Short-term effect of orthodontic clear aligners on muscular activity and occlusal contacts: A cohort study

The effect of clear aligner treatment on masticatory muscles (masseter, temporalis) activity in adults: a systematic review and meta-analysis

BACKGROUND

The use of clear aligners is becoming more common for aesthetic orthodontic treatment, but there are still concerns about how they affect mastication biomechanics in the short and long term. The clear aligners treatment (CAT) mechanism changes the position of the mandible and maxilla, especially impacting the masseter muscle. Surface electromyography (sEMG) proves to be a useful method to evaluate masticatory muscle activity (MMA).

OBJECTIVES

To analyze the effect of clear aligners treatment on alterations in masticatory muscles (masseter, temporalis) using surface electromyography.

SEARCH METHODS

Five databases (PubMed, Web of Science, SCOPUS, Cochrane Library, and Google Scholar) were searched up to March 2024.

SELECTION CRITERIA

Studies in which MMA was evaluated after the installation of orthodontic clear aligners.

DATA COLLECTION AND ANALYSIS

Screening, data extraction, and quality assessments were performed by four investigators independently. The data, which evaluated temporalis and masseter muscle characteristics during CAT using surface electromyography, was extracted, and the quality of the studies was evaluated. The risk of bias was assessed using the Newcastle-Ottawa Scale (NOS).

RESULTS

Six studies (two prospective cohort studies, three observational longitudinal studies, and one observational longitudinal case-control study) with low and moderate risk of bias were included in the qualitative synthesis. Six of these were also included in the meta-analysis. Our study investigated the dynamics of masseter and temporalis muscle activity during CAT. The results show that during maximal voluntary clenching, the masseter muscle demonstrated a significant initial increase (P < .05) followed by a subsequent non-significant decrease. It also showed that submaximal voluntary clenching consistently exhibited a significant reduction in muscle activity throughout the study period (P < .01). Assessment of muscle activity at the mandibular resting position revealed a variety of responses, with some participants showing a significant increase while others exhibited non-significant changes (P < .05, P > .05, respectively). However, the meta-analysis showed a non-significant difference in measuring masseter and temporalis muscles activity during CAT.

CONCLUSIONS

Based on existing evidence, it is reasonable to conclude that CAT affected MMA. During maximal voluntary clenching, masseter muscle activity initially increased but later decreased, while temporalis muscle activity showed mixed results. Submaximal voluntary clenching revealed a consistent decrease in muscle activity over time. Mandibular resting position assessments showed both increases and no significant changes in muscle activity. However, the existing literature is insufficient to draw concrete conclusions; therefore, well-conducted further research is needed to confirm this statement.

REGISTRATION

This systematic review and meta-analysis were registered in the International Prospective Register of Systematic Reviews (PROSPERO CRD42024522231).

Assessment of Food Masticatory Capability with Clear Aligners

Nowadays, aligners represent a possible therapeutical approach that combines both esthetic and function in order to address dental malocclusion. However, they require a significant level of compliance from the patient. According to the manufacturer, at least 22 h of wearing a day is demanded to reach the optimal therapeutical level; hence, aligners can only be removed during meals. Patients' compliance might increase and the duration of the treatment might decrease if they were allowed to eat with aligners on. The idea of patients keeping the aligners on during meals has been contemplated, not only to favor patients' compliance but also treatment effectiveness. This study aims to assess the degree of chewing difficulty that aligners cause when eating certain kinds of food and the quantity of residue left.

MATERIAL AND METHODS

A questionnaire titled "Questionnaire for the Assessment of Masticatory Function with Aligners" was administered using Google Forms to 240 patients in treatment with clear aligners. The survey was validated through the reliability test using the test-retest method. This method had a higher correlation coefficient of 0.9 across all items (with a cutoff of 0.8) with statistical significance, and an excellent internal correlation coefficient (α > 0.9). The statistical analysis performed consisted of descriptive analysis, frequencies, percentages, Pearson's correlation test and Friedman's test.

RESULTS

Pearson's test showed a statistically significant correlation between all items except between meat or clams and yogurt or ice cream and with mozzarella or soft cheese regarding food chewing difficulties. Pearson's test showed a statistically significant correlation between all items regarding food residues. A total of 69.2% of the cohort reported some movements of aligners during mastication. In total, 88.3% of them affirmed not to have perceived deformations or breakage of aligners during chewing. Furthermore, 79.2% of them declared that they would continue to eat if eating with aligners was proven to speed up treatment.

CONCLUSIONS

Wearing clear aligners while chewing foods such as yogurt, ice cream, soft cheese, bread, rice, etc., can be possible and can help shorten the duration of orthodontic treatment, benefiting both the patient and the orthodontist. However, further research using qualitative methods is needed to understand the barriers and facilitators to chewing food with aligners.

Does short-term treatment with clear aligner therapy induce changes in muscular activity?

DESIGN

Prospective cohort study.

COHORT SELECTION

The inclusion criteria comprised patients older than 18 years who attended the orthodontic department at the University of L'Aquila (L'Aquila, Italy) and requested treatment with clear orthodontic appliance. Only patients with Class I malocclusion with mild (0-3 mm) or moderate (4-6 mm) crowding were included. The exclusion criteria comprised patients who had medical condition that preclude the use of surface EMG (sEMG), patients who had anterior or posterior open bite, patients who had cross bite that include more than one tooth, patients who had edentulous ridges and patients with temporomandibular disorder.

DATA ANALYSIS

Included patients were assessed at three points in time: at the start of treatment (T0), after 3 months (T1), and after 6 months (T2). The patients were instructed during the study to wear each set of aligners (Nuvola; Gruppo Europeo di Ortodonzia S.r.l., Rome, Italy) for 2 weeks and 22 h/d and to remove them while eating and drinking (except for still water). The following two assessments were done for each patient at T0, T1, and T2: surface electromyography (sEMG) evaluation and a T-Scan digital occlusal analysis evaluation. Both evaluations were performed in centric occlusion and with aligners worn. The assessments were done by the same operator. The sEMG evaluation aimed to assess the muscular symmetry and balance. The sEMG evaluation was performed using an instrument that recorded analogical sEMG signal (BTS TMJOINT, Teethan S.p.a., Garbagnate Milanese, Milano, Italy). The patients were instructed to clench as much as possible and to maintain the same level of contraction during the test. The software automatically selected the 3 s with the most stable sEMG signal. The following outcomes were assessed using the sEMG: 1. Masseter percentage overlapping coefficient (POC), temporalis POC, and mean POC: which is an index of the symmetrical distribution of sEMG potentials within homologous muscular couples. 2. Torque coefficient (TC): compares the activity of the temporalis muscle to that of the contralateral masseter muscle. 3. Activity index (Ac): compares the activity of the temporalis muscle to that of the homolateral masseter muscle 4. Asymmetry index (Asym): compares the activity of the temporalis and masseter muscles of the right side to those of the left side. The digital occlusal analysis was performed using a 100μ thin, flexible horseshoe-shaped Mylar sensor (Novus HD sensor, Tekscan, Inc. S. Boston, MA, USA) that is used to analyze the dynamic and real-time distribution and timing of occlusal contacts and can reproduce 256 levels of varying occlusal force. A first recording was made without aligners, then a second with the aligners worn on the dental arches. The following outcomes were assessed using the digital occlusal analysis: 1. Position of the center of occlusal force (COF). 2. The maximal intercuspation time (MIC) (i.e., the time interval expressed in seconds between the first dental contact and the maximal intercuspation). The null hypothesis was that no difference exists for the sEMG indexes between the 3-time points and the two occlusal conditions.

RESULTS

Twenty-six female adult patients (mean age, 33.67 ± 13.33 years) were enrolled in this study. Statistically significant differences were observed in terms of POC for the temporalis and masseter muscles, as well as the mean POC across the three time points and between occlusal conditions (with or without aligner). Hence, null hypothesis was rejected. On the other hand, TC, Ac, Asym, and MIC variables did not exhibit any statistically significant differences. This confirms that the stable positioning of COF on the transverse plane was accompanied by the absence of torquing muscular couples or imbalances in muscular activation. Symmetry in muscular couples' activation (indicated by POC value) remained consistent over the 6-month follow-up period for the occlusion with aligners, and in fact, demonstrated some improvements. In contrast, there was a decline in POC over time during centric occlusion. Statistically significant variation in COF position was observed in the sagittal plane, but not in the transverse plane. This shift in COF position coincided with changes in muscular balance as assessed by surface electromyography. Regarding the anteroposterior position of the COF, an overall slight posterior shift was observed when aligners were worn. After 3 months of full-time aligner usage, an anterior COF position was detectable in centric occlusion, which exhibited statistical significance when compared with the occlusion involving aligners at T0 and T1.

CONCLUSIONS

The use of clear aligners led to an anterior displacement of the COF during biting in centric occlusion, along with a posterior shift while the aligners were worn in healthy female subjects over a 6-month monitoring period. No significant asymmetries in the COF position on the transverse plane were observed. The alterations in occlusal contact were subsequently followed by a short-term improvement in muscular balance when aligners were used, but a worsening muscular balance over time in centric occlusion condition.

Curve of Spee modification in different vertical skeletal patterns after clear aligner therapy: a 3D set-up retrospective study

OBJECTIVE

The purpose of the present study was to evaluate: (1) changes in the curve of Spee (COS) after clear aligner therapy and (2) whether such changes correlated with the patient's skeletal patterns.

MATERIALS AND METHODS

Three-dimensional mandibular models of 106 patients (47 males and 59 females; mean age 22.3 SD ± 3.4 years) treated with clear aligners were retrospectively analysed. The perpendicular distance between the occlusal reference plane and the buccal cusp tip of each lateral tooth was measured. Five angular cephalometric measurements (PP-MP, PP-OP, OP-MP, AFH, and PFH) were performed and correlated with the T1-T0 difference in linear distances. The patients were divided into three groups according to facial divergence. Variance analysis with Tukey post hoc correction was performed to evaluate the differences among groups.

RESULTS

The one-way ANOVA test showed a statistically significant difference for all analysed variables, except for the change in the distance of the second molar from the occlusal reference plane. Tukey's HSD test showed the following differences: In normodivergents, the T1-T0 difference in the distance of the first molar from the occlusal plane was 1 mm greater than that observed in the hyperdivergent group (p < 0.05); in the normodivergent group, the T1-T0 difference in the distance of the second premolar from the occlusal plane was 1.23 mm greater than that observed in the hyperdivergent group (p < 0.05), while in the hypodivergent group, it was 1.08 mm greater than in the hyperdivergent group (p < 0.05); finally, in normodivergents, the T1-T0 difference in the distance of the first premolar from the occlusal plane was 0.97 mm greater than that observed in the hyperdivergent group (p < 0.05).

CONCLUSION

Treatment with aligners did not lead to a clinically significant change in COS depth. However, when dividing the sample into groups based on craniofacial divergence, COS depth change differed significantly between the three groups.