Introduction
Bite-raising posterior occlusal attachments are integrated into clear aligner treatments to prevent posterior extrusion and promote intrusion by enhancing masticatory muscle activity (MMA) and occlusal forces. However, their effect remains anecdotal. The study aimed to (1) investigate the effect of passive aligners with posterior occlusal attachments on daily MMA compared with control aligners, and (2) determine if bite raising induces transient temporomandibular disorder (TMD); explore whether any early intrusive effects on teeth could be detected.
Methods
Twelve volunteers participated in a randomized, crossover experiment. MMA recordings (amplitude, duration, frequency, and duty time) were collected using a wearable electromyography device over 5-hour sessions in natural settings while wearing (1) no aligners, (2) passive aligners for 8 days, and (3) aligners with occlusal attachments for 8 days. Participants’ freeway space (FWS), occlusal discomfort, perceived stress levels, TMD symptoms, and intraoral scans were monitored. Data were analyzed with linear mixed modeling.
Results
Bite-raising attachments increased contraction episodes per hour by 55% (95% confidence interval, 12%-67%) from days 1 to 8, but not compared with baseline. Both aligners significantly increased FWS, with bite-raising aligners showing a 114% increase (95% confidence interval, 85%-144%). Significant occlusal discomfort was reported in the first 3 days for both aligner types. No participants were diagnosed with TMD. No discernible intrusion was detected on serial intraoral scans.
Conclusions
Aligners encroaching into the FWS transiently increase MMA in healthy young adults, despite adaptation and diminishing discomfort. However, we found no convincing evidence to support the hypothesis that occlusal attachments contribute to vertical control or enable intrusion in the short term. More research is needed to assess long-term effects and determine whether intrusions become clinically relevant.
Highlights
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Clear aligners with posterior attachments temporarily increased masticatory muscle activity.
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The aligners caused an increase in freeway space and transient discomfort.
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No significant intrusion or TMD symptoms observed with short-term aligner use.
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Future research should focus on long-term aligner effects in high parafunction patients.
Clear aligner therapy offers a more esthetic and comfortable alternative to traditional fixed appliances. Among the various indications of clear aligners, their role in vertical control has attracted considerable interest, particularly for the management of patients with anterior open bites or with hyperdivergent facial patterns. It has been suggested that aligners may prevent unwanted extrusion of posterior teeth or even promote slight intrusion. This effect could be further enhanced by the application of unfilled occlusal attachments or bite-raising pads to the posterior occlusal surfaces of clear aligners. ,
The proposed mechanism is that occlusal attachments encroach on the freeway space (FWS), thus promoting masticatory muscle contractions and tooth clenching. This, in turn, may increase occlusal forces, improving vertical control or causing posterior teeth intrusion. However, the effects of aligners with occlusal pads on habitual muscle activity remain largely unknown. As a matter of fact, little is known about the potential side effects of sudden increases in occlusal vertical dimension (OVD) induced by clear aligners. Anecdotal reports suggest that the wear of aligners may cause masticatory muscle pain and transient signs or symptoms of temporomandibular disorder (TMD).
Masticatory muscle activity (MMA) refers to the electrical bursts generated by the masticatory muscles during functional and nonfunctional activities. These bursts can be measured noninvasively using surface electromyography (EMG) by placing electrodes on the skin over targeted muscles. Using surface EMG, it is possible to assess the frequency, amplitude (intensity), and duration of muscle contractions, thus providing valuable insights into masticatory muscle function. The available evidence concerning the influence of orthodontic appliances on MMA, jaw function, and oral parafunctional behaviors is both limited and inconclusive. , Recently, the introduction of small, wearable EMG devices has facilitated research into MMA by enabling real-time monitoring in natural environments over extended periods. Leveraging this approach, this study focuses on habitual MMA, investigating jaw muscle activity during daily life to explore the potential effects of posterior occlusal attachments.
The study aimed to investigate the short-term effect of aligners with bite-raising occlusal attachments, compared with standard aligners without occlusal attachments, on habitual activity of the masticatory muscles. A secondary objective was to determine whether the increase in OVD associated with occlusal attachments would cause a transient appearance of signs and symptoms of TMD. A third objective was to explore whether any early intrusive effects on teeth could be detected.
We hypothesized that aligners with occlusal attachments, through encroachment on the FWS, would increase MMA to levels beyond those of standard aligners.
Material and methods
This study was designed as a crossover randomized controlled experiment with each participant acting as their control. This report adheres to the Consolidated Standards of Reporting Trials guidelines. The flow chart of sample recruitment is summarized in Figure 1 . MMA data were collected from a group of young adults in a laboratory and in a routine life setting, using an EMG wearable device. The laboratory part of the study took place at the Craniofacial Research Laboratory of the Faculty of Dentistry, the University of Otago. Before study commencement, approval was obtained from the local ethics committee (H22/156).
Consolidated Standards of Reporting Trials flow diagram detailing participant and group allocation.
Emails were sent out to 92 postgraduate and 570 undergraduate students at the University of Otago, Faculty of Dentistry. Recruitment was stratified and balanced for sex, so that the sample included the same number of male and female participants within each group. Participants were selected on a first-come basis, and the final sample of selected participants consisted of 6 males and 6 females. Data collection was completed between July 2023 and April 2024.
At the screening appointment, inclusion and exclusion criteria were applied to confirm participant eligibility, and informed consent was obtained. Adults aged >18 years with a healthy dentition free of active caries and periodontal disease were eligible for inclusion. Exclusion criteria included currently having orthodontic treatment, acute pain condition affecting the mouth such as caries and active periodontal disease, >5 missing teeth (excluding third molars), presence of any crossbites (anterior or posterior), presence of mandibular functional shifts >1.5 mm, any underlying disease or pathology which affects normal masticatory muscle function, history of TMD and/or orofacial pain over the past month not including asymptomatic clicks, reported allergies to aligner materials, use of medications that affect the central nervous system and/or motor skills, and facial hair or make-up material participants are unwilling to remove which would interfere with the placement and/or signal of the wireless EMG.
The baseline characteristics of psychosocial factors, oral behaviors, signs and symptoms of TMD, vertical facial type, and alcohol and caffeine consumption are summarized in the Table . Heightened somatization is a risk factor for the development of TMD symptoms. , Parafunctional behaviors can put undue strain on the muscles and joints, and there is a close relationship between the MMA and oral parafunctional behaviors of an individual. ,, Patients with TMD present with reduced masticatory efficiency along with ease of fatigue compared with healthy subjects. Low-angle patients exhibit greater muscle activity and strength compared with high-angle patients. ,,,,, Nonetheless, research examining habitual MMA in patients with long and short facial morphologies under natural conditions has not identified any significant differences between the 2 groups. Participants’ alcohol and caffeine consumption was evaluated using an alcohol and caffeine questionnaire. Both substances are known to affect muscle activity and the central nervous system, potentially confounding outcomes. ,,,
Table
Psychosocial variables, Oral Behavior Checklist, baseline TMD symptom score, vertical facial characteristics, and alcohol and caffeine consumption in the sample investigated
| Variables | Values |
|---|---|
| Psychosocial variables (median [IQR]) | |
| Somatisation questionnaire | 1.0 (0.8-5.5) |
| Somatosensory amplification scale | 17.0 (8.8-22.3) |
| Oral Behavior Checklist (median [IQR]) | |
| Nonfunctional | 3.5 (3-4.3) |
| Diurnal | 12.0 (11-15.8) |
| Total | 16.5 (15.0-19.8) |
| Baseline TMD symptom score (median [IQR]) | 0.5 (0-1) |
| Lower anterior face height (%) (mean [SD]) | 57.7 (2.6) |
| Alcohol and caffeine consumption (n [%]) | |
| Regular alcohol consumption | 1.0 (8.3) |
| Regular caffeine consumption | 11.0 (91.7) |
IQR, interquartile range; SD , standard deviation.
Note. Oral Behavior Checklist scores derived by summation of weighted responses to specific items of the questionnaire (nonfunctional score range, 1-6; diurnal score range, 8-22; total range, 12-24); regular alcohol/caffeine consumption defined as product consumed ≥1 per week.
The targeted sample size was determined based on prior assessments of daily MMA variability using a repeated measurement study design approach. The α error was set at 0.05, the β error at 0.2 (ie, 80% power), and a 15% change in EMG outcome variables was considered clinically relevant. With 10 subjects per group, the design would achieve 70%-88% power. We aimed to recruit 12 participants to account for possible dropouts.
Assessments were taken over 5 study sessions ( Fig 2 ). Digital scans were taken in centric occlusion using an intraoral scanner (iTero Element Plus Series; Align Technology, Calif) to allow fabrication of the aligners. Dental scans were obtained at baseline, after 1 week of wearing passive aligners (PAs), and after 1 week of wearing the occlusal attachment aligners (OAAs). To enhance the external validity of the intervention, special attention was paid to ensure that the material chosen for the construction of aligners closely matched the most prevalent aligner materials at the time. Two sets of maxillary aligners were produced per participant using 0.76 mm Zendura FLX (thermoplastic polyethylene copolymer; Bay Materials LLC, Fremont, Calif). These sheets were thermoformed over 2 sets of models (OAA with bite-blocks measuring 5 mm in width by 6 mm in length that exceeded the participants’ FWS by 1 mm over the occlusal surfaces of the maxillary first and second molars, and one without any blocks). Zendura FLX was selected because of its previous use as an aligner simulant, its comparable thickness to most popular aligner options, and its cost-effectiveness. Participants were randomly assigned to 1 of 2 treatment sequences by drawing from a container stratified for sex ( Fig 2 ). Half of the participants began by wearing a standard PA without occlusal attachments, serving as the control appliance, whereas the other half wore an OAA first. Participants were instructed to wear the aligners full-time except when eating and drinking liquids other than water. Both groups had their jaw muscle activity recorded over 5 nonconsecutive days: firstly, at baseline with no appliance in place; secondly, on the day the appliance was first delivered; thirdly, after 7 days of consecutive aligner wear. After a 7-day washout period without aligners, participants switched; those who started with the control PAs were issued OAAs, and vice versa. Muscle activity was measured again on the day that aligners were issued and after 7 days of consecutive wear.
Brief schematic detailing the study sessions for the crossover study.
A small wireless surface EMG device (MREMG, Dunedin, New Zealand) was used for this study. The device is placed on the skin overlying the masseter muscle and contains a miniature electromyographic unit ( Fig 3 ) powered by a lithium battery, and is capable of continuously transmitting data to a smartphone. The technical specifications and procedure outlined below pertaining to the use of this equipment have been described in our previous reports. ,, The device is small and minimally invasive, weighing 4.1 g and measuring 28 × 35 × 10 mm in size, based on an oval circuit board which facilitates connections to the various components. Power is supplied by a rechargeable 3.7 Vnom, 55 mAh lithium battery measuring 4 × 10 × 20 mm. The primary hardware employed for recording EMG activity consists of a programmable gain right leg drive amplifier. Sampling was conducted at a rate of 1000 Hz, with subsequent high-pass filtering using a 70 Hz cutoff frequency. The resulting analog data was then converted into binary code with a 24-bit resolution, subjected to rectification using a 125 ms root mean square window, and transmitted wirelessly via Bluetooth to a smartphone for storage. During each study session, participants were provided with an Android smartphone (Samsung Galaxy A04) preinstalled with a customized application for calibration, assessment, and storage of the EMG activity.
Wearable EMG device in situ.
For the primary outcome measures, a bespoke software (BruxiView, Dunedin, New Zealand) was used to detect episodes of MMA including the frequency of contraction episodes (number of contractions per hour), episode mean amplitude (as a percentage of a participant’s maximum voluntary contraction [MVC]), episode duration (measured in seconds) and duty time (ie, muscle workload), representing the primary outcome measures of the study. ,,, An MMA episode was defined as any muscular activations surpassing 5% of MVC, and persisting for a minimum of 2 seconds. These episodes were allowed to include subthreshold signals, provided their duration did not exceed the standby time of 2 seconds. In essence, identified MMA episodes could still manifest muscle activity below the threshold value, provided such activity did not last beyond 2 seconds. These threshold parameters were implemented to mitigate the influence of background noise and minimize the risk of false-positive episode identifications stemming from confounding activities such as swallowing, speaking, or movement artifacts. In addition, periods characterized by sustained elevated muscle activity attributable to mastication were identified and excluded from the recording using the software. Duty time was defined as the total amount of time that the masseter muscle was active above a 5% MVC threshold during the recording period, and was expressed as a percentage of the total time.
The FWS of each participant was evaluated in a seated position in a dental chair using a standardized technique using a Willis gauge. The FWS is calculated by subtracting the measurement of an individual’s lower facial height at rest without aligners (eg, immediately postswallowing) from their facial height during maximum intercuspation. Each participant’s FWS was determined based on the mean of 3 consecutive measurements. The measurement process was repeated throughout the study at baseline and before and after each aligner session to monitor potential adaptations to the aligners. The method error for FWS was derived from our previous study that employed the same measurement protocol. According to the Dahlberg formula, the method error was 0.5 mm, corresponding to 15.8% when expressed as a percentage.
Self-reported stress and occlusal discomfort (OD) were evaluated using separate 100-point visual analog scales (VAS) adapted from previous research. On the scale, 0 mm indicated no stress or any discomfort in participants’ teeth, and 100 indicated maximal stress or discomfort. Signs and symptoms of TMD were assessed using a standardized examination form derived from the diagnostic criteria for the TMD workgroup.
The model scans were imported into Geomagic Control X metrology software (Geomagic, Morrisville, NC) to compare the occlusal surfaces at baseline, after wearing the PAs, and after wearing the OAAs. Superimposition was performed using best-fit alignment with an 80%-point sampling ratio and 50 iterations, followed by regional refinement based on the anterior palatal region, including the palatal rugae, as a stable reference. The percentage of surface points falling within a ± 0.1-mm tolerance was calculated using Geomagic Control X to provide a quantitative assessment of displacement. This analysis focused on the crowns of the first and second maxillary molars (teeth 16, 17, 26, and 27) across all 3 time points, offering a concise numerical summary of the extent of positional changes in each condition. Color-coded deviation maps were generated to qualitatively visualize the possible occurrence of localized intrusion.
Statistical analyses were performed using SPSS (version 28.0.1.1; IBM, Armonk, NY). A linear mixed model was used to analyze the data. This analysis was conducted to examine potential differences in the characteristics of contraction episodes between the PAs and OAAs. Dependent variables were episode amplitude, duration, frequency, and duty time, whereas fixed factors were condition (no aligner/baseline, CA, and OAA) and sex. Linear mixed model analysis was also used for the secondary outcome measures assessed multiple times, including stress, OD, and FWS. The level of significance was set at 0.05.
Results
The sample recruited included 6 males and 6 females; their mean age (± standard deviation) was 28.6 ± 1.2 years, with 7 Asian, 2 New Zealand European, and the remaining 3 belonging to other ethnicities.
The primary outcome measures included mean episode amplitude, duration, frequency, and duty time ( Fig 4 ). No participants were excluded from the analysis of primary outcomes, and no data were missing from the 12 participants. PAs were worn on average 21.3 ± 2.0 h/d, and OAAs on average 21.2 ± 1.9 h/d. The difference between the 2 was not statistically significant ( P = 0.947). The average length of the EMG recording time per day was 5.2 ± 0.4 hours (313.9 ± 24.0 minutes).
Estimated marginal means of episode duration ( A ), episode amplitude ( B ), episode frequency ( C ), and duty time ( D ) for the 2 aligners investigated over time; adjusted for sex and ethnicity; error bars: standard error of the mean; ∗ P = 0.029 indicates a statistically significant difference between days 1 and 8 in the occlusal attachment condition.
The mean amplitude of contraction episodes did not differ significantly between baseline, control, and OAAs conditions (F = 0.9; P = 0.496), even though the mean amplitude appeared to decrease slightly over time.
Compared with the aligner-free baseline condition, episode duration also showed an apparent decrease over time, which was not statistically significant (F = 1.2; P = 0.329) with no significant difference between the 2 aligner conditions ( P ≥0.135).
The mean number of contraction episodes at baseline was around 40 episodes per hour (95% confidence interval [CI], 23.4-54.8) and did not change significantly across time points while wearing the PAs ( P ≥0.902). When wearing the OAA, the mean number of contraction episodes per hour on day one showed a modest, but nonsignificant decrease of approximately 10 episodes per hour compared with baseline ( P = 0.242). By day 8, the frequency increased significantly to 46.9 episodes per hour (95% CI, 31.2-62.6), with a statistically significant difference observed between days 1 and 8 ( P = 0.029). However, the change from baseline to day 8 was not statistically significant ( P = 0.291).
Duty time showed a time profile very similar to the mean number of contraction episodes, but the difference between days 1 and 8 while wearing OAA only approached statistical significance ( P = 0.059).
There was no significant sequence effect. In other words, none of the main outcome variables were significantly influenced by the order in which the 2 aligners were delivered (F ≤1.8; P ≥0.214). Linear mixed model analysis revealed that there were no significant associations between any of the analyzed EMG variables with sex or ethnicity ( P ≥0.205).
All participants were included in the analysis of secondary outcomes, and no data were missing. The measures of FWS and OD are displayed in Figure 5 .
