2.1. Literature Search
An electronic search was conducted using MEDLINE (January 1990 to September 2010) to identify pertinent literature. The search strategy applied was as follows: dental caries AND ((parental smoking) OR (maternal smoking) OR (household smoking) OR (paternal smoking) OR (secondhand smoke) OR (involuntary smoking) OR (passive smoking)). In addition, the reference lists in articles that we read completely and the latest articles, of which we were informed by a newsletter (the MDLinx, M3 Inc., Tokyo), were considered.
2.2. Outcome and Exposure
The primary outcome of interest was dental caries. The dmft and DMFT indices have been used most frequently for this purpose. Numbers of decayed (unfilled) tooth, missing teeth because of dental caries, and filled teeth were calculated for “d”, “m” and “f” categories for early childhood caries. The DMFT index was applied to permanent teeth. These indices are indicators of the caries experience of each individual. Therefore, prevalence based on these indices indicates the population with dental caries experience. Because the reason for missing teeth in primary dentition is difficult to distinguish from natural replacement with permanent teeth, “m” is not used for children aged five years or older. Several variables such as serum cotinine levels, maternal smoking during pregnancy, and paternal, maternal, and household smoking can be used as a measure of SHS.
2.3. Eligibility Criteria and Searching Process
The inclusion criteria for studies were English language and analysis of the relationship between SHS and dental caries utilizing multivariate models. Literature reviews and basic science studies were excluded. Search results were stored using management software (iPubMedMaker 7, Sapporo, Japan) for initial screening based on the title and abstract. Two calibrated reviewers independently screened the results. Disagreements between reviewers were resolved by discussion until a consensus was reached. Final screening consisted of evaluating full-text reports, assessing studies that could evaluate the independent association.
2.4. Methodological Quality Assessment
We used the modified Newcastle–Ottawa Scale (NOS), which assessed each criterion for eight items regarding the methodology of observational studies [8
]. One star was given when the criterion of an item was satisfied. The grouping items of NOS consist of the following three categories: selection, comparability, and ascertainment of exposure (for case-control format) or outcome (for cohort format). A maximum of two stars were given for comparable categories; one star was given if the data were adjusted for at least one traditional risk factors regarding oral health behavior, and a second star was given if a variable for socioeconomic status (SES) or dental visit was used for adjustment. A non-response rate of less than 25% and a follow-up period of one year or more along with a dropout rate of less than 25% were used for the assessment of case-control and cohort format studies, respectively. Two reviewers independently coded the items in the modified NOS. Disagreements between reviewers were resolved by discussion until a consensus was reached. Overall quality was evaluated according to the total number of stars, and studies that were given seven stars or more were considered high-quality studies.
2.5. Data Abstraction
The abstracted elements in the characteristics of the studies were citation and focal factor(s) regarding the association (SHS only or various factors including SHS), analytical design (case-control or cohort format), setting (number, sex, age range, country, residency, and representativeness), analytical model and factors considered for multivariate analysis, non-respondent rate for case-control format studies, observational length and follow-up rates for cohort format studies, and funding source. Subsequently, we abstracted effect size with confidence intervals and definitions of dental caries and SHS in addition to the prevalence. One reviewer abstracted these data, which were verified independently by another reviewer. Disagreements between reviewers were resolved by discussion until a consensus was reached.
2.6. Evaluation of Causal Association
The three elements that were extracted and used according to the Bradford Hill criteria [9
] and the Surgeon General’s report [3
] are as follows: strength of association, biological gradient, and natural experiment. Common descriptors for the strength of association that were defined using effect size are as follows: weak association, ≤1.49; moderate association, 1.50–2.99; and strong association, ≥3.00 [10
]. The element of biological gradient was assessed by dose-response relationship. The Bradford Hill criterion of the experiment [9
] was evaluated in this review by comparing the strength of association between previous and current exposure relative to non-exposure. This criterion was named as “natural experiment” in the Surgeon General’s report [3
] because conducting interventional studies in which a scientist determines subjects to be removed from exposure in a manner that does not depend on any of the subject’s characteristics is difficult in humans. Although differences from current exposure should be evaluated, the reference was usually set as the non-exposure group.
Evidence concerning a causal association between SHS and dental caries in children was evaluated on the basis of the three elements by collecting evidence with regard to consistency in terms of study quality and design and by considering shortcomings of the evidence.
2.7. Evidence Synthesis of Consistency according to Study Quality
Results in each element were evaluated according to study quality. The following descriptors were used for interpretation of consistency [11
]: strong evidence, consistent findings among multiple high-quality studies; moderate evidence, consistent findings among multiple low-quality studies and/or one high-quality study; limited evidence, one low-quality study; conflicting evidence, inconsistent findings among multiple studies; and no evidence among studies.
2.8. Evidence Synthesis of Association according to Study Design
Evidence synthesis was further performed by considering study design according to modification of the standardized descriptions [12
]. Intervention study was replaced with natural experiment. This review excluded descriptions of biological plausibility, and the following criteria were applied:
- Convincing: Evidence is based on epidemiological studies showing consistent associations between exposure and disease, with little or no evidence to the contrary. The available evidence is based on a substantial number of studies including prospective observational studies, and where relevant, natural experiments of sufficient size, duration, and quality showing consistent effects.
- Probable: Evidence is based on epidemiological studies showing fairly consistent associations between exposure and disease, but there are perceived shortcomings in the available evidence or some evidence to the contrary, precluding a more definite judgment. Shortcomings in the evidence may be any of the following: insufficient duration of natural experiment (or studies), insufficient experiments (or studies) available, inadequate sample sizes, and incomplete follow-up.
- Possible: Evidence is mainly based on the findings of case-control and cohort formats studies. Insufficient numbers of natural experiments or observational studies are available. Additional natural experiments are required to support the tentative associations.
- Insufficient: Evidence is based on the findings of a few studies that are suggestive but insufficient to establish an association between exposure and disease. Limited or no evidence is available from natural experiments. Additional well-designed research is required to support the tentative associations.