Mary P. Walker

Molecular Structure of Acid-etched Dentin Smear Layers-in situ Study:

It is commonly reported that acid etchants remove the smear layer, but to date, there has been no chemical evidence to support these observations. The purpose of this study was to determine the molecular structure of acid-etched carbide- and diamond-bur-created smear layers. This project tested the null hypothesis that such smear layers are totally removed with current etchants. Smear layer/demineralized/mineralized dentin interfaces were analyzed at 1- m intervals by micro-Raman spectroscopy. Features associated with the organic component were substantially broadened with loss of fine structure, and mineral peaks were clearly evident in the spectra of acid-etched smear layers. The organic features in the spectra of the EDTAtreated smear layer showed relative intensity ratios similar to demineralized dentin without contribution from the mineral phase. The disorganized collagen within the smear layer was not removed but was denatured by the acid treatment; the mineral was trapped in this gelatinous matrix and shielded from complete reaction.

Dimensional accuracy and surface detail reproduction of two hydrophilic vinyl polysiloxane impression materials tested under dry, moist, and wet conditions☆

STATEMENT OF PROBLEM A major limitation of vinyl polysiloxane (VPS) impression materials is their hydrophobicity. There are 2 aspects to this problem, the wettability of the polymerized impression by dental gypsum materials and the ability of the unpolymerized material to wet intraoral tissues. To address this problem, manufacturers have added surfactants and labeled the new products as hydrophilic vinyl polysiloxane. PURPOSE The purpose of this investigation was to compare dimensional accuracy and surface detail reproduction of 2 hydrophilic VPS impression materials, when used under dry, moist, and wet conditions. MATERIALS AND METHODS A total of 102 impressions were made of stainless steel metal dies similar to those described in American Dental Association (ADA) specification 19. The dies had 2 vertical and 3 horizontal lines inscribed on their superior surfaces. Impressions were made under dry, moist, and wet conditions. Dimensional accuracy was measured by comparing the average length of the middle horizontal line in each impression to the same line on the metal die, by use of a measuring microscope with an accuracy of 0.001 mm. A 2-way analysis of variance and least significant difference post hoc test were used to compare mean dimensional changes (alpha=.05). Surface detail reproduction was evaluated in 2 ways: (1) by use of criteria similar to ADA specification 19 for detail reproduction, continuous replication of at least 2 of the 3 horizontal lines, and (2) by use of a method developed for this study that categorized the impressions as satisfactory or unsatisfactory based on their surface characteristics: presence of pits, voids, or roughness. Pearson chi(2) (alpha=.05) was used to compare detail reproduction results. RESULTS Conditions (dry, moist, and wet) did not cause significant adverse effects on the dimensional accuracy of either material. The mean dimensional change and SD were 0.005% +/- 0.002% or less. With both surface detail analyses, dry, moist, and wet conditions had a significant effect on the detail reproduction of both materials (P<.05). Only under dry conditions did both impression materials continuously replicate at least 2 of the 3 horizontal lines 100% of the time. Under moist conditions, 82% of the Aquasil impressions and 100% of the Reprosil impressions were judged satisfactory, while under wet conditions, only 47% Aquasil and 11% Reprosil impressions were satisfactory. With the additional surface detail characterization, only under dry conditions were impressions produced with clinically acceptable surface quality (Aquasil 77% and Reprosil 100% satisfactory). CONCLUSIONS Dimensional accuracy of both materials tested was well within ADA standards. Best surface detail results were obtained only under dry conditions for both materials.

Morphological and chemical characterization of bonding hydrophobic adhesive to dentin using ethanol wet bonding technique

OBJECTIVE BisGMA, a widely used component in dentin adhesive has very good mechanical properties after curing, but is relatively hydrophobic and thus, does not adequately infiltrate the water wet demineralized dentin collagen. Developing techniques that would lead to optimum infiltration of the hydrophobic component into the demineralized dentin matrix is very important. The purpose of this study was to evaluate interfacial morphological and chemical characteristics of the resultant adhesive-dentin interface when the ethanol wet bonding technique is used with hydrophobic adhesives. MATERIALS AND METHODS The occlusal one-third of the crown was removed from six unerupted human third molars; a uniform smear layer was created with 600 grit SiC. The dentin surface was etched with 35% phosphoric acid for 15s before applying BisGMA/HEMA model adhesive using either water wet or ethanol wet bonding technique. Five-micro-thick sections of adhesive/dentin interface specimens were cut and stained with Goldners trichrome for light microscopy. Companion slabs were analyzed with SEM and micro-Raman spectroscopy. RESULTS The presence of ethanol in the demineralized dentin increased adhesive collagen encapsulation as indicated by trichrome staining. The SEM results confirmed that the ethanol wet bonding improved the quality of the interface. Micro-Raman spectral analysis of the dentin/adhesive interface indicated there was a gradual decrease in penetration of BisGMA component for specimens using water wet bonding, while relatively homogeneous distribution of the hydrophobic BisGMA component was noted in the interface with ethanol wet bonding. SIGNIFICANCE Wet bonding with ethanol instead of water permits better BisGMA infiltration improving the quality of interface. We speculate that the higher infiltration of hydrophobic BisGMA and better collagen encapsulation observed from the specimens using ethanol wet bonding would lead to more durable bonds because of improved resistance to hydrolytic attack.

Grape seed proanthocyanidins increase collagen biodegradation resistance in the dentin/adhesive interface when included in an adhesive

OBJECTIVES Contemporary methods of dentin bonding could create hybrid layers (HLs) containing voids and exposed, demineralised collagen fibres. Proanthocyanidins (PA) have been shown to cross-link and strengthen demineralised dentin collagen, but their effects on collagen degradation within the HL have not been widely studied. The purpose of this study was to compare the morphological differences of HLs created by BisGMA/HEMA model adhesives with and without the addition of grape seed extract PA under conditions of enzymatic collagen degradation. METHODS Model adhesives formulated with and without 5% PA were bonded to the acid etched dentin. 5-μm-thick sections cut from the bonded specimens were stained with Goldners trichrome. The specimens were then exposed to 0.1% collagenase solution for 0, 1, or 6 days. Following collagenase treatment, the specimens were analysed with SEM/TEM. RESULTS Staining did not reveal a difference in the HLs created with the two adhesives. SEM showed the presence of intact collagen fibrils in all collagenase treatment conditions for specimens bonded with adhesive containing PA. These integral collagen fibrils were not observed in the specimens bonded with adhesive without PA after the same collagenase treatment. TEM confirmed that the specimens containing PA still showed normal collagen fibril organisation and dimensions after treatment with collagenase solution. In contrast, disorganised collagen fibrils in the interfacial zone lacked the typical cross-banding of normal collagen after collagenase treatment for specimens without PA. CONCLUSIONS The presence of grape seed extract PA in dental adhesives may inhibit the biodegradation of unprotected collagen fibrils within the HL.

Mechanical Properties and Surface Characterization of Beta Titanium and Stainless Steel Orthodontic Wire Following Topical Fluoride Treatment

OBJECTIVE To study the effect of fluoride prophylactic agents on the loading and unloading mechanical properties and surface quality of beta titanium and stainless steel orthodontic wires. MATERIALS AND METHODS Rectangular beta titanium and stainless steel wires were immersed in either an acidulated fluoride agent, a neutral fluoride agent, or distilled water (control) for 1.5 hours at 37 degrees C. After immersion, the loading and unloading elastic modulus and yield strength of the wires were measured using a 3-point bend test in a water bath at 37 degrees C. A one-way analysis of variance and Dunnetts post hoc, alpha = .05, were used to analyze the mechanical testing data. Scanning electron microscopy was also used to qualitatively evaluate the wire topography as a function of the fluoride treatments. RESULTS Unloading mechanical properties of beta titanium and stainless steel wires were significantly decreased (P <or= .05) after exposure to both fluoride agents. Corrosive changes in surface topography were also observed after exposure to both the neutral and the acidulated phosphate fluoride agents. CONCLUSIONS The results suggest that using topical fluoride agents with beta titanium and stainless steel wire could decrease the functional unloading mechanical properties of the wires and potentially contribute to prolonged orthodontic treatment.

Mechanical property characterization of resin cement after aqueous aging with and without cyclic loading

OBJECTIVE The purpose of this study was to determine changes in flexural properties of resin cement under cyclic loading and aqueous aging. METHODS Panavia F flexural modulus and strength were measured by static loading to failure after 48-h, 31-d, and 60-d aqueous aging at 37 degrees C with and without cyclic loading at clinical stress levels. Six specimens were used for each experimental condition. Scanning electron microscopy (SEM) was also used to characterize the morphology of the fractured surfaces to potentially identify resin cement components particularly susceptible to degradation under simulated clinical function. RESULTS A two-factor ANOVA (p< or =0.05) and Least Significant Difference post hoc test indicated that cyclic loading produced a significant increase in the flexural modulus with no significant effect on the flexural strength. In contrast, aqueous aging time produced a significant decrease in flexural strength with no effect on the flexural modulus. The SEM fracture analysis indicated that resin matrix fracture occurred in static-aqueous specimens; while in the aqueous-cycled specimens, resin matrix fracture occurred in addition to an increasing proportion of filler/resin interface fracture as a result of both increased aqueous aging and increased flexural cycles. SIGNIFICANCE Evidence suggests that after aqueous aging with cyclic loading to simulate resin cement clinical function, initial degradation may be related to breakdown of the filler/resin interface bond. Such breakdown is potentially reflective of slow crack propagation that may contribute to in vivo resin cement cohesive failure.

Dimensional change over time of extended-storage alginate impression materials.

OBJECTIVES To evaluate the dimensional change over time of two extended-storage alginate impression materials. METHODS Impressions were made of stainless steel dies in accordance with ADA Specification No. 18 using three alginates: two extended-storage alginates and one conventional alginate. The impressions were stored for 30 minutes, 48 hours, or 100 hours (n = 10 impressions/material/storage time). Following the respective storage times, dimensional change was measured by comparing the length of the middle horizontal line in the impression with the same line on the die and computing percent difference. RESULTS Significant differences in dimensional change were noted between materials across time (P < .05). All materials exhibited shrinkage after 30 minutes, with the conventional alginate continuing to shrink over time and the extended-storage alginates expanding with increased storage time. The conventional alginate was most accurate after 30 minutes. In contrast, one extended-storage alginate demonstrated minimal dimensional change at all storage times, and another was most accurate after 100-hour storage. CONCLUSIONS Evidence suggests that delayed pouring with dental gypsum should not adversely affect dimensional accuracy of the generated casts with both extended-storage alginates. However, only one of the extended-storage materials appears suitable for both short-term and extended-storage applications.

Fracture Strength of Ceramic Bracket Tie Wings Subjected to Tension

One of the most common areas of ceramic bracket fracture is within the tie-wing complex. When an archwire is ligated into position, tensile forces are placed under the tie wing. However, no study, to date, has focused specifically on the fracture resistance of the tie-wing complex. The aim of this study is to compare the tensile fracture strength of seven currently available ceramic brackets (Inspire, Fascination, Mystique, InVu, Clarity, Virage, and Luxi) as a function of bracket brand and bracket configuration, semitwin vs true-twin. Based on a power analysis of pilot data, 10 maxillary central incisor brackets per group were tested to failure with a tensile load placed directly under the distoincisal tie wing. The results ranged from a maximum mean fracture strength of 147.71 (5.87) MPa with Fascination brackets to a minimum mean fracture strength of 84.28 (7.01) MPa with Luxi brackets. The statistical analysis indicated a significant effect on fracture strength as a function of bracket brand (P < .05) and that semitwin brackets, Fascination, Mystique, and Virage, had significantly higher fracture strength than true-twin brackets, Clarity, lnVu, and Luxi (P < .05). Interestingly, the only monocrystalline bracket in the study, Inspire, could not be fractured using the investigation protocol. In fact, the steel ligature fixture wire would break before tie-wing fracture at a mean fixture failure of 198.65 MPa.

Radiotherapy effect on nano-mechanical properties and chemical composition of enamel and dentine.

OBJECTIVE To understand radiotherapy-induced dental lesions characterized by enamel loss or delamination near the dentine-enamel junction (DEJ), this study evaluated enamel and dentine nano-mechanical properties and chemical composition before and after simulated oral cancer radiotherapy. DESIGN Sections from seven non-carious third molars were exposed to 2 Gy fractions, 5 days/week for 7 weeks for a total of 70 Gy. Nanoindentation was used to evaluate Youngs modulus, while Raman microspectroscopy was used to measure protein/mineral ratios, carbonate/phosphate ratios, and phosphate peak width. All measures were completed prior to and following radiation at the same four buccal and lingual sites 500 and 30 μm from the DEJ in enamel and dentine (E-500, E-30, D-30 and D-500). RESULTS The elastic modulus of enamel and dentine was significantly increased (P ≤ 0.05) following radiation. Based on Raman spectroscopic analysis, there was a significant decrease in the protein to mineral ratio (2931/430 cm(-1)) following radiation at all sites tested except at D-500, while the carbonate to phosphate ratio (1070/960 cm(-1)) increased at E-30 and decreased at D-500. Finally, phosphate peak width as measured by FWHM at 960 cm(-1) significantly decreased at both D-30 and D-500 following radiation. CONCLUSIONS Simulated radiotherapy produced an increase in the stiffness of enamel and dentine near the DEJ. Increased stiffness is speculated to be the result of the radiation-induced decrease in the protein content, with the percent reduction much greater in the enamel sites. Such changes in mechanical properties and chemical composition could potentially contribute to DEJ biomechanical failure leading to enamel delamination that occurs post-radiotherapy. However, other analyses are required for a better understanding of radiotherapy-induced effects on tooth structure to improve preventive and restorative treatments for oral cancer patients.

Type IV Collagen is a Novel DEJ Biomarker that is Reduced by Radiotherapy

The dental basement membrane (BM) is composed of collagen types IV, VI, VII, and XVII, fibronectin, and laminin and plays an inductive role in epithelial-mesenchymal interactions during tooth development. The BM is degraded and removed during later-stage tooth morphogenesis; however, its original position defines the location of the dentin-enamel junction (DEJ) in mature teeth. We recently demonstrated that type VII collagen is a novel component of the inner enamel organic matrix layer contiguous with the DEJ. Since it is frequently co-expressed with and forms functional complexes with type VII collagen, we hypothesized that type IV collagen should also be localized to the DEJ in mature human teeth. To identify collagen IV, we first evaluated defect-free erupted teeth from various donors. To investigate a possible stabilizing role, we also evaluated extracted teeth exposed to high-dose radiotherapy – teeth that manifest post-radiotherapy DEJ instability. We now show that type IV collagen is a component within the morphological DEJ of posterior and anterior teeth from individuals aged 18 to 80 yr. Confocal microscopy revealed that immunostained type IV collagen was restricted to the 5- to 10-µm-wide optical DEJ, while collagenase treatment or previous in vivo tooth-level exposure to > 60 Gray irradiation severely reduced immunoreactivity. This assignment was confirmed by Western blotting with whole-tooth crown and enamel extracts. Without reduction, type IV collagen contained macromolecular α-chains of 225 and 250 kDa. Compositionally, our results identify type IV collagen as the first macromolecular biomarker of the morphological DEJ of mature teeth. Given its network structure and propensity to stabilize the dermal-epidermal junction, we propose that a collagen-IV-enriched DEJ may, in part, explain its well-known fracture toughness, crack propagation resistance, and stability. In contrast, loss of type IV collagen may represent a biochemical rationale for the DEJ instability observed following oral cancer radiotherapy.