Noninvasive Therapy: 10 Caries Management by Modifying the Biofilm
There′s an old saying in dentistry that “a clean tooth never decays.” Accordingly, the Keyes rings ( Fig. 2.1 , p. 22) illustrate plaque as an essential etiological factor of dental caries. If bacteria like Streptococcus mutans are one of the main causes of caries, it seems reasonable at first glance to eliminate as many of them as possible from the oral cavity by intense oral hygiene and chemical agents. However, this goal can neither be accomplished nor would it be helpful in maintaining oral health.
To understand better the role of oral hygiene in the etiology and treatment of caries, we should call to mind that bacteria have been with us since time immemorial. It is estimated that our body consists of ten times as many prokaryotic cells (bacteria) as eukaryotic (human) cells.1 The physiological flora plays an important role in the physiology of the human body. It is involved in the digestion of nutrients and defense against potentially pathogenic microbes such as fungi. This also makes clear that a complete elimination of bacteria from the oral cavity is neither possible nor reasonable. Therefore, a control or beneficial modification rather than an elimination of bacteria/plaque seems to be the goal of our caries preventive strategies.
There are two classical ways of controlling plaque: first, by a mechanical disturbance of the biofilm; and second, by the chemical action of antimicrobial agents. Neither strategy is a human invention, since both have been established during our co-evolution with oral bacteria. The complex anatomy of the teeth and surrounding soft tissues allows a very efficient mechanical self-cleaning during mastication and movement of oral tissues. Moreover, the physiological exfoliation of oral epithelia limits the adhesion of bacteria. Besides these mechanical actions, the human body also provides a range of chemical agents with antimicrobial activity, including the innate and acquired immune system. In the past years a vast number of antimicrobial peptides has been identified,2 and we are just beginning to understand what important roles these antimicrobial substances play in maintaining the ecological equilibrium between the host and the microorganisms.
Humans have always supported natural cleaning of the oral cavity by employing mechanical aids as well as chemical agents to keep their teeth healthy. The first known oral hygiene devices were tooth sticks (Miswak) that were already in use more than 5000 years ago ( Fig. 10.1 ).3 Antimicrobial substances derived from plants may have an even longer history in human culture.4 In modern times, patients and dental professionals clean teeth with sophisticated mechanical techniques and use different antimicrobial agents to control growth and quality of dental plaque.
A third and more recent approach to control plaque arose from the idea that oral diseases such as caries and periodontitis are most probably caused by an ecological shift within the dental plaque (ecological plaque hypothesis). Consequently, biological approaches aim to alter the plaque composition by specific modification of the oral ecology, for example, by administration of harmless (probiotic) microorganisms that compete with pathogenic microbes.
This chapter aims to give the reader an insight into various strategies to prevent and treat caries by biofilm modification/control, and will cover:
Dental plaque as a biofilm
Methods to mechanically remove the oral biofilm
Different chemical agents to control the oral biofilm
Biological approaches for altering the biofilm composition
Dental Plaque as a Biofilm
The Dutch scientist Antonie van Leeuwenhoek first showed in 1683 that the sticky mass of dental plaque consists of tiny bacteria. He also showed that only the surface layers of bacteria were vulnerable, whereas the deeper layers resisted the vinegar he used to clean his teeth. Later, acid-producing bacteria in dental plaque like S.mutans were identified as being involved in caries initiation and consequently the reduction of dental plaque became one of the main strategies of caries prevention. In recent years new technologies have given scientists a deeper insight into the structure and microbiology of dental plaque, which has been shown to be much more complex than previously thought. In dental plaque, clusters of live and dead bacteria are embedded in an extra-cellular matrix of polymers, which are of host and bacterial origin.5 The matrix gives mechanical stability and serves as an extracellular nutrition storage which helps bacteria to survive in times of starvation (see Chapter 11). Dental plaque shows a relatively open architecture with channels and voids that allow diffusion of liquids. A similar accretion of microorganisms can be found on many solid surfaces in nonsterile aqueous environments like rivers, aquaria, or the tubing systems of dental units. In biology these microbial aggregations are called “biofilms” and because many findings from biofilms in general can be transferred to dental plaque, today dental plaque is often referred to as an oral biofilm.
Bacteria associated in biofilms show different properties than planktonic (swimming, growing in liquid) bacteria with regard to their gene expression profiles and their antimicrobial resistance, which limits the informative value of studies on planktonic bacterial cultures vis-àvis biofilms. Moreover, multispecies biofilms like dental plaque react differently compared to monospecies cultures which (for reasons of simplicity) are often used in laboratory experiments. The various microorganisms in dental biofilms show very complex interactions as they exchange signaling molecules and genetic information and thus can react better to environmental changes. Being protected in such biofilms, up to 300-fold concentrations of antibiotics are needed to kill those bacteria, exceeding by far in some cases the lethal dose for human beings.6 Today, more than 700 different microbes and phylotypes have been identified in oral biofilms.7 High throughput technologies showed more than 19000 different species in healthy mouths.8 The composition of species varies between individuals and various tooth sites and even within different locations in the plaque. This complexity of oral biofilms should be considered whenever one thinks about approaches to eliminate dental plaque or to modify its composition and structure with the aim of caries prevention.
Dental plaque is a complex oral biofilm. Bacteria associated in biofilms are highly resistant to antimicrobial substances, which limits the efficacy of chemotherapeutic approaches to control dental plaque.
Mechanical Biofilm Control
Correlation between Oral Hygiene and Caries
With regard to the etiology of caries one might assume a strong correlation exists between the oral hygiene of patients and the occurrence of dental caries. Indeed, it has been shown in several studies that children with good oral cleanliness tend to develop less caries than those with poor oral hygiene. However, interestingly, the differences were rather small.9,10 This is not so surprising, if one takes into account the multifactorial nature of caries (see Fig. 4.1 , page 67) in which oral hygiene is just one of the many factors. Significant differences in caries incidence between groups with good or bad oral hygiene were mainly observed for erupting teeth, free smooth surfaces, and front teeth, which are all relatively easy to clean,11–15 whereas differences for approximal and occlusal surfaces were rather small. It seems that good oral hygiene is efficacious at preventing caries only to a certain degree. Even if good oral hygiene is performed, the oral cavity is never free from microbial deposits and hence in many patients factors other than oral hygiene may play a dominant role, especially in tooth sites that are hard to clean, even by highly motivated individuals. This is important to consider, as oral hygiene education should be a major, but not the only, focus of caries prevention. Moreover, measures to improve oral hygiene should be focused on sites where the best preventive effect can be achieved.
In general, mechanical plaque control can be performed by the patient (self-applied) and by dental personnel (professional tooth cleaning).
Oral hygiene is most effective in easily accessible sites such as buccal surfaces and front teeth. In tooth sites that are hard to reach (e.g., approximal surfaces) oral hygiene is less effective.
Self-Applied Mechanical Biofilm Control
Tooth brushing with either manual or powered toothbrushes is the most common oral hygiene method in most countries. It is mostly performed using either manual or powered plastic brushes and in combination with a toothpaste.
It seems obvious that the quality of tooth brushing, that is, the efficacy of removing plaque, is important for its caries preventive effect. To evaluate whether the efficacy of tooth brushing can be improved, several studies investigated the effect of supervision of tooth brushing in children.14,16,17 In these studies the overall effect on caries was modest and most apparent in front teeth and free smooth surfaces, since these were most easily reached by the brush10 ( Fig. 10.2 ). For scarcely accessible surfaces, however, only highly motivated individuals could maintain a high standard of plaque control by self-applied oral hygiene.9
Intensified oral hygiene has been shown to be efficacious in the occlusal surfaces of erupting molars.18 During the eruption period the natural self-cleaning by occlusal mastication and anatomical shape, which keep most of the fissure and buccal surfaces free of plaque in fully erupted teeth, is often inefficient. Hence, during eruption particularly molars are susceptible to caries. Therefore, during this time special attention should be given to the cleaning. Brushing of erupting molars in a bucco-lingual direction (cross brushing) is often more efficacious than normal brushing techniques and should be advocated ( Fig. 10.3 ).
Also during the orthodontic treatment with fixed appliances (self-) cleaning is often considerably impaired. To avoid an iatrogenic increase of caries risk it is necessary to compensate this impairment of self-cleaning with intensified oral hygiene during the treatment period. It has been shown that oral health education and promotion can improve oral hygiene in orthodontic patients at least in the short term.19
Erupting molars and teeth under fixed orthodontic appliances are especially susceptible to caries, because (self-) cleaning is impaired. For these situations, special measures should be recommended.
Brushing Technique and Brush Used
The efficacy of plaque removal depends on the toothbrush and brushing technique used. In general today, a multi-tufted brush with a short head with end rounded plastic bristles is recommended. The use of natural brushes is discouraged because bristles tend to split, harbor many bacteria, and may even injure the gingiva.
In the past century, several tooth brushing techniques have been developed and advocated (summarized in Table 10.1 ). The plane bristle field of most manual brushes makes it difficult to reach the approximal areas. Therefore, most brushing techniques suggest holding the brush at about a 45° angle to better penetrate proximal areas ( Fig. 10.4 ). Some advanced toothbrush models have cross-angulated bristles, which seems to slightly improve the approximal cleaning.20 Although it has been shown that the employment of certain tooth brushing techniques results in better plaque removal compared with unstructured brushing,21 such technical superiority with regard to caries prevention is yet to be demonstrated. Moreover, it should be kept in mind that most of the advocated techniques require a high level of dexterity and thus are not suitable for every patient.
For this reason powered toothbrushes have become more and more popular in recent years. Powered brushes simulate parts of the brushing motion, thus reducing and simplifying the manual movements that are needed to clean the teeth. Early concerns that powered toothbrushes might injure the gingiva and lead to gingival recessions could be neither corroborated by clinical experience nor by scientific evidence. In addition, the efficacy of plaque removal could be significantly increased and different designs have been developed ( Fig. 10.5 ).
Most powered toothbrushes seem to be at least as effective as manual brushing in reducing plaque and gingivitis, but fail to show superior results. Only powered brushes with a rotating, oscillating/multidimensional motion seem consistently to remove significantly more plaque than manual brushes.22 However, keeping in mind the only modest association between oral cleanliness and the incidence of caries, it is still unclear whether the use of powered brushes also results in lower caries (and periodontitis) incidence compared with manual brushes. Moreover, it should be considered that most available studies investigated the tooth cleaning efficacy under the controlled conditions of clinical studies. The effectiveness (in real world conditions) of toothbrushes might differ from the efficacy, as patients’ personal perceptions and preferences might also play an important role. For these reasons, no clear recommendations with respect to the superiority of either manual or powered toothbrushes can be made.22 For the individual patient, personal preference should be considered as well as manual dexterity and adherence.
Powered toothbrushes are more efficacious then manual brushes in removing dental plaque. However, it is unclear whether this results in lower incidence of caries and periodontitis.
The Effect of Fluoride
In most countries tooth brushing is usually performed with fluoride toothpaste. Therefore, it is often not clear whether the caries preventive effect of tooth brushing is mainly due to the disturbance of plaque, or rather by the action of fluoride. In one study, which investigated the effect tooth brushing and fluoride separately, the caries increment of 9–11-year-old children was evaluated over a period of three years.16 In one group, children brushed their teeth every school day under supervision using fluoride toothpaste. In a second group, children brushed their teeth with non-fluoride toothpaste also under supervision. In a control group, no intervention was performed and children carried on with their established oral hygiene (mainly non-fluoride toothpaste). To assess the effect of fluoride alone, in another control group, children only rinsed with 0.5% NaF solution fortnightly. In children brushing with non-fluoride toothpaste the relative reduction of caries incidence in the intervention compared with the control group was only modest (8%). When supervised brushing was performed with fluoride toothpaste, however, a significantly lower caries incidence (51% less compared with the nonsupervised control) was observed ( Fig. 10.6 ). In conclusion, the combination of tooth brushing with the local application of fluoride (by using fluoride toothpaste) seems to be most effective in controlling caries.
Tooth brushing is most effective in combination with fluoride toothpaste. Tooth brushing with fluoride-free toothpaste has only very limited caries preventive effect.
Tooth brushing: How Often, When, and How Long?
How often? It is widely recommended to brush the teeth at least twice daily. But what is the rationale behind it? Dental plaque grows quite slowly and the first subclinical tissue changes beneath plaque can be observed only after more than 24 hours of plaque accumulation.23 As it seems unlikely that with an increase of brushing frequency the quality of brushing increases (see below), in consequence easily accessible sites are cleaned more often than necessary and plaque stagnation sites that are difficult to reach often remain untouched. As negative side effects of brushing, like wedge-shaped defects and erosion/abrasion (see below), may increase—for some highly motivated patients, too much oral hygiene is even detrimental. Besides cosmetic considerations, the recommendation to brush twice daily is mainly based on the fact that tooth brushing is the most efficient way to apply fluorides locally. It has been shown that patients brushing twice daily develop less caries compared with those brushing just once daily.24,25
When? It is often recommended to brush the teeth directly after meals26 and especially before going to bed,27 but there is no strong scientific evidence for this. On the one hand, brushing after meals might help to clear carbohydrates from the oral cavity but on the other hand removal of plaque before meals would leave no biofilm to ferment carbohydrates.28 Brushing directly after acidic meals can trigger erosion, as the softened enamel is easily brushed away and has no time to remineralize.28 With regard to this, a safety period of more than one hour seems to be necessary before brushing after erosive meals.29,30 However, there is little evidence behind these statements and it is currently doubted that such general advice is of any clinical relevance, as long as the teeth do not already show proven erosive damage. The rationale behind the recommendation to brush before going to bed is that salivary flow is decreased during sleep.31 However, since at night usually no cariogenic food is consumed, it is questionable whether this has a clinical impact. As the main motivation of patients for tooth brushing is cosmetic, it makes sense to support this intrinsic motivation and to recommend that patients brush their teeth whenever they are most willing to in their daily routine. This might have a greater effect than theoretical considerations.
How long? Increasing the tooth brushing time more than one minute seems to have only limited effect on the quality of plaque removal, most probably because patients miss the same areas again.32,33 However, when brushed professionally, manual brushing reaches its limit after six minutes, whereas brushing with oscillating–rotating brushes achieves this efficacy after two minutes. Longer brushing, however, also results in longer contact times with fluorides when fluoride toothpaste is used, which might be beneficial. Retention of fluoride in the mouth also depends on rinsing after brushing.28 It has been shown that the caries preventive effect seems to be higher if less toothpaste is expectorated after brushing.25,34 Thus, mouth-rinsing with water after brushing should be reduced to a minimum. The amount of toothpaste used, however, does not seem to have an important effect.24
Most patients should brush their teeth twice daily with fluoride toothpaste. To enhance the fluoride effect the toothpaste slurry should not be rinsed with water. The individual′s adherence should be considered when giving recommendations.