Bernard Ciucchi

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.

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.

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.

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.

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.

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.

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.

Resin diffusion through demineralized dentin matrix

This paper has focused on the factors that may affect the permeability of adhesive resins into the demineralized dentin matrix during the development of the bonding process. The effects of surface moisture are discussed respectively to the adhesive systems, and the problems related to incomplete hybrid layer formation presented.

CLINICAL AND HISTOLOGICAL OBSERVATIONS OF HUMAN TEETH WITH CERVICALLY EXPOSED DENTINE

We have now attempted to correlate sensitivity with light- and scanning electron-microscopical observations on 65 caries-free human teeth with varying degrees of cervicafly exposed dentine. Teeth were grouped according to patient age ( 49 years) and sensitivity status. Teeth were then carefully extracted and randomly assigned, 31 for light and 34 for scanning electron microscopy. Observations by light microscopy were: depth of dentine loss ( 3 mm), open tubules at surface, material inside tubules, and thickness of reaction dentine ( 430 pm). Sections prepared for scanning electron microscopy were observed for open tubular orifices and material in tubules beneath the surface. Relations between observations were assessed by the Fisher-Irwin exact test. Because of limited sample sizes, none of the associations tested was statistically significant; therefore the results will be discussed in terms of general trends (with 0.05 <p < 0.1 or 0.1 <p <0.2). Sensitivity was noted in 35% of teeth, which is higher than the 7-30% reported by Addy (1992), but we expected more sensitivity because the sample contained only teeth with cervically exposed dentine. The observed incidence of sensitive teeth may be age related as more than 50% of the teeth came from older subjects, who tend to have iess sensitivity (Seltzer and Bender, 1984). In agreement with other studies, sensitive teeth were found predominantly in younger subjects. Most sensitive teeth (1 l/14) exhibited open tubules at the surface and this is consistent with previous in vivo reports (Absi, Addy and Adams, 1989) and with the hydrodynamic hypothesis of pain induction through open tubules (Hirvonen and Narhi, 1984). However, surface openings were also observed sensitivity, histopathology.