Dental India Newsletter dated 20th June 2004

Tensile strength and microhardness of treated human dentin

Victoria Fuentesa, Laura Ceballosa, Raquel Osorioa, Manuel Toledanoa,
Ricardo M. Carvalhob,*, David H. Pashleyc
aDepartment of Dental Materials, School of Dentistry Granada, University of Granada, Spain
bDepartment of Operative Dentistry, Endodontics and Dental Materials, Bauru School of Dentistry,
University of Sao Paulo, Brazil
cDepartment of Oral Biology and Maxillofacial Pathology, School of Dentistry, Medical College of Georgia,
Augusta, GA, USA
Received 5 September 2002; received in revised form 10 April 2003; accepted 29 May 2003

Summary Objectives. To determine the ultimate tensile strength and Knoop hardness of mineralized, EDTA-treated, sodium hypochlorite (NaOCl)-treated, EDTA-treated resin-infiltrated, and NaOCl-treated resin-infiltrated dentin.

Methods. Dumbell-shaped specimens with a cross-sectional area of 0.5 mm2 were prepared from the crowns of extracted human third molars. Specimens were randomly assigned to the following experimental groups:

(1) mineralized dentin;

(2) 0.5 M EDTAdemineralized dentin, pH 7/5 days;

(3) 5% NaOCl-deproteinized dentin/2 days;

(4) EDTA-treated, Single Bond resin-infiltrated dentin;

(5) NaOCl-treated, Single Bond resin-infiltrated dentin.

All specimens were tested in tension in a Vitrodyne testing machine at 0.6 mm/min. Knoop microhardness was measured on the fractured edges of specimens in groups 1, 3, 4, and 5. Results were analyzed by ANOVA and SNK tests p , 0:05:

Results. Both EDTA and NaOCl treatments caused significant reductions in the tensile strength and microhardness of mineralized dentin p , 0:05; with the largest reductions observed after NaOCl treatment p , 0:05: Resin infiltration of treated
dentin resulted in moderate increase of its tensile strength and microhardness, however, the original mineralized values were not recovered p , 0:05:

Significance. Whenever dentin surfaces are treated with EDTA or NaOCl prior to a
clinical bonding procedure, clinicians must be aware that a weak layer may be present
at the interface, which may lead to premature failures of resin/dentin bonds.

Introduction: Most current dentin bonding procedures rely on partial demineralization of the dentin with acidic solutions to remove smear layers, open dentinal tubule orifices, and expose the collagen matrix for penetration of hydrophilic monomers to form a
hybrid layer. The presence of the collagen fibril network in demineralized dentin may contribute to the technique-sensitivity of the so-called total-etch bonding procedures. After demineralization, water occupies the interfibrillar spaces left by solubilization of the mineral phase and maintains the collagen network in an expanded state. If dentin is air-dried, the water evaporates from the
interfibrillar spaces causing the collapse of collagen network reducing the size of the spaces necessary for resin infiltration, thus reducing the uptake of resin. Conversely, excess moisture on the dentin surface can result in phase separation in resins and
in the presence of microscopic voids at the resin dentin interface. These phenomena result in an incomplete resin infiltration within the demineralized dentin and leave a collagen-rich zone that may be susceptible to leakage and its deleterious
consequences. The total-etch technique may alter the structure of the collagen matrix. Several authors reported that the exposed collagen fibrils are in a destabilized state, susceptible to proteolytic degradation and its durability over time is a questionable
issue. To avoid the potential problems of such unprotected collagen fibrils, sodium hypochlorite (NaOCl) treatment has been suggested to be used after etching to remove the collagen network before bonding to dentin. NaOCl is a wellknown nonspecific proteolytic agent capable of removing organic material, as well as magnesium and carbonate ions from the dentin. Several researchers have evaluated the effects of NaOCl on dentin adhesion. The application of a deproteinizing agent alters the ultra-morphology of an etched dentin surface by removing the exposed collagen fibrils, exposing a network of secondary
lateral canals and anastomoses, and widening the aperture of the dentinal tubules. Demineralized collagen-rich dentin is transformed into a porous collagen-depleted structure with multiple irregularities, with good mechanical retention of adhesive
resins into the modified dentin substrate. Although the effects of both demineralizing and deproteinizing agents on resin/dentin bond strength have been extensively evaluated, little is known regarding the effects of such solutions on the mechanical properties of the dentin substrate. Stress distribution at the resin/dentin interface depends on the mechanical properties of its components.Chemical pre-treatments induce considerable changes in the surface morphology and in the physical properties of dentin. Such changes may alter the distribution of stresses along the interface and may determine the preferential location of failures. While it is expected that depletion of mineral by EDTA or protein by NaOCl will cause significant decrease in dentin strength, little is known as to whether subsequent infiltration with resins would restore dentin original strength.

The aim of this study was to evaluate the effects of demineralization by EDTA treatment, and deproteinization by NaOCl treatment on the ultimate tensile strength (UTS) and Knoop hardness (KHN) of dentin. Additionally, UTS and KHN were determined following the infiltration of EDTA- and NaOCltreated dentin with an adhesive resin. The null hypothesis tested was that resin-infiltration of EDTA- or NaOCl-treated dentin will not restore the original strength of mineralized dentin.

Materials and methods Specimen preparation Human third molars stored in isotonic saline containing a few crystals of thymol at 4 8C were used in this study. Teeth were collected after obtaining the patients informed consent under a protocol approved by the University of Granada Institutional Review Board. Slices of mid-coronal dentin (0.7 mm in thickness) were obtained by
transversally sectioning the crowns with a lowspeed diamond saw (Extec Co., Einfield, CT, USA) under water irrigation. Each slab was trimmed into an hourglass shape with a cross-sectional area at the neck of about 0.5 mm2 by means of a diamond
bur under copious air water spray. Trimmed specimens were then randomly assigned to the following groups of 10 specimens each:

(1) Mineralized or untreated dentin. Specimens in this group received no further treatment and were stored in water until tested.
(2) EDTA-treated dentin. Specimens had their ends covered with nail varnish and were immersed in 0.5 M EDTA (pH 7) solution for 5 days in a gyratory shaker to completely demineralize the central area. After demineralization, they were removed from the EDTA, thoroughly washed, the varnish scraped off, and the specimens stored in distilled water until tested.
(3) NaOCl-treated dentin. Specimens had their ends covered as in group 2 and were immersed in a 5% NaOCl solution for 2 days under agitation. Preliminary experiments showed that longer immersion time resulted in spontaneous fracture of several specimens while in the solution or during manipulation before testing. They were then thoroughly washed and stored in distilled water until tested.
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Table 1 Tensile strength (UTS) and microhardness (KHN) of dentin under several conditions.

                                                                      UTS (MPa)                                      KHN
Mineralized dentin                                      86.6 (18.8) N  10a           63.9 (8.5) N  10A
EDTA-treated dentin                                   18.8 (5.8) N  10c                              
NaOCl-treated dentin                                  4.8 (1.7) N  7d                 30 (3.7) N  7C
EDTA-treated, resin-infiltrated dentin            35.8 (11.3) N  10b           11.9 (2.7) N  10D
NaOCl-treated, resin-infiltrated dentin           16.4 (2.4) N  5c              34.2 (7.1) N  5B

Identical letters indicate no significant differences among groups p . 0:05:

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(4) EDTA-treated, resin-infiltrated dentin. EDTAtreated specimens (as described above) were resin-infiltrated following a modified procedure as described by Sano et al. The adhesive system approximately 47% of its mineralized value p ,
0:05: Subsequent infiltration with resin increased its KHN about 7%, which was significant p , 0:05: However, that value was still much lower than the KHN of mineralized dentin p , 0:05: The infiltration of EDTA-treated dentin with resin produced
measurable KHN, however, the mean value was the lowest among all the groups p , 0:05; being lower than 20% of the original mineralized value.

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