Subjects
Eighteen adolescents with SAOB (4 males, 14 females; age: 12–18 years) and eighteen adolescents with normal occlusion (2 males, 16 females; age: 12–18 years) were selected for comparison of the OMS. Three female adolescents with SAOB were lost to follow-up during OMT, so a total of fifteen adolescents with SAOB (4 males, 11 females; age: 12–18 years) finished the OMT regimen. All of the subjects were diagnosed at the orthodontic department of the Hospital of Stomatology, Sun Yat-sen University. This study was performed in accordance with the Declaration of Helsinki, and the Medical Ethics Committee of the Hospital of Stomatology, Sun Yat-sen University, approved the study (Protocol Title: The Electromyographic Activity Of Oromaxillofacial Muscles In Adolescents With Skeletal Anterior Open Bite, No. KQEC-2020-53-01). The purpose and methods of the study were explained to all subjects, each of whom provided informed consent to participate.
The selection criteria for the experimental group were as follows: (1) age from 12–18 years; (2) full permanent dentition with second molar occlusion; (3) lack of contact between the lower incisors and the upper incisors or palate, and a vertical distance between the lower and upper incisal edge of at least 0.5 mm; (4) hyperdivergent facial type: SN-MP ≥ 40°; and (5) no missing teeth. The selection criteria for the control group were as follows: (1) age from 12 to 18 years; (2) full permanent dentition with second molar occlusion; (3) normal overbite: vertical distance between the lower and upper incisal edge of 1–4 mm; (4) normodivergent facial type: 30° < SN-MP < 40°; (5) no missing teeth; and (6) bilateral neutral molar relationship. The exclusion criteria for both groups were as follows: (1) previous orthodontic treatment or orthognathic surgery; (2) symptoms of temporomandibular joint disorder; (3) severe skeletal facial asymmetry; and (4) unilateral masticatory habit.
EMG examination
EMG recordings of the oromaxillofacial muscles were made using a BioEMG electromyographic amplifier (BioEMG, Bioresearch, Inc., Milwaukee, WI, USA) and BioPAC software (BioPAC Systems, Inc., Santa Barbara, CA, USA) (Fig. 1). The EMG examinations were conducted in the EMG laboratory of the orthodontic department of the Hospital of Stomatology, Sun Yat-sen University. Data were recorded with each subject seated in a dental chair; the head was supported with the Frankfort horizontal plane parallel to the ground. The position of differential active electrodes, determined by palpation and anatomical location, was parallel with the muscle fibres’ main direction, and skin impedance was reduced by wiping the skin with alcohol. In every case, a ground electrode was placed near the 7th vertebra to assist the bipolar electrode configuration. In this way, EMGA associated with mastication, including the activity of anterior temporalis, masseter muscle, and anterior digastric, and EMGA associated with the closed mouth state, including the activity of upper orbicularis and mentalis muscle, were recorded (Fig. 2).
The EMG recordings were carried out under the following clinical conditions: mandibular rest (REST), maximum voluntary contraction in the intercuspal position (ICP), lip sealing (LIP), and swallowing (SWA). The protocols were as follows: REST: Subjects were asked to relax facial muscles for 5 min with the mandible in the rest position and the lips and teeth not in contact. ICP: The teeth were clenched in the ICP as tightly as possible for 2 s. LIP: Before testing, the lips were ensured to be moist and the mandible was in the rest position; then, the lips were slightly closed lips to achieve natural contact for 2 s while receiving instruction. SWA: First, 20 ml of water was held in the mouth and the facial muscles were relaxed for at least 5 s; then, the water was swallowed in one gulp while receiving instruction. EMGA under each condition was obtained three times, resting EMGA was measured for 5 s each time, EMGA under the other three conditions was assessed 2 s each time, with an interval of 1 min in order to prevent muscular fatigue. The mean values of EMGA of those muscles under each condition were calculated and recorded.
Morphological characteristic evaluation
Before orthodontic treatment, the craniofacial morphological characteristics of all 36 subjects were measured using lateral cephalograms. Radiographs were obtained when the upper and lower dentition were in the centric occlusion position and the Frankfort horizontal plane was parallel to the ground. The radiographs were digitally traced using Quick Ceph Studio software (Quick Ceph Systems, San Diego, CA, USA).
The following cephalometric measurements were traced and measured to express different anatomic relationships: anteroposterior jaw relationships: SNA, SNB, and ANB; protrusion of incisors: Mx1-SN, Md1-MP, and angle of the axis of the upper and lower central incisors; mandibular plane inclination: FMA and GoGn-SN; facial height: N-Me, ANS-Me, Go-S, and Go-Ar; positional relations between the upper and lower incisors: overbite and overjet; and alveolar height: U1-PP, L1-MP, U6-PP, and L6-MP.
OMT
According to the myofunctional and morphological characteristics of the SAOB subjects, the perioral muscles, tongue and masticatory muscles were specially trained to achieve a better myofunctional status. The protocol was as follows: The perioral muscles were strengthened by pressing the lips tightly together for nasal breathing for at least 0.5 h a day (Fig. 3a). The masticatory muscles and tongue were trained for 2 h a day by chewing gum vigorously with posterior teeth and shaping the softened chewing gum into a ball on the front of the tongue, lifting the tip of the tongue and sticking the gum to the hard palate, pressing the tongue against the hard palate and flattening the gum, swallowing saliva while simultaneously pressing the tongue against the gum, and repeating the above steps (Fig. 3b).
The OMT regimen lasted for 3 months and subjects received follow-up contact by telephone every 2 weeks and paid a return visit every month. Each subject received an EMGA record before and after the 3-month OMT treatment.
Statistical analysis
Wilcoxon signed rank tests and t-tests were performed to evaluate myofunctional and morphological differences between the normal and SAOB subjects. In order to select representative items, factor analysis was performed for 18 cephalometric measurements and 20 EMG measurements; then, the main factors from each group were extracted. Furthermore, we investigated the correlations between muscle activity and morphological characteristics by performing Pearson or Spearman correlation analysis of the main factors from among the cephalometric and EMG measurements. The OMS of SAOB subjects before and after receiving OMT was compared using a Wilcoxon signed rank test or t-test to investigate the effect of OMT. Wilcoxon signed rank tests were used for data with a skewed distribution, and t-tests were used for data with a normal distribution; α = 0.05. Data are presented as the median (25%, 75%) or mean ± standard deviation. A P value less than 0.05 was considered statistically significant. All tests were conducted using SPSS 19 software (SPSS, Chicago, IL, USA).