Attachment geometry and clinical predictability in aligner treatment: A systematic review

Criteria for considering studies for this review

Search methods for identification of studies

MEDLINE (PubMed), ScienceDirect, and Cochrane Library were screened using a combination of mesh terms for eligible studies related to the focused question [Table 1]. A complementary hand search was done in some journal databases. No restrictions on language or year of publication were placed.

Selection of studies

A systematic search was performed using a Population, Intervention, Comparison, and Outcome approach in the following databases: MEDLINE, PubMed, Cochrane Library, and Elsevier-ScienceDirect for eligible studies related to the focused question. No time restrictions were applied for the screening process. The last electronic search was performed in November 2023. In addition, a complementary hand search was done in some journal databases. No restrictions on language or year of publication were placed.

Data collection and analysis

Two review authors (H. Ben Mohimd and H. Benyahia) separately examined the title and abstract of each article identified. If the title or abstract did not provide sufficient information regarding the inclusion criteria, it was then necessary to proceed to a full-text reading before including or excluding the article.

Inclusion and exclusion criteria

Only clinical trials, prospective and retrospective studies, published in English or French, were included [Table 2].

Data extraction and management

Full-text articles were checked according to eligibility criteria. Studies, that did not meet all the inclusion criteria, were excluded from the review. Any disagreements in terms of study selection were resolved by discussion with the third author.

The following information was determined for all articles:

• Names of the authors

• Year of publication

• Study design

• Number and groupings of participants

• Type of intervention

• Comparative groups, and

• Type of clinical outcome.

Quality of the studies

To determine the methodological quality and the level of evidence, two authors (B-M. H and B. H) independently analyzed all studies to identify existing biases. In case of disagreement, the judgment of a third reviewer (O. Y) was planned.

Quality assessment of the included studies was performed using the revised tool to assess the risk of bias 2 (RoB 2) in randomized trials and Joanna Briggs Institute (JBI) RoB quality assessment for prospective and retrospective cohort and case-control studies.

According to RoB 2, the assessment was based on five domains (randomization process, deviations from intended interventions, missing outcome data, measurement of the outcome, and selection of the reported result). Within each domain, one or more signaling questions were answered. These answers allowed us to formulate a judgment of “low risk of bias,” “some concerns,” or “high risk of bias.” The judgments within each domain lead to an overall RoB judgment for the result being assessed.

Study selection

The database search identified a total of 193 literature references on MEDLINE (PubMed), 725 on ScienceDirect, 37 on the Cochrane Library, and 15 by manual research with a total of 970 reference titles. After title and abstract reading, 853 titles were removed including all duplicates and those considered as irrelevant. Finally, after full-text reading and analyzing, only 19 studies met the pre-established eligibility criteria [Figure 1].

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Figure 1:

Flow chart of the systematic review.

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Study characteristics

The characteristics of each study are presented in [Table 3]. [Table 4] gives an overview of the results of the included studies regarding the clinical parameters of each type of orthodontic movement.

Among the studies included, one study was a randomized clinical trial, five studies were prospective, 13 studies were retrospective, and one was a case-control study. The selection process of articles is presented in the Preferred Reporting Items for Systematic Reviews and Meta-Analyses flow diagram.

The sample size in the 19 studies included ranged from 10 to 100 patients with a total of 803 patients, mostly adults. All the studies used the Invisalign system. Articles that did not specify the type of attachment used were excluded.

Attachment efficiency was assessed in the majority of included studies through the comparison of the movement planned on Clincheck and the movement obtained except for two studies that compared pre- and post-treatment cephalometric values.

Attachments used in the studies were conventional rectangular, rectangular beveled, ellipsoid, and optimized. The movements evaluated by the studies were derotation, extrusion, intrusion, torque, and radicular control in mesio-distal movement. Expansion-contraction movement and posterior bucco-lingual tip were not studied.

The analysis of the results obtained was studied for each type of orthodontic movement. Regarding root control movement during mesio-distal displacements, we chose to evaluate separate cases with and without extraction. Thus, attachment efficacy was assessed first for distalization movement and the correction of dental axes and secondly for anchorage management and canine and incisor retraction.

RoB assessment

Quality assessment of the nineteen studies was performed using different tools for RoB assessment based on the study type. The number of included studies is limited compared to published reports in medical sciences. The adopted search strategy identified one randomized controlled trials (RCTs), seventeen cohort studies, and one case-control. 13 studies were considered according to JBI at a medium RoB, 2 at low, and one at high risk [Table 5a and b].

Concerning the one RCT included, the randomization process was well described and was at low RoB, deviations from intended interventions and missing outcome data were also at low RoB. However, the measurement of the outcome and the selection of the reported results raised some concerns [Table 5c].

Table 5c: Risk of bias 2 tool for risk of bias quality assessment for randomized clinical trial.

Derotation movement

Derotation is one of the challenging movements in aligner biomechanics. To improve its predictability, the use of attachments will allow the aligner to lock in the dental crown, reducing considerably the sliding effect and emphasizing the force system through the increase of the lever arm.^[9,10] According to our results, 7 included studies evaluated attachment efficiency in derotation movement.

Among these 7 studies, two compared the accuracy of derotation movement of the canines^[11] and premolar (PM)^[12] with and without attachment and reported no significant difference between the 2 groups with a mean accuracy of 35.8% and 39.95%, respectively. These results can be explained by several elements, first of all, the cylindrical morphology of canines and PM which classifies them among the most difficult teeth to derotate due to the absence of interproximal undercuts and the reduction of force application surface causing the aligner to slip when it attempts to rotate. In addition, the majority use of ellipsoidal attachment shape in the study of Kravitz et al.^[11] remains unsuitable for rotational movement. At last, the aligner material used was EX30 whose biomechanical properties are less compared to the SmartTrack.

The other 5 studies^[13-17] assessed the accuracy of derotation movement with optimized and/or conventional attachments with or not a control group without attachments and reported variable predictability percentages ranging from 46% to 86%. That said, despite this difference, we noted an improvement over time in the predictability even for lower C and PM which can be explained by the evolution of the smartforce and SmartTrack functionality. Concerning the impact of attachment type on the accuracy of movement, there was no statistically significant difference between the use of optimized and conventional attachments but both Karras et al.,^[15] and Stephens et al.^[17] reported a more precise movement expression with optimized attachments.

Extrusion movement

The correction of several malocclusions requires recourse to extrusion movement (open bite, deepbite, ectopic teeth). The absence of a surface on which the aligner can rest to extrude the tooth causes it to slip. The use of attachment is essential so that the aligner rests on the active surface and brings the tooth vertically.^[7] According to our results, 4 studies^{[13,15,18,19]} evaluated the effectiveness of attachments in extrusion movement. Concerning the anterior teeth, the results showed an improvement in the percent of predictability, especially for the maxillary incisors with an increase from 56% to 79%. Indeed in comparison to the Kravitz et al. study^[20] where movement accuracy was 18% for maxillary incisors, we noted an evolution in the percent (Haouili et al. 2020^[13]: 55%, Karras et al. 2021^[15]: 66.3%) reaching 79% according to Groody et al.^[18] This result implies a better orthodontic management of open bite cases.

Concerning posterior teeth, Haouili et al.^[13] reported only a 40% accuracy for maxillary and mandibular molars with optimized extrusion attachments. In fact, posterior extrusion could be difficult to obtain due to the bite block effect of aligners or to the patient’s muscles.

The comparison of conventional and optimized attachments showed no statistically significant difference; however, Groody et al.^[18] reported the superiority of the conventional one in the maxillary lateral incisor extrusion with a 22% improvement using a horizontal attachment design. This can be related to the larger active surface of horizontal rectangular attachments allowing the aligner to push on, thus improving biomechanical properties.

Intrusion movement

One of the difficult movements to achieve in orthodontics is intrusion. In aligner treatment, the use of passive or active attachments on the buccal surface of the first and second PM increases retention and provides anchorage for the intrusion of the anterior teeth.^[21]

Only 3 studies assessed the accuracy of intrusion movement and reported variability in the % of predictability not exceeding 50%, in fact, Haouili et al.^[13] using the G5 protocol reported a low accuracy of mandibular incisor intrusion with a mean average of 35%, Al-Balaa et al.^[22] found 51% and Burashed and El Sebai^[23] found 30–40%. This low accuracy of intrusion movement of the anterior region could be related to the lack of posterior anchorage especially since all studies used only passive deepbite optimized attachment on the first premolar. In addition, the use of other auxiliaries like bite ramps is frequently indicated to enhance the correction of deepbite; however, none of the studies reported using it in their patients. Finally, it is important to control the incisor’s axis during the movement which might be difficult with aligner material.

Concerning the impact of the type of attachments used, Burashed and El Sebai^[23] reported that optimized attachments are no more effective than using conventional attachments in reducing deep overbite.

Torque

Anterior root torque is one of the challenging movements to obtain with aligner treatment.^[24] The use of some auxiliaries such as attachments or mostly power ridge is important to accomplish better root control. According to our results, only one study evaluated root torque efficiency with aligners using attachments and reported the ability of aligners to induce root displacement. However, a torque loss (up to 50%) was noticed. Moreover, the comparison of the attachment group with the power ridge showed no differences if the upper central incisor torque was supported with a horizontal ellipsoid attachment or with a Power Ridge.^[12]

Radicular control in mesiodistal displacement (non-extraction cases)

Radicular control in extraction cases

The management of extraction cases requires a good control of the anchorage and incisor torque allowing to reach class IC and M and to re-establish a functional anterior guide.

Baldwin et al. 2008^[32] reported a lack in the control of tooth movement using aligners, with significant tipping of the teeth adjacent to the extraction sites in both the maxilla and mandible. A few years later, Li et al. 2015^[33] reported a good root angulation with the Invisalign system thanks to the use of adequate attachments with some limitations regarding occlusal contacts and buccolingual inclination. In fact, Invisalign proposed the G6 protocol in extraction cases, in which they use optimized anchorage attachment on posterior teeth and optimized retraction attachment on the canine achieving maximum anchorage control.^[34]

According to our results, 4 studies assessed the efficacy of attachments in extraction cases, Dai et al.^[35] studied only maxillary first molars and central incisors after monomaxillary extraction, Ren et al.^[36] added maxillary canine distalization, Dai et al.^[37] worked on molar anchorage and canine and incisor retraction in both the maxilla and mandible and Tang et al.^[38] and al evaluated mandibular anchorage loss and canine and incisor retraction efficacy between first and second premolar extraction cases.

Study limits

Given the results of our systematic review, further clinical trials are necessary to assess the impact of attachments on the effectiveness of aligners during different orthodontic movements.

Our results emphasize that attachments can produce significant therapeutic effects allowing the improvement of aligner biomechanics. However, despite these results, it is not without limitations, often related to the nature of studies that included sample size, the absence of no attachment control group, and variability of means of outcome assessment (cephalometry, superposition of 3D models, facial superposition). The literature has shown great heterogeneity; there are few clinical trials, and the risk of bias at the level of the included studies was variable with the majority having a medium RoB. Other well-designed prospective studies are necessary with a high level of proof, respecting the randomization protocol, with a homogeneous population and no-attachment control group, and representative sample size is needed to better define the role of attachments in aligner therapy and to clarify what is the best configuration for each movement. It would also be interesting to introduce cone beam and facial superposition to maximize the stability of superposition structures, improve the reproducibility of measurements, and benefit from artificial intelligence to ensure an effective location of attachments.