Response of masticatory muscles to treatment with orthodontic aligners: a preliminary prospective longitudinal study

ABSTRACT Introduction: The emergence of orthodontic aligners has provided an aesthetic and comfortable option for orthodontic treatment. However, the encapsulated design of the aligners can influence the masticatory muscles, and might compromise safe treatment. Objective: This preliminary longitudinal study aimed to investigate whether the use of orthodontic aligners affects the biting force and myoelectric activity of the superficial masseter and anterior temporal muscles. Methods: Ten subjects participated in the study and underwent treatment during an 8-month follow-up period. The root mean square (RMS), the median power frequency (MPF) of the surface electromyography (sEMG) signals, and the biting force (kgf) were recorded and normalized relative to the pretreatment condition. The data were analyzed by repeated-measure analysis of variance (ANOVA), with the significance level set at 5%. Results: Both the superficial masseter and the anterior temporal muscles presented an increase in sEMG signal activity during the treatment, with a marked increase in the latter compared to the former (p<0.05). Moreover, a significant decrease in bite force was evidenced (p<0.05). Conclusions: This preliminary study observed that the orthodontic aligners affected the muscle recruitment pattern of masticatory muscles, and reduced biting performance during the 8-month follow-up period.


INTRODUCTION
The relevance of aesthetic values is reflected by the popularity of orthodontic aligners as a viable therapeutic approach to meet society's current demands. 1 However, the short and longterm effects of this new therapy on mastication biomechanics are still unclear.
The encapsulated occlusal devices used for bruxism and temporomandibular disorders (TMD) have a myorelaxant effect on masticatory muscle activity. 2 Unlike the occlusal devices also used in the treatment of bruxism, encapsulated orthodontic retainers used in the short-term have not been found to cause changes in muscle recruitment. 3 However, orthodontic retainers and orthodontic aligners cannot be compared with occlusal devices, despite their similar encapsulated design, because aligners are usually not adjusted for more stable occlusal contact, during clinical practice. Quantitative analysis of the bite force produced is also a wellknown clinical parameter used to assess chewing performance.
Neuromuscular adaptation in bite force has been reported in the orthodontic retention phase. 6 Treatment with Invisalign ® has been found to help reduce the painful symptomatology of TMD patients. 7 Moreover, the use of aligners in individuals with sleep bruxism (SB) did not influence the SB index, and was able to increase masseter phasic contractions. 8 An increase in the myoelectric activity of the masseter after short periods of aligner use has also been reported. 9 However, to our knowledge, there is no previous account of the effect of orthodontic aligners used for long-term periods on the response of other masticatory muscles during different tasks in asymptomatic subjects.
To this end, this preliminary longitudinal study aimed to investigate whether orthodontic aligners can affect the biting force and myoelectric activity of superficial masseter and anterior temporal muscles. The null hypothesis tested was that separation of the dental arches and lack of occlusal adjustment prompted by the aligner would change mastication muscle recruitment during orthodontic treatment with Invisalign.

MATERIAL AND METHODS
This prospective longitudinal clinical study included 10 participants (7 women; mean age 29.9 ± 5.5 years). This experimental protocol for the study was approved by the local ethical committee (process number 2.096.512/2017), following the principles of the Declaration of Helsinki. All the subjects gave their written informed consent to participate in the study.
The selected subjects had Angle Class I or Class II malocclusions, crowding of teeth ≤ 5.0 mm, a good vertical facial pattern (SN.GoGn = 27º to 37º), and a functional state of general and periodontal health. The individuals who were excluded comprised those under orthodontic treatment, and indicated for tooth extraction or auxiliary mechanics (mini-implants, miniplates, buttons, precision cuts or intermaxillary elastic), or else affected by syndromes with blood-related or dentofacial manifestations, temporomandibular joint dysfunctions, crossbite or open bite, and reporting routine use of analgesics, anti-inflammatory drugs, muscle relaxants or anxiolytics.
All participants were instructed to use the aligners for at least 22 hours daily. The following pairs of aligners were changed every two weeks after that, as defined in the treatment plan.
The time of daily use and the duration of each pair of aligners for 14 days followed the manufacturer's instructions.
Surface electromyography (sEMG) was used to evaluate the bilateral recruitment of superficial masseter and anterior temporal muscles. The sEMG signals were recorded at predefined time intervals during an 8-month follow-up period. The initial exam (T0) was performed one week before using the first pair of aligners, and defined as the baseline condition (pretreatment muscle parameters). The sEMG signal was recorded with and  was obtained from the mandibular dynamometer. The 30% MVBF was calculated and defined for the third task, i.e., SVBF.
The raw sEMG signals were recorded during the three recruitments levels, whenever measurements were made (T0-T8), and each recording lasted 20 seconds.
The sEMG signals were processed by the SignalHunter software The median frequency was also extracted from the power spectrum of the sEMG signal using the following equation: In which f indicates the frequencies represented in the energy power spectrum (P) of the sEMG signal, separating two regions of similar power.
The RMS amplitude and the median power frequency (MPF) were calculated for all the muscles evaluated, and from the 2-s window selected from the signal referring to the dynamometer channel.

STATISTICAL ANALYSIS
The Shapiro-Wilk test was used to assess whether the data distribution was Gaussian. Based on the results of the normality test, the data were analyzed using parametric tests. RMS ampli-

RESULTS
The data of the descriptive statistics of the muscles studied, the side and the level of recruitment during the eight months of follow-up are available in Table 1. The RMS amplitude variation for the superficial masseter muscle is shown in Figure 2A tasks decreased by about 30%, following the opposite variation observed in the RMS amplitude ( Fig 2B).
Paes-Souza SA, Garcia MAC, Souza VH, Morais LS, Nojima LI, Nojima MCG -Response of masticatory muscles to treatment with orthodontic aligners: a preliminary prospective longitudinal study Table 1: Median and standard deviation of both the normalized data of the root mean square (RMS amplitude), and the median power frequency (MPF) of the superficial masseter muscles and anterior temporalis. The data referred to the three tasks of muscular activity, their respective hemifaces, and their variations over the 8-month follow-up period. There was a significant increase in the RMS amplitude (F (8,728) =24.675; p=0.000) for the anterior temporal muscle, gradually from T0 to T4-T8 for all tasks (Fig 2C). At the end of the follow-up period, muscle recruitment increased by about 80% for the mandibular rest task, 70% for the MVBF, and 90% for the SVBF task. The MPF (F (8,752) =17.119; p=0.000) also decreased by about 30%, following the opposite variation observed in the RMS amplitude (Fig 2D). The mean level of bite force decreased significantly (~20%), from T0 to T6-T8 (F (8,264) =7.42; p<0.05) (Fig 3). No interaction was observed between the recordings performed with or without aligners (T1-T8) in the buccal cavity in the RMS amplitude (F (8,736)   It is well known that the neuromuscular activation pattern depends on the specific task that the muscle undertakes to develop. The masseter muscle function promotes mandibular closure, and provides greater isometric force during maximum clenching. 5 This muscle is minimally recruited during mandibular rest, and is markedly called into play as the mandible closes. 13 The present results showed that the myoelectric activity of the superficial masseter muscle increased relative to the baseline (T0), followed by a decrease in the median frequency content in the sEMG signal. Lou et al 9 also observed an increase in the sEMG activity of the masseter muscle following the use of aligners, and a consequent decrease after four weeks of use.
Our follow-up period was longer; it recorded the highest averages of myoelectric activity after 4 weeks of use, and a decrease in activity after 16 weeks of use. In this case, the present results suggest a hypothetical reorganization of the synergic pattern of masseter muscles in controlling the mandible position.
In turn, the function of the anterior temporal muscle is marked by mandibular balance and posture maintenance, and is more sensitive to changes in dental occlusion. During rest, the anterior temporal muscle also presents minimal recruitment.
Despite the minimum recruitment of both muscles during rest, the anterior temporal muscle contributes more significantly for maintaining postural balance than the masseter muscle. 13 In the present study, the anterior temporal muscle also showed an increase in sEMG signal amplitude, compared to the baseline values, even during rest. It is interesting to highlight that the sEMG signal (RMS) amplitude increased by about 70% from the baseline (T0) to the end of the treatment (T8), in comparison with all muscle recruitment levels studied herein (mandibular rest position, MVBF, and SVBF). We may conjecture that this increase resulted from the lack of occlusal stability, which overloaded the postural balance of the jaw muscle.
Significant changes in the myoelectric activity of orthodontic aligner usage were identified over time, concerning the baseline values of each subject. Non-synergistic masticatory muscle activation has been observed in previous studies. [14][15][16] According to the present results, the use of the orthodontic aligners presumably led to new activation patterns adopted to control the stomatognathic system. The difference in the muscle recruitment pattern suggests that the biomechanics of the stomatognathic system was upset when greater recruitment was required of the temporal muscle in tasks that are usually performed in conjunction with superficial masseter. Based on this assumption, we can presume that the increase in the myoelectric activity of the anterior temporal muscle may be attributed to a reorganization in the protective reflex of the masseter muscle to prevent the teeth from being damaged by excessive bite force; however, the authors recognize that this hypothesis needs to be further examined. The increase in relative RMS amplitude exhibited by both muscles during mandibular rest and contraction suggests that additional recruitment was required to keep the system in balance.
Periodontal pressoreceptors are known to provide feedback to the chewing muscles. 17 It can be expected that periodontal mechanoreceptors will be continuously activated during bite tasks, thus allowing larger individual motor units to be recruited quickly, thereby developing larger closing forces without much effort. 18 We can speculate that the encapsulated design of the aligners and the application of orthodontic force generated cavity. This likely disruption can be observed when unaccustomed individuals start performing a given task using a more extensive and less coordinated muscle chain, and thus trigger a greater firing rate and number of motor units recruited as a compensatory strategy. 22,23 Hence, the reduction in bite force may refer to specific biomechanical effects that elicit different neural drive strategies on muscle recruitment, evoked by the dental position imposed by each new pair of aligners, even though the subjects did not complain of tooth or muscle pain when acquiring the aligners.
Finally, the aligners seem to cause a maladaptive synergism between temporal and masseter muscles when performing biting tasks, thus leading to a reduction in an individual's maximum ability to exert force between the teeth.
In this study, the dental and skeletal variables used to select the inclusion and/or exclusion criteria did not contraindicate treatment with orthodontic aligners. However, they were chosen with the goal to establish cases with slight tooth movement since these characteristics may influence masticatory muscle recruitment patterns and function as a confounding factor. It is noteworthy to mention that the absence of a control group with conventional fixed appliances, and of a follow-up during orthodontic treatment were the main limitations of the present study. Nevertheless, in the event of a control group, the biomechanics and the time interval between the application of orthodontic forces would differ between the groups, since the two treatment modalities do not use similar apparatuses.
A procedure using both groups would necessarily have various effects on the masticatory muscles, thus invalidating the direct comparison results of this study.
For this reason, we believe that the present study results were not affected by the lack of a control group, insofar as we did not aim to compare distinct treatment modalities. An ideal control group would be individuals using passive aligners manufac-