Control of transverse dental arch expansion is often fundamental to orthodontic treatment planning for various reasons. Inadequate transverse expansion can result in insufficient space for relief of crowding, may impact smile aesthetics,1,2 and may be implicated in the abnormal path of closure from the retruded contact position into maximum intercuspation.1,3 Moreover, improper buccolingual inclination of both posterior and anterior teeth may result in a less stable dentition and less harmonious occlusal relationships.4,5

The effectiveness of clear aligner therapy in achieving maxillary buccal expansion remains unclear, primarily owing to an absence of high-quality studies. However, a retrospective study by Zhou and Guo6 have indicated that although the Invisalign^® appliance could achieve expansion, excessive tipping of the maxillary dentition could likely occur. A systematic review by Papadimitriou et al.7 also demonstrated that Invisalign^® struggled with bodily tooth movement.

The release of SmartTrack^® material by Align Technology^® (San Jose, CA, USA) in 2013 was accompanied with claims of superior elasticity, potentially allowing the aligner to remain active for longer periods, which may result in improved clinical performance.8 By contrast, the material it superseded, known as EX30^®, may be less elastic in vitro,9 potentially being less resilient over time. In 2016, Align Technology^® recommended a change from the original 2-week aligner change regime, which was considered necessary to allow sufficient time for safe remodelling of the periodontium, to 1-week changes. Unfortunately, evidence exploring these policy changes in maxillary dental expansion is limited.

In 2003, a randomized controlled trial series10-12 has revealed that patients who change aligners weekly had less effective outcomes than those with 2-week changes in terms of peer assessment rating score and treatment completion rates. More recently a randomized clinical trial by Al-Nadawi et al.13 has indicated 2-week aligner changes to be more accurate than 10-day or 7-day changes for more complicated tooth movements, such as root torque, crown tip, and rotation. Finally, a retrospective study by Stephens et al.14 noted no significant difference between 1-week and 2-week aligner change regimens in the expression of mandibular canine rotation with the Invisalign^® appliance. However, changes in material and aligner change regimen have yet to be explored concurrently, and with an ever-increasing plethora of aligner materials available from various companies, the superiority of different materials and treatment regimens requires further investigation.

Inaccuracies in the expression of transverse expansion and buccolingual crown inclination may result in clinical failures and inefficient treatment. This research aims to assess 1) the effects of SmartTrack^® and EX30^® and 2) the effects of 1-week versus 2-week aligner changes, on the accuracy of maxillary buccal expansion and buccolingual inclination of maxillary first molars, premolars, and canines with Invisalign^® aligners.

This retrospective study was granted ethical approval by the School of Dentistry, University of Queensland Human Research Ethics Committee (approval number: 2020000610). Consent forms were signed by all study participants. Data were sourced from the Australasian Aligner Research Database which comprises > 10,000 cases. This database contains all case data from numerous experienced private orthodontic practitioners in Australia, New Zealand, and the United States, using Invisalign^® aligners only, with a minimum of ≥ 10 years of aligner treatment experience. Each orthodontist must have completed at least 300 cases between 2013 and 2020.15-17 The database requires that all Invisalign^® cases are submitted, whether successful or unsuccessful, by all included practitioners who contributed to reduce the risk of selection bias within the dataset.

Consecutive participants for this study were selected randomly, following the identification of cases requiring transverse maxillary expansion, according to the inclusion and exclusion criteria (Tables 1 and 2). Each case had a minimum of 0.75 mm of expansion across all tooth levels. Following this, a ledger was created wherein all participants were grouped according to alphabetical order for each group with the first 40 cases selected for each of the three groups (Table 3). Stereolithography (STL) files of the maxillary dentition of each participant were obtained at three time points as follows: (T1) initial scan (pre-treatment), (T2) ClinCheck^® prediction, and (T3) actual outcome (scan of the clinical outcome after the initial series of Invisalign^® aligners, before refinement). All participants were de-identified for the purposes of the study. Three maxillary dental models per participant were recorded as STL files and stored in a password-encrypted hard drive. The STL files were then imported into the Geomagic^® Control X™ software (version 2017.0.3; 3D systems, Rock Hill, NC, USA).

Table 1 . Inclusion criteria.

Dual arch treatment

Completion of the prescribed initial series of aligners

Stereolithography files available at all 3 timepoints: initial (T1), predicted (T2), and achieved (T3)

Adult patients (> 18 yr of age)

Non-extraction treatment

No previous history of orthodontic treatment

Buccal expansion planned in maxillary arch

Full permanent dentition (excluding the third molars teeth)

Table 2 . Exclusion criteria.

Ongoing pharmacological treatment which may affect tooth movement (i.e. bisphosphonates, prostaglandin inhibitors)

Prescribed interproximal reduction (IPR)

Active periodontal disease

Dental pathology, hyper/hypodontia

Use of auxiliaries such as intermaxillary cross elastics

Table 3 . Groups with aligner regimens that were studied.

Groups studied	Material	Change regimen
ST1	SmartTrack®	1-week changes
ST2	SmartTrack®	2-week changes
EX2	EX30®	2-week changes

To eliminate any error of assessment bias, the examiner (JOC) was blinded to the patient’s name, sex, and age. Eight repeatable points were then identified on each cast, a point on the buccal cusp tip (mesio-buccal cusp tip on the first molar) and above (0.1 mm) the gingival margin of the canine, first premolar, second premolar, and first molar in the mesiodistal centre of the tooth for cuspids and bicuspids and the mesio-distal centre of the anterior half of the first molar, delineated by the buccal groove (Figure 1). Occlusal points were selected from the occlusal perspective, whereas buccal points were selected from the buccal view of the tooth in question after standardized orientation of the digital casts. The method for measurement of buccal expansion was adapted with modifications from Solano-Mendoza et al.18 (Figures 2 and 3). The buccolingual inclination for each tooth was then assessed by measuring the angle formed by two constructed lines; the line formed by the gingival points on opposite sides of the arch, with the line formed from the cusp tip point to the gingival point on the same tooth, representing the buccolingual inclination for the tooth in question (Figure 3).

Figure 1. A, Digital cast with all occlusal and gingival points added and horizontal vector highlighted between right and left gingival points at the canine level with a digital cast. B, Further addition of the left canine vector is observed without the digital cast. Dots appear blue when being used for measurement while dots appear red when not being used for measurement.

Figure 2. A, All vectors added to a sample digital cast with vectors at the canine level highlighted. B, Measurement of the linear distance between the right and left canines is also observed without the digital cast.

Figure 3. A second sample with all vectors and measurements added, with all vectors at the canine level highlighted, displayed with (A) and without the digital cast (B).

Statistical analysis

The sample size for this project was based on a pilot study conducted on 10 patients, using the methods described with the PASS 2019 software (version 19.0.3; NCSS LLC., Kaysville, UT, USA). A power analysis indicated that a minimum of 32 samples were required, with a bilateral confidence interval (CI) of 95% and a standard deviation of 30%, to detect an inaccuracy rate of 20%, and power of 0.90. Intraclass correlation coefficient (ICC) and paired t tests were used to calculate the test–retest reliability of the methodology. Eight angles and four distances at two time-points were measured. The analyses of intra- and interexaminer reliability are presented in Tables 4 and 5. Reliability analyses were performed using the Stata version 15 (StataCorp, College Station, TX, USA).

Table 4 . Intra-examiner reliability.

Tooth	Intraclass correlation coefficient (95% confidence interval)	P value
Buccolingual inclinations
RCanineI	0.86 (0.72–0.96)	< 0.001
RPm1I	0.97 (0.89–0.99)	< 0.001
RPm2I	0.96 (0.85–0.99)	< 0.001
RMolarI	0.93 (0.75–0.98)	< 0.001
LCanineI	0.96 (0.84–0.99)	< 0.001
LPm1I	0.83 (0.77–0.96)	< 0.001
LPm2I	0.94 (0.80–0.99)	< 0.001
LMolarI	0.99 (0.95–1.00)	< 0.001
Buccal expansion
CanineE	0.95 (0.83–0.99)	< 0.001
Pm1E	0.92 (0.74–0.98)	< 0.001
Pm2E	0.99 (0.96–1.00)	< 0.001
MolarE	0.99 (0.95–1.00)	< 0.001

RCanineI, RHS canine buccolingual crown inclination; Pm1I, first premolar buccolingual crown inclination; Pm2I, second premolar buccolingual crown inclination; MolarI, first molar buccolingual crown inclination; R, right; L, left; CanineE, canine buccal expansion; Pm1E, first premolar buccal expansion; Pm2E, second premolar buccal expansion; MolarE, molar buccal expansion..

Table 5 . Inter-examiner reliability.

Tooth	Intraclass correlation coefficient (95% confidence interval)	P value
Buccolingual inclinations
RCanineI	0.84 (0.78–0.97)	< 0.001
RPm1I	0.97 (0.85–0.99)	< 0.001
RPm2I	0.98 (0.84–1.00)	< 0.001
RMolarI	0.99 (0.94–1.00)	< 0.001
LCanineI	0.92 (0.93–1.00)	< 0.001
LPm1I	0.90 (0.87–0.97)	< 0.001
LPm2I	0.95 (0.87–1.00)	< 0.001
LMolarI	0.97 (0.89–0.99)	< 0.001
Buccal expansion
CanineE	0.97 (0.88–0.99)	< 0.001
Pm1E	0.95 (0.82–0.99)	< 0.001
Pm2E	0.98 (0.93–1.00)	< 0.001
MolarE	0.98 (0.94–1.00)	< 0.001

RCanineI, RHS canine buccolingual crown inclination; Pm1I, first premolar buccolingual crown inclination; Pm2I, second premolar buccolingual crown inclination; MolarI, first molar buccolingual crown inclination; R, right; L, left; CanineE, canine buccal expansion; Pm1E, first premolar buccal expansion; Pm2E, second premolar buccal expansion; MolarE, molar buccal expansion..

The percentage of clinically significant inaccuracies (CSI) was then calculated for each of the groups from absolute values. The threshold for clinical significance was based on the American Board of Orthodontists model grading system for case evaluation and has been used similarly by Grünheid et al.19 Clinical significance was considered when linear inaccuracies are outside the range of 0 ± 0.5 mm and when buccolingual inclination inaccuracies are outside the range of 0 ± 2° from that predicted. Those that did not reach this threshold of inaccuracy were deemed clinically accurate. This method of assessment was selected instead of a percentage of overall accuracy for example, because of the high proportion of both positive and negative values (indicating over- and underexpression of the prescription, respectively) which would have resulted in a skewing of the accuracy calculation toward zero.

Following measurement of the casts at each time point, CSI were identified, and their rates were calculated for each linear and buccolingual inclination measurement at each tooth level for the three groups. Statistics for the rates of CSI for each tooth level were derived from 80 measurements for each buccolingual inclination (left and right sides combined) and 40 measurements for linear maxillary buccal expansion. The rate of CSI was then calculated and expressed as a percentage. The mean amount of CSI was also calculated, with those deemed clinically accurate not contributing to the calculations.

To identify significant differences between the groups, 95% CI tests were performed to compare the CSI rates between the three groups at all tooth levels for buccal expansion and buccolingual inclination. Finally, dropline plots were used to display the difference between predicted and achieved expansion values for each group in Figures 4-6. Patient samples were ordered by the quantity of the predicted expansion, with linear regression fit lines for predicted and achieved dental expansion against this ordering of patients displayed on the plots. Statistical analyses were performed using the R Statistical package (version 4.0.2) and Stata (version 17; StataCorp).

Figure 4. Dropline plots for the SmartTrack^® 1-week group distances. Plots illustrate predicted expansion (blue) and achieved expansion (grey open circles), arranged by predicted expansion. A linear regression best fit line for each plot is also observed.
Pm1E, first premolar buccal expansion; Pm2E, second premolar buccal expansion.

Figure 5. Dropline plots for the SmartTrack^® 2-week group distances. Plots illustrate predicted rotation (blue) and achieved expansion (grey open circles), arranged by predicted expansion. A linear regression best fit line for each plot is also observed.
Pm1E, first premolar buccal expansion; Pm2E, second premolar buccal expansion.

Figure 6. Dropline plot for the EX30^® 2-week group distances. Plots illustrate predicted rotation (blue) and achieved expansion (grey open circles), arranged by predicted expansion. A linear regression best fit line for each plot is also observed.
Pm1E, first premolar buccal expansion; Pm2E, second premolar buccal expansion.

Intraexaminer agreement test–retest reliabilities were excellent (ICC > 0.75) for all angle and linear measurements (Table 4). The test–retest reliabilities ranged from 0.83 to 0.99 for buccolingual inclinations and 0.92 to 0.99 for linear measurements. For the interexaminer agreement, the test–retest reliabilities were also excellent (ICC > 0.75) for all buccolingual inclinations and buccal expansion measurements (Table 5). No evidence of systematic differences between any measures at the two time points was identified. A final sample of 119 was included with one case being excluded from the SmartTrack^® 2-week changes (ST2) group because of a defective dataset.

SmartTrack^® 1-week

Clinically significant inaccuracies outside the range of 0 ± 0.5 mm between predicted and achieved transverse expansion were noted across all four distance measurements, with a particular bias toward underexpression of the prescribed buccal expansion. Canines (CanineE) demonstrated CSI in 67.5% of cases, while only 32.5% of cases were clinically accurate. Higher CSI rates were noted at the first premolar (Pm1E) and second premolar (Pm2E) levels (87.5% and 85.0%, respectively). Molar buccal expansion (MolarE) also demonstrated CSI in 87.5% of cases. The mean inaccuracy of absolute values of all clinically significant cases increased from anterior to posterior with 1.28 mm at CanineE, 1.60 mm at Pm1E, 1.79 mm at Pm2E, and 1.89 mm at MolarE recorded (Table 6).

Table 6 . Proportion of clinically significant inaccuracies with mean amount of inaccuracy for distances.

Group	CanineE			Pm1E			Pm2E			MolarE
	N (%)	Mean ± SD (mm)		N (%)	Mean ± SD (mm)		N (%)	Mean ± SD (mm)		N (%)	Mean ± SD (mm)
ST1	27 (67.5)	1.28 ± 0.76		35 (87.5)	1.60 ± 1.01		34 (85.0)	1.79 ± 1.20		35 (87.5)	1.89 ± 1.47
ST2	22 (56.4)	1.22 ± 0.77		24 (61.5)	1.32 ± 0.82		23 (59.0)	1.40 ± 0.77		23 (59.0)	1.27 ± 0.66
EX2	22 (55.0)	1.26 ± 0.98		28 (70.0)	1.70 ± 1.40		25 (62.5)	2.00 ± 1.31		28 (70.0)	2.01 ± 1.77

CanineE, canine buccal expansion; Pm1E, first premolar buccal expansion; Pm2E, second premolar buccal expansion; MolarE, molar buccal expansion; SD, standard deviation; ST1, SmartTrack^® 1-week changes; ST2, SmartTrack^® 2-week changes; EX2, EX30^® 2-week changes..

Canine buccolingual inclination (CanineI) revealed a CSI rate of 62.5%, with only 37.5% of teeth considered clinically accurate (Table 7). High CSI rates were also noted for first premolar buccolingual crown inclination (Pm1I) (71.3%) and second premolar buccolingual crown inclination (Pm2I) (75%). First molar buccolingual inclinations (MolarI) demonstrated CSI in 72.5% of the measurements. The mean CSI was 4.44° at CanineI, 6.47° at Pm1I, and 5.30° at Pm2I, whereas at MolarI, 5.08° of inaccuracy was noted, which were all calculated from absolute values (Table 7).

Table 7 . Proportion of clinically significant inaccuracies with mean amount of inaccuracy for buccolingual inclination.

Group	CanineI			Pm1I			Pm2I			MolarI
	N (%)	Mean ± SD (°)		N (%)	Mean ± SD (°)		N (%)	Mean ± SD (°)		N (%)	Mean ± SD (°)
ST1	52 (62.5)	4.44 ± 2.94		57 (71.3)	6.47 ± 3.42		60 (75.0)	5.30 ± 2.63		58 (72.5)	5.08 ± 3.03
ST2	37 (47.4)	4.21 ± 1.87		52 (66.6)	4.59 ± 2.11		55 (70.5)	4.97 ± 2.84		51 (65.4)	5.72 ± 3.04
EX2	46 (57.5)	3.56 ± 1.21		47 (58.8)	5.33 ± 2.92		48 (60.0)	5.33 ± 3.33		49 (61.3)	5.43 ± 3.06

CanineI, canine buccolingual inclination; Pm1I, first premolar buccolingual inclination; Pm2I, second premolar buccolingual inclination; MolarI, molar buccolingual inclination; SD, standard deviation; ST1, SmartTrack^® 1-week changes; ST2, SmartTrack^® 2-week changes; EX2, EX30^® 2-week changes..

A significant tendency toward underexpression of transverse expansion across all measured tooth pairs. Of the cases with CSI, 92.6% at the canine level, 94.3% at the first premolars, 97.2% at the second premolars, and 91.4% at the first molars were because of inadequate expression of buccal expansion. The remaining inaccuracies were attributed to overexpression of buccal expansion (Table 8).

Table 8 . Proportions of direction of expression for all clinically significant inaccuracies at each tooth level for buccal expansion and buccolingual inclination.

Tooth	ST1			ST2			EX2
	Under	Over		Under	Over		Under	Over
Buccal expansion
CanineE	92.6	7.4		89.9	10.1		64.0	36.0
Pm1E	94.3	5.7		75.0	25.0		89.3	10.7
Pm2E	97.2	2.8		78.2	21.8		92.0	8.0
MolarE	91.4	8.6		89.6	10.4		89.6	10.4
Buccolingual inclinations
CanineI	21.9	78.1		40.5	59.5		31.9	68.1
Pm1I	36.7	63.3		12.2	87.8		20.0	80.0
Pm2I	29.8	70.2		13.9	86.1		15.7	84.3
MolarI	44.1	55.9		19.6	80.4		19.0	81.0

Underexpression (under) refers to the lingual side, and overexpression (over) refers to the buccal side, expressed as a percentage (%)..

CanineE, canine buccal expansion; Pm1E, first premolar buccal expansion; Pm2E, second premolar buccal expansion; MolarE, molar buccal expansion; CanineI, canine buccolingual inclination; Pm1I, first premolar buccolingual crown inclination; Pm2I, second premolar buccolingual crown inclination; MolarI, first molar buccolingual inclinations; ST1, SmartTrack^® 1-week changes; ST2, SmartTrack^® 2-week changes; EX2, EX30^® 2-week changes..

A tendency toward overexpression of buccal crown inclination was noted at all four levels. Of the teeth that demonstrated significant inaccuracies, 78.1% at the canines, 63.3% at the first premolars, 70.2% at the second premolars, and 55.9% at the first molars were because of an overexpression of buccal crown inclination. The remaining inaccuracies were attributed to underexpression of buccal crown inclination (Table 8).

SmartTrack^® 2-week

Clinically significant inaccuracies between predicted and achieved movements were noted across all 4 distance measurements with a particular bias toward underexpression of the prescribed buccolingual width (Table 6). CanineE demonstrated CSI in 56.4% of cases, whereas only 43.6% of cases fell within the range of acceptable clinical accuracy (Table 6). Of the 56.4% of measurements that exhibited CSI, 28.2% had CSI of ≥ 1 mm. Higher CSI rates were noted at Pm1E (61.5%) and Pm2E (59.0%). Molar buccal expansion also demonstrated CSI in 59.0% of cases. The mean inaccuracy from absolute values of all clinically significant cases was 1.22 mm at CanineE, 1.32 mm at Pm1E, 1.40 mm at Pm2E, and 1.27 mm at MolarE (Table 6).

CanineI demonstrated CSI between predicted and achieved in 47.4% of teeth (Table 7). High CSI rates were also noted at Pm1I (66.6%) and Pm2I (70.5%). MolarI also exhibited CSI in 65.4% of measurements. The mean inaccuracy at each level was 4.21° at CanineI, 4.59° at Pm1I, and 4.97° at Pm2I, while MolarI had an inaccuracy of 5.72° (Table 7).

A significant tendency toward underexpression of transverse expansion was observed across all four measured tooth pairs. Of the cases that had CSI, 89.9% at the canines, 75.0% at the first premolars, 78.2% at the second premolars, and 89.6% at the first molars were caused by inadequate expression of buccal expansion, while the remaining cases were caused by overexpression of the buccal expansion (Table 8).

A tendency toward overexpression of buccal crown inclination was noted at all levels. Of the teeth that exhibited significant inaccuracies, 59.5% at the canines, 87.8% at the first premolars, 86.1% at the second premolars, and 80.4% at the first molars were caused by an overexpression of buccal crown inclination, and the remaining cases were caused by underexpression of the buccal crown inclination (Table 8).

EX30^® 2-week

Clinically significant inaccuracies between predicted and achieved expansion were noted across all distance measurements with a particular bias toward underexpression of the prescribed buccal expansion. CanineE showed CSI in 55% of cases (Table 6).

Higher CSI rates were noted in Pm1E (70%) and Pm2E (62.5%, while MolarE demonstrated significant inaccuracies in 70.0% of the cases. The mean amount of CSI increased from anterior to posterior with 1.26 mm at CanineE, 1.70 mm at Pm1E, 2.0 mm at Pm2E, and 2.01 mm at MolarE, which was calculated from absolute values (Table 6).

Clinically significant inaccuracies were noted in CanineI in 57.5% of the teeth, while only 42.5% of the teeth were clinically accurate (Table 7). Higher CSI rates were also noted at Pm1I (58.8%) and Pm2I (60.0%), while MolarI also demonstrated CSI in 61.3% of measurements. The mean CSIs were 3.56° at CanineI, 5.33° at Pm1I, 5.33° at Pm2I, and 5.43° at MolarI (Table 7).

A tendency toward underexpression of transverse buccal expansion across all four tooth pairs was observed. Of the cases that exhibited significant inaccuracies, 64% at the canine level, 89.3% at the first premolars, 92% at the second premolars, and 89.6% at the first molars were caused by inadequate expression of buccal expansion. The remaining cases were attributed to overexpression of buccal expansion (Table 8).

Of the teeth that demonstrated significant inaccuracies, 68.1% at the canines, 80.0% at the first premolars, 84.3% at the second premolars, and 81.0% at the molar level were caused by an overexpression of buccal crown inclination (Table 8).

Comparative analysis

For buccal expansion , significant differences in CSI rates were observed between the SmartTrack^® 1-week changes (ST1) and ST2 groups across the first premolars (P = 0.003), second premolars (P = 0.004), and molars (P = 0.001) (Table 9). Significant differences between the ST1 and EX30^® 2-week changes (EX2) groups were noted at the second premolar level (P = 0.018) (Table 9). For buccolingual inclination, significant differences in CSI rates were noted between the ST1 and ST2 groups at the canine level (P = 0.049), and between the ST1 and EX2 groups at the second premolar level (P = 0.040) (Table 9).

Table 9 . Statistically significant differences between groups for rates of clinically significant inaccuracies, with 95% confidence intervals where significance was considered at P < 0.05.

	Tooth level		Mean	Lower limit	Upper limit	P value
	Group 1	Group 2
Buccal expansion	Pm1E, ST1	Pm1E, ST2	0.274	0.092	0.458	0.003
Buccal expansion	Pm2E, ST1	Pm2E, ST2	0.275	0.086	0.464	0.004
Buccal expansion	Pm2E, ST1	Pm2E, EX2	0.225	0.039	0.411	0.018
Buccal expansion	MolarE, ST1	MolarE, ST2	0.300	0.116	0.484	0.001
Buccolingual inclination	CanineI, ST1	CanineI, ST2	0.152	0.002	0.302	0.049
Buccolingual inclination	Pm2I, ST1	Pm2I, EX2	0.150	0.007	0.293	0.040

CanineI, canine buccolingual inclination; Pm1E, first premolar buccal expansion; Pm2E, second premolar buccal expansion; MolarE, molar buccal expansion; ST1, SmartTrack^® 1-week changes; ST2, SmartTrack^® 2-week changes; EX2, EX30^® 2-week changes..

Finally, the average expansion attempted per group is presented in Table 10.

Table 10 . Attempted expansion per tooth and per group.

	CanineE	Pm1E	Pm2E	MolarE
ST1 (mm)	2.74	4.59	4.39	3.33
ST2 (mm)	2.65	4.18	3.87	3.20
EX2 (mm)	2.32	4.63	4.34	3.19

CanineE, canine buccal expansion; Pm1E, first premolar buccal expansion; Pm2E, second premolar buccal expansion; MolarE, molar buccal expansion; ST1, SmartTrack^® 1-week changes; ST2, SmartTrack^® 2-week changes; EX2, EX30^® 2-week changes..

This research highlights CSI in the achieved clinical expression of both buccolingual crown inclination and linear buccal expansion in all three study groups at all tooth levels studied. Similar observations have been reported in previous studies by Solano-Mendoza et al.,18 Houle et al.,20 Grünheid et al.,19 and most recently Lione et al.21 However, this is the first study to date to compare different aligner change regimens and different aligner materials concurrently.

With respect to maxillary buccal expansion, CSIs were noted for each study group, although the rate of CSI was not equal in all groups. For CanineE, the ST2 group (56.4%) and EX2 group (55%) exhibited similar rates of CSI, whereas the ST1 group demonstrated CSI in 67.5% of the cases. The rate of CSI increased from anterior to posterior in the ST1 group, although it reached 87.5% at Pm1D and 85% both at Pm2D and MolarE. The mean amount of CSI also increased from anterior to posterior in this group, almost doubling from the canine to the molar level. The rate of CSI in the EX2 group increased posteriorly, although this increase was not to the same extent as that in the ST1 group. By contrast, both the ST2 and EX2 groups exhibited ≤ 70% inaccuracies at Pm1D, Pm2D, and MolarE, thus highlighting the superiority of these regimens in posterior areas.

This data suggests that 2-week change regimens offer an advantage in cases requiring buccal expansion. The most inaccurate group was the ST1 group followed by the EX2 group, with the ST2 group demonstrating an advantage over the other groups. Interestingly, an advantage was noted with ST2 changes compared with EX2 changes with respect to linear buccal expansion, although the difference was minor. This may seem counterintuitive, as EX30^® is a more rigid material and based on conventional principles, may be expected to demonstrate increased effectiveness especially in posterior regions. This result may be explained by the improvement in clinician skill and aligner biomechanics, and improved comfort with SmartTrack^® aligners,9,22 resulting in better patient compliance. The apparently improved expression observed with two-week changes in comparison to 1-week changes is attributed to the provision of sufficient time for the bone to remodel before the reintroduction of a new aligner.

Comparative analysis revealed that the differences between the ST2 and ST1 groups were significant in the first premolars (P = 0.003), second premolars (P = 0.004), and molars (P = 0.001) (Table 9), indicating a clear superiority of the SmartTrack^® 2-week group over the SmartTrack^® 1-week group. Significant differences were only identified between the ST1 and EX2 groups at the second premolar level (P = 0.018) (Table 9), whereas the differences in CSI rates between the ST2 and EX2 groups were not significant at any level studied. Hence, 1-week changes with the SmartTrack^® material performed the worst, whereas 2-week changes with the SmartTrack^® or EX30^® material may be more reliable at expressing posterior dental expansion. The mean inaccuracies for the ST2 group were lower than those of the other groups at all levels most notably at the molar level suggesting that this group outperformed the other two groups.

Regarding buccolingual crown inclination, significant unpredictability was noted across all tooth movements (Table 8). The ST1 group exhibited the highest rate of CSI at all levels studied and the highest mean amount of inaccuracy at the canine and first premolar levels. Large inaccuracies may lead to a failure of movements to track as prescribed, resulting in clinical inefficiencies.19,23,24 Clinically significant inaccuracies were predominantly caused by excess buccal crown inclination for all groups. However, CSI due to underexpression of buccal crown inclination also highlighted the unpredictability or lack of control of buccolingual inclination in all groups studied. This was notable at the molar level in the ST1 group, where of the observed CSI, 44.1% were due to an underexpression of buccal crown inclination (55.9% due to overexpression of buccal crown inclination), compared with only 19.6% in the ST2 group and 19% in the EX2 group at the same level. The variation observed in this pattern for the MolarI in the ST1 group suggests an increase in cases that failed to track. This unpredictability may be explained by torquing forces on teeth acting in one plane causing unintended forces in other planes in the complex biomechanical situation of a dental arch.

Similar findings have been reported in previous studies.18,19 Recently, Zhou and Guo6 reported that buccal crown inclination was often overexpressed in a cone-beam computed tomography study where crown and root positions were assessed. This study assessed left and right molar crown inclination separately, with significant asymmetry noted in the expression of inclination. In this study, although a tendency toward underexpression of expansion was noted, some cases did exhibit overexpansion beyond what was programmed which may be partly explained by the variability in biological systems. This variability in expression has been reported in other studies involving aligner treatment.14,15

Comparative analysis indicated that the rates of CSI between the groups for buccolingual inclination were significant between the ST1 group and the ST2 group at the canine level (P = 0.49), and between the ST1 group and the EX2 group at the second premolar level (P = 0.040) (Table 9). This suggests that all materials struggled with control of buccolingual inclination.

High rates of CSI were noted across all levels for buccal expansion and buccolingual inclinations in all the groups studied. With respect to buccal expansion, the ST2 group exhibited the lowest rate of CSI of most groups studied followed by the EX2 group, with the ST1 group demonstrating the highest rate of inaccuracy across most levels. The ST2 group also exhibited the lowest mean amount of inaccuracy across most levels studied. Differences in the expression of buccal expansion at the first premolar, second premolar, and molar levels were significant between the ST2 and ST1 groups at the first premolar, second premolar and molar levels for expression of buccal expansion. Differences in expansion were significant between the EX2 and ST1 groups at the second premolar level only. No significant differences in the rate of CSI for buccolingual inclinations or buccal expansion were noted between the ST2 and EX2 groups. All the groups struggled similarly with control of buccolingual inclinations during expansion.

Conceptualization: JO, TW, EF, BK. Data curation: TW. Formal analysis: JO, TW. Funding acquisition: JO, EF, BK. Investigation: JO, TW. Methodology: JO, TW. Project administration: JO, TW. Resources: TW, EF, BK. Supervision: TW, EF, BK. Validation: JO, TW. Visualization: JO, TW. Writing – original draft: JO. Writing – review & editing: JO, TW, EF, BK.

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

None to declare.