Introduction
Orthodontic tooth movement occurs within a functional craniofacial complex that includes the masticatory muscles. Clear aligners cover the dental occlusion and may alter muscle function and patient perception. This study evaluated changes in masticatory muscle activity with aligner use and the associated impact on oral health–related quality of life.
Methods
Twenty-four adult patients (5 males and 19 females; mean age 38.2 ± 13.4 years) undergoing clear aligner treatment were assessed in a cross-sectional study. Surface electromyography (EMG) of the masseter and anterior temporalis muscles was recorded under 2 conditions: with and without the aligners in place. EMG indexes of overall muscle effort (impact) and intensity were analyzed, along with the distribution of bite forces among the 4 muscles. Each patient’s EMG signals were normalized against a maximal clench on cotton rolls. Participants also completed the 14-item Oral Health Impact Profile questionnaire. Paired statistical comparisons, correlation analyses, and 1-sample tests for muscle balance were performed (α = 0.05).
Results
Aligner use was associated with a significant increase in EMG activity. The impact index was 27.6% higher with aligners than without ( P <0.001), and the EMG intensity was modestly but significantly elevated with aligners (4.1%, P = 0.007). Aligner wear produced a more balanced distribution of muscle activity: with aligners, no significant deviation from an equal 25% contribution per muscle was observed, whereas without aligners, the muscle contributions were uneven ( P <0.001 for imbalance in each muscle). The 14-item Oral Health Impact Profile scores were low (median 4.5 on a 0–56 scale) and did not correlate with muscle activity ( P = 0.806). No significant differences in muscle indexes were found between sexes, and longer duration of aligner use showed no significant correlation with muscle activity levels.
Conclusions
Aligners tended to distribute occlusal forces more evenly across muscles, potentially reducing localized overload. These findings suggest that clinicians should anticipate a period of adaptation in which masticatory muscles respond to the altered occlusion.
Highlights
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Clear aligners increase masticatory muscle activity during clenching.
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Aligner use improves occlusal force distribution symmetry.
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No correlation exists between muscle activity and oral health–related quality of life.
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Transient muscular adaptation occurs without compromising patient comfort.
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It is important to monitor muscle function during clear aligner treatment.
Orthodontic treatment outcomes depend not only on tooth movement and skeletal changes but also on the function and adaptation of the craniofacial musculature. The masticatory muscles can influence facial growth patterns, dental arch form, occlusal relationships, and long-term posttreatment stability. Indeed, individual variability in muscle function—including differences between males and females—may affect how patients respond to orthodontic therapy. , Optimal orthodontic planning should therefore incorporate an understanding of the patient’s muscular baseline and functional capacity, aiming to achieve not only desired dental alignment but also functional harmony within the stomatognathic system.
The masseter and temporalis have been the focus of most orthodontic-related muscle studies mainly because they are easily accessible for surface electromyography (EMG) recordings. Other muscles, such as the medial pterygoids, which are equally important for mandibular elevation, and the lateral pterygoids, which are critical for chewing, also play key roles in craniofacial function, but their evaluation is limited by technical challenges in electrode placement. A practical, noninvasive method to assess muscle function is surface EMG, which detects and records the electrical activity during muscle contraction. EMG has been used in dentistry since as early as 1949, when Moyers first analyzed muscle contraction patterns in relation to malocclusion. Advances in EMG technology and protocols have improved the quality of muscle activity data, enabling more precise insights into muscle behavior during orthodontic treatment.
Clear aligner therapy has become increasingly popular in contemporary orthodontics, but its biomechanical effects extend beyond tooth movement alone. The aligners fully cover the occlusal surfaces of the teeth, introducing an occlusal layer that increases the vertical dimension during wear. This occlusal coverage alters the sensory input from periodontal and temporomandibular receptors and may require neuromuscular adaptation. Parrini et al demonstrated that clear aligners can produce changes in posture and occlusion, noting alterations in vertical dimension and masticatory muscle activity because of the presence of the appliance. Given these occlusal modifications, it has been hypothesized that aligner wear could lead to transient changes in the activity or coordination of the masticatory muscles. Some clinical reports and patient feedback have linked aligner use with masticatory muscle fatigue or tension, particularly during the initial adaptation period or with nighttime use. , In general, oral appliances that cover the teeth have been known to influence jaw muscle behavior: for example, “soft” occlusal splints (made of pliable thermoplastic similar to aligner material) tend to increase masseter activity, whereas rigid acrylic splints can reduce muscle activity. , In a small-sample trial, Al-Quran and Lyons found that a soft night guard produced a significant increase in masseter EMG activity compared with baseline, whereas a rigid acrylic splint led to a slight, nonsignificant reduction. Similarly, Okeson reported that soft occlusal appliances were associated with higher jaw muscle activity in some patients relative to rigid splints.
In contrast, not all studies have observed a detrimental effect of aligner-like appliances on muscle function. Manfredini et al evaluated patients wearing clear orthodontic retainers during sleep and found no statistically significant difference in sleep bruxism episode frequency between nights with and without the retainer. This indicates that for some parameters, aligner wear might be neutral with respect to parafunctional muscle activity, at least in the long term or at night. Overall, the current evidence is mixed, and the impact of clear aligners on masticatory muscle activity remains an area of active investigation. A recent systematic review concluded that aligner therapy can significantly affect the muscles of mastication, particularly noting an increase in muscle activity and discomfort in the initial days of appliance use, which tends to normalize as treatment progresses. However, because of the limited and heterogeneous data, consensus has yet to be reached on the extent and clinical relevance of these effects. Furthermore, one multiple-day observational study using portable EMG found that aligners did not substantially alter 24-hour masseter muscle activity in most subjects, suggesting that any effect may be highly individual or context-dependent. These divergent findings underscore the need for controlled studies that directly measure muscle function with and without aligners under standardized conditions.
In addition to biomechanical effects, the patient’s subjective experience and quality of life are important considerations in evaluating any orthodontic treatment. Oral health–related quality of life (OHRQOL) can be evaluated via validated questionnaires, among which the Oral Health Impact Profile (OHIP) is one of the most comprehensive. The simplified 14-item version (OHIP-14) covers functional limitation, physical pain, psychological discomfort, physical disability, psychological disability, social disability, and handicap. Orthodontic treatment can impose various impacts in these domains (eg, pain, difficulty chewing, speech alterations, or psychosocial effects because of appliance visibility). Clear aligners are often advertised as a more comfortable and esthetic option compared with fixed appliances, which might translate to a less negative impact on daily life. Indeed, prior studies have indicated that patients with aligners report less oral dysfunction and pain over treatment than those with conventional brackets. However, if aligners do alter masticatory function or introduce muscle fatigue, this could negatively influence certain aspects of OHRQOL, such as chewing efficiency or jaw comfort. Thus, it is worthwhile to correlate objective muscle activity data with patients’ reported quality of life.
This study aimed to evaluate and compare the activity of the masticatory muscles, masseter and temporalis, in orthodontic patients during aligner wear vs without aligners, using surface EMG. We also assessed whether clear aligner use leads to measurable changes in the balance of muscle activity and examined differences between male and female patients. Furthermore, we investigated the relationship between masticatory muscle activity and patients’ self-reported oral health impact, as measured by the OHIP-14 questionnaire.
Material and methods
This cross-sectional observational study was conducted on a sample of adults in active orthodontic treatment with clear aligners. The research protocol was reviewed and approved by the institutional ethics committee (CAAE: 64459122.9.0000.5336). All participants provided informed consent before enrollment.
Patients were recruited from the orthodontic clinic of a university and a private dental practice. The inclusion criteria were age of 18-70 years, currently in treatment with clear aligners (at any stage of treatment), and the presence of all permanent teeth (excluding third molars). Patients were excluded if they had active caries or periodontal disease, any signs or symptoms of temporomandibular joint disorder, any craniofacial deformity or syndrome, or if their treatment plan included orthognathic surgery. A total of 24 patients (5 men and 19 women) met the criteria and were included. The sample size was predetermined by power calculation, which indicated that 21 subjects would be required in each condition to detect a 25% difference in EMG outcomes (power 80%, α = 0.05). Our final sample of 24 provided adequate power for the primary comparisons.
In addition to the inclusion criteria described above, further clinical data were collected to characterize the sample. Dental classification was as follows: 14 participants were classified as Class I, 8 as Class II, and 2 as Class III malocclusion. All patients had attachments bonded to the teeth as part of their aligner treatment, although the number and distribution of attachments varied according to individual treatment plans. Periodontal screening was conducted before data collection, and all participants were free of active periodontal disease. Radiographic examinations, cephalometric analysis, and digital dental impressions were not included in the study protocol, as the primary focus was on functional electromyographic evaluation. Clinical images illustrating the data collection process (EMG setup and procedures) have been added to the Supplementary Material to enhance methodological transparency.
Regarding the history of parafunctional habits, we investigated whether participants had previous episodes of nocturnal bruxism. Three out of the 24 patients reported a history of sleep bruxism and had used night guards before initiating orthodontic treatment. The use of these occlusal splints was suspended during the course of aligner therapy and reintroduced after treatment, in combination with retention protocols. All patients were asymptomatic for temporomandibular disorder (TMD) at the time of enrollment, and this was confirmed through clinical screening based on the absence of pain, joint noises, or functional limitations.
Each participant contributed data under 2 conditions: (1) no aligner (control)—without aligners, in maximum intercuspation on natural dentition; and (2) with aligner—wearing their orthodontic aligners in maximum intercuspation. Thus, each subject served as their own control for evaluating the immediate effect of aligner presence. At the time of testing, the median duration of aligner use in this sample was 12 months (interquartile range [IQR] 7-19 months), ensuring that all patients were well adapted to aligner therapy ( Table I ). The specific brands of aligners used included Invisalign (n = 22 patients), Click Aligners (n = 1), and ClearCorrect (n = 1). All aligners were thermoformed from clear polyurethane sheets approximately 0.75 mm thick, fully covering the occlusal and incisal surfaces of both maxillary and mandibular teeth.
Table I
Sample characterization (n = 24)
| Variables | Description |
|---|---|
| Age (y), mean ± SD | 38.2 ± 13.4 |
| Sex, n (%) | |
| Male | 5 (20.8) |
| Female | 19 (79.2) |
| Treatment duration (mo), median (Q1, Q3), (Min-Max) | 12 (7, 19), (1-32) |
| OHIP-14 score, median (Q1, Q3), (Min-Max) | 4.5 (2, 8), (1-21) |
SD , standard deviation; Q1 , first quartile; Q3 , third quartile; Min, minimum; Max , maximum.
Surface EMG recordings were obtained using a wireless EMG unit (Teethan; BTS Bioengineering, Italy) with 4 surface electrode channels to record bilateral masseter and anterior temporalis muscle activity. All measurements were performed by the same calibrated operator with the subject seated upright in a dental chair, head in natural position, and back supported, to standardize posture. Preparation of the skin and placement of electrodes followed established guidelines to ensure signal quality and repeatability. Briefly, the skin over each muscle belly was first cleansed with 70% isopropyl alcohol to reduce impedance, and any facial hair in those areas was shaved if present (per surface EMG for the noninvasive assessment of muscles recommendations). Disposable self-adhesive surface electrodes were positioned on the skin parallel to the muscle fiber direction over the center of each muscle, in accordance with the protocol described by Ferrario et al ,
The EMG device sampled muscle activity at 1000 Hz. The raw signals were amplified with a gain of 150 and band-pass filtered (10 Hz high-pass, 500 Hz low-pass) by the hardware to eliminate movement artifacts and noise. The processed signals were transmitted in real time to a computer running the BTS Dental Contact Analyzer software (BTS Bioengineering), which computed specific EMG indexes and stored the data for analysis. ,
Each patient underwent a standardized clenching protocol consisting of 2 types of bite tasks: First, for EMG normalization, the patient performed a maximum voluntary clench on a pair of cotton rolls (10 mm diameter)—1 roll on each side placed on the occlusal surfaces of the mandibular molars and premolars (cotton roll test). The cotton rolls distribute force evenly, allowing maximum muscle contraction without pain, thereby serving as a reference for 100% muscle activity. The patient was instructed to bite as hard as possible on the rolls for 5 seconds. After a rest period, the patient then performed a maximum intercuspation clench (bilateral teeth contact in habitual occlusion) for 5 seconds without cotton rolls. This intercuspation clench was executed twice in succession: once without the aligners in the mouth (the no-aligner condition, also referred to as “Occlusion” in our data) and once with the aligners in place (with-aligner condition). Sufficient rest (approximately 1-2 minutes) was given between clenching trials to avoid muscle fatigue. The order of the 2 conditions was not randomized: all subjects first clenched without aligners and then with aligners.
The EMG software calculated the muscular work indexes for each trial. For each muscle (left temporalis, right temporalis, left masseter, and right masseter), the EMG potential during the intercuspation clench was expressed as a percentage of that muscle’s activity in the cotton roll reference clench. Two summary indexes provided by the Teethan system were used for analysis: impact (IMP) and intensity. IMP is a proprietary composite index reflecting the overall muscular effort and balance during the clench (considering the activity of all 4 muscle channels), whereas intensity refers to the average level of muscle activation (as a percentage of maximum) during the clench. In simpler terms, IMP captures the global muscle work output and how it is distributed, whereas intensity captures how strongly the muscles contracted on average. In addition, the percentage contribution of each of the 4 muscles to the total clenching effort was recorded for each condition, which allows assessment of the muscle activity distribution.
To control for potential bias related to the timing of aligner force activation, no electromyographic data collection was performed within the first 24 hours after an aligner change. All participants followed a 10-day aligner replacement cycle in accordance with manufacturer and clinical guidelines. EMG recordings were conducted on various days within this cycle, but always beyond the initial 24-hour adaptation window. This approach was adopted to avoid transient increases in muscular activity or discomfort commonly reported immediately after aligner insertion, which could confound the assessment of baseline neuromuscular response during clenching tasks.
After the EMG tests, participants were asked to complete the OHIP-14 questionnaire in its validated Brazilian Portuguese version. The OHIP-14 consists of 14 questions probing the frequency of problems in various domains related to oral health and function (including chewing discomfort, speech difficulty, pain, tension, embarrassment, etc., in the past few weeks). Responses are given on a 5-point Likert frequency scale: 0, never; 1, rarely; 2, sometimes; 3, often; 4, always. Participants filled out the questionnaire independently, without input from the investigator. For scoring, the simple additive method was used: the response values for the 14 items were summed to yield a total OHIP-14 score ranging from 0 (no impact of oral health issues on quality of life) to 56 (maximum negative impact). We recorded each patient’s total OHIP-14 score as a measure of their perceived treatment impact or discomfort at that point in time.
The primary outcomes of interest were the EMG-derived muscle activity indexes (IMP and intensity) and the distribution of muscle activity (percentage contribution of each muscle) in the no-aligner vs with-aligner conditions. Secondary outcomes included the OHIP-14 score and its correlation with muscle activity, as well as differences in muscle activity by sex and by duration of aligner use.
Statistical analysis
Data analysis was performed using SPSS Statistics software (version 27.0; IBM, Armonk, NY). Descriptive statistics were calculated for all variables. Continuous variables were checked for normality using the Shapiro-Wilk test. For approximately normally distributed variables, mean and standard deviation values are reported; for nonnormal variables, medians and IQRs are reported. Categorical variables (such as sex) are summarized by absolute and relative frequencies.
Comparisons between the no-aligner and with-aligner conditions were made using paired-sample t tests (2-tailed). This was applied to the IMP index when the aligner was in place. In addition, a paired t test was used to compare each muscle’s percentage contribution against its counterpart condition (however, the primary analysis of muscle distribution was done via 1-sample tests described below).
To evaluate the distribution of muscle activity, the contribution of each muscle was set and examined to the equal 25%. One-sample t tests were then conducted to test whether the mean difference was significantly different from 0 (no deviation from 25%). A significant result ( P <0.05) for a muscle would indicate an imbalanced contribution (either above or below 25%) in that condition on average. This analysis was done separately for the no-aligner and aligner conditions to observe in which state the muscle work was more evenly distributed.
Sex differences were explored by comparing male and female subjects. Given the small number of male patients, these comparisons were considered exploratory. Independent-samples t tests were used to compare the mean IMP (and intensity) between males and females in each condition.
Correlations between numeric variables were assessed using Pearson’s correlation coefficient if both variables were normally distributed, or Spearman’s rank correlation if not. The analysis specifically looked at the correlation between OHIP-14 scores and muscle activity (IMP) with aligners, to test if higher muscle activity was associated with worse reported quality of life. The analysis also examined the correlation between the duration of aligner use (in months) and the muscle activity indexes, to see if patients who had been in treatment longer exhibited different muscle activation levels (potentially reflecting adaptation over time).
For all tests, a significance level of α = 0.05 was adopted. Given the exploratory nature of some analyses (eg, sex differences), adjustments for multiple comparisons were not made; instead, those findings were interpreted with caution. The results are presented in tables corresponding to key outcome measures ( Tables I-III ).
Table II
Comparison of IMP and intensity between occlusion and aligner
| Variables | Occlusion, mean ± SD | Aligner, mean ± SD |
Difference
(95% CI) |
P value |
|---|---|---|---|---|
| IMP | 68.4 ± 32.1 | 96.0 ± 24.8 | 27.6 (13.6-41.6) | <0.001 |
| Intensity | 80.2 ± 7.0 | 84.3 ± 6.2 | 4.08 (1.24-6.93) | 0.007 |
SD , standard deviation; CI, confidence interval.
Table III
Comparison of force distribution deviations
| Muscle | Mean ± SD | P value |
|---|---|---|
| Right anterior temporalis: occlusion | 4.38 ± 5.62 | <0.001 |
| Right anterior temporalis: aligner | 0.25 ± 5.03 | 0.810 |
| Left anterior temporalis: occlusion | 6.08 ± 7.80 | <0.001 |
| Left anterior temporalis: aligner | −0.67 ± 5.86 | 0.583 |
| Right masseter: occlusion | −5.12 ± 5.81 | <0.001 |
| Right masseter: aligner | 1.42 ± 5.36 | 0.208 |
| Left masseter: occlusion | −5.33 ± 7.38 | 0.002 |
| Left masseter: aligner | −0.96 ± 6.58 | 0.483 |
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