Introduction
Dental abnormalities are a broad range of disorders in the number, morphology, position, size, structure, and eruption of the teeth which require special attention for their treatment, since they can result in aesthetic and functional problems that can interfere with the treatment outcomes particularly in orthodontic patients [1, 2]. Co-occurrence of these dental and craniofacial abnormalities can further complicate the treatment procedures [3]. Although various studies have assessed the overall prevalence of dental abnormalities [1, 4, 5], there is scant research on the relationship between some types of skeletal malocclusion and dental abnormalities [6]. The current study aims to compare the prevalence of dental abnormalities among patients with skeletal malocclusion class I, II, and III in northern Iran. We also assed the prevalence of dental abnormalities based on the patients’ age and gender.
Methods
Participants were 310 patients aged 12-40 years attending the orthodontic clinic of the School of Dentistry, Guilan University of Medical Sciences during 2017-2019. Inclusion criteria were the availability of full dental records including panoramic radiographs and lateral cephalograms, diagnostic casts, intra-oral images, and detailed dental clinical history especially about tooth extraction, prosthodontic rehabilitation, and dentoalveolar trauma. Patients with cleft lip/palate, craniofacial syndromes, systemic or metabolic disorders, and a history of orthodontic treatment were excluded. 
Patients’ skeletal classes were characterized by measuring the sagittal inter-maxillary angle (ANB) according to Steiner [7]. Dental abnormalities were diagnosed mainly through evaluation of the pre-treatment panoramic radiographs. Two oral & maxillofacial radiologists and one orthodontist who were totally unaware of the patients’ skeletal classes performed the assessments. Pearson’s chi-square test, Fisher’s exact test, and independent t-test were applied for comparing the prevalence of dental abnormalities in patients with different malocclusion classes, gender, and ages. P< 0.05 was statistically significant.
Results
A total of 310 patients (121 males and 189 females) participated. 102(32.9%) had class I, 105(33.9%) had class II, and 103(33.2%) had class III skeletal malocclusion. Mean age of the patients was 18.41±7.54 years. It was found that 102 patients had at least one dental abnormality, and the overall prevalence rate was 33%. Although there was no statistically significant difference (P=0.105), the overall prevalence was higher in the class III group (40.8%). Table 1 presents the frequency and percentage of each dental abnormality for three malocclusion groups.


The only abnormality that showed a statistically significant difference among the groups was the impacted tooth (P<0.001), where the class III and II groups had the highest and lowest rates, respectively. In assessment of dental abnormalities in terms of gender, it was reported that hypodontia was significantly more prevalent in males (P=0.040), which was 1.88 times more than in females. Subjects with impacted tooth were older (P=0.004), whereas those with microdontia were found to be younger (P=0.002).
Discussion
The overall prevalence of dental abnormalities in northern Iran regardless of the skeletal classes of malocclusion was 33%. Different prevalence rates of dental abnormalities have been reported in different studies, ranging from 15.7 to 40.8%. [4, 5]. These discrepancies are mainly attributed to differences in ethnicity, sample size, defined criteria, evaluation method, and type of abnormalities assessed in each study. In our study, prevalence of dental abnormalities was higher in the class III group (40.8%), although the difference was not statistically significant. Basdra et al. [6] and Fernandez et al. [8] also reported the highest prevalence of abnormalities in the class III individuals. 
Regarding the type of dental abnormalities, impacted tooth was found to be the only abnormality with significant difference among the malocclusion classes. Class III patients had the highest prevalence of impacted tooth (23.3%), followed by class I (15.68%) and class II groups (4.76%). By excluding the third molar teeth, Uslu et al. [9] and Pedreira et al. [10] found that dental impaction occurs significantly more in class I and III malocclusions and maxillary canines are mostly affected. Therefore, the two classes of malocclusion (I and III) might differ in terms of impacted third molar teeth. In other words, impaction of third molars might be more associated with the class III malocclusion which was observed in our study.
Comparison of dental abnormalities by gender revealed that hypodontia was more prevalent in males. Fernandez et al. also reported that impaction and fusion were more frequent in males [8]. Furthermore, we observed that impaction and microdontia were significantly associated with age. The former was more frequent in older patients, while the latter was more in younger patients. An explanation to this finding is that dental impaction is more diagnosed at higher ages. Furthermore, people at higher ages usually seek restorative treatments for microdontia due to aesthetic concerns; therefore, this anomaly was more frequent in younger patients.
In conclusion, dental abnormalities, including impacted tooth, are higher in patients with the skeletal class III malocclusion living in northern Iran. Dental practitioners should be aware of dental abnormalities especially in patients with skeletal class III malocclusion. 

Ethical Considerations
Compliance with ethical guidelines
This study was approved by the Research Ethics Committee of Guilan University of Medical Sciences (Code: IR.GUMS.REC.1398.511). Ethical guidelines were carefully followed during the study.

Funding
This research did not receive any grant from funding agencies in the public, commercial, or non-profit sectors. 

Authors' contributions
Study concept and design: Negar Khosravifard, Faegheh Gholinia; Acquisition, analysis, or interpretation of data: Negar Khosravifard, Farzaneh Ostovarrad, Zahra Dalili Kajan, Hamideh Mohammadi, Parmida Farzam; Drafting of the manuscript: Negar Khosravifard; Critical revision of the manuscript for important intellectual content: Faegheh Gholinia, Farzaneh Ostovarrad, Zahra Dalili Kajan.

Conflicts of interest
The authors declare that there is no conflict of interest.

Acknowledgements
The authors would like to thank Mohammad Ebrahim Ghaffari (Department of Biostatistics, Dental Sciences Research Center, Guilan University of Medical Sciences) for the statistical consults.


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