This study was approved by the ethics committee of our hospital (no. 2018(009)). Parents of all participants who sought treatment in the Women and children’s hospital of Chongqing Medical University provided written informed consent and comprehended the study’s details.

Using convenience sampling, 210 preschool children who sought treatment in the Women and children’s hospital of Chongqing Medical University were recruited. The recommended sample size was 7 times the number of items (8 items*7 = 56), but the confirmatory factor analysis (CFA) required the sample size to be greater than 200 [14]. Thus, the final sample size was 210, which was in compliance with the standard. The exclusion criteria were toothache caused by dental caries, a history of dental trauma one month before the clinical examination and the use of orthodontic appliances in the past.

The questionnaire was applied to all preschool-aged children in the presence of their guardians before the dental examination. If the participants had any questions, the investigators could be contacted at any time. It took approximately 10 min to complete each assessment. The oral examination was performed by an experienced dentist in our hospital (CQ). WHO criteria, i.e., the decayed, missing and filled teeth (dmft) index, was adopted to evaluate caries, which were classified based on number: no caries, dmft = 0; and caries, dmft > 0 [15]. Malocclusion was assessed using previously published standards proposed by Foster et al. [16]. For clinical calibration, repeated examinations were performed on 20 preschool children after one week to confirm the intra-examiner agreement. The Kappa values of the intra-examiner agreement were 0.92 and 0.95, for caries and malocclusion, respectively.

The MIS-EC was compiled and studied by Homem et al. [13]. The scale includes 8 items in 2 domains: the Child Impact Sect. (6 items) and the Family Impact Sect. (2 items). For scoring, a Likert scale was used, with scores ranging from 0 (never) to 4 (very often). If the response is “I don’t know,” the item is not scored and considered missing data. The higher the score is, the greater the impact; the total score of the scale ranges from 0 to 32 points. To test the convergent validity, a general oral health question (“How would you evaluate the health of your child’s teeth, mouth, lips and jaws (bones of the oral cavity)?“) was added at the end of the MIS-EC. The possible answers to the general oral health question are “very good (0),“ “good (1),“ “fair (2)” “poor (3)” and “very poor (4).”

In accordance with standard guidelines [17], I translated and cross-culturally adapted the questionnaire in 5 steps. In the first step, the questionnaire was translated into Chinese by two bilingual translators with dental experience. Two drafts of the questionnaire were then generated. Both translators were proficient in Chinese and English. A second step was to translate the two Chinese versions back into English by two local dental experts who were not familiar with the MIS-EC and resolve the differences between the two versions. Two dental experts and a medical English professor evaluated the translation quality, resulting the second Chinese version. A team of experts studied the conceptual and semantic equivalence of the second draft, revised the draft, and conducted a preassessment with 30 participants. Each item of the evaluation was analyzed separately after the evaluation. We modified the translated version (MIS-EC/C) to keep the original meaning. This was done to avoid any confusion.

Statistical analysis was performed using SPSS software 25.0 and AMOS 25.0 (IBM Corp. NY; USA).A statistical significance level of 0.05 was used.

Internal consistency and test-retest reliability were used to evaluate the reliability of the MIS-EC/C. Internal consistency was evaluated by calculating Cronbach’s α value. Two weeks after completing the MIS-EC/C for the first time, 30 preschool children were randomly selected, and test-retest reliability was evaluated by calculating the intraclass correlation coefficient (ICC). Prior to completing the MIS-EC/C survey, all participants received no treatment.

In general, when the Cronbach’s α value is 0.70 or higher, the difference between the test value and the retest value is comparable. It is recommended, however, the Cronbach’s α value does not exceed 0.95 in order to avoid redundancy [14]. ICC values range from 0 to 1. In general, repeatability is better when the ICC value is high. ICCs can be divided into 4 categories: poor (< 0.50), moderate (0.50–0.75), good (0.75–0.90), and excellent (> 0.90) [18].

Cross-cultural validity, discriminant validity, and convergent validity were used to evaluate the validity of the MIS-EC/C. Confirmatory factor analysis (CFA) was used to assess cross-cultural validity. To evaluate the goodness of fit between the model and the data, we used the following parameters: the ratio of chi-square to degrees of freedom (chi-square/DF), the goodness-of-fit index (GFI), the Tucker‒Lewis index (TLI), the comparative fit index (CFI), and root mean square error of approximation (RMSEA). According to traditional standards, an acceptable model has a chi-square/DF lower than 3.0, a TLI greater than 0.90, and a RMSEA less than 0.08 [19].

The discriminant validity of the MIS-EC/C was evaluated using the Mann‒Whitney U test, and the questionnaire scores for participants with and without malocclusion were compared. The convergent validity was evaluated by calculating the correlation coefficient between the MIS-EC/C score and the general oral health question score. Spearman coefficients of 0-0.20, 0.21–0.40, 0.61–0.80, and 0.81-1.0 are broken down into weak, fair, good, and excellent correlations [14]. On the basis of previous study [13], we predicted that there was a good positive correlation between the MIS-EC/C score and the total score of overall oral health problems.