INTRODUCTION

Clear aligner treatment (CAT) has become an accepted treatment modality for the orthodontic correction of malocclusion by orthodontists. Invisalign (Align Technology, San Jose, Calif) has become a commonly prescribed CAT appliance among dental clinicians globally.¹ Treatment with the appliance involves the manufacture of a series of aligners from impressions or an intraoral three-dimensional (3D) image of the patient’s dentition. The clinician can adjust the treatment plan provided by the aligner manufacturer with the use of propriety software. The software can aid the clinician in the treatment-planning processes as it enables a visual representation of the predicted outcome of the treatment plan. Once satisfied with the digital treatment plan (DTP), the clinician can initiate the manufacture of the initial treatment series.²

Among the strategies adopted to facilitate effective use of the appliance are the deliberate positioning of bonded composite resin (CR) attachments with distinct geometric shapes onto the teeth and the interproximal reduction (IPR) of enamel.^3–7 Studies, however, have shown that the actual (achieved) treatment outcome after an initial series of aligners rarely matches the predicted outcome.^8–11 Consequently, patients are commonly required to undergo one or more refinement phases by which additional aligners are prescribed to achieve the desired outcome.^12,13

The treatment planning processes related to Invisalign have been minimally explored in the literature.¹² Recent studies indicated that the initial DTP provided by Align Technology is rarely accepted by the treating clinician.^1,12 Adjustments by the clinician to the initial plan are, therefore, commonly employed before acceptance.¹³ Further new information regarding digital treatment-planning processes may enable the orthodontist to formulate more effective DTPs resulting in improved treatment protocols and a more cost-effective use of orthodontist time.¹² The aim of the present study, therefore, was to investigate the DTP processes associated with the Invisalign appliance by determining the differences between the initial plan provided by its provider, Align Technology, and the final accepted plan by the orthodontist. A secondary aim was to determine whether these numbers/differences vary significantly between subjects who underwent extraction of permanent teeth as a necessary part of their orthodontic treatment and those who did not.

MATERIALS AND METHODS

Ethical approval was granted by the University of Adelaide Human Research Ethics Committee (H-2022-197) for this retrospective investigation. The information for the study was obtained from the Australasian Aligner Research Database (AARD). AARD consists of data pertaining to more than 17,000 patients treated with the Invisalign appliance by 16 orthodontists practicing in Australia, New Zealand, and the United States, experienced in the use of the appliance. Before treatment, each patient had consented for their demographic and treatment data to be used for research purposes. Patients who had commenced treatment between 2018 and 2022 were considered for participation in the study.

Five hundred subjects were randomly selected using a random sequence of integer generator (https://www.random.org/sequences/) for the present investigation from the database, and the following inclusion and exclusion criteria were applied: (1) treated with the Invisalign appliance only no earlier than 2018, (2) in the permanent dentition, (3) without medical conditions and/or prescribed medication that may have altered bone metabolism, and (4) treatment-planning process completed and aligners manufactured for the initial treatment phase.

Information regarding subject gender, age, and whether each had undergone extraction of teeth as part of the orthodontic treatment was recorded on a Microsoft Excel (Microsoft, Redmond, Wash) spreadsheet. In addition, data regarding the following were collected: (1) total number of DTPs before acceptance by the orthodontist, (2) total number of aligners from Align Technology’s initial proposed plan, (3) total number aligners from the accepted plan by the orthodontist, (4) total number of teeth to which CR attachments were placed and IPR contact points prescribed by Align in the initial plan, and (5) total number teeth to which CR attachments were placed and IPR contact points prescribed in the accepted plan by the orthodontist.

Descriptive statistics were calculated via GraphPad Prism 9.0 (GraphPad Software Inc, La Jolla, Calif). The normality of all groups and subgroups, including those with prescribed orthodontic extractions (extraction group) and those with no prescribed extractions (nonextraction group) within the study, were assessed with the Shapiro-Wilk test. As all groups and subgroups had a nonnormal distribution, Wilcoxon rank-sum and Mann-Whitney tests were adopted to determine significant differences between groups and subgroups. The Spearman correlation test was used to determine the association between groups and subgroups. The data of 40 randomly chosen subjects were checked via intracorrelation coefficient (ICC) testing to evaluate intrarater reliability in data input to the Excel spreadsheet.

RESULTS

Most of the 431 subjects who satisfied inclusion and exclusion criteria were female (72.85%, n = 314). The ICC scores for data input were excellent (0.99 to 1). Table 1 outlines the demographic data of the subjects included in the study. Table 2 shows the number of extracted teeth per arch per subject in the extraction group.

Table 1. Demographic Statistics of Subjects (n = 431)^a

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Table 2. Number of Extracted Teeth per Arch Per Subject (n = 101)^a

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The initial DTP was accepted by the orthodontist for 54 (16.4%) subjects in the nonextraction group, whereas the initial DTPs of two (2%) subjects only in the extraction group were accepted by the orthodontist. The median (interquartile range [IQR]) number of DTPs per subject before acceptance was 3 (2, 4; minimum: 1, maximum 14). A Mann-Whitney test indicated that there were more median DTPs before acceptance (P < .0001) among the extraction group (Figure 1).

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Table 3 shows that there was a significant increase in the number of aligners prescribed between the initial and accepted DTP in all groups. Five subjects had no prescribed CR attachments in the accepted DTP of the nonextraction group. The minimum number of prescribed CR attachments per tooth in the accepted DTP of the extraction group was 12.

Table 3. Comparison of Initial and Accepted DTP Details^a

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Table 4 shows that there was no significant change in the number of teeth prescribed with CR attachments in the accepted plan compared with the initial plan in those subjects with a prescribed 1-week aligner change protocol and extraction of permanent teeth as part of the orthodontic treatment plan.

Table 4. Intragroup Comparisons Between the Initial and Accepted DTP Details of a 1-Week Aligner Change Protocol for Extraction Cases (n = 49)^a

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Table 5 indicates that there was a significant increase in the number of teeth prescribed with CR attachments in the accepted plan compared with the initial plan in those subjects with a prescribed 2-week aligner change protocol.

Table 5. Intragroup Comparison Between the Initial and Accepted DTP Details of a 2-Week Aligner Change Protocol (n = 52)^a

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One hundred twenty-eight (38.67%) subjects in the nonextraction group had no prescribed IPR in their accepted DTP. The median (IQR) number of IPR contact points among the 18 subjects in the extraction group who had IPR was 3 (1.5, 7; minimum: 1, maximum: 18), whereas the median (IQR) number of IPR contact points among the nonextraction group who had IPR was 5 (3, 9; minimum: 1, maximum: 22).

A greater number of DTPs was not correlated with an increased number of aligners in the accepted DTP in either the extraction group (r = 0.128; 95% CI: −0.08 to 0.32) or the nonextraction group (r = 0.13; 95% CI: −0.063 to 0.32).

The Mann-Whitney tests indicated that there were more teeth with CR attachments in the extraction treatment compared with the nonextraction treatment group (P < .0001) in the initial plan and that there were more CR attachments in the extraction treatment group compared with the nonextraction treatment group (P < .0001) in the accepted DTP. There was no correlation between the number of teeth extracted and the number of teeth with CR attachments (r = 0.15; 95% CI: −0.05 to 0.34) and the total number of CR attachments (r = 0.23; 95% CI: 0.027 to 0.41).

There was a weak correlation between the number of the final IPR contact points and the number of extractions prescribed in the final treatment plan (r = −0.27; 95% CI: −0.45 to −0.078).

DISCUSSION

The global popularity of CAT and the paucity of available evidence regarding adjunctive DTP strategies emphasize the relevance of the present investigation. The overall sample of 431 subjects was higher than the 75–355 evaluated in comparable studies.^7,13,14 A large sample was chosen, as there are little available data with which to calculate a number to power the study. Most subjects were female, and the average age was 30.55 years, corresponding to the demographic frequently treated with CAT.^15,16

The median number of DTPs required for the subjects in this study before acceptance of the plan was 3. This compared with means of 3.07–7 reported in recent studies.^12,17 It also corresponded with the outcome of a recent national survey in which almost 90% of orthodontists reported that they made up to six changes to the initial DTP before acceptance.¹ Interestingly, in the current study, a significantly greater number of DTPs were required for cases with orthodontic extractions prescribed as part of the treatment compared with nonextraction cases. This may have reflected the extra complexity associated with malocclusion requiring extraction and/or difficulties with tooth movement into spaces created by the extractions. It may also help to explain why many orthodontists do not carry out CAT in patients requiring orthodontic extractions.¹

The median number of initial aligners in the initial DTP for the nonextraction group was 24.5. A significant increase to 26 was observed in the accepted plan. This compared with the accepted prescription of a mean of 19–22 initial aligners in a 2021 randomized clinical trial and 23.2 in a recent retrospective cohort study.^13,18 However, data are lacking in the literature regarding the changes clinicians make in the number of aligners, and why those changes are made, between the initial DTP and the plan ultimately accepted.

This investigation is one of the first to evaluate the prescription of CR attachments and IPR as part of the treatment-planning processes with the Invisalign appliance. A recent survey among orthodontists identified CR attachments as being a factor that always or mostly required change from the initial plan by more than 60% of respondents.¹

Less than 1% of subjects had no prescribed CR attachments in the investigation presented here. This appeared to confirm recognition that CR attachments are deemed to be required for effective control of tooth movement.^3,8 The median number of teeth with attachments in the present study (15) compared with an overall mean of 12 in a 2020 prospective clinical study.¹⁹ In the present investigation, significantly more teeth had attachments placed on them in the accepted DTP compared with the initial plan. Future research is required to investigate the number, type, and precise positioning of CR attachments to facilitate optimal treatment processes.

Almost 62% of the subjects in the nonextraction group were prescribed IPR in their accepted DTP. This compared with 71% among 500 subjects who had nonextraction CAT in a 2022 study.⁴ It also exceeded the 55.3% of their annual CAT caseload that orthodontists reported necessitated IPR to achieve treatment goals.¹ The median number of the prescribed IPR contact points among those subjects in the nonextraction group who had IPR in the study presented here was five, which corresponded with the mean 6.92 contact points reported in the 2022 study.⁴

There was a significant increase in the number of IPR contact points from the initial to the accepted DTP among nonextraction subjects. The use of IPR is not surprising as it is a commonly adopted nonextraction strategy to relieve crowding, with almost 10 mm of space obtainable in some orthodontic patients via the technique.^2,20

Interestingly, IPR was also carried out in almost 18% of the subjects in the extraction group in the present study. This may have suggested significant space requirements to address the subjects’ malocclusion. Alternatively, it may have indicated a desire to minimize unwanted transverse expansion and manage open gingival embrasures.^1,21 However, information regarding the change in prescribed IPR from the initial to the accepted DTP appears to be lacking in the literature. Further research, therefore, is required to investigate how IPR can be effectively integrated into CAT, as current evidence suggests that the planned removal of interproximal enamel does not match that removed by the clinician during treatment.^4–6,21

The shortcomings of this investigation are acknowledged. The findings of this study are applicable to the Invisalign appliance only and not CAT appliances from other providers. The risk of potential bias associated with retrospective studies, however, was lessened by the rigorously applied inclusion and exclusion criteria for subject selection.

Further research is required to investigate the additional changes to DTPs made by orthodontists prior to acceptance, including those related to the final position of individual teeth and changes in movement staging such as how the teeth move from their initial position to their final alignment. In addition, future investigation is required to assess the modifications required to achieve specific treatment objectives such as desired changes in overbite, transverse expansion, and root parallelism associated with extraction sites.

The outcomes of this investigation, however, have provided new information regarding the digital treatment-planning processes related to the Invisalign appliance. The findings emphasize the need for diligent review and appropriate modification of the DTP by the orthodontist. Reliance on the DTP proposed by Align may have been deemed to result in inappropriate treatment planning and unsatisfactory treatment outcomes. The study presented indicated that significant changes regarding IPR contact points and CR attachments were required after the receipt of the initial plan from Align. In addition, up to 14 iterations of DTPs per patient were required before acceptance, which illustrated the need for the expertise of the orthodontist in formulating an acceptable treatment plan.

Despite the changes in treatment protocols observed in this study, achieved outcomes do not routinely match predicted outcomes.^8–10,22 Knowledge, therefore, such as that determined in the present study, may help inform future research regarding the identification and management of the factors necessary to enhance treatment-planning processes.

CONCLUSIONS

• Significant changes regarding the required number of DTPs and differences in the prescription of aligners, CR attachments, and IPR contact points were observed between the initial and accepted DTPs in the study presented here.

• Diligent appraisal of the DTP provided by the CAT provider is essential.

• Further research is required to enable the identification and management of the factors necessary to increase the effectiveness of DTP planning processes.