Are bristle stiffness and bristle end-shape related to adverse effects on soft tissues during toothbrushing? A systematic review

Abstract

Background: Toothbrushing might be associated with the development of soft tissue lesions. This systemic review aimed to examine soft tissue lesions caused by different bristle stiffnesses and bristle end-shapes in manual toothbrushes in adult individuals. Materials and methods: Scopus, EMBASE and PubMed databases were searched to find controlled trials that evaluated manual toothbrush bristle stiffness and/or end-shape in regard to soft tissue safety. The grey literature was also included in the search strategy. Two reviewers independently performed the screening, final selection and data extraction. To be included, studies had to have been performed using at least two manual toothbrushes differing in bristle stiffness and/or end-shape, and had to report any adverse effects on oral soft tissues after at least 7 days of follow-up. A meta-ethnography strategy was used for qualitative data synthesis. The Cochrane Collaboration tool was assessed to evaluate the risk of bias. Results: Thirteen studies were included from the 1,945 initially retrieved. Toothbrush bristle end-shape was investigated in six studies, bristle stiffness in two, and both features were investigated in five studies. Hard-bristle toothbrushes produced more gingival lesions than medium- and soft-bristle brushes. A slight gingival recession width increase was identified in the end-rounded group, compared with the tapered group. Only four studies presented adverse effects as the primary outcome. Conclusion: Soft and extra-soft toothbrushes tend to be safer. Oral soft tissue injuries are similar for both tapered and end-rounded bristles. Further studies investigating adverse effects as a primary outcome are recommended.

INTRODUCTION

It is well established that the presence of supragingival biofilm is necessary for the occurrence of both dental caries¹ and gingivitis². Manual toothbrushes are the most widely used instruments to remove biofilm³. Both manual toothbrush and dentifrice are commonly used twice a day in populations of developed countries⁴.

Good standards of oral hygiene have frequently been associated with soft tissue lesions, which may be a consequence of traumatic toothbrushing5., 6., 7.. One common soft tissue lesion caused by toothbrushing is gingival abrasion. Several randomised controlled trials (RCTs) have reported the development of this condition following manual toothbrushing8., 9., 10., 11..

It is hypothesised that development and progression of gingival recession are caused by repeated gingival abrasion lesions¹². However, there is no clear evidence to support this hypothesis13., 14.. Furthermore, the evidence supporting traumatic toothbrushing as a contributing factor in gingival recession is inconclusive¹⁵. As a consequence of gingival recession, exposed roots may become susceptible to dentine hypersensitivity¹⁶. Toothbrushing in the absence of a dentifrice appears to have little effect on dentine hypersensitivity¹⁷. However, toothbrushing combined with an intake of acidic food tends to intensify the erosive action of dietary acids17., 18..

Studies have reported that the stiffer the toothbrush, the better the quality of dental plaque removal¹⁹. Nevertheless, the use of medium or hard toothbrushes has been linked to adverse oral soft tissue effects11., 19., 20.. Similarly, end-rounded bristles may be superior in removing dental plaque, when compared with tapered bristles²¹. Additionally, end-rounded bristles may be less harmful than tapered bristles, as fewer gingival lesions are expected in comparison to non-rounded bristles8., 9..

Considering the broad range of toothbrush availability and use in mechanical biofilm removal, the present systematic review aimed at evaluating adverse events on soft tissues of adult individuals after use of different manual toothbrush bristle stiffness and end-shapes.

MATERIALS AND METHODS

This systematic review followed the PRISMA guidelines²². The focused question was given as follows: ‘Are bristle stiffness and toothbrush bristle end-shapes related to adverse soft tissue effects during toothbrushing in adult individuals?’ Therefore, the PICO was as follows:

• Patients (P): adult individuals;

• Intervention (I): any bristle stiffness and/or toothbrush bristle end-shapes, such as soft and end-rounded, respectively;

• Comparison (C): different bristle stiffness and/or toothbrush bristle end-shapes, such as extra-soft, medium or hard and tapered, respectively;

• Outcome (O): any adverse effect on oral soft tissues, such as gingival abrasion, gingival recession, gingival fissures, sensitivity or any other injury to the soft tissues.

Information sources and search strategy

Three scientific literature databases were used in this systematic review, which included MEDLINE-PubMed, EMBASE and Scopus. Controlled clinical trials were used from online databases up to September 2017. The search strategy for EMBASE and SCOPUS was adapted from that of PubMed, which is described in Figure 1.

Figure 1.

Search strategy performed on PubMed database.

The authors of the included studies were contacted in order to find any potential relevant grey literature. Additionally, Trip Database, Google Scholar, Centre for Reviews and Dissemination, and NYAM Grey Literature were searched. Protocols in the Clinical Trials website were screened for eligibility. An adapted search strategy was used as described in Figure 1.

Eligibility criteria and risk of bias assessment

Papers had to fulfill the following criteria to be included:

• •RCTs or non-RCTs
• •Subjects aged 18+ years
• •Comparison between at least two different manual toothbrushes, and report of any adverse effect on oral soft tissues, such as gingival abrasion, gingival recession, gingival fissures, gingival sensitivity and gingival bleeding
• •Toothbrush bristle stiffness could be hard, medium, soft or extra-soft bristles
• •The toothbrush bristle end-shape could be tapered or end-rounded
• •Minimum of 7 days of follow-up.

No restrictions were made due to date and publication language. However, studies presenting one of the following characteristics were excluded:

• •Comparison between powered and only one manual toothbrush
• •Comparison between manual toothbrushes with the same bristle stiffness and end form
• •Single-use clinical trials (those in which brushing was performed only once)
• •Case reports, case series, animal models, letters to the editor, in vitro studies and reviews.

Study selection

Two reviewers independently performed the study selection (NR and FWMGM). Full-text analysis was performed only if sufficient information was available in the title and abstract. When an abstract was not available, the study was included after screening. A third researcher (CKR) was involved in this process to provide consensus after disagreement between examiners. The references of all selected studies and related systematic reviews13., 15., 23., 24., 25. were also screened for eligibility.

Assessment of risk of bias

The Cochrane Collaboration tool was used to evaluate the studies²⁶. Seven criteria were used to independently evaluate each selected study by two reviewers (FWMGM and NR). In case of detailed information or description, the study was attributed low risk of bias. The article was considered as having a high risk of bias when any missing information was detected. When neither high nor low risk of bias could be assigned, an unclear risk of bias was attributed. Extensive discussion was used to solve any discrepancies between the reviewers.

Data collection

Two reviewers (FWMGM and NR) independently conducted the data collection, using a spreadsheet specifically created for this study. Data collected included author, year of publication, country, number of participants included, toothbrush bristle stiffness, bristle end-shape, toothbrush brand, use of dentifrice with abrasives, allowance of other types of oral products for mechanical removal of biofilm (such as dental floss or toothpicks), assessment of adverse effects reported in the methodology, study follow-up, type of oral soft tissue lesion assessed, study design, sampling, brushing technique, clinical procedures performed prior to the beginning of the study, toothbrushing frequency per day, total number of oral soft tissue lesions, smoking exposure, conflicts of interest, and any additional observations.

The two reviewers discussed each variable collected in order to extract the most appropriate one. In case of disagreement, a third reviewer was involved in the process (CKR). A meta-analysis was not viable, considering the heterogeneity among the studies. Therefore, the results are presented descriptively in the tables.

Assessment of quality of evidence

The main outcomes assessed (toothbrush bristle stiffness and end-shape) were rated using the GRADE approach²⁷ to evaluate the overall quality of evidence.

RESULTS

Study selection

Figure 2 presents the flowchart of the study selection. From the 1,945 individual studies retrieved, 13 unique studies were included in this study19., 28., 29., 30., 31., 32., 33., 34., 35., 36., 37., 38., 39..

Figure 2.

Flow of articles through review and selection.

General characteristics of the included studies

Two studies analysed toothbrush bristle stiffness19., 28., six studies compared bristle end-shape29., 30., 31., 32., 33., 34., and five studies compared both bristle stiffness and end-shape35., 36., 37., 38., 39.. Information regarding how bristle end-shapes were evaluated was available from the authors in five studies29., 33., 34., 36., 39.. Meanwhile, the bristle end-shapes of the six remaining studies were determined by contacting the manufacturer30., 31., 32., 35., 37., 38.. No specific method of determining bristle end-shape was reported in any of the included studies.

All articles included in this study were fully or partially sponsored by the industry. In addition, all clinical trials used a standard dentifrice throughout the evaluation period. Eight studies included the use of toothbrush and dentifrice throughout the entire study period19., 32., 33., 34., 35., 37., 38., 39.. Seven articles did not mention if subjects were current smokers19., 29., 30., 31., 34., 35., 37., and six articles described subjects as non-smokers28., 32., 33., 36., 38., 39. (Table 1).

Table 1.

Description of the included articles presenting safety as primary or secondary outcomes

	Reference	Design/treatment performed/evaluation period	Subject population and number of subjects that completed the study (n)	Outcome parameters	Test groups brand (n)/gender (n) and age/bristle stiffness and end-shape and brushing methodology	Safety results
Safety as a primary outcome	Checchi et al.29	•RCT, parallel, single-blind; •Baseline dental debridement; •6 months	•Dental students. •No periodontal disease or history of periodontal surgery. •n = 30	•Plaque •Gingivitis •Gingival Recession	•Meridol® (15): S & TP: Male: 8; Female: 7; Mean age: 21.7 •ADA (15): S & ER: Male: 7; Female: 8; Mean age: 21.8 •Bass technique; Brushing twice a day for at least 2 minutes	•No presence of new gingival recession lesions; •Meridol® presented 0.5 mm reduction in width of gingival recession of 30% and 14%, while ADA showed 0.5 mm increase in width of gingival recession of 40% and 6% at 3 and 6 months, respectively
	Sgan-Cohen et al.37	•RCT, parallel, single-blind; •Supragingival debridement 1 week before baseline; •8 weeks	•Generally healthy adults that had at least minimal dentine exposure and sensitivity; •n = 107	•Plaque •Soft lesions adverse effects: irritation, sensitivity, gingival abrasion	•Elmex® Sensitive Extra Soft (54): ES & TP: Male: ?; Female: ?; Mean age: 45.4 •ADA (53): S & ER: Male: ?; Female: ?; Mean age: 45.8 •No oral hygiene instruction was provided; Brushing twice per day for 2 minutes	No adverse effects attributable to using either of the toothbrushes were reported by the subjects or found by the clinician during the study
	Versteeg et al.33	•RCT, split mouth, single-blind; •Dental prophylaxis at baseline; •4 weeks	•Generally healthy adults with minimum of 5 teeth in each quadrant of lower jaw; non-smokers; no oral lesions and no periodontal pockets >5 mm; a level of gingival bleeding of more than 25%; •n = 32	•Plaque •Gingivitis •Gingival abrasion (disclosing solution)	•Meridol® (32): S & TP: Male: 9; Female: 23; Mean age: 24 •ADA (32): S & ER: Male: 9; Female: 23; Mean age: 24 •Bass technique; Brushing twice per day for 1 minute	•Presence of new gingival abrasion lesions; •Mean (SD) gingival abrasion scores: Baseline: Meridol® = 0.94 (1.52); ADA = 1.28 (1.87); 1 week: Meridol® = 0.81 (1.71); ADA = 2.16 (2.97); 2 weeks: Meridol® = 1.06 (1.42); ADA = 2.44 (2.11); 4 weeks: Meridol® = 1.03 (1.79); ADA = 1.44 (1.85); There were no statistically significant differences between the two groups (P = 0.1009)
	Zimmer et al.19	•RCT, parallel, single-blind; •Supragingival debridement at the screening examination; •8 weeks	•Generally healthy adults with >16 teeth and no history of diabetes and severe periodontal disease; PBI per tooth:0.5 and QHI per tooth:2.0; •n = 120	•Plaque •Gingivitis •Gingival lesions (DI)	Dr Best Plus® Toothbrush – TP &: •Group 1 (40): H: Male: ?; Female: ?; Mean age: ? •Group 2 (40): M: Male: ?; Female: ?; Mean age: ? •Group 3 (40): S: Male: ?; Female: ?; Mean age: ? Modified Bass technique; Brushing twice per day for 2 minutes	•Absolute number of ≤ 1 gingival lesions according to each group tested (4 weeks/8 weeks): Bristle stiffness: 4 weeks: (H) = 17; (M) = 5; (S) = 5 8 weeks: (H) = 20; (M) = 6; (S) = 2 •After 4 and 8 weeks, hard toothbrushes showed more gingival lesions compared with soft and medium toothbrushes (P < 0.01), whereas there were no statistically significant differences between the latter two groups
Safety as a secondary outcome	Harpenau et al.36	•RCT, crossover, single-blind; •Dental debridement at baseline; •5 weeks (1 week wash-out)	•Dental hygiene students, non-smokers, with plaque score ≥ 1.8; •n = 21	•Plaque *Oral soft tissue examination at baseline	•Curvex® (21): ES & TP: Male: ? Female: ? Mean age: ? •Oral-B® Indicator® 40 (21): S & ER: Male: ? Female: ? Mean age: ? •Bass technique; Brushing twice per day for 2 minutes	No abrasions of the soft tissues or bristle ‘splaying’ were observed for either brush in this study
	Gallob et al.34	•RCT, parallel, single-blind; •None; •12 weeks	•Generally healthy adults with ≥ 20 teeth and a mean of QHI ≥ 1.5 and LSGI ≥ 1.0; •n = 75	•Plaque •Gingivitis *Oral cavity examined for adverse effects at each visit	•Colgate SlimSoft (36): S & TP: Male: 10; Female: 24; Mean age: 45.5 •ADA (39): S & ER: Male: 7; Female: 30; Mean age: 53.0 •No oral hygiene instruction was provided; Brushing twice per day	One adverse effect related to product usage was reported in the S & TP toothbrush. The adverse effect was not specified
	Mankodi et al.30	•RCT, parallel, single-blind; •Not reported; •4 weeks	•Generally healthy adults with ≥ 20 teeth and a mean of RMNPI ≥ 0.6 and a mean LSGI ≥ 1.0; •n = 81	•Plaque •Gingivitis •The study did not describe oral soft tissue examination in the methodology	•Colgate® 360°™(39): S & TP: Male: 13; Female: 26; Mean age: 24 •Oral-B® Indicator® (42): S & ER: Male: 5; Female: 37; Mean age: 48.2 •No oral hygiene instruction was provided; Brushing twice per day for 1 minute	No adverse effects attributable to using either of the toothbrushes were reported by the subjects or found by the clinicians during the study
	Nathoo et al.31	•RCT, parallel, single-blind; •Not reported; •4 weeks	•Generally healthy adults with ≥ 20 teeth and with RMNPI ≥ 0.6 and LSGI ≥ 1.0; •n = 78	•Plaque •Gingivitis •The study did not describe oral soft tissue examination in the methodology	•Colgate® 360°™(39): S & TP: Male: 15; Female: 24; Mean age: 38.3 •Oral-B® CrossAction® (39): S & ER: Male: 16; Female: 23; Mean age: 33.7 •No oral hygiene instruction was provided; Brushing twice per day for 1 minute	No adverse effects attributable to using either of the toothbrushes were reported by the subjects or found by the clinician during the study
	Ren et al.39	•RCT, parallel, single-blind; •None; •4 weeks	•Generally healthy adults, non-smokers, with ≥ 20 teeth except third molars and moderate gingivitis (LSGI ≥ 1.0); •n = 84	•Plaque •Gingivitis *Oral cavity examined for adverse effects as all as patients were asked about irritation, tooth sensitivity and soft tissue trauma during each examination	•Elmex® Sensitive Extra Soft (28): ES & TP: Male: 13; Female: 15; Mean age: 32.6 •Elmex® Sensitive Soft (27): S & TP/ER: Male: 12; Female: 15; Mean age: 33.4 •ADA (29): S & ER: Male: 13; Female: 16; Mean age: 32.1 •No oral hygiene instruction was provided; Brushing twice per day	There were no signs of tissue trauma or abrasion from clinical evaluations
	Rösing et al.35	•RCT, parallel, single-blind; •None; •1 week	•Generally healthy adults, with ≥ 20 teeth, except third molars, and RMNPI of ≥ 0.6; •n = 70	•Plaque •Oral soft tissue examination at each measurement period	•Colgate SlimSoft (35): S & TP: Male: 13; Female: 22; Mean age: 31.89 •Curaprox CS5460 (35): EX & ER: Male: 19; Female: 16; Mean age: 33.26 •No oral hygiene instruction was provided; Brushing twice per day	Tooth sensitivity was reported in the S & TP toothbrush. Burning sensation, gingival sensitivity, gingival bleeding and development of aphthae were reported in the ES & ER toothbrushes
	Sgan-Cohen et al.28	•RCT, parallel, single-blind; •Dental debridement at baseline; •4 weeks	•Generally healthy adults, non-smokers, with ≥ 20 teeth and with no hard or soft tissue pathologies; •n = 112	•Plaque •Gingivitis •Oral soft tissue examination at each measurement period	•Elmex® interX medium (56): M & ER: Male: ?; Female: ?; Mean age: ? •ADA (56): S & ER: Male: ?; Female: ?; Mean age: ?; •Subjects randomised to be given oral hygiene instruction; Brushing twice per day	No adverse effects attributable to using either of the toothbrushes were reported by the subjects or found by the clinician during the study
	Sgan-Cohen et al.32	•RCT, parallel, single-blind; •Dental debridement at baseline; •8 weeks	•Generally healthy adults with ≥ 20 teeth, non-smokers, and no periodontal disease; •n = 68	•Plaque •Gingivitis •Oral soft tissue examination at each examination period (irritation, sensitivity)	•Meridol®(35): S & TP: Male: 15; Female: 20; Mean age: 36 •ADA (33): S & ER: Male: 14; Female: 19; Mean age: 35.5 •No oral hygiene instruction was provided; Brushing twice per day for 2 minutes	No adverse effects were detected using either of the toothbrushes provided in the study
	Sharma et al. Study 138	•RCT, parallel, single-blind; •Dental prophylaxis at baseline; •12 weeks	•Generally healthy adults, non-smokers, with ≥ 18 teeth and a baseline whole mouth RMNPI of 0.6 and MGI of at least 1.80; •n = 99	•Plaque •Gingivitis •Safety assessment for hard and soft tissues, during the study	•Dr Best® InterDent (51): S & TP: Male: 22; Female: 29; Mean age: 35.31 •Oral-B CrossAction (48): S & ER: Male: 15; Female: 33; Mean age: 34.75 •No oral hygiene instruction was provided; Brushing twice per day	No adverse effects were detected using either of the toothbrushes provided in the study

RCT, randomised controlled trial; QHI, Quigley and Hein plaque index; RMNPI, Rustogi Modification of the Navy Plaque Index; PBI, Papillary Bleeding Index; MGI, Modified Gingival Index; LSGI, Löe-Silness Gingival Index; DI, Danser gingival abrasion Index; ES, extra-soft; S, soft; M, medium; H, hard; TP, tapered; ER, end-rounded; ?, no information provided in the article.

Risk of bias assessment

Six articles showed low risk of bias in random sequence generation29., 33., 35., 36., 38., 39., and the other seven studies were unclear (Figure 3). In all studies, the volunteers and researchers were not blinded properly, presenting a high risk of bias in these criteria. Conversely, a low risk of bias was detected in the 11 included studies19., 28., 29., 31., 33., 34., 35., 36., 37., 38., 39. regarding incomplete outcome data, blinding of outcome assessment, and selective reporting.

Figure 3.

Risk of bias for the selected studies.

Bristles stiffness and soft tissue lesions

The results of the safety analysis for each group according to the differences in bristle characteristics are specified in Table 2. One study found more gingival lesions in those that use hard-bristled toothbrushes in comparison to both medium and soft toothbrushes¹⁹. Rösing et al.³⁵ observed tooth sensitivity with the use of soft toothbrushes (end-rounded bristles), while the extra-soft toothbrush (tapered) caused a burning sensation, gingival sensitivity, gingival bleeding and the development of aphthae. All studies evaluating bristle stiffness observed no adverse effects over time28., 36., 37., 38., 39..

Table 2.

Safety analysis of the studies grouped by bristle characteristics

Groups	Reference	Description of bristle end-shape/stiffness	Conclusion
Bristle stiffness	Zimmer et al.19	Hard vs. medium vs. soft (all TP)	More gingival lesions are shown from hard toothbrushes in comparison to soft and medium toothbrushes (P < 0.01) after 4 and 8 weeks. No significant differences between medium and soft groups
	Sgan-Cohen et al.28	Medium vs. soft (both ER)	Over 4 weeks, no AE were reported
Bristle end-shape	Checchi et al.29	TP vs. ER (both soft)	The majority of gingival recessions remained stable over 6 months. However, a slight gingival recession width reduction was observed in the TP group, and an increase in gingival recession width observed in the ER group
	Mankodi et al.30	TP vs. ER (both soft)	No AE reported over 4 weeks
	Nathoo et al.31	TP vs. ER (both soft)	No AE reported over 4 weeks
	Sgan-Cohen et al.32	TP vs. ER (both soft)	No AE reported over 2 months
	Versteeg et al.33	TP vs. ER (both soft)	There was an inclination toward fewer sites with gingival abrasion with the use of an ER toothbrush in comparison to a TP toothbrush over 4 weeks
	Gallob et al.34	TP vs. ER (both soft)	One AE related to product usage was reported with the TP toothbrush. The AE was not specified
Bristle stiffness and bristle end-shape	Harpenau et al.36	Extra-soft and TP vs. soft and ER	No AE reported over 5 weeks
	Sgan-Cohen et al.37	Extra-soft and TP vs. soft and ER	No AE reported over 8 weeks
	Sharma et al. Study 138	Soft and TP vs. soft and ER	No AE reported over 12 weeks
	Ren et al.39	Extra-soft and TP vs. soft and TP/ER vs. soft and ER	No AE reported over 4 weeks
	Rösing et al.35	Soft and TP vs. extra-soft and ER	Tooth sensitivity was reported with the soft and TP toothbrushes. Burning sensation, gingival sensitivity, gingival bleeding and aphthae development were reported with the extra-soft and ER toothbrushes

AE, adverse event; ER, end-rounded; TP, tapered.

Bristle end-shape and soft tissue lesions

Eleven studies analysed toothbrush end-shape and adverse events29., 30., 31., 32., 33., 34., 35., 36., 37., 38., 39.. Of these, seven clinical trials exclusively analysed bristle end-shape using soft toothbrushes with end-rounded or tapered filaments29., 30., 31., 32., 33., 34., 36.. Of these seven, three studies29., 32., 33. investigated a tapered toothbrush branded Meridol® (GABA Intl., AG, Switzerland) with a control toothbrush (ADA) with end-rounded filaments. Checchi et al.²⁹ observed that gingival recession width was slightly increased in the end-rounded group, and Versteeg et al.³³ showed a tendency toward gingival abrasion with the use of end-rounded toothbrushes. Gallob et al.³⁴ observed an adverse event with the use of a tapered toothbrush, but it was not specified in the article. Rösing et al.³⁵ also observed adverse effects with both tapered and end-rounded toothbrush bristles. All other studies did not observe any adverse effects with the use of tapered or end-rounded toothbrushes over time30., 31., 32., 36., 37., 38., 39..

Soft tissue safety as a primary outcome

From the 13 selected studies in this systematic review, four19., 29., 33., 37. clinical trials presented soft tissue safety as a primary outcome, and the characteristics are described in Table 1. Of those, three studies described an association between bristle stiffness¹⁹ and end-shape29., 33. with adverse effects, including soft tissue trauma, gingival recession and gingival abrasion, respectively. In these studies, a high level of heterogeneity was noticed in the evaluation of adverse effects and soft tissue lesions, as well as in the study duration and brushing method. Checchi et al.²⁹ evaluated gingival recession, while Versteeg et al.³³ and Zimmer et al.¹⁹ evaluated gingival abrasions using two different scores for lesion measurement, both using a disclosing solution. Sgan-Cohen et al.³⁷ observed soft tissue lesions as adverse effects, including irritation, sensitivity (subjective questionnaire) and gingival abrasion, which was not observed with the disclosing solution. The total length ranged from 4 weeks³³ to 6 months²⁹. Three studies used the Bass technique or a modified version in the brushing methodology19., 29., 33.. One study did not provide any oral hygiene instruction³⁷.

Soft tissue safety as a secondary outcome

When safety was assessed as a secondary outcome, two studies identified adverse effects34., 35.. Seven studies did not report any adverse effects28., 30., 31., 32., 36., 38., 39.. A high level of heterogeneity was evident in these studies, especially in the analysis of adverse effects. Oral soft tissue examination was analysed at the baseline³⁶ or at each examination period28., 32., 34., 35., 38., 39.. Two studies did not describe safety examination in their methodology30., 31.. The clinical trial duration varied from 7 days to 12 weeks, and the majority of the studies did not provide oral hygiene instruction. These studies were all described as examiner-masked RCTs.

Quality of evidence at the review level

Table 3 summarises the quality of the evidence at the review level for both toothbrush bristle stiffness and end-shape analysis, and it was considered low for both outcomes.

Table 3.

Summary of the quality assessment of toothbrush bristle stiffness and end-shape analyses

Quality assessment							Summary of findings
No. of studies	Study design	Risk of bias	Inconsistency	Indirectness	Imprecision	Other considerations	Impact	Quality	Importance
Toothbrush bristle stiffness and oral soft tissue lesions (follow-up: range from 7 days to 12 weeks)
7	Randomised trials	Very serious*	Not serious	Serious*	Not serious	Dose response gradient	The risk of bias detected in the studies and the absence of clear methodologies to assess oral soft tissue lesions may impact the results	⨁⨁◯◯ LOW	Critical
Toothbrush end-shape and oral soft tissue lesions (follow-up: range from 7 days to 6 months)
11	Randomised trials	Very serious†	Not serious	Serious†	Not serious	Dose response gradient	The risk of bias detected in the studies and the absence of clear methodologies to assess oral soft tissue lesions may impact the results	⨁⨁◯◯ LOW	Critical

^*Allocation concealment is not properly reported in all studies. Moreover, the number of smokers in each group is not well described in each study. The lack of specific methodologies to assess oral soft tissue lesions may impact the main results. ^†The allocation concealment and blinding of outcome assessments are not properly reported in all studies. Moreover, the number of smokers in each group is not well described in each study. The lack of specific methodologies to assess oral soft tissue lesions may impact the main results.

DISCUSSION

Different systematic reviews have evaluated potential adverse effects caused by both manual and powered toothbrushes15., 24., 25.. However, this is the first systematic review assessing potential effects of manual toothbrush bristle stiffness and end-shape on oral soft tissues.

It should be emphasised that the included studies were heterogeneous; therefore, it was not possible to statistically merge the results for a meta-analysis. Moreover, considering the whole body of selected evidence, gingival recession has not been thoroughly studied, due to ethical problems, as well as the necessity for large and lengthy studies that have not yet been performed.

Previously, RCTs using a toothbrush only once per day (single-use RCT) identified an association between gingival abrasion and toothbrush bristle stiffness¹¹ or end-shape8., 9., 21.. However, single-use RCTs were excluded in the present systematic review, as the analysis was not performed over time. We assume that pre- and post-brushing single assessments would not represent a satisfactory period for the development of adverse soft tissue effects, such as sensitivity and gingival recession.

Zimmer et al.¹⁹ showed a significant difference in the presence of gingival lesions with hard-bristled toothbrush use in comparison to medium- and soft-bristled toothbrushes. The use of hard40., 41. and medium toothbrushes⁴² has previously been reported to produce more soft tissue injuries leading to gingival recession than soft toothbrushes. In this systematic review, gingival recession was not an adverse effect evaluated according to the different bristle stiffness levels. Nevertheless, gingival recession was assessed by comparing soft-bristle toothbrushes with tapered and end-round bristles²⁹. It was concluded that the majority of gingival recession lesions were stable over 6 months, but with a tendency toward increased width with the use of an end-rounded toothbrush. In the same way, Versteeg et al.³³ evaluated gingival abrasion over 4 weeks and found more lesions after the use of end-rounded toothbrushes when compared with tapered ones.

Gingival abrasion lesions are caused by toothbrushing trauma that removes a portion of the surface epithelium. Thus, it is plausible that repeated gingival abrasion lesions might result in gingival recession12., 43.. However, there is no clear evidence that gingival abrasion caused by toothbrushing results in recession13., 14.. In contrast, a systematic review assessing gingival recession progression and development caused by the effects of toothbrushing determined that the association between gingival recession and toothbrushing is inconclusive¹⁵.

Sgan-Cohen et al.³⁷ classified their study as a double-blind RCT. However, all studies herein were characterised as being single-blind in design, as it is not possible to mask different toothbrushes. In addition, two studies did not describe safety in the methodology30., 31., and this may have contributed to unnoticed lesions already present at the time of evaluation.

Therefore, the criteria of blinding of participants and personnel presented a high risk of bias in all articles. Furthermore, other biases were defined as unclear in the majority of cases if smoking habits were not clearly described in the sample demographics, and if the calibration process was not performed or reported. Additionally, one study did not blind the examiner³⁰; thus, the outcome assessments presented a high risk of bias. A blinded examiner was reported in the other selected studies.

This is the first systematic review assessing the adverse events of toothbrush bristle stiffness and/or end-shape. Both tapered and end-rounded toothbrushes were considered to be safe, and there was no evidence to indicate one type of toothbrush over another. Moreover, other strengths may be highlighted in the studies, such as the majority masking the examiners and some having adequately estimated sample sizes.

None of the articles assessing safety as a secondary outcome showed an association between bristle stiffness or end-shape and adverse effects. This might be explained by the fact that some gingival lesions could only be detected by meticulous methodology. An example is the use of disclosing agents that stain gingival lesions to differentiate abrasions from non-traumatised tissue8., 44., or even the use of photography to analyse gingival abrasion¹¹.

High heterogeneity was detected between the studies; therefore, no meta-analysis could be performed. Moreover, only four studies presented adverse effects as primary outcomes. These are the main limitations of the current literature. Additionally, the quality of evidence for both outcomes assessed is low, suggesting that better-designed studies, with a lower risk of bias, are necessary. It must be highlighted that some methodological issues may have impaired the results herein. Several of the articles included in the present systematic review did not properly report how the random sequences were generated or how the allocation concealment occurred. Furthermore, it is not possible to blind the participant. More studies assessing soft tissue lesions as a primary outcome, with specific methodologies to verify gingival abrasion, are warranted.

Therefore, the reviewed literature is still inconclusive in terms of adverse events. Gingival recession could be considered a true outcome of these studies and gingival abrasion a surrogate. However, this could be due to an absence of evidence. Thus, it is recommended that all studies of plaque control using toothbrushes should include more detailed reports of adverse events. Long-term studies are also recommended with gingival recession as an outcome.

The safety of manual toothbrush usage over time is more often described as a secondary outcome than a primary outcome. Generally, the descriptive analysis of this systematic review suggests that soft and extra-soft toothbrushes tend to be safe, and both tapered and end-rounded bristles present no clinical relevance concerning soft oral tissue damage. Therefore, further RCTs should assess toothbrush safety over time as a primary outcome parameter.

Acknowledgements

This study was self-funded, and the authors report no conflict of interest.