Failures of Sealed Molars: Three-Year Results from a Multi-Centre, Prospective Study in Public Dental Service in Norway

Occlusal surfaces of first permanent molars (FPMs) are the most susceptible sites to dental caries in the developing permanent dentition [1]. During their eruption period, biofilm accumulation is facilitated by their limited mechanical function and difficulties associated with tooth brushing [2]. Therefore, early caries control of these surfaces is of utmost importance. Where basic caries control measures including instruction in tooth brushing technique and dietary counselling have not been successful, fissure sealants (FS) may be applied as an effective measure to arrest caries progression and protect at-risk surfaces during the eruption phase given adequate moisture control. FS provide a physical barrier preventing growth of biofilm and reduce the occlusal relief, making surfaces easier to clean [2, 3]. Therapeutic FS are applied over an active lesion to arrest caries progression, whereas preventive FS are applied over sound occlusal surfaces to prevent initiation of caries. Both preventive and therapeutic FS aim at controlling progression of caries lesions soon after tooth eruption [2, 3]. FS are routinely used for caries control in children, and safety and effectiveness of FS is well documented [4, 5].

Most studies that have proved the efficacy of FS are randomized controlled trials (RCTs), under strictly controlled conditions. A Cochrane systematic review from 2017 demonstrated that FS were effective in preventing occlusal surface caries in children and adolescents; FS reduced caries by 11–51% compared to no FS at 24 months [4]. However, the effectiveness of FS depends on a long-term retention, and cost-effectiveness is reduced if FS require resealing [6]. An RCT examining FS failures demonstrated that 15% of resin-based FS were resealed after 3 years when applied on second molars among 12- to 16-year-old high-risk children [7].

Application of FS in children is considered a technique sensitive procedure [8], and it has been demonstrated that patient’s younger age, sex, and questionable occlusal surface influence the failure rate [9]. It has been speculated if FS effectiveness in real life might be lower due to higher rate of FS failures than in controlled experimental settings [9]. A 12-month follow-up study conducted in a real-life setting showed a FS failure rate of 41% [10]. The environment, time resources, clinician, and other uncontrolled factors can potentially modify the caries-preventive effect of FS in natural settings; however, the literature on the risk factors is scarce [11].

Despite decline in the prevalence of dental caries in Norway and most high-income countries, one in five Norwegian 5-year-olds still have dmfs >0 and are considered at high risk [12]. Public dental service (PDS) in Norway provides free dental care for all children, and Norwegian guidelines recommend FS as a first choice for control of occlusal caries in permanent molars of children at high and moderate caries risk, in addition to individual caries control measures, including instruction in tooth brushing technique and dietary counselling. FS, preferably resin-based, should be placed as soon as possible after eruption, and follow-up of sealed molars at each appointment is recommended [13].

Evaluation of success and retention rates of FS as dental public health programs have received attention for several decades [14]. However, there is scarce evidence from prospective pragmatic studies of success rates of sealed molars. Therefore, the aim of this study was to prospectively examine the rate of failures of sealed FPMs and associated risk factors in a practice-based setting in PDS in Norway.

The present study was based on a secondary data analysis of a practice-based clinical split-mouth trial (ClinicalTrials.gov ID: NCT03315312) comparing the effectiveness of fluoride varnish (FV) and resin-based FS when applied on FPMs with sound occlusal surfaces or initial caries lesions (referring to both active and inactive non-cavitated lesions) in 6- to 10-year-old children at high caries risk [15]. This study was based on prospective follow-up of FPMs that received FS. The participants were recruited during routine examinations at nine PDS clinics by dentists and dental hygienists. The sealant applications were performed from February 2017 to November 2019, and a 3-year follow-up was completed in November 2022. Prior to the study, clinicians received information and training in the study protocol and diagnostic criteria for baseline caries assessment.

The detailed, validated criteria for caries registration are nationally adopted by dental education institutions and PDS and were as follows: grade 1: caries characterized by white or brown discolouring without substance loss. No radiographic findings; grade 2: little substance loss with break in the enamel surface or discolouring fissure with grey/opaque adjacent enamel and/or caries restricted to enamel radiographically; grade 3: moderate substance loss and/or caries in the external 1/3 of the dentine radiographically; grade 4: considerable substance loss and/or caries in the middle 1/3 of the dentine radiographically; grade 5: great substance loss and/or caries in the internal 1/3 of the dentine radiographically [16, 17]. In addition, written instructions for participant recruitment and follow-up, and a pictorial illustration guide with caries registration criteria were provided to the participating clinics. Dentists and dental hygienists were instructed to use FV and resin-based FS materials available at their clinics and to follow their conventional procedures for applying and maintaining them [15]. High-risk children (having dental caries experience at the time of enrolment (D₃MFT/d₃mft >0) with two fully erupted FPMs in the same jaw, with sound occlusal surfaces or initial carious lesions (caries grade 1–2), and adequate moisture control were included in the study [16].

Background characteristics including sociodemographic characteristics (birth date, sex, clinic’s county) and baseline oral health status (caries experience at age 5, plaque level) were recorded during the recruitment. Participants’ caries experience at age 5 was retrieved from the electronic dental record form as a sum of decayed, missing, and filled teeth (d₃mft).

Oral health-related behaviours were sugar-containing diet and tooth brushing collected at baseline. Diet was covered by three food/drink item groups, namely, sugar-containing drinks, pastry, and sweets (0 – “less often than once per week”, 1 – “once per week”, 2 – “several times per week”, 3 – “several times per day”). Tooth brushing was covered by two questions, self-brushing frequency and if parents help brushing (0 – “never,” 1 – “sometimes,” 2 – “once a day,” 3 – “twice a day”). For statistical analyses, sugar-containing diet score was calculated by summing up frequency consumption scores of all three sugar-containing items. It was used as a continuous variable ranging between 0 and 7, with a higher score indicating higher sugar-containing diet consumption frequency. Tooth brushing score was calculated in the same way, adding frequency of self-brushing score and parents’ help score. In statistical analysis, it was used also as a continuous variable ranging from 2 to 6, with a higher score indicating more favourable tooth brushing behaviour. Sealed FPM characteristics, i.e., tooth type (upper, lower) and baseline occlusal surface status, were recorded during the recruitment.

FS was applied at baseline and maintained/reapplied if considered necessary by the clinician. The frequency of FS additionally treated with FV application during 6-, 12-, and 24-month follow-up was calculated. In statistical analyses, it was used as continuous variable and for Kaplan-Meier survival curves as binary (FV application vs. no FV application). Clinician’s identification code was recorded at 6, 12, 24, and 36 months, and information on whether the child was seen by the same clinician throughout the study period was dichotomized as “same clinician” versus “different clinician.”

The outcome was occlusal surface status, computed based on a combination of occlusal surface/FS status and the following treatment at each follow-up (Fig. 1). The outcome was categorized into minor failure (FS defect or initial caries that were not treated, i.e., lesions that did not receive chairside preventive measures and were left for monitoring by clinicians without reapplying FS or FV application), failure (FPM occlusal surface was resealed, developed dentine caries, or was restored), or success. Minor failure category was treated as success in the main outcome in the primary analysis.

Inter-examiner reliability of the clinicians was assessed using 24 clinical intraoral images at two time points, in the beginning of the study (spring 2019, n = 40) and repeated later (autumn 2021, n = 25). Weighted kappa values for inter-examiner reliability for sound, initial caries, dentine caries, and sealed surfaces were 0.62 (CI: 0.58–0.66) for the first and 0.66 (CI: 0.63–0.69) for the second calibration, respectively, which were considered as substantial [18].

Sample size was calculated for the main study. For this, the calculation was performed to detect clinically significant difference of 10% between FV and FS. The calculations were based on the assumptions that 80% of included teeth will not develop dentine caries and that the between-methods correlation is 0.3. Based on the estimation, 180 participants were needed at the final follow-up for the split-mouth trial, with 80% of power and 5% of significance. To compensate for a potential dropout between 20 and 25% over the 3-year follow-up, enrolment of a total of 400 participants was needed.

SPSS version 25 (IBM, Armonk, NY, USA) was used for statistical analyses. Mean and standard deviation (SD) and median and interquartile range (IQR) were calculated for continuous variables and frequencies for categorical variables. χ², Fisher’s exact, and likelihood ratio tests for categorical and Mann-Whitney U, independent sample t test, one-way ANOVA, and Kruskal-Wallis tests were used for continuous variables.

As a primary analysis to investigate potential factors contributing to the main outcome (failure or success), we performed univariable and multivariable Cox survival analysis. Cases with missing data were excluded. Multivariable model included factors which resulted in p < 0.2 in univariable analysis [19]. Sensitivity analysis included all factors as independent variables in multivariable Cox survival analysis (online suppl. Table 2; for all online suppl. material, see https://doi.org/10.1159/000544068). Kaplan-Meier survival curves visualized failures of sealed FPM stratified by factors that were significant in multivariable Cox survival analysis (FPM occlusal surface baselines status, FV over FS, and clinic’s county). The level of significance was set at p < 0.05, and crude hazard ratios (crude HR) and adjusted HR (adHR) are presented with their 95% confidence intervals (CIs). The manuscript was prepared in accordance with the STROBE guidelines.

Out of 409 participants recruited, 4% (16) were lost during the follow-up. Thus, this study included 393 participants, 48% girls, mean age 7.7 (SD = 1.1) years at the time of recruitment. Participants and dropouts were similar in all characteristics, except that dropouts were older (Table 1).

Out of 393 sealed FPMs, 19% (75) were evaluated as failure, 72% (284) as success, and 9% (34) as minor failure during the study period (Table 2). Of 75 FPMs that failed, 13% (51) received FS reapplication and 6% developed dentine caries or were restored. Of FPMs that had initial caries lesions at baseline, 8 (11%) developed dentine caries lesions or were restored compared to 14 (4%) of FPMs that were sound at baseline. A higher rate of minor failures was observed at 36-month follow-up (chi-square test 13.873, df 6, p 0.031) (online suppl. Table 1).

One third (121, 30%) of FPMs received additional FV application over FS and the proportion of FPMs that received FV over FS increased at each follow-up time (chi-square test 14.750, df 3, p 0.002) (online suppl. Table 1). Two thirds (245, 62%) of sealed FPMs were followed by different clinicians during the study period (Table 1). Kaplan-Meier survival curves demonstrated that FPMs with initial caries on occlusal surface, those that did not received additional FV application over FS and those that were sealed in Ostfold county had statistically significantly lower survival (Fig. 2a–c).

According to the multivariable Cox survival analysis, caries grade 1–2 at occlusal FPM surface at baseline doubled hazards for sealed FPM failure (adHR 1.9, 95% CI: 1.1–3.1) (Table 3). Each additional FV application over FS decreased hazards for failure by 60% (adHR 0.4, 95% CI: 0.3–0.7). Hedmark and Oppland counties (vs. Ostfold) had lower hazards for failure (adHR: 0.2, 95% CI: 0.1–0.5 and adHR: 0.6, 95% CI: 0.4–0.9, respectively). Sensitivity analysis showed similar estimates (online suppl. Table 2).

The present study investigated failures of sealed FPMs and risk factors during a 3-year follow-up in a practice-based PDS setting following conventional routines. At the end of the study period, nearly one in five sealed FPMs had failed; intact occlusal surface at baseline and FV application over FS reduced risk for the failure. In addition, regional differences for risk of failure were observed.

In our study, the failure rate over 3 years was 19% including FS reapplication and dentine caries or restoration. All FS were applied in children at high caries risk and previous studies demonstrated that sealed teeth failed more often in high-risk children [20, 21]. In the present study, over 3 years of follow-up, 13% of sealed FPMs received FS reapplication. Similarly, a study performed in one private clinic under strict protocol found that 17% of sealed FPM were resealed after 36 months in higher caries risk 6- to 8-year-olds [20, 21]. Our findings are also in line with a Nordic RCT demonstrating 15% reseal rate over 3 years in high-risk second permanent molars [7].

In our study, 6% of FPMs developed dentine caries or were restored. Double higher rates (14%) were demonstrated by a prospective study following a strict protocol [20, 21], while double lower rates of restorations in sealed FPMs (3%) after 24 and 36 months were found in other two older real-life setting studies from North America [14, 22]. The discrepancy in the results could be explained by the fact that the latter studies were retrospective and based on records, thus potentially more prone to bias.

A recent prospective study with 24 months of follow-up evaluated the effectiveness of resin-based FS on sound versus initial caries lesions of FPMs and found 9% of caries lesions on occlusal surfaces that were sound at baseline (ICDAS 0) and 13% on carious surfaces coded ICDAS 1–3 [23]. In our study, similar failure rate of FPMs that had initial lesions at baseline were observed.

The effectiveness of FS is related to their retention [4], and an initial caries lesion under a tight FS is unlikely to progress [24]. However, in the present study, initial caries lesions on occlusal surfaces increased risk for FS failure. This finding is in line with other previous studies where initial caries lesions increased risk for dentine caries lesion development and FS reapplication [25, 26]. The long-term success of sealants placed over initial occlusal caries lesions has been questioned in earlier studies, due to alterations in enamel structure and reduced bonding conditions, which may increase the risk of microleakage [26]. Sealant microleakage has been shown to be higher in questionable carious occlusal surfaces than in sound occlusal surfaces [27]. Another possible explanation for increased risk of failures in FPMs with initial lesions might be that the causes of the caries disease on individual level were not successfully managed. Nevertheless, this result calls for closer FS monitoring, especially when applied over initial caries lesions.

The current study showed that FS receiving FV applications at least once during follow-up had lower hazards for failures. This finding is in line with another study that compared FV over FS versus FV and demonstrated clinically significantly higher effectiveness of FV over FS at 2-year follow-up [28]. The results suggested that the effect of FV over FS could be an additional factor contributing to higher effectiveness of FS. These findings draw attention to the potential need to reevaluate the cost-effectiveness of FS, FV over FS and FV applications.

FPMs that were sealed in Hedmark and Oppland counties (vs. Ostfold) had lower hazard for failure. It might be speculated that FS are more likely subject to failures when applied by clinicians who use the technique less often. Practice variation in FS application routines between the three counties have been previously documented; a significantly lower proportion of clinicians in Ostfold county reported frequent use of sealants compared to the other two counties which could have influenced failure rates of sealed FPM [8].

The strengths of this study were the prospective longitudinal design, the rather long follow-up period, the low dropout rates, and multiple centres in PDS. Moreover, it was implemented in a practice-based setting, where clinicians followed their conventional procedures for applying and maintaining FS, thus for allowing broader external validity of the results. Given the pragmatic nature of the study, calibration was conducted using intraoral images. The large number of participating clinicians made patient examinations impractical due to logistical challenges (geographical area). Furthermore, the second calibration was carried out during pandemic restrictions, making in-person calibration on patients unfeasible. However, it has been shown that good quality photographs alone may be used for training and calibration among challenging populations or settings without adversely affecting data quality [29].

The study was based on a secondary analysis of practice-based clinical trial data, which was not purposely collected to test the present research question and may be considered as limitation. On the other hand, using existing data allowed exploring our research questions rapidly and saving resources for additional data collection. However, our analysis may have been limited by data availability, as FS application techniques were not investigated. Misclassification bias cannot be ruled out, as the size or severity of defected FS was not recorded, but if clinician did not reseal it, we judged it as a minor failure and consequently as success in the main analysis. It may introduce bias. Bias could have been introduced also because in one third of the children the same dentists who performed the treatment were those who evaluated it during the study period. We have used survival but not interval-censored survival analysis. On the other hand, COVID-19 pandemic restrictions caused some deviations regarding the follow-up periods; thus, time to event varied.

The findings from the present study indicate a need for closer monitoring of FS applied in FPMs over initial caries. Moreover, the cost-effectiveness evaluation of FS versus FV over FS versus FV after more than 24 months separate for intact and surfaces with initial caries lesions is warranted.

After 3-year follow-up, nearly one in five sealed FPMs failed, i.e., they had to be resealed, developed dentine caries, or were restored. Initial caries lesion on occlusal surface increased while FV application over FS reduced risk for failure. Moreover, regional differences in sealed FPM failures were observed.

We would like to thank clinical personnel at Fredrikstad, Gjøvik, Halden, Kongsvinger, Lena, Lillehammer, Raufoss, Valdres public dental service clinics and the participants for their contribution to the study, and Nina J. Wang for her contribution during study planning phase.

This study was performed in compliance with Good Clinical Practice and the Declaration of Helsinki. Approval was obtained from the Regional Committee for Medical Research Ethics South East Norway (Approval No. 2016/2002/REK sør-øst). Participation was voluntary and based on a parental signed written informed consent form.

The authors have no conflicts of interest to declare.

None.

Marte-Mari Uhlen-Strand, Ingrid Volden Klepaker, and Rasa Skudutyte-Rysstad made substantial contributions to the conception of the current work. Lina Stangvaltaite-Mouhat and Rasa Skudutyte-Rysstad contributed to the design of the study. Lina Stangvaltaite-Mouhat performed statistical analyses and together with Rasa Skudutyte-Rysstad and Marte-Mari Uhlen-Strand interpreted them. Lina Stangvaltaite-Mouhat drafted the manuscript, and Marte-Mari Uhlen-Strand, Ingrid Volden Klepaker, and Rasa Skudutyte-Rysstad revised the manuscript. All authors approved the final version of the manuscript and agreed to be personally accountable for their contribution and to ensure that questions related to the accuracy or integrity of any part of the work, even parts in which the authors were not personally involved, are appropriately investigated, resolved, and the resolution documented in the literature.

The data that support the findings of this study are not publicly available due to containing information that could compromise the privacy of research participants but are available from the corresponding author (R.S.R.).