Patients undergoing clear aligner therapy (CAT) report muscle tenderness and produce wear facets on their aligner trays. However, little is known about the masticatory muscle response to clear aligners. Here, we measured the activity of the masseter during CAT using ambulatory electromyography. We also explored whether psychological traits modulate the masticatory muscle response to CAT.
Using portable data loggers, we recorded the electromyographic (EMG) activity in the right masseter muscle of 17 healthy adults without temporomandibular disorder (16 females, 1 male; mean age ± standard deviation, 35.3 ± 17.6 years) commencing treatment with CAT over 4 weeks, under the following conditions: week 1 without aligners (baseline), week 2 with a passive aligner (dummy), week 3 with their first active aligner (active1), and week 4 with their second active aligner (active2). We used a mixed-effect model to test differences in EMG activity over the 4-weeks and a general linear model to test the effect of psychological traits on EMG activity.
The EMG activity of the masseter increased significantly with aligners compared with baseline. The largest relative increase in EMG activity was seen during the dummy (152%; P <0.001) and active1 (155%; P <0.001) stages. During active2, the activity of the masseter decreased significantly toward baseline levels. Participants’ trait anxiety was positively associated with increases in EMG activity ( P = 0.027).
CAT is associated with a transient increase in masticatory muscle activity, possibly because of an increase in wake-time parafunctional tooth clenching. Temporomandibular disorder-free patients adapt well to CAT as the masticatory muscle activity decreases toward baseline levels after 2 weeks.
Clear aligner therapy (CAT) produces a transient increase in masticatory muscle activity.
Trait anxiety is associated with an increase in masticatory muscle activity during CAT.
Healthy temporomandibular disorder-free subjects adapt well to CAT.
Clear aligner therapy (CAT) has seen rapid growth and advancements over the past several years, making it a popular treatment modality in contemporary orthodontics. This trend is largely because of the growing demand among prospective patients for esthetic treatment alternatives to traditional brackets and wires. , The primary advantages of CAT over traditional fixed-edge appliances are its esthetics, , removability, and comfort.
The muscles of mastication are capable of adapting to the various functional demands imposed on them. These adaptive changes include altering their physical size, fiber properties, muscle activity, and force of contraction. However, despite the rising popularity of CAT, little is known about the response of masticatory muscles to clear aligners.
A few studies have shown that patients undergoing CAT have increased frequency of wake-time tooth clenching episodes, report jaw muscle tenderness, and produce wear facets on their aligner trays. In contrast, orthodontic treatment with a fixed appliance can lead to patients avoiding tooth contact to reduce tooth pain related to orthodontic tooth movement. Repetitive clenching on aligners might be an acquired behavior acting as a conditioning stimulus to reduce the perception of the orthodontic nociceptive stimuli. Indeed, similarly to plastic wafers, clenching on the aligners could induce a temporary displacement of the teeth and promote blood flow through the compressed areas of the periodontal ligament; thus, preventing the accumulation of proalgesic mediators in the periodontal ligament space and promoting pain relief.
There is substantial evidence of a positive association between awake bruxism—repetitive masticatory muscle activity involving tooth clenching—and impaired mood. , For example, awake bruxism is highly prevalent in anxious individuals, , and experimental tooth clenching reduces salivary cortisol concentrations. Therefore, tooth clenching may be a maladaptive coping mechanism to manage stress.
Surface electromyography (sEMG) is an objective method for measuring the activity of a muscle of interest through the placement of electrodes over the skin. Its simplicity and noninvasive nature have brought wide-spread use to researchers in dentistry for both basic science and clinical studies. For instance, sEMG has been used to study temporomandibular disorder (TMD), , detect muscle hyper- and hypoactivity, , imbalance, and fatigue.
Current literature about the response of masticatory muscles to CAT during wakefulness is limited to patients’ self-report. In this study, we aimed to evaluate the masticatory muscle response to CAT using ambulatory sEMG during the daytime. We also explored whether the masticatory muscle response to CAT is modulated by psychological traits. It was hypothesized that (1) patients subjected to CAT would have a transient increase in masseter muscle activity, (2) that this increase in masseter activity is related to orthodontic tooth movement, and (3) is dependent on psychological traits.
Material and methods
Subjects aged 17 years or older with a plan to undergo CAT, were recruited from the graduate orthodontic clinics at the University of Toronto (Toronto, Ontario, Canada) and the University of Western Ontario (London, Ontario, Canada). Ethics approval was obtained from the corresponding Research Ethics Boards at each Institution, and informed consent was acquired from each subject before entering the study.
Each potential participant completed an initial screening questionnaire using the TMD-Pain screener. In addition, each subject underwent a preliminary TMD examination at each center according to the diagnostic criteria for TMD clinical protocol. Exclusion criteria consisted of current TMD or orofacial pain, current use of muscle relaxants or other medications affecting masticatory muscle activity, presence of any systemic disorders affecting motor behaviors and pain perception, and daily use of any analgesics.
Seventeen subjects were recruited (16 females, 1 male; mean age ± standard deviation, 35.3 ± 17.6 years). All participants were treated using Invisalign clear aligners (Align Technology, San Jose, Calif), made of the latest generation of multilayer thermoplastic polyurethane-based material, SmartTrack (Align Technology). Using the ClinCheck Pro software (Align Technology), the first stage of aligners for all participants consisted of maxillary and mandibular aligners programmed with no active tooth movements (passive aligners). Active tooth movements were programmed for the subsequent stages at the standard rate recommended by the ClinCheck Pro algorithms. All participants had Class I or mild Class II malocclusion with mild to moderate crowding or spacing in the maxillary and mandibular dental arches (nonextraction cases). Chewies to improve aligner seating and intermaxillary elastics were not used.
To determine the effect of psychological traits and self-reported parafunctional oral behaviors on masticatory muscle activity during CAT, we asked each participant complete the Oral Behavior Checklist (OBC), the State-Trait Anxiety Inventory, the Somatosensory Amplification Scale (SSAS), and the Beck Depression Inventory (BDI).
The OBC includes 21 items assessing the awareness and self-reported frequency of oral behaviors on a 5-point scale (range of scores, 0-84). The State-Trait Anxiety Inventory includes 20 items to assess state anxiety and 20 statements to assess trait anxiety on a 4-point scale (range of scores for each subscale, 20-80). For this study, only trait anxiety was used. The SSAS includes 10 statements investigating participants’ sensitivity to bodily sensations on a 5-point scale (range of scores, 10-50), and it is an estimate of bodily hypervigilance. , The BDI investigates 21 depressive symptoms, scored between 0 and 3 (range of scores, 0-63).
The study design is depicted in Figure 1 . We recorded the electromyographic (EMG) activity in the right masseter muscle of subjects commencing treatment with CAT over 4 weeks, with sampling done 3 d/wk (day 1, day 3, and day 5 of each week). EMG data were collected before the start of CAT with no aligners (week 1, baseline stage), for 1 week while participants were wearing a passive aligner (week 2, dummy stage), for 1 week while participants were wearing their first active aligner (week 3, active1 stage), and finally for 1 week with their second active aligner (week 4, active2 stage).
Participants were given a kit containing a portable EMG device (MicroEMG; OT Bioelettronica, Turin, Italy), disposable bipolar self-adhesive concentric EMG electrodes (Code 2.0; Spes Medica, Genova, Italy) with a radius of 2 cm, rectangular electrodes (3.5 × 4 cm, Red Dot; 3M, Saint Paul, Minn), disposable batteries, secure digital (SD) cards, and alcohol pads. The kit also contained a personalized calendar to help remind participants of which day they had to perform the EMG recording and a paper diary. The SD cards were labeled; that is, they reported the date on which they had to be used.
Before starting the experiment, each participant received detailed instructions and demonstrations on the proper usage of the EMG device. A video tutorial, as well as a procedure manual and checklist, were also provided.
In summary, participants were instructed as following: (1) not to use make-up on the day of the EMG recording (and to shave for male participants); (2) to insert a new battery and a new SD card in the device before the daily EMG recording; (3) to rub their right cheek with the alcohol pads provided for at least 20 seconds before using the EMG device; (4) to place the EMG electrode on the right cheek and the device on the right clavicle ( Fig 2 ); (5) to start the recording by pressing the red button on the device in the afternoon of the predetermined recording day; (6) to clench their teeth in maximum intercuspation and at their maximum voluntary contraction for 3 times lasting 3 seconds each, and separated by 5-second intervals, at the beginning of each recording session; (7) to stop the recording after 4 hours by pressing the same red button; (8) to store the SD card and dispose of the battery; and (9) to repeat procedures 1-8 at the next recording day, as reported in their calendar.
Participants were asked to place the concentric electrode on the right masseter muscle, along a line projecting from the mandibular angle to the lateral canthus of the eye, approximately 20 mm above the mandibular angle, and to connect the electrode to the device. The electrode was located on the most prominent belly of the muscle as evaluated by palpation during maximum voluntary contraction (ie, the part of the muscle which closely approximates the largest muscle “bulge” when they clenched; Fig 2 ).
The device was connected to a reference electrode (3.5 × 4 cm, Red Dot; 3M) on the middle point of the right clavicle ( Fig 2 ). The concentric ring systems of the electrodes show higher spatial selectivity with respect to the traditional detection systems and reduce the problem of electrode location because they are insensitive to rotations and reduce EMG cross talk.
To reduce artifacts during EMG recordings, we instructed all participants to avoid exercising, chewing, and eating during recording sessions. If these activities did occur, participants were instructed to record the time of the corresponding day in the diary.
Before starting the 4-week recording, the operator carefully checked that the participant could place the electrodes and perform the EMG recording correctly. All participants were asked to try the procedures beforehand at our clinics. Specifically, we asked them to position both the electrode on the masseter and the EMG device and complete a recording lasting a few minutes. All participants were informed that they could contact the investigators if they had questions regarding the use of the device, or for any technical reason.
Participants returned their kit to the research unit at the end of week 4. After that, a TMD examination was performed. All participants received financial compensation for participating in this study.
EMG signal processing
EMG signals were sampled at 1024 Hz, amplified, and bandpass filtered between 10-550 Hz. All EMG raw data were downloaded from the SD cards, sorted, and imported in a worksheet including the day of recording (day 1, day 3, or day 5), condition (baseline, dummy, active1, or active2), and subject identification number, using a custom-made computer algorithm. This algorithm was written using macros for Microsoft Excel using Visual Basic programming language (version 16.0; Microsoft Corp, Redmond, Wash).
Using software (OTBioLab; OT Bioelettronica), 2 operators (T.L. and J.T.) visualized and processed all the EMG signals recorded during the experiments, identified and removed EMG artifacts on the basis of information provided by the participants and careful examination of all EMG signals. The operators were trained by an investigator (I.C.) with over 15 years of experience in EMG data analysis. Any disagreement between the operators was resolved by the senior investigator (I.C.). Finally, EMG data were standardized across participants and conditions by computing z-scores.
A generalized linear mixed-effect model with Bonferroni-corrected Wilcoxon paired signed rank test was used to test differences in EMG z-scores between the different experimental conditions (baseline, dummy, active1, and active2), and between recording days (day 1, day 3, day 5) of each condition. The interaction day of recording by condition was tested and included in the model since it was statistically significant. Therefore, the fixed factors included in the final statistical model were the recording day, the experimental condition, and their interaction. The participant identification number was included as a random factor.
A general linear model was used to test the effect of behavioral (OBC) and psychological factors (trait anxiety, SSAS, BDI) on the EMG relative changes from baseline EMG measurements. The statistical significance was set at P <0.05. The statistical analysis was conducted using SPSS (version 24; IBM, Armonk, NY) by a single operator (I.C.).
On the basis of a previous study, 16 participants were required to detect a 10% change in EMG activity with the Invisalign appliance, with a large effect size (d = 0.85), alpha and beta error set at 0.05 and 0.1, respectively.
There were no participant dropouts during the experimental period. The compliance of the participants in the research protocol was satisfactory. Over the 4-week experimental period, the expected recording time was 48 hours over 12 days (4 weeks, 3 d/wk, 4 h/d). The average EMG wear time of the participants was 45.93 ± 14.60 hours. No participant developed symptoms of TMD after the 4-week recording.
Masseter muscle activity increases with CAT
The EMG activity of the masseter during the 4 experimental conditions is reported in Figure 3 , A . The activity of the masseter was affected by the experimental condition (F = 130.14; P <0.001), the recording day (F = 6.95; P = 0.001), and the interaction condition by day (F = 13.41; P <0.001).