Immediate effects of temporary bite-raising with light-cured orthodontic band cement on the electromyographic response of masticatory muscles

Abstract Objective: To assess the immediate effects of temporary bite-raising using light-cured orthodontic band cement on the superficial masseter and anterior temporalis electromyography (EMG) activity in healthy adults. Materials and Methods: Surface EMG signals were recorded bilaterally from the superficial masseter and anterior temporalis muscles of 30 volunteers with a normal occlusion, before and after having temporary bite-raising. The bite-raising was done by adding light-cured orthodontic band cement (3x5x2 mm WxLxH) on the lingual cusps of both upper first molars. The measurements were recorded (i) at rest, (ii) while clenching in centric occluding position and (iii) while chewing on an artificial test food. The EMG activity at rest and during clenching, the maximum voltage, and the duration of the identified EMG signal burst while chewing the artificial test food before and after temporary bite-raising were statistically compared using the paired t-test or the Wilcoxon signed-rank test based on the normality of the variables. The significance level was set at 5%. Results: After temporary bite-raising, we found no significant change in integral EMG activity at rest position for the superficial masseter (mean difference (MD)=7.5 μVs) and for the anterior temporalis muscle (MD=36.8 μVs); however, the integral EMG activity during clenching was significantly reduced for the superficial masseter (MD=201.2 μVs) and for the anterior temporalis muscle (MD=151.8 μVs). During mastication, the maximum voltage of the identified burst was significantly reduced on the preferred chewing side of the superficial masseter and anterior temporalis muscles (MD=127.9 and 47.7 μV, respectively), while no significant change was found for the duration of the identified burst (MD=-34.1 and 3.4 ms, respectively) after temporary bite-raising. Conclusion: The results point to an altered neuromuscular behavior during clenching and chewing immediately after temporary bite-raising with light-cured orthodontic band cement. This information is relevant for orthodontists to inform their patients what will happen to their masticatory muscle activity when this bite-raising method is used.

are shown in Figure 1. The mean overbite after temporary bite-raising was -0.57±0.70 mm (mean± S.D.). Consequently, the mean interincisal distance change was 2.90±0.20 mm (mean ± S.D.). After the EMG recordings, the bite-raising material was removed using adhesive removing pliers.

Surface EMG recordings
Surface EMGs were simultaneously recorded from the left and right superficial masseter and anterior temporalis muscles by a researcher using the ML866 For the anterior temporalis muscles, the electrodes were placed 1.5-2 cm posterior to the lateral canthus of the eye, slightly above the zygomatic arch. For the superficial masseter muscles, the electrodes were placed on the midpoint of the bisection of the anteroposterior and inferosuperior dimensions of the muscle belly, which was determined by asking the subject to clench the teeth according to the method described by Teenier, Throckmorton and Ellis 21 (1991).
The inter-electrode distances were recorded for each subject for the left and right sides and used to place the electrode in the latter phase. A ground electrode was placed on the sternal end of the clavicle. External noise was controlled to avoid artifacts caused by smiling or other facial expressions.
After placing the electrodes and practicing for the measurement, the subjects were given a five minutes rest period to relax and for the skin to absorb the conductive gel. The EMG recordings were then made as follows: Test A -physiological rest position The purpose of this test was to monitor and quantify the amount of electrical activity generated by the superficial masseter and anterior temporalis muscles when at rest.

Test B -maximum voluntary clench (MVC)
The subjects were instructed to clench their teeth in centric occluding position using as much force as possible without causing pain and to hold this force intensity until instructed to relax after 2 seconds.

Test C -mastication
The subjects were instructed to chew an artificial test food for 20 strokes on their preferred chewing side. When the subjects were not sure of their preferred chewing side, they were instructed to use the right side. The artificial test food was made with a dental impression material, OptoSil (Heraeus Kulzer GmbH, Hanau, Germany), using the standardized production of an artificial test food protocol 2 and cut into four quarters.
We performed two experimental sessions as

Intra-examiner reliability
The baseline recordings for both muscles at rest were used to evaluate intra-examiner reliability, they were conducted in two visits, with a week interval between them. Both recordings were performed by the same examiner and in the same period of the day, according to the procedure mentioned above.

Statistical analysis
Statistical analysis was performed using the software SPSS version 17.00 (SPSS Inc., Chicago, IL, USA). The normality of the variables was verified by the Kolmogorov-Smirnov test. For comparisons between before and after bite-raising, the paired t-test or the Wilcoxon signed-rank test was used based on the normality of the data. We tested 16 pairs of beforeafter data. The paired t-test was performed for the integral superficial masseter and anterior temporalis muscles EMG activity during MVC, the normalized superficial masseter muscle EMG activity during MVC, the maximum voltage of the identified burst on the superficial masseter preferred chewing side, the duration of the identified burst on the superficial masseter and anterior temporalis preferred chewing side, and the duration of the identified burst on the anterior temporalis non-chewing side. The Wilcoxon signed-rank test was performed on the integral superficial masseter and anterior temporalis muscles EMG activity during rest, the normalized superficial masseter and anterior temporalis muscles EMG activity during rest, the normalized anterior temporalis muscle

Results
The intraclass correlation coefficient for the superficial masseter and anterior temporalis muscles measurements were 0.93 and 0.91, respectively, indicating an excellent reproducibility of the EMG measurements. No significant differences were found between the superficial masseter integral EMG activity or normalized EMG activity before and after temporary bite-raising at rest. Similar results were J Appl Oral Sci. 2018;26:e20170214 5/8 also found for the anterior temporalis muscle at rest. However, integral EMG activity and normalized EMG activity for both muscles was significantly reduced during clenching after temporary bite-raising (Table 1 and 2). The effect size of the integral and normalized EMG activity for the superficial masseter muscle was 114.0 μVs and 16.7%, respectively. The effect size of the integral and normalized EMG activity for the anterior temporalis muscle was 120.8 μVs and 21.4%,

respectively.
Representative EMG recordings for the superficial masseter and anterior temporalis muscles during mastication are shown in Figure 2. The maximum voltage in both muscles was significantly reduced on both the preferred chewing side and non-chewing side after temporary bite-raising ( Table 3). The effect size of the maximum voltage for the superficial masseter muscle on the preferred chewing side and the non-   respectively. However, no significant difference was found for its duration (Table 4).

Discussion
This study investigated the immediate effect of temporary bite-raising using light-cured orthodontic band cement on the occlusal surface of the upper first molars on the EMG activity of the superficial masseter and anterior temporalis muscles. We found that there was no change in EMG activity at rest, however, EMG activity was reduced during MCV and chewing. Based on these results, the null hypothesis was not rejected at rest, but it was rejected during MVC and chewing.
Except for sex, the factors affecting masticatory muscle EMG activity, such as age, facial type, malocclusion, and the EMG recording period, were controlled in this study 5,25 . Men and women were not on even numbers in our study. However, the paired t-test or Wilcoxon Signed-Ranks Test was used for within-subject and within-muscle comparisons, thus, the variable EMG responses between participants were eliminated.
The difference was not significant on EMG activity for both muscles at rest, despite being reduced immediately after temporary bite-raising. The cause for this could be the opening distance chosen for this study, set at 2.5-3 mm, which is close to the physiological rest position and is usually adequate for treating orthodontic patients. However, several studies found that increasing the occlusal vertical dimension by ≥3 mm by other means affects the EMG activity of the superficial masseter and anterior temporalis muscles 4,11 . Therefore, the EMG response to biteraising by this method at greater opening distances should be further investigated.
The EMG activity of both muscles during MVC decreased significantly after temporary bite-raising.
As reported by Jimenez 10 (1987), if the occlusion does not result in mandible stability, the jaw-closing muscles will contribute to the stabilization by reducing its EMG activity to avoid damage to other structures. There were only two occlusal contact areas after temporary   Since bite-raising by this method caused a reduced occlusal contact area, our results showed that the superficial masseter and anterior temporalis muscles maximum voltage on the preferred chewing side was significantly reduced after temporary bite-raising. This is consistent with the study of Tomonari, et al. 22 (2014) who found lower EMG activity for both muscles during chewing on the preferred chewing side of subjects with reduced occlusal surface contacts. The maximum voltage for both muscles was significantly reduced on the non-chewing side after temporary bite-raising. This could be the result from a protective mechanism to control jaw balance, since chewing on one side could cause the mandible to deform and/or tilt around the sagittal axis 13 . The increase on interocclusal distance would cause similar muscle activity on both sides, which might lift the mandible on the non-chewing side, causing excessive temporomandibular joint loading.
The short period of investigation is a limitation of this study. Habituation should be considered when evaluating the effect of bite-raising, since muscle physiology and function may adapt if longer observation periods are allowed. In animal experiments, bite-raising for 2 weeks significantly reduced masseter muscle spindle sensitivity; however, no significant differences were found between control and after more than 6 weeks of bite-raising 26 . However, a long-term investigation with healthy subjects was not possible in our study model due to ethical standards. Therefore, to investigate the precise effect of biteraising by this method on masticatory muscle activity, a future clinical study should evaluate subjects over an extended period.

Conclusion
Our results revealed that temporary bite-raising by placing orthodontic band cement on the occlusal surface of the upper first molars had no immediate effect on EMG activity at rest, however, superficial masseter and anterior temporalis muscles EMG activity reduced during MVC and mastication. This information is useful for orthodontists to inform their patients about what will happen to their masticatory muscle activity when this bite-raising method is used.
Furthermore, the effects of this type of bite-raising on more clinical-related aspects, such as masticatory performance and masticatory ability should be investigated in future studies. 2018;26:e20170214 8/8