Selective caries removal: The importance of finding and using the right adhesive

Minimally invasive dentistry is becoming increasingly popular among dental practitioners worldwide and has had a decisive impact on many fields of dentistry, including restorative procedures. Unlike traditional tooth preparation strategies that require a complete removal of carious tissues – and sometimes even an extension into sound enamel and dentin (“extension for prevention” once promoted by G.V. Black) – modern approaches are based on selective caries removal. Depending on the depth of the lesion, selective removal to hard dentin in the peripheral areas and to firm dentin in the center (in lesions with shallow or moderate depth) or to soft dentin in the center (in deep lesions close to the vital pulp) is recommended. A prerequisite for long-term success of this strategy, however, is proper sealing of the residual carious tissue, as this measure blocks bacteria from its nutrient supply and stops caries from progressing.

After selective caries removal, the bonding surface of the tooth typically consists largely of caries-affected dentin. The morphological, chemical, and physical characteristics of caries-affected dentin differ from those of sound dentin, leading to differences in bonding, bond strength, and bond durability. For this reason, it is essential to check if your adhesive system is suitable for bonding and sealing of caries-affected dentin.

BONDING PROPERTIES OF CARIES-AFFECTED DENTIN

According to Nakajima et al., typical characteristics of caries-affected dentin compared to sound dentin include:

• Scattered and randomly distributed mineral crystals
• Mineral crystals partly occluding dentinal tubules
• Lower magnesium content, higher water content and lower permeability
• A thicker smear layer with more organic components¹

It is mainly due to these characteristics that caries-affected dentin shows a different reaction than sound dentin when treated with many currently available adhesives – regardless of whether they are applied using the etch-and-rinse or self-etch technique. Etching with phosphoric acid removes the smear layer completely and creates a deep demineralized zone. This large, demineralized area, along with the higher water content and the mineral crystals partly occluding the dentinal tubules, seem to hinder resin monomer infiltration and resin tag formation (both of which are key to an effective marginal seal) when the adhesive is applied. While the hybrid layer formed on the surface is comparatively thick, its bottom zone is usually porous. The same is true for the use of when using self-etch adhesives, which show incomplete adhesive monomer infiltration into the deep demineralized zone under a thicker-than-usual hybrid layer. Together, these issues can lead to a lower quality dentin-adhesive interface, which typically results in lower initial bond strength and a higher susceptibility to hydrolytic degradation.¹

Figure 1: Schematic illustration of dentin-adhesive interfaces –
A: Sound dentin
B: Caries-affected dentin – developed after the application of an etch-and-rinse adhesive. While penetration of the dentinal tubules is possible and resin tags are formed on sound dentin, mineral deposits in the tubules and the thicker hybrid layer lead to an incomplete resin monomer infiltration in caries-affected dentin. (Source: modified from Nakajima et al.¹)

IMPROVING THE BONDING POTENTIAL

In this context, several researchers and manufacturers have begun developing strategies to improve dental adhesives’ infiltration ability and bonding performance on caries-affected dentin. Possible approaches include the use of a chemical cross-linker prior to application of etch-and-rinse adhesives and cleaning with mildly acidic hypochlorous acid before applying self-etch adhesives.^2,3 Applying a chemical cross-linker (glutaraldehyde or grape seed extract) to the etched dentin surface seems to increase the stability of dentin collagen, which may inhibit enzymatic degradation, thereby helping improve the bond’s durability.² In a laboratory study, the pre-treatment of caries-affected dentin with mildly acidic hypochlorous acid had a positive effect on the micro-tensile bond strength of a subsequently applied self-etch adhesive, presumably by removing gelatinized collagen from the surface.³ None of the pretreatments demonstrated a negative effect on adhesion to normal dentin. In order to eliminate the need for additional components and work steps, 3M focused on developing an adhesive with a new chemical composition designed to improve bonding and sealing caries-affected dentin.

At Piracicaba Dental School (UNICAMP) in Sao Paulo, Brazil, we decided to test the bonding performance of the new 3M™ Scotchbond™ Universal Plus Adhesive to caries-affected dentin. We began by applying the product to prepared samples of caries-affected dentin and sound dentin (as a control) in the etch-and-rinse or self-etch mode. Then the microtensile bond strengths were determined and the dentin-adhesive interfaces were evaluated with the aid of confocal laser scanning microscopy. The results show that the new adhesive delivers reliable bond strength to caries-affected dentin independent of the selected application mode, forms a continuous hybrid layer, and infiltrates deeply into the dentinal tubules to provide the required seal.

Figure 2: Confocal laser scanning microscopic images illustrating the morphological characteristics of 3M™ Scotchbond™ Universal Plus Adhesive (left) and its predecessor 3M™ Scotchbond™ Universal Adhesive (right) bonding to caries-affected dentin after application in etch-and-rinse mode. Scotchbond Universal Plus Adhesive shows a particularly deep diffusion of the resin monomers into the demineralized tissue.

HL = Hybrid layer
RT = resin tags
A = adhesive
RC = resin composite
CAD = caries-affected dentin
(Images courtesy of Dr. Mario De Goes and Dra. Carolina Garfias.)

Figure 3: Confocal laser scanning microscopic images illustrating the morphological characteristics of 3M™ Scotchbond™ Universal Plus Adhesive (left) and its predecessor 3M™ Scotchbond™ Universal Adhesive (right) bonding to caries-affected dentin after application in self-etch mode. Scotchbond Universal Plus Adhesive shows a particularly deep diffusion of the resin monomers in the demineralized tissue.

HL = Hybrid layer
RT = resin tags
A = adhesive
RC = resin composite
CAD = caries-affected dentin

In fact, Scotchbond Universal Plus Adhesive seemed to infiltrate deeper into the inter- and intra-tubular dentinal areas in the sound and caries-affected dentin compared to the control (3M™ Scotchbond™ Universal Adhesive), independent of the application mode, promoting a complete dentin seal in both. This is exactly what is needed to ensure reliable bonding outcomes in minimally invasive dentistry using selective caries removal techniques. The microtensile bond strength was above 30 MPa for both etching modes, which is on the same level as gold standard adhesives like 3M™ Scotchbond™ Multipurpose Adhesive or CLEARFIL™ SE Bond (Kuraray Noritake) on sound dentin, indicating a reliable performance.⁴

(Images courtesy of Dr. Mario De Goes and Dra. Carolina Garfias.)

CONCLUSION

Clinicians opting for minimally invasive preparation techniques with selective caries removal should check if their adhesive is able to deliver the performance necessary for reliable long-term clinical outcomes on caries-affected dentin. The improvement of the bonding potential in modern adhesives to caries-affected dentin, as found in Scotchbond Universal Plus Adhesive, could reinforce the tooth-composite restoration complex, protecting the underlying tooth structure from secondary caries and tooth fracture – and ultimately, setting you and your minimally invasive procedure up for success.