In Chile, the National Health Fund (FONASA) is responsible for providing health care to those 70% of the Chilean population who do not have a private insurance [19-21]. The health care needs of the most affluent 30% of the population are covered by the private insurance system [22]. According to the Chilean legislation, the beneficiaries of FONASA have the right to receive primary dental care free of charge. This dental care includes dental examinations, common intraoral radiographs, emergency treatments, extractions, dental fillings, supragingival scaling and polishing, sealants in permanent molars, topical fluoride, pulpotomies, and endodontic treatment in permanent teeth. Most adolescents are therefore covered by the public oral health-care system.

Second-stage sampling: The size of the schools varied noticeably and a second sampling stage was applied. For schools with few students or fewer than 4 classes, all classes were included in the study. In larger schools, where the number of students in the last four grades was >100 and the number of classes was >3, three classes were randomly selected [23,26]. A total of 310 classes were finally selected and included in the study.

Participation approval was obtained from the headmasters of each selected high school, and informed consent was obtained from the parents of the students. Students were informed about their right to withdraw from the study at any point in time.

A total of 9,203 students aged 12–21 years present in the selected classes were invited to participate and were offered a toothbrush for their participation. All students accepted to fill a brief questionnaire on oral health-related behaviors and conditions [23], while 40 students refused to participate in the clinical examinations. The questionnaire included information on tooth brushing frequency (How often do you brush your teeth? Less than once a day, once a day, more than once a day), smoking habits (Do you smoke cigarettes? No; Yes, sometimes; Yes, daily), their last dental visit (When was the last time you visited a dentist? Less than 6 months ago; 6 to 12 months ago; more than a year ago; never seen a dentist) and the reason for the last dental visit. Why did you visit the dentist?) [23]. The students who received a clinical examination also filled an additional questionnaire on several dimensions of their socioeconomic position. A full description of the questionnaire variables can be found in previous publications [23,27]. Previous analyses showed that some of these indicators were associated with several poor oral health outcomes, including tooth loss, periodontal attachment loss and necrotizing ulcerative gingival lesions [27,28] and these were therefore used in the present analysis. These socioeconomic indicators include the monthly paternal income in thousands of Chilean pesos (no income; <$100; $100–$299; $300–$499; $500–$999; ≥$1000); and the level of education achieved by each of the parents (no education; incomplete primary school; primary school completed; incomplete high school; high school completed; incomplete technical education; technical education completed; incomplete university education; university education completed). The headmasters of the participating schools provided information on the monthly tuition and the annual school fees and this information was used to derive a single 'annual education expense' variable, which was used as an indicator of wealth. Three dichotomous outcome variables were considered. DENTVIS was coded 0 if the student reported to have visited a dentist within the last year, and 1 if this was not the case. SYMPTOM was coded 1 if the student reported that the last dental visits had been due to symptoms, such as pain, bleeding and infection, and coded 0 if the visit was not prompted by the presence of symptoms. NEVER was coded 1 if the student reported to never have visited a dentist, and coded 0 if this was not the case.

Univariable logistic regression analyses were carried out for the three outcomes investigated and the covariates gender; tooth brushing frequency; smoking habits; self-perceived oral health status, and each of the socioeconomic indicators investigated. Variables showing a P-value < 0.25 in the univariable analyses were selected to be included as covariates in age-adjusted multivariable logistic regression analyses for each outcome. The models were built by the consecutive exclusion of one variable from each full model using the likelihood ratio test as described by Hosmer and Lemeshow [29], refitting and verifying the stability of the model after each deletion.

In order to use this modeling approach, only subjects for whom complete data were available for all variables in all models were included in the analyses. At most this led to the exclusion of 11% of the total study population.

The logistic regression analyses showed that students who had visited a dentist more than a year ago were more likely to be boys (OR = 1.3); to attend a relatively poor high school (OR = 1.4); to have a father without income (OR = 1.8); to have a father (OR = 1.2), or a mother (OR = 1.5) who only had achieved primary school education; just as they were more likely to report a poor oral health status (OR = 2.0), Table ​Table22.

Students who had never attended a dentist were more likely to have a relatively lower socioeconomic position than those who had visited a dentist at least once, and this finding was consistent whether socioeconomic position was assessed using paternal income, parental achieved education or annual education expenses, just as they were more likely to report that they brush their teeth only once a day (OR = 1.7) or less than once a day (OR = 1.9), (Table ​(Table22).

The use of self-reported social indicators and self-reported data on dental visits in this study could be seen as a weakness. However, previous studies have shown that adolescents are able to supply valid and reliable information on their parents' socioeconomic position [32-34]; and a study conducted among adults indicate that self-reported data on dental care habits are accurate [35].

The health care needs of socioeconomically deprived Chilean adolescents are thought to be covered by the public system [19-21], and it may seem surprising that dental attendance patterns are strongly associated with social position even in a population that is offered public dental care free of charge for those in the lower end of the social hierarchy. However, this may in part reflect the effect of additional factors such as negative beliefs about dentists [36]; lack of knowledge, and cultural and parental values about the importance of oral health [9,37]; limited access to transportation to and from the health care centers; and the extent to which these health care services are really available. Hence, there are many observations of the public health care system being unable to cope with the current demands for treatment.

Considerably more adolescents in this study population have never visited a dentist or have attended more than one year ago than what has been reported previously for a US American adolescent population [13]. The percentage of subjects who reported their last dental visit being due to symptoms appears to be comparable with that reported for US American and Scottish adolescent populations [9,13].

Our results corroborate previous reports on the role of social inequalities in dental attendance patterns among adolescents [4,9-11,13]. This result strengthens the idea that free access to health care does not guarantee dental care provision among the young, the findings of a study revealing significant ethnic disparities in the utilization of dental services among US American children and adolescents even among Medicaid-eligible subjects points to the same direction [8].

Girls were more likely to visit frequently the dentist than did boys and were more likely to consult the dentist because of symptoms. These results, though not new [8-13], raise questions in connection with the results of previous analyses showing that girls have fewer remaining teeth than do boys in this study population [28]. A plausible explanation for this finding is that visiting a dentist with symptoms increases the probability of tooth extractions because "the reason for seeking dental treatment care influences the treatment likely to be received" [38]. This agrees with the results of a previous study showing that children and young adolescents who consulted a dentist because of symptoms were likely to have more teeth missing in later adolescence than were asymptomatic dental attendants, despite a similar number of decayed or filled teeth for both groups at baseline [39].