Clinical Treatment of Deep Caries

Clinically, the transitions between these histological zones may be difficult to detect, but they may be more easily described by the pioneering work of Kuboki et al⁷ and Fusayama and Terachima⁸ as the “outer carious dentin” (also called “infected dentin”), and the “inner carious dentin” (also called “affected dentin”). The two layers were differentiated by a staining dye and it was determined the infected dentin contained irreversible destruction of collagen cross-linking⁷ and was unable to undergo remineralization.⁸ It was later reported the infected dentin had 1300 times greater bacterial concentrations than the nonstained affected dentin.⁹

CARIOUS DENTIN DIFFERENTIATION

Many general dentists consider dentin hardness as the most important criteria for caries excavation.¹⁰ Infected dentin is soft due to the lack of mineral content or collagen network. Its consistency can be likened to cottage cheese (Figure 2). Affected dentin is firmer than infected dentin due to the presence of the collagen network and some mineral content. Its consistency may be referred to as leatherlike (Figure 3). Healthy dentin is often described as hard dentin or sound dentin.

Because the terms used to describe dentin hardness can be subjective, manipulation with dental instruments can help differentiate soft (infected), firm (affected), and hard (healthy) dentin.⁵ Soft dentin can be deformed by pressing an instrument into it, and it can easily be removed by gently scraping its surface with minimal force. Firm, leatherlike dentin does not deform when an instrument is placed into it, but it can be scraped off with medium pressure. Hard dentin requires a strong force to engage the dentin, and it cannot be removed without a sharp cutting edge or bur. A “scratchy” sound can be heard when dragging an explorer across hard dentin.

Aside from the use of tactile sensation, some clinicians rely on alternative methods to identify and remove soft dentin. A recent survey reported the majority of general dentists use a metal bur (69%) or hand excavator (66%) to remove deep caries, but some use ceramic (4%) and polymer (1%) burs.¹⁰ Polymer (or plastic) burs have a hardness that is lower than sound dentin, but harder than carious dentin. Although the reduced hardness of the bur and less aggressive blade design help prevent removal of sound dentin, polymer burs have been reported to leave remnant infected dentin.¹¹ Made from zirconia, ceramic burs have been shown to remove caries as efficiently as metal burs.¹¹ Similar to metal burs, however, they are harder than sound dentin, and their ability to discriminate between infected and affected dentin is unproven.¹¹

Another method of identifying carious dentin is the use of caries detecting dyes. These dyes are typically composed of a propylene glycol solvent and either a red or green dye. The dyes have been shown to stain demineralized organic matrix, not the actual bacteria.¹² Circumpulpal dentin and the dentin-enamel junction (DEJ) have also been shown to stain with these dyes (owing to a reduced level of mineralization).¹³ Therefore, injudicious use of caries detecting dyes will not truly differentiate affected and infected dentin, and may promote unnecessary removal of sound or affected dentin or even expose the pulp. Some evidence suggests that lightly stained dentin (termed “pink-haze” with the use of a red dye) represents affected dentin.¹⁴

EXCAVATION LEVEL

The decision of how much infected dentin to remove is a balance between removing enough carious dentin to obtain a favorable bond and effective seal for the restoration, while maintaining sufficient dentin over the pulp to ensure pulpal health. The long-taught dogma of complete caries removal until hard dentin is achieved — even at the expense of pulpal exposure — is being challenged with more conservative, selective caries-removal protocols.⁵ Selective caries removal has a 77% lower incidence of exposing the pulp than complete caries removal; however, there is insufficient evidence to determine if there is a decrease in signs and symptoms of pulpal disease with this approach.¹ The argument for leaving carious dentin over the pulp is to avoid introducing caries-infected dentin into the pulp during excavation,¹⁵ and preserve a dentin thickness of at least 0.25 mm to prevent injury to the underlying odontoblasts.¹⁶ A 10-year study demonstrated that bonded restorations placed over carious dentin can arrest their progression.²

Prior to initiating a procedure in which a deep lesion is to be removed, a pulpal diagnosis should be obtained. Restorative treatment may be initiated with a normal pulp (immediate dissipation of cold sensation when stimulus is removed) or reversible pulpitis (sharp, quick pain that ends when cold stimulus is removed). Teeth with irreversible pulpitis (lingering pain to hot or cold beyond five seconds, or spontaneous pain) or pulpal necrosis (no response to vitality testing) require endodontic treatment.¹⁷

ONE- OR TWO-STEP PROCESS

Deep caries may be selectively removed in either a one- or two-step process. In a one-step process, the lesion on the pulpal floor is removed until firm, leatherlike dentin is encountered. This firm, affected dentin may be remineralized¹⁸ and serves to protect pulpal health. The exception to this stopping point is if the lesion extends radiographically beyond the inner third of the dentin. For very deep caries, soft, infected dentin may be left over the pulpal floor, as a permanent restoration will seal remnant bacteria from any source of nutrition, causing bacterial death or dormancy (Figures 4A through 4F).^2,19

In a two-step process, the lesion is removed until soft dentin is achieved, after which a provisional restoration is placed. At six weeks to a year after placement of the provisional restoration, the patient’s pulp vitality and caries risk are reassessed, the caries is reexcavated to achieve firm dentin, and the tooth is permanently restored.⁴ Some evidence suggests the one-step caries removal technique may be more effective than the two-step technique.^1,19,20 The one-step technique has resulted in 77% fewer pulpal exposures than complete caries removal, whereas the two-step technique has led to 55% fewer exposures.¹

Although leaving affected or infected dentin over the pulpal floor can be advantageous, achieving a periphery of sound, hard dentin and enamel is necessary to achieve optimal bonding and an effective restorative seal. Bond strength to affected dentin is 33% lower than when bonding to hard dentin, and the bond to infected dentin is 78% lower.²¹ Bonding to caries-infected dentin is ineffective due to its low cohesive strength from mineral loss and collagen degradation. By the same token, bonding to caries-affected dentin is inferior to bonding to hard dentin due to deposition of mineral content within its dentinal tubules, which cause an altered etch pattern and decreased resin penetration.¹¹ Therefore, the peripheral area of the preparation should be composed entirely of hard dentin. Some authors have recommended a width of 1 to 2 mm of hard dentin.³    

One area of carious tissue removal that may pose a clinical challenge is at the DEJ. Once a lesion is cavitated and the dentin is colonized with bacteria, the lesion can spread along the DEJ. At the periphery of the lesion, sound enamel may actually help shelter bacteria, creating sound enamel undermined by carious dentin.¹¹ When achieving a peripheral zone of hard dentin, there may be parts of the margin in which normal enamel is lined by carious dentin. The clinician must choose between removing normal enamel with carious dentin, or removing only the dentin at the DEJ — leaving some unsupported enamel (Figure 5, Figure 6 and Figure 7). In their study, Kidd et al²² concluded that caries tissue at the DEJ should be removed if it is soft, but not necessarily stain-free.

INDICATIONS FOR A LINER

If pulpal exposure has occurred via iatrogenic damage or trauma, a direct pulp cap may be performed with a calcium hydroxide or calcium silicate liner. There is some evidence to suggest calcium silicate liners, such as mineral trioxide aggregate, perform better than calcium hydroxide liners.²³ As noted previously, caries lesions approaching the pulp should not be exposed in order to prevent bacterial contamination of the pulp. When carious dentin is left near the pulp, some clinical guidelines suggest a liner should be placed using an indirect pulp cap technique,⁴ while others recommend the direct application of an adhesively bonded restoration.³ If a calcium hydroxide or calcium silicate liner is placed, its purpose is to stimulate the release of growth factors and, consequently, the formation of reparative dentin to protect the pulp from thermal, electrical or chemical stimuli.²⁴ A recent systematic review reported the use of calcium hydroxide liner had no positive effect on the clinical outcome in the treatment of deep caries.²⁴

SUMMARY

In conclusion, embracing a minimally invasive approach in managing deep caries will help avoid complications with pulpal involvement. Current evidence suggests the endpoint for caries removal should be leathery, firm affected dentin on the pulpal floor, with a periphery of sound, hard dentin and enamel. If a deep lesion extends to the inner third of the dentin, a minimal amount of soft, infected dentin may be left on the pulpal floor to prevent pulpal exposure. While a liner may be placed over the deep pulpal dentin, it may not be necessary. Finally, a bonded restoration should be carefully placed to create an effective seal that will prevent further lesion progression.

KEY TAKEAWAYS

• Prior to initiating a procedure in which a deep lesion is to be removed, a pulpal diagnosis should be obtained.
• Selective caries removal will reduce issues with pulpal exposure.¹
• Long-term evidence has demonstrated that bonded restorations placed over carious dentin can arrest lesion progression.²
• A cross section of carious dentin has distinct histological and structural zones that represent how far along the caries process has progressed within the dentin.
• Infected dentin is soft due to the lack of mineral content or collagen network. Its consistency can be likened to cottage cheese.
• Affected dentin is firmer than infected dentin due to the presence of the collagen network and some mineral content. Its consistency may be referred to as leatherlike.
• Healthy dentin is often described as hard dentin or sound dentin.
• Another method of identifying carious dentin is the use of caries detecting dyes, but injudicious use of dyes will not truly differentiate affected and infected dentin, and may promote unnecessary removal of dentin, or even expose the pulp.
• The decision of how much infected dentin to remove is a balance between removing enough carious dentin to obtain a favorable bond and effective seal for the restoration, while maintaining sufficient dentin over the pulp to ensure pulpal health.

REFERENCES

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Nathaniel Lawson, DMD, PhD, discloses he has received grants, research support or honoraria from 3M, Ivoclar, Dentsply Sirona, DMG America, GC America, Kuraray, and Pulpdent. Augusto Robles, DDS, MS, has no commercial conflicts of interest to disclose.