Introduction

The post-World War II generation, known as the baby boomers [1], are now entering old age and represents the largest proportion of the older adult population in many countries. In Scandinavia, the majority of this cohort has nearly complete dentitions [2,3,4], and higher expectations of function and aesthetics, compared to previous cohorts [5]. These factors are likely to increase the demand for dental health services in the older adult population.

Old age is strongly associated with multimorbidity and increased medication use [6], both of which may increase the risk of dental caries due to oral dryness [7] and reduced ability to self-care [8]. In addition, age-related changes in the oral cavity, such as gingival recession and subsequent exposure of root surfaces, result in more tooth surfaces becoming susceptible to caries [9]. Along with periodontal disease, dental caries and its sequelae are one of the main causes of tooth loss among adults [10].

Dental caries is a multifactorial disease, determined by interactions between biological, behavioural, psychosocial, and environmental factors [11]. Inadequate oral hygiene [12], and frequent sugar intake [13, 14] are well known caries risk indicators. Furthermore, low income and/or lower level of education attainment are also associated with higher levels of untreated dental caries [15,16,17,18]. In addition, demographic variables, such as male gender [4, 19, 20] and country of origin [16], have also been reported to influence caries experience. The explanation may be that sociodemographic factors influence oral health behaviour, such as oral hygiene, dietary habits, and use of dental health services [18]. Moreover, financial hardship may be a reason for not attending routine dental check-ups [21], which in turn may influence caries control and management. The hierarchical arrangement of proximal (e.g. salivary status), intermediate (e.g. oral health-related behaviour), and distal (e.g. socio-demographic) effects should be taken into account in multivariable analyses [22].

Effect size of established caries risk indicators related to sociodemographic, behavioural, and biological factors, may vary depending on the population of interest. As fluoride toothpaste was first introduced in the early 1970’s, the current young-elderly population did not have access to fluoridated toothpaste until well after their permanent teeth had erupted [23]. Furthermore, when the current young-elderly were young adults, dentists had a more operative approach to treating dental caries [24]. In Norway, the criteria for restorative treatment of dental caries changed during the 1980s, from operative treatment of enamel caries lesions to delaying operative intervention until the lesion could be clearly registered radiographically in the dentine [25]. Limited use of fluoride toothpastes and the more invasive treatment approach may have contributed to more caries, dental restorations and tooth fractures in the current young-elderly population compared to younger generations. Therefore, caries risk indicators found in studies on younger age groups should also be explored in older age groups.

In Norway, dental health services are divided into a private and public sector [26, 27]. The majority of adults receive oral care from private general dental practitioners, mainly paid by the patients. Dental treatment provided by the Public Dental Health Service is free of charge for patients aged 0–18 years, mentally disabled adults, and older adults living in an institution or receiving home nursing care.

Age 65 years is defined as the beginning of old age [28, 29]. Currently, there are few studies describing caries experience in the young-elderly population, and epidemiological studies on the caries status of older adults, including assessment of behavioural and socioeconomic risk indicators, are much needed [30]. From a public health perspective, it is important to map caries experience in the young-elderly population for planning of future dental health services for this age cohort. In addition, knowledge about caries risk indicators in this age group is essential for the planning of oral health education and prevention programmes. The aims of the present study were therefore to investigate the caries experience among 65-year-olds in Oslo, Norway, and to explore the associations between having decayed teeth and sociodemographic, behavioural, and biological factors, accounting for hierarchical relationships between these variables.

Materials and methods

Study design and setting

The present cross-sectional study was part of a larger study investigating oral diseases and conditions in 65-year-olds in Oslo, Norway (the OM65 study). As previously described (31), the participants were examined at the Research Clinic, Faculty of Dentistry, University of Oslo, between February and December 2019. The Norwegian Regional Committee for Research Ethics approved the study protocol (REK 2018/1383), and the study was performed in compliance with the tenets of the Declaration of Helsinki. All participants signed a written informed consent form. The present paper was written using the ‘strengthening the reporting of observational studies in epidemiology’ (STROBE) guidelines [32].

Participants

The inclusion criteria were being 65 years old (born in 1954), resident in Oslo, Norway, and being reachable by phone. The latter criteria was set for practical reasons in relation to booking of appointments for the examinations. There were no exclusion criteria. Assuming that the prevalences of the oral conditions of interest are 10% or more, and accounting for planned longitudinal follow-up studies with a potential total dropout rate of 70%, the calculated sample size was 450 participants. Eligible individuals were randomly selected from the Norwegian National Population Register, and invitation letters were sent out consecutively to these individuals until the calculated sample size was reached. At the end of the recruitment process we had sent out a total of 1230 invitation letters to these 65-year-olds. Within 2 weeks, all individuals were contacted by telephone, and individuals who answered were asked if they were interested in participating in the study.

Questionnaire

Prior to attending the clinical examination at the Research Clinic, individuals who agreed to participate received an electronic link to a self-administered online questionnaire (Nettskjema®, University of Oslo). The participants were asked about their gender (male or female), country of birth, educational attainment, financial capacity related to affording dental treatment, smoking habits (never, former or current), dental attendance pattern, frequency of intake of sugary food (specifically, chocolate, candy, ice cream, cakes, cookies, jam, marmalade, chocolate spread, and yoghurt) and/or drinks (specifically, hot drinks, soda, sports drink, and juice), and frequency of tooth brushing. The participants’ country of birth was dichotomised into ‘western’ (Nordic countries, Western Europe, North America, and Australia) and ‘non-western’ (the rest of the world), and the level of education was dichotomised into ‘higher education’ (university/college) and ‘basic education’ (high school, elementary school, or lower). Financial capacity related to affording dental treatment was assessed by the question “Have you experienced that dental treatment was affected by, or had to be postponed, due to the cost?” (yes or no). Furthermore, dental attendance pattern was dichotomised into ‘regular’ (regular check-ups at least every second year) and ‘irregular’ (occasionally, only emergency visits, or never). Frequency of sugary intake was dichotomised into ‘twice a week or less’ (seldom/never, 1–2 times per month, 1–2 times per week), and ‘several times a week/daily’ (3–4 times per week, daily, several times daily) [33]. Frequency of tooth brushing was dichotomised into ‘twice daily or more’ (more than once a day), and ‘less than twice daily’ (never, not every day, once a day).

Clinical and radiographic examinations

The clinical and radiographic examinations were performed by two calibrated dentists (MTD and ATTS). Both dentists were present during all the examinations and reached a joint decision in cases of doubt related to caries registrations. For calibration, seven of the patient examinations (840 tooth surfaces) were performed by both examiners. The kappa value was 0.95 (95% CI: 0.94–0.96) for inter-examiner agreement on decayed, missing and filled surfaces, indicating almost perfect agreement.

Participants were instructed to refrain from eating, drinking, and smoking for at least 1 h before the clinical examination. Prior to the clinical examination, unstimulated whole saliva (UWS) was collected using a standardised protocol. Participants were instructed to sit relaxed and swallow any saliva in their mouth. The participants were asked to avoid swallowing saliva during the collection time, and they were instructed to spit into the test cup before swallowing, if they felt the urge to swallow. After 5 min, the participants were asked to spit any remaining saliva into the test cup. Pathologically low salivary secretion rate, hyposalivation, was defined as secretion rate of ≤ 0.1 ml/min for UWS [34].

Two bitewing radiographs were then taken, using an intraoral imaging unit with a rectangular 30.5 cm collimator (MINIRAY, SOREDEX, PaloDEx Group Oy, Tuusula, Finland). Additional bitewing radiographs were taken if the distal surface of the maxillary canines or any posterior approximal surface were not visible.

All participants then underwent a thorough clinical examination in a dental chair. Primary- and secondary coronal and root caries lesions, and dental restorations were recorded at the level of the tooth surface aided by the radiographic examinations. A five-grade scale was used to grade the coronal primary caries lesions, according to the diagnostic criteria described by Amarante and co-workers and used in several Norwegian epidemiological studies [4, 13, 19, 35]. Coronal primary caries lesions in enamel were registered as either grades 1 (outer half of the enamel) or 2 (inner half of the enamel), and dentine caries lesions as grades 3, 4 or 5 (outer, middle and inner third of the dentine). Secondary caries lesions were recorded without grading and were all classified as dentine caries. Root caries lesions were graded according to the International Caries Detection and Assessment System (ICDAS) for root caries [36]. Root surfaces were recorded as either (1) sound root surface without discolouration, (2) clearly demarcated discoloured area on the root surface but with no cavitation (loss of anatomical contour < 0.5 mm), and (3) root surface with discolouration and cavitation (loss of anatomical contour ≥ 0.5 mm).

Caries experience was quantified using the DMF-index (decayed, missing, filled) in accordance with the criteria by the World Health Organization [37], combining the recordings from both the crown and root surfaces as follows: The D-component included coronal dentine caries lesions, root caries lesions with ≥ 0.5 mm cavitation, and root remnants. Root remnants that were visible less than 2 mm above the gingival margin, were registered as four decayed surfaces (DS) for anterior teeth and five decayed surfaces (DS) for premolars and molars. If a tooth had a coronal caries lesion and/or a root caries lesion on the same surface, it was counted as having only one decayed surface when calculating the total DMFS score. The M-component comprised all missing teeth, regardless of the reason for the loss. Finally, the F-component comprised all restorations on coronal and root surfaces, including direct fillings and crowns, but not fixed dental prosthesis abutments. If a tooth had a coronal and/or root restoration on the same surface, it was counted as only having one filled surface (FS) when calculating the DMFS index. Third molars were not included in the registrations.

Statistical analyses

Registrations were entered in the Oral Data Collector (ODC) sheet specifically designed for data entry in the present study. The ODC sheet was developed in Microsoft Excel 2016 (Microsoft Corporation, Redmond, Washington, USA), and processed using openpyxl 3.0.4 and pandas 1.1.0 in Python 3.8 (Python Software Foundation, https://www.python.org/) and was imported into STATA (Stata version 16.1; College Station, TX, USA) for statistical analysis. The data were securely stored and analysed within the Service for Sensitive Data (TSD) module (Centre for Information Technology Services, University of Oslo).

The outcome measures were mean number of remaining teeth, DMFT/S, and the presence of decayed teeth (DT > 0). The Mann-Whitney U test was used to determine significant differences in the distribution of caries experience related to the explanatory variables. For the further analyses, it was decided to focus on the parameter that reflected current disease, i.e. decayed teeth. Edentulous individuals (n = 2) were excluded from the bivariate and multivariable analyses where the presence of decayed teeth was the outcome. The Chi-square test was used to investigate the associations between the explanatory factors and the presence of decayed teeth. Furthermore, multivariable logistic regression analyses were used to further explore the associations between the explanatory factors and the presence of decayed teeth. Separate models for distant (sociodemographic variables, Table 1, Model 1), intermediate (behavioural variables, Table 1, model 2), and proximal biological influence (hyposalivation, Table 1, unadjusted odds ratio) are presented, accounting for hierarchical relationships between these independent variables. Finally, all explanatory variables were included in the same model (Table 1, Model 3). The results from the regression analyses are presented as unadjusted and adjusted odds ratios with a 95% confidence interval. The level of significance was set to p < 0.05.

Table 1 Logistic regression analysis: Presence of decayed teeth (DT > 0) in relation to selected explanatory variables among a sample of Norwegian older adults (n = 455)
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Results

Of the 797 eligible participants who were reached by telephone after having received the invitation letter, 460 attended the clinical and radiographic examination (response rate of 58%). Three individuals were excluded from the analyses due to missing questionnaire data, and the final sample therefore comprised 457 individuals. The sample population was characterised by an even gender distribution (52% men, 48% women). They were predominantly born in a western country (91%) and had higher education (67%) (Table 2). 8% of the participants had hyposalivation. The participants had a mean number of 25 teeth (SD: 4, range: 0–28) and 94% had more than 20 remaining teeth. Two individuals were edentulous. Ninety-nine per cent of the participants had one or more exposed root surfaces, and the mean number of exposed root surfaces was 25 (SD: 18).

Table 2 Caries experience of the study participants in relation to selected explanatory variables among a sample of Norwegian older adults (n = 457)
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Caries experience (DMFT/S)

The participant’s caries experience in relation to selected explanatory variables is presented as mean values in Table 2 and as median values in a supplementary table (Table S1). The mean DMFT for the study sample was 19.4 (SD: 4.7). The mean DT was 0.8 (SD: 1.6) and the mean MT was 2.5 (SD: 3.9). 37% of the individuals had at least one decayed tooth (DT > 0). A higher DT/DS was significantly associated with male gender, non-western origin, basic level of education, irregular dental attendance, limited financial capacity, toothbrushing less frequently than twice daily, and hyposalivation. The filled teeth (FT) component made up the largest part of the overall mean DMFT score, with an average of 16.1 filled teeth per person (SD: 5.4). A higher FT/FS was significantly associated with female gender, western origin, higher level of education, not having limited financial capacity, being a former smoker (compared to current smokers) and regular dental attendance.

Prevalence of coronal and root caries

The prevalence of enamel caries was 35%, and the prevalence of the other caries classifications was as follows: primary coronal dentine caries 12%, secondary coronal caries in dentine 33%, root caries without cavitation 17%, and root caries with cavitation 7% (Table 3). Thirteen participants (3%) had one or more root remnants (range: 1–9).

Table 3 Distribution of participants according to the presence of coronal and root caries lesions among a sample of Norwegian older adults (n = 457)
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Risk indicators for decayed teeth

The prevalence of decayed teeth (DT > 0) was 37%, and its distribution according to sociodemographic, behavioural, and biological factors is presented in Table 4. Multivariable logistic regression analyses showed that male gender and basic level of education were significant risk indicators of having decayed teeth in the model including only the sociodemographic variables (Table 1, model 1), while irregular dental attendance and toothbrushing less than twice daily were significant risk indicators in the model including only the behavioural variables (Table 1, model 2). When the sociodemographic, behavioural, and biological variables were all included in the same model (Table 1, model 3), having male gender, a basic level of education, irregular dental attendance and hyposalivation were significantly related to having one or more decayed teeth.

Table 4 Bivariate associations between the prevalence of decayed teeth (DT > 0) and selected explanatory variables among a sample of Norwegian older adults (n = 455)
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Discussion

This paper describes the caries experience in a random sample of 65-year-old urban Norwegians. In addition, associations between having decayed teeth and sociodemographic- and behavioural variables as well as hyposalivation were explored. To our knowledge, few studies have explored the association between caries prevalence and clinically measured hyposalivation in a study sample from the general population.

Caries experience

A recent review has summarized the caries experience in European citizens aged 65–74 years that has been reported in studies in the period 1996–2016 [38]. The mean DMFT scores varied among 20 countries between 14.6 and 25.8. The authors observed a trend towards lower MT scores and higher FT scores during this period. Regardless, the MT component represented the largest component of the DMFT score in almost all countries in this age group, unlike in the present study, where the FT component was the most predominant component. Restored tooth surfaces have a higher risk of developing caries than sound tooth surfaces [39]. Hence, this trend towards the retention of more teeth later in life (lower MT), and more restorative therapy (higher FT) [38], is likely to increase the need for maintenance of restored teeth in older adults as this cohort ages. The high retention of teeth in the present young-elderly in Norway compared to many other European countries, may be partly attributed to the good access to, and high utilization of dental services in Norway, where 10 years ago 80% of adults were reported to have visited the dentist within the last year [40].

The caries experience in the present study population in Oslo (DMFT 19.4) was slightly lower than in similar studies from the central (DMFT 21.0) and northern (DMFT 21.5–22.5) parts of Norway [4, 13, 19]. Although the mean number of decayed teeth in the present study (DT = 0.8) was within the range reported in the other Norwegian studies (0.2–1.5) [4, 13, 19], in the present study the mean number of missing teeth was lower (MT = 2.5 versus 6.1–9.2), and the mean number of filled teeth was higher (FT = 16.1 versus 12.4–15.2). The lower level of caries experience in the city of Oslo compared to the other Norwegian studies may be partly explained by the fact that those study samples were of somewhat older age groups (65–74 years). Furthermore, the other study samples were from more rural areas that are known to have higher levels of dental caries [13]. Moreover, there is better access to dental care in the Oslo area compared to in the central and northern parts of Norway where patients may need to travel some distance to clinics [27], which may have contributed to the lower numbers of MT and higher numbers of FT in Oslo. A study from Northern Norway showed that 32% of adults 30–59 years and 26% of adults ≥ 60 years used dental health services at least every second year [41]. These are lower proportions compared to the corresponding figure in the present study on 65-year-olds in Oslo (66%).

In comparison, the mean DFT score in the present study (16.8) was very similar to that reported in 70-year-olds in Sweden (16.6) [2]. However, the mean DMFT scores among comparable age groups in Denmark (age 65–74 y: mean DMFT = 23.1) [3] and Finland (age 65–74 y: mean DMFT = 25.6) [42] were higher than in the present study. The Danish and Finnish studies were conducted more than ten years ago, and in older age groups, and the DMFT scores in these countries may therefore be somewhat lower now.

A recent report from the United States (US) showed a mean DMFT score of 15.9 among dentate 65-74-year-olds in the resident, civilian, noninstitutionalized US population. The mean DT (0.3) and FT (9.9) were lower than in the present study, and the mean MT was higher (5.6) [43]. The prevalence of edentulism in the US study was 13% compared to 0.4% in the present study, so the inclusion of missing teeth in the edentulous population would have increased the mean DMFT score estimate to 17.4. In general, more tooth extractions and less restorative caries treatment in the US compared to the general practice in Norway, may be partly explained by less affordable dental health services in the US [44].

Prevalence of coronal and root caries

In the present study, both enamel and dentine caries lesions were recorded, but only caries lesions that had reached dentine were included in the DMFT score. Enamel caries- and root caries lesions without cavitation are therefore reported separately.

More than one of three individuals in our study had untreated dentine caries (DT > 0), which is in line with recent data from Northern Norway (35–39%) [13, 19]. During the last decade, apart from the Norwegian studies, few European studies have reported caries prevalence in this age group from a general population sample. A caries prevalence of 49% was recently reported in Polish 65-74-year-olds [45], indicating a higher caries treatment need in this age group than in Norway. Although similar figures from the US showed a caries prevalence of 15% among dentate 65–74 year-olds [43], root caries was not included in these figures. In the present study, teeth with secondary caries lesions represented the largest proportion of decayed teeth. This was not surprising considering the high proportion of filled tooth surfaces among the participants.

The overall prevalence of root caries (20%) in the present study was higher than reported in previous Norwegian studies (14–15%) [19, 20], but lower than in studies from Turkey (28%) [46] and Greece (38%) [47], from similar age groups. In the present study, exposed root surfaces were frequent, and more than one-third of participants had unstable periodontitis [48]. Gingival retractions and subsequently exposed root surfaces are preconditions for root caries and have been reported as a root caries risk indicator [49, 50]. Therefore, the periodontal status in the present study sample may indicate that one can expect an increased prevalence of root caries as this population ages further.

Risk indicators for decayed teeth

In the present study, 8% of the participants had hyposalivation with respect to unstimulated whole saliva secretion. Saliva protects against caries as it is a reservoir of tooth minerals, clears food and helps to neutralise plaque pH after eating, and contains antimicrobial components [51]. In line with a previous study [52], the present study showed an association between hyposalivation and prevalence of decayed teeth (Table 1, model 3). Furthermore, the use of multiple medications, which is increasingly common among older adults [53], is associated with hyposalivation [31, 54]. Therefore, hyposalivation and accompanying problems, such as a higher risk of developing root caries [49], may be expected to increase in this population as they get older.

Previous studies have reported associations between the presence of dental caries and male gender [4, 19, 55], lower level of education [13, 56], irregular dental attendance [13, 57], and toothbrushing less than twice daily [13]. These findings were confirmed in the present study. Logistic regression analyses showed that having male gender and a basic level of education were significantly associated with having decayed teeth, after adjustment for the other sociodemographic variables (Table 1, model 1). Furthermore, irregular dental attendance and toothbrushing less than twice daily were significantly associated with having decayed teeth after adjustment for the other behavioural variables (Table 1, model 2). The association between the sociodemographic factors and caries may be partly explained by behavioural patterns [18, 58]. In the present study, males, and individuals with only a basic level of education visited the dentist less regularly, and brushed their teeth less often than their counterparts, which may have contributed to the higher prevalence of decayed teeth in these groups. Of those who reported brushing their teeth at least twice daily in the present study (n = 386), the majority reported using fluoridated toothpaste on a daily basis (89%). It has been suggested that biological and behavioural factors serve as stronger predictors of caries than sociodemographic factors because they have a more direct effect on the development of the disease [22]. This concurs with the present study where irregular dental attendance and hyposalivation were slightly stronger risk indicators for having decayed teeth than the sociodemographic variables (Table 1, model 3). Regular dental attendance may lead to a more successful prevention of caries and also early intervention with restorative treatment when necessary, thus reducing the caries prevalence in that group.

Limitations of the study

In line with many studies that are performed in specific population groups, the response rate of 58% reflected a considerable proportion of non-attenders. This figure is comparable to previous studies with similar recruitment procedures (51–64%) [59, 60], and due to restrictions from the ethics committee, we were not permitted to ask non-attenders why they declined to participate. Therefore, to explore potential selection bias, the gender distribution and education level of the study sample were compared with the reported data on 65-year-olds living in Oslo, retrieved from Statistics Norway. Although gender distribution was similar, the proportion of participants with a higher level of education in the present study sample was higher than in the source population. Thus, selection bias may have affected the caries-related prevalence estimates. Given that the DT- and MT components were higher in those with only basic level of education, the actual prevalence of decayed and missing teeth in the present study may have been underestimated.

Conclusions

The present study indicates that while the young-elderly population in Oslo have few decayed and missing teeth, they have many restored teeth. Coronal secondary dentine caries lesions comprised the largest proportion of the total burden of decay. Irregular dental attendance and hyposalivation were the strongest risk indicators for having decayed teeth in this population. Furthermore, exposed root surfaces that are at risk of developing root caries lesions, were very common. Based on the present results, it will be important to ensure access to regular dental care and to increase the emphasis on caries preventive measures, especially for individuals with hyposalivation, in this age group.