Extrinsic and intrinsic chemical factors relating to tooth wear

Human enamel is resilient. It is capable of withstanding a compression strength of 363 Mega Pascals, the equivalent of 3630 kg of weight pressing on an area of 1 cm², and so is one of the strongest substances in the body until exposed to acids.¹ When exposed to acids, the ionic crystalline bonds, which make up 98% of the enamel structure, can dissociate and breakdown. Once softened, any mechanical action can promote tissue loss.² Until recently, enamel softening was recognized predominantly as a surface phenomenon with limited subsurface effect. However, recent investigations have shown that acid challenge can result in 3–4 microns of wear, with subsurface softening observed up to 50 microns below the surface.³

Clinically and academically, we are increasingly using the term ‘erosive tooth wear’ to recognize that severe tooth wear rarely happens without some degree of exposure to acids.⁴ This article discusses the intrinsic and extrinsic chemical factors relating to tooth wear, and how we are increasingly recognizing the impact of medical comorbidities on tooth wear. It is impossible to distinguish from the appearance of worn teeth the origin of the acid, particularly as it may be coming from more than one source. Therefore a thorough history and understanding is important. The source of the patient's wear in Figure 1 was predominantly dietary, but the patient was referred for gastro-oesophageal reflux disease (GORD) because the dentist had not diagnosed the source of dietary acid.

Extrinsic factors

The biggest contributor towards tooth wear for most of the population are the acids present in our diet. However, there are several studies showing no association between the diet and erosive tooth wear. This is often because the food frequency questionnaires used in research are often based on the frequency <1/week, <1/day but >1 per week, 1/day, >1/day. This is insufficient detail for tooth wear because we know that those with tooth wear progression are likely to be consuming several acids a day and not necessarily the same type of acid. One case-control study on 600 age-matched participants observed that the risk for developing erosive tooth wear increased with three or higher dietary acids a day.⁵ Another longitudinal clinical trial reported that those who consumed four dietary acids a day had significant tooth wear progression after 7 years.⁶ Many of us have one or two acid challenges a day, but few will have three to four. This also includes snacking on fruit. However, a healthcare provider needs to balance the knowledge that most of the population do not consume sufficient fruit, and it may be within the patients' best interest to keep eating fruit to maintain overall health. There is good evidence to suggest that keeping fruit to mealtimes is not associated with increased tooth wear progression.⁵ When taking a dietary history, it is important to establish the pattern of consumption rather than singling out individual items, and count overall frequency of any acid consumption. For example, if someone consumes a cordial every evening over a 3–4 hour period, this sustained acid challenge can also increase the risk of developing tooth wear.

When establishing a dietary history, it is important to ask:

• What do they tend to drink throughout the day?
• What do they snack on throughout the day, focusing on fruit, fruit-flavour items or vinegar products?
• Do they consume it over a long period of time, or have a habit where contact with the teeth is prolonged, such as swishing a drink or retaining the dietary acid in their mouth?

There is evidence to suggest that if the behaviour causing the tooth wear is stopped, tooth wear progression can slow to normal rates. In a randomized controlled clinical trial, participants who were given a brief behaviour change intervention reduced their frequency of dietary acid intake and their tooth wear progression.⁷Figure 2 shows an example of an if–then plan behaviour change intervention. It helps the patient to think about how they are going to make the change, i.e. plan an appealing but non-erosive substitute, not purchase the dietary acid in the weekly shop, etc.

Simple dietary advice does not work⁷ and it is important for the patients to know that they do not need to completely give up on the things they enjoy as long as they look at overall patterns of consumption. It is fine to have orange juice for breakfast, provided other erosive foods or drink are not consumed regularly throughout the day. It is interesting to note that drinking tea/coffee seemed to have a protective effect.⁸ This may be because a patient may prefer tea or coffee to an erosive beverage on a daily basis. Laboratory research has also reported that the tannins present in teas/coffee may have a protective effect when incorporated into the salivary pellicle.⁹ Further clinical research is required, but it is likely that there is a dental and overall health advantage in choosing tea or water over a cola or carbonated energy drink.

Another commonly quoted source of external acid damage is from chlorinated swimming pools. Swimming pools are typically maintained at a specific pH level between 7.2 and 7.8 to ensure the chlorine is effective and irritation is reduced. If maintenance of the pool is poor, and the pH of the pool is not adjusted properly, prolonged exposure to the reduced pH can erode the teeth.¹⁰ In modern commercial swimming pools, maintenance of pH is heavily regulated. This is not an area of concern and exercise is one of the single most important things we can do to maintain health. Unless there are regularly other symptoms with swimming, such as burning eyes and skin dermatitis, this is unlikely to be the cause of erosive tooth wear. A more common issue among competitive swimmers and athletes in general is the sipping of energy drinks and frequent snacking during training sessions instead of water, which will increase frequency of dietary acid intake and potentially result in erosive tooth wear.¹¹

A further discussion point is alcohol consumption. All alcoholic drinks are acidic, although some have a relatively low titratable acidity, such as beer. This means that although acidic, the pH returns to normal quite quickly. This contrasts with any high titratable acidity alcoholic drinks containing citric or tartaric acids, such as wine or fruit-flavoured drinks. High titratable acidity drinks reduce pH and keep it lower for longer.¹² Carbonation of drinks also increases their erosive potential.¹² Alcohol consumption on an irregular basis will not harm teeth, and again healthcare providers can remind patients about government recommendations for overall health. If alcohol is consumed every night, over a long period of time the potential to cause damage increases. Particularly if the alcoholic drink is held or swished around the mouth, such as seen in professional wine tasters.¹³ Alcohol abuse is also associated with increased risk of regular vomiting and gastro-oesophageal reflux disease.¹⁴ This can mean there is an internal source of acid exposure in addition to an external one.

A final external source of acid can be medications. While it is important to not influence medication intake prescribed by a medical doctor, it is equally important to be mindful when medication is acidic or creates an oral environment conducive to acid damage. You can advise your patient that this will reduce their ability to consume dietary acids without risk of acid damage. Effervescent tablets, particularly those containing vitamin C can be erosive.¹⁵ Over-the-counter dissolvable medications for cold and flu can also be acidic, but the damage is unlikely in the small periods of time over which the medications are taken.¹⁵ For children, many liquid medications are fruit flavoured, and even sugar-free versions can be acidic. This becomes important when they are consumed every day, such as liquid beta-blockers.¹⁶ A second potential mechanism is the use of long-term asthma control medications. Many inhalers result in a drop in intra-oral pH immediately upon use.¹⁷ Dry powder inhalers are more acidic than metered dose inhalers, with potential to cause greater drops in salivary and plaque pH.¹⁷ At the same time, it is known that long-acting beta-agonists can affect the amount and quality of saliva.¹⁸ Although the true protective effect of saliva against erosive tooth wear is relatively unknown, reduced salivary flow, or quality, could potentially affect the protective role that proteins and chemical composition in the salivary pellicle can have on wear.¹⁹ In addition, those with reduced salivary flow may increase consumption of drinks to compensate.²⁰ In an old, but nationally representative dataset, having asthma by itself was not associated with an increased risk of tooth wear, but those on long-term medication for asthma control were.²⁰ This is still association data, and we do not know the underlying mechanism of action for this. Asthma is a serious condition and the benefits of these medications far offset the increased risk for erosive tooth wear. However, explaining this potential for increased risk to the patient may motivate the patient to make less erosive food and drink choices.

To summarize, the main extrinsic risk factor for erosive tooth wear is frequent or prolonged consumption of dietary acids. Behaviour change interventions aimed at working out plans on how to reduce consumption, replacing dietary acids with more tooth-friendly substitutions will help to reduce erosive tooth wear progression regardless of other external factors.

Intrinsic factors

Stomach acid is extremely acidic. To give an analogy of acids on teeth, citric acid could be described as a toddler in a china shop who is left in there all day, hydrochloric acid is the bull in the china shop rampaging through, rarer but more destructive. The two commonly discussed intrinsic factors associated with erosive tooth wear are gastro-oesophageal reflux disease and vomiting eating disorders. However, it is worth bearing in mind that increased risk for erosive tooth wear is not a reason to intervene or offer opinion on the medical management of any condition. Gastro-oesophageal reflux is associated with erosive tooth wear. However, not everybody with gastro-oesophageal reflux will get tooth wear. The fluid/gas originating from the stomach, known as refluxate, can remain in the oesophagus and not travel as high as the oral cavity. When patients undergo 24-hour pH monitoring, they normally have their pH monitored at two regions, the proximal oesophagus located at 5 cm above the lower oesophageal sphincter and the distal oesophagus located at 15 cm above the lower oesophageal sphincter. For most, the refluxate does enter the distal oesophagus, but when it does, the refluxate is not as acidic, often with a pH above 4.2. When the refluxate travels beyond the distal oesophagus, it is termed extra-oesophageal reflux. This is the type of reflux that is most likely to cause erosive tooth wear. If it backtracks into the laryngo-pharyngeal area it is called laryngo-pharyngeal reflux (LPR) and is associated with a host of respiratory problems, such as chronic cough, exacerbated asthma, COPD and other pulmonary conditions. So far there is no specific test for LPR; however, clear signs of the acid entering the oral cavity may be a good clinical indicator of extra-oesophageal reflux. LPR does not often present as typical GORD.²¹ Globus sensation, throat clearing, hoarseness, excess throat mucus or postnasal drip being the most common presentations.²¹ A patient should always be counselled to follow up with their GP if they present with a globus sensation and their GP is not aware of it.

When gastric acid is introduced to the oesophagus, it has been reported to trigger a bruxism response or increase in rhythmic masticatory muscle activity (RMMA).²² Theoretically, this is possibly because of the similar embryological origin and neural innervation of the oesophagus and bronchial tree with the vagus nerve.²³ There are those that believe this to be an evolutionary advantage, opening up the airway and helping to reduce obstructive sleep apnoea, or helping to clear the acid from the throat.²⁴ What is known is that the prevalence of sleep bruxism in those with sleep-related gastro-oesophageal reflux is high,²⁴ and potentially up to 73.7%.²⁵ Obstructive sleep apnoea is a common condition affecting 4% of adults in the UK. Over 40% of these patients will have both bruxism and gastro-oesophageal reflux.²⁶ This possible combination of repeated, concurrent acid damage with mechanical damage can lead to accelerated tooth wear.

Asthma affects more than 300 million people globally.²⁷ There is a bidirectional association between asthma and GORD.²⁸ Extra-oesophageal acid may irritate the airways. However acid stimuli in the distal oesophagus can potentially lead to bronchial symptoms via the vagally mediated reflexes mentioned above.²³ It has also been shown that asthma and asthma medication can trigger reflux. Lung hyperinflation and an increased pressure gradient between the thorax and abdomen cause asthmatic individuals to work harder to breath and can result in the lower oesophageal sphincter to relax increasing reflux events.²⁹ Some bronchodilators, for example theophylline, relax smooth muscles, including the gastro-oesophageal sphincter.²⁹ The range of other conditions that are also associated with GORD, such as Parkinson's disease, alcoholism, obesity and the increase in sleep bruxism associated with these conditions, the increase in the prevalence of tooth wear is not surprising.

Rumination syndrome is a disorder of gut–brain interaction in which patients deliberately, and repeatedly, regurgitate recently ingested food and swallow it again. It has not been well researched, but recent studies suggest that it is more prevalent than was once thought with a global prevalence of 3.1%.³⁰ These studies suggest the condition is more common in children, adolescents, and patients with developmental, or psychiatric disorders.³⁰ As resultant erosive tooth wear can be quite severe, it can be worth enquiring about this disorder if other causes have been excluded.

Vomiting-associated eating disorders and bulimia may be increasing in prevalence with bulimia nervosa having a current global prevalence of 0.8–2.6% in women and 0.1–0.2% in men.³¹ These punishing conditions can involve forced vomiting multiple times a day. Some who have these conditions have erosive tooth wear and others do not. One systematic review reported that patients with an eating disorder had five times the odds of developing dental erosion compared with controls (95% confidence interval (CI) 3.31–7.58) and those with self-induced vomiting had seven times the odds (odds ratio (OR) 7.32).³² Patients also had reduced salivary flow (OR 2.24; 95% CI 1.44–3.51).³² The frequency of vomiting, diet and treatment impact on the rate of progression of tooth wear. In one study where participants were asked ‘how many times per day do you vomit when you feel particularly bad regarding your eating disorder disease’, the mean number of episodes was 3.8, but ranged from one episode per day to 10 per day.³³ Brushing immediately after vomiting can potentially do harm, but it is also important to brush teeth, particularly if the patient also has a high caries risk.³⁴

Figure 3 shows a patient flow diagram when examining risk for wear and suitable follow-up periods. If there is substantial wear progression after 1 year, the patient may want to consider further medical management of their condition. Intervening with restorations is not required at any stage unless the patient's quality of life is affected by the reduced aesthetics of worn teeth.³⁴

Conclusion

For all patients, it is important to provide information on their condition, how it may progress given their current risk factors, support, and preventive advice. A patient may find it easier to reduce their likelihood of wear progression by managing their dietary acidic intake and using preventive techniques if they are finding a medical or psychological condition difficult to control. Either way, patients should be reassured that there is good evidence to suggest that tooth wear in itself is not a pathology, and pulpal health can remain intact even in severe wear cases³⁵ if they are currently finding their medical condition difficult to manage. However, to improve quality of life and reduce future restorative complications, prevention for these patients should focus on reducing the frequency of dietary acid intakes, switching to a stannous-based fluoride toothpaste and maintaining a healthy lifestyle.³⁶