Efficacy of Invisalign attachments: A retrospective study

Introduction

The purpose of this study was to compare the efficacy of Invisalign’s (Align Technology, Santa Clara, Calif) optimized and conventional attachments on rotational and extrusive tooth movements.

Methods

Initial, predicted, and achieved digital dental models from 100 orthodontic patients were exported from Invisalign’s ClinCheck software as stereolithography files and subsequently imported into the Slicer CMF program (version 4.7.0; www.slicer.org ) for superimpositions on posterior teeth with no planned movement. Rotational and extrusive measurements for both optimized and conventional attachments were made on 382 teeth from the superimposition of the initial and predicted models (predicted movement) and from the superimposed initial and achieved models (achieved movement). Predicted and achieved movements were compared along with movements of teeth with optimized and conventional attachments.

Results

Differences between accuracies of tooth movements using optimized vs conventional attachments for both rotation and extrusion were neither statistically nor clinically significant. Mean predicted values were larger than mean achieved values for all attachment types and movements ( P < 0.0001). For extrusion, the mean difference between predicted and achieved movements was clinically significant (0.40 mm and 0.62 mm for optimized and conventional attachments, respectively). Overall, the mean accuracy was 57.2%. Mean accuracy was 63.2% for rotation and 47.6% for extrusion. Interproximal reduction or spacing did not significantly affect accuracy.

Conclusions

Conventional attachment types may be just as effective as Invisalign’s proprietary optimized attachments for rotations of canines and premolars and extrusion of incisors and canines. Clinicians should consider overcorrecting tooth movements, especially anterior tooth extrusion.

Highlights

  • A clinically detectable amount of tooth movement was chosen to be 15° and 0.2 mm.

  • The efficacy of Invisalign’s optimized and conventional attachments was similar.

  • Mean accuracy was 63.2% for rotation and 47.6% for extrusion.

  • Clinicians may overcorrect extrusion by 0.4 mm to 0.6 mm and rotations by 4° to 6°.

Although fixed orthodontic appliances are still widely used today, the advent of removable clear aligners has undoubtedly revolutionized the field of orthodontics in recent years. In 1997, Align Technology (Santa Clara, Calif) developed Invisalign, which is arguably the most used and recognizable clear aligner system today. Initially, each Invisalign aligner was programmed to move a tooth 0.25 to 0.33 mm over 14 days. , In 2016, Invisalign changed its protocol from two-week wear to weekly aligner switches, decreasing treatment time by up to 50%. Each aligner is to be worn for 20-22 hours a day to be effective. ,

Several studies have evaluated the accuracy of the Invisalign system by superimposing predicted and achieved virtual models over unmoved posterior teeth using 3-dimensional (3D) superimposition software. , Although it is possible that the teeth superimposed on may move during treatment, more stable landmarks (ie, palatal rugae) are not available on Invisalign’s predicted models because they only illustrate teeth and attached gingiva. In addition, most of these studies were conducted before the release of Align Technology’s SmartTrack (LD30; Align Technology) material developed in 2013 and before weekly aligner switches were recommended in 2016.

A recent systematic review concluded that Invisalign could predictably level, tip, and derotate anterior teeth, but not canines and premolars. The authors found that limitations of Invisalign also include posterior arch expansion through bodily tooth movement, closure of extraction spaces, improvement of occlusal contacts, extrusion of maxillary incisors, and correction of large anteroposterior and vertical discrepancies. To increase effectiveness, composite attachments are bonded to teeth so that the aligner can be more retentive and to facilitate tooth movement.

The first Invisalign attachments were conventional attachments that were either ellipsoid or rectangular in shape. The ellipsoid shape is considered the least effective attachment today because of its small size and lack of a defined active surface. Conventional rectangular attachment dimensions, prominence, degree of beveling, and position on the tooth may be changed according to clinician preference in the ClinCheck Pro software (Align Technology) and are still widely used today. Optimized attachments, a type of SmartForce feature introduced in 2009, are engineered and patented by Align Technology to create precise biomechanical forces on teeth, thus increasing the predictability of tooth movement. They vary by shape and are automatically placed by the ClinCheck software when a certain amount and type of planned tooth movement is detected. Optimized rotation attachments are automatically placed onto canines or premolars when a rotation of ≥5° is detected. Maximum rotational velocity is 2° per stage. Optimized extrusion attachments are applied on to incisors or canines when ≥0.5 mm extrusion is detected by the software. Maximum linear velocity is 0.25 mm per stage.

Unlike optimized attachments, conventional attachments are not unique to Invisalign and are used by other companies offering clear aligners or software to create in-office aligners using 3D printers. Although the precision of orthodontic tooth movements with Invisalign has been studied, the effectiveness of the different attachment types, among other aligner variables, has not been considered. This research aimed to compare the efficacy of optimized and conventional attachment types on rotations of canines and premolars and extrusion of anterior teeth—two movements reported to be the most difficult to achieve predictably with Invisalign. Results can help guide dentists in their choice of attachment types or in considering any overcorrection of movements when treatment planning with Invisalign or another clear aligner software.

Material and methods

This retrospective study consisted of 382 teeth from digital dental models of 100 orthodontic patients aged 11-63 years (32 males and 68 females with a mean age of 28 years 2 months). The sample teeth were derived from 97 maxillary arches and 60 mandibular arches. Some patients were used more than once because they had a refinement scan available with qualifying teeth for a total of 120 subjects. All patients were treated with Invisalign (Align Technology) by 1 of 2 orthodontists in private practice outside of Milwaukee, Wis and Chicago, Ill between October 2016 and August 2018. Both orthodontists had been providing Invisalign for at least 5 years before when the patients were started. A power analysis indicated that a sample size of at least 64 teeth per group would be needed to have a power of 95% with a significance level (α) of 0.05. The number of attachment types were: 163 optimized rotation (43%), 72 conventional rotation (19%), 81 optimized extrusion (21%), and 66 conventional extrusion (17%). Aligners were changed once a week according to the manufacturer’s and clinician’s recommendations at the time. The average number of aligners per series was 20, corresponding to an average treatment time of 5 months. Spacing was present or interproximal reduction (IPR) performed on either side of 61 out of the 382 teeth studied (16%). The study protocol was approved by the Institutional Review Board of Marquette University.

The main inclusion criteria were as follows: (1) presence of optimized or conventional rotation or extrusion attachments in the planned ClinCheck; (2) completion of the initial series of aligners, resulting in either a refinement or final scan; (3) no planned movement of at least one posterior tooth per side of the dental arch; (4) good compliance reported with aligner wear; (5) full permanent dentition; and (6) treatment beginning in 2016 or later. The exclusion criteria were: (1) patients in the primary or mixed dentition; (2) new dental restorations or extractions during treatment; (3) the use of any auxiliaries, such as elastics or vibrational devices; and (4) patients with any orofacial syndromes or malformations.

To detect which teeth had conventional attachments placed primarily for rotation or extrusion, the previous unaccepted ClinChecks were reviewed to confirm that an optimized rotation or extrusion attachment was removed and replaced by a conventional one. Removal and replacement of an optimized attachment would indicate that conventional attachments were placed on teeth with planned rotations of ≥5° or planned extrusion of ≥0.5 mm, which are the thresholds for optimized attachments to be placed. Predicted rotation was divided into mild (<45°), moderate (45°-55°), or advanced (>55°), whereas predicted extrusion was also divided into mild (<2.5 mm), moderate (2.5-3.5 mm), or advanced (>3.5 mm), according to Align Technology’s classifications.

Initial, predicted, and achieved digital dental models were exported from the ClinCheck software as stereolithography files. The initial and final models from the original ClinCheck were labeled as “initial” and “predicted,” respectively. The models from the midtreatment refinement scan or the models from the final scan at the end of treatment (whichever came first) were labeled as “achieved.” The stereolithography files were then imported into the 3D Slicer CMF program (version 4.7.0; www.slicer.org ) for superimpositions and measurements. Fiducial markers were placed on the central pits of posterior teeth planned to have no movement, and a region of interest was selected to include the entire occlusal surface, at a minimum, to superimpose on. Gingival margins were not included as superimposition landmarks because the virtual gingiva in treatment simulations may be inaccurate and misleading. Initial and predicted models were superimposed to measure predicted tooth movements, whereas initial and achieved models were superimposed to measure achieved movements ( Fig 1 ).

Fig 1
3D model superimposition using 3D Slicer CMF: A, regions of interest on unmoved second molars of initial ( top ) and predicted ( bottom ) maxillary arches; B, arches after they were superimposed.

Measurements were made on the teeth as follows: (1) for rotations of canines and premolars, two landmarks were manually placed on each tooth, the points were automatically connected to form a straight line, and the angle (yaw) between the two lines from each model was calculated by the software in degrees (°) ( Fig 2 ). The landmarks used were usually buccal and lingual cusp tips on premolars or a cusp tip and cingulum on canines. If the cusp tips or cingula were ill-defined or the points not reproducible, the most mesial and distal points of each tooth were used; and (2) for extrusion of incisors and canines, one point was chosen near the center of the incisal edge or cusp tip of each tooth, and the vertical distance between the two points on each model was calculated in millimeters (mm) ( Fig 3 ).

Fig 2
Rotational measurements.

Fig 3
Extrusive measurements.

To account for any error in model superimposition because of inadvertent vertical movement of posterior teeth superimposed on, all achieved extrusive measurements were adjusted by comparing them to a control tooth. The control teeth were typically directly adjacent to those being measured so that they were roughly in the same anteroposterior position along the dental arch. Control teeth were measured to confirm no predicted vertical movement (0 ± 0.05 mm). If the movement was achieved even though no movement was predicted, it was assumed this was because of either intrusion or eruption of the teeth superimposed on. The achieved value from a control tooth was subtracted from the achieved value of the adjacent tooth of interest to calculate the true extrusion of the latter.

Statistical analysis

To calibrate the principal investigator to a uniform measuring method, all of the measurements were performed only after initially completing several measurements as a practice exercise. The same examiner repeated 40 of the rotational measurements and 40 of the extrusive measurements by random within a 3-week interval to assess intraexaminer reliability. The intraclass correlation coefficient was excellent, with a score of 0.970 (95% confidence interval [CI], 0.944-0.984) for overall mean difference values. For rotation, Cronbach’s alpha was 0.965 (95% CI, 0.914-0.986). For extrusion, intrarater reliability had a value of 0.907 (95% CI, 0.780-0.962).

Any tooth measured to have a negative achieved value for a vertical movement, indicating intrusion, was changed to 0 mm because no extrusion was achieved. This was done to avoid large negative percentages when calculating accuracy (% accuracy = 100 − [(|predicted − achieved|)/(|predicted|) × 100]). In this equation, the absolute value of the difference between predicted and achieved movements was taken to ensure that percent accuracy never exceeded 100% for the teeth that achieved movements beyond what was predicted. To account for this same situation, the absolute value was also taken when calculating the discrepancy between predicted and achieved measurements in degrees and millimeters to avoid yielding negative values that would affect the mean without accounting for directionality.

To reduce the number of variables, similar types of teeth were grouped, including contralateral teeth, maxillary first and second premolars, mandibular first and second premolars, and mandibular central and lateral incisors. Independent t tests (two-tailed) were used to compare mean predicted and achieved movements between optimized and conventional attachments. Paired t tests (two-tailed) were used to compare mean predicted and mean achieved movements within groups. A one-way analysis of variance was used to compare the mean accuracies of movements among tooth types. Data analysis was performed using Statistical Analysis Software (version 9.4; SAS, Cary, NC) at a significance level of P < 0.05.

Results

Descriptive statistics for both rotation and extrusion with optimized and conventional attachment types are presented in Tables I–IV . When comparing the efficacy of optimized and conventional attachments, the mean differences in raw values were higher for conventional attachments, and mean percent accuracies were higher for optimized attachments, but this did not reach statistical significance for both rotation and extrusion ( P >0.05) ( Table V ).

Table I
Descriptive statistics for optimized rotation attachments
Tooth Movement n Mean Standard deviation
Maxillary canine Predicted (°) 38 14.26 9.97
Achieved (°) 9.71 7.37
|Predicted − achieved| (°) 4.94 6.62
Accuracy (%) 65.9 22.9
Maxillary premolar Predicted (°) 36 12.65 12.73
Achieved (°) 9.68 8.83
|Predicted − achieved| (°) 3.54 5.86
Accuracy (%) 72.8 23.6
Mandibular canine Predicted (°) 35 15.49 11.04
Achieved (°) 12.23 9.61
|Predicted − achieved| (°) 3.89 4.60
Accuracy (%) 68.0 25.9
Mandibular premolar Predicted (°) 54 14.42 8.49
Achieved (°) 9.62 7.95
|Predicted − achieved| (°) 5.74 5.59
Accuracy (%) 58.6 28.3

Table II
Descriptive statistics for conventional rotation attachments
Tooth Movement n Mean Standard deviation
Maxillary canine Predicted (°) 17 11.18 7.29
Achieved (°) 7.11 6.61
|Predicted − achieved| (°) 4.45 4.59
Accuracy (%) 57.9 29.8
Maxillary premolar Predicted (°) 10 11.94 8.79
Achieved (°) 5.08 2.83
|Predicted − achieved| (°) 6.86 7.73
Accuracy (%) 48.1 23.4
Mandibular canine Predicted (°) 19 15.78 8.53
Achieved (°) 9.50 7.71
|Predicted − achieved| (°) 6.84 6.39
Accuracy (%) 60.5 25.0
Mandibular premolar Predicted (°) 26 14.41 8.93
Achieved (°) 7.66 4.80
|Predicted − achieved| (°) 6.84 8.23
Accuracy (%) 58.6 28.3
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Aug 14, 2021 | Posted by in Orthodontics | Comments Off on Efficacy of Invisalign attachments: A retrospective study
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