Is plaque regrowth inhibited by dentifrice?

Abstract Objectives The aim of this systematic review was to establish in studies with human participants the effect of a regular fluoride dentifrice compared to water or saline on dental plaque inhibition. Methods MEDLINE‐PubMed, Cochrane‐CENTRAL, EMBASE and other electronic databases were searched, up to April 2018. The inclusion criteria were controlled clinical trials among participants aged ≥18 years with good general health. Papers that evaluated the effect of dentifrice slurry compared with water or saline on plaque regrowth during a 4‐day nonbrushing period were included. Data were extracted from the eligible studies, the risk of bias was assessed, and a meta‐analysis was performed where feasible. Result The search retrieved eight eligible publications including 25 comparisons. The estimated potential risk of bias was low for all studies. Based on three different indices, overall plaque regrowth was significantly (P < 0.01) inhibited for 0.25 or more by the use of a dentifrice slurry as compared to water. All subanalysis on specific dentifrice ingredients and the overall descriptive analysis supported these findings. Conclusion The results of this review demonstrate moderate‐quality evidence for a weak inhibitory effect on plaque regrowth in favour of the use of a dentifrice intended for daily use.

mechanical action of the toothbrush during a test period obscures the antiplaque effect of the dentifrice by itself. 13 Also, the Hawthorne effect, whereby oral hygiene practices are improved irrespective of the test product, can easily occur in oral hygiene study designs. To some incalculable degree, it could mask the true adjunctive effect of the dentifrice, 14 making it impossible to determine whether the reduction in plaque regrowth results from very efficient brushing or from a chemical antiplaque effect of the dentifrice. 15 One proposed alternative is to assess the effects of dentifrice ingredients on plaque regrowth independently of those of mechanical cleaning effect of a toothbrush by delivering the dentifrice formulation as a slurry in mouthwash form. 15,16 To obtain a slurry, the dentifrices are mixed with water so that simple rinsing reproduces the quantity of active substance present in the oral cavity during normal toothbrushing, without the mechanical cleaning effect of toothbrushing. 17 A suitable research model for investigating whether dentifrice can play a role as plaque-reducing agent seems to be the 4-day nonbrushing model developed by Addy et al 15 This design has been used extensively and allows the chemotherapeutic activity of dentifrice products on dental plaque to be rapidly determined. 18 The objective of this systematic review (SR) was therefore to systematically and critically appraise the literature on 4-day nonbrushing models that compared the efficacy on plaque regrowth of a dentifrice for daily use with that of water or saline only.

| MATERIAL S AND ME THODS
This SR was prepared and described in accordance with the Cochrane Handbook for Systematic Reviews of Interventions 19 and the guidelines of Transparent Reporting of Systematic Reviews and Meta-analyses (PRISMA statement). 20 The protocol that details the review method was developed "a priori" following an initial discussion among the members of the research team.

| Focused question
What is the efficacy of a regular dentifrice intended for daily use on regrowth of dental plaque used as a slurry in comparison with that of water or (sterile) saline in healthy adults?  Table 1.

| Screening and selection
The titles and abstracts of the studies obtained from the searches were screened independently by two reviewers (C.V. and D.E.S.) to select studies that potentially met the inclusion criteria. No language restrictions were imposed. Based on the title and abstract, the fulltext versions of potentially relevant papers were obtained. These papers were categorized (by C.V. and D.E.S.) as definitely eligible, definitely not eligible or questionable. Disagreements concerning eligibility were resolved by consensus, and if disagreement persisted, the decision was resolved through arbitration by a third reviewer (G.A.W.). The papers that fulfilled all the inclusion criteria were processed for data extraction.
The included studies were considered to meet the following criteria: (a) the study design was either a randomized controlled clinical trial (RCT) or a controlled clinical trial (CCT). (b) The studies were conducted with humans, who were not institutionalized and were 18 years of age or older. (c) The studies only included participants who were in good general health (no systemic disorders) and were without orthodontic appliances and/or removable prostheses. (d) The studies used a nonbrushing 4-day plaque regrowth model. (e) The intervention was a slurry from a regular dentifrice, and the comparator was water or saline. (f) The studies evaluated any plaque scores. (g) The publications were available as full reports.

| Assessment of heterogeneity
The following factors were used to evaluate the clinical and methodological heterogeneity of the outcomes of the different studies: study design, subject characteristics, study group details, side effects and industry funding.

| Study quality and risk of bias assessment
Two reviewers (C.V. and D.E.S.) independently scored the individual methodological qualities of the included studies using the checklist presented in Appendix S2. In short, a study was classified as having a low risk of bias when random allocation, defined inclusion/exclusion criteria, blinding to patient and examiner, balanced experimental groups, identical treatment between groups (except for the intervention) and reporting of a follow-up were present. Studies that met six of these seven criteria were considered to have a potential moderate risk of bias. If two or more of these seven criteria were absent, the study was considered to have a high risk of bias as proposed by Van der Weijden et al 21 23 Where applicable, inches were converted to centimetres and totals to averages. For those papers that provided insufficient data to be included in the analysis, the first or corresponding author was contacted to request additional data.

| Grading the "body of evidence"
The Grading of Recommendations Assessment, Development and Evaluation (GRADE) system was used as proposed by the GRADE-working group 43 to appraise the evidence emerging from this review. Two reviewers (G.A.W. and D.E.S.) rated the quality of the evidence, and any disagreement between the two reviewers was resolved after additional discussion with a third reviewer (C.V.).

| Search and selection results
The search of the MEDLINE-PubMed, Cochrane-CENTRAL and EMBASE databases resulted in 195 unique papers (for details, see

| Study characteristics, heterogeneity and funding
The included studies exhibited moderate heterogeneity. Information regarding the study characteristics is provided in detail in Appendix S1. All the studies had a crossover design. The washout periods between treatments herein varied from 3 to 10 days. 44,45 The dentifrices used in the studies exhibited a large variation in brands, compositions and concentrations of the ingredients. All but one study 15 provided the name of the dentifrice brands. Two noncommercial dentifrices were used in one study. 46 Rinsing under supervision was performed in four of the studies. 44,45,47,48 The rinsing time was 1 minute in all but one study. 44  Three studies did not mention support or assistance from a commercial partner. 15,17,45 Two studies acknowledged support and assistance from industry 46 (Colgate-Palmolive Company), 48 (Procter and Gamble). None of the studies included a disclosure statement for conflict of (financial) interests. However, several authors in the studies mentioned affiliations with industry 44,47,48 (Procter and Gamble).

| Methodological quality and risk of bias assessment
To estimate the potential risk of bias, the methodological qualities of the included studies were used, as assessed in the checklist presented in Appendix S2. The procedures for allocation concealment were not described in any of the selected studies. Because the current study was focused on the adjunctive use of a dentifrice, blinding to the intervention was not applicable. All studies provided a professional prophylaxis to remove all plaque, stains and calculus at baseline. Two studies performed a sample size as well as power calculation and mentioned an intention-to-treat analysis. 44,45 Four of the studies did not provide information about examiner calibration. 15,16,45,46 Based on a summary of the proposed criteria, the estimated potential risk of bias was low for all studies.

| Study outcomes/results
Appendix S3 presents the results of the data extraction per index used. The outcomes by and relative to the Plaque Index as well as the plaque area score are presented in the current study. Table 2 provides a summary of the differences reported to be significant between rinsing with a dentifrice slurry as compared to rinsing with water or (sterile) saline alone, as reported by the original authors.

| Descriptive analysis
Twenty-two comparisons out of 25 demonstrated a significant difference between interventions in favour of the use of a dentifrice slurry on the Plaque Index used. Only three comparisons indicated no significant difference. 15,16,44 All but one 16 of the 21 comparisons also exhibited a significant difference in favour of the use of a dentifrice slurry according to the Plaque Area Index. This was irrespective of the specific dentifrice ingredient or when a comparison as saline or water used.

| Meta-analysis
All studies except one 44  All but one subanalysis based on the active ingredients for all indices of interest were statistically significant in favour of the dentifrice slurry compared to saline/water (for details, see Table 3).
Unexplained heterogeneity in the meta-analyses was high for the

| Trial sequential analysis
Appendix S7 presents the results of the trial sequential analysis (TSA) per index used. TSA of this MA showed that the effect was conclusive and reliable and that additional data are unlikely to affect the summary effect. 39

| Evidence profile
The data gathered are indirect as the model of interest is a research model for a proof of principle. However, the data are rather consistent and precise. Table 4 shows a summary of the various factors used to rate the quality of evidence and strength of recommendations according to GRADE. 43 The strength of a recommendation based on the quality of the evidence emerging from this review is estimated to be moderate. Given that the effect is small, the direction of recommendation emerging from this SR is weak in favour of the use of a dentifrice with the intention to inhibit regrowth of dental plaque.

| D ISCUSS I ON
Over recent decades, dentifrice formulations have been developed to deliver chemical and physical mediated benefits. 54 Despite these efforts, a recent SR indicated that dentifrice appears not to provide an adjuvant mechanical action of toothbrushing on the instant removal of plaque. 8 Traditionally, dentifrices have played an important role in the sense of a fresh mouth and in tooth discoloration control. 55,56 In August 1960, the American Dental Association (ADA) for the first time recognized a dentifrice with fluoride to have therapeutic value in fighting tooth decay. 57 Since fluoride dentifrices first became available, many formulation changes regarding fluoride type, concentration and abrasive systems have been made to improve stability, compatibility and bioavailability of active ingredients. 58 Even chemical agents have been added for the improved treatment of bad breath, staining, caries, gingivitis, dental plaque, dental calculus, demineralization and dentinal hypersensitivity. 56,59 Because plaque control plays a paramount role in the aetiology of caries and periodontal disease 60 and plaque formation on teeth cannot be stopped, disturbing plaque accumulation is of major importance. 61 The aim of the present review was to investigate whether dentifrice can play a role as plaque-reducing agent. Nearly all the dentifrices in the included studies of this SR appeared to provide a significant inhibiting effect on plaque regrowth in comparison with rinsing with water or saline.
The 4-day nonbrushing model design, developed by Addy et al, 15 has been extensively used to investigate the effects of mouthrinses or dentifrice slurries. For the latter, the model utilizes an aqueous dentifrice slurry and examines the effects of such treatments on plaque regrowth over a 4-day period of no oral hygiene following a dental prophylaxis. By comparison with controls, the relative biological effects of antimicrobial ingredients incorporated into dentifrices can be determined. This design approximates the dilatation TA B L E 2 A descriptive summary of the statistical significance of individual study outcomes related to the effect of rinsing with a dentifrice slurry or with water on dental plaque in a 4-day plaque regrowth design of a dentifrice with saliva that occurs with normal use of such products. 18,62 This study design prevents the complicating effects of mechanical toothbrushing. 15,18,63,64 Consequently, the Hawthorne effect, the effect often cited as being responsible for oral health improvements of control groups that receive placebo treatments, 65 may be absent or limited. One could question whether a slurry achieves the same antibacterial effect as that obtained by the original dentifrice. Addy et al 15 attempted to produce dentifrice slurries of comparable concentration to that delivered by toothbrush.
Therefore, 3 g/10 mL of each paste was employed, based on the normal quantity of toothpaste used on a brush was reported to be 1.45 g 62 which is diluted approximately 1 in 4 by saliva. 15 Moran et al 66 have pointed out that an antimicrobial product that is proved ineffective in such a study would also have no effect if used with a toothpaste and mechanical cleaning. 15 The results of this SR agree with those of other studies which do include the mechanical action of toothbrushing. Experiments over a 24-hour duration confirmed toothbrushing with dentifrice to form less plaque postbrushing compared with brushing with water alone. 10,11,67,68 Also, experiments ranging from four days to five weeks exhibited higher inhibition of plaque regrowth by brushing with dentifrices as opposed to that by brushing with water alone. 9,12,[69][70][71][72] In the meta-analyses of this SR, a high heterogeneity was demonstrated for the studies that evaluated the products according to the PI of Q&H Turesky et al 26  The meta-analysis allowed for a subgroup analysis on the reported dentifrice ingredients some of which have claimed antiplaque activity. These were sodium fluoride (NaF), sodium monofluorophosphate (MFP), stannous fluoride (SnF), triclosan (Tcs) and baking soda. Irrespective of the Plaque Index used (Q&H Turesky et al, 26 Greene and Vermillion, 24 Plaque Area 15 ), the Tcs product numerically exhibited the highest inhibition of plaque regrowth. Interestingly, both NaF and MFP products, which contained no specific ingredients brought forward for their antimicrobial effect, exhibited, irrespective of the Plaque Index used in all the meta-analysis (Appendices S4, S5, and S6), a significant effect on the regrowth of plaque. Evidently, dentifrices contain more ingredients which exhibit inhibition of plaque regrowth of which SLS is the most commonly used ingredient.
Besides difference in means (DiffM) and 95% confidence intervals, we calculated also 95% prediction intervals. The advantage of also using prediction intervals is that it is more informative. It reflects the variation in treatment effects over different settings, including what effect is to be expected in future patients, such as the patients that a clinician is interested to treat. 29 The prediction intervals were all below zero and suggest that dentifrice will be beneficial when applied in at least 95% of the individual study settings, an important finding for clinical practice. 80,81 Most systematic reviews with meta-analyses are underpowered. 82,83 Trial sequential analysis (TSA) is a cumulative randomeffects meta-analysis method that estimates a "required information size" (ie, required meta-analysis sample size) using the same framework as sample size calculation for an individual RCT, but additionally accounting for heterogeneity and multiple comparisons when new RCTs are added. Also, before the required information size is reached, TSA constructs monitoring boundaries to determine when an estimated effect is so convincingly large (or small) that the conclusions are unlikely to change with more evidence. 42,82,84 The TSA of the Greene and Vermillion 24

| Post hoc sensitivity analysis
In a crossover trial, each participant serves as his/her own control.
A correlation coefficient describes how similar different measurements on interventions are within a participant.
Since the results of crossover trials are generally similar to those of parallel-arm trials, 85  Other limitations are described in detail in the Appendix S9.
In summary, plaque scores of the dentifrices slurries in the 4day nonbrushing models demonstrate a reduction in plaque formation. The question is whether this effect is noticeable under normal home-use conditions. Small reductions in plaque regrowth may reduce gingivitis and caries to a certain extent. Based on the findings of the present review, it is recommended that, with respect to plaque regrowth inhibition, a dentifrice should be used during toothbrushing. Future research may focus more specifically on active ingredients of dentifrices with assumed impact on dental plaque regrowth in different study models or on other reasons for using a dentifrice. In the future, dentifrice manufactures may reinforce its role as a nurturing dental cream.

| CON CLUS ION
The results of this review demonstrate moderate-quality evidence for a weak inhibitory effect on plaque regrowth in favour of the use of a dentifrice.

| Scientific rationale for the study
Twice-daily toothbrushing with a fluoride dentifrice is a universal recommendation for personal oral care. A recent review has indicated that dentifrice does not provide an additional effect to toothbrushing with respect to plaque removal. The plaque regrowth inhibitory property of dentifrices has not yet been systematically studied.

| Principal findings
This review demonstrates that toothpaste contributes to a reduction in plaque regrowth following a professional prophylaxis.

| Practical implications
Although dentifrice does not contribute to the mechanical plaque removing efficacy, the addition of dentifrice enhances the lasting effect of toothbrushing. Active plaque-inhibiting ingredients support the daily use of a regular dentifrice.

ACK N OWLED G EM ENTS
The authors gratefully acknowledge the support of Joost Bouwman, the head librarian of the ACTA, and Franc van der Vlugt, the head librarian of the KNMT mediacentrum, who helped retrieve full-text papers. The authors are also grateful to Prof. Dr. M. Addy for his responses, time and effort in searching for additional data. In addition, independent statistician Sharon Klaassen is acknowledged for the help in converting the imperial measurements.

CO N FLI C T O F I NTE R E S T A N D S O U RCE O F FU N D I N G S TAT E M E N T
The authors declare that they have no conflict of interests.