Anatomy

Interdental space

Evidence: Medium Anatomy Reading time: approx. 5 min. Reviewed: 10.08.2026 · KEERN Editorial Team

The interdental space is the gap between two neighboring teeth. It accounts for roughly 30 to 40 percent of all tooth surfaces, yet is not fully accessible to toothbrushes. Germany's S3 guideline considers regular interdental cleaning biologically plausible for caries protection and recommends it several times weekly, even though the evidence for individual tools is inconsistent.

Related terms:BiofilmPlaqueCariesGingivitisInterdental brush

Definition

The interdental space is the gap between two neighboring teeth. According to Germany's S3 guideline on caries prevention, roughly 30 to 40 percent of all tooth surfaces lie within interdental spaces: a substantial share that is easily overlooked when brushing. Toothbrushes cannot fully reach this area due to their shape, even with careful technique.

Exactly where the toothbrush cannot reach, biofilm tends to accumulate especially reliably, with corresponding consequences for caries risk on the contact surfaces of teeth.

In short

The interdental space is every toothbrush's blind spot. Additional tools (floss, interdental brushes, or similar products) are biologically sensible for reaching this area. Which tool fits best in an individual case depends on the size of the interdental spaces and personal handling.

What surprises many people

It is often reported that the scientific evidence for floss is ‘weak,’ a finding frequently misread as ‘floss does not do anything.’ What is actually lacking, above all, are well-controlled long-term studies examining caries directly as an endpoint; most studies instead measure plaque reduction as a substitute measure. But an absence of evidence for a benefit is not the same as evidence of an absent benefit; the biological rationale for interdental cleaning remains plausible, and newer individual studies do find an association with reduced caries.

Why does the guideline recommend it anyway?

This can seem contradictory at first: weak study evidence, yet a recommendation anyway. Guidelines can incorporate biological plausibility and the totality of available evidence, not only direct caries-endpoint RCTs. In this case, the German S3 guideline considers regular interdental cleaning biologically plausible and recommends it several times per week.

Tools at a glance

  • Floss: flexible to use, often more practical where the contact point between teeth is tight
  • Interdental brushes: may be a better anatomical fit for larger or more open spaces; comparative studies also suggest a possible advantage over floss for some gingival outcomes, though the certainty of this evidence is low to very low and no single tool is superior for every patient or outcome
  • Wooden or rubber cleaning sticks: simple to handle, particularly for larger spaces
  • Oral irrigators: pleasant to use; the evidence available does not show a clear advantage over toothbrushing alone on the endpoints studied

How often and how

Germany's S3 guideline recommends regular interdental cleaning several times a week. Whether it happens before or after the rest of toothbrushing is not decisive; what matters more is correct, regular use. An introduction from a dental practice can help find the right tool and improve technique.

Key takeaways

  • According to Germany's S3 guideline, the interdental space accounts for 30 to 40 percent of all tooth surfaces and is not fully accessible to toothbrushes
  • Floss, interdental brushes, and similar tools specifically supplement toothbrushing at this spot
  • Weak study evidence does not mean interdental cleaning is ineffective: guidelines can also weigh biological plausibility and the totality of available evidence
  • Interdental brushes may be a better anatomical fit for larger spaces and may outperform floss for some gingival outcomes, though the certainty of that comparative evidence is low to very low
  • Regular use several times a week is recommended

Frequently Asked Questions

Why isn't toothbrushing alone enough?

Toothbrushes barely reach the contact surfaces between teeth. Biofilm accumulates there without being removed during normal brushing, no matter how thoroughly someone brushes.

Floss or interdental brush: which is better?

It depends partly on anatomical fit and partly on the outcome you're asking about. For tight contact points, floss is often more practical. For larger or more open spaces, an interdental brush may be a better fit. Comparative studies also suggest interdental brushes may outperform floss for some gingival outcomes, but the certainty of that evidence is low to very low, and no single tool is superior for every patient or outcome. A dental practice can help with the selection.

Is it true that floss doesn't work?

No, that is a misconception. The study evidence is weak, mainly because long-term studies using caries as a direct endpoint are lacking. That does not mean floss is ineffective; the biological rationale remains plausible, and some newer studies find an association with reduced caries.

Are oral irrigators a good alternative?

Oral irrigators are pleasant to use. The evidence available, however, does not show a clear advantage over toothbrushing alone on the endpoints studied. They can still be a sensible addition, but do not replace mechanical cleaning of the contact surfaces.

Before or after toothbrushing?

The order does not seem to matter much. What matters is regular, correct use, several times a week, as recommended by Germany's S3 guideline.

KEERN Perspective

The interdental space exemplifies, for KEERN, why good oral care does not end at the toothbrush. An area that accounts for 30 to 40 percent of tooth surfaces deserves the same care as the readily visible surfaces, even though it is easily overlooked.

Which tool fits an individual case is most reliably clarified in a dental practice.

Professional perspective

Clinical relevance

  • Counseling on individually appropriate interdental tools based on space size
  • Nuanced framing of the evidence for patients, without unsettling them through oversimplified media coverage
  • Instruction on correct technique as a key factor in effectiveness

Evidence in detail

The Cochrane review by Worthington et al. (2019) summarizes 35 randomized controlled trials on home-use interdental tools. Not a single study arm examined caries as a direct endpoint; plaque reduction served throughout as the surrogate parameter. In overview: floss in addition to toothbrushing reduces plaque more than toothbrushing alone (very low-certainty evidence, study duration 3 to 6 months); for interdental brushes, the evidence is low-certainty; wooden sticks showed no additional benefit for plaque or biofilm reduction in the included short-term studies (3 months); for rubber/elastomeric cleaning sticks, the evidence is very low-certainty (1 month); for oral irrigators, only low-certainty evidence exists, and it does not show a clear additional effect beyond toothbrushing alone on the endpoints evaluated. In head-to-head comparisons between tools, very low-certainty evidence suggests interdental brushes may outperform floss for some gingival outcomes; no single tool is established as superior for every patient or outcome.

More recent studies with caries as the endpoint

A small number of individual studies, not yet incorporated into systematic reviews, report an association with caries prevention. In a cross-sectional study of 2,031 first-grade schoolchildren in Iran, floss use was significantly associated with lower dmft and DMFT index values, alongside factors such as brushing frequency, parental education, and family size (Kamiab et al., 2021). Using NHANES data, self-reported interdental cleaning in US adults was associated with a significantly lower prevalence of caries, periodontitis, and edentulism (Marchesan et al., 2018), and a comparable, though somewhat smaller, association was found in a separate NHANES-based analysis of older adults (Marchesan et al., 2020). All three studies are observational and cross-sectional, establishing association rather than controlled intervention effect; they require confirmation through further high-quality studies before they can be incorporated into updated systematic reviews.

Guideline status

Germany's S3 guideline ‘Caries Prevention in Permanent Teeth’ (AWMF 083-021, Chapter 6.1.6) issues a recommendation on this evidence base (evidence-based recommendation 4, new in 2023, Grade B, strong consensus 14/0/0): it is biologically plausible that regular interdental cleaning has a caries-protective effect, and it should therefore be carried out regularly, several times a week. Evidence basis: 1++ (Worthington et al., 2019), 2+ (Marchesan et al., 2018, 2020; Kamiab et al., 2021). Guidelines can incorporate biological plausibility and the totality of available evidence, not only direct caries-endpoint RCTs; Grade B here reflects that broader basis alongside the study evidence.

What current evidence does not show

  • None of the studies included in the systematic reviews examined caries directly as an endpoint; the evaluation relies predominantly on the surrogate parameter of plaque reduction
  • An absence of evidence for the benefit of individual tools is not equivalent to evidence of an absent benefit; the guideline explicitly emphasizes this
  • No clear superiority of any single tool for all patients or all outcomes; individual suitability depends on anatomical factors and handling
  • The observational studies with caries as a direct endpoint establish association, not controlled intervention effect, and have not yet been incorporated into systematic reviews

Evidence summary

What current evidence supports

  • Interdental spaces account for a substantial share of tooth surfaces (roughly 30 to 40 percent) that toothbrushes cannot fully reach, a well-established anatomical basis for recommending supplementary cleaning
  • The Cochrane review (Worthington et al., 2019) found low- to very low-certainty evidence that floss and interdental brushes, used alongside toothbrushing, reduce plaque more than toothbrushing alone
  • Newer observational studies using caries as a direct endpoint (Kamiab et al., 2021; Marchesan et al., 2018, 2020) found a significant association between interdental cleaning and lower caries prevalence across different age groups and populations
  • Germany's S3 guideline recommends regular interdental cleaning by incorporating biological plausibility alongside the available study evidence, not on high-certainty direct caries evidence alone

Why this matters

Understanding that weak direct-caries evidence reflects a gap in study design, namely the near-total reliance on plaque as a surrogate endpoint, rather than a demonstrated absence of benefit, matters clinically: it supports continuing to recommend interdental cleaning on biological plausibility grounds while newer, caries-endpoint studies accumulate toward the higher-certainty evidence a future guideline update would require. It also matters that anatomical fit (which tool suits a given space) and comparative clinical efficacy (which tool performs better on a given outcome) are separate questions; a tool can be the more practical choice for a given space without being established as clinically superior overall.

What remains uncertain

  • Whether the associations found in observational studies with caries as an endpoint would hold up in controlled intervention trials
  • Which specific interdental tool is most effective for caries prevention specifically, as opposed to plaque reduction
  • How the evidence base will shift once ongoing or future high-quality studies with caries as a direct endpoint are incorporated into systematic reviews

Worthington, MacDonald, Poklepovic Pericic, et al. (2019): Home use of interdental cleaning devices, in addition to toothbrushing, for preventing and controlling periodontal diseases and dental caries. Cochrane Database of Systematic Reviews, 2019(4), CD012018. Systematic review of 35 RCTs; no study arm examined caries directly, plaque reduction served throughout as the surrogate parameter; primary evidence basis for the guideline recommendation.

Kamiab, Mohammadi Kamalabadi, Sheikh Fathollahi (2021): DMFT of the First Permanent Molars, dmft and Related Factors among All First-Grade Primary School Students in Rafsanjan Urban Area. Journal of Dentistry (Shiraz), 22(2), 109-117. Cross-sectional study of 2,031 schoolchildren; floss use significantly associated with lower caries indices.

Marchesan, Morelli, Moss, et al. (2018): Interdental cleaning is associated with decreased oral disease prevalence. Journal of Dental Research, 97(7), 773-778. NHANES-based observational study in US adults.

Marchesan, Byrd, Moss, et al. (2020): Flossing is associated with improved oral health in older adults. Journal of Dental Research, 99(9), 1047-1053. NHANES-based observational study in older US adults.

Sources

Guideline:
📋 DGZ / DGZMK: S3 Clinical Guideline on Caries Prevention in Permanent Teeth, AWMF 083-021, Version 2.0, valid until 27 January 2030, Chapter 6.1.6, Evidence-based recommendation 4.

📚 Worthington HV, MacDonald L, Poklepovic Pericic T, Sambunjak D, Johnson TM, Imai P, Clarkson JE (2019): Home use of interdental cleaning devices, in addition to toothbrushing, for preventing and controlling periodontal diseases and dental caries. Cochrane Database of Systematic Reviews, 2019(4), CD012018.
📚 Kamiab N, Mohammadi Kamalabadi Y, Sheikh Fathollahi M (2021): DMFT of the First Permanent Molars, dmft and Related Factors among All First-Grade Primary School Students in Rafsanjan Urban Area. Journal of Dentistry (Shiraz), 22(2), 109-117.
📚 Marchesan JT, Morelli T, Moss K, et al. (2018): Interdental cleaning is associated with decreased oral disease prevalence. Journal of Dental Research, 97(7), 773-778.
📚 Marchesan JT, Byrd KM, Moss K, et al. (2020): Flossing is associated with improved oral health in older adults. Journal of Dental Research, 99(9), 1047-1053.

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The KEERN Lexicon provides evidence-based educational information about oral health. It is intended to support, not replace, individual advice from a dentist, physician, or pharmacist. Diagnosis and treatment decisions should always be based on a personal clinical assessment.