Objective

The purpose of this review was to test the null hypothesis of no difference in marginal bone loss or implant failure rates between smokers and non-smokers, depending on the follow-up period.

Search strategy

A comprehensive electronic search was conducted, with no date or language restrictions, in PubMed, Web of Science, and the Cochrane Central Register of Controlled Trials, up to February 2015. The search strategy and the PICO tool are presented in Table 1 . In addition, references of the studies included (cross-referencing) was also performed to obtain further new studies.

Selection criteria

This review sought prospective and retrospective cohort studies, as well as randomized controlled trials (RCTs) that compared the marginal bone loss and implant failure rates between smokers and non-smokers. For this review, implant failure was regarded as the absolute loss of the implant. The exclusion criteria included animal studies, in vitro studies, and case series, case reports, and reviews. In addition, studies conducted on volunteers with unbalanced metabolic diseases, or with periodontal disease without prior treatment, were excluded.

Screening process

The research and screening processes were conducted by both reviewers (V.M.F. and E.P.B.). The titles and abstracts were first analyzed. The second step involved the selection of full papers for careful reading; these were analyzed according to the eligibility criteria (inclusion/exclusion) for future data extraction. Disagreements between the reviewers were resolved through careful and detailed discussions. Agreement between the two reviewers was evaluated statistically with Cohen’s kappa ( κ ) test. The authors of the studies were contacted via e-mail for clarification of any points, when required.

Quality assessment

The analysis of quality of the non-randomized studies (prospective and retrospective cohort studies) included in this review was performed using the Newcastle–Ottawa scale (NOS). For the categories of ‘selection’ and ‘outcome’, studies may obtain a star/point for each item. For the ‘comparability’ category, two stars/points may be awarded. The highest score that could be assigned to a study according to the NOS was nine stars/points (highest scientific evidence). Studies scoring six stars/points and above were considered to be of high quality.

Data extraction

The following data were extracted from the studies included (when available): authors, publication year, follow-up period, number of subjects, gender and age of the subjects, smoking status, number of implants placed, implant system, implant length and diameter, healing period, days of antibiotic prophylaxis, use of mouth rinse, marginal bone loss, implant survival, failed and placed implants, P -value for implant failure rate, and the number of drop-outs. This review regarded anyone who consumed any quantity of tobacco at any time during the surgery and recovery period to be a smoker.

Statistical analysis

Binary and continuous variables from the studies included were analyzed through meta-analysis when the same type of data was assessed by at least two studies. For binary outcomes (e.g. implant failure), the estimate of the intervention effect was expressed in the form of an odds ratio (OR) with a confidence interval (CI) of 95%. For continuous outcomes (e.g. marginal bone loss), the average and standard deviation (SD) were used to calculate the standardized mean difference (SMD) with a 95% CI. The results were pooled using the fixed-effects model (Mantel–Haenszel–Peto test) or random-effects model (DerSimonian–Laird test). The I ² statistical test was used to express the percentage of heterogeneity in the studies. Values up to 25% were classified as indicating low heterogeneity, values of 50% as indicating medium heterogeneity, and values of ≥70% as indicating high heterogeneity. The results of the random-effects model were validated when significant heterogeneity was observed ( P < 0.10). The fixed-effects model was considered when low heterogeneity was observed. The level of statistical significance was set at P < 0.05.

Publication bias was explored graphically through a funnel plot. Asymmetry in the funnel plot may indicate possible publication bias.

All data were analysed using the statistical software Review Manager version 5.2.8 (The Nordic Cochrane Centre, The Cochrane Collaboration, Copenhagen, Denmark; 2014).

Literature search

The initial search yielded 965 titles from Medline/PubMed, 85 titles from the Cochrane Central Register of Controlled Trials, and 240 titles from the Web of Science. After the initial evaluation, 29 full papers were selected. Fourteen studies were excluded after careful reading, as they did not conform to the eligibility criteria of this review. Therefore, 15 studies published between 1993 and 2013 were included in this meta-analysis. The selection process and the grounds for exclusion of studies are presented in Fig. 1 .

Flow diagram (PRISMA format) of the screening and selection process.

The κ value for agreement between the reviewers for the inclusion of potential studies (titles and abstracts) was 0.79 and for selected articles was 0.85, demonstrating a substantial agreement between the reviewers for inclusion of potential articles and an ‘almost perfect’ concordance for studies selected, according to the criteria proposed by Landis and Koch.

Study characteristics

The characteristics of the studies included are presented in Table 2 . Five prospective studies and 10 retrospective cohort studies were included. The number of participants in the studies ranged from 60 to 1727, and the average age was 52.5 years. The follow-up period ranged from 8 to 240 months. The number of implants installed in smokers was 5840 and in non-smokers was 14,683. The number of implants installed in the groups was not clearly reported in one article. Implants were installed without surface treatment in four studies. The Brånemark system (Nobel Biocare AB, Göteborg, Sweden) was the most commonly used implant system. In one study, all patients received implants in areas of bone regeneration ; another study included patients susceptible to periodontal disease (which was clinically controlled).

Ten studies reported a statistically significant difference in the average number of implant failures between smokers and non-smokers ; the difference was not statistically significant in only one study. With regards to post-surgical care, two studies reported the use of antibiotic prophylaxis, while only one provided information related to the prescription of chlorhexidine mouthwash.

Quality assessment

Only two studies obtained a score of less than six stars. The scores for each study are summarized in Table 3 .

Table 3

Quality assessment of the studies using the Newcastle–Ottawa scale.

Authors (year)	Selection				Comparability	Outcome
	Representativeness of the exposed cohort	Selection of external control	Ascertainment of exposure	Outcome of interest not present at start	Comparability of cohorts on the basis of the design or analysis a	Assessment of outcome	Was follow-up long enough for outcomes to occur? b	Adequacy of follow-up of cohorts	Total 9/9
Bain and Moy (1993)	0	*	*	*	* 0	*	*	0	6/9
Haas et al. (1996)	0	*	*	*	* 0	*	*	0	6/9
Kan et al. (1999)	0	*	*	*	* 0	*	*	*	7/9
Lambert et al. (2000)	*	*	*	*	* 0	*	*	0	7/9
Kumar et al. (2002)	0	*	*	*	* 0	*	0	0	5/9
Schwartz-Arad et al. (2002)	0	*	*	*	* 0	*	*	0	6/9
Nitzan et al. (2005)	0	*	*	*	* 0	*	*	0	6/9
DeLuca and Zarb (2006)	0	*	*	*	* 0	*	*	0	6/9
Sánchez-Pérez et al. (2007)	*	*	*	*	**	*	*	0	8/9
Balshe et al. (2008)	0	*	*	*	* 0	*	*	*	7/9
Sverzut et al. (2008)	0	*	*	*	* 0	*	0	0	5/9
Cavalcanti et al. (2011)	*	*	*	*	* 0	*	*	*	8/9
Vandeweghe and De Bruyn (2011)	0	*	*	*	**	*	*	0	7/9
Vervaeke et al. (2012)	0	*	*	*	**	*	*	0	7/9
Sayardoust et al. (2013)	0	*	*	*	**	*	*	0	7/9

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