New Options for Restoring a Deep Carious Lesion

INTRODUCTION
Operative dentistry not only requires technical expertise and an in-depth understanding of materials science, but knowledge in cariology and pulp biology is also essential. We no longer need to create a preparation that may far exceed the boundaries of the diseased tooth structure. Instead, with the advances in adhesive dentistry, tooth preparation can be much more conservative. Even the amount of potentially diseased tooth structure that must be removed is being called into question.
This article will explore management of the deep carious lesions and look at some of the following questions being considered in the dental literature. Do all caries need to be removed (when the excavation extends near the pulp) in every clinical case? Can caries progression be arrested? What type of restorative materials should be used to ensure the best outcome for the patient?

Traditional Concepts
In his classic text (1908), G. V. Black said, “It is better to expose the pulp of a tooth than to leave it covered only with softened dentine.” However, G. V. Black also stated that it is imperative that dentists understand the pathology of the caries process or they will only be considered as mechanics. Obviously, dental science has advanced our understanding of the carious process and the ability to create a restoration with well-sealed margins and associated grooves and fissures.^1,2 G. V. Black may have altered this assertion 100 years later.
The traditional concept of complete caries removal in very deep preparations has been challenged.² Complete carious dentin removal may not be a prerequisite to arrest caries progression. It is well known that bacteria in the dentin cause pulpal inflammation. This inflammatory process may be a benefit for pulp regeneration.³ Leaving some carious tissue beneath a restoration does not necessarily interfere with treatment success. Short-term studies of 36 to 45 months, in which carious dentin was sealed in, showed lack of progression of the lesion and a decrease in the number of microorganisms. Remineralization of the remaining carious dentin was detected both biochemically and radiographically. Calcium hydroxide liners were used in these studies.⁴
Leaving caries beneath a restoration is a very controversial issue. The traditional concept of an indirect pulp cap usually required that the dentist place some type of interim restoration. After a period of weeks or months, the tooth is then reentered, the remaining caries excavated, and a definitive restoration placed. Frequently a calcium hydroxide liner is placed beneath the interim and definitive restorations.
In indirect pulp treatment, demineralized carious tissue is left at the deepest sites of the cavity preparation so as not to expose the pulp. Indirect pulp treatment is limited to teeth that have no signs or symptoms of irreversible pulpal pathology. Complete removal of all carious tissue from the pulpal walls is essential to control microleakage. The lesion may be slowly or rapidly progressing. Clinical, radiographic, and bacteriologic studies have shown that caries can be arrested. A well-sealed restoration, without marginal defects, is essential.⁵
The classic indirect pulp cap has a relatively high clinical success rate, pulp exposure is usually avoided, and the tooth is usually asymptomatic. Dentin upon re-entry is described as dryer, harder, and darker. From a microbiological perspective, there is a substantial decrease in any cultivatable flora. There is a possibility that the dental material may have an effect on the outcome, but very few studies address this in a controlled manner. “Indeed, a cautious approach may be preferable to vigorous excavation because fewer pulps will be exposed and sealing the dentin from the oral environment encourages arrest of the lesion progression. The reparative process of tubular sclerosis and tertiary dentine are encouraged, thus reducing the permeability of the remaining dentin. The remaining microorganisms are entombed by the seal of the restoration on one side and the reduced permeability of the remaining dentin on the other.”¹

Changing Evidence
It has been well accepted that sealants protect the underlying tooth structure, preventing plaque accumulation and dissolution of minerals. More recently, an increasing body of evidence indicates that arrest of noncavitated carious lesions is possible with dental sealants. Yet noninvasive management of caries has not been widely adopted in these instances by dental practitioners.^6,7
The outcome of stepwise excavation versus direct complete excavation has been compared. In this study,⁸ when a temporary restoration could be properly placed, no further excavation was carried out; leaving soft, wet, and discolored dentin centrally on the pulpal floor. A calcium hydroxide lining material was applied over the remaining carious dentin and the cavity was temporarily sealed with glass ionomer. After 8 to 12 weeks, the cavity was reentered and the final excavation was carried out. A calcium hydroxide liner was applied and the cavities were restored with composite resin. The authors8 observed significantly fewer pulp exposures after stepwise excavation than after direct complete excavation in adult teeth. Also, a significantly better success rate was found for stepwise excavation at one year of follow-up versus direct complete excavation, when considering unexposed pulps with sustained vitality without apical radiolucency. This observation emphasizes the importance of maintaining an unbroken dentin barrier against the pulp, even if there remains some caries.⁸
Most recently, the need for stepwise excavation is being questioned. Indirect pulp capping is simple, more patient friendly, and less expensive than root canal treatment. Although some microorganisms may survive, these are rarely enough for the caries to progress, and tertiary dentin is deposited resulting in mineral gain in the radiolucent zone is promoted.³ Infected dentin should be removed completely from the preparation walls but selectively from the pulpal floor or axial wall. Removal of all of the infected dentin in deep carious lesions may not be required in every clinical situation, “provided that the restoration can seal the lesion from the oral environment effectively.”²

Dental Liners: Effects on the Pulp
Liners are frequently used beneath dental restorations to reduce the potential for postoperative sensitivity. Several properties are necessary for an ideal liner: (1) the ability of the material to kill bacteria, (2) induce mineralization, and (3) establish a tight bacterial seal.⁹ Postoperative sensitivity is partially related to the remaining dentin thickness (RDT) following cavity preparation and presence of bacteria on the cavity walls. There is no material better to protect the pulp than dentin. The remaining dentin thickness between the base of the cavity preparation and the pulp is one of the most important factors in protecting the pulp from toxins.⁹ A 0.5 mm thickness reduces the effect of toxins by 75%. A 1.0-mm thickness reduces the effect of toxins by 90%. Remaining dentin thickness of 2.0 mm (or more) results in little or no pulpal reaction. It is at the 0.5 mm region that liners become most important. As the remaining dentin thickness decreases, odontoblast survival and pulp—dentin repair becomes compromised.⁹
Reactionary dentin deposition was observed beneath cavities with a RDT above 0.5 mm as well as beneath cavities with a RDT below 0.25 mm; however, maximal reactionary dentin appeared to be beneath cavities with an RDT between 0.5 to 0.25 mm. The area of reactionary repair was also influenced by the choice of restoration material (from greatest to least: calcium hydroxide, composite, resin-modified glass ionomer [RMGI] cement, and zinc oxide-eugenol). Odontoblast numbers were maintained beneath cavities with a RDT above 0.25 mm.^9,10
Calcium hydroxide has been used as a lining material since the 1920s. Because of the basic pH of about 11, calcium hydroxide is both antibacterial and can neutralize the acidic bacterial byproducts. The high pH creates an environment conducive to the formation of reparative dentin. In addition, calcium hydroxide has the capacity to mobilize growth factors from the dentin matrix, causing the formation of new dentin. Calcium hydroxide is an ideal lining material for the very deep cavity preparation and also continues to represent an option for both the indirect and direct pulp cap.^10,11
Adhesive resins can be acidic and cause pulpal irritation. Many dentin bonding agents and resin-reinforced glass ionomers are actually detrimental to the pulpal tissues. In contrast, calcium hydroxide has been shown to provide a significantly improved potential for pulpal repair compared to adhesive resins.¹²
Unfortunately, the self-setting calcium hydroxide liners are highly soluble and subject to dissolution over time.¹³ Traditional calcium hydroxide liners are easily lost during acid etching. Dentin bonding agents that contain water, acetone, or alcohol can also detrimentally affect the properties of calcium hydroxide. A hermetic seal of the cavity may stabilize the lesion and arrest caries progression. Therefore, when a restoration of composite resin is planned, glass ionomer cement should line the cavity preparation, sealing over the calcium hydroxide material, if used.¹⁴
Restorative materials that exhibit antimicrobial benefits are useful in minimal intervention and other types of dentistry.¹⁵ Some studies show that RMGIs were about equal to conventional calcium hydroxide liners.³ Remaining softened, demineralized dentin covered by glass ionomer becomes remineralized, presumably under the influence of the fluoride release and the presence of calcium and phosphate ions from the cement. This phenomenon is also referred to as the “healing of affected dentin.” However, in other studies, RMGIs were found to cause the greatest reduction in odontoblast numbers. It is frequently recommend that a thin liner of calcium hydroxide be applied to the cavity floor of deep preparations before RMGI is placed.16 This appears to provide improved pulpal protection from injury and bacterial microleakage.¹⁷
In recent years, mineral trioxide aggregate (MTA) preparations have been introduced (ProRoot MTA [DENTSPLY Tulsa Dental Specialties]). These silicate cements are antibacterial, biocompatible, have a high pH, and are able to aid in the release of bioactive dentin matrix proteins. MTA is a powder consisting of fine hydrophilic particles of tricalcium silicate, tricalcium aluminate, tricalcium oxide, and silicate oxide. It also contains small amounts of other mineral oxides, which modify its chemical and physical properties. Hydration of the powder results in formation of colloidal gel with a pH value equal to 12.5 (similar to calcium hydroxide) that solidifies to form a strong impermeable hard solid barrier in approximately 3 to 4 hours. It is hypothesized that the tricalcium oxide reacts with tissue fluids to form calcium hydroxide.¹³
The material has a low solubility and a radiopacity slightly greater than that of dentin. Because it has low compressive strength, it should not be placed in functional areas. Another significant disadvantage for the restorative dentist is that the setting times may take several hours. As a result, 2-step procedures are frequently necessary, requiring interim restorations. As previously discussed, recent evidence indicates that an indirect pulp cap should be performed in a single treatment appointment. Any immediate restoration will require coverage with a layer of RMGI cement.18 MTA is an excellent material for direct vital pulp exposures and numerous endodontic applications. The material has good long-term sealing capabilities, and some studies show greater success than conventional calcium hydroxide.¹⁹

CASE REPORTS
Case 1
A 30-year-old patient presented with a large carious lesion in the lower first molar (Figure 1). The patient reported that the tooth was sensitive to cold, but otherwise asymptomatic. A cold test did verify the transient sensitivity to cold, which dissipated at approximately 30 seconds. A periapical radiograph confirmed that the caries was in close proximity to the pulp. The caries was excavated until the preparation was excavated to a depth estimated at less than 1.0 mm from the dental pulp (Figure 2). Biodentine (Septodont) was used as the therapeutic interim restoration (Figure 3). The tooth was allowed to heal for about 4 months. During this time, the tooth was asymptomatic. A cold test was used to verify pulpal vitality, and a periapical x-ray was taken (Figure 4). It was decided to restore the tooth with a direct distal occlusal composite as a definitive restoration (Figure 5).

Figure 1. Large carious lesion in a tooth with minimal symptoms.	Figure 2. Partial caries excavation was carried out, leaving soft, wet, and discolored dentin.
Figure 3. Biodentine (Septodont) interim restoration.	Figure 4. Radiograph of the Biodentine restored tooth. Remaining caries is evident on the radiograph. Depending on the clinical symptoms, the dentist may choose to prepare this tooth, leaving some of the Biodentine as a base beneath the definitive restoration.
Figure 5. Definitive distal occlusal composite restoration.	Figure 6. Deep excavation with discolored dentin of an uncertain quality.

Figure 7. TheraCal (BISCO Dental Products), a light-cured resin-modified calcium silicate lining material, is delivered into the preparation.

Material Discussion
In the author’s experience, Biodentine is a relatively more user-friendly material for the restorative dentist as compared to traditional MTA preparations. Biodentine is an active biosilicate material useful in direct and indirect pulp capping, and it also has endodontic applications. In addition, it is different from the usual MTA formulations. The manufacturing process of the active biosilicate technology eliminates the metal impurities. The setting reaction is a hydration of tricalcium silicate, which produces a calcium-silicate gel and calcium hydroxide. In contact with phosphate ions, it creates precipitates that resemble hydroxyapatite. “An evaluation of the dentin-Biodentine interface demonstrated an increase in the carbonate content of interfacial dentin, which suggested intertubular diffusion and mineral tags of Biodentine hydration products creating a hybrid zone.”²⁰ Biodentine resists microleakage similar to RMGI cements²¹ and also has an antibacterial effect.²²
Biodentine reaches a final set in about 10 to 12 minutes. This is much faster than the time required for MTA and also demonstrates higher compressive strength. Therefore, it can serve as an excellent interim restoration. As of yet, the manufacturer recommends the material be used in a 2-step procedure. At a subsequent appointment, the clinician cuts the ideal cavity preparation, leaving the Biodentine as a liner or a dentin substitute base under the definitive restorative material.²³

Case 2
The lower molar in Figure 6 had an existing defective amalgam restoration with recurrent caries. The restoration was removed and the caries excavated. A thin layer of TheraCal LC (BISCO Dental Products) (a light-cured, resin-modified calcium silicate lining material) was placed over the deepest portions of the preparation using the convenient syringe delivery system (Figure 7) and then light-cured for 20 seconds. The cavity preparation was etched (Figure 8), rinsed, coated with adhesive, and then air-thinned and light-cured (Figure 9); ready for composite resin placement to complete the restoration.

Material Discussion
TheraCal LC, another recently introduced material, is in simplistic chemical terms, a material that offers the pulpal/dentin benefits of both calcium hydroxide and RMGI in one product. The material demonstrates strong physical properties with low water solubility and can be light-cured up to a thickness of 1.0 mm. TheraCal LC is FDA approved as an apatite stimulating liner with the ability to induce apatite crystal formation, similar to commercially available self-curing MTA products.^24,25
Dentinal fluid absorbed by TheraCal LC results in a sustained release of calcium and hydroxide ions. Calcium is necessary for rapid apatite stimulation, and hydroxide ions are required for providing an “alkalinizing effect” for wound healing propagation. TheraCal LC is intended for use as an internally placed pulpal protectant liner over both occlusal and axial dentin. When performing a direct pulp cap, use of a rubber dam to reduce bacterial contamination, apex development, and control of pulpal hemorrhage are among the essential factors for success. This material will appeal to the clinician who wants the ease of placement of conventional calcium hydroxide liners along with the biological benefits of the newer calcium silicates. TheraCal LC allows for the one-step indirect pulp cap.

IN SUMMARY
The ADA describes evidence-based dentistry as researching the best available clinical evidence, using one’s clinical skills and judgment along with careful considerations of the patient’s needs and preferences. Today’s dentist needs to follow these principles in order to practice the best dentistry possible and provide optimum patient care. There exists a significant amount of literature indicating the overall success of the indirect pulp cap. On the other hand, literature reviews of direct pulp caps, especially in mature teeth, do not describe the same success in avoiding the need for root canal therapy.²⁶ There may have been times when the dentist chose to place an indirect pulp cap and reenter the tooth at a later time to excavate the remaining caries.
As this article demonstrates, stepwise excavation is not always necessary. The newer calcium silicate cements may be especially useful in achieving even greater success in these cases. More clinical studies will be necessary before the dental profession generally accepts this concept. There are certain instances when root canals cannot easily be accomplished, such as the frail, elderly, severely medically compromised, or developmentally disabled populations. In addition, indirect pulp capping may be a reasonable choice in specific public health settings where root canals are not financially feasible. Dentists should be aware that there are times when the indirect pulp cap can now be incorporated into a definitive restoration without reservation.

References

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Dr. Rada received his DDS from the University of Illinois College (UIC) of Dentistry and completed a general practice residency at Loyola University. He is currently a clinical associate professor in the department of oral medicine and diagnostic sciences at UIC and maintains a private practice in LaGrange, Ill, focusing on restorative and aesthetic dentistry, as well as dentistry for special needs patients and hospital dentistry. He can be reached at (708) 482-3636 or at bobr759@sbcglobal.net.

Disclosure: Dr. Rada reports no disclosures.