The aims of this study were to determine the difference in electromyography (EMG) of the orbicularis oris muscles between subjects with lip incompetence and lip competence and to elucidate the effectiveness of hypoxic lip training with EMG.
Twenty-five subjects (14 men, 11 women; mean age, 24.5 ± 2.6 years) were divided into 2 groups by sealed lip ratio: 13 subjects with lip incompetence (LI) and 12 subjects with lip competence (LC). Integral values of EMG for the orbicularis oris muscle during lip closing were estimated (baseline, T1). Then the subjects in the LI group performed the hypoxic lip training every day for 4 weeks. The EMG recordings were repeated during the training (T2), immediately after the training (T3), 4 weeks after the training (T4), and 8 weeks after the training (T5).
In the LI group, the EMG values during lip closing were significantly higher than those of the LC group, and the values of EMG during lip closing at T3 were significantly lower than those at T1. In the posttraining period, the values of EMG at T4 and T5 were significantly lower than those at T1. The results suggested that subjects with lip incompetence need greater orbicularis oris muscle activity to keep their lips closed.
The suggested standardized lip training could be used as a training method for patients with lip incompetence.
Hypoxic lip training is effective for improving lip incompetence.
Patients with lip incompetence require higher orbicularis oris muscle activity to keep their lips closed.
Hypoxic lip training enables patients with lip incompetence to relax their orbicularis oris muscle as measured by EMG activity.
Some studies have shown that patients with malocclusion have lip incompetence, and lip incompetence has been thought to have a negative influence on the growth and development of the craniofacial complex. Lip competence is the condition in which the lips are in light contact when the mandible is in its clinical rest position. Hillesund et al reported that visual estimation of orofacial muscular activity is variable and not reliable. Wagaiyu and Ashley reported that mouth breathing, increased lip separation, and decreased upper lip coverage at rest were all associated with higher levels of plaque and gingival inflammation. Tomiyama et al reported that subjects with lip incompetence have difficulty chewing while their lips are relaxed and that the inability to seal the lips is a lip dysfunction and can affect masticatory function. Therefore, improving lip incompetence would be effective for overcoming these negative effects on normal growth and the development of the craniofacial complex.
Myofunctional therapy is training with the goal of achieving an equilibrium of the orofacial muscles and improving stomatognathic functions such as swallowing, phonation, chewing, and respiration. The use of myofunctional therapy in growing patients, combined with conventional orthodontic therapy, is an important aid in achieving harmonious orofacial development. In orthodontic treatment, it is generally agreed that assessments of perioral muscle function and lip posture are important for preventing relapse of malocclusion.
Electromyography (EMG) has been used to evaluate lip competence. A standardized method of training has shown good results. However, the effectiveness of hypoxic lip training for lip incompetence was not estimated by using EMG. The aims of this study were to determine the difference in EMG of the orbicularis oris muscle between subjects with lip incompetence and lip competence with normal occlusion and to elucidate the effectiveness of hypoxic lip training with EMG.
Material and methods
All subjects received an explanation of the study and signed the informed consent forms. The research protocol was approved by the ethics review committee for clinical and epidemiological study, Graduate School of Dental Medicine, Hokkaido University in Sapporo, Japan (approval number 9 ).
Twenty-five subjects (14 men, 11 women; 24.5 ± 2.6 years old) participated in this study. They were all generally healthy and were undergraduate or graduate students of Hokkaido University. Exclusion criteria were nasopharyngeal complaints, previous orthodontic treatment, missing anterior teeth, skeletal malocclusion (ANB: < 3.0° or >5.5°), or abnormal overbite or overjet (<2.0 mm or >3.0 mm).
The subjects were divided into 2 groups according to the sealed lip ratio (percentage of lip-sealing time/total recorded time) and lip incompetence less than 30.1% at rest and less than 13.2% during a concentration task. The lip incompetence (LI) group included 13 subjects (5 men, 8 women; mean age, 23.4 ± 2.2 years). The lip competence (LC) group included 12 subjects (9 men; 3 women; mean age, 25.7 ± 2.6 years). Electromyographic (EMG) activities of the orbicularis oris during lip opening and closing were recorded (T1). The subjects in the LI group performed the standardized hypoxic lip training every day for 4 weeks. Their orbicularis oris EMG values were recorded and evaluated before the training (T1), during the training (T2), immediately after the training (T3), 4 weeks after the training (T4), and 8 weeks after the training (T5) ( Fig 1 ).
Each subject was seated on a chair with his or her head and chin stabilized in natural head position. The subject’s skin was cleaned with 80% ethyl alcohol, and surface electrodes (DL-141, S&ME, Tokyo, Japan; Fig 2 , A ) greased with conductor gel were fixed with tape. One electrode was attached on the right side above the vermillion border of the upper lip, and another electrode was attached on the right side below the vermillion border of the lower lip ( Fig 2 , B ). The center electrode was attached to the subject’s right wrist.
Figure 3 shows the measurement schedules. The EMG activities were recorded (1) in the mandibular rest position with the lips opening (interval I), (2) in the mandibular rest position with the lips lightly closing (interval II), and (3) in centric occlusion with the lips closing with effort for strong contact (interval III). The EMG values were recorded (BiologDL 2000; S&ME) at a sampling frequency of 100 Hz. Data were converted to full-rectified waveform, and the integrated waveform was obtained using software (LabChart7.2.4; ADInstruments, Bella Vista, New South Wales, Australia). The integrated waveform of interval II during a period of 5 seconds, recorded from 1 second after starting motion, and the mean of 2 times of interval II as the lip closing EMG value were used for the analysis.
The subjects of the LI group performed the standardized hypoxic lip training. First, the weight for hypoxic lip training was determined by measuring the subject’s orbicularis oris muscle strength. Each subject was seated on a chair with his or her head and chin stabilized in natural head position. A traction plate (30 × 50 × 3 mm) ( Fig 4 , A ) made of a thermoplastic material (Bioplast; Scheu Dental, Iserlohn, Germany) was inserted in the oral vestibule, and the plate was pulled by the testing machine (Digital Force Gauge ZP & Motorized Test Stands MH-1000N; Imada, Toyohashi, Japan) via a string at a constant velocity of 45 mm per minute. The subjects were instructed to bite with their molars and not to suck on the plate. The force required for the plate to come out of the oral vestibule was measured as the tensile strength of the orbicularis oris. Each subject performed the test 3 times with 10-minute rest intervals, and the weight to be used for hypoxic lip training was determined as 80% of the maximum tensile strength value in the 3 measurements.