Bonding characteristics to dentin walls of class II cavities, in vitro.

OBJECTIVES: The objective of this study was to test the hypothesis that bond strengths to intact class II cavity wall surfaces are lower than those measured on corresponding flat surfaces isolated by cutting away the rest of the cavity walls. METHODS: Class II (MOD) cavities were prepared in extracted human third molars and then divided into two groups: Intact cavity bonding group or flat surface group. All prepared surfaces were acid-etched and bonded with Scotchbond Multi-Purpose Plus adhesive system and restored with Z100 resin composite. After storage in water for 2 days, the teeth were divided mesio-distally into two equal halves. One half was vertically serially sectioned, while the other half was horizontally serially sectioned to yield a series of 0.5mm thick slabs. Each slab was trimmed into an hour-glass outline with the narrowest cross-sectional area at the region of interest (i.e., axial resin-dentin interface). RESULTS: The mean bond strengths obtained in the cavity bonding group were significantly lower (p<0.05) than those of the flat bonding group. However, within either group, there were no significant consistent differences among the various regions. SIGNIFICANCE: The large flat surfaces used in most laboratory bonding studies may overestimate the bond strengths that can be achieved in complex cavities prepared and restored under clinically relevant conditions.

Bond strength of composite to dentin using conventional, one-step, and self-etching adhesive systems.

OBJECTIVES: This in vitro study compared the dentin bonding performance of eight adhesive systems using a microtensile bond strength test. METHODS: Thirty bovine teeth were ground to 600-grit to obtain flat root-dentin surfaces. Two conventional adhesive systems (Scotchbond Multipurpose Plus, OptiBond FL), four one-step adhesive systems (Scotchbond 1, Asba S.A.C., Prime and Bond NT, Excite) and two self-etching adhesive materials (Clearfil Liner Bond 2 V and Prompt L-Pop) were evaluated. Each bonding system was applied according to manufacturer's instructions and followed by composite (Z100) application. Immediately after bonding, the teeth were prepared for microtensile testing. Bond strength to dentin was measured using a Vitrodyne V-1000 universal tester. There were 14 replicates for each material. Fractured specimens were further observed by SEM. RESULTS: Scotchbond Multipurpose Plus exhibited significantly (p<0.05) higher bond strength values (30.3+/-9.4 MPa) than all other materials. The bond strengths of the other materials were (from highest to lowest): Opitbond FL (22.4+/-4.3 MPa); Scotchbond 1(18.9+/-3.2); Clearfil Liner Bond 2 V (18.9+/-3.0); Prime and Bond NT (18.3+/-6.9); Asba S.A.C. (14.4+/-2.9); Excite (13.8+/-3.7); and Prompt L-Pop (9.1+/-3.3). Statistical comparisons frequently overlapped, but Optibond was significantly (p<0.05) greater than Asba, Excite, and Prompt L-Pop; whereas, Scotchbond 1 was only significantly (p<0.05) greater than Prompt L-Pop. Asba, Excite and Prompt L-Pop were not significantly different. The fracture modes were mostly adhesive. CONCLUSIONS: The conventional adhesive systems produced higher bond strengths to root dentin than most one-step adhesives and one self-etching adhesive; with the exception of one material in each respective system.

Ability of adhesive systems to seal dentin surfaces: an in vitro study.

PURPOSE: The objective of this study was to evaluate the ability of three adhesive systems to seal dentin by measuring the permeability of dentin before and after bonding procedures. MATERIALS AND METHODS: Thirty-six flat dentin surfaces were prepared from human extracted teeth and connected to a fluid-filled system for permeability measurements. The hydraulic conductance of the flat occlusal surface of dentin was measured by the filtration technique, before and after application of ScotchBond MP Plus, Prime & Bond 2.0, and All-Bond 2. Bonding procedures were followed by P-50 resin composite application. The specimens were then perfused with silver nitrate to permit morphologic localization of microleakage pathways using SEM. RESULTS: The results showed that the best dentin seal was produced by All-Bond 2, followed by Prime & Bond 2.0, and ScotchBond MP Plus. However, none of the three bonding systems tested was able to totally prevent the passage of fluid across the dentin in every specimen. When observed with SEM, each adhesive system exhibited different patterns of leakage. CONCLUSION: The inability of all three materials to create a perfect seal in vitro raises concerns about the ability of these adhesive systems to provide completely sealed restorations in vivo. However, because the current study applied pressure 3 to 6 times higher than physiologically normal, it is likely that the leakage represents a worst-case scenario.

The influence of dentine permeability on cytotoxicity of four dentine bonding systems, in vitro.

Dentine adhesives are often placed directly on dentine from which the smear layer has been removed, the thickness of the dentine is minimal and the potential for diffusion of adhesive components into the pulp is greatest. The permeability of the dentine is one factor that should be critical to whether sufficient diffusion of adhesive components occurs to cause damage to pulpal cells. Dentine discs were prepared and divided into those with low-, medium-, and high-permeability. They were then treated with four different dentine adhesives, after which the pulpal side of the dentine was placed in contact with 1 mL of cell-culture medium. The medium was collected at 24 h intervals for 168 h, and was then placed on monolayers of human pulpal fibroblasts for 24 h. The response of the cells was assessed by succinic dehydrogenase activity (MTT method). The results showed that four dentine adhesive systems released sufficient components to cause suppression of cellular metabolism through dentine. High-permeability dentine generally allowed more diffusion of these components, but the effect of dentine permeability depended on the material. On the other hand, the time interval between the application of the bonding agent and collection of the eluant was consistently important for all materials. Materials were most cytotoxic at early intervals, and were generally less cytotoxic at later intervals, although there were exceptions and there was persistent (> 15%) suppression of cellular metabolism even at late (168 h) intervals. The results suggest that application of these materials to dentine, and particularly dentine with high permeability, poses a potential risk to the health of pulpal tissues.

Volume of the internal gap formed under composite restorations in vitro.

OBJECTIVES: The gap that develops at the interface of dentin composite restoration during the polymerization of the resin can be subsequently filled by fluid filtrating from the pulp via the dentinal tubules. This in vitro study was designed to determine the volume of such a gap, at the occlusal floor of class I restoration and as a result of different dentin treatments and restoration procedures. METHODS: Fifty-six human third molars had their pulp chambers first sealed and connected to a hydraulic apparatus permitting microlitre fluid shift recordings. The teeth then received class I cavities of uniform dimensions and were sampled into nine groups for three dentin treatments (bonding with a dentin bonding agent, lining with a resin modified light-cured glass ionomer, lining with a zinc phosphate cement) and three restoration procedures (Bulk placement of the composite material, Multilayer, Indirect inlay). Fluid displacements were recorded during the filling procedures and stopped 30 min after the completion of the restorations. RESULTS: Dentin bonding agent treated cavities consistently presented the smallest gap volumes, followed by the GI and the ZnPO4 lined specimen. Multilayer and Indirect restoration techniques reduced the formation of gaps. CONCLUSIONS: None of the materials or techniques tested assured a gap-free interface and more effort should be directed at increasing the adhesive and sealing properties of restorative materials to be placed on the dentin.

Regional strengths of bonding agents to cervical sclerotic root dentin.

The regional bond strengths of three current-generation bonding systems (All Bond 2, Scotchbond Multi-Purpose, and Clearfil Liner Bond 2) were measured in natural wedge-shaped defects in the cervical area of extracted human teeth. A microtensile testing method was used to compare the strengths of resin bonds made to occlusal margins with those made to gingival margins. Controls consisted of normal teeth which had artificial wedge-shaped defects, of the same depth and dimension, created with a high-speed bur. The results indicated that there were no regional differences in bond strength, although bonds made to natural lesions were from 20 to 45% lower than those made to normal dentin in artificially created wedge-shaped defects, depending on the bonding agent. Scanning electron microscopy revealed that Clearfil Liner Bond 2 created the thinnest hybrid layers, which were difficult to measure in the natural lesions. The natural lesions contained sclerotic dentin, whereas the artificial lesions were composed of normal dentin. Although the bond strengths to sclerotic dentin were lower than those to normal dentin, the absolute values (ca. 16 to 17 MPa) were high relative to previous-generation bonding agents.

In vitro cytotoxicity and dentin permeability of HEMA.

An in vitro diffusion chamber was used to measure the diffusion of 2-hydroxyethyl methacrylate (HEMA) through etched human dentin disks. Concentrations of HEMA, which diffused through dentin, were measured by ultraviolet spectroscopy, and the effect of initial HEMA concentration, dentin thickness, and back pressure on diffusion were assessed. The cytotoxicity of HEMA was determined using BALB/c 3T3 mouse fibroblasts in direct contact with HEMA for 12 or 24 h. HEMA diffused rapidly through dentin under all conditions, but increased thickness, back pressure, or decreased initial concentration all reduced diffusion. The permeability coefficient of HEMA was approximately 0.0003 cm/min, and diffusion through 0.5 mm of dentin reduced the HEMA concentration by a factor of approximately 6,000 (with 10 cm of H2O back pressure). It was concluded that the risk of acute cytotoxicity to HEMA through dentin was probably low, but that decreased dentin thickness, lack of polymerization, or extended exposure times might increase the risk significantly.

Bond strength versus dentine structure: a modelling approach.

Bond strengths of a hypothetical hydrophilic dentine-bonding agent were calculated as a function of dentine depth and resin strength to evaluate the importance of several variables in a simple model. The tested hypothesis was that the total bond strength was the sum of the strengths of resin tags, hybrid layer and surface adhesion. Each of these three variables has a range of values that can influence its relative contribution. The resulting calculations indicate the potential for higher bond strengths to deep dentine than to superficial dentine in non-vital dentine and the importance of resin strength in the development of strong bonds. Comparison of the calculated bonds with published values indicated that they were within the same order of magnitude. Such theoretical modelling of dentine bonding can identify the relative importance of variables involved in the substrate, resins and surface adhesion.

Tensile bond strength and SEM evaluation of caries-affected dentin using dentin adhesives.

Tensile bond strength measurements are commonly used for the evaluation of dentin adhesive systems. Most tests are performed using extracted non-carious human or bovine dentin. However, the adhesion of resins to caries-affected dentin is still unclear. The objectives of this study were to test the hypothesis that bonding to caries-affected dentin is inferior to bonding to normal dentin, and that the quality of the hybrid layer plays a major role in creating good adhesion. We used a micro-tensile bond strength test to compare test bond strengths made to either caries-affected dentin or normal dentin, using three commercial adhesive systems (All Bond 2, Scotchbond Multi-Purpose, and Clearfil Liner Bond II). For scanning electron microscopy, the polished interfaces between the adhesive bond and dentin were subjected to brief exposure to 10% phosphoric acid solution and 5% sodium hypochlorite, so that the quality of the hybrid layers could be observed. Bonding to normal dentin with either All Bond 2 (26.9 +/- 8.8 MPa) or Clearfil Liner Bond II (29.5 +/- 10.9 MPa) showed tensile bond strengths higher than those to caries-affected dentin (13.0 +/- 3.6 MPa and 14.0 +/- 4.3 MPa, respectively). The tensile bond strengths obtained with Scotchbond Multi-Purpose were similar in normal and caries-affected dentin (20.3 +/- 5.5 MPa and 18.5 +/- 4.0 MPa, respectively). The hybrid layers created by All Bond 2 in normal dentin and by Clearfil Liner Bond II in normal or caries-affected dentin showed phosphoric acid and sodium hypochlorite resistance, whereas the hybrid layers created by All Bond 2 in caries-affected dentin and those created by Scotchbond Multi-Purpose to normal and caries-affected dentin showed partial susceptibility to the acid and sodium hypochlorite treatment. The results indicate that the strength of adhesion to dentin depends upon both the adhesive system used and the type of dentin. Moreover, the quality of the hybrid layer may not always contribute significantly to tensile bond strength.

Comparative SEM and TEM observations of nanoleakage within the hybrid layer.

Most adhesive interface studies have involved SEM demonstration of the penetration of adhesive resins into demineralized dentin surfaces with subsequent creation of hybrid layers. Nanoleakage is a term that describes the diffusion of small ions or molecules within the hybrid layer in the absence of gap formation. The present microscopic study examined the nanoleakage of the hybrid layer using a silver nitrate staining technique. Adhesive dentin sandwiches, which were immersed in a silver nitrate solution, were prepared for both SEM and TEM examination using both the Clearfil Liner Bond and All-Bond 2 adhesive systems. Both systems demonstrated silver accumulation within the hybrid layers. Clearfil Liner Bond System showed scattered silver particles at the bottom two-thirds of the hybrid layer by both SEM and TEM observation, whereas All-Bond 2 revealed stained fiber-like structures within the full thickness of the hybrid layer. To evaluate the quality of the hybrid layer, the utilization of tracer molecules such as silver nitrate that are detectable by both SEM and TEM is proposed. It is important to determine the location and morphology of these nanometer-sized porosities that may permit the hydrolysis of collagen fibers and degradation of adhesive monomers.

Tensile properties of resin-infiltrated demineralized human dentin.

The ability of adhesive resins to restore the physical properties of demineralized dentin has not been well-documented. The unfilled resins that are used for adhesion have relatively low moduli of elasticity and limited ability to increase dentin stiffness, although they may increase the ultimate tensile strength of dentin. This study tested the hypothesis that resin infiltration of demineralized dentin can restore its tensile properties to those of mineralized dentin. Small (ca. 0.5 mm thick x 0.5 mm wide) specimens of demineralized human dentin were infiltrated with one of five different dentin bonding resins over many hours, to determine how these resins altered the tensile properties of dentin. Tensile stress and strain were measured in these and control (mineralized and demineralized) specimens until their ultimate failure. The results indicate that some adhesive resins, after infiltrating demineralized dentin, can restore and even exceed the ultimate tensile strength of mineralized dentin. These resins increased the modulus of elasticity of resin-infiltrated dentin to values equal to or greater than those of the resins but far below those of mineralized dentin. Although the conditions in this experiment were far removed from the manufacturer's recommendations or clinical practice, the results support the potential of resin infiltration for reinforcing dentin.

Dentinal fluid dynamics in human teeth, in vivo.

Cavities were prepared in human premolars scheduled for extraction for orthodontic reasons. The smear layer was removed from the dentin surface by acid etching, and the cavity was sealed using a hollow chamber. The chamber was filled with sterile saline solution and connected via tubing to a hydraulic circuit featuring an adjustable pressure reservoir and a device that measures fluid movement across dentin. In the absence of any exogenous pressure, all cavities exhibited an outward fluid flow rate of 0.36 microliters min-1 cm-2. As exogenous pressure was applied to the cavity, the outward flow slowed. The exogenous pressure that stopped outward fluid flow was taken to be equal to normal pulpal tissue pressure. The mean value was 14.1 cm H2O in five teeth. This simple method permits measurement of dentinal fluid flux, the hydraulic conductance of dentin, and estimates pulpal tissue pressure.

Adhesion testing of dentin bonding agents: a review.

Adhesion testing of dentin bonding agents was reviewed starting with the adhesion substrate, dentin, the variables involved in etching, priming and bonding, storage variables and testing variables. Several recent reports attempting to standardize many of these variables were discussed. Recent advances in the development of new bonding systems have resulted in bond strengths on the order of 20-30 MPa. At these high bond strengths, most of the bond failure modes have been cohesive in dentin. As this precludes measurement of interfacial bond strength, new testing methods must be developed. One such new method, a microtensile method, was described along with preliminary results that have been obtained. The last decade has produced major advances in dentin bonding. The next decade should prove to be even more exciting.

Nanoleakage: leakage within the hybrid layer.

Most microleakage studies involve quantitating the magnitude of movement of a tracer molecule through a gap between restorative materials and the wall of cavity preparations. The present microscopic study examined the migration of silver nitrate into the interface between dentin and five different dentin bonding agents used to restore class 5 cavities, in the absence of gap formation. Several different leakage patterns were seen, but they all indicated leakage within the hybrid layer when viewed by SEM. The ranking of microleakage from most to least was: All-Bond 2 > Suberbond C&B > Scotchbond Multi-Purpose > Clearfil Liner Bond System > Kuraray Experimental System, KB-200. To distinguish this special type of microleakage within the basal, porous region of the hybrid layer in the absence of gap formation, we propose the term nanoleakage.

Relationship between surface area for adhesion and tensile bond strength--evaluation of a micro-tensile bond test.

OBJECTIVES: The purpose of this study was to test the null hypothesis that there is no relationship between the bonded surface area of dentin and the tensile strength of adhesive materials. METHODS: The enamel was removed from the occlusal surface of extracted human third molars, and the entire flat surface was covered with resin composite bonded to the dentin to form a flat resin composite crown. Twenty-four hours later, the bonded specimens were sectioned parallel to the long axis of the tooth into 10-20 thin sections whose upper part was composed of resin composite with the lower half being dentin. These small sections were trimmed using a high speed diamond bur into an hourglass shape with the narrowest portion at the bonded interface. Surface area was varied by altering the specimen thickness and width. Tensile bond strength was measured using custom-made grips in a universal testing machine. RESULTS: Tensile bond strength was inversely related to bonded surface area. At surface areas below 0.4 mm2, the tensile bond strengths were about 55 MPa for Clearfil Liner Bond 2 (Kuraray Co., Ltd.), 38 MPa for Scotchbond MP (3M Dental Products), and 20 MPa for Vitremer (3M Dental Products). At these small surface areas all of the bond failures were adhesive in nature. SIGNIFICANCE: This new method permits measurement of high bond strengths without cohesive failure of dentin. It also permits multiple measurements to be made within a single tooth.

Tensile properties of mineralized and demineralized human and bovine dentin.

The relative contribution of the matrix of dentin to the physical properties of dentin is unknown but thought to be small. The objective of this study was to test the hypothesis that the demineralized matrix of dentin contributes little to the strength of dentin by measuring and comparing the ultimate tensile strength and modulus of elasticity of mineralized and demineralized dentin. Small slabs (4 x 0.5 x 0.5 mm) of bovine and human dentin were tested in a microtensile testing device in vitro. Human coronal mineralized dentin gave a mean ultimate tensile strength (UTS) of 104 MPa. Bovine incisor coronal dentin exhibited a UTS of 91 MPa, and bovine root dentin failed at 129 MPa. The modulus of elasticity of mineralized bovine and human dentin varied from 13 to 15 MPa. When dentin specimens were demineralized in EDTA, the UTS and modulus of elasticity fell to 26-32 MPa and 0.25 GPa, respectively, depending on dentin species. The results indicate that collagen contributes about 30% of the UTS of mineralized dentin, which is higher than was expected.

Permeability of dentin to adhesive agents.

The permeability of dentin to adhesive agents is of crucial importance in obtaining good dentinal bonding. In those systems that remove the smear layer, the opportunity exists for resin to infiltrate both tubules and intertubular dentin. Resin penetration into tubules can effectively seal the tubules and can contribute to bond strength if the resin bonds to the tubule wall. Resin infiltration into intertubular dentin can only occur if the mineral phase of dentin is removed by acidic conditioners or chelators. This is more easily accomplished in fractured dentin than in smear layer-covered dentin because of the residual collagen debris that remains on the surface following acid etching of smear layers. The channels for resin infiltration are the perifibrillar spaces created around the collagen fibers of dentin following removal of apatite mineral by acids. The diffusion of adhesive resins through these narrow, tortuous, long channels in 1 to 2 minutes offers a number of challenges that require further research.