Oral impacts mediate the association between reduced dentition and self-perceived need for complete dentures

Background: To evaluate the direct association and oral impact-mediated association between a reduced dentition and the self-perceived need for complete dentures (CD) in dentate adults. Methods: Data from the Brazilian National Oral Health Survey (2010) were analyzed. Self-perceived need for complete dentures was investigated by the question "Do you think you need to wear CD?". The hierarchical dental functional classification defined reduced dentitions: at least one tooth in each arch, 10 teeth in each arch, 12 anterior teeth, posterior occluding pairs (POPs) of premolars (≥3) and molars (≥1 bilateral). Presence of all these criteria were defined as functional dentition (FD ClassV ). Oral impacts were assessed using the Oral Impacts on Daily Performances scale. Associations were investigated using logistic regression and structural equations. Results: FD (ClassV) was associated with less self-perceived need for CD both directly (p<0.001) and mediated by oral impacts (p=0.003). Dentitions without molar POPs were associated with the outcome mediated by oral impacts (p=0.002). Individuals with 10 teeth in each arch (with or without anterior teeth and POPs) had similar frequencies of self-perceived denture needs. Individuals with <10 teeth in each arch and 12 posterior teeth did not have a higher frequency of self-perceived denture need. Conclusions: Oral impacts mediated the association between reduced dentitions and self-perceived denture needs. Clinical relevance: Individuals with tooth loss may report need for CD, even when they have dental configurations compatible with functionality. The dentist must taken into account patient-centered measures to make decision about oral health prevention and treatment.

and social aspects of oral health beyond strictly clinical and normative measures [3,4]. This approach considers oral health to be a determinant of quality of life [1] and is especially important to the investigation of reduced dentitions with an emphasis on the functional components of oral health.
Indeed, associations have been found between reduced dentitions and general quality of life [5] as well as oral health-related quality of life (OHRQoL) [3,[6][7][8]. Reduced dentitions have been assessed using different definitions based only on the number or the type, location and function of remaining teeth. There is a consensus in the literature that reduced dentitions with the preservation of the anterior teeth and posterior occlusal units are compatible with quality of life and satisfaction [3,9]. In contrast, a greater number of missing teeth is associated with a negative perception of oral health [10,11] and general health [12] as well as lower general quality of life [5] and OHRQoL [4].
In cases of tooth loss, rehabilitation with complete dentures may be perceived as a definitive solution for oral problems. Studies with a qualitative approach have found that tooth extraction and the placement of complete dentures is often perceived as the only solution when tooth loss compromises basic oral functions, such as chewing, aesthetics and speech [13]. It is possible that this perception changes depending on the different types of reduced dentition and the occurrence of oral impacts. To the best of our knowledge, however, no analytical epidemiological studies have investigated the direct association between different reduced dentitions and the self-perceived need for complete dentures or whether oral impacts mediate this association. Considering consistent evidence of the influence of socio-demographic and economic characteristics on clinical conditions and perceptions regarding oral health [14][15][16], such an analysis should consider other social determinants of health.
The results of this study could contribute to the identification of priority groups for health promotion actions and more conservative rehabilitative treatment that value the preservation of the remaining teeth throughout life, despite the occurrence of missing teeth. Such strategies could contribute to reducing the prevalence of edentulism among adults and seniors, which is a situation that affects approximately 150 million people around the world [17].
In the present investigation, we tested the hypothesis that adults with dentitions that have fewer functionality criteria in terms of esthetics and occlusion as well as more oral impacts perceive the need for complete dentures (CD) with greater frequency. Thus, the aim of this study was to evaluate the direct association and the association mediated by oral impacts between reduced dentitions and the self-perceived need for CD among dentate adults.

Sample and study design
A cross-sectional study was conducted using secondary data from the 2010 National Oral Health Survey (SBBrasil 2010) developed by the Brazilian Health Ministry in the five regions of the country [18]. The geographic division of the country into regions was determined by the Brazilian Institute of Geography and Statistics and is adopted in epidemiological studies with a national scope. These regions were part of the sampling plan with the 27 state capitals (including the Federal District), totaling 32 domains formed by 177 municipalities (the 27 capitals + 30 municipalities in each region).
The sample was obtained through a random selection of municipalities and census sectors, characterizing multi-stage cluster sampling with probability proportional to population size [19].
Detailed information on the sample planning can be found elsewhere [19,20].
The participants of the present study were dentate adults between 35 and 44 years of age who did not wear any type of fixed or removable denture. Edentulous individuals and those who wore dentures were excluded because the system adopted for the classification of reduced dentition, which was that proposed by   [21], is based on only natural remaining teeth. Moreover, this exclusion criterion was chosen considering the unavailability of information on satisfaction with one's dentures and therefore the impossibility of controlling for the effect of this variable on selfreported perceptions of denture need.

Data collection
Data collection involved oral examinations and interviews using questionnaires addressing demographic and socioeconomic characteristics as well as perceptions regarding oral health. The field teams were formed by examiners who had undergone calibration exercises (Kappa >0.65) and trained annotators [20]. The oral examinations were performed following the guidelines of the World Health Organization (WHO) manual for epidemiological studies [22], using the Decayed, Missing and Filled Teeth (DMFT) index, Community Periodontal Index and clinical attachment loss for the determination of the dental and periodontal status, respectively. The total number of teeth was determined by the number of teeth present, excluding codes 4 and 5 (missing) and 8 (unerupted) of the DMFT index. A posterior occluding pair (POP) was defined as a pair of antagonist posterior teeth on each side of the mouth, such as the pair formed by teeth 16 and 46, for example.

Response variable
The self-perceived need for CD was determined by the answer to the following question: "In your opinion, do you need complete dentures or need to exchange your current dentures?" [18], for which the response options were "yes", "no" or "does not know/did not answer". As denture wearers were excluded from the study, the response referred to the self-perceived need for complete dentures. This criterion eliminated those who reported wanting to exchange their CD.

Assessment of dentition
A dentition classification system was employed for the definition of reduced dentitions. This system consists of five hierarchical levels based on the functionality of the teeth in terms of esthetics and occlusion [21,23]. A functional dentition (FD ClassV ) [21,23] was recorded when the dentition had all five of the following levels: Level I -at least one natural tooth in the maxilla and mandible; Level IIat least 10 teeth in each arch to enable nine to 10 pairs of opposing teeth; Level III -all 12 anterior teeth present; Level IV -at least three pre-molar POPs; and Level V -at least one molar POP bilaterally. Based on this system, the following categories were defined: a) 10 teeth in each arch, <12 anterior teeth, with or without ≥3 premolar POPs, with or without ≥1 molar POP bilaterally (reference category) b) Functional dentition (FD ClassV ) c) 10 teeth in each arch, 12 anterior teeth, ≥3 premolar POPs, without ≥1 molar POP bilaterally d) 10 teeth in each arch, 12 anterior teeth, without ≥3 premolar POPs, with or without ≥1 molar POP bilaterally e) Less than 10 teeth in each arch, 12 anterior teeth, with or without ≥3 premolar POPs, with or without ≥1 molar POP bilaterally f) Less than 10 teeth in each arch, <12 anterior teeth, ≥3 premolar POPs, with or without ≥1 molar POP bilaterally g) Less than 10 teeth in each arch, <12 anterior teeth, without ≥3 premolar POPs, ≥1 molar POP bilaterally h) Less than 10 teeth in each arch, <12 anterior teeth, without ≥3 premolar POPs, without ≥1 molar POP bilaterally i) Less than one tooth in each arch The reference category was adopted based on the concept of "20 well-distributed teeth", with at least 10 teeth in each arch, which is believed to ensure minimum functionality. This category is also based only on the number of teeth, without considering tooth type and occlusal functions and was used for the purposes of comparison with other dentitions considering all or some of the functionality criteria defined by NGUYEN et al. (2011) [21]. This comparison enables the identification of specific criteria from which an association with a self-perceived need for CD may be found.

Assessment of oral impacts
Oral impacts were assessed using nine items of the Oral Impacts on Daily Performance (OIDP) scale, which is widely used for the assessment of OHRQoL [24]. The OIDP addresses problems caused by the teeth in the previous six months regarding the following aspects of daily living: eating, cleaning the teeth, affected mood, having fun, practicing physical activities, speaking, smiling without embarrassment, working and sleeping. Each item has dichotomous response options (0: absence of impact; 1: presence of impact). Oral impact was recorded for any individual who reported that problems with the teeth affected one or more aspects of daily living (OIDP >1).

Covariables
The covariables were demographic characteristics (sex and self-declared skin color [white, black, brown, yellow/indigenous]), socioeconomic characteristics (income and schooling), perceptions regarding oral health and symptoms (satisfaction with teeth/mouth and reports of dental pain in the previous six months) and clinical variables (gingival bleeding, calculus, shallow periodontal pocket [4-5 mm] and deep pocket [6 mm]). Income was the sum of monthly family earnings determined in Brazilian currency and converted into American dollars (mean exchange rate in 2010: R$1.76 = US$1.00) in the following categories: ≤ US $ 284, US $ 285 to US $ 852, US $ 853 to US $ 2557 and > US $ 2557). Schooling was categorized based on complete years of study (≤ four years, five to eight years, nine to 11 years, and 12 or more years). Satisfaction with the teeth/mouth was assessed using the following question: "Regarding your teeth, would you say you are very satisfied, satisfied, neither satisfied nor dissatisfied, dissatisfied or very dissatisfied?" [14] The response options were grouped into satisfied (very satisfied + satisfied) and dissatisfied (neither satisfied nor dissatisfied + dissatisfied + very dissatisfied).

Statistical analysis
Descriptive analysis was performed to characterize the sample and obtain the frequency of adults according to the categories of the variables investigated. The chi-square test was used to compare denture wearers and non-wearers. Crude and adjusted logistic regression models were then used to estimate the association between dentition and the self-perceived need for CD. The variables maintained in the final multiple model were those for which the association with the response variable was statistically significant (p ≤0.05) or that contributed to the fit of the model and had importance recognized in the literature (sex and schooling). The goodness of fit of the model was evaluated using the Hosmer-Lemeshow test. Variables that remained in the final model were used in a structural equation model to evaluate direct and indirect associations between a reduced dentition and the selfperceived need for complete dentures.
Structural equation modeling consists of a measurement model that establishes how latent constructs are measured and a structural model used to analyze associations between variables. In the present study, the latent variable (represented by a circle) was obtained through the nine OIDP items using confirmatory factor analysis. The parameters were estimated using weighted least squares and variance estimates. We estimated the total effects, which are composed of both direct effects (a direct path from one variable to another [e.g., dentition → self-perceived denture need]) and indirect effects (path mediated by other variables [e.g., income → self-perceived denture need via oral impacts]). The estimates of the parameters of direct and indirect associations and 95% confidence intervals (CI) were determined using the bootstrapping method with 1500 iterations. The goodness of fit of the model was evaluated using the root mean square error of approximation (RMSEA), the comparative fit index (CFI) and the goodness-of-fit index (GFI). RMSEA <0.05 indicates a strong fit, 0.5 to 0.08 indicates a reasonable fit and >0.1 indicates an inadequate fit. CFI and GFI of 1.0 indicate a complete fit of the model and CFI and GFI >0.95 indicate a good fit [25,26].
All analyses were performed using Stata ®

Results
The total sample was composed of 9547 adults, 24 of whom were completely edentulous and were excluded. Among the dentate individuals, 6083 (63.7%) did not use fixed, removable or complete dentures and composed the final sample of the present study. The excluded group had significantly more women (p = 0.000), individuals with brown skin color (p = 0.004), income up to R$500 (p = 0.000) and oral impacts (OIDP ≥1) (p = 0.000). The majority of adults did not perceive a need for CD (72.2%). The characterization of the sample is displayed in Table 1. Individuals with a functional dentition (FD ClassV ), those with oral impacts and those who were dissatisfied with their teeth/mouth comprised the majority of the sample (58.8%, 53.3% and 56.8%, respectively) ( Table 1).
The additional file 1 displays the proportion of the self-perceived need for CD as well as the mean number of teeth and POPs according to the levels of the dental functional classification system.
Among the entire sample, 6028 adults (99%) had at least one tooth in each arch (Level I). The selfperceived need for CD was higher among those with 10 teeth in each arch and the absence of all anterior teeth (36.1%). Among those with the complete anterior sextant, 23.4% perceived a need for CD. The proportion of self-perceived need for CD was higher among individuals who did not successively meet the criteria of the dental functional classification system (right side of additional file 1). Individuals with a functional dentition (FD ClassV ) had an average of 28.9 teeth and 7.6 POPs.
Those with a dentition that did not meet the criteria for Levels II to V had an average of 16 teeth and 1.2 POPs.
Note: Level I -at least 1 tooth in each arch; Level II -≥10 teeth in each arch; Level III -12 anterior teeth; Level IV -≥3 premolar POPs; Level V -≥1 molar POP bilaterally. The left side of figure corresponds to individuals who met the criteria and those on the right side did not meet the criteria.
The results of the final multiple logistic regression models demonstrated a significant association between a reduced dentition and the self-perceived need for CD in the presence of the adjustment variables (sex, skin color, income and schooling). Greater self-perceived denture need was found among those whose dentitions were missing all anterior teeth with or without the presence of POPs (Table 2). Adults with a functional dentition (FD ClassV ) had a 56% lower chance of perceiving a need for CD (OR = 0.44; 95% CI: 0.28 to 0.70) compared to those with 10 well-distributed teeth, less than 12 anterior teeth and with or without satisfactory posterior occlusal contact. The odds of the selfperceived need for CD among adults with less than one tooth in each arch was 6.29-fold higher compared to those who had 10 well-distributed teeth in each arch (OR = 6.29; 95% CI: 1.55 to 25.52).
A significant association was also found between oral impacts and the self-perceived need for CD. Individuals with OIDP ≥1 had a 107% greater chance of a self-perceived need for CD than those  Having a functional dentition (FD ClassV ) was associated a lower frequency of a self-perceived need for complete dentures directly ([SC] = -0.250; p <0.001) and via oral impacts (p = 0.003). Having a functional dentition was also directly associated with a lower OIDP [FD classV [SC] = -0.206; p <0.001) and the dentition category '10 teeth in each arch + 12 anterior teeth + ≥3 premolar POPs and without molar POPs' ([SC] = -0.149; p <0.001)]. Dentitions with missing anterior teeth and/or POPs (with the exception of the category 'less than 10 teeth in each arch, <12 anterior teeth, ≥3 premolar POPs, with or without ≥1 molar POP bilaterally') were associated with a greater frequency of a selfperceived need for CD (direct positive association). The category '10 teeth in each arch, 12 anterior teeth, ≥ 3 premolar POPs, without ≥ 1 molar POP bilaterally' was not directly associated with the outcome, but an oral impact-mediated association was found (p = 0.002). The coefficients of the latent variable OIDP ranged from 0.688 to 0.882 and a significant direct effect of oral impacts was found on the self-perceived need for CD (p = 0.042). Significant direct and indirect (via oral impacts) effects were found among some skin color and income categories, which also had significant direct effects on the OIDP (Figure 1 and Table 3).

Discussion
Individuals with less tooth loss (FD ClassV ) had fewer oral impacts and a lower frequency of selfperceived needs for complete dentures. Oral impacts mediated the association between a reduced dentition and the self-perceived need for CD. This mediation indicates that aspects of daily living related to the teeth and mouth should be addressed to gain a better understanding of the selfperceived need for CD and should be considered when developing educational actions directed at adults with the aim of preserving the teeth and their functions. This finding is consistent with previous evidence of greater satisfaction with oral health and a greater frequency of an absence of impact (OIDP = 0) among adults with a functional dentition (FD ClassV ) [3].
Oral impacts also mediated the association between a reduced dentition characterized by the absence of molar POPs bilaterally and the self-perceived need for CD. Individuals with this type of dentition did not have a greater self-perception of CD needs, but reported fewer oral impacts, which, in turn, were associated with the self-perception of the need for CD. The direct negative association between a dentition with an absence of molar POPs and OIDP is consistent with previous findings that a dentition with three premolar POPs, complete anterior region and no molar POPs is sufficient to satisfy the majority of individuals with regards to oral health [27]. Likewise, previous studies found that OHRQoL (mediated by oral impacts) did not differ between individuals with dentitions containing anterior teeth and premolars (reduced dental arch) and more complete dentitions [6,7].
The direct positive association between oral impacts and a self-perceived need for CD also reveals the importance of evaluating oral functions in person-centered care. The self-perception of denture needs by a dentate individual may be the consequence of the impact of the teeth and mouth on aspects of daily living and does not necessarily signify a need for CD defined by the clinical condition. A previous study conducted with the same sample of adults found that the measure of the self-perceived need for CD overestimated the normative need by about 30% [28], demonstrating that this is a measure of little usefulness in the estimation of normal dental needs. Moreover, the self-perceived denture need may be influenced by cultural aspects, beliefs and values regarding tooth loss, which is often seen as a natural part of aging [13]. The results of the present study show that the loss of a greater number of teeth resulting in dentitions with less than 10 teeth in each arch and no anterior teeth (even in the presence of POPs) was directly associated with a greater frequency of oral impacts and a selfperceived need for CD. Therefore, advanced tooth loss and its impacts may limit the perception of partial dentures as a rehabilitation option for the recovery of function with the preservation of natural teeth.
The logistic regression model revealed that the frequency of the self-perception of the need for CD was similar among individuals with 10 teeth in each arch, independently of the presence/absence of anterior teeth and premolar and molar POPs. This finding is consistent with the concept that 20 welldistributed teeth ensure adequate oral function by providing nine to ten occlusal pairs [27]. In a population-based sample of Vietnamese adults, the distribution of teeth based on the limit of 10 teeth in each arch included greater percentages of complete anterior regions as well as sufficient premolar and molar regions [21]. These findings suggest that the criterion based on the number of teeth is associated with a dentition that ensures the basic functions of chewing and esthetics. The lack of a significant direct association or the negative direct association between dentitions with 10 teeth in each arch and OIDP in the structural equation model lend support to this finding.
Individuals with "less than 10 teeth in each arch, 12 anterior teeth, with or without > 3 premolar POPs, with or without > 1 molar POP bilaterally" did not have a significantly greater frequency of a self-perceived need for CD compared to those with "10 teeth in each arch, < 12 anterior teeth, with or without > 3 premolar POPs, with or without > 1 molar POP bilaterally". This finding suggests that the presence of anterior teeth favors a more positive perception of the teeth even when individuals do not have 10 teeth in each arch, which may be explained by the importance attributed to esthetics and the strong associations between the anterior teeth and satisfaction with one's mouth, psychofunctional wellbeing and the absence of impact [3,[29][30][31][32]]. This may be due to the key role the anterior teeth play in the personal image of individuals [32] and their social relations [27], as the frequency of impact on smiling/speaking is lower among those who have complete anterior sextants [3].
Regarding demographic characteristics, adults who declared having brown or yellow/indigenous skin had greater impact measured by the OIDP. Individuals with brown skin also had a greater frequency of a self-perceived need for CD and this association was mediated by greater oral impact measured using the OIDP. These results may be partially explained by the greater dissatisfaction with oral health expressed by non-whites of different nationalities [15,33]. More dissatisfied individuals likely perceive impact and the need for CD more often.
In terms of socioeconomic characteristics, both the final regression model and structural equations identified that the self-perceived need for CD and oral impacts were less frequent among individuals with a higher income compared to those in a lower economic class. The association between a higher income and the lower frequency of perceived denture needs was both direct and mediated by oral impacts. Indeed, there seems to be a direct association between low income and dissatisfaction, as found among adults in northeastern Brazil [15].
These results underscore the occurrence of inequities in health, as individuals with a lower income and non-whites have greater oral impacts and a greater frequency of a self-perceived need for CD.
The identification of such inequities is coherent with the recognition that socially and economically disadvantaged groups have worse health conditions and experience a greater impact of oral problems on quality of life [34], demonstrating that the present findings are in line with the theory of social determinants of health. Moreover, worse socioeconomic conditions are associated with the irregular use of healthcare services. As a result, individuals only seek care in urgent situations involving pain and complex oral problems, which culminate in tooth loss [35]. This is due to the fact that options for maintaining one's natural teeth are costly 35 and often not available at public healthcare services. Moreover, the acceptance in different cultures, especially those in developing countries, of tooth loss as a natural part of life [13] is related to this social vulnerability. It is therefore understandable that individuals with a lower income and non-whites perceive a greater need for CD.
This study evaluated a sample of Brazilian adults based on the methods standardized by the WHO (1997) [22] but has limitations that should be considered. The cross-sectional design does not enable the establishment of causal relations or the evaluation of the temporal relation assumed in the model tested (reduced dentition → oral impacts on daily performance → self-perceived need for CD). Another limitation is the possible selection bias of the participants, as the exclusion of denture wearers led to a significant loss of the representativeness of the sample. The associations between self-perceived denture needs and skin color, income and oral impacts may have been underestimated due to the profile of the sample included in the study. Moreover, individuals with a complete dentition (35.69%) were included among those classified as having a functional dentition (FD ClassV ). Such individuals may have an even lower frequency of oral impacts and self-perceived needs for CD compared to those that met all the functionality criteria defined by the dental functional classification system employed in the present study even with the loss of one or more teeth. Thus, the association between FD (ClassV) and self-perceived denture needs may be underestimated due to this potential positive effect of a complete dentition.
The strengths of the present study include the estimates generated by the statistical analysis, which considered sample weights and the complex sampling design. We used established indices that indicated the goodness of fit of the model to the data when compared to reference values [25,26].
Moreover, the present study addressed an issue that remains under-explored in the literature, considering the association between different reduced dentitions and the self-perceived need for CD in a sample of Brazilian adults.
The present results suggest that individuals in situations of greater social disadvantage and those with greater tooth loss could benefit from educational approaches that highlight the importance of preserving their natural teeth and oral functions throughout life. Such actions should be accompanied by an increase in access to conservative treatment and rehabilitation with partial dentures. Informed consent was obtained from all individual participants included in this study.

Consent for publication
Not applicable.

Availability of data and materials
The data that support the findings of this study are available from the Brazilian Health Ministry, but restrictions apply to the availability of these data, which were used under license for the current study and are therefore not publicly available. However, data are available from the authors upon reasonable request and with permission of the Brazilian Health Ministry.

Additional file
File name: Additional file 1 Distribution of individuals according to hierarchical dental functional classification system on five levels perceived a need for CD. The proportion of self-perceived need for CD was higher among individuals who did not successively meet the criteria of the dental functional classification system (right side of additional file 1). Individuals with a functional dentition (FD ClassV ) had an average of 28.9 teeth and 7.6 POPs. Those with a dentition that did not meet the criteria for Levels II to V had an average of 16 teeth and 1.2 POPs.
Note: Level I -at least 1 tooth in each arch; Level II -≥10 teeth in each arch; Level III -12 anterior teeth; Level IV -≥3 premolar POPs; Level V -≥1 molar POP bilaterally. Left side of figure corresponds to individuals who met the criteria and those on the right side did not meet the criteria.

Competing interests
The authors RCF is a member of the editorial board of the BMC Oral Health.

Authors' contributions
FLC is the principal researcher, has made substantial contributions to: obtaining, analyzing and interpreting data; writing of the article and relevant critical review of the intellectual content; final approval of the version to be published; participated of all aspects of work. IK made substantial contributions to: interpretation of data for the study; relevant critical review of intellectual content; final approval of the version to be published; participated of all aspects of work. GACR made substantial contributions to: obtaining and interpreting data for the study; writing of the article; final approval of the version to be published; participated of all aspects of work. AAS made substantial contributions to: interpretation of data for the study; relevant critical review of intellectual content; final approval of the version to be published; participated of all aspects of work. LLFH made substantial contributions to: analysis and interpretation of data for the study; writing of the article and relevant critical review of intellectual content; final approval of the version to be published; participated of all aspects of work. RCF is the study supervisor, made substantial contributions to: data collection, analysis and interpretation; writing of the article and relevant critical review of the intellectual content; final approval of the version to be published; participated of all aspects of work.
All authors read and approved the final manuscript. *total n equal to 5,886 due to missing dependent variable self-perceived need for complete dentures (3,2%) Final model obtained from the adjustment sequence: Model 1 -reduced dentition, satisfaction, impact, pain, gingival bleeding, calculus, shallow periodontal pocket, deep pocket, sex, income, schooling and skin color; Model 2 -reduced dentition, satisfaction, impact, sex, income, schooling and skin color; Final Model -shortened dentition, impact, sex, income, schooling and skin color.

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