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Does Psychological Stress Mediate Social Deprivation in Tooth Loss?

¹ Institute of Dentistry, Barts and the London, Queen Mary University of London, London, UK;
² Australian Research Centre for Population Oral Health (ARCPOH), The University of Adelaide, Adelaide, Australia, 5005; and
³ School of Public Health, Queensland University of Technology, Brisbane, Australia

Correspondence: ^* corresponding author, anne.sanders{at}adelaide.edu.au

	ABSTRACT

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It is unclear which theoretical dimension of psychological stress affects health status. We hypothesized that both distress and coping mediate the relationship between socio-economic position and tooth loss. Cross-sectional data from 2915 middle-aged adults evaluated retention of < 20 teeth, behaviors, psychological stress, and sociodemographic characteristics. Principal components analysis of the Perceived Stress Scale (PSS) extracted ’distress’ (a = 0.85) and ’coping’ (a =0.83) factors, consistent with theory. Hierarchical entry of explanatory variables into age- and sex-adjusted logistic regression models estimated odds ratios (OR) and 95% confidence intervals [95% CI] for retention of < 20 teeth. Analysis of the separate contributions of distress and coping revealed a significant main effect of coping (OR = 0.7 [95% CI = 0.7–0.8]), but no effect for distress (OR = 1.0 [95% CI = 0.9–1.1]) or for the interaction of coping and distress. Behavior and psychological stress only modestly attenuated socio-economic inequality in retention of < 20 teeth, providing evidence to support a mediating role of coping.

Key Words: psychological stress • tooth loss • mediator • health inequalities • risk behavior

	INTRODUCTION

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The causes of tooth loss are attributed as much to social-behavioral circumstances as to disease-related factors (Burt et al., 1990). Compared with more advantaged adults, deprived adults have more dental risk behaviors (Sanders et al., 2005), greater prevalence of dental caries and periodontitis, and higher rates of tooth loss (Thomson et al., 2004). Despite risk behavior being causally related to dental conditions, there is little evidence that variation in behavior accounts for socio-economic inequality in tooth loss among adults (Sanders et al., 2006) or in dental caries among children (Slade et al., 2006). Consequently, efforts to explain socioeconomic inequality in oral health should be focused on mechanisms earlier than behavior in the hypothesized causal chain.

A diverse set of social and psychological factors is associated with tooth loss, including negative life events, low prestige, depression, needing help from others (Drake et al., 1995), and having hobbies (Morita et al., 2006). What is needed is an evidence-based theoretical framework to integrate these factors and to help develop their causal link. Among the proliferation of factors, psychological stress has emerged as a significant risk indicator of clinical signs of periodontitis (Genco et al., 1999; Ng and Keung Leung, 2006), which is one of the main causes of tooth loss.

Psychological stress is theorized to involve two cognitive processes: primary appraisal, which involves interpretation of whether a stimulus is threatening; and secondary appraisal, which assesses the adequacy of available resources to manage a threatening encounter (Lazarus, 1966). According to stress and coping theory, distress arises when a stress stimulus is appraised as exceeding an individual’s coping resources. When resources are deemed adequate, the stressor is managed successfully, and the accompanying emotions are appropriately governed (Lazarus and Folkman, 1984).

We present this relationship conceptually in the Fig. Distress arises when insufficient or inadequate coping resources undermine an individual’s capacity for healthy behavior. In these circumstances, individuals are more likely to use dental services episodically, to be less vigilant of oral hygiene, and to smoke. Together, these behaviors elevate the risk of clinically significant levels of tooth loss. Conversely, people whose coping strategies avert the onset of distress are less prone to risk behavior. We hypothesized that distress and coping account, in part, for socio-economic inequality in tooth loss, and hence represent a mediator of this relationship.

View larger version (11K): [in this window] [in a new window]   Figure. Conceptual framework showing the psychological response (distress and coping dimensions) to stress stimulus as a mediator between socio-economic deprivation and retention of < 20 teeth via a risk behavioral pathway. Where the stress stimulus is perceived as exceeding available resources, the distress that arises is positively associated with risk behaviors and tooth loss. Coping is negatively associated with risk behaviors.

	METHODS

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Dependent Variable
Tooth loss was defined as retention of fewer than 20 teeth. In their review of evidence from more than 50 published papers that examined aspects of tooth retention and oral function, Elias and Sheiham (1998) concluded that 20 teeth were sufficient for satisfactory chewing function. Other research has shown that diet and nutritional status (Sheiham et al., 2001) are affected with fewer than 20 teeth, and that fewer teeth are associated with impaired quality of life (McGrath and Bedi, 2002). Self-reported numbers of teeth were dichotomized to ’fewer than 20’ vs. ’20 or more’ teeth. Tooth loss was self-reported, and misclassification was a possibility. Yet at least ten studies have reported that the general public provides valid estimates of retained teeth in surveys by questionnaire and telephone interview modes (e.g., see Lahti et al., 1989; Gilbert et al., 1997; Pitiphat et al., 2002).

Independent Variables
Participants were defined as ’socially deprived’ if eligible for state-government-funded health care, including dental care. Eligibility is means-tested, and recipients experience financial hardship and may have other forms of disadvantage, such as a disability.

Behaviors assessed dental attendance, oral hygiene, and smoking. Variables were re-coded into binary variables comprising a risk and a non-risk category, except for smoking, which was classified into three groups: current smokers, former smokers, and those who never smoked.

The Perceived Stress Scale (PSS) evaluates the degree to which people perceive stress stimuli as threatening in light of their available resources (Cohen et al., 1983). Seven items are negatively worded, e.g., ’... felt upset because of something that happened unexpectedly?’ The remainder are posed positively, as in ’... dealt successfully with irritating life hassles?’. Cohen et al.(1983) recommended the reverse scoring of negatively worded questions to compute a single score. Prominent stress theorists have criticized biological and behavioral research that conceptualizes psychological stress as a ’single unidimensional environmental variable’ (Gruen et al., 1988). Consequently, this study first examined psychological stress as a single variable consistent with the conventional approach, and then examined separately its two dimensions, namely, distress and coping.

Analytic Methods
Weighted analysis conducted in SUDAAN software corrected for differences in sampling and response, and adjusted for the clustered design, so that estimates were representative of the population aged 43–57 yrs in Adelaide.

We used a conceptually driven technique of hierarchical modeling (Victora et al., 1997) to test the study hypothesis. Blocks of explanatory variables were entered into an age- and sex-adjusted binary logistic regression model in three steps, labeled Models 1 to 3. In these models, the dependent variable was (log) odds of retention of < 20 teeth. Deprivation was entered in Model 1, with main effect presented as an odds ratio (OR) with its 95% confidence interval (95%CI). The Z-transformed sum PSS score was entered in Model 2, and the four risk behaviors were entered in Model 3. Blocks of explanatory variables were entered in this sequence so that we could evaluate pathways depicted in the Fig. Specifically, we reasoned that if any block of explanatory variables was a mediator of the effect of deprivation on tooth loss, we would observe attenuation of the odds ratio for deprivation in the model containing that block of explanatory variables.

To test empirically its theoretical components of distress and coping, we entered PSS items into a principal components analysis (PCA) with orthogonal rotation. The internal consistency of extracted factors was examined by Cronbach’s alpha. In a second hierarchical model, the sum PSS score was replaced with the z-transformed scores for two derived factors, distress and coping.

To demonstrate that a variable mediates a relationship, it must cause variation in the dependent variable, and, itself, be caused to vary by the independent variable—hence, be statistically related to both factors (Last et al., 2001). Associations between the four behavior variables and sum PSS or its two subscales (distress and coping), as well as between deprived and non-deprived groups, were tested with the independent samples t test. Differences in mean scores on the PSS or its two subscales in relation to deprived and non-deprived groups were also tested by the independent samples t test.

Ethical approval for the study was obtained from the Human Research Ethics Committee of the University of Adelaide.

	RESULTS

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The response rate was 69.4% (n = 2915). Males contributed 48.2%, and the mean age was 50.1 yrs. Deprived adults comprised 19.3% (n = 553), non-deprived 79.4% (n = 2313), and data were missing for 1.7%. Only 5% of deprived adults had household income higher than AU$52,000, and 25.2% were permanently unable to work.

Prevalence of < 20 retained teeth was 11.9%. An examination of 21 pair-wise correlations between explanatory variables revealed weak-to-moderate correlations ranging from rho = 0.01 to rho = 0.36.

Retention of < 20 teeth was positively related to the four risk behaviors (all OR > 2.7), PSS scores (OR = 1.4; CI = 1.3 to 1.5), and deprivation (OR = 2.99; CI = 2.3 to 3.9) (Table 1). These associations remained statistically significant when all variables were entered into the model showing an independent effect on retention of < 20 teeth (Table 2, model 3). Hierarchical entry of PPS scores in Model 2 and dental behaviors in Model 3 revealed partial attenuation of the odds ratio of deprivation on retention of < 20 teeth. This suggests that these variables accounted for some of the variation in retention of < 20 teeth attributed to socioeconomic inequality evident in Model 1 (Table 2).

Deprived adults had higher overall scores on the PSS than did non-deprived adults (Appendix Table 2), as well as higher distress scores and lower scores for coping (Table 3, Appendix Table 2). Odds of retaining < 20 teeth were elevated two-fold among the deprived adults, and greater than two-fold among smokers and among adults who brushed their teeth fewer than 7 times a wk.

	DISCUSSION

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This study confirmed empirically the theoretical view of psychological stress as bidimensional. Deprived adults experienced higher levels of psychological stress, risk behavior and retention of < 20 teeth, when examined as single dimensions. When examined separately, only one dimension was associated with retention of < 20 teeth. Distress, while associated with both deprivation and risk behavior, was not associated with retention of < 20 teeth. Coping, in contrast, was inversely associated with retention of < 20 teeth. We interpret this to mean that while distress covaries with deprivation and risk behavior, the relationship is not causally associated with tooth loss. By contrast, coping had a protective effect against tooth loss and may operate via a behavioral pathway.

Findings only partly supported the hypothesis that psychological stress mediates the relationship between socioeconomic position and tooth loss, inasmuch as coping appeared protective against tooth loss. While psychological stress reduced the elevated odds of retention of < 20 teeth for deprived adults, the effect was only modest.

One or both of the two dominant explanations for social inequality in health may explain a mediating role of coping. The neo-materialist explanation contends that social groups have differential capacity to secure material resources essential for health (Dunn et al., 2006). In the context of this study, adults with better coping scores had greater capacity to utilize dental services.

The psychosocial explanation contends that people attach meaning to their social circumstances. Under deprivation, bleak perceptions are likely to undermine coping resources such as health self-efficacy. It is possible that disadvantaged people under stress may ’trade’ against their future health for the immediate satisfaction of smoking (Hornik, 1990). Smoking is a recognized predictor of tooth loss (Ahlqwist et al., 1989; Slade et al., 1997). Likewise, they may resist investment in future health through dental attendance and oral hygiene, choosing to direct their coping toward more urgent survival needs.

The cross-sectional design neither permits causal inference about the protective effect of coping, nor confirms the temporal sequence specified in the conceptual framework.

Reversed causation, whereby tooth loss contributes to tooth loss, cannot be discounted. However, the extensive literature about social determinants of disease would suggest that the association observed here between deprivation and tooth loss could be attributed predominantly to the effects of deprivation on risk of dental disease, and on decisions to treat dental disease with extraction. Furthermore, we draw on findings from prospective cohort studies showing that deprivation precedes behavioral adjustment in childhood and psychosocial functioning in adulthood (Schoon et al., 2003).

Several earlier studies reporting a relationship between psychological stress and oral diseases suggest a biological and/or behavioral pathway. Among financially strained adults, those with poor coping skills had a higher risk of periodontal attachment loss and alveolar bone loss than those with low financial strain in the same coping group (Genco et al., 1999). A study of care-givers of persons with dementia found that salivary cortisol—an index of stress reactivity—was positively associated with clinical attachment level and probing depth, but was not associated with psychological stress (Hilgert et al., 2006). The authors commented that the null finding might be explained by not assessing coping, and they cited Bohnen et al.(1991), who found a negative association among stress, coping, and cortisol reactivity.

We broke with convention in examining the separate components of the PSS, and found that this was more informative about their individual effects on tooth loss. Until now, studies of health inequality have emphasized the distress dimension, giving less attention to the protective role of coping.

Inequalities in oral status cannot be fully explained by differences in coping resources or risk behavior. These are not the root determinants of health, but are themselves determined by material and social conditions of society. Future research needs to integrate these macro-social determinants into a unifying explanation, along with both psychosocial and biological determinants, and establish temporal relationships between these variables and oral health outcomes.

	ACKNOWLEDGMENTS

 
The National Health and Medical Research Council (NHMRC) funded this study (project grant #250315). Dr. Sanders is supported by a NHMRC Sidney Sax (overseas) Public Health Fellowship (#399222). Dr. Turrell is supported by a NHMRC Senior Research Fellowship (#390109).

	FOOTNOTES

 
A supplemental appendix to this article is published electronically only at http://www.dentalresearch.org.

Received for publication September 12, 2006. Revision received June 26, 2007. Accepted for publication September 9, 2007.

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Journal of Dental Research, Vol. 86, No. 12, 1166-1170 (2007)
DOI: 10.1177/154405910708601205


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