Should new operating procedures arising from COVID-19 make us re-think our management of deep caries?

Learning Objectives

Introduction

Concerns about aerosol generating procedures (AGPs) occurring in dental surgeries such as using an air rotor to prepare enamel and dentine make it timely to reflect on how carious tooth tissue should be managed. As the production of aerosols during dental treatment is considered to be a risk for the transmission of severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2), also known as the coronavirus, leading to conditions such as COVID-19, measures have been introduced to reduce the risk of transmission in the dental surgery. ¹ These include: introduction of ‘fallow’ periods following AGPs and an increase in the use of special personal protective equipment (PPE) which adds considerable time and cost pressures on treatment. ²

Dental caries is still very prevalent in our society. The Oral health survey of adults attending dental practices, published by Public Health England, found that 27% of attending adults had tooth decay. ³ In addition, it has been reported that the prevalence of dental caries will increase as a result of an increase in the number of ‘older’ adults among the UK population. ⁴ There has been considerable published research stressing the need for a minimally interventive approach to managing caries, and the emphasis must be on prevention. Adopting a patient-centred, evidence-based approach which focusses on promoting and maintaining oral-health that is tailored to the patients’ needs is recommended. ⁵ It is therefore appropriate to review the evidence that supports a ‘biological’ approach to managing dentine caries and options for restoring the resulting cavity.

Despite evidence-based recommendations supporting a non-interventive or minimally interventive approach to managing dental caries, many clinicians still completely remove carious tooth tissue, thus risking pulpal exposure and loss of vitality of the tooth; this illustrates the large gap that still exists between what is published and what many clinicians do in practice.^6,7 For example, over 20 years ago Mertz-Fairhurst et al. (1999) showed that dentine caries could be sealed by an adhesive restoration, which was bonded to enamel. ⁸ Patients were recalled ten years following adhesive bonding of composite and amalgam restorations to carious cavities, which were left with softened carious dentine and modified enamel margins. ⁸ Although the recall rate was 50%, teeth which had softened carious dentine remaining had a similar outcome to teeth which were managed by complete caries removal. ⁸ This therefore begs the question, why are some clinicans not following an evidence-based, biological approach to managing caries?

Partial caries removal approach

It is accepted contemporary operative practice to manage dentine carious lesions by a ‘partial-caries’ removal approach. To help us understand what is meant by ‘partial caries removal’ we have the work undertaken by Fusayama in the late 1970s to guide us. ⁹ This seminal work resulted in the dentine carious lesion being classified into two zones: an outer “caries-infected” dentine zone which was heavily infected with bacteria and demineralised, and an inner “caries-affected” zone which could remineralise and was shown to have dentinal tubules filled with whitlockite crystals potentially creating a seal in the tubules. ⁹ Fusayama advocated removing the outer “caries-infected” dentine and leaving the “caries-affected” zone. ⁹ Clinically, it is difficult to distinguish between the two as there is a gradual ‘hardening’ of the carious tissue, which can be detected if an excavator is used to remove the softened carious dentine. Kidd coined the phrase “remove the soft stuff” and this serves as a very practical guide to knowing what carious tissue to remove and what to leave behind. ¹⁰ Importantly, to date, there is no robust evidence that supports complete removal of ‘softened’ dentine.

When should operative intervention take place?

Research shows us that once the lesion has become cavitated and is not cleansable then interventive operative treatment is indicated. Assessing whether caries occurring proximally has resulted in cavitation is more challenging to determine and it is recommended that when caries is seen to extend beyond the outer-third of dentine on a radiograph then interventive management should be undertaken, ¹¹ particularly once the caries has extended beyond a third into dentine.

Recent research by Sergis et al. (2020), has shown that keeping the operation speed of a speed-increasing electric handpiece to between 80,000 rpm and 100,000 rpm, can eliminate aerosol production while maintaining adequate water cooling. ² However, a reduction in cutting efficiency was observed. ² When managing dentine caries, access to the carious dentine often has to be accomplished initially inevitably resulting in the cutting of sound enamel and possibly dentine. However, if caries is managed biologically and there is minimal removal of sound tooth tissue, this can have important biological benefits for the tooth in terms of tooth tissue preservation and, consequently, for the patient. Preserving as much healthy enamel and dentine as possible must be a key aim of caries management.

Applying the current concepts of operatively managing dentine caries when treating a carious tooth would mean that even if a speed-increasing electric handpiece is used at a speed of 80,000 rpm to 100,000 rpm, this should not be a significant issue as the only healthy enamel and dentine that should be removed is that necessary to gain access for the use of instruments, such as an excavator, and to remove any grossly unsupported enamel after caries management. This might seem a strange concept to a clinician who considers that all carious dentine should be removed and then proceeds to remove peripheral sound tooth tissue creating diverging cavity walls for an ‘indirect’ restoration. This increased and unnecessary drilling of enamel and dentine could result in potentially excessive sacrifice of sound tooth tissue which may irreversibly compromise both pulp vitality and the longevity of the tooth itself.

Something that is often forgotten is that Fusayama also advocated a “painless” method of caries removal. ⁹ This idea was initially refuted by clinicians until it was realised that careful excavation of only the soft carious dentine can be achieved without the need for local anaesthesia. If, as mentioned above, only sufficient enamel is removed to gain access to the caries lesion then this would also be painless. Gaining access to proximal caries lesions would require local anaesthesia as sound dentine would have to be cut when accessing through the marginal ridge. By managing the dentine caries lesion using an evidenced-based, biological approach, the procedure can be potentially painless, resulting in minimal risk to pulpal exposure potentially affecting tooth vitality. Additionally, if access is accomplished using a speed-increasing electric handpiece running at 80,000 rpm to 100,000 rpm, this potentially results in no aerosol generation. ²

Restorative options

There are many restorative options for managing the cavity after ‘partial’ caries removal. As highlighted above, a biological approach should be considered whenever possible. Even if a tooth with a deep caries lesion is asymptomatic and does not exhibit periapical pathology, as diagnosed radiographically, it must not be considered completely healthy. Abou-Rass introduced the concept of the “stressed pulp” when teeth are compromised by caries. ¹² When an asymptomatic tooth with a deep caries lesion is being managed, consideration should be given to placing a provisional bioactive restoration in the first instance, such as a bioactive calcium silicate cement. Bioactive calcium silicate cement has been shown to be therapeutic to the pulp as result of its alkaline setting reaction and encourages remineralisation at the cavity periphery. ¹³ If a couple of weeks are allowed for complete maturation of the setting reaction, which would also allow a period of post-treatment assessment, the bioactive provisional restoration only requires cutting back to allow space for an adhesive restoration such as direct composite resin. It is not necessary to completely remove the bioactive calcium silicate cement. This can act as a ‘dentine’ replacement and will seal any residual caries, and possibly encourage remineralisation and therapeutic healing of the pulpal tissue. The disadvantage of bringing the patient back is the possibility of an AGP as the glass ionomer and bioactive silicate provisional restorations set hard. An alternative would be to wait a little longer for these materials to set at the initial appointment, fill to the enamel-dentinal junction and then overlay with direct composite resin micro-mechanically bonded to the peripheral enamel with an appropriate adhesive.

What about omitting initial provisionalisation with bioactive calcium silicate cement and placing an adhesive direct composite resin restoration? Kidd coined another helpful phrase when managing caries extending into dentine and that is “the seal is the deal”. ¹⁰ Supporting this concept, is the previously mentioned clinical trial of Mertz-Fairhurst et al., which sealed in dentine caries with adhesive direct composite and amalgam restorations. ⁸ Further confirmatory work supporting the concept of controlling a carious lesion’s activity by sealing it off from the surrounding environment rather than removing the lesion itself has recently been published. ¹⁴

Placing an appropriate dental adhesive, according to manufacturers’ instructions, will create a hybrid layer of resin-infiltrated dentine in the surface dentine. ¹⁵ There are several different types, including three-step etch, prime and bond; etch and rinse; self-etching adhesive; and all-in-one or universal adhesives. A full discussion of the pros and cons of each type of adhesive is beyond the scope of this article. Notwithstanding the different properties of commercially available adhesive bonding systems, it is important to note that whichever adhesive system is chosen, it forms a good quality hybrid layer at the adhesive-dentine interface. A hybrid layer whereby the resin has infiltrated and polymerised a few micrometers into the dentine cavity floor and cavity walls will seal any exposed tubules and remaining microorganisms and resist progression of dental caries. ¹⁵ It must be emphasised that the routine use of liners under adhesively bonded composite resin restorations is not recommended and should be limited to cases where a pulp capping agent, such as calcium hydroxide, has been applied.^16-18

Conclusion

Teeth with dentine caries can be effectively managed with the avoidance of an AGP, using an approach that combines partial-caries removal, maximum preservation of healthy enamel and dentine, followed by the placement of a bioactive provisional restoration if the caries is deep, or the use of a dental adhesive to seal the cavity floor and walls through the formation of a hybrid layer in the surface dentine and a direct composite resin restoration.