the appliance. Most patients were prescribed a 1-week WP (N = 46; 65.7%). The mean ± SD number of aligners prescribed per patient was 41 ± 13.

Table 2. Demographic Statistics According to Subgroups (N = 70 Incisors; 35 Patients)^a

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The overall accuracy of the LRT expression was 58.2%. A Bland-Altman plot indicated that the differences between planned and achieved changes were approximately normally distributed (Figure 2). A cluster toward the lower end of average values and below the mean suggested that a pretreatment lower torque angle led to better accuracy.

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Figure 3 illustrates the difference between the planned and achieved LRT changes for each patient.

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Underexpression of the planned changes was observed in most incisors (N = 66; 94.3%), overexpression was observed in 2.9% (N = 2), and no difference was noted in two incisors (2.9%). To determine the percentage of incisors that showed a clinically significant shortfall of LRT expression, the level of underexpression was subdivided into increments of 5°. A clinically significant shortfall was observed in 48 incisors (68.6%), most of which under expressed between 10° and 20° (Table 3).

Table 3. Breakdown of Clinically Significant Torque Under Expression per Mandibular Central Incisor

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Linear regression analyses indicated no statistically significant effect regarding patient age, sex, WP, or the presence of PR on the difference between the planned and achieved LRT (Table 4).

Table 4. Linear Regression Analysis of Discrepancy Between Planned and Achieved Torque of Mandibular Central Incisors Using Selected Patient and Clinical Factors^a

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Figure 4 shows that no difference was found in the accuracy of torque expression between presence and absence of PRs, weekly WP, and combined PR and WP groups.

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Table 5 shows that 93.3–100% of mandibular central incisors with LRT or uncontrolled tipping movements planned achieved that OTM type.

Table 5. Planned and Achieved OTM According to PR and WWP Factors^a

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A trend toward greater accuracy was observed in those prescribed PRs, the 2-week WP, and the combined 2-week WP/PRs groups. However, this was not statistically significant (F = 0.92, P = .44).

ICC scores between 0.91 and 0.95 were achieved, indicating excellent reliability.

DISCUSSION

We are the first to investigate the efficacy of lower central incisor moderate to severe LRT changes using the Invisalign appliance. The findings indicated that the achieved changes were less than 60% of those planned and that WP and the presence of PRs made little difference to the accuracy of changes. The null hypothesis was rejected. The findings provide clinically relevant data concerning the management of lower central incisors, requiring considerable LRT changes with the Invisalign appliance.

The overall accuracy of torque expression was 58.2% of the planned change in LRT in lower central incisors. This reflected the challenges observed in fixed appliance therapy regarding the delivery of optimal torque.²⁶ With clinical significance defined as a ≥5° difference between planned and achieved outcomes in the present study, approximately 70% of the incisors showed a clinically significant shortfall of the planned changes. Most investigations regarding CAT torque efficacy focus on upper incisor root torque. However, authors of two studies^12,20 have examined the efficacy of CAT in producing desired torque changes in lower incisors. The current findings indicated greater efficacy than the landmark prospective study by Kravitz et al.,¹⁷ who reported an overall mean accuracy of 41%. That study, however, was carried out before the introduction of the currently used SmartTrack material. Interestingly, Gaddam et al.¹² demonstrated that the planned root torque changes were more reliably expressed in the lower incisors than the upper incisors, with a mean accuracy of 64.7% for lower labial crown torque being recorded. This was reportedly due to incisor proclination resulting from leveling of the curve of Spee. However, the definition of torque in that study was based on the labiolingual position of the crown, and the sample was subdivided depending on the direction of overexpression or underexpression. By contrast, 86.1% of the planned lower incisor torque changes was achieved in a 2017 study.²⁰ In that study, however, the authors used a different CAT system, and the torque movement evaluated was not clearly defined.

Linear regression analysis was conducted to determine the influence of patient and clinical factors. Although a trend toward greater accuracy was observed in those that had PRs, the 2-week WP, and the combined 2-week WP/PRs groups, one-way ANOVA indicated that this was not significant. These findings regarding WPs were like those in the study by Smith et al.²¹ concerning lower incisor mesiodistal root tip. However, in the present study, we suggest that an increased number of prescribed aligners increased the accuracy of torque expression. Further research is required to explore whether more frequent aligner change may improve the efficacy of planned torque changes.¹

The retrospective nature of the study risks selection bias. However, this was minimized by screening all patients from a very large database and adopting strict inclusion criteria. Although the criteria aimed to reduce the effect of potential confounding factors, any rotational corrections, the initial amount of spacing, or crowding present in which transverse corrections were required, as well as the underestimation of the mesiodistal widths of the teeth by ClinCheck, may favor forces to be directed more lingually or labially. Additionally, the findings of the present study are relevant to the initial series of Invisalign aligners and are not applicable to additional series of Invisalign aligners or other CAT brands.

CONCLUSIONS

• Achieved LRT changes in mandibular central incisors were 58.2% of those planned when ≥10° of change in LRT was planned with an initial series of Invisalign aligners.

• Most incisors (68.6%) showed a clinically significant (≥5°) underexpression of their planned LRT changes.

• The presence/absence of PRs, 1-weekly/2-weekly WPs, and combined WP and PR groups made little difference to the accuracy of planned LRT changes.

• An increased number of prescribed aligners appeared to increase the accuracy of LRT expression.