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INTRODUCTION
Skeletal class III malocclusion is the incidence of oral disease in the high incidence of malformations, the Asian population, especially children have a higher incidence of Chinese morbidity in the 5-12 % (Sun et al., 2015; Wu et al., 2017). Skeletal class III malocclusion often manifested as anterior crossbite or arch crossbite, compensatory lip of the upper anterior teeth and compensatory tongue of the lower anterior teeth, mesial occlusion of molar, soft tissue profile for concave type, hypoplasia of middle-face 1/3, lower lip protruding (Liou, 2005; Akbari et al., 2016; Cabrita et al., 2017). Skeletal class III malocclusion is not only a significant effect on the function of children's oral and maxillofacial system, but also one of the common causes of temporomandibular joint dysfunction unsightly teeth and facial type will seriously affect children's mental health, resulting in children's inferiority complex (Song & Wu, 2007; Heymann et al., 2010). Genetic factors and bad habits are the main cause of skeletal class III malocclusion, in the period of primary dentition and mixed dentition, the need for timely intervention in the treatment, among them, the patients in mixed dentition were treated with protraction (Ghassemi et al., 2014). In this study, we investigate the morphological changes of skeletal class III malocclusion in mixed dentition through light force protraction combined activities, to explore the improvement of the profile of the patients by this treatment, so as to guide the clinical work.
MATERIAL AND METHOD
Materials selection. Thirty patients (15 males and 15 females) were selected from September 2014 to August 2017 in the Department of orthodontics, Shanxi Medical University Stomatological Hospital.Inclusion criteria: 1) ages: 6-10 years; 2) skeletal class III malocclusion; 3) mesial molars relationship; 4) anterior crossbite and reverse overjer. Exclusion criteria: 1) history of orthodontic treatment or trauma; 2) oral and maxillofacial deformities and other systemic diseases affecting oral and maxillofacial development.
Instruments and equipment. There were head positioning X-ray camera (Siemens, Germany) and Winceph 8.0 cephalometric analysis system (Rise, Japan).
Experimental method. Thirty patients underwent a maxillary casting type of pre-traction device, according to the condition of maxillary dentition to select the corresponding activities of the movable expansion appliance, each side of the force of about 150g-200 g, requiring patients to wear 10-12 hours a day, the appliance should be removed after reaching the normal occlusion. The correlation cephalometric profile of the cranial lateral radiographs before and after orthodontic treatment was measured. 1) All patients were photographed by the same technician from the Department of Radiology of our hospital with the head positioning X-ray camera under the same condition; 2) The subject stood erect and looked straight ahead. With the head positioning X-ray camera, in intercuspal position, the face relaxed. The sagittal plane was parallel to the cassette; 3)Lateral radiographs of the skull were introduced into the Winceph 8.0 software. Cephalometric measurements were repeated three times by the same researcher at different times, the average value was calculated.
Measurement index of hard tissue. 1) SNA: sella-nasionpoint A angle, representing the positional relationship (anterior and posterior) of the maxilla with respect to the skull; 2)SNB: sella-nasion-point B angle, representing the positional relationship (anterior and posterior) of the mandible relative to the skull; 3) ANB: AB plane angle, representing the positional relationship of the maxilla and mandible to the skull; 4) A-Ptm: representing the length of maxillary body; 5) MPSN: representing the steepness of the mandibular body; 6) ANS-Me/N-Me: lower face height to total face height ratio; 7) U1-SN: the inclination of the maxillary incisor relative to the SN plane; 8) L1-MP: The inclination of the mandibular incisor relative to the mandibular plane (Fig. 1).
Measurement indexes of soft tissue.1) Ns-Sn-Pos: representing facial profile of soft tissue profile in maxillofacial region; 2) TUL-E: representing the upper lip protrusion; 3)TLL-E: representing the lower lip protrusion; 4) Nasolabial angle: the angle between the lower edge of the nose and the leading edge of the upper lip 5) Upper lip base thickness: representing the upper lip thickness and protrusion (Fig. 2).
Statistical analysis. The data were processed by SPSS 22.0 software,the measurement data were expressed as (x ± s), and the paired t test was used before and after treatment. The difference was statistically significant (P <0.05).
RESULTS
Clinical orthodontic treatment effect. Thirty patients with light force protraction combined activities for 6-10 months (mean 8.8 months) can make the mixed dentition of skeletal class III malocclusion in patients with a significant improvement in profile appearance, all patients of concave type became direct type. Twenty-one patients achieved normal occlusion, nine patients had shallow overbite.
Changes of cephalometric measurement indexes after orthodontic treatment. After treatment, SNA increased by 2.514° (P <0.05), ANB increased 3.452° (P <0.05), indicating that the maxillary move forward; A-Ptm increased 2.121 mm, indicating that the maxilla also occurred growth. SNB decreased by 0.766° (P> 0.05), indicating that the sagittal relationship between the Maxilla and mandible was significantly improved.MP-SN increased by 2.215° (P<0.05). ANS-Me / N-Me increased by 1.091 % (P<0.05), which may be caused by clockwise rotation of mandible and growth and development during treatment.U1-SN increased 3.126° (P <0.05), indicating that the anterior teeth forward, the upper anterior teeth are slightly lip.L1-MP decreased by 1.102° (P> 0.05), indicating that the lower anterior teeth no obvious after the shift and tilt. Ns-Sn-Pos increased by 5.784° (P < 0.05), the upper lip forward 3.442 mm (P <0.05), the lower lip retreated 0.363 mm (P> 0.05), the upper lip thickness decreased 1.782 mm (P <0.05), the Nasolabial angle decreased by 5.629° ( P <0.05), indicating that the posterior side of the traction convex degree increased, the upper lip changes more obvious, and lower lip changes are not obvious (Table I).
Typical cases. Changes of facial and lateral radiographs before and after treatment of typical cases (Figs. 3 and 4).
DISCUSSION
Because the craniofacial morphology is complex, and the formation of skeletal class III malocclusion is closely related to individual inheritance, growth and development, environment and bad oral habits, clinical orthodontic treatment is more difficult (Lee et al., 1997; Gautam et al., 2009). Maxillary protraction is an important means of orthodontic treatment of skeletal class III malocclusion in mixed dentition, suitable for maxillary hypoplasia, maxillary retrusion of the patients, can promote the growth and development of the maxilla, this method has also been proved to have obvious growth and improvement effect for providing a more favorable environment for the normal development of the maxilla and mandible (Cordasco et al., 2014; Ngan & Moon, 2015; Scherer et al., 2015).The timing of the treatment is about 8 years old, or after the eruption of the maxillary central incisor and the lateral incisor. Saadia & Torres (2000) think that the maxillary deficiency cause of the skeletal class III malocclusion deformity through protraction treatment, in 3 children between the ages of 12 bones have changed, the deciduous dentition and early treatment time is relatively short and profile aesthetic changes more significantly. Mixed dentition is the rapid growth of the maxillofacial development period, the plasticity of the jaw is large and the degree of malformations is relatively mild protraction combined the maxillary expansion device can solve the maxillary width at the same time (Lee et al., 2010; Kim et al., 2017). In the treatment process, we must pay attention to relieve the bad habits of the mouth, correct the wrong sleeping posture. In order to avoid the rotation of the palatal plane counterclockwise, the direction of traction should be inclined with the plane before the 15 -30°, the force of the line as far as possible through the center of the nasal maxillary complex, so that the maxillary movement of the overall movement to prevent the phenomenon of jaw opening. Scholars believe that the front of the soft tissue after the protraction correction straight, lip position improved, which is due to the establishment of the normal cover of the incisors, which makes the upper lip obtain better tension and morphology (Kiliçoglu & Kirliç, 1998; Ozbilek et al., 2017). It was also found that the amount of forward movement of the maxillary soft tissue after protraction was 50-79 % of the amount of hard tissue moving forward, and the amount of anterior movement of the mandibular soft tissue was 71-81 % of the amount of hard tissue movement. After the treatment of soft tissue facial corners decreased, the upper lip more prominent, the upper lip can move forward 4.5 mm, soft tissue pogonion backward shift, so that the face tends to balance. The upper anterior teeth forward 1.25 mm can make the upper lip corresponding forward 1 mm, the thickness of upper lip decreased and the depth of upper lip sulcus was less, but the lower lip area change is not significant, which may be related to the lower lip position affected by a variety of factors.
The study of the pretraction force between 150-200 g, which reduces the damage caused by adverse gravity on the muscles and joints, While improving patient comfort and appliance retention performance, reducing the appliance off rate. Protraction combined activity expansion appliance to solve the maxillary sagittal and lateral problems, for protraction to achieve better results. Activity expansion device is also conducive to maintaining oral hygiene, easier access for parents and patients with the understanding and cooperation. The patients in the study in the 6 to 10 months of the correction process. A point of occurrence of significant forward, maxillary hypoplasia has been improved to varying degrees, the maxillary has moved forward and increased, the upper lip inclination is improved. The increase of MP-SN and ANSMe / N-Me is statistically significant, which indicates that the mandibular plane angle increases with the anterior and inferior hyperextension and the mandibular rotation occurs clockwise, which should be the result of the synergistic effect and growth and development. This suggests that the process of growth and development of patients with correction, should be combined with growth characteristics of correction. The cephalometric results showed the upper lip forward and lower lip backward, which makes up the deficiency of skeletal class III malocclusion concave profile, have been greatly improved.
In short, the light force protraction combined activities of the expansion of orthodontic treatment of skeletal class III malocclusion in mixed dentition, can create a good environment for the growth of maxilla and mandible, is beneficial to the normal growth of patients with maxillofacial development, improves the lateral appearance, increases patient self-confidence, reduces the difficulty of re-treatment of patients.
