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 S19,20. 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.