Effect of orofacial myofunctional therapy on the middle-mixed dentition patients with incompetent lips

Abstract

Background: Various pre-formed early orthodontic appliances for correcting bad oral habits and training orofacial muscles have emerged on the market. However, reports on the orofacial myofunctional training effects of the appliances are few.

Methods: This retrospective study selected children with incompetent lips in the middle-mixed dentition from all the patients treated at the Pediatric Dentistry Department of Shanghai Ninth people’s Hospital from 2016 to 2018. Patients were divided into two groups according to whether they had done the orofacial myofunctional exercises. For each subject of the two groups, initial (pre-treatment, T1) and final (post-treatment, T2) internal and external photos, dental casts, lateral cephalograms, and orthopantograms(OPG) were taken, and lip strength was measured. Dolphin Imaging Cephalometric Analysis Software measured SNA, SNB, ANB, APDI, FMA, U1SN, and IMPA before and after treatment. The hyoid bone position was also recorded. Significant between-group differences were tested with an independent sample t-test (P < 0.05).

Results: A total of 109 children (54 males, 55 females; age range:7-10 years, mean age: 8.2 years) were selected from a parent sample of 870 patients. The first group consisted of 56 subjects (30 females; 26 males) with a mean age of 8.1 years (SD 1.1 years) treated with orofacial myofunctional treatment and preformed appliances. The second group consisted of 53 subjects (25 females; 28 males) with a mean age of 8.2 years (SD 1.0 years) treated with conventional early orthodontic treatment without orofacial myofunctional therapy. In the first group, statistically significant forward movement of the mandible was indicated by an increase of SNB by -1.06 degrees (P < 0.01) and an increase of APDI by -2.23 degrees (P < 0.01). The increase of IMPA (-3.21 degrees, P < 0.01) demonstrated a statistically significant proclination of the lower incisors. The increase in HC3 (-1 mm, P < 0.01) and HFH (-2.95 mm, P < 0.01) implied the forward and downward movement of the hyoid bone. The OMT resulted in a statistically significant increase in lip strength (-2.30, P < 0.05). There was a significant difference in LS, SNB and IMPA between the two groups (P < 0.05).

Conclusions: Orofacial myofunctional therapy was effective for middle-mixed dentition patients with incompetent lips. It can result in significant improvement of lip strength and forward movement of the mandible, which can perfect the jaw relationship.

Introduction

Besides the regular function of eating, the oral cavity also assists nasal breathing and pronunciation, which requires the synergy of many muscles, such as the mouth, face, and neck. Relationships between functional behavior and dentitional development have been discussed since the approval of orthodontics. Previously, Angle had realized what influences such as the digressive functions of cheeks, tongue, and lips have on the occurrence and persistence of malocclusions¹. In addition to genetic factors, environmental factors such as the morphology, position, and abnormal function of the orofacial muscles, lips, cheeks, tongue, and chewing muscles, for instance, are also important causes.

The prevalence of malocclusion in children and adolescents is high. Normal occlusal relationships were found only in 25.3% of children in the primary dentition. The frequency of children with normal dentitions fell significantly in the mixed dentition (7.3%) ². Franka reported that every fourth child in primary dentition and every third in mixed dentition presented three or more orofacial dysfunctions. The frequency rates of orofacial dysfunction in primary and mixed dentition were 61.6% and 80.8%. The most widely-studied functional factors in the literature thought to influence dentitional development are oral habits. 62.0% of children with primary dentition and 63.5% with mixed dentition showed unphysiological swallowing, which is the most frequently diagnosed orofacial dysfunction. While habitual open mouth posture was founded in 37.3% of the primary dentition children and 42.0% of those in mixed dentition.

Rosemarie reported that compared with children with normal dentition, children with frontal open bite, lateral crossbite, and increased overjet presented significantly more static functional disturbances, such as the open mouth and non-physiological tongue postures. Postural anomalies or faulty postural habits are most likely to influence the development of dental occlusion significantly in all three dimensions⁴.

Theories regarding the growth of the facial skull emphasize the influence of function. The stability of the final position of the teeth depends on the harmony of the oral and maxillofacial system and respiratory system. Imbalance in neuromuscular forces resulted in malocclusion⁵. The results of orthodontic treatment based on abnormal muscle function are unstable. Therefore, all clinicians and scientists need to correct the orofacial dysfunctions as soon as possible.

Orofacial myofunctional therapy (OMT) aims to establish a new neuromuscular pattern and correct abnormal functional and resting postures. The objectives of OMT are strengthening the orofacial muscles to pave the way for mouth closure, establishing nasal breathing, and learning a physiological swallowing pattern⁶.

Nowadays, there are a variety of pre-formed early orthodontic appliances for correcting bad oral habits and orofacial muscle training on the market. However, few reports have been made on the orofacial myofunctional training effects of the appliances. Lip strength, hyoid position, and early orthodontic effect are quantitatively measured to evaluate the result of orofacial myofunctional training on correcting middle-mixed dentition malocclusions to guide the clinical early orthodontic work better provide the theoretical basis for large-scale clinical randomized controlled trials in the future.

Materials And Methods

Ethical Consideration 

This project was approved by the Ethics Committee in Shanghai Ninth People’s Hospital, Shanghai Jiao Tong University School of Medicine (No. SH9H-2021-T332-1 ).

Subjects grouping and treatment modalities

Patients with incompetent lips in the middle-mixed dentition were collected from all the patients treated at the Pediatric Dentistry Department of Shanghai Ninth people’s Hospital from 2016 to 2018. All patients in the final sample were included according to the following criteria (Table 1).

All the children were instructed to do active lips and tongue training at home every day at the first visit, which had to be supervised and recorded by their parents. (First, the lip muscle training: 1. Lip closure and competency exercise: tightly closing the lips together until the upper and lower lip redness is not visible with a plastic sheet for 1 hour a day. 2. "Lip kisses" 100 times a day. Second, tongue muscle training: first, place chewing gum on the tip of the tongue. Then lift the tongue against the roof behind the upper front teeth and squeeze the gum to its extent by the tongue for 30 minutes a day. Patients were divided into two groups according to whether they had done the orofacial myofunctional exercises. Patients in the first group had done the exercises persistently using pre-formed appliances (MRC Myofunctional Research Co. Queensland, Australia) for correction. They were worn at night (≥8h) during sleep and continuously worn for 2 hours during the day. Patients visited dentists monthly, and next stage appliances were replaced in time. Patients were asked to visit once a month and replace the next stage appliance in time until the alignment of the anterior teeth.

Patients in the second group received conventional early orthodontic treatment without persistently doing the orofacial myofunctional exercises. Different appliances were used according to malocclusions, such as arch expansion devices, "2*4" local fixed appliances etc. The treatment ended until the alignment of the anterior teeth.

Evaluation of treatment outcomes

For each subject of the two groups, initial (pre-treatment, T1) and final (post-treatment, T2) internal and external photos, dental casts, lateral cephalograms, and orthopantograms (OPG) were taken, and initial and final lip strength were collected.

For each patient, lateral cephalograms were taken before (T1) and after (T2) treatment in the Department of Oral Radiology, Shanghai Ninth Hospital. Dolphin Imaging Cephalometric Analysis Software (Dolphin, America) measured SNA, SNB, ANB, APDI, FMA, U1SN, and IMPA before and after treatment. The position of the hyoid bone was described and recorded as follows (Table 2, Figure 1).

Data collection and Statistical analysis

To determine the method error, tracing and measurements of the cephalometric radiographs were performed by one trained examiner and repeated after an interval of approximately one week. Cohen’s Kappa was used to compare the two measurements (systematic error).

Descriptive statistics were calculated for all the measurements in each group. A paired t-test was performed on the measurement items of each group. Significant between-group differences were tested with independent sample t-tests. P<0.05 indicated that the difference was statistically significant. The means and standard deviations of the measurements were calculated, and a paired t-test and independent sample t-tests were performed using the Statistical Package for Social Sciences version 25.0 (SPSS Inc., Chicago, Illinois, USA).

Results

No systematic error was found between the repeated measurements.

A total of 109 children (54 males,55 females, age range:7-10 years, mean age:8.2 years) were selected from a parent sample of 870 patients treated at the Pediatric Dentistry Department of Shanghai Ninth people’s Hospital from 2016 to 2018.

The first group comprised 56 subjects (30 females, 26 males) with a mean age of 8.1 years (SD 1.1 years) treated with orofacial myofunctional treatment and preformed appliances. The second group comprised 53 subjects (25 females, 28 males) with a mean age of 8.2 years (SD 1.0 years) treated with conventional early orthodontic treatment without orofacial myofunctional training. There was no statistically significant difference in gender, an average age, and primary lip strength between the two groups. 

Statistical comparisons between the boys and girls after the treatment in each group were shown in Tables 3-4.

The cephalometric variables and lip strength data at T1 and T2 and the differences between the two groups were shown in Table 5.

No statistically significant difference in the measurement after the treatment had been found between the boys and girls in each group(Tables 3,4).

The cephalometric measurements in the first group revealed the statistically significant forward movement of the mandible, indicated by an increase of SNB by -1.06 degrees (P < 0.01) and an increase of APDI by -2.23 degrees (P < 0.01). The increase of IMPA (-3.21 degrees, P < 0.01) indicated a statistically significant proclination of the lower incisors. Forward and downward movement of the hyoid bone was evidenced by an increase in HC3 (-1 mm, P < 0.01) and HFH (-2.95 mm, P < 0.01). U1SN slightly decreased, and FMA slightly increased, but no statistically significant changes were found (P > 0.05). 

A group t-test was performed on the data of the two groups before and after the treatment. Compared with the second group, the orofacial myofunctional exercises showed a statistically significant increase in lip strength (-2.30, P < 0.05). The cephalometric measurements in the first group revealed the statistically significant forward movement of the mandible, indicated by an increase of SNB by -2.1 degrees (P < 0.05). However, there was no significant difference in the position of the hyoid bone between the two groups after treatment(P > 0.05) (Table 5).

Discussions

The relationship between the lip strength and the position of the anterior teeth and malocclusion

The abnormal shape, position, and function of the lip and tongue and other perioral muscles were one of the critical causes of malocclusion. Cattoni⁷ showed that the lips were naturally closed during normal nasal breathing, the tongue was lifted, and the maxillofacial region was well developed. At the same time, abnormal airflow enters through the oral cavity in patients who breathe for a long time with their mouth. The upper lip was short and upturned. The upper anterior teeth were proclined, the tongue was lowered, and the upper dental arch was narrow, which further aggravated the protrusion of the upper anterior teeth. Insufficient upper lip muscle strength can easily lead to incompetent lips. Hassan⁸ showed that the strength of the labial muscles is closely related to the position of the anterior teeth. Burstone⁹ found that the muscle strength of the lower lip played an essential role in maintaining the normal position of the upper and lower anterior teeth at rest. When the pressure of the lower lip muscle was too high, retroclination of lower incisors, the mandible retraction, and Angle Class Ⅱ division 2 malocclusion would appear. When the lower lip muscle strength was too weak, proclination of upper incisors would appear.

Rogers¹⁰ once reported that the perioral muscle functional training method could correct malformations and deformities. He believed that the perioral muscles could be changed after training, which could affect the position of the teeth and change the shape of the dental arch.

In this study, the lip strength of the 56 children in the treatment group who received active orofacial myofunctional therapy significantly increased after the treatment. The difference between the two groups was significant (P < 0.05). Compared with the children in the second group, patients in the first group who had received early orthodontic treatment with active orofacial myofunctional therapy showed more mandibular advancement, which was more beneficial to maxillofacial development and profile improvement. This result was consistent with the study by Usumez¹¹. The mandible of the second group also moved forward after the treatment, which may be related to the expansion of the upper dental arch and the alignment of the anterior teeth after the treatment. 

Relationship between abnormal tongue position, hyoid bone position and Malocclusion

Urzal¹² and others believed that the tongue could exert a force up to 500g, while less than 2g was enough to move the incisors. At rest, the tip of the tongue should be located at the incisor papilla 5mm behind the upper incisor, and the back of the tongue should be held against the palate. The pressure exerted by the tongue was one of the main factors in maintaining the position of teeth. The lateral force exerted by the tongue promoted the width development of the maxillary dental arch. The balance of labiolingual muscle strength determined the position of anterior teeth, while the balance of buccal-lingual muscle strength determined the position of posterior teeth. Mason¹³ pointed out that the posture at rest was more important than the functional position of the tongue. Abnormal tongue position at rest could easily lead to malocclusion, such as upper dental arch stenosis. Establishing the correct resting tongue position was beneficial for stabilising the outcome after correction of the malocclusion and preventing relapse.

Abnormal dynamic tongue position mainly refers to abnormal swallowing, which was the tongue extended forward to form a seal with the lip or extended out of the mouth for swallowing. Abnormal swallowing could be simple tongue swallowing or tongue swallowing combined with complex maxillofacial muscle movement. For simple swallowing, the tongue extended between the upper and lower teeth, with force continuously acting on the teeth, affecting the normal eruption of the teeth, which could cause open bite deformity. Rogers¹⁰ proposed that tongue swallowing, combined with excessive contraction of labial muscle, buccal muscle and mental muscle, easily led to narrow upper arch, upper incisors protrusion, a lingual inclination of lower incisors, mandibular retraction and deep overjet of anterior teeth, etc.

The hyoid bone is a vital structure around the tongue body. The tongue body was attached to the attachment point on the hyoid bone through muscles such as hyoid muscle, which were independent and connected with each other¹⁴. The hyoid bone position affected the tongue body's position in the oral cavity, and the abnormal position of the hyoid bone also accompanied the abnormal position of the tongue body. Haralabakis¹⁵ found that the position of the hyoid bone was closely related to the tongue body position, and the position of the hyoid bone in open bite patients was relatively higher. Gokce and other scholars¹⁶ have found that in patients with skeletal Class III malocclusion, there was a specific abnormal position of the hyoid bone, closely related to abnormal swallowing activities. There was a relationship between mandibular positional changes and the position of the hyoid bone¹⁷. The position of the hyoid bone in Class III malocclusion patients was lower than that of the Class I patients¹⁸.

In this study, in patients in the first group after active orofacial myofunctional therapy, the position of the mandible moved forward, the jaw relationship improved, and the hyoid bone moved forward and downward, and the difference was statistically significant. It was consistent with the study by Zhou Li and Yassaei S^19,20. They found that the position of the hyoid bone moved forward and downward after the treatment of Class II division 1 patients with functional appliances. The mandible of the second group also moved forward after the treatment, which may be related to the expansion of the upper dental arch and the alignment of the anterior teeth after the treatment. 

The collection of lip strength

In the 1970s, Posen had described a method of measuring the strength of the lips for clinical use²¹. Over the past decades, many different measurement systems for lip strength have been developed. These had been roughly classified into three main types: 1) tension gauge type, 2) balloon type and 3) strain gauge type²². These systems, however, were difficult to operate and use clinically, especially for children. Saitoh had reported that lip strength in children had two different stages, one was a period of development (3-6 years old), and the other was a stable period (7-12 years old)²². In our study, the lip strength of the patients before and after treatment was collected by a digital medical strain gauge (Lipplekun, Shofu, Kyoto, Japan), which had been reported to be a reliable measuring device²³. In our study, the patients’ age range was 7 to 10 years old, which was in a stable period. Significant improvement in the lip strength in the first group of our study was mainly attributed to muscle function training. 

Orofacial myofunctional therapy

Orofacial myofunctional therapy is a method of assessment, diagnosis, and treatment for patients with abnormal orofacial muscle function²⁴. Through re-education of the nerves and muscles of the oral and maxillofacial region, the patients' bad oral habits can be broken, and the normal development of the craniofacial structure and the coordination and stability of the function of the oral and maxillofacial system can be promoted. Good patient and parental compliance are the keys to the treatment of orofacial muscle function, and it takes time and the accumulation of treatment to be effective. In addition, it is crucial to control the indications and contraindications of the orofacial myofunctional therapy. It has limited effects in treating severe skeletal malocclusions, open bite, and severe tooth and bone volume irregularity. Conventional orthodontics and even the combination of orthognathic surgery with orthodontics are still required for the following treatment of permanent teeth. Moreover, this study only investigated the hyoid bone position. The comparison of tongue position can be added in the later study to evaluate the comprehensive effect of orofacial myofunctional therapy more accurately.

Conclusions

Early orthodontic treatment with orofacial myofunctional therapy was effective for middlemixed dentition patients with incompetent lips. It can result in significant improvement of lip strength and forward movement of mandible, which can perfect the jaw relationship. But the lower incisors protrusion was obvious after the treatment, suggesting that more attention should be paid to the labial gingiva of the lower anterior teeth during the treatment.

Declarations

Funding

This work was supported by Shanghai Municipal Health Commission under Grant 20164Y0054. 

Author contributions

Xue Yang designed the study, performed the clinical operation, analyzed and evaluated the treatment outcomes, collected the data, and wrote the article. Guangyun Lai helped modify and write the article. Jun Wang helped guide in the process of study and reviewed the manuscript. 

Acknowledgements

We thank Dr. Yan Cai for statistical analysis of the data. 

Ethics approval and consent to participate

All experimental protocols were carried out according to relevant guidelines and regulations or declaration of helsinki and were approved by the Ethics Committee in Shanghai Ninth People’s Hospital, Shanghai Jiao Tong University School of Medicine (No. SH9H-2021-T332-1). The written informed consent was obtained from legal guardian of children. 

Availability of data and materials

The data set underlying this article will be available from the corresponding author on reasonable request. 

Consent for publication

Not applicable. 

Competing interests

The authors declare no competing interests. 

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Tables

Table 1. Inclusion and exclusion criteria

Table 2. The position of the hyoid bone

Table 3. Statistical comparison between the boys and girls in the first group 

Abbreviation: SD, standard deviation.

Gender:1 boys 2 girls 

Table 4. Statistical comparison between the boys and girls in the second group 

Abbreviation: SD, standard deviation.

Gender:1 boys 2 girls

Table 5. Variables before(T1) and after(T2) the treatment between the two groups

Abbreviation: SD, standard deviation.

* P<0.05，** P<0.01