| Sign In to gain access to subscriptions and/or personal tools. |
Evaluation of a Self-limiting Concept in Dentinal Caries Removal
1 Department of Prosthodontics and Biomaterials, New York University College of Dentistry, 345 East 24th Street, Room 804, New York, NY 10010, USA; Correspondence: * corresponding author, van.thompson{at}nyu.edu
The mechanical removal of dentinal caries traditionally involves the use of tactile sensation and/or caries-indicating dyes. This study tested the hypothesis that self-limiting polymer burs are as effective as conventional carbide burs in creating substrates for dentin bonding. Carious dentin from extracted human molars was removed with carbide or polymer burs, with dental explorer hardness as the end-point for caries removal. Dentin substrates were bonded with etch-and-rinse or self-etch adhesives and prepared for microtensile bond testing and transmission electron microscopy. For each bur type, there was no difference in bond strength between adhesives. However, the polymer bur surface exhibited significantly lower bond strengths than the carbide bur, and both were lower than flat, non-carious dentin controls. TEM revealed areas of incompletely removed, denatured caries-infected dentin in the polymer bur specimens. These first-generation polymer burs might best be utilized for deep caries removal where pulpal exposure is a concern.
Key Words: polymer bur • caries-affected dentin • caries-infected dentin • microtensile bond strength • transparent zone
Disease prevention is the ultimate goal in restorative dentistry (Featherstone, 2000; Pitts, 2004). Ideally, caries removal should be accomplished with minimal patient discomfort during and after the restorative procedures (Anusavice and Kincheloe, 1987; Malmström et al., 2003). The existence of two layers of carious dentin has been well-reported in the literature (Massler, 1967; Fusayama, 1979). The superficial layer of caries-infected dentin is grossly denatured and is a poor substrate for adhesion of restorative materials (Nakajima et al., 1995; Yoshiyama et al., 2002). The underlying layer of partially demineralized caries-affected dentin contains dentinal tubules that are usually filled with Whitlockite caries crystals, rendering it highly impermeable to dentinal fluid transudation (Lee et al., 2003), or the creation of rapid fluid shifts (Brännstrom, 1986) that may stimulate the underlying A-  nerve fibers and cause post-operative sensitivity (Närhi et al., 1994). Since caries-affected dentin contains intact, undenatured collagen fibrils and is amenable to remineralization (ten Cate, 2001), there is a general consensus for this layer to be preserved during caries excavation (McComb, 2001; Kidd, 2004). Traditionally, carious dentin may be removed mechanically with burs, hand excavators, and air-abrasion, or chemomechanically with the adjunctive use of a deproteinizing agent alone, or in combination with amino acids (Yip and Samaranayake, 1998; Banerjee et al., 2000). Since diamond and tungsten carbide burs are indiscriminant in their removal of carious tissues, they can remove caries-infected and caries-affected dentin simultaneously, with possible extension into the underlying sound dentin. This may be accompanied by pain and necessitates the application of local analgesia during treatment (Rafique et al., 2003). A novel, recently proposed, self-limiting concept in mechanical caries removal (Boston, 2003) has been brought to fruition by the introduction of a polymer bur (SmartPrep, SS White Burs, Inc., Lakewood, NJ, USA). The paddle-shaped bur has a unique flute design, and is constructed from a medical-grade polyether-ketone-ketone (PEKK), with a particular hardness and wear resistance that reportedly enable it to remove only the soft caries-infected dentin, leaving the caries-affected dentin intact (Boston, 2000, 2002). Utilized exclusively at low speed (500–800 rpm), the bur quickly dulls and vibrates when it encounters the more highly calcified caries-affected dentin. Although the self-limiting concept of caries removal appears to have potential merits, and its use without local anesthetic is accepted by patients (Allen et al., 2005), the ability of the polymer bur to remove infectious carious tissues and produce optimal bonding substrates in the remaining dentin has not been established. Thus, the objective of this study was to evaluate the efficacy of the polymer bur in removing carious-infected dentin by examining the ultrastructural features of the dentin substrate available for bonding following caries removal with the polymer bur, and by comparing microtensile bond strengths achieved with etch-and-rinse and a self-etch adhesive in polymer bur- and conventional carbide bur-prepared carious dentin. The research hypothesis tested was that polymer burs are as effective as carbide burs in caries removal and in creating optimal substrates for dentin bonding.
Sixteen extracted human molars with extensive occlusal caries and 8 sound third molars were collected after the patients’ informed consent had been obtained under a protocol reviewed and approved by the NYU College of Medicine Institutional Review Board. The teeth were sterilized with gamma radiation and then stored in water for not more than 3 mos until use.
Specimen Preparation We removed the remaining enamel and sound dentin from each tooth by abrading the entire occlusal surface with 240-grit followed by 600-grit silicon carbide papers on a metallurgical polisher (Buehler Ltd., Lake Bluff, IL, USA), until a flat surface was created close to the original excavated caries lesion. The flattened sound dentin was marked with indelible ink to delimit the carious regions where the polymer or carbide burs had been used. A separate microtensile bond strength group (8 teeth) was completed with flat and sound dentin substrates obtained by the use of 600-grit silicon carbide paper to permit correlation to the carbide and polymer group findings. Half of the total specimens (8 carious and 4 sound) were bonded with Single Bond (3M ESPE, St. Paul, MN, USA), a representative ethanol-based etch-and-rinse adhesive, and the other half were bonded with a well-known self-etching system, Clearfil SE Bond (Kuraray Medical Inc., Tokyo, Japan), resulting in 6 subgroups (N = 4). Bonding agent selection was based upon use in a previous study on carious dentin (Yoshiyama et al., 2002). The bonded surfaces were coupled with a hybrid resin composite (Z-100, 3M ESPE) that was applied in 2-mm-thick increments and polymerized in a quartz-tungsten-halogen light-curing unit at 500 mW/cm2 (Curing Light 2500, 3M ESPE) to form 4-mm-thick cores. The teeth with composite build-ups were stored in water at 37°C for 7 days to ensure hydration.
Microtensile Bond Testing
The beams were stressed to failure under tension in a Bencor Multi-T device (Danville Engineering, San Ramon, CA, USA) in a universal testing machine (Model TSD 500, Chatillon-Ametek, Agawam, MA, USA) at a crosshead speed of 1.0 mm/min. The exact beam dimensions were measured after bond-testing for the compilation of the mean bond strengths. We analyzed the data with a two-way ANOVA design, to examine the effects of bur-type (polymer vs. carbide burs) and adhesive-type (etch-and-rinse vs. self-etch), as well as the interaction of these two factors on microtensile bond strengths referenced to the values obtained on sound flat dentin substrates. We performed multiple-comparison tests for polymer vs. carbide bur using the Tukey test. Statistical significance was set at The failure mode of each specimen was determined under a stereomicroscope (SXZ-ILLB 100, Olympus, Tokyo, Japan) and designated as adhesive, mixed, or cohesive failures in either dentin or resin composite. Subsequently, selected specimens were analyzed via Transmission Electron Microscopy (TEM). One operator performed the caries removal, bonding procedures, and microtensile testing for all groups.
Transmission Electron Microscopy
Statistical analysis of the bond strength data derived from the polymer and carbide subgroups (Table  ) showed that microtensile bond strengths of carious dentin were affected by the type of bur used for caries removal (P < 0.05), but not by the type of adhesive (P = 0.28), with the polymer bur specimens exhibiting significantly lower bond strengths than the carbide bur specimens. There was no significant interaction between bur type and adhesive type (P = 0.63). A mixed failure mode was observed in 92.1% of all specimens: 3.3% of the specimens exhibited cohesive failure in composite, 2.7% exhibited cohesive failure in dentin, and 1.9% exhibited adhesive failure.
The flat sound dentin groups (Table  ) showed higher bond strength values for each respective polymer and carbide group.
At the ultrastructural level, incompletely removed caries-infected dentin was seen in all unbonded dentin surfaces following caries removal with the polymer bur (Fig. 1A
Application of the etch-and-rinse adhesive Single Bond to dentin following caries removal with polymer burs produced 12- to 15-µm-thick hybrid layers within the incompletely removed caries-infected dentin (Fig. 2A ). Although these hybrid layers were abnormally thick when compared with those from sound dentin (Yoshiyama et al., 2002), resin infiltration was incomplete, as a result of the depth of the highly porous sub-surface translucent dentin zone (Fig. 2B). Similarly, abnormally thick hybrid layers (ca. 8-12 µm) (Yoshiyama et al., 2002; Doi et al., 2004) created by the self-etch adhesive Clearfil SE Bond could be seen within the incompletely removed caries-infected dentin after caries removal with the polymer bur (Fig. 3A). Complete resin infiltration of the dentin substrate by an adhesive that etches and primes simultaneously was not achieved. This is also likely the result of the depth of the highly porous underlying transparent dentin zone (Fig. 3B).
Cavity preparation should be completed with minimal tissue removal (Fusayama, 1993). This conservation of sound tissue promotes the longevity of the restored tooth. Banerjee et al.(2003) have compared caries excavation techniques using decalcified dentin autofluorescence to determine the removal of caries to an adequate depth. Their conclusion was that carbide burs routinely overprepared the carious dentin, extending well into the affected dentin. That study indicated that hand excavation, while not time-efficient, was the most reliable method to prevent overpreparation of the dentin. The polymer bur instrumentation could be a new method to achieve this goal. The dentin surface remaining after polymer bur removal of caries was found to be hard but discolored/pigmented. This is to be expected, as pointed out in another study of caries depth relating microhardness and autofluorescence of dentin (Banerjee et al., 1999). Banerjee and coworkers (1999) found that non-infected enamel and dentin did not autofluoresce. Autofluorescence distribution from carious dentin correlated with the highly softened tissue (detected by the Knoop indenter) and terminated at a level within the affected dentin, superficial to the translucent zone (low permeability zone). This zone was still pigmented. Normal, sound dentin hardness levels were found deep to the translucent zone. These authors found a correlation existing between the zone of autofluorescence and carious dentin that was markedly softened by the carious process. These findings highlighted the possibility that the autofluorescence might be used as an in vitro, objective histological and clinical marker for the softened, carious dentin requiring clinical excavation (Lennon, 2003). The bond strengths to the surfaces prepared with the polymer bur, as compared with the carbide bur, were significantly lower for the bonding agents (p < 0.05). The bond strengths to carbide-bur-prepared affected dentin were less than those observed in other, more recent, studies with Single Bond and Clearfil SE Bond (Yoshiyama et al., 2002). The carbide bur is difficult to control because of its high cutting efficiency for dentin, removing the infected, affected, and sound tissues with little tactile feedback. It must be noted that the specimens prepared with either the polymer or carbide bur do not have the same perpendicular interfaces as the usual microtensile bond specimens. This could contribute to both the lower bond strengths and the variability observed. The effect of bond angle on microtensile bond strength is being investigated. The maintenance of a seal at the cavosurface margin of restorations is the key to prevention of residual bacteria from proliferation. It remains to be determined if the bond strengths to polymer instrument-prepared carious dentin are sufficient to prevent resin-based composite debonding and subsequent cavosurface microleakage. It is not known whether the low elastic modulus of caries-affected dentin is sufficient to allow for its deformation and subsequent relief of polymerization strain caused by composite resin shrinkage. Comparative leakage studies between carious tooth restorations are advised, but are very difficult to design and conduct with proper controls. Initial clinical findings with Class I restorations are promising (Allen et al., 2005). There are several problems that may affect bonding efficacy when self-etch and etch-and-rinse adhesive systems are used on caries-affected and caries-infected tissues. Carious intertubular dentin exhibits a higher degree of porosity than sound dentin, due to loss of mineral (Yoshiyama et al., 2002). Therefore, the lower bond strengths found for the tested adhesives are probably due to the decrease in modulus of elasticity and the lower cohesive strength of the caries-affected dentin. Additionally, acid-etched, caries-affected dentin contains a significantly higher amount of water that makes its replacement by adhesive resins during bonding more difficult. Remaining water at the bonded interface would interfere with the polymerization of the adhesive, thereby compromising the bond strength (D.H. Pashley, personal communication). Based upon our findings, this novel self-limiting concept of caries removal should go forward with modifications in the bur composition/hardness, to ensure complete removal of infected tissue. Since the bur is fabricated with a medical polymer (PEKK), the hardness and/or the shape of this preliminary bur might not be adequate to remove the infected tissue completely. A new, re-designed version of the polymer bur is already on the market, and further studies are warranted.
The polymer burs and tungsten carbide burs used in this study were generously provided by SS White Burs, Inc. The authors are grateful to the Department of Prosthodontics and Department of Biomaterials of NYUCD, CNPQ (Grants 201471/2003–5, 300481/95–0, and 474226/03–4), and the Department of Prosthodontics of Bauru School of Dentistry, FOB-USP. The TEM part of this study was supported by grant 10204604/07840/08004/324/01, Faculty of Dentistry, the University of Hong Kong, Pokfulam. We thank Elizabeth Clark and Carlos Augusto de Oliveira Fernandes (University of Ceara, Brazil) for laboratory and editorial support.
A supplemental appendix to this article is published electronically only at http://www.dentalresearch.org. Received for publication November 22, 2004. Revision received June 17, 2005. Accepted for publication November 2, 2005.
Journal of Dental Research, Vol. 85, No. 3,
282-286 (2006)
|
|
|||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||
 |
|
|
 |
|
Sign In to gain access to subscriptions and/or personal tools.
TOP
ABSTRACT
INTRODUCTION
= 0.05. 
