Prevalence of malocclusion in Turkish children and adolescents: A systematic review and meta‐analysis

Abstract Objectives The aim of this article is to establish a comprehensive nationwide prevalence of malocclusion traits on the sagittal, vertical, and transverse planes of space in the Turkish population. Material and Methods A systematic search of PubMed, Scopus, and Web of Science was supplemented by manual searches of Google Scholar and the reference lists of included studies. Original Turkish health studies of any age were included. Strengthening the Reporting of Observational Studies in Epidemiology assessed study quality and bias (STROBE). Sagittal, vertical, and transverse malocclusion features were retrieved and gathered. Results Eleven studies were selected from 434 titles. Two studies showed a high risk of bias, eight low and one moderate. Thirteen thousand two hundred seventy‐one individuals were investigated from early childhood to late adulthood. Most studies were sampled from universities and dental (nonorthodontic) clinics. The pooled malocclusion prevalence was 56% for Class I (95% confidence interval (CI): 44−68%), 31% for Class II (CI: 6–42%), and 11% for Class III (CI: 21–37%). The other common types of malocclusions were crowding (41%, CI: 18–65%), overjet (34%, CI: 21–50%), negative overjet (13%, CI: 7–20%), and crossbite (11%, CI: 7–15%). Additionally, there was no significant difference in Class I (relative risk [RR] = 1.00, [0.96–1.05]), Class II ([RR] = 0.97, [0.92–1.03]), and Class III ([RR] = 1.08, [0.96–1.225]) malocclusion by gender. Conclusions This study showed Class I malocclusion has a high prevalence among the Turkish population followed by Class II and Class III malocclusions. In addition, crowding and overjet were the most prevalent malocclusions among Turkish individuals. There were no significant differences in the prevalence of malocclusions between males and females.


| INTRODUCTION
Any variation from the teeth's normal occlusion is what Angle refers to as malocclusion (Sandhu et al., 2012). According to the World Health Organization in 1987, malocclusion is an abnormality that causes disfigurement or affects function and requires treatment if it is an obstacle to the patient's physical or mental well-being (Sharma et al., 2019). Malocclusions are the third most common oral health issue, behind dental caries and periodontal disorders, (Guo et al., 2016) that significantly affects people's self-esteem and social acceptance (Balachandran & Janakiram, 2021). Malocclusion must be detected and treated, especially in children, because it affects oral activities like mastication, swallowing and speaking. It also, leads to the development of periodontitis, dental caries, temporomandibular disorders, and trauma (Sanadhya et al., 2014). In addition to enamel defects like Molar-incisor hypomineralization, (Amrollahi et al., 2023) malocclusion exerts a detrimental influence on oral health-related quality of life (Baskaradoss et al., 2022).
Malocclusion has a multifactorial etiology and many factors have been attributed to its cause. The main contributing factors are genetic, environmental, and ethnic. Moreover, other causes are the geographical location and the prevalence of malocclusion in the population (Alhammadi et al., 2018). The malocclusion prevalence of Class I, Class II, and Class III was assessed to be 58%, 24%, and 4%, in Denmark (Proffit et al., 1998). On the other hand, the prevalence of Class I malocclusion was between 50 and 55%. The Class II and Class III malocclusion prevalence was 15% and 1%, respectively in the United States (Mills, 1966). Various studies have assessed the prevalence of malocclusion or the Turkish population. While one study showed that the most prevalent malocclusion in Turkish people was Class II division 1 malocclusion, (Gelgör et al., 2007) others demonstrated that Class I malocclusion has the most prevalence (Celikoglu et al., 2010;Gungor et al., 2016;Sayin & Türkkahraman, 2004).
For a multifactorial condition such as malocclusion, accurate epidemiological data is essential. Epidemiology studies, including the prevalence of malocclusion, are beneficial for estimating the size of health issues. It also provides the necessary information and analyzes a potential hypothesis. It establishes the priorities of health programs to plan future actions (Alhammadi et al., 2018;Foster & Menezes, 1976).
Due to the reasons mentioned above, this systematic review and meta-analysis study aims to generate nationally representative data on individuals' malocclusion in turkey.

| METHODS AND MATERIALS
This systematic review followed the Preferred Reporting Items for Systematic Reviews and Meta-Analysis Protocol (PRISMA). Registered in PROSPERO with the following ID: CRD42023409379.
The study period was conducted in accordance with PRISMA standards for performing a systematic review (Mehta et al., 2022).

| Information sources and literature search
The literature was systematically searched for relevant materials in PubMed/Medline, Web of Science, and Scopus. Data was evaluated through February 2023, with no time restrictions. The studies were searched manually for potential materials. The following search strategies were used for each database, which is shown in Table 1.
The included articles were in English and the Turkish language.
Pertinent text words and the medical subject heading (MeSH) were used to build the search strategy. The adopted database search approach is displayed in Table 1.
Relevant studies were searched independently by two authors (M. R. and Q. P.). Abstracts, titles, and entire texts were searched.
Before the start of the study, discussion sessions were held to calibrate the reviewers. A list of inclusion and exclusion criteria was prepared and both reviewers were instructed to analyze 22 abstracts.  Angle's classification: class I, II, III, and normal occlusion Primary dentition: mesial step, distal step, and flush terminal. covering various aspects of the methodology such as sample size, study design, sampling method, population, data collection methods, tools, examining sample methods, statistical analysis, and reporting findings based on objectives. Each item corresponded to "yes = 1" or "no = 0". Therefore, each study was allocated a score of 0−12.

| Statistical analysis
Statistical analyses were performed in STATA v.17 software. Heterogeneity between the eligible studies was calculated using Cochrane Q and I 2 Based on the level of heterogeneity, random or fixed effect models were utilized to estimate the overall prevalence of malocclusion. For each category of indices and classification utilized in the review's scoring criteria. Also, for the overall prevalence, pertinent forest plots were produced, and the confidence interval was maintained at 95%. The data from the included studies were used to create a gender-specific pooled estimate of prevalence. For publication bias of studies in this metaanalysis the eager test was performed and the bias was modified using the trim and fill method.

| Search strategy and study quality
The PRISMA flow diagram for the study selection process through different stages is shown in Figure 1, and a completed PRISMA checklist is shown in Appendix A. The initial search yielded 434 results and a total of 381 studies remained after duplication removal. The remaining materials were excluded after reading their titles and abstracts (if necessary). Full texts were retrieved for the remaining papers. Eleven papers were excluded after reading the full text. Figure 1 shows the reason for the exclusion of each paper.
The remaining Papers were included in the systematic review and in the meta-analysis. Based on the STORB appraisal tool (Table 2), the majority of studies had a low risk of bias. However, one study showed a moderate risk of bias, (Akbulut, 2020) and two studies showed a high risk of bias (Kaygisiz et al., 2015;Sari et al., 2003).

| Study characteristics
The included studies had diverse age groups ranging from 0 to 38 years old with a total sample size of 13,271 participants. Most of the studies were conducted only on permanent dentition (Akbulut, 2020;Başçiftçi et al., 2002;Celikoglu et al., 2010;Gelgör et al., 2007;Nur et al., 2014;Sayin & Türkkahraman, 2004;Ugur, 1998;Uzuner et al., 2015). However, a few of the studies recruited their sample from universities and dental (nonorthodontic) clinics. Table 3 presents a detailed summary of the main characteristics of each included study.

| Prevalence of malocclusion traits
The following provides a summary of the malocclusion characteristics found in the studies.
The only study which determined the molar relation of primary dentition showed that 88% of individuals had flash terminal moral relation, 7% had a distal step, and 5% had a mesial step molar relation (Sayin & Türkkahraman, 2004).

| Skeletal sagittal relation
Merely one study determined the prevalence of various kinds of skeletal malocclusion in the Turkish population (Akbulut, 2020). Most of the individuals in this study had class I skeletal relation (64%), followed by class II (26%), and (10%) class III relations.
F I G U R E 1 PRISMA flowchart illustrates the included studies' systematic screening/selection process.
T A B L E 2 Risk of bias assessments using STROB tool. Are the research questions clearly stated?
Is the study context Is the sampling method clearly described?
Is the sampling strategy appropriate for the research question?
Is the method of data collection clearly described?
Is the data collection method appropriate Is the method of analysis Are the main characteristics o the Is the method of analysis appropriate for the research question?   (Gardiner, 1982) and Egyptian, (El-Mangoury & Mostafa, 1990) the Turkish population Angel's class I malocclusion has the highest prevalence followed by class II and class III malocclusion. However, two studies that determined the overall prevalence of malocclusion among Saudi Arabian and Chinese individuals showed that class III malocclusion is more prevalent compared to class II malocclusion in these populations (Almotairy & Almutairi, 2022;Shen et al., 2018). The higher predominance of Class III malocclusion in these two populations than others could be attributed to the ethnic differences in craniofacial morphology, genetic predisposition, environmental factors, (Aldrees, 2011;Fleming et al., 2008) and methodological variations such as the participants' age and sample size. Moreover, like the dental relation, the skeletal relation of the two jaws were mainly class I followed by Class II and Class III relations.
There have been controversial results about the occurrence of malocclusion in different populations. Some previous studies reported malocclusions pervasiveness between the two genders were significantly different (Akbari et al., 2016;Lew et al., 1993). On the other hand, the results of the present study indicated that there was no significant difference in malocclusion prevalence between the two genders. In accordance with the present study, the Shen et al. study showed the same result in the Chinese population (Shen et al., 2018).
However, due to these controversial outcomes, further gender-based cohort studies are required to confirm this assumption in the Turkish population.
The major molar relation in Turkish primary dentition was flush terminal molar relation like other studies. The second most common molar relation was a distal step in Turkish children unlike the previous studies which were conducted on Chinese, Indian, and Saudi-Arabian children (Almotairy & Almutairi, 2022;Lochib et al., 2015;Shen et al., 2018). The results of the current study showed that the majority of children in the Turkish population had a flush terminal molar relation (88%), followed by distal step relations (7%) and mesial step relations (5%). On the other hand, the occurrence of molar relationships in India was flush terminal plane (66.0%), mesial step (12.8%), and distal step (2.4%) (Lochib et al., 2015). The molar position in primary dentition can indicate the future dental relationship of individuals' permanent dentition (Hegde et al., 2012).  mandible and mesial migration of the mandibular arch (Onyeaso & Isiekwe, 2008). However, the frequency of flush terminal relation (88%) among children was higher compared to the incidence of class I malocclusion (56%). Since there was only one study that determined the molar relationship in Turkish children compared to the ten studies on the permanent dentition more studies need to be done.
The (34%) overjet prevalence in the present review was closely associated with the occurrence of (31%) Angle's Class II relationship.
Moreover, the overall incidence of Angle's Class III dental relationship (11%) and the negative overjet (13%) also were near each other. The meta-analysis study on Saudi Arabian population showed the same result regarding the accordance between Class II malocclusion and overjet and the prevalence of Class III and negative overjet malocclusions (Almotairy & Almutairi, 2022). These findings demonstrated that differences between two jaws can lead to Class II and Class III malocclusion which results in an increased or reverse overjet.
However, in some cases, the teeth can reduce the relationship between two jaws, especially in Class III individuals.
One study used the DAI, which is advised by the WHO, to standardize epidemiological data on malocclusion and treatment requirements (Organization, 2013). However, the DAI is more of an indicator of the need for esthetic treatment and does not evaluate occlusal factors such as crossbite, asymmetry or impacted teeth (Singh et al., 2011). Furthermore, another study utilized TPI for determining the severity of malocclusions. Although the TPI and DAI both score some orthodontic aspects and evaluate the need for orthodontic treatment, they do not show the incidence of orthodontic features (Jenny & Cons, 1996).
In accordance with previous studies, (Dimberg, Lennartsson, et al., 2015;Proffit et al., 1998) teeth crowding was shown to affect 41% of individuals. The increased frequency of tooth crowding during permanent dentition can be attributed to several factors. Primary teeth serve as the body's natural space maintainers for developing permanent teeth during early childhood. However, if the primary teeth are lost too early or hold on until the eruption of the permanent teeth, the erupting permanent teeth will eventually be pushed aside, leading to crowding of the teeth (Almotairy & Almutairi, 2022).
It is important to realize that studies assessing the prevalence of malocclusion frequently have obvious methodological limitations.
These include variations in sample sizes, ages, the source of the sample's recruitment and the process used to register malocclusion traits (Rakhshan, 2013 Page 3,15, Table 1 Selection process 8 Specify the methods used to decide whether a study met the inclusion criteria of the review, including how many reviewers screened each record and each report retrieved, whether they worked independently, and if applicable, details of automation tools used in the process.
Page 3,4 Data collection process 9 Specify the methods used to collect data from reports, including how many reviewers collected data from each report, whether they worked independently, any processes for obtaining or confirming data from study investigators, and if applicable, details of automation tools used in the process.
Page 3

Data items 10a
List and define all outcomes for which data were sought. Specify whether all results that were compatible with each outcome domain in each study were sought (e.g., for all measures, time points, analyses), and if not, the methods used to decide which results to collect.

Page 4 10b
List and define all other variables for which data were sought (e.g., participant and intervention characteristics, funding sources). Describe any assumptions made about any missing or unclear information.
Page 3,4 Study risk of bias assessment 11 Specify the methods used to assess risk of bias in the included studies, including details of the tool(s) used, how many reviewers assessed each study and whether they worked independently, and if applicable, details of automation tools used in the process.
Page 4 Effect measures 12 Specify for each outcome the effect measure(s) (e.g., risk ratio, mean difference) used in the synthesis or presentation of results.
Page 5 Synthesis methods 13a Describe the processes used to decide which studies were eligible for each synthesis (e.g., tabulating the study intervention characteristics and comparing against the planned groups for each synthesis (item #5)).
Page 3,4 13b Describe any methods required to prepare the data for presentation or synthesis, such as handling of missing summary statistics, or data conversions.
(Continues) 13d Describe any methods used to synthesize results and provide a rationale for the choice(s). If meta-analysis was performed, describe the model(s), method(s) to identify the presence and extent of statistical heterogeneity, and software package(s) used.
Page 5   13e Describe any methods used to explore possible causes of heterogeneity among study results (e.g., subgroup analysis, meta-regression).
Page 5   13f Describe any sensitivity analyses conducted to assess robustness of the synthesized results.
Reporting bias assessment 14 Describe any methods used to assess risk of bias due to missing results in a synthesis (arising from reporting biases).
Page 4 Certainty assessment 15 Describe any methods used to assess certainty (or confidence) in the body of evidence for an outcome.
Page 5

Study selection 16a
Describe the results of the search and selection process, from the number of records identified in the search to the number of studies included in the review, ideally using a flow diagram.
Page 6, Page 14, Figure 01 16b Cite studies that might appear to meet the inclusion criteria, but which were excluded, and explain why they were excluded.
Page 6, Page 14, Figure 01 Study characteristics 17 Cite each included study and present its characteristics. Page 6, Page 17,18, Table 3 Risk of bias in studies 18 Present assessments of risk of bias for each included study. Page 6, Page 16, Table 2 Results of individual studies 19 For all outcomes, present, for each study: (a) summary statistics for each group (where appropriate) and (b) an effect estimate and its precision (e.g., confidence/credible interval), ideally using structured tables or plots.
Page 6,7, Page 17,18, Table 3 Results of syntheses 20a For each synthesis, briefly summarize the characteristics and risk of bias among contributing studies.
Page 16, Table 2 20b Present results of all statistical syntheses conducted. If meta-analysis was done, present for each the summary estimate and its precision (e.g., confidence/credible interval) and measures of statistical heterogeneity. If comparing groups, describe the direction of the effect.
Page 19,20, Table 4,5 20c Present results of all investigations of possible causes of heterogeneity among study results. 20d Present results of all sensitivity analyses conducted to assess the robustness of the synthesized results.

Reporting biases 21
Present assessments of risk of bias due to missing results (arising from reporting biases) for each synthesis assessed.
Page 16, Table 2 Certainty of evidence 22 Present assessments of certainty (or confidence) in the body of evidence for each outcome assessed.

Discussion 23a
Provide a general interpretation of the results in the context of other evidence. Page 8,9 23b Discuss any limitations of the evidence included in the review. Page 10 23c Discuss any limitations of the review processes used.
23d Discuss implications of the results for practice, policy, and future research. Page 10

Other information
Registration and protocol 24a Provide registration information for the review, including register name and registration number, or state that the review was not registered.

24b
Indicate where the review protocol can be accessed, or state that a protocol was not prepared. 24c Describe and explain any amendments to information provided at registration or in the protocol.

Support 25
Describe sources of financial or nonfinancial support for the review, and the role of the funders or sponsors in the review.

None
Competing interests 26 Declare any competing interests of review authors. None Availability of data, code and other materials 27 Report which of the following are publicly available and where they can be found: template data collection forms; data extracted from included studies; data used for all analyses; analytic code; any other materials used in the review.