Biomedical subjects

D C Watts

Publications and source records attributed to D C Watts.

At least 37 records · Page 2Linked to original sources

PubMed2000

Resistance of two dentin-bonding agents and a dentin densensitizer to acid erosion in vitro.

OBJECTIVES: To determine the resistance of two dentin-bonding agents (DBAs) and a dentin desensitizer to simulated dental erosion. METHODS: Twenty teeth in each of three groups of selected premolars, with exposed cervical dentin, were treated with one of three dentin surface treatment systems: Gluma Desensitizer (Heraeus Kulzer (UK) Ltd), One Coat Bond (Whaledent (UK) Ltd), and Optibond FL (Kerr (UK) Ltd) and exposed to water (control) and an erosive challenge (Coca Cola) for 14 days. Sequential tracings of the specimens were taken, superimposed and the mean reduction in densensitizer or DBA thickness and or tooth loss determined. RESULTS: A mean loss of 77 microns (s.d. 14.6 microns) of tooth tissue over the untreated sites following 14 days exposure to Coca Cola was observed. The application of DBAs, as opposed to Gluma Desensistizer alone, over exposed dentin surfaces afforded substantial protection against erosion by Coca Cola over the period of study. The mean change in vertical profile of teeth treated with a DBA was 20 microns (s.d. 14.7 microns), almost four times less than that seen over untreated sites. SIGNIFICANCE: With the exception of Gluma Desensitizer, which was lost completely, One Coat Bond and Optibond FL demonstrated levels of acid dissolution resistance, which would be of potential value clinically.

Analysis of Variance↗

PubMed2000

Optimal specimen geometry in bonded-disk shrinkage-strain measurements on light-cured biomaterials.

OBJECTIVES: The aim was to determine the effect on apparent shrinkage-strain values of varying specimen aspect ratio in the bonded-disk configuration. Thereby one source of possible inter-laboratory variation might be understood and eliminated. METHODS: The bonded-disk procedure was followed and applied to three representative resin-composites. However, specimen sub-groups were examined, each of different diameters (3.5, 5.0, 7.0 and 9.0 mm), corresponding to C-factors in the range 0.45-7.0. RESULTS: For each material, no statistically significant difference was apparent between data obtained with 7 and 9 mm diameter specimens. However, statistically significant reductions (p < 0.05, Scheffé and SNK) were apparent at 5.0 and 3.5 mm diameter, in the range 14-18% of the maximum 'true' value. For each material, the diameter (d)-dependence of apparent shrinkage-strain (epsilon) was given by the curve fit expression: epsilon = A - B/d2. SIGNIFICANCE: For accurate determination of maximum final equilibrium shrinkage-strain values, a high aspect ratio (7-9:1) of the bonded-disk should be utilised. This is important for accurate comparison of data from proposed low-shrinkage amalgam-replacement materials.

Analysis of Variance↗

PubMed2000

High pressure liquid chromatography of dentin primers and bonding agents.

OBJECTIVE: The purpose of this study was to analyse the composition of representative dentin primers and bonding agents, and to investigate the relationship between chromatographic retention times and partition coefficient (log P) values. METHODS: Dentin bonding systems (DBS) were analysed with reversed phase high-performance liquid chromatography (HPLC). The log P values were obtained computationally with the advanced chemistry development software. RESULTS: The DBS were analysed and the monomers were identified. The log P values were calculated and the relationship between log P and retention times for the monomers was described by the equation: log P = 2.436R(t) - 3.636, with a correlation value (r) of r = 0.9095. SIGNIFICANCE: The components of the DBS were successfully resolved and identified, thus illustrating the analytical power of HPLC regarding those systems. Also the log P values correlated with the retention times of monomers. Thus, they can be used as a prediction tool in future analysis. These findings are important for a mechanistic understanding of Primer and Adhesive actions in the bonding to the dentin.

Algorithms↗

PubMed2000

Dimensional changes of resin/ionomer restoratives in aqueous and neutral media.

OBJECTIVES: The aim was to study dimensional time-dependence of resin/ionomer formulations from 5 min age to one month and to separate out the intrinsic setting shrinkage and hygroscopic expansion effects, by using non-aqueous and aqueous storage media, respectively. METHODS: Materials studied included: A: resin-, B: metal- and C: polyphosphonate-modified glass-ionomer cements [GICs]; and controls of D: poly-acid modified composite [compomer]; and E: resin-composite. Separate specimen groups (n = 5) were stored in different storage-media: (i) silicone fluid; (ii) de-ionized water. Experiments were repeated at 23 and 37 degrees C. Volumetric changes of specimens (4 mm diameter x 6 mm height) were obtained via accurate mass measurements (to 10(-4) g), using Archimedes principle, with silicone or water also used as the Archimedean fluid. These measurements were made periodically over a 30 day period, post fabrication. RESULTS: In silicone at 23 degrees C, all materials underwent further gradual intrinsic shrinkage (after 5 min from mix). This was highly significant (p < 0.05) for the RM-GIC (A). At 37 degrees C, however the RM-GIC expanded, indicating that its cure is temperature-sensitive. In water, at 23 and 37 degrees C, the shrinkage was either partially offset (materials C, D, E), or replaced by appreciable expansions (materials A and B). Differences between RM-GIC (A) and MM-GIC (B) were significant (p < 0.05). SIGNIFICANCE: The deployment of dual storage media made an important contribution to the separate analysis of the volumetric changes due to the on-going setting chemistry in these systems and the time-dependent effects of an aqueous environment.

Analysis of Variance↗

PubMed2000

Degree of cure of orthodontic adhesives with various polymerization initiation modes.

The purpose of this study was to estimate the degree of cure (DC) of a light-cured, and a two- and a one-phase (no-mix) chemically-cured, as well as a dual-cured commercially available orthodontic adhesive resin. Forty stainless steel brackets were divided into four groups of 10 brackets each, and the bracket bases were covered with a standardized volume of adhesive. They were then pressed firmly onto a yellowish background surface of 75 per cent reflectance covered with cellulose film to facilitate detachment of the system and recovery of the set material. The visible light- and dual-cured adhesives were photopolymerized by irradiation from the incisal and cervical edges of the bracket for 10 seconds each, while another group of ceramic brackets was used to assess the differential interference of transparent relative to opaque material in the DC. Micro-multiple internal reflectance Fourier transform infrared spectroscopy was employed for the estimation of the DC of the adhesives (alpha = 0.05, n = 10), utilizing a method proposed previously. The dual-cured product demonstrated the highest DC followed by the light-cured combined with the ceramic bracket, and the no-mix and the chemically-cured adhesives. The combination of the metallic bracket with the light-cured product resulted in a DC comparable with that of the chemically-cured material.

Analysis of Variance↗

PubMed2000

Removal of porcelain veneers aided by a fluorescing luting cement.

PURPOSE: Porcelain veneers are a safe and effective treatment modality for selected teeth that have poor esthetics. However, removal of porcelain veneers that have failed may be time-consuming and involve considerable removal of sound tooth structure adjacent to the veneer. The aim of this study was to evaluate a novel method of porcelain laminate veneer removal by incorporating a fluorescent dye into the luting cement that allows the practitioner to visualize the cement on the tooth and remove the veneer without causing residual damage to the adjacent tooth substance. MATERIALS AND METHODS: Porcelain veneers were luted on extracted teeth with a luting cement modified with a fluorescing agent. Faculty teaching staff and final year dental undergraduates were asked to remove the veneers with the aid of a curing light to render the luting cement visible by fluorescing. They were then asked to compare the removal of comparable veneers without this visual aid and to complete a standard questionnaire. The depth of cure of the conventional and modified cements was measured using a penetrometer. RESULTS: Results of the questionnaire indicated that the operators found removing the veneer cemented with the modified (fluorescing) cement considerably easier than removing the veneer cemented with the conventional cement. Microscopy indicated that more damage was caused to the underlying tooth during removal of conventionally cemented veneers. The incorporation of the dye into the cement reduced the depth of cure from 4.238 mm (SD = 0.025) to 3.761 mm (SD = 0.096).

Dental Debonding↗

PubMed2000

Strength of a 'no-bottle' adhesive system bonded to enamel and dentine.

Existing bonding systems permit effective bonding to enamel or dentine. Bonding to dentine is mainly achieved through the hybridization of dentine with resin. However, despite their success, the 'three-bottle' systems do have drawbacks--the large number of steps involved may be confusing and prone to errors of application, as well as being time consuming. Recently developed systems have significantly reduced the number of steps and the total treatment time, but deliver a reliable outcome.

Adhesiveness↗

PubMed2000

A chemomechanical method for caries removal.

Over recent years the dental profession has shifted towards practising preventive dentistry and adopting more conservative and tooth-preserving procedures. Such progression is considered to be a response to the decline in the level of dental caries and advances in materials science. This shift in caries management will no doubt continue over the coming decades, based on rational clinical and scientific principles. Chemomechanical caries removal involves the application of a solution that selectively softens the carious dentine, thus facilitating its removal. This limits the removal of sound tooth structure, the cutting of open dentinal tubules, pulpal irritation and pain compared with conventional mechanical methods.

Dental Caries↗

PubMed1999

Intrinsic 'soft-start' polymerisation shrinkage-kinetics in an acrylate-based resin-composite.

OBJECTIVES: A resin-composite based on multi-acrylate monomers was to be evaluated for any differences in the setting shrinkage-strain kinetics relative to more conventional formulations based on di-methacrylate monomers. METHODS: Four resin-composites were examined for shrinkage-strain over time periods up to 60 min, from initial irradiation, using a 'bonded disk' measurement device. One material was evaluated with a standard and also a two-level (low to high) blue light irradiation regime. The test material (Solitaire), and the other two controls were studied under fixed-(normal) level irradiation by blue light. Measurements were made at 23 and 37 degrees C. RESULTS: The low-high light irradiation applied to one di-methacrylate composite led to a highly significant (p < 0.001) reduction in shrinkage-strain to a minimal level for the initial 0.2 min. The fixed-level irradiation with the acrylate-composite also led to a formally identical, but intrinsic 'soft-start' shrinkage-delay regime; significant at p < 0.001. Di-methacrylate composites under the same fixed-level light regime did not exhibit shrinkage delay. Both di-methacrylate and multi-acrylate composites exhibited increases in the range 18-29% in final equilibrium shrinkage-strain by increase of specimen temperature. SIGNIFICANCE: Reductions in the rate of initial shrinkage of light-cured restoratives may have clinical benefits for restoration bond-integrity nearly as important as reductions in the final equilibrium shrinkage-strain. This may be achieved either by special light irradiation regimes (low to high, or ramped) or in favourable cases by novel monomer-composite formulations and setting chemistry (e.g. Solitaire). The bonded disk shrinkage-strain measurement technique is suitable for the elucidation of such rapid kinetic and temperature-dependent events during photo-polymerisation setting processes.

Acrylic Resins↗

PubMed1999

Comparison of two stylus methods for measuring surface texture.

OBJECTIVES: The purpose of this study was to evaluate and compare two "stylus" methods for measuring surface texture of dental tissues and materials. METHODS: The two styli chosen were a contact diamond stylus and a non-contact focussed laser stylus attached to the same measuring apparatus. RESULTS: These indicate that there are significant differences between those obtained from surface texture measurements using a non-contact laser stylus and a diamond contact stylus method despite being mounted in the same profilometer. This is valid for both the test specimens of known surface texture, provided by the manufacturers, and for a "real world" simulation using contoured and finished Dicor ceramic blocks. The only significant agreement between the two styli was found for the Ra parameter. This should not be used alone to describe the roughness of a surface because the parameter is not sensitive to profile shape. Owing to the properties of the stylus used it is essential that the limitations of surface profilometry be recognised. SIGNIFICANCE: Caution should be exercised when comparing the results of surface texture studies of dental hard tissues and restorative materials using varying types of stylus attached to a surface profilometer.

Dental Equipment↗

PubMed1999

Creep and visco-elastic recovery of cured and secondary-cured composites and resin-modified glass-ionomers.

OBJECTIVE: The aim of this investigation was to study whether secondary curing had any effect or not on the visco-elastic stability of resin-based restoratives. MATERIALS: Five resin composite restorative materials (Tetric, Z100, Lite-fil, Palfique, Graft LC) and two resin-modified glass-ionomers (RM-GIC) (Fuji II LC&Photac-Bond), were chosen as representative of those clinically available. METHODS: Specimens were fabricated as short cylinders (6 mm x 4 mm), with and without secondary oven cure (120 degrees C, 7 min). A creep measurement apparatus was used to subject each specimen in turn to a cycle of constant compressive stress of 50 MPa for 6 h followed by 6 h of strain recovery, after load removal. Time-dependent creep and recovery were recorded. RESULTS: The creep curves of these VLC materials during loading and unloading were characterised by an initial rapid elastic deformation, greater than 1%, followed by a visco-elastic response. During unloading there was an initial elastic recovery, followed by the creep recovery, and finally permanent set. Secondary-cure had no statistically significant effect (p > 0.05) on maximum creep strain (Y1) which was 1-2% for resin-composites and 3-4% for RM-GIC. Permanent set for the RM-GIC were 0.68-0.77% and for resin-composites ranged from 0.13-0.46%. The influence of secondary-cure on creep recovery and permanent set was limited. However, some materials (Z100. Lite-fil and Photac-Bond) had permanent set significantly reduced (p < 0.05) by application of a second cure. SIGNIFICANCE: Visco-elastic creep values of less than 2% are acceptable for composites at the stress-levels applied. However the two-fold increase in creep response by RM-GIC suggests that these materials are unsuitable for stress-bearing areas.

Composite Resins↗

PubMed1999

Mechanical properties of direct core build-up materials.

OBJECTIVE: This work was undertaken to measure mechanical properties of a diverse group of materials used for direct core build-ups, including a high copper amalgam, a silver cermet cement, a VLC resin composite and two composites specifically developed for this application. METHODS: Compressive strength, elastic modulus, diametral tensile strength and flexural strength and modulus were measured for each material as a function of time up to 3 months, using standard specification tests designed for the materials. RESULTS: All the materials were found to meet the minimum specification requirements except in terms of flexural strength for the amalgam after 1 h and the silver cermet at all time intervals. SIGNIFICANCE: There proved to be no obvious superior material in all respects for core build-ups, and the need exists for a specification to be established specifically for this application.

Analysis of Variance↗

PubMed1999

Rheology of urethane dimethacrylate and diluent formulations.

OBJECTIVE: The purpose of this study was to measure the viscosity of resin formulations and investigate the relationship between diluent percentage and temperature. METHODS: Resin formulations were prepared using UDMA and either TEGDMA or HEMA, or HPMA, as diluents at different percentages. Viscosities were obtained with a Bohlin Rheometer at 23 degrees C for all formulations, and at 37 and 60 degrees C for selected formulations. RESULTS: The viscosity of the formulations exhibited Newtonian behaviour and decreased as the percentage of the diluent monomer increased, especially up to 40% of diluent. Linear regressions relating log(viscosity) and mass percent diluent, were obtained in the range 20-80% diluent. Viscosity also decreased as the temperature increased. Activation energies were calculated from Arrhenius plots, and ranged from 16.2-23.4 kJ mol-1. SIGNIFICANCE: The viscosity of the resins controls the flow properties and thus has a significant effect on the deployment of the respective monomers.

Methacrylates↗

PubMed1999

Residual monomer concentrations in denture-base acrylic resin after an additional, soft-liner, heat-cure cycle.

OBJECTIVES: To determine the effects on methylmethacrylate (MMA) monomer concentration of a second heat-cure cycle introduced for the purpose of processing a denture soft-liner. METHODS: Two denture-base resins (Lucitone199 and Trevalon; Dentsply International, York, PA, USA) were selected. Heat-cure cycles were S (73 degrees C/90 min + 100 degrees C/30 min); L (73 degrees C/9 h); SM (S + 100 degrees C/2 h); LM (L + 100 degrees C/2 h). Cycles SM and LM thus simulated the effect of a further, soft-liner, heat-cure cycle. Groups (n = 6) of the specimens of the denture-base were prepared in the form of thin polymer disks. Monomer was then obtained from the broken fragments of disks by acetone extraction. Concentrations of MMA monomer were determined by reversed-phase high performance liquid chromatography (HPLC). Two-way ANOVA and independent sample t-tests were used to compare the results. RESULTS: The cycle S, applied to Lucitone199, gave a mean concentration of 1.95 wt% MMA (SD 0.24). The cycle L gave the highest level of residual concentration of MMA monomer in both materials. Cycles SM and LM, applied to Trevalon and Lucitone199, gave significant reductions of residual concentration of MMA monomer (p < 0.05). SIGNIFICANCE: The HPLC method was suitable for determining the amount of residual monomer in the denture-base acrylic resin. A further (soft-liner) heat-cure cycle had a statistically significant effect on reducing residual monomer concentrations, and this may have an effect upon mechanical properties.

Acrylic Resins↗

PubMed1999

Aluminium radiopacity standards for dentistry: an international survey.

OBJECTIVE: The aims of this study were to determine the appropriate composition and purity limits of aluminium alloy radiopacity standards for dental biomaterials, taking into account the current status in representative laboratories worldwide, and to formalise the calculation procedure for processing radiopacity data. METHODS: A series of aluminium step-wedges were obtained from academic and industrial research laboratories. These were analysed by energy dispersive X-ray procedures. The set of step-wedges were independently characterised for radiopacity at two University dental schools. RESULTS: The percentage of aluminium together with the percentages of minor alloying elements (magnesium, manganese, iron and silicon) have been determined as mass percentages. For each centre, the optical density versus step-height for all wedges was judged to fall on a common curve, with the main exception of one step-wedge that was found to contain 4% copper, by mass. A suitable calculation procedure was described explicitly. CONCLUSION: Of the examined representative step-wedges, all but the aluminium--4% copper material were satisfactory and gave results comparable with wedges of high purity. Limits should therefore be revised for composition of aluminium radiopacity step-wedges or test blocks to a more realistic level of purity. Alloys with more than 0.05% copper or 1.0% iron should be excluded and the aluminium content should be at least 98% by mass. In the calculations deployed for deriving radiopacity as 'equivalent aluminium thicknesses', an appropriate linear regression procedure should be employed and considerable care taken in the evaluation of materials of low or borderline radiopacity, relative to specification limits. The method described is principally suitable for materials with radiopacity greater than 1 mm. For materials with radiopacity less than 1 mm aluminium, thicker specimens (2 mm) should be used.

Aluminum↗

PubMed1999

Surface fine structure of treated dentine investigated with tapping mode atomic force microscopy (TMAFM).

OBJECTIVES: The objective of this study was to investigate the surface morphology and roughness of human dentine treated with different dentine conditioning agents with tapping mode atomic force microscopy (TMAFM). METHODS: Dentine samples from third permanent molar teeth were prepared as flat disks. The samples were treated with different conditioning agents for 15-30 s: aqueous phosphoric acid and those supplied with three commercial adhesive products. The surface morphology and roughness was measured with tapping mode atomic force microscopy, a highly resolving method which minimises unwanted side effects of conventional AFM. RESULTS: The tubules, the areas between the tubules and the entire surface showed variations in surface morphology and roughness, depending on the conditioning agent used. These differences might affect the bonding properties of the agents in dentine/polymer systems. CONCLUSIONS: The surface structure, such as the tubules diameters and depths, the local height differences of the surface between the tubules and the overall mean surface roughness Ra varies between samples treated with different conditioning agents. All conditioning agents, however, led to exposed and open tubules. CLINICAL RELEVANCE: The direct observation of effects of conditioning agents on dentine is important in order to judge their performance under clinically relevant conditions-free of desiccation artefacts. A vast number of conditioning agents is available today. The differences in their actual effects on dentine surfaces, however, are less than clear to many users.

Acid Etching, Dental↗

PubMed1999

Dental materials: 1997 literature review.

This review of the published literature on dental materials for the year 1997 has been compiled by the Dental Materials Panel of UK. It continues a series of annual reviews started in 1973. Emphasis has been placed upon publications, which report upon the materials science or clinical performance of the materials. The review has been divided by accepted materials classifications (fissure sealants, glass polyalkenoate cements, dentine bonding, dental amalgam, endodontic materials, casting alloys, ceramometallic restorations and resin-bonded bridges, ceramics, denture base resins and soft lining materials, impression materials, dental implant materials, orthodontic materials, biomechanics and image processing, resin composites, and casting investment materials and waxes). Three hundred and thirty three articles have been reviewed.

Dental Materials↗

PubMed1999

Structural conformation of in vitro and in vivo aged orthodontic elastomeric modules.

The objectives of this study were to investigate the structural characteristics of open and closed elastomeric modules in the as-received condition, and following (a) 24-hour in vitro loading determined as a 50 per cent extension relative to their original length; (b) 24-hour intra-oral exposure; (c) 3-week intra-oral exposure. Ten specimens of each type (open and closed) of the three brands of elastic chains were included in the study (total of 60). All specimens were subjected to (a) optical transmission microscopy utilizing phase contrast and polarized light modes, (b) micro MIR FT-IR spectroscopy, and (c) scanning electron microscopy and electron probe X-ray microanalysis. Stretched elastomers demonstrated a honeycomb pattern of filament detachment corresponding to strained areas. The chains possessing a well-differentiated inter-modular link presented higher strain concentration gradients located in the link. Phase contrast imaging revealed that the stress adsorption mechanism of these materials involved macromolecular chain orientation and elongation co-axially to the applied load. The specimens retrieved after 24 hours had acquired a proteinaceous film rich in alcohol groups with minimal evidence of phosphate mineralization, while the 3-week retrieved specimens presented precipitation of Ca- and P-forming calcium phosphates. The high protein content of the biofilm organized on the surface of these materials, as well as the calcification pattern found, were similar to a non-specific mechanism of film adsorption of biomaterials exposed to body fluids. The results have clinical implications for aspects of retraction control through sliding mechanics with the use of elastomeric ligatures, and the potential detrimental effects on dental and periodontal tissues such as decalcification and gingival inflammation, respectively. In addition, the experimental protocols adopted for the in vitro study of the force decay of elastomerics should be modified to include parameters pertinent to adsorption phenomena and calcification processes documented to occur in vivo.

Biofilms↗