1. Introduction
Hypodontia is a condition of missing 6 or more teeth. Hypodontia is among most prevalent dental anomalies [1]. A combination of genetic and environment factors may lead to hypodontia [2, 3]. A series of recent studies has indicated that hypodontia can affect individual’s esthetic, function and oral health related quality of life [3-5]. Some authors have driven the further development that hypodontia can lead to tooth delay eruption, tooth size anomaly, canines malposition, peg shape laterals and taourodontism [4-7]. 
It has been reported that hypodontia was also in charge of periodontal diseases, malocclussion and, alveolar bone growth reduction [8]. Previous studies have emphasized that the most common complain among patients with hypodontia consisted of unesthetic appearance and spaces in their dental arch [9].
Early diagnosis and suitable treatment could play an important role in prevention of its physiological, functional and, aesthetical complications [8]. Kreczi et al. have demonstrated that children with hypodontia experienced increased overjet and overbite compared to the normal population of children [10]. The prevalence rate of hypodontia in orthodontic patients were reported 8.7% in Shiraz and, 9.1% in Tehran [8, 11]. Also, the prevalence of hypodontia have been reported 6.02% and 9% in dental clinics of Puerto Rico and Italy, respectively [12, 13]. Previous studies confirmed a significant correlation between malocclusion and hypodontia [8, 11].
Mandibular second premolar was recognized as the most frequent hypodotia teeth in the Japanese population [14]. Prevalence of hypodontia varies in different studies, due to varieties in applied methods of assessment, clinical examination, gender, age, geographic area and ethnics [15]. Hypodontia requires extensive treatment from simple restoration to multiple treatments, and the key point to the hypodontia treatment is to consider a multidisciplinary approach. Orthodontic treatment is essential for patients with hypodontia, especially in case of lower incisor hypodontia [16]. The aim of the present study was to investigate the prevalence of congenital missing tooth in patients, referred to Orthodontic Department of Kerman Dental School, as well as some private dental care centers.

2. Materials and Methods
The current descriptive retrospective cross-sectional study conducted on 1883 patients referred to the Orthodontics Department of Kerman Dental School and other private Orthodontics offices during September 2010 to September 2015. The research was approved by the Ethics Committee of the Kerman Medical University under the ethical code of IR.KMU.REC.1393.494. A signed informed consent was obtained from all patient who participated in our study.
A total number of 1883 records of patients including dental casts, intraoral photographs, lateral cephalometric and panoramic radiographs were evaluated. Patients with any syndrome, periodontal diseases, trauma, past orthodontic treatment history, dental caries and tooth extraction were excluded from the setting of the study. Poor quality radiographies, incomplete files of patients and third molars, were also excluded. Patients information regarding age, gender, number and type of hypodontia, location of hypodontia (maxilla or mandible, right or left side, unilateral or bilateral) were recorded. 
Type of occlusion were assessed by patients cephalographythrough ANB anglemeasurement method. ANB angle between 2 and 4 degrees were categorized as class I occlusion/malocclusion and higher and lower ANB angles were categorized as class II and class III malocclusions, respectively. All measures were evaluated by two independent clinicians. Statistical analyses were performed using the SPSS software version 21. The data were analyzed by Chi‑square test. A significance level of 0.05 was considered. 

3. Results
Of all cases examined (1883), 22.31% were males and 77.69% were females. The mean age of patients were 17.93±6.14 years. One hundred and one (5.36%) patients had congenital missing. Also, of all of the patients with missing teeth, 62 were females (61.38%) and 39 were males (38.61%). Our findings showed no significant differences between gender and hypodontia (P=0.939). Our results demonstrated that mandibular second premolar was the most frequently affected teeth followed by maxillary lateral incisor .In the present study, congenital molars hypodontia were not found (Table 1). In addition, class II malocclusion was the most frequent malocclusion (39.60%) .
Bilateral Congenital absence was observed in 59 (3.13%) patients. The most frequent bilateral missing were maxillary lateral incisors. Of all 101 patients with hypodontia, 36.6% were classified into class I, 39.6% into class II, and 23.8% were of class III malocclusions. According to our data, there was no significant correlation between type of occlusion and, type and number of congenital hypodontia (P=0.91) (Table 2). Hypodontia was higher in the posterior of mandible, compared to the other site of jaws, which this difference was statistically significant (P=0.000).
 

 

4. Discussion
The prevalence of hypodontia was calculated 5.4% in the present study . Results of the study conducted by Hedayati et al. showed that 7.6% of orthodontic patients suffered from congenital missing teeth [8].
 

 

 Furthermore, same value in the research by Vahid-Dastjerdi et al. was reported 9.1%, higher than our study [11]. Iran is a vast country with various ethnic groups. Therefore, this might explain the reason to such epidemiologic differences.
Prevalence of hypodontia obtained in our study is comparable to the results of Medina in pediatric orthodontic population in Venezuela (4%), and Celikoglu 4.6% in Turkish orthodontic patients which was lower than findings of Endo et al. (8.5%), and Gracco et al. 9% [13, 14, 17, 18]. Such differences might be due to varieties in study designs, geographic characteristics, gender, races, and genetics differences in the criteria of selection among various investigations.
The prevalence of hypodontia calculated in the present study was within the broad range reported in previous studies on Asian populations (2.6 to 11.2%) [15]. The most prevalent missing teeth consisted of lower second premolars, in the present research (42%). This data is comparable with the reports of Endo et al. and Gracco et al. that showed the most prevalent congenital hypodontia in orthodontic patients were second premolars [13, 14].
Based on our findings, maxillary lateral incisors were the second most frequent congenital missing teeth .This result is in consistent with findings of, Endo et al. and Gracco et al. [13, 14]. In addition, Hedayati et al., Amini et al., and Gomes et al. studies documented that the prevalence of maxillary lateral incisor was the most frequent hypodontia [7, 8, 15]. Such differences might be due to disparate sample population of different studies. From these results it is clear that hypodontia in posterior of mandible were significantly higher than anterior segment, contrary to the findings of Vahid Dasjerdi study [11]. As we found second premolar, the most prevalent teeth mandibular, posterior of the mandible was the most frequent segment for hypodontia.
Our results highlighted that the frequency of hypodontia was greater in females. Also, no significant correlation was obsereved between gender and hypodontia. This finding is in line with the studies of Gracco et al., Endo et al., Vahid-Dastjerdi et al., Gomes et al., Hedayati et al., and Fekonja et al. [7, 8, 11, 13, 14, 19]. Based on the type of occlusion in the present study, occlussion class II was the most frequent one in individuales with hypodontia. Our data suggested no significant correlation between hypodontia and type of occlussion. This finding is compatible with the results of Uslu et al., and Hedayati et al. studies [8, 20]. However, Bauer et al. did not find any significant correlation between craniofacial growth and congenital permanent teeth missing [21]. 
The pattern and prevalence of hypodontia varies among different races and ethnic groups. The prevalence of hypodontia/ was calculated 5.4% in the current study and no statistically significant differences were observed between males and females. The second mandibular premolar was recognized as the most frequently missing tooth. Tooth agenesis in the lower arch was more prevalent. Class II malocclusion was the most among the malocclusions, in patients with hypodontia. However, this finding was not statistically significant. Early detection of congenital missing teeth and, intervention by a a multidisciplinary team should be considered and planned in order to minimize such complications.

Ethical Considerations
Compliance with ethical guidelines
The research was approved by the Ethics Committee of the Kerman Medical University under the ethical code of IR.KMU.REC.1393.494. A signed informed consent was obtained from all patient who participated in our study.

Funding
This research was financially support by Deputy of Research and Technology of Kerman University of Medical Sciences.

Conflict of interest
Authors have no conflict of interest to declare.



References