The two main dental diseases, caries and periodontal disease, are associated with biofilms and dysbiosis.¹ For caries, a repeated provision of fermentable carbohydrates leads to a change in supragingival biofilm composition, from nonacidogenic and nonaciduric to acidogenic, aciduric, and hence cariogenic bacteria, resulting in the generation of relevant organic acids leading to mineral loss of the hard tissues. For periodontal disease, pathogenic supragingival and, more so, subgingival biofilms are similarly characterized by a dysbiosis, provoking an inflammation of the periodontal tissue, ie gingivitis or, in later stages, periodontitis.

Mechanical biofilm removal is one main, causal strategy in prevention of both diseases, and in managing them. This mechanical biofilm removal is mainly performed via tooth brushing: Tooth brushing removes biofilm, and also serves to deliver fluoride as one main preventive strategy for caries prevention (see Chapter 3). However, tooth brushing is, even if performed with high quality, not able to remove biofilm sheltered in dental niches, mainly in interdental areas.^2,3 To allow mechanical biofilm removal in these interdental areas, interdental oral hygiene using either floss or interdental brushes has been recommended.

Flossing is performed by moving a dental floss, either waxed or nonwaxed, up- and downwards along the interdental tooth surfaces (Fig 7-1). Floss should not be used in a saw-like action. The application of floss into the interdental space is not easy for most patients, as some dexterity and training is needed. Conventionally, flossing is recommended for interdental cleaning of either very narrow interdental spaces (eg, crowded anterior mandibular region) or, more often, for interdental spaces with an intact gingival papilla. Interdental brushing, in contrast, is slightly easier to use for most patients, especially in anterior areas (up to premolars). The interdental brush should fill out the interdental space, and should not be too small, as its bristles need to be in contact with the dental surfaces to realize any cleaning effect. The brush is then used in a bottle-cleaning like motion of mainly fore and back movements, and also minimal rotations. Interdental brushes are available in different sizes to allow application in narrower and wider spaces; very small sizes can also be used in interdental areas with only minimal gingival recession.

The evidence for flossing

Recently, the evidence base of flossing has been re-examined after two systematic reviews questioned its preventive effect for caries, for plaque removal, and for the prevention of gingivitis.^4,5 In the case of caries, the plausibility of flossing as a preventive measure is generally accepted (see Chapter 4). There is evidence that the additional plaque removal induced by flossing generates only low caries-preventive effects, which are mainly conferred by fluoride toothpaste instead (see Chapter 6, Fig 6-2).⁶ There is ample evidence that plaque removal itself may have only limited preventive effectiveness, especially when fluoridated toothpaste is not a key part.⁷

Two more recent systematic reviews on interdental cleaning and its effect on plaque, gingivitis, and caries stress that no relevant studies on caries can be found, and that flossing is less effective for plaque removal than interdental brushes.^8,9 There is weak evidence that flossing, in addition to tooth brushing, may reduce gingivitis.⁹

From a practical point of view, flossing is one of the least adapted oral preventive measures: Both the sales figures for dental floss as well as the self-reported use of floss remain disappointing after decades of promoting its use.^10,11 Even in clinical trials on proximal caries lesion management, where patients are specifically instructed how to use floss, only a limited number of patients truly adopt,¹¹ and consequently, interproximal plaque and gingivitis scores do not improve.¹²

A more detailed look into the evidence on flossing was provided by a systematic review on the effect of dental flossing on interproximal caries.⁴ The review included six randomized clinical trials, which are preferable over cohort studies, where patients were assigned randomly to the test or control group, hence reducing the risk of a selection bias. For example, groups where dental floss is used regularly could also have a higher dental awareness and health literacy and overall lower caries risk. The different studies were conducted in various settings, which allow for a judgement of potential cofactors (Table 7-1):

• Professional flossing on schooldays shows clear caries preventive effects and feasibility.
• High-frequency professional flossing in schools would, however, be cost-intensive. On the other hand, a reduced frequency of flossing only every 3 months does not have a clinically relevant effect any more.
• Self-performed and even supervised flossing does not lead to relevant caries reductions, potentially due to inadequate application.

Table 7-1 Randomized clinical trials on the efficacy of dental flossing on interproximal caries (Hujoel et al⁴)

In addition, it may be added that flossing has some effectiveness to remove plaque and prevent gingivitis, again depending on patients’ compliance and application.

The evidence for interdental brushes

A recent systematic review found very limited evidence supporting the use of interdental brushes.⁹ For caries prevention, no study could be included at all. There are two studies on a total of 93 individuals demonstrating that these brushes are more effective in plaque removal than tooth brushing (Table 7-2). Consequently, gingivitis, measured by Gingival Index or Bleeding Index, was significantly reduced by interdental brushing; again, the same two studies (93 individuals) assessed these outcomes. Interdental brushes have also been found more effective than flossing for plaque removal and gingivitis prevention.

Table 7-2 Comparison of interdental brushes plus tooth brushing versus tooth brushing alone (from Worthington et al⁹ including references)

Clinical considerations

It seems that under ideal conditions, flossing can reduce dental plaque and subsequently also gingivitis. For caries, this association between plaque reduction and caries prevention seems less clear. However, the major problem of dental flossing is the compliance and performance. Children are generally not able to perform flossing at a decent quality and although data on adults are not available, controlled trials on caries lesion arrest in proximal areas demonstrate low motivation, adherence, and performance quality of flossing in adults.^12,20

Flossing was, in the past, mainly recommended for children, adolescents, and young adults with intact interproximal papillae. It is likely that these individuals will show limited caries increment anyway, at least in countries with effective caries-preventive systems or water fluoridation.²¹ Hence, for most low-risk individuals, it is generally questionable if dental flossing will have any preventive effect. On the other hand, improper use of flossing has been found to come with the risk of inducing “flossing clefts.”^22,23

For the remaining population of high-risk individuals, who bear the majority of caries experience, more basic oral hygiene measures including regular tooth brushing with fluoridated toothpaste²⁴ often needs to be implemented first. Flossing will have limited priority in these groups.

In cases with existing initial caries lesion on proximal surfaces, flossing can be a component of noninvasive caries treatment, possibly together with the application of fluoride varnishes, especially in case of lesions restricted to enamel or more compliant individuals.¹² Otherwise, micro-invasive measures (sealing, infiltration) may need to be applied.

If flossing is practiced by patients, it should not be demotivated by dental professionals, but monitored to increase its effectiveness and reduce the risk of harm.

For interdental brushes, there is evidence supporting their use to remove plaque and prevent gingivitis. For caries, but also for periodontitis, studies have not found any beneficial effect, likely due to too short follow-up periods or, for caries, also a limited plausibility. Interdental brushes further provide a more effective plaque removal and gingivitis prevention than flossing. Overall, the evidence supporting interdental brushes is more consistent than that on flossing. Dental practitioners should instruct patients on using interdental brushes, especially for patients who already suffer from periodontal diseases. For high-risk caries patients, other measures such as like fluoride provision may be prioritized, if needed, over the use of interdental brushes. Again, the use of interdental brushes requires professional training, including motivation and instruction/demonstration.

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