Biomedical subjects

A C Shortall

Publications and source records attributed to A C Shortall.

At least 19 recordsLinked to original sources

PubMed2006

Photoinitiation chemistry affects light transmission and degree of conversion of curing experimental dental resin composites.

INTRODUCTION: The effect of photoinitiator and co-initiator chemistry on the setting reaction and degree of conversion of dental resin-based composites (RBCs) has rarely been determined explicitly. This work examines the effect of type and concentration of photoinitiator and co-initiator on the rate of change of light transmission throughout polymerisation and degree of conversion of model RBC formulations. METHODS: Bisphenol-A diglycidyl ether dimethacrylate (bis-GMA) and triethylene glycol dimethacrylate (TEGDMA) (6:4 molar ratio) resins filled with silanized glass filler (74wt.%) and containing various photoinitiators (camphorquinone; CQ, 1-phenyl-1,2-propanedione; PPD, benzil; BZ), co-initiator types (N,N-dimethyl-p-amino benzoic acid ethyl ester; DABE, N,N-cyanoethyl methylaniline; CEMA, N,N-diethanol-p-toluidine; DEPT) and concentration (0.0-0.3% DABE) were polymerised using a halogen or LED light curing-unit (LCU) for 10, 20 and 40s. The setting reaction was monitored in real-time by measuring the light transmittance through the curing specimen and bulk degree of conversion (DC) evaluated using Fourier transform infra-red spectroscopy. RESULTS: Specimens containing CQ and PPD cured with the halogen LCU did not have a significant effect on DC or changes in light transmission, although a significant increase in DC was observed for CQ compared with PPD specimens cured with the LED LCU. DABE and CEMA were more effective co-initiators than DEPT. Although DC was not limited by co-initiator concentration, the absence of a co-initiator resulted in marked differences in light transmission and decreased DC throughout 40s irradiation with each LCU type. CONCLUSIONS: The spectral range emitted from different types of LCU and absorption characteristics of the photoinitiator chemistry of light-activated resin-based composites play a critical role in the efficiency of polymerisation.

Analysis of Variance↗

PubMed2005

How light source and product shade influence cure depth for a contemporary composite.

Directly placed light activated resin based composite restorations are becoming increasingly popular for restoring cavities in posterior teeth. Marketing and patient as factors well as operator preference are behind this trend. Achieving adequate depth of cure is critical to the success of these restorations. Recently a number of second generation light emitting diode (LED) light activation units have been marketed and the manufacturer of one of these claims that it is capable of curing in half the time of its predecessor. This study tested that claim using a companion composite from the same manufacturer. The relationship between cure depth, shade changes on curing and opacity were also assessed. Under the limitations of the current investigation the results indicated that the second generation LED unit in question met the manufacturer's claim for halving cure time. Depths of cure approached those of a control halogen unit in half the 40 s radiation time of the latter. For the product tested depth of cure is strongly linked to material opacity.

Color↗

PubMed2003

Influence of filler loading on the two-body wear of a dental composite.

The purpose of the study was to explore the fundamental wear behaviour of a dental composite with different filler loadings under two-body wear conditions. The parent resin and filler components were mixed according to different weight ratios to produce experimental composites with filler loadings ranging from 20 to 87.5% by weight. A two-body wear test was conducted on the experimental composites using a wear-testing machine. The machine was designed to simulate the impact of the direct cyclic masticatory loading that occurs in the occlusal contact area in vivo. The results showed that there was little increase in the rate of wear with filler loadings below 60 wt%, but a sharp increase between 80 and 87.5 wt% in filler loading. Wide striations and bulk loss of material were apparent on the wear surfaces at higher filler loadings. Coefficients of friction increased with filler loading and followed the increase in rate of wear loss closely. It was concluded that, under two-body wear conditions, addition of high levels of filler particles into the resin matrix could reduce the wear resistance of dental composites. This finding may help when designing future dental composites for use in particular clinical settings.

Bisphenol A-Glycidyl Methacrylate↗

PubMed2002

A pilot investigation of operator variability during intra-oral light curing.

OBJECTIVE: To test the hypothesis that operator experience influences the efficacy of light curing in a typical posterior intra-oral location. To investigate whether short cure cycles affect performance. DESIGN: A cross-sectional single-centre study designed to assess the efficacy of experienced and inexperienced operators when undertaking simulated intra-oral curing. SETTING: An in vitro laboratory based investigation conducted in a dental school during 2001. MATERIALS AND METHODS: A computer-based technique was used to monitor light intensity in a clinical simulation. Dentists and student operators were tested for their ability to cure a posterior restoration effectively. Relative light intensity was assessed against time for each operator and test run. RESULTS: Experienced (qualified) operators produced more effective and consistent cure results than less experienced undergraduate students. Operator performance was not affected by variations in irradiation time. CONCLUSIONS: This cross-sectional pilot investigation demonstrates that operator experience is a factor in successful clinical photo-curing of posterior restorations. Stable and accurate light guide positioning are required throughout the entire irradiation cycle to optimise intra-oral cure of light-activated restorations. Further investigations are planned to assess the potential of this novel method of assessment for use as a routine teaching aid in clinical practice.

Analysis of Variance↗

PubMed2002

Wear of three dental composites under different testing conditions.

Although a great effort has been made to date with research on in vitro wear simulation testing, it is difficult to identify the relationship between the results of in vitro testing and in vivo observations. An intensive study into wear mechanisms is necessary for wear testing simulation. The objective of this research was to study the wear behaviour of three selected dental composites under different wear conditions to provide a more rational explanation for their wear mechanisms. Two typical wear conditions, two- and three-body wear, were conducted on the composites. The results showed that the wear losses had different rankings between two wear conditions. It is therefore unreliable to predict the clinical performance merely by wear loss ranking from in vitro wear testing. Further analysis on the wear surfaces indicated that three dental composites experienced different wear mechanisms under different wear conditions. It is concluded that in vitro analysis of wear mechanisms may lead to a better understanding of in vivo failure patterns. Similar wear mechanisms should be the premise for any correlation between the results of in vitro and in vivo studies.

Analysis of Variance↗

PubMed2001

A comparison of the mechanical properties of a gallium-based alloy with a spherical high-copper amalgam.

OBJECTIVES: The aim of the present study was to investigate how the mechanical properties of a palladium free gallium-based alloy (Galloy) compare with a leading spherical high-copper amalgam (Tytin). METHODS: Cylindrical specimens were mechanically condensed, according to the ISO 1559:1986 standard, to measure compressive strength, Vickers hardness, static creep and dimensional change on setting. Disc and beam shaped specimens were manually prepared to assess the diametral tensile and flexural strengths of the investigated alloys. RESULTS: The mean hardness, 1h compressive fracture strength, 24 h diametral tensile and 24h flexural strengths of Galloy were significantly lower (P<0.001) than Tytin. No significant differences in modulus of elasticity, creep, dimensional change on setting, 24 and 168 h compressive fracture strength for the two alloys were identified. SIGNIFICANCE: The significant reduction in the 1 h mean compressive fracture strength and hardness identified for Galloy compared with Tytin possibly indicate a slower setting reaction in the gallium-based alloy. Manual condensation of the gallium-based alloy produced specimens with inferior mechanical properties possibly due to the increased likelihood of introducing voids within the test specimens. Previous reports indicating poor corrosion resistance and moisture sensitivity of gallium-based alloys highlight the need for further research to investigate the effect of the oral environment on the gallium-based alloy.

Compressive Strength↗

PubMed2001

Effect of exposure intensity and post-cure temperature storage on hardness of contemporary photo-activated composites.

OBJECTIVES: The effect of variation in post-exposure storage temperature (18 vs. 37 degrees C) and light intensity (200 vs. 500mW/cm(2)) on micro-hardness of seven light-activated resin composite materials, cured with a Prismetics Mk II (Dentsply) light activation unit, were studied. METHODS: Hardness values at the upper and lower surfaces of 2mm thick disc shaped specimens of seven light-cured resin composite materials (Herculite XRV and Prodigy/Kerr, Z100 and Silux Plus/3M, TPH/Dentsply, Pertac-Hybrid/Espe, and Charisma/Kulzer), which had been stored dry, were determined 24h after irradiation with a Prismetics Mk II (Dentsply) light activation unit. RESULTS: Hardness values varied with product, surface, storage temperature, and curing light intensity. In no case did the hardness at the lower surface equal that of the upper surface, and the combination of 500mW/cm(2) intensity and 37 degrees C storage produced the best hardness results at the lower surface. CONCLUSIONS: Material composition had a significant influence on surface hardness. Only one of the seven products (TPH) produced a mean hardness values at the lower surface >80% of the maximum mean upper surface hardness obtained for the corresponding product at 500mW/cm(2) intensity/37 degrees C storage temperature when subjected to all four test regimes. Despite optimum post-cure storage conditions, 200mW/cm(2) intensity curing for 40s will not produce acceptable hardness at the lower surface of 2mm increments of the majority of products tested.

Analysis of Variance↗

PubMed2001

Handling characteristics of a palladium-free gallium-based alloy compared with a high copper dental amalgam in a simulated clinical trial.

The aim of the current study was to compare the handling characteristics of a palladium-free gallium-based alloys (Galloy) with those of a high-copper amalgam (Permite C). The study had a particular interest in the evaluation of the direct placement delivery system used with both alloys. Ten dentists participated in the current study. Each placed two amalgam and two gallium-based alloy restorations in conventional class II cavities prepared in acrylic typodont teeth. None of the participating dentists had used the direct placement delivery system or had any previous experience with gallium-based alloy and no practice was allowed beforehand. The restorations were evaluated according to the following criteria: ease of loading the cavity (delivery system), ease of condensation, capacity to produce and sustain contact area, ease of carving, resistance to damage during removal of the matrix band, overall quality of the restoration and the available working time. Each criterion was given a score on a scale of 1-5 (1: very poor, 2: poor, 3: fair, 4: good, 5: very good). The results showed no statistically significant difference in the evaluated criteria between the two alloys (P > 0.05) except for criterion number 2 (ease of condensation, P=0.0005).

Acrylic Resins↗

PubMed2001

The influence of fibre placement and position on the efficiency of reinforcement of fibre reinforced composite bridgework.

The effect of placement of ultra-high molecular weight polyethylene (UHMWPE) fibres on the flexural properties and fracture resistance of a direct dental composite was investigated. The UHMWPE fibres are increasingly being used for the reinforcement of laboratory fabricated resin composite crown and bridgework. The aim of this study was to assess the effect of a commonly used laboratory fabrication variable on the in vitro strength of beam shaped specimen simulating a three-unit fixed bridge. Four groups (10 specimens per group) of Herculite XRV were prepared for flexural modulus and strength testing after reinforcement with UHMWPE fibres. Two groups of control specimens were prepared without any fibre reinforcement. Half the specimen groups were stored in distilled water and the other groups were stored dry, both at 37 degrees C for 2 weeks before testing. The results of this study showed that placement of fibre at or slightly away from the tensile side improved the flexural properties of the composite in comparison with the unreinforced control specimen groups whilst the mode of failure differed according to fibre position. Scanning electron microscope (SEM) investigation revealed that placement of the fibre slightly away from the tensile side favoured crack development and propagation within the resin bridging the interfibre spaces in addition to debonding parallel to the direction of fibre placement. Laboratory fabrication variables may effect the strength of fibre reinforced bridgework significantly.

Analysis of Variance↗

PubMed2001

Successful restoration of load-bearing cavities in posterior teeth with direct-replacement resin-based composite.

Successful restoration of posterior teeth with resin-based restorative materials requires the clinician to be aware of the factors influencing success. These include correct patient and cavity selection, correct choice of material, identifying a matrix system that will achieve a tight contact, and correct placement methods. A means of avoiding stress due to polymerization shrinkage should also be used.

Bicuspid↗

PubMed2001

Relative merits of narrow versus standard diameter light guides.

The purpose of this study was to assess the relative effectiveness of narrow versus standard diameter light guides at curing the initial increment of composite in a deep cavity. The micro-hardness of 2 mm thick samples of a light activated microfilled posterior composite was determined. Composite was irradiated at 0, 1 or 6 mm light guide tip/composite surface distances. Light intensity was measured with a computer-based radiometer. Lower surface cure was reduced with increasing light guide tip distances. Narrow diameter guides offered no advantage over standard guides for polymerisation of the initial increment of composite in the base of a cavity.

Analysis of Variance↗

PubMed2000

In vitro evaluation of the effect of freshly mixed amalgam and gallium-based alloy on the viability of primary periosteal and osteoblast cell cultures.

The effect of freshly mixed specimens of a gallium-based alloy Galloy and a high copper spherical amalgam Tytin on the viability of primary periosteal and osteoblast cells was investigated. The cells were grown from the parietal bones of 2-3 day old Albino Wistar rats and were seeded in multi-well plates and exposed to the test materials for 1 or 6 days. The number of viable cells in each test group was determined using the Trypan blue dye exclusion test and compared with the controls. The area of cell death around the test specimens was also measured. Statistical analysis (ANOVA, Tukey's pairwise comparisons) showed a significant effect of the test materials on cell viability (P < 0.01). The viability of cultures containing Tytin was significantly lower than cultures with Galloy and the controls (P < 0.05). The viability of cultures containing Galloy was significantly lower (P < 0.05) than controls except for periosteal cells after 1 day. The culture media was analysed using atomic absorption spectrophotometry (AAS) for metal ion content. The results suggested that mercury (Hg) was the main element released from the dental amalgam followed by copper (Cu) and silver (Ag) whilst gallium (Ga) was the main element released from the gallium-based alloy followed by indium (In), Ag and Cu.

Alloys↗

PubMed1999

Two-body in vitro wear study of some current dental composites and amalgams.

STATEMENT OF PROBLEM: Wear resistance of restorative materials is a major concern in clinical practice for restorations involving occlusion. The relatively poor wear resistance of dental composites in stress-bearing posterior situations has restricted even wider clinical application of this type of restorative material. PURPOSE: This in vitro study compared the relative wear resistance of a selection of current dental composites and amalgams under cyclic loading to explore the wear mechanisms operating on these materials and to assess their relative potential clinical wear resistance under variable masticatory loads. MATERIAL AND METHODS: A 2-body in vitro wear test was undertaken on a selection of 2 Ultrafine Compact-Filled composites, 1 microfilled composite, and 3 dental amalgams. An alternating sine curve load (0.5 to 15 N) was applied to the wear surface through a rotating countersample with a purpose-built system to simulate the variable loading pattern that occurs during mastication. RESULTS: Under conditions simulating the repetitive cyclic loading pattern that may occur intraorally in high stress occlusal contact situations, the Ultrafine Compact-Filled composites exhibited a delamination wear pattern and had a significantly lower wear resistance than the amalgams. By comparison, the microfilled composite (Silux Plus) displayed an improved wear resistance that was superior to some amalgams. Dispersalloy had the best wear resistance among the materials selected. CONCLUSION: The wear of Ultrafine Compact-Filled composite and microfilled composite differed and reflect different operative wear mechanisms. For amalgams, the size, shape, and composition of the particles had an effect on the wear resistance of the materials.

Analysis of Variance↗

PubMed1999

The influence of clinically induced variability on the distribution of compressive fracture strengths of a hand-mixed zinc phosphate dental cement.

OBJECTIVES: Conventional approaches to comparing dental cements use standard property tests under manufacturers' specified conditions. Zinc phosphate cements are supplied in powder/liquid form and manipulation frequently involves mixing the components by eye so a range of mixing ratios will inevitably occur in practice. Unfortunately, the physical, chemical, biological and mechanical properties of cements are known to be dependent on the mixing ratio. METHODS: Forty dental nurses prepared a series of three cement samples to a luting consistency they considered acceptable for use in practice. RESULTS: It was found that each dental nurse produced consistent cement mixes, although, the mixing ratios varied from 1.7 to 3.2 g/ml between nurses. The mean compressive strength, standard deviation and associated Weibull Moduli (m) of the cements were determined as a function of this mixing ratio range and showed considerable variation ranging from 33.5 +/- 3.2 MPa (m = 11.0) at 1.7 g/ml to 71.4 +/- 8.4 MPa (m = 8.6) at 2.6 g/ml and 42.5 +/- 10.0 MPa (m = 5.0) at 3.2 g/ml. SIGNIFICANCE: An analytical approach was adopted which facilitated an integrated analysis of the mixing ratio variability with the strength data. It was found for the test group of dental nurses that 25% of cement mixes produced would have achieved strengths below 40 MPa whilst strengths below the standard value were produced in at least 70% of mixes. These results indicate that a simple analysis of the properties of cements manipulated under optimum conditions, provides little information on the material characteristics obtained in practice because of clinically induced variability.

Analysis of Variance↗

PubMed1999

The influence of cyclic loading on the wear of a dental composite.

The load applied to a tooth surface during mastication is not a constant force and the objective of the research reported in this paper was to evaluate the impact of variable masticatory loads on the mechanisms involved in the wear of a dental composite. A new wear testing machine has therefore been developed, which incorporates a sine cam mechanical system to generate an alternating sine curve load at the wear surfaces to simulate the loading produced by masticatory process. The basic wear pattern is generated by positioning a disc shaped specimen against a rotating cylindrical ring with a normal load applied to the upper surface of the specimen. This approach allows different combinations of static and sine curve loads to be applied, thereby facilitating the study of different wear situations. It was found that the wear behaviour associated with variable loading patterns differed from that of static loading.

Composite Resins↗

PubMed1999

Encapsulated verses hand-mixed zinc phosphate dental cement.

Zinc phosphate cements are commonly supplied as two components, powder and liquid, and the proportions of the constituents are determined by operator experience. A capsulated system which is mechanically mixed has been marketed and this study investigated the performance of the encapsulated cement system. The mean fracture strength, standard deviation and associated Weibull Moduli (m) of encapsulated cements were determined by compressive fracturing 20 cement specimens filled directly from the mixing syringe or from narrower cement tubes. Pore distribution within the cylindrical specimens was determined using image analysis to assess the influence of the method of mould filling with the cement. The strength data showed variation in magnitude and consistency ranging from 44.6+/-13.7 MPa (m = 3.18+/-0.71) for cements filled directly from the syringe to 61.0+/-7.8 MPa (m = 8.35+/-1.87) for cements filled from cement tubes. Larger pores were found in specimens consolidated directly from the cement syringe. Mechanical mixing of the encapsulated cement resulted in air entrapment in the cement mix which manifested itself as large pores (over 200 microm diameter) within the cylindrical specimens. The smaller orifice of the cement tube compared with the syringe was considered to be responsible for eliminating the majority of the air entrapped in the cement mass during mixing. Whilst mechanical mixing of encapsulated cements is quicker and more convenient, the encapsulated specimens consolidated according to the manufacturers instructions from the syringe offered no significant advantage in terms of reliability or strength over hand-mixed cements in this investigation.

Biocompatible Materials↗

PubMed1998

Temperature rise during polymerization of light-activated resin composites.

There has been a steady rise in the use of resin-based composites in the restoration of large posterior cavities. Visible light-activated materials have come to dominate the market for direct posterior composites because of the convenience offered to the operator by their command set nature. Manufacturers have produced more powerful light-activation units in response to concerns over inadequate depth of cure. However, some authors have warned of the danger to the dental pulp induced by excessive heat generated during polymerization. Variables governing heat generation include output intensity of the light-activation unit, quality of light filter, irradiation time, material composition and material surface-light guide exit window distance, residual dentine thickness and cavity dimensions. This in vitro study assessed the relative influence of some of these factors under controlled conditions. The findings suggest that minimum irradiation times should be used when curing bonding agents in unlined cavities where residual dentine thickness is limited and high output intensity light-activation units are employed. Material composition is identified as an important variable in determining the magnitude of the temperature rise.

Analysis of Variance↗

PubMed1997

Effectiveness of battery powered light activation units.

OBJECTIVE: To investigate the effectiveness of chargeable light curing units and their ability to sustain a high radiant output following successive discharge cycles. SETTING: University based research investigation assessing the output intensity of three battery powered light activation units and the effect of successive discharge cycles, without intervening periods of recharging, on light output and depth of cure of resin composite. MATERIALS AND METHODS: Mean light intensity (mW/cm2), through a 4 mm aperture, was measured with a computer based radiometer. Two commercially available resin composites (one hybrid and one microfilled resin) were used for the depth of cure determinations using a digital penetrometer. RESULTS: Depths of cure were significantly greater for the hybrid resin composite. Output intensity diminished over the period of battery discharge for two of the three units and this finding was paralleled in the depth of cure results. The battery light units tested varied significantly with regard to their length of discharge cycle and recharge times from complete discharge. CONCLUSIONS: Output intensity diminishes over the period of battery discharge for some makes of battery light units. However, many other factors including handpiece size and weight and unit cost and reliability are important considerations when choosing a unit.

Composite Resins↗