, 1994), with the section of nicotine use and dependence based on the Composite International Diagnostic Interview (Cottler et following website al., 1991). The Finnish NAG subsample contains 445 twin individuals, of whom data on bruxism were available from the 1990 questionnaire. The mean age of the participants in the NAG Finland Study was 53.7 years (SD = 4.6). The ethical committee of the Hospital District of Helsinki and Uusimaa approved the study protocol in 2001. In addition to the standard epidemiological analyses of the relationship of smoking with bruxism in individuals (see below), we utilized the twin data to analyze the risk of weekly bruxism using twin pairs discordant for smoking status, viz.

, examining the ratio of the number of pairs in which a smoking twin reports bruxism (at least weekly), while the co-twin neither smokes nor experiences bruxism, contrasted with the number of pairs in which the opposite was true: A smoking twin does not report bruxism, while the co-twin does not smoke but experiences bruxism. We identified 142 twin pairs discordant for bruxism, among whom the distribution of smoking status was examined. Statistical Methods We studied associations between smoking and bruxism using cross-tabulations and the Pearson chi-square test of independence, corrected for clustered sampling of twins within pairs, which is expressed as an F ratio (Rao, 1984). The association of tobacco use and bruxism was assessed using multinomial logistic regression models (Hosmer & Lemeshow, 2000) that controlled for age and sex as there were three outcome categories (at least weekly, monthly, and rarely) and never bruxism as the reference category.

Odds ratios (ORs) of all models were adjusted for correlated observations within twin pairs by means of the statistical software package Stata 9.0 (StataCorp, 2005), using a robust estimator of variance. Conditional logistic regression models were used to obtain ORs for the risk of bruxism in relation to tobacco use and nicotine dependence in twin pairs discordant for bruxism. In the absence of covariates, this is equivalent to a McNemar chi-square test for matched pairs. Results Of the current smokers, 27.8 % of males and 19.9 % of females smoked 20 or more cigarettes daily, while 15.4% of males and 36.7% of females smoked less than 10 cigarettes daily. Bruxism was more frequent among cigarette smokers in both genders.

Similarly, bruxism was more frequent among current heavy smokers than among current light smokers (Table 1). Table 1. Percentages of Cigarette Smoking Status and Amount Smoked in Adulthood For ��Weekly,�� ��Monthly,�� ��Rarely,�� and ��never�� Bruxing in Males and Females In the age- and gender-controlled multinomial logistic regression, both monthly and rarely reported Batimastat bruxism associated with both current cigarette smoking (OR = 1.74 [95% CI = 1.37�C2.22] and1.64 [95% CI = 1.44�C1.