Selection of investigation methods and standards
There is no unified standard for the epidemiological investigation of malocclusion in the world. At present, the commonly used investigation standards include Angle's classification standard, Dental Aesthetic Index (DAI), and Index of Orthodontic Treatment Need (IOTN). The latter two are widely used in European and American countries. Taking into account the limitations of DAI and IOTN, and ensuring consistency with the standards of most studies at home and abroad, this survey used the Angle's classification as the standard that reduced the impact of the survey standards on the research results, and allowed the comparison with the results of local surveys.
Prevalence of malocclusion
In recent years, many investigations and studies have been published on the prevalence of malocclusion; some of which were related to the prevalence of malocclusion in children with mixed dentition. Perhaps due to the differences in race, geographical environment, eating habits, and economic status, the prevalence characteristics of malocclusion in the same age group are different in different regions and countries such as 46.5% in Japan , 83.3% in India , and 87% in Iran . In China, Fu Minkui and others investigated in 2000 that the malocclusion of children in the period of mixed dentition stage was as high as 71.21% . In the past decade, the survey results have been very different across the country, 79.40% in Shanghai , and 69.38% in Hailing District, Taizhou City .
The results of this survey showed that the prevalence rate of malocclusion in children aged 6−12 years old in Jinzhou was 67.90%, which belongs to a high level in foreign and domestic reports. The pathology and related factors of malocclusion are complex. It is known that genetic factors are highly correlated with craniofacial abnormalities . In addition to genetic factors, environmental factors can also affect the normal development of occlusal. For example, finger sucking can cause anterior teeth to open bite . This study found that the Angle class I malocclusion had the highest prevalence rate among all kinds of malocclusion, and dentition crowding was the most common clinical manifestation of all kinds of malocclusion, which was the same as that reported previosuly [11, 12, 15]. It can be seen that most of the malocclusion is dentition crowding in which the molar relationship is neutral, which is in line with the human evolutionary process of ethnic evolution, the masticatory organs are reduced and degenerated, and the degeneration of alveolus and jaws is faster than that of teeth. In addition, with the improvement of the level of modern food processing, the frequency of eating tough food is reduced, the jaws are not stimulated, and the number of teeth is greater than that of bone, resulting in dentition crowding. For example, the prevalence rates of malocclusion and dentition crowding in Tibetan  adolescents are lower than those in Han nationality, mainly because they eat beef, mutton, and other high toughness, crude fiber food and strong grinding food for a long time, which makes the degeneration of masticatory muscle and jaw more slowly, and the imbalance of tooth and bone mass is alleviated.
Differences of malocclusion in gender, age, nationality, urban and rural areas
This study showed that there was no significant difference in the prevalence rate of malocclusion between men and women, consistent with previous results . Previous data indicate differences in the prevalence of malocclusion among different age groups; the prevalence rate increases with age. The prevalence rate of deciduous tooth stage is the lowest, followed by the period of mixed dentition stage, and the highest in the early stage of permanent dentition. Because the dentition of the selected samples in this study were in the mixed dentition stage, the final results showed that the prevalence rate of malocclusion was higher in children aged 6−8 years, and less in children aged 9−12 years old. This could be due to the fact that the children from 6−8 years old were in the early stage of mixed dentition. With the increase of age, the jaws continue to develop, and their length and width increase. It is beneficial to adjust the position of permanent teeth and improve the symptoms of malocclusion. There was no difference in prevalence rate between urban and rural areas, which is consistent with the results of the recent survey . With the improvement of people's living standards, the differences between the urban and rural areas are shrinking, and the dietary structure is gradually converging between urban and rural areas. Moreover, people's oral hygiene habits are also being strengthened. No matters in urban or rural areas, people pay more attention to oral health, especially children's dental health, and children's awareness of oral health care and medical treatment are increasing. In addition, there was no difference in the prevalence rate between different ethnic groups, which is different from some domestic research results [11, 18].
Manifestations of malocclusion in different categories
The malocclusion was divided into groups according to Angle classification, and the results showed that the highest prevalence rate of Angle class I was 46.48%, and that of Angle class II and III was 9.39% and 12.03%, respectively. The results were similar to the study of Brazilian  children. The results showed that Angle Class I was the main malocclusion, accounting for 35.89%, Angle Class II and Class III are basically the same, which are 5.82% and 6.36%, respectively. The differences between the results of each study may be due to differences in diagnostic criteria, race, age and lifestyle of the subjects. It should be noted that this survey takes individual normal occlusion as the judgment standard, and according to the national unified orthodontic textbooks, any slight deformities that do not interfere with the physiological process can be included in the category of normal occlusion, therefore, the judgment of individual normal occlusion and malocclusion will be affected by subjective factors, and the prevalence rate of each Angle classification may be distorted to a certain extent.
The survey found that the prevalence of dentition crowding was the highest among local abnormalities (71.79%), which was related to the unbalanced degenerative changes in the craniofacial region. Jaw degeneration is faster than teeth, resulting in tooth mass greater than bone mass. The result of this survey shows that the incidence of spacing is 12.07%. The gap in deciduous dentition and mixed dentition is beneficial to the eruption and establishment of permanent dentition, but it is found that 9 of the 16 subjects do not have deciduous tooth space, but their permanent dentition is not crowded, indicating that it may not be necessary to use developmental space and primate space to solve the problem of permanent dentition crowding. The relationship between crowding, spacing and malocclusion needs to be confirmed by other longitudinal data.
Compared with the national average level, Jinzhou children not only have a high prevalence of malocclusion, but also have complex and diverse types of malocclusion. The survey found that except for 694 (32.10%) individual normal occlusion, 8.14% of the children had only one malocclusion. Most children (47.46%) showed 2 or 4 malocclusion at the same time, and 12.30% of the children had 5 or more malocclusion types at the same time. Complex and diverse manifestations of malocclusion will increase the treatment difficulty of malocclusion and prolong the treatment time of malocclusion, which also calls for early prevention and treatment of malocclusion in children. At the same time, Jinzhou City should strengthen policy support and financial investment for the prevention and treatment of children's malocclusion.
With the eruption of the second molars and the growth and development of the mandibular ramus, the deep overbite can be relieved by itself . Because there is no corresponding auxiliary examination means, we have included all the deep overbite into the category of malocclusion and increased the prevalence of deep overbite (39.32%). The vertical malocclusion also includes anterior edge-to-edge occlusion (4.02%) and open bite (2.08%). The prevalence of anterior edge-to-edge occlusion is similar to that of a survey in Xi'an , but lower than that of other foreign studies[21, 22] .
Deep overjet (8.93%) is the most common sagittal malocclusion. According to a morphological longitudinal study, with the increase of age, because the mandible grows forward more than the maxilla, deep overjet is likely to decrease . The study found that the prevalence rate of anterior crossbite in children in Jinzhou is 9.62%. It will affect the growth of craniofacial region and have a certain effect on joints. With the increase of age, anterior crossbite will become more and more serious .
Malformed frenulum of lip mainly include low labial frenulum and short lingual frenulum. This examination found that the probability of abnormal frenulum in children in Jinzhou is 5.83%. Too low maxillary labial frenum will lead to scattered space between maxillary central incisors, and too short lingual frenulum will lead to mesial torsion of mandibular central incisor. The results showed that delayed loss of primary teeth was closely related to malocclusion (p = 0.001). The causes of deciduous tooth retention included pulp lesions of deciduous teeth or malposition of germs of succeeding permanent teeth, which makes the root of deciduous teeth stick to alveolar bone or slow absorption. Retention of deciduous teeth will lead to dislocation eruption or embedded impaction of succeeding permanent teeth, resulting in individual tooth crossbite and occlusal interference. Therefore, we suggest that parents should pay attention to the situation of children's replacement teeth. Daily diet should not be too fine, and when the eruption of permanent teeth is found, the retained deciduous teeth should be removed as soon as possible.
Analysis of risk factors related to malocclusion
It is believed in the existing literature that bad oral habits are closely related to the occurrence of malocclusion, and the prevalence rate of malocclusion in children with bad oral habits is higher than that in normal children. Bad habits such as lip biting, finger sucking and tongue sticking will lead to the occurrence of malocclusion such as tilted teeth, narrow dental arch, anterior teeth open bite and so on . The results of this survey are similar, the prevalence rate of malocclusion in children with bad oral habits is higher than that in normal children, and the difference is statistically significant (p > 0.05). Higher frequency and duration of oral bad habits was associated with greater possibility of malocclusion. This study found that the habit of biting the lower lip increased the risk of deep overbite and increased the incidence of Angle II malocclusion. When biting the lower lip, the lower lip is located between the upper anterior teeth and the lower anterior teeth, which makes the upper anterior teeth tilted to the labial side and suppresses the lower anterior teeth to the lingual side, which is easy to make the lower dentition crowded and form a deep overjet in the anterior tooth area. Biting the lower lip restricts the development of the mandible and increases the occurrence of Angle II malocclusion. The habit of biting the upper lip and protruding the mandible can easily lead to malocclusion of anterior crossbite and Angle III malocclusion. Its mechanism is opposite to that of lower lip bite, which can promote the growth of mandible forward, and is prone to malformations such as anterior crossbite; mandibular protrusion and mesial malocclusion.Oral breathing habits increased the deformities of deep overbite, anterior open bite and Angle II malocclusion. During oral breathing, the tongue is pulled downward, the strength of the tongue muscle to the upper dental arch is weakened, the strength of the buccal muscle to it is enhanced, and the internal and external muscle balance of the maxillary arch is lost, resulting in stenosis of the upper dental arch, protrusion of the upper anterior teeth, and deep overjet. In addition, during oral breathing, the mandible is usually in the backward position, which can cause mandibular retraction over a long time, and the molars are in distal relationships. Oral breathing continues to increase the facial height, the posterior teeth erupt excessively, and the mandible rotates backward and downward, resulting in the open bite and occlusion of the anterior teeth. When children prop up their cheeks to read and think, the occlusal, jaw and facial growth and development are subjected to abnormal external forces, which may lead to the occurrence of malocclusion. In this study, the habit of propping up cheeks can increase the prevalence of deep overbite. In addition, in the course of this study, it was found that retained deciduous teeth and low maxillary labial frenulum would lead to an increase in the prevalence of malocclusion, and low labial frenulum would increase the prevalence of space between maxillary central incisors.
It is not clear whether there is a correlation between dental caries and malocclusion. According to the cross-sectional study on the relationship between dental caries and malocclusion in children aged 6–12 years old in Jinzhou, it was found that there was a correlation between dental caries and malocclusion (p < 0.05).In addition, it is difficult to rule out the influence of genetic and other factors on malocclusion. Future epidemiological studies on malocclusion need a unified examination standard, a larger and more systematic sample size, and controlling other variables to analyze the relationship between a dependent variable and malocclusion.
A large number of studies have discussed the relationship between dental caries and malocclusion. Some scholars believe that dental caries is a risk factor for malocclusion. Because dental caries can change the height and width of the crown, cause dentition crowding and changes in occlusal surface, and affect joints. The incidence of malocclusion in children with deciduous teeth is 2.04 times higher than that in children without dental caries  .However, some scholars have pointed out that the occurrence of malocclusion is closely related to the time and severity of caries, and only the caries of mixed dentition and permanent dentition may lead to malocclusion . This survey found that the prevalence of malocclusion is related to dental caries. The total prevalence rate of dental caries in children is as high as 61.05%, indicating that the oral health status of children in Jinzhou is poor and the awareness of health care is not enough. The prevalence rate of malocclusion in patients with dental caries was higher than that in normal subjects, independent of age, sex, area and other factors. It is well known that deciduous caries lead to reduction of mesial and distal crown diameter and early loss of dental caries, resulting in insufficient eruption space or abnormal eruption of inherited permanent teeth, which can cause dentition crowding; extensive dental caries make oral mastication function insufficient, jaws can not get physiological stimulation, and can also cause malocclusion.
Effect of malocclusion on oral health-related quality of life
The term "quality of life" originates from the medical field and is defined as "people's perception of their status in life according to their goals, expectations, standards and concerns in the context of the culture and value system of their lives."Health-related quality of life is a term used to assess pain/discomfort and how physical, psychological and social functions affect well-being.The impact of oral health on quality of life is called oral health-related quality of life, which is defined as "symptoms, functions and psychosocial effects caused by oral diseases and disorders".
Previous studies have shown that malocclusion has a significant impact on children's oral function, oral symptoms, psychology, social contact and facial beauty.In 2010, Locker D et al  used the Children's perception questionnaire to study the oral function and psychology of 66 children aged 11 ~ 14 years old. The results showed that malocclusion had a significant impact on children's oral function and psychology. The results showed that the severity of malocclusion could affect most adolescents' self-perception of dental cosmetology. For example, domestic scholar ELYASKHIL M, SHAFAI N A A and MOKHTAR N  investigated and analyzed the malocclusion of 255 malay school children. The results showed that malocclusion can not only cause the disorder of children's masticatory system, but also affect children's facial beauty. It has an important impact on students' social communication and psychological behavior.
The Chinese version of OHIP-14 scale was used to evaluate oral health-related quality of life  .OHIP-14 measures the frequency of oral effects in seven conceptual areas, each with two problems, namely, functional limitation, physical pain, psychological discomfort, physical disability, psychological disability, social disability and handicap. The answer options were Likert scale: 0 = never, 1 = almost never, 2 = occasionally, 3 = often, 4 = very often. Add up the project scores to determine the total score. The sum of scores can range from 0 to 56. A score of zero (0) means no effect, while a score of 56 indicates the opposite, that is, oral health has the greatest impact on children's quality of life. The results of this survey showed that there was no difference in the scores of oral health-related quality of life among children aged 6 to 12 in Jinzhou city in terms of gender, age, nationality and socio-economic conditions (p > 0.05). This result is different from that of foreign studies . The reason for the analysis may be due to different inclusion criteria, ethnic groups and children's living habits. The survey found that "physical pain" is the most common negative impact, accounting for 3.65%, followed by "psychological discomfort" is also a common occurrence in the population, accounting for 3.56%, the item with the least impact on individuals is "social disability", accounting for only 1.61%. However, on the whole, malocclusion has little effect on children's oral health-related quality of life, which may be due to the younger age, lower aesthetic requirements and less degree of malocclusion in the investigated children; it doesn't have a significant impact on the quality of life. In the course of this survey, the influencing factors of oral health-related quality of life were analyzed. OHIP-14 was divided into dependent variables, and multiple linear regression analysis was conducted with anterior open bite, deep overbite, deep overjet, BMI index, economic level and other independent variables. Among them, anterior open bite, deep overbite and deep overjet had influence on oral health-related quality of life, while BMI index and economic level had no effect.
On the whole, malocclusion affected the oral health-related quality of life of children in Jinzhou, mainly in pain and psychology. In addition, multiple linear regression analysis found that anterior open bite, deep overbite, deep overjet were significantly associated with oral health related quality of life. The results of this survey showed that there was no difference in the scores of oral health-related quality of life among different genders, ages, nationalities, and socio-economic conditions. Since the severity of malocclusion was not classified by IOTN, it could be mainly considered that the overall prevalence of malocclusion of children in Jinzhou City is high, but the severity of malocclusion is relatively low, and hence, the impact on children is relatively small.