Reducing white spot lesion incidence during fixed appliance therapy

What are white spot lesions?

White spot lesions occur as a consequence of repeated episodes of mineral loss from the enamel into the surrounding saliva and plaque. The mineral is initially lost from the enamel surface, however, subsurface minerals serve to maintain the integrity of the surface layer, resulting in a subsurface lesion. The process is dynamic, with periods of demineralization and remineralization occurring, depending on the changing conditions of the oral environment.4 Whilst the surface layer remains intact, there is still the possibility of arrest or reversal of a lesion. However, if demineralization exceeds remineralization over a prolonged period of time, a critical point is reached where the surface layer cannot be maintained and cavitation occurs.5

In a demineralized lesion, water partially replaces mineral, reducing the refractive index.6 This lower refractive index means that light does not penetrate as far into the enamel before being back-scattered and it is the increase in back-scatter of white light which leads to the whiter appearance of a demineralized lesion.7 When an area of demineralization is dried, the water is replaced by air, which has an even lower refractive index, making the lesion appear even whiter (Figure 1).7

Remineralization of demineralized enamel does not recreate the original enamel prism structure. Instead, a dense region of calcium, phosphate and fluoride forms on the surface. This highly dense layer has been found to be more resistant to further demineralization than normal enamel.8 As enamel remineralization commences from the surface layer inwards, with the centre of the lesion being the last area to remineralize, this dense surface layer can act as a barrier to penetration of minerals into the subsurface lesion. This can leave the subsurface lesion demineralized and the characteristic white appearance may remain.9 It has been suggested that high concentration fluorides can result in hypermineralization of the surface layer,10 potentially ‘locking in’ the white appearance. A more appropriate approach may be slow remineralization by saliva or low dose fluoride, which allows penetration of the deeper layers of enamel.10

Fixed appliances and white spot lesions

White spot lesions require fermentable carbohydrate, plaque bacteria, a susceptible surface and time to occur. Food debris has a tendency to become trapped when fixed appliances are in place and plaque control is more difficult (Figure 2). Poor plaque control can lead to prolonged breakdown of carbohydrate, which in turn can increase acid produced by plaque bacteria and favour demineralization. In addition, plaque can act as a barrier to acid diffusion away from the enamel surface and mineral diffusion towards the enamel surface, tipping the balance towards demineralization.4 Studies have suggested that fixed appliances lead to increased numbers of Streptococcus mutans and Lactobacilli in the oral cavity,11 micro-organisms which have been linked to increased caries incidence.12

Unfortunately, maxillary incisors, the most visible teeth, are those most frequently affected by WSLs.2 Poor plaque control in this area causes demineralization and caries activity may be accelerated by reduced oral clearance,2 with these teeth having less exposure to saliva.4 The maxillary lateral incisor is reportedly the most frequently affected tooth,9^,13^,14 and this may be due to the decreased distance between the bracket and gingival margin associated with these teeth, making the area more difficult to keep clean (Figure 3).2^,9

Predictors of white spot lesion development

It is reported that WSLs can develop in as little as four weeks.15 Orthodontic appointments are commonly at six-week intervals, therefore WSLs can develop between bonding of an appliance and first review, making it imperative that the risk of WSL development is assessed prior to commencement of treatment.

An individual's susceptibility to WSL development is likely to be determined by his/hers diet, oral hygiene, salivary composition and oral flora.11 There are a number of recognized means to assess caries risk that general dental practitioners and orthodontists can utilize. If a patient is identified as being at high caries risk then this should be addressed prior to referral for, or embarking on, orthodontic treatment.

In the first instance, each potential orthodontic patient should have a full examination carried out. The presence of gingivitis is an indicator of poor oral hygiene. The teeth should be cleaned and dried to allow any cavitated lesions or pre-existing white spot lesions to be observed. If noted, these may suggest that the patient is at high risk of caries development and not yet ready to commence orthodontic treatment.

The American Academy of Paediatric Dentistry (AAPD)16 caries risk assessment tool (CAT) can be used to categorize patients as being at low, moderate or high caries risk, depending on clinical, environmental and health conditions. Similarly, the Scottish Intercollegiate Guidelines Network (SIGN) guideline 4717 includes a caries risk assessment for children based on:

Each factor in turn is analysed and used to classify patients as low, moderate or high caries risk. Both the AAPD CAT and SIGN guideline 47 suggest that previous caries experience is a useful clinical indicator of current caries risk. It should be noted that CAT categorizes patients as moderate risk if they have pre-existing WSLs. Once fixed appliances are in place, patients are categorized as being at moderate caries risk, regardless of any other clinical, environmental or general health conditions.

Diet diaries may be useful in high caries risk patients to allow dietary changes to be implemented prior to commencing orthodontic treatment. The Department of Health guidelines18 suggest that, generally, sugary foods should be restricted to mealtimes, food with added sugars should be consumed a maximum of four times per day and that sugars should make up less than 10% of the energy provided by the diet. This advice should be borne in mind when analysing diet diaries.

It has been suggested that salivary pH, flow rate and buffering capacity influence the amount of demineralization that occurs when the pH of the oral environment is reduced and the amount of remineralization that subsequently occurs.19 Therefore, testing these parameters prior to orthodontic treatment may give an indication of susceptibility to demineralization. It has been demonstrated that salivary flow rate often increases when fixed appliances are placed,20^,21 as does salivary pH and buffering capacity,21 which may explain why some patients with sub-optimal plaque control do not develop WSLs.

It has been suggested that the presence of early proximal lesions on bitewing radiographs at the time of fixed appliance placement is a predictor for WSL development during treatment.22 Bitewing radiographs have often been taken in the period prior to orthodontic treatment and, if proximal enamel lesions are detected at this stage, then it might be prudent to delay commencing orthodontic treatment while the cause is addressed.

Prevention of WSL development

Once fixed appliances are in place, there are a number of strategies which can be implemented to help prevent the development of WSLs, most of which aim to achieve good plaque control or increase exposure to low dose fluoride.

Ionic fluoride can be taken up by enamel to form fluorhydroxyapatite or calcium fluoride.23 Fluorhydroxyapatite forms on the outermost layer of enamel when the fluoride concentration is low and the environment acidic. The more time the enamel spends in an acidic environment in the presence of fluoride, the greater the fluoride concentration in the enamel surface.23 Fluorhydroxyapatite is very resistant to dissolution,23 protecting against further demineralization.

In an acidic environment when the fluoride concentration is above 100 ppm, eg when topical fluoride is applied, enamel dissolution occurs, releasing calcium. Substantial amounts of calcium fluoride can then form on the enamel surface and precipitate in plaque and on the pellicle. This calcium fluoride can then act as a reservoir of calcium and fluoride ions, which can gradually be released to promote remineralization.14

Plaque control

The Department of Health suggest that toothbrushing be performed twice daily, once at night before bed and at one other time during the day and that children and young adults undergoing orthodontic treatment use a ‘small-headed toothbrush with soft (ISO 8627:1987 standard 1–3) round-ended filaments, a compact angle arrangement of long and short filaments and a handle which is comfortable’.18 If patients choose to use a powered toothbrush then it is suggested that a powered brush with ‘an oscillating/rotating action removes plaque more effectively’.

A Cochrane Review on interspace/interdental brushes for oral hygiene in orthodontic patients with fixed appliances has been carried out.24 The review considered plaque removal, health of teeth and supporting tissues, cost and adverse effects. Unfortunately, the review failed to identify any studies that met the inclusion criteria highlighting a lack of high quality evidence to support the recommendation of using interdental/interspace brushes in patients undergoing fixed appliance treatment. However, in the absence of evidence, common sense would suggest that more thorough cleaning, particularly interdentally, is likely to be of benefit and certainly of little harm. However, the increased cost of oral hygiene products should be considered when recommending them. We would suggest that, once fixed appliances are in place, a dental professional assesses the patient to determine if interdental brushes are appropriate and, if so, what sizes are required and what technique is to be employed.

A Cochrane Review of manual versus powered toothrushing in participants not wearing fixed appliances concluded that only rotation oscillation powered brushes were better than manual brushes at removing plaque and decreasing gingivitis.25 Once again, cost implications should be considered before recommending a specific type of brush to patients.

We recommend that oral hygiene should be of a high standard prior to placement of any fixed appliances. A visit to the hygienist to achieve this is likely to be required. Once appliances are in place, we would recommend a minimum of twice daily toothbrushing, morning and night, with the toothbrush the patient finds most comfortable and effective, supplemented with interdental brushing as prescribed by an appropriate dental professional (Figure 4). A follow-up appointment with the hygienist once the fixed appliances are in place is helpful to ensure a good level of plaque control is being maintained.

Fluoride varnishes

Varnishes are often advocated as a means of delivering a slow release of fluoride over a prolonged period of time, providing low concentrations of fluoride at the enamel surface and in plaque.13 Application of 1.23% acidulated phosphate fluoride (APF), 2% sodium fluoride, 8% stannous fluoride or 5% sodium fluoride varnishes has been suggested in orthodontic patients to help prevent WSLs,30 however the evidence to support this is weak. A Cochrane Review of fluoride varnishes for the prevention of caries in children and adolescents concluded that fluoride varnishes have a substantial caries-inhibiting effect and application should be considered two to four times a year.31 Although the participants in the studies included in this review were not wearing fixed appliances, it would seem reasonable to assume a similar caries-inhibiting effect would also be seen in orthodontic treatment patients.

The Department of Health suggest children and young adults undergoing orthodontic treatment should have 2.2% fluoride varnish applied to their teeth 3–4 times yearly (Figure 5).18 We recommend application of 2.2% fluoride varnish 3–4 times a year during orthodontic treatment and suggest that the orthodontist and the patient's GDP liaise to determine who will apply the varnish and when.

Role of the general dental practitioner in preventing white spot lesions

General dental practitioners and orthodontists should ensure that patients referred for and commencing orthodontic treatment are motivated and of low caries risk. Once fixed appliance treatment commences, we would suggest examinations every six months and a review of oral hygiene at every appointment, with advice tailored to the individual. Patients should be advised of the minimum fluoride concentrations of toothpaste and mouthrinse to use and at least twice daily toothbrushing should be advocated. Application of fluoride varnish should be carried out approximately three to four times per year.

If concerns regarding caries risk arise during orthodontic treatment, then tailored oral hygiene advice should be provided and follow-up arranged until oral hygiene is of a satisfactory standard. It should be ensured that patients are aware of the fluoride concentration of toothpaste and mouthrinse they should be using and diet diaries can be utilized, where appropriate. If concerns remain after exhausting all of the above, then it may be prudent to discuss early removal of the appliances with the orthodontist.

Conclusions

Patient selection is key to preventing WSL development. Oral hygiene and dental health should be monitored closely prior to and throughout treatment, with advice and support provided by both GDP and orthodontist, as required.

Patients undergoing orthodontic treatment should be advised to use 1,350 ppm or above fluoridated toothpaste for a minimum of twice daily toothbrushing, accompanied by 0.05% sodium fluoride mouthrinses daily at a different time from brushing and 2.2% fluoride varnish application three to four times a year. The use of interdental brushes and powered toothbrushes should be discussed with patients individually, with advice tailored to suit each individual.