D.H. Pashley

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.

Zymographic Analysis and Characterization of MMP-2 and -9 Forms in Human Sound Dentin

The role and function of dentin matrix metalloproteinases (MMPs) are not well-understood, but they may play a key role in dentinal caries and the degradation of resin-bonded dentin matrices. To test the null hypothesis that MMP-9 is not found in dentin matrix, we used gelatin zymography to extract and isolate all molecular forms of gelatinolytic MMPs in demineralized mature sound dentin powder obtained from extracted human molars, characterizing and identifying the enzymes by Western blotting. Gelatinolytic MMPs were detected in extracts of demineralized dentin matrix and identified as MMP-2 and MMP-9. Acidic extracts (pH 2.3) yielded 3–8 times more MMP activity than did EDTA (pH 7.4). Their activation may contribute to dentin matrix degradation, which occurs during caries progression and following resin bonding. Inhibition of MMP-2 and -9 proteolytic activity may slow caries progression and increase the durability of resin-dentin bonds.

Effect of water content on the physical properties of model dentine primer and bonding resins

OBJECTIVE Primers and adhesives containing hydroxyethyl methacrylate (HEMA) are placed on moist dentine even though several studies indicate that water may interfere with the polymerization reaction. The purpose of this study was to evaluate the influence of increasing amounts of water on the physical properties of a model dentine primer resin (HEMA) and model dentine bonding resin (a mixture of HEMA and Bis-GMA). METHODS Miniature (ca 10 x 0.5 mm (long x thick) hour-glass shaped and parallel-sided specimens were created by casting the monomer or comonomer mixtures into appropriately shaped moulds. The water content was either 0, 5, 9, 17 or 29 vol%. One hour after polymerization, half the specimens were subjected to physical testing under dry conditions. The other half were immersed in water for 24 h and then tested while wet. The ultimate tensile strength (UTS), modulus of elasticity (E), percent elongation at failure, and toughness were calculated. RESULTS The properties of the dry-stored primer and bonding resins were not altered by water incorporation up to 9 vol%. Higher water content lowered the physical properties (P < 0.05). Immersion of primers polymers in water for 24 h significantly decreased their properties. Water immersion of the bonding resins was unaffected only for specimens containing 0 or 5 vol% water. CONCLUSIONS The plasticizing effects of extrinsic water are far more important than the effects of intrinsic water in poly-HEMA resin. Hybrid layers composed primarily of poly-HEMA would be expected to be more elastic than those made with bifunctional, cross-linked polymer chains.

Regional Strengths of Bonding Agents to Cervical Sclerotic Root Dentin

The regional bond strengths of three current-generation bonding systems (All Bond 2, Scotchbond MultiPurpose, 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.

Reversal of Compromised Bonding in Bleached Enamel

Oxygen inhibits polymerization of resin-based materials. We hypothesized that compromised bonding to bleached enamel can be reversed with sodium ascorbate, an anti-oxidant. Sandblasted human enamel specimens were treated with distilled water (control) and 10% carbamide peroxide gel with or without further treatment with 10% sodium ascorbate. They were bonded with Single Bond (3M-ESPE) or Prime&Bond NT (Dentsply DeTrey) and restored with a composite. Specimens were prepared for microtensile bond testing and transmission electron microscopy after immersion in ammoniacal silver nitrate for nanoleakage evaluation. Bond strengths of both adhesives were reduced after bleaching but were reversed following sodium ascorbate treatment (P < 0.001). Resin-enamel interfaces in bleached enamel exhibited more extensive nanoleakage in the form of isolated silver grains and bubble-like silver deposits. Reduction of resin-enamel bond strength in bleached etched enamel is likely to be caused by a delayed release of oxygen that affects the polymerization of resin components.

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.

The effects of oxalate treatment on the smear layer of ground surfaces of human dentine

The layer was evaluated by scanning electron microscopy and by measurement of hydraulic conductance before and after 2-min topical treatment with potassium chloride, neutral potassium oxalate, half-neutralized oxalic acid or both neutral and acidic oxalates. The treated smear layers were then re-evaluated microscopically and functionally both before and after acid challenge. The layers treated with KCl were not altered either microscopically or functionally and were susceptible to acid etching. Dentine surfaces treated with either oxalate solutions became less permeable and were acid-resistant.

Dentin Permeability: Determinants of Hydraulic Conductance

A technique is described which permits measurement of the ease with which fluid permeates dentin. This value, the hydraulic conductance of dentin, increased as surface area increases and/or as dentin thickness decreases. It increased 32-fold when dentin was acid etched due to removal of surface debris occluding the tubules.

Effects of Distance from the Pulp and Thickness on the Hydraulic Conductance of Human Radicular Dentin

The purposes of this study were: (1) to measure the effect of distance from the pulp on the hydraulic conductance of human radicular dentin ; (2) to determine the influence of dentin thickness on the rates of fluid flow; and (3) to attempt to correlate dentinal tubule densities and diameters with root dentin hydraulic conductance. Dentin slabs prepared from extracted, unerupted, human third molar teeth were placed in a split-chamber device to permit quantitation of fluid filtration rate (hydraulic conductance). In the SEM portion of the study, dentinal tubule numbers and diameters were recorded. The results indicated that radicular dentin hydraulic conductance decreased with distance from the pulp and with increasing dentin thickness. Tubule density and diameter correlated well with the measured hydraulic conductances. The relatively low hydraulic conductance of outer root dentin makes it a significant barrier to fluid movement across root structure.

Scanning electron microscopy of the substructure of smear layers in human dentine

Smear-layer debris was sonicated from dentine surfaces and trapped on microfilters for microscopy, which showed that the layers are composed, in part, of aggregates of globular subunits approx. 0.05-0.1 micron in diameter. Such globules were also seen in smear layers in situ and on fractured dentine surfaces. The composition of smear layers may reflect the substructure of dentine matrix.