Abnormal oral functions and their impact on craniofacial growth and morphology are widely studied. However, their effects on the growth of the mandible are still a subject of interest and controversy.^[1]

Myofunctional appliances have been introduced into the dental practice to treat functional disorders and to reestablish muscle balance. One of the main goals of the interceptive treatment is correction of the abnormal activity of the lips, the mentalis muscle and the tongue. The main reason for using myofunctional appliances is to achieve muscular balance, eliminate oral dysfunction, and to reduce or correct maxillary muscle hyper- and hypoactivity.^[2] These appliances have been successfully used for many years due to their simplicity and ease of use, for carefully selected cases and by experienced operators.^[3]

One of the widely used myofunctional appliances is T4K (Pre-Orthodontic Trainer, Myofunctional Research Co., Australia). It is a useful appliance for treating class II/1 malocclusions, which according to Class II/1 malocclusion is one of the most common malocclusions in clinical practice^[4] and, according to the Index of Malocclusion Severity Assessment, is rated as severe and very severe malocclusion depending on the severity of the distal bite.^[5]

The aim of this study was to examine electromyographically the response of the masseter, anterior temporal, orbicularis oris, and mentalis muscles to the treatment with the T4K Pre-Orthodontic Trainer.

Sixteen patients (mean age, 10±1.4 years) were selected for treatment with the T4K Pre-Orthodontic Trainer First Phase (Myofunctional Research Co., Australia). The inclusion criterion was that the children should have class II/1 malocclusion. The main exclusion criteria were that the children had not previously received orthodontic treatment, did not have any temporomandibular disorders (TMD), and did not experience orofacial pain while clenching.

The children were instructed to wear the Т4К Pre-Orthodontic Trainer First Phase, for one hour during the daytime and at night while sleeping (Fig. 1). The follow-up period lasted 9 months.

Surface EMG recordings were performed during three activities: maximal clenching, swallowing water taken in the mouth in advance, and sucking through a straw while the other end was closed with a finger. Each EMG recording lasted 3 seconds for every contraction except swallowing.

The surface EMG activity of the right and the left masseter and the anterior temporal muscle were recorded during maximal clenching for 3 seconds. The EMG recordings during swallowing and sucking through a straw were performed using bipolar electrodes positioned over the musculus orbicularis oris and musculus mentalis. The intervals between all muscle contractions were one minute. All activities were repeated three times. As these contractions are voluntary, we used the contraction with maximal value for the statistical analysis.

The patient sits in a dental chair with head unsupported and upright back. A two-channel electromyograph (Neuro-EMG-Micro-2) with Neuro-MEP-Ω software was used to perform the recordings. For each contraction, the appliance gives two values: maximal and mean value of the whole separate contraction. Before the recording, the skin above the examined muscles was cleaned with 70% alcohol and after drying up, silver/silver chloride bipolar electrodes were positioned. EMG recordings were performed at the beginning and at the end of the treatment (mean treatment period of 9.5±1.4 months).

The EMG recordings were made during maximal clench in centric occlusion, during swallowing water and during sucking through a straw. The patients made three attempts of every task. The activity of masseter and anterior temporalis muscles bilaterally were detected during maximal clench while the muscular activity of m. orbicularis oris and m. mentalis were tested during swallowing and sucking.

The EMG values of the pre- and post-treatment muscle activity are shown in Tables 1, 2.

Both the maximal and mean values of the muscle activity of m. temporalis and m. masseter showed statistically significant increase during clenching at the end of the observation period compared with the pretreatment values (p<0.05, p<0.02, respectively).

Table 2 presents the EMG activity of orbicularis oris and mentalis muscles during swallowing and sucking.

The results showed that statistically significant differences during swallowing and sucking at the end of the treatment period were observed only in the orbicularis oris muscle. Both maximal and mean EMG values were increased (p<0.05). The EMG activity of the mental muscle did not change after Trainer application and the difference between the pre- and post-treatment values was insignificant.

The aim of the study was to investigate the activity of the masticatory and perioral muscles as a result of a Trainer treatment.

The soft tissues, the masticatory and other facial muscles influence not only the bone growth, but also have an impact on the treatment results and stability after orthodontic treatment. The basic method to evaluate the changes of the muscle activity/function is electromyography^[6], hence appropriate application of non-invasive EMG provides objective information about the results and usefulness of the orthodontic appliances. The reliability of EMG studies on the activity of elevator and facial muscles has been evaluated in previous studies.^{[2, 7]} In this study, we examined the effects of myofunctional treatment on orofacial muscle activity registered using surface EMG during the oral functions of clenching, sucking, and swallowing.

In our study, we found that the activity of the anterior temporal and masseter muscles during voluntary maximal clenching significantly increased at the end of the observation period compared with the pre-treatment values. This is valid both for the maximal and mean values of the EMG activity. Early studies by Pancherz et al. reported that in patients with distal occlusion, the reduced activity of the masseter and temporal muscles after treatment with the activator and Herbst appliance gradually increased, approaching the values obtained with normal occlusion, which is compatible with the results of our study.^[8] Our data also correlate with the results of a study by Satygo et al.^[9] who reported an identical increase in the activity of these muscles at clench. The results obtained by Petrovic et al.^[10] are similar: they found that the electrical activity of the masticatory muscles in Angle Class II/2 malocclusion significantly increased during the first year of treatment with the activator compared to the start of the therapy. Increasing potential action is approximately equal for both observed muscles. On the contrary, Uysal et al.^[11] found that after an 8-month Pre-Orthodontic Trainer treatment, the activity of the anterior temporal, mental, and masseter muscles was decreased. Tallgren et al.^[2] reported similar changes in the EMG activity of anterior temporal muscle at maximal clenching during myofunctional appliance treatment. Uner et al.^[12] evaluated masticatory muscles in distal occlusion and found an increased activity in the masseter muscle in the physiologic rest position of the mandible at the beginning of treatment, and the reduction of the activity at the end of treatment with an activator.

During sucking through a straw with a closed opposite end, the results of the EMG recording showed a statistically significant increase in the muscular activity of the orbicularis oris muscle. During sucking, Uysal et al.^[11] found increase in the activity of orbicularis oris muscle, while Yagci et al.^[13] reported a statistically significant decrease. Our results support the findings of Uysal et al.^[11] In our study, when the patients were instructed to swallow, the EMG activity of the orbicularis oris muscle significantly increased, while other authors reported no changes before and after treatment.^[11]

We did not observe significant changes during sucking and swallowing in pre-and post-treatment values of the EMG activity of the mentalis muscle. Similar results reported Uysal et al.^[11]

The results of the prospective study of T4K-treated patients showed significant changes of the muscle activity as detected via surface electromyography. No changes after treatment were observed in the mentalis muscle. The temporalis and masseter muscles, which keep the appliance in the mouth, and the orbicularis oris muscle, which is responsible for touching the lips together with the Trainer, increased their activity.