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Biomedical subjects

C L Davidson

Publications and source records attributed to C L Davidson.

At least 37 records · Page 2Linked to original sources
PubMed1998

The effect of the photopolymerization method on the quality of composite resin samples.

An optimal degree of conversion and minimal polymerization shrinkage are generally antagonistic goals, as increased monomer conversion invariably leads to elevated polymerization shrinkage values. However, both parameters are indispensable for an optimal resin composite restoration. A number of approaches have been used to reduce the stress on the restoration cavity wall interface, such as dentine bonding agents to counteract polymerization shrinkage, stress-absorbing lining materials and low-intensity curing lights to control the flow capacity of the material during polymerization. However, the configuration of the cavity and cohesive fractures of the material and surrounding tooth tissues are still a problem in day-to-day clinical practice. A new photopolymerization light source, pulsed laser, ensures a higher degree of conversion and lower polymerization shrinkage, and differentiates this technique from standard polymerization methods and continuous-wave argon laser polymerization. The coherence and monochromacity of pulsed laser light set at 468 nm and the far greater intensity of laser nanopulses produce a saturation effect in the depths of the composite, thus resulting in higher monomer conversion. The total amount of energy illuminating the sample surface, which is only one-fifth of that of conventional methods, and the cooling and relaxation of the material between nanopulses may be responsible for the reduced net polymerization shrinkage.

Bisphenol A-Glycidyl Methacrylate↗
PubMed1998

Structural integrity of resin-modified glass ionomers as affected by the delay or omission of light activation.

Since light activation of resin-modified glass ionomers as a means of polymerizing the HEMA is usually done shortly after mixing occurs, the acid-base reaction will proceed mainly within a formed HEMA-polymer matrix. Delaying or omitting light activation may alter the structure and consequently its integrity. The aim of this study was to investigate the effect on the structural integrity of Fuji II LC, Photac-Fil, and Vitremer by delaying or omitting light initiation as compared with the integrity when light activation is performed 2 min after mixing occurs. We evaluated integrity by three-body wear experiments, conducted 8 hrs after sample preparation, to establish the integrity in the early phase of hardening, as well as after 1 wk and after 4 mos, to follow the materials throughout the process of maturation. When light activation was delayed for 1 hr, the structural integrity of Fuji II LC and Photac-Fil improved significantly in the early stages of hardening. In the case of Vitremer, an hour's delay of light activation significantly decreased integrity, which declined further when light activation was omitted. Fuji II LC was not affected by the omission of light activation, while Photac-Fil was markedly weakened. After 4 mos of aging, most of the samples of each product which had been cured by the different methods attained equal integrity, with the exception of the non-light-activated Vitremer samples, which remained weaker. We concluded that the structural integrity of resin-modified glass-ionomer cements benefits from a chemical integration of the polyalkenoate and poly-HEMA networks, as in Vitremer. Improvement in the structural integrity in the early phase for cements with a mechanical entanglement of the matrices, as in Fuji II LC and Photac-Fil, requires an acid-base reaction, a considerable portion of which may take place before activation of the HEMA polymerization.

Composite Resins↗
PubMed1998

Bonding efficiency and interfacial morphology of one-bottle adhesives to contaminated dentin surfaces.

PURPOSE: To evaluate the effect of dentin contamination with water, saliva or blood on the bond strength of one-bottle dentin adhesives and interfacial morphology. MATERIALS AND METHODS: The tested materials were Scotchbond 1, One-Step, Prime & Bond 2.1 and Syntac SC. The three-step bonding system Scotchbond Multi-Purpose Plus (SMP) was also included as control. Extracted human molars were used. For each tooth, enamel was removed by sectioning with a diamond saw and a smear layer was created by grinding on wet 600 grit SiC paper. Before the adhesive materials were applied and cured, the dentin was dried with air for 3 s, dried with a damp cotton pellet, or contaminated with saliva or blood. Cylinders of the composite resin Z100 were applied onto the adhesive dentin surfaces using Teflon split molds. After 24-hour water storage, the shear bond strength of the specimens was determined using an Instron testing machine at a crosshead speed of 0.5 mm/min. The morphology of the dentin-resin interface was examined using SEM. RESULTS: Wetting of the substrate significantly increased the bond strength produced by One-Step, while the bond strength of Syntac SC was significantly decreased. The bond strength produced by Scotchbond 1, Prime & Bond 2.1 and SMP showed a slight increase or decrease, which was not significant. Saliva contamination decreased the bond strength significantly of only Syntac SC, while blood contamination significantly decreased the bond strengths of all materials. SEM evaluation of the dentin-resin interface showed a relation between resin tag formation and bond strength.

Adhesives↗
PubMed1998

Handling of mechanical stresses in composite restorations.

The origin of stress in adhesive resin composite restorations is attributed to restrained shrinkage during polymerization and is dependent on the configuration of the restoration. Moreover, non-homogeneous deformations during functional loading can damage the interface as well as the coherence of the material. Damage from these stresses can be reduced by application of an elastic lining at the adhesive interfaces and by slowing the initial conversion by two-step light initiation of the resin. The various factors that mediate flow and compliance are discussed.

Composite Resins↗
PubMed1997

Induction of tolerance with nondepleting anti-CD4 monoclonal antibodies is associated with down-regulation of TH2 cytokines.

BACKGROUND: Induction of tolerance with anti-CD4 has mainly focused on monoclonal antibodies (mAbs) that deplete CD4+ T cells. In this study, the mechanisms by which nondepleting anti-CD4 mAbs induce tolerance in the Dark Agouti to PVG rat heart graft model were examined. METHODS: Five anti-CD4 mAbs were tested. Immunohistology and cytokine mRNA profiles were analyzed within grafts. Effects of combining anti-CD4 therapy with alloantibody (alloAb), interleukin (IL)-4, and anti-IL-4 mAb were also examined. RESULTS: All mAbs tested induced indefinite graft survival (>150 days), with blocking of alloAb production. Exogenous alloAb did not restore rejection. Similar T cell receptor alphabeta+, CD8+, IL-2 receptor+ T cell, macrophage, and natural killer cell infiltration and comparable MHC II and intercellular adhesion molecule-1 levels were seen in rejecting and tolerant grafts. mRNA for IL-2, interferon-gamma, lymphotoxin, tumor necrosis factor-alpha, transforming growth factor-beta, cytolysin, and granzyme-A/B was comparable, although inducible nitric oxide synthase was slightly reduced in tolerant grafts. IL-4 and IL-5 were significantly reduced in tolerant grafts, although IL-6, IL-10, and IL-13 levels were similar; this was consistent with partial T helper (Th)2 response inhibition, which was also manifested by inhibited alloAb. The combination of alloAb, IL-4, or anti-IL-4 mAb with anti-CD4 did not prevent tolerance induction. CONCLUSIONS: This study demonstrated that anti-CD4 mAb therapy did not inhibit activation and infiltration of Th1 and CD8+ effector T cells. Preferential induction of Th2 responses, especially IL-4, was not essential for the induction of tolerance. Our studies also found no evidence to support induction of anergy or transforming growth factor-beta as mechanisms of tolerance induction. These results question whether IL-4 is required for induction of transplantation tolerance.

Animals↗
PubMed1997

The effect of curing light variations on bulk curing and wall-to-wall quality of two types and various shades of resin composites.

OBJECTIVES: This study evaluated the influence of light intensity and irradiation time variations on the curing efficacy of two types and various shades of resin composites and the effect of reduced light intensity on the preservation of wall-to-wall continuity. MATERIALS AND METHODS: Three microfilled composites (in three different shades) and one hybrid composite were used in this study. Polymerization shrinkage, and the hardness and adaptation of adhesive restorations in dentin cavities were determined at light intensities of 175 and 700 mW/cm2 and irradiation times of 10 and 60 s. Data were compared using in a general linear model analysis. RESULTS: Shrinkage measurements were the indication of conversion and conversion rate. Reduced intensity slowed down the rate of polymerization but did not reduce the conversion as long as an irradiation time of 60 s was employed. High-energy irradiation caused increased separation of the composite from the tooth structure. On the basis of obtaining optimal conversion and adaption, it was demonstrated that the irradiation time to be more effective than irradiation energy. SIGNIFICANCE: Light-cured composites require an understanding of their structure, pigmentation and irradiation parameters to obtain optimal performance. High intensity light-curing does not necessarily lead to optimal quality.

Analysis of Variance↗
PubMed1997

Sealing ability of eight resin bonding systems in a Class II restoration after mechanical fatiguing.

OBJECTIVE: The purpose of this in vitro study was to evaluate eight different adhesive systems to determine whether functional loading affects the marginal integrity of Class II restorations. METHODS: In 80 extracted upper human premolars, Class II cavities were prepared and restored with eight different commercially available adhesive resin restorative systems. The occlusal margins were located in enamel and the gingival margins in dentin. During immersion in a dye solution, half of the restored teeth were exposed to a load-cycling procedure, after which all the specimens were sectioned, and the extent of dye penetration was visually examined using a stereo microscope. The results were analyzed statistically using the Mann-Whitney U-test and Wilcoxons Rank test. RESULTS: Exposure of the restorations to load cycling significantly reduced the marginal integrity cervically. Perfect sealing was found practically at all occlusal margins. Significant differences (p < 0.05) were found in the cervical sealing quality of the various adhesive systems. SIGNIFICANCE: The results of this study show that functional loading damages the cervical marginal seal of adhesive Class II restorations. For many adhesive systems, the setting shrinkage stress alone was not able to damage the marginal integrity cervically. Therefore, mechanical fatiguing distinguishes the sealing quality of adhesive systems in a more expressive way. There is a substantial difference in sealing quality between the various resin restorative systems tested.

Bite Force↗
PubMed1997

Polymerization contraction stress in thin resin composite layers as a function of layer thickness.

OBJECTIVES: In the present study, the effect of layer thickness on the curing stress in thin resin composite layers was investigated. Since the value of the contraction stress is dependent on the compliance of the measuring equipment (especially for thin films), a method to determine the compliance of the test apparatus was tested. METHODS: A chemically initiated resin composite (Clearfil F2, Kuraray) was inserted between two sandblasted and silane-coated stainless steel discs in a tensilometer. The curing contraction of the cylindrical samples was continuously counteracted by feedback displacement of the tensilometer crosshead, and the curing stress development was registered. After 20 min, the samples were loaded in tension until fracture. The curing stress was determined for layer thicknesses of 50, 100, 200, 300, 400, 500, 600, 700 microns, 1.4 mm and 2.7 mm. The compliance of the apparatus was calculated with the aid of a non-linear regression analysis, using an equation derived from Hooke's Law as the model. RESULTS: None of the samples fractured due to contraction stress prior to tensile loading. The contraction stress after 20 min decreased from 23.3 +/- 5.3 MPa for the 50 microns layer to 5.5 +/- 0.6 MPa for the 2.7 mm layer. The compliance on the apparatus was 0.029 mm/MPa. SIGNIFICANCE: A measuring method was developed which was found to be suitable for the determination of axial polymerization contraction stress in this films of chemically initiated resin composites. The method makes it possible to estimate the stress levels that occur in resin composite films in the clinical situation.

Analysis of Variance↗
PubMed1997

Resistance of cementum in Class II and V cavities to penetration by an adhesive system.

OBJECTIVES: As the cervical margin located in cementum-dentin is still the most unpredictable area of an adhesive resin restoration, the aim of this investigation was to evaluate the morphology of the cementum layer at the cervical margins of Class V and Class II cavities and the impregnability of this layer to resin bonding systems. METHODS: Three different types of in vitro investigations of the cervical margins were performed by scanning electron microscopy: 1) direct anatomical observation of conditioned cavities; 2) observation of resin replicas; and 3) observation of resin infiltration. During direct observation, the presence of opened tubules was evaluated; in the resin replicas, the presence of resin tags and their density were observed; in the observation of resin infiltration, the presence of an acid-resistant interdiffusion was investigated. RESULTS: From direct observation, cut tubules were seen 200 microns from the cervical margin. After treatment with a dentin bonding system, the outer layer was infiltrated by the resin. In the resin replica, the presence of resin tags was detectable 150-200 microns from the margin. In Class II samples, the presence of an outer layer at the cervical margin, which could not be identified as bulk dentin or cementum, was clearly detectable by both direct and indirect observation. In the Class V samples, the border between this layer and bulk dentin was less evident. SIGNIFICANCE: The presence of a cementum layer of approximately 150-200 microns at the cervical margins of cavities may pose a serious clinical problem for reliable bonding. Although in the present study the observation of a zone of resin-impregnated cementum may confirm the improvement obtained with the last generation of hybridizing dentin bonding systems, the effectiveness of the bond is still unclear.

Composite Resins↗
PubMed1997

Influence of compliance of the substrate materials on polymerization contraction stress in thin resin composite layers.

The present study determined in a laboratory set-up the influence of compliance of the substrate material on polymerisation contraction stress for various thicknesses of bonded dental resin composite films. When the compliance of the tensilometer set-up was increased from 0.029 micron MPa-1 to 0.150 micron MPa-1, the contraction stress in films with a thickness of 100 microns and a diameter of 5.35 mm decreased from 22 to 7 MPa. For the 700 microns samples the stress decreased from 12 to 11 MPa. It was concluded that if compliance from the substrate materials is possible, a thinner resin composite film may effect a more reliable bond.

Chemical Phenomena↗
PubMed1997

Mapping of tubule and intertubule surface areas available for bonding in Class V and Class II preparations.

OBJECTIVES AND METHODS: The aim of this in vitro study was to evaluate the morphology of dentine in Class V and the Class II preparation walls in terms of tubule orientation, density and increase in surface area after conditioning. Six circular V-shaped preparations were cut at the cementum-enamel junction (CEJ) of anterior teeth and six Class II cavities with the cervical margin 1 mm below the CEJ were prepared in posterior teeth. The preparations were conditioned with 10% maleic acid. The samples were directly studied by SEM. The observations were organized according to location in the preparation, tubule density was counted and the increase in area available for bonding after acid etching was calculated. RESULTS: Dentine morphology of Class V and Class II preparation walls was mostly regular according to predictable patterns. Between the tubules, along the walls of the cavity, the etched dentine surface exhibited a porous network of collagen fibers. More than 50% of Class V and Class II cavity surface presented oblique or parallel tubule orientation with respect to the cut surface. Because of the presence of a structureless cementum layer. the morphology of the cervical area turned out to be less predictable. The tubule density varied considerably in different regions of the preparations. The intertubular dentine surface area increased after conditioning in Class V preparations from 20.9% to 50.3% on the walls where tubules were cut parallel to their long axis, and from 5.8% to 47.8% where tubules were cut perpendicularly. These same values for Class II preparations ranged from 0.6% to 46.4% on the walls where tubules were cut parallel to their long axis, and 29.8% of the tubules were cut perpendicularly. CONCLUSIONS: Based on this morphological investigation, the increase in intertubular dentine surface area might very well be responsible for the enhanced bond strength after acid etching of dentine, but not all areas exhibited equal responses to etching. In particular, the bonding substrate at the gingival margins may contribute little in terms of micromechanical retention.

Acid Etching, Dental↗
PubMed1997

Polymerization shrinkage and polymerization shrinkage stress in polymer-based restoratives.

OBJECTIVES: This paper is intended to contribute to the recognition and understanding of problems related to polymerization shrinkage. DATA SOURCES: Scientific publications of relevance with regard to this subject were critically reviewed. STUDY SELECTION: The dimensional changes which develop during the curing of resin composites and glass polyalkenoate cements are studied, with special reference to methods of determining shrinkage, shrinkage stress and stress relief. CONCLUSIONS: As no method for handling the adhesive restorative materials has yet been described which guarantees a leakproof restoration, the practitioner has to accept the problem of polymerization shrinkage and destructive shrinkage stress. Only a proper understanding of the mechanisms that cause these problems and the techniques that may reduce their effects will enable the practitioner to derive maximum benefit from the application of resin composites and glass polyalkenoate cements in restorative dentistry.

Composite Resins↗
PubMed1997

[Dental restoration materials in pediatric dentistry].

Restorative materials in pediatric dentistry have to fulfill special requirements. They should be easy to handle and applicable in a not always dry mouth. They should potentially be adhesive in order to avoid too much mechanical preparation. They do not have to be extremely wear resistant as the dwell time of the restorations is relatively short. Glass-ionomer cements and in particular the resin modified types possess properties which make them almost ideal for the required purpose.

Child↗
PubMed1997

[Gallium: an alternative for amalgam?].

Some of the physical properties of gallium-based dental alloys for restorative goals equal those of dental silver amalgam, but the results of the relatively few and short-term studies of their clinical behaviour differ. Corrosion, discoloration, rough surface, expansion and fractures of the margins and possibly the teeth, may pose serious problems and are assessed to make the clinical use premature. Handling characteristics are less favourable than for silver amalgam. Gallium alloys release a substantial amount of gallium. Results of toxicity studies with cell cultures differ. Compared to some brands of silver amalgams the alloys seem to be somewhat more cytotoxic during a longer period of time. However, the gallium released by the restorations would be insufficient to harm the patients. Implants of (early) alloys evoked more serious reactions than silver amalgam. Sensitization is not reported, but few restorations have been made and allergic studies are scarce.

Chemical Phenomena↗
PubMed1997

Fracture resistance of endodontically-treated premolars adhesively restored.

PURPOSE: To investigate the cusp fracture resistance of endodontically treated teeth, adhesively restored with various materials. MATERIALS AND METHODS: MOD preparations and endodontic treatment was carried out on extracted sound maxillary premolars. The cavities were restored with the amalgam Valiant in combination with Superbond or Panavia bonding, the resin composites Z100, Herculite XRV or Clearfil RP with their respective bonding systems, Z100 in combination with the glass ionomers Ketac Fil, Fuji II and Vitremer, and Tetric in combination with Compoglass. Fracture resistance was measured by axial loading in an Instron testing machine. RESULTS: One of the restorative methods, resin composites in combination with dentin bonding systems in beveled MOD preparations rendered the tooth a cusp fracture resistance which did not differ significantly from that of sound natural teeth. Two other restorative methods, bonded amalgam and a sandwich of glass ionomer cement/resin composite in beveled preparations were significantly weaker in resisting cusp fracture than sound natural teeth, but still significantly stronger than the unrestored tooth with a MOD preparation. It was statistically apparent that several adhesive restorative systems could satisfactorily be used to restore teeth after endodontic therapy.

Analysis of Variance↗
PubMed1997

Effect of two etching times on the sealing ability of Clearfil Liner Bond 2 in Class V restorations.

PURPOSE: To investigate the sealing ability of the Clearfil Liner Bond 2 self-etching primer in Class V restorations placed under clinical and laboratory conditions. MATERIALS AND METHODS: Firstly, the influence of two etching times of the self-etching primer were tested in vitro. In 10 Class V cavities (Group 1), the self-etching primer was applied for 30 seconds on the enamel and dentin, while in another 10 Class V cavities (Group 2) the etching time was 60 seconds. Since the Group 2 samples showed the best in vitro sealing ability at both the enamel and cementum-dentin sites, the teeth of Group 3 (10 restorations) were restored as in Group 2 under clinical conditions, applying the self-etching primer for 60 seconds. The sample teeth were extracted after 65-90 days, and then processed for leakage. Two other groups of 10 samples each were prepared under laboratory conditions to investigate hybrid layer, resin tags and adhesive lateral branch formation. A flat buccal surface on each extracted tooth was prepared. Group 4 samples were conditioned for 30 seconds, and those of Group 5 for 60 seconds. RESULTS: In Group 1, 40% of the restorations showed leakage at the cementum-dentin site and 40% at the enamel site. Groups 2 and 3 showed statistically significant difference (less leakage) than Group 1. In Group 2 and 3 no leakage was found at the enamel site, and only a moderate leakage (10% and 20% of restorations, respectively) was found at the cementum-dentin site. The scanning electron microscopy observations of Group 4 samples showed a thin hybrid layer, with well-fitting and smooth resin tags and adhesive lateral branches only sporadically. Group 5 samples presented a thicker hybrid layer, rough and deep resin tags with many adhesive lateral branches. The etch enamel pattern was more uniform and rougher in Group 5 than in Group 4. The 60-second application time of the self-etching primer seems to be more reliable than a shorter conditioning time in day-to-day practice.

Acid Etching, Dental↗
PubMed1997

Interdiffusion of a traditional glass ionomer cement into conditioned dentin.

PURPOSE: To investigate whether a new auto-curing glass ionomer cement (Fuji IX) in combination with polyacrylic acid (Cavity Conditioner) forms an interdiffusion zone both under laboratory and clinical conditions. MATERIALS AND METHODS: Seven vital human posterior teeth were selected. A flat dentin surface was created on the buccal surface in order to apply the glass ionomer material. Seven other extracted human molars were prepared following the same procedure as with the in vivo experimental teeth. After extraction of the in vivo restored teeth, the samples were cut and prepared for hybrid layer observation under scanning electron microscope. RESULTS: An interdiffusion zone, of approximately 6 microns thick, was observed both under in vivo and in vitro conditions. At the interface between the two substrates, the interdiffusion zone was acid resistant and uniform.

Acrylic Resins↗