Evaluation of Masticatory Muscle Activity in Pediatric Patients Undergoing Rapid Maxillary Expansion

Transverse maxillary constriction subsists when the distance between upper first molars palatal cuspids is lower than the distance between lower first molars vestibular central fossae. It can be clinically expressed with a narrow and high palatal vault, a unilateral or bilateral crossbite, dental crowding and/or reduced volume of the nasal cavities. It is one of the most common malocclusions in children, with a prevalence of 8 to 22% among orthodontic patients in primary and mixed dentition and 5 to 15% among general population. The hypothesis of this study is that transverse maxillary constriction correction by Rapid Maxillary Expander (RME) affects masticatory muscles activation potential and improves activation symmetry in patient with unilateral crossbite. The aim of this study is to evaluate electromyographic activity of masseters, anterior temporalis and suprahyoid muscles in clenching, chewing and swallowing in patients undergoing maxillary expansion with RME. The electromyographic evaluation is carried out before treatment (T0), at the end of the expansion (T1) and 6 months later (T2). Detailed Description: Transverse maxillary constriction subsists when the distance between upper first molars palatal cuspids is lower than the distance between lower first molars vestibular central fossae. It can be clinically expressed with a narrow and high palatal vault, a unilateral or bilateral crossbite, dental crowding and/or reduced volume of the nasal cavities. It is one of the most common malocclusions in children, with a prevalence of 8 to 22% among orthodontic patients in primary and mixed dentition and 5 to 15% among general population. One of the most frequent expression of transverse maxillary constriction is unilateral crossbite, that consists in an inverted bucco-lingual relationship between one or more posterior teeth (canine to molars) with their antagonists; it can be present both in deciduous and permanent dentition. There is evidence that an altered relationship between upper and lower teeth is associated with asymmetric masticatory function that has been related to asymmetric contraction of masticatory muscles, decreased thickness of the cross-sided masseter muscle and altered masticatory pattern. This condition could lead to an asymmetric development of the mandibular bone during growth. Many authors conclude that early treatment of transverse maxillary constriction with RME would be recommended to reduce the risk of development of skeletal asymmetries and temporomandibular disorders (TMD). Muscle activity is commonly recorded by means of surface electromyography (sEMG). However sEMG data can be affected by various artifacts, resulting in questionable interpretation of the results. A standardisation procedure allows to reduce variability of the assessment of masticatory muscle activity during static and dynamic tasks.

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The hypotheses of this study are:

Patients with transverse maxillary constriction improve muscles activation potential after rapid maxillary expansion; correction of unilateral posterior crossbite by rapid maxillary expansion improves muscles activation potential symmetry.

The main objectives of this study are:

• to evaluate the activity of superficial masseters, anterior temporalis and suprahyoid muscles in patients with traverse maxillary constriction, using surface electromyography with a standardized protocol. The test consists of detecting muscles activity in maximum clenching (on cottons and on teeth), during swallowing and chewing;
• to evaluate whether the presence of unilateral posterior crossbite is associated with asymmetrical activation of superficial masseters, anterior temporalis and suprahyoid muscles;
• to compare intra-patient results before and after traverse maxillary constriction correction. The electromyograph that will be used is Easy-MYo EMG Tracer of 3 Technology Srl; Udine, Italy. It records muscular activity of superficial masseters, anterior temporalis and suprahyoid muscles. Disposable bipolar surface electrodes (21 × 41 mm, 20 mm inter-electrode distance; F3010; Fiab) will be used. The patient's skin will be clean with cotton gauze soaked in alcohol before electrodes placement to reduce skin impedance. The operator will palpate the muscle belly while the patient clenches his teeth and will position surface electrodes in parallel to muscular fibres.

Doing so the position of the electrodes results as follows:

Masseters electrodes will be fixed parallel to the exocanthion-gonion line and with the upper pole of the electrode under the tragus-labial commissural line. Temporalis electrodes will be positioned along the anterior margin of the muscle (corresponding to the frontoparietal suture). Suprahyoid muscles electrodes will be placed in the submental area nearly 1 cm posterior to the mental symphysis, paramedian to the midline and lightly diverging. A reference electrode will be applied on the forehead of the patient.

The sEMG analysis will be composed of four parts:

Masticatory muscle standardisation procedures (repeated thrice):

two 10mm thick cotton rolls will be positioned on the mandibular posterior teeth of each participant, and a 5 second maximum voluntary contraction will be recorded to standardize anterior temporalis and superficial masseters sEMG signals. The mean sEMG potential obtained in the first acquisition was set at 100%, and all further ssEMG potentials will be expressed as a percentage of this value (μV/μV × 100); Maximum voluntary teeth clenching: patients will be asked to clench their teeth in maximum intercuspation as hard as possible for 5 seconds.

Submental muscle standardisation procedures: participants will be asked to push their tongue against the palate (without teeth clenching), and a 5 seconds sEMG suprahyoid muscles activity will be recorded. All further sEMG potentials will be expressed as a percentage of this value (μV/μV × 100).

Saliva swallowing: participants will be asked to keep their mouth open to accumulate saliva and, when needed, to swallow "freely" (as usual) and a 5 seconds sEMG activity will be recorded.

Three EMG sessions will be recorded:

1. before starting treatment with Rapid Maxillary Expander (RME) (T0);
2. at the end of RME activation (T1), established by the clinician, generally looking for hypercorrection, that is contact between upper molar palatal cuspid and lower molar buccal cuspid.
3. after 6 months from T1 (T2).