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Biomedical subjects

D C Watts

Publications and source records attributed to D C Watts.

At least 19 recordsLinked to original sources
PubMed1995

Mechanical behaviour and structure of light-cured special tray materials.

OBJECTIVES: This study relates some structural characteristics of representative special tray visible light cured (VLC) resin composites to their creep-recovery, hardness and flexural strength properties. VLC prosthodontic materials have not yet achieved the same acceptance as the VLC resin composites, perhaps because less is known of their structural and performance characteristics. METHODS: The VLC resin composites used were Triad, Convertray and Palatray. The inorganic composition was analysed quantitatively by heating known amounts of resin composite to 800 degrees C, and weighing the ash. Elemental composition was determined using X-ray microanalysis. Mechanical properties, including hardness, flexural behaviour and creep recovery, were obtained by standard methods. RESULTS: The inorganic filler content for Palatray was higher than that of Convertray or Triad, and the flexural modulus and flexural strength of Palatray were superior to that of Triad or Convertray. The mean maximum creep strain for the Palatray specimens was only 2.45% after 5 h loading under 37 MPa and there was a permanent set of 1.5% after 7 h recovery. CONCLUSIONS: Palatray exhibited superior overall properties and this evidently related primarily to the high volume fraction of filler.

Aluminum↗
PubMed1995

Comparison of methods for measuring surface roughness of ceramic.

The aim of this investigation was to compare two methods of assessing the surface texture of finished dental ceramic; a laser reflectivity method (LSR) and a contact stylus tracing method. Identical ceramic blocks (Dicor MGC) were finished using a variety of techniques and devices, and the surface texture characterized by LSR and contact stylus tracing to enable comparisons to be made between the two methods. The results indicate that there is little correlation between the two measuring methods. Therefore, laser reflectivity should not be exclusively used to measure the surface texture of contoured and finished ceramic materials. The development of sophisticated surface characterization parameters suggest that the surface characteristics of dental restorations and other related surfaces should be described using more than one surface measurement parameter. Surface parameters should be chosen which can both quantify surface roughness and provide information on the shape of the surface under investigation.

Dental Polishing↗
PubMed1995

Development and assessment of an objective method of colour change measurement for acrylic denture base resins.

Current standard specifications for the laboratory evaluation of denture base materials require subjective assessment of colour stability. This study evaluates a new objective measurement technique for translucent denture base materials, by comparing quantitative data with the results obtained from a standard subjective method. Preliminary work on three representative commercial acrylic materials resulted in the selection of a white background for the quantification of colour change of these translucent materials. One half of each sample was exposed to artificial sunlight for 24 h. The colour of both exposed and unexposed resin was measured on the CIE L*a*b* scale using a photo-electric colorimeter. Significant changes in the b* and E* values (P < 0.01) were observed following light exposure. Objective data for a wide range of commercial materials were compared with the results of a subjective evaluation and it was shown that a change in b* of 1.5 units was discernable subjectively by eye for 100% of the cases. It is suggested that this method and limit value could form the basis of a new objective colour change specification for polymeric dental materials.

Acrylic Resins↗
PubMed1995

Indirect composite preparation width and depth and tooth fracture resistance.

PURPOSE: To evaluate the influence of preparation width and depth on the fracture resistance of teeth restored with one type of indirect composite restoration. MATERIALS AND METHODS: Variations of depth and width were superimposed on a preparation of dentin design and dimensions with other variables including tooth size and morphology being controlled. Indirect composite restorations were constructed and placed in accordance with manufacturer's instructions. The restored teeth were subjected to compressive loading with fracture being induced by application of a 4 mm steel rod to the specimens in a universal testing machine, at a cross-head speed of 1 mm/minute. RESULTS: Preparation depths of 3 to 4 mm were found to be preferable, but when the prevalence of severe tooth fracture was assessed, the shallower preparation depth was found to be indicated. Cavity depths of one-third and one-half bucco-lingual width were evaluated, the fracturing forces for the group with the narrower width being statistically lower.

Analysis of Variance↗
PubMed1994

Composite inlays.

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Composite Resins↗
PubMed1994

Aesthetic inlays.

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Composite Resins↗
PubMed1994

A review of dental injuries and the use of mouthguards in contact team sports.

Dental injuries are the commonest type of orofacial injury sustained during participation in sports. Of the types which are currently available, custom-made mouthguards fitted by a dentist have been demonstrated to provide the greatest protection from dental injuries. Such mouthguards should therefore be recommended for use by those who participate in contact sports, and their provision should be actively encouraged by dental care workers.

Athletic Injuries↗
PubMed1994

Analysis of optical transmission by 400-500 nm visible light into aesthetic dental biomaterials.

The penetration of visible light into dental biomaterials is an essential factor in photoinitiation of setting reactions and in the optical aspects of dental aesthetics. Light of visible blue wavelengths, 400-500 nm, has been applied at normal angles to 0.2-5.0 mm sections of human dentine and representative ceramic, polymerceramic composites and hybrid glass-polyalkenoate materials. The integrated optical transmission has been determined for each material section. The data have been converted to absorbance values and analysed to check for mathematical conformity to the Beer-Lambert Law. It is found that conformity (typically, P < 0.01) to the linear Beer-Lambert Law is only attained by making a substantial correction for the intensity of light reflected from the surface of aesthetic biomaterials. This is otherwise expressed by distinguishing between true and apparent absorbance. From linear regression of apparent absorbance with section thickness, the intercept depends upon the logarithm of the surface-reflection ratio. This factor ranges from 30% to 90% in the materials investigated. It follows that there is a high degree of inefficiency in the transmission of visible light into and through aesthetic biomaterials for the purposes of photoactivation using existing technology. Means by which this limitation and inefficiency may be reduced are discussed. While the reflectivity of aesthetic biomaterials has been perceived by dental practitioners, the magnitude of this effect and its implications in connection with light-cured materials have not been analysed and emphasized hitherto.

Absorption↗
PubMed1994

Elastic moduli and visco-elastic relaxation.

The elastic moduli of dental biomaterials are discussed in relation to the various intrinsic and extrinsic factors that can influence the magnitude of these parameters and hence the extent of modulus-matching with oral tissues. The time-dependent visco-elastic response is shown to be an important feature of both natural and many synthetic biomaterials. This modifies the stress magnitudes sustained in the restored unit, especially when polymeric chains or networks are a major constituent of the overall structure. An understanding of both moduli and visco-elasticity is valuable in the appraisal of clinical performance of biomaterials.

Acrylic Resins↗
PubMed1994

Hybridization of matrix-bound MM-creatine kinase with BB-creatine kinase and arginine kinase.

Dimeric rabbit muscle creatine kinase (MM-CK) was bound to CNBr-activated Sepharose 4B by one of its subunits (MM-CKA). Treatment of MM-CKA with guanidine hydrochloride released the unbound subunit to yield the matrix-bound monomer (M-CKB). M-CKB recombined with dissociated MM-CK soluble subunits to reconstitute a matrix-bound dimer (MM-CKC). M-CKB also associated with dissociated subunits of BB-CK from crude extracts of rabbit brain and of arginine kinase from sea cucumber muscle (MM-AK) to form the matrix-bound heterohybrids MB-CKC and M-CK/M-AKC, respectively. Guanidine hydrochloride gradient elution studies showed that MM-CKA, MM-CKC and MB-CKC were all dissociated at the same concentration of the denaturant (0.96 M), while the M-CK/M-AKC heterohybrid was less stable, dissociating at 0.5 M. The specific interaction between subunits of echinoderm and mammalian phosphagen kinases to form a hybrid enzyme of dual substrate specificity supports the view that these enzymes had a common evolutionary origin.

Animals↗
PubMed1994

Fracture resistance of teeth restored with dentin-bonded crowns.

While dentin-bonded crowns may hold esthetic advantages over metal-ceramic restorations, and minimal tooth preparation may also be appropriate, resistance to fracture of the restored unit is an important consideration. Dentin-bonded crowns were placed on standardized preparations in two groups of 10 maxillary premolar teeth in vitro. The preparations differed only in degree of taper. The restored specimens were subjected to compressive loading at 1 mm/min from a steel bar placed along the midline fissure. Mean fracture loads of 0.77 kN (6-degree taper) and 0.71 kN (8-degree taper) were recorded for the groups of prepared teeth, and a mean fracture load of 0.97 kN was obtained for a group of unprepared controls. Statistical analysis indicated that there was no significant difference between the fracture load of the 6-degree taper group and that of the sound control teeth.

Analysis of Variance↗
PubMed1994

Fracture resistance of teeth restored with indirect composite resins: the effect of alternative luting procedures.

Some variations in luting technique from those recommended by manufacturers may hold clinical advantages, but it is also necessary to evaluate other parameters of the performance of the restored unit. Among these is fracture resistance. Accordingly, indirect composite resin restorations were placed in standardized preparations under standardized conditions. Restorations were placed in three groups of 10 teeth, each group with a different luting procedure: luting with a composite resin restorative material, conventional luting but without enamel etching, and luting with a 4-META material. In another group the restorations were luted in accordance with the manufacturer's recommendations, while two groups of sound teeth acted as control. Following compressive loading on a universal testing machine, the force required to cause fracture was recorded. The results indicated that optimal fracture resistance of teeth restored with indirect composite resin restorations was obtained when the 4-META-based luting material was used.

Acrylic Resins↗
PubMed1993

The effect of time of trimming on the surface finish of anterior composite resins.

This in vitro study investigates whether the surface smoothness of a finished composite resin is a function of the time at which it is trimmed. Five materials were studied. Visiodispers (VIS) is a sintered microfine, the remainder were hybrid formulations: Miradapt (MIR), Command Ultrafine (COM) and two experimental formulations-'EXP (2)' and 'EXP (15)'-with larger filler particles. All except MIR were photo-activated. Samples were placed and cured in holes machined in Perspex rods, ensuring excess of material and moulded to shape with cellulose acetate matrix strip. The strips were removed immediately after light curing and after 5 min in the case of MIR. Sample groups were finished after the following intervals: 10, 15, 30, 45s, 1, 2, 3, 4, 5 min, dry, and after 1 and 7 days in 37 degrees C water. The finishing procedure used plain-cut tungsten-carbide burs and coarse, medium and fine finishing discs, which were applied for standardized times, directions and pressures. Surface roughness was assessed visually and with a surface profilling instrument. COM and VIS materials significantly improved in surface finish when the procedure was delayed for 7 days. However, EXP (2), EXP (15) and MIR surface finishes were independent of finishing time. This may result from a rapid attainment of optimum surface hardness, whereas with COM and VIS incomplete curing leads to smearing of resins and filler across the surface.

Composite Resins↗
PubMed1992

Bonding characteristics of a phosphonated anaerobic adhesive to amalgam.

The shear bond strength of a phosphonated anaerobic resin dental adhesive (Panavia-Ex) to amalgam was determined and clinically important parameters affecting the bond strength: amalgam type, surface finish and corrosion, were investigated. It is possible to bond to high copper set amalgam using this adhesive with a clinically useful shear bond strength. Three copper-enriched, single composition alloys were studied. One was a spherical alloy and two were lathe cut. The bond strength to one of the lathe-cut alloys was significantly lower than to the other two alloys. A significantly higher bond strength to aged lathe-cut amalgam than to fresh amalgam was found. However, Weibull analysis indicated that these latter changes were less consistent and indicated patchy, localized changes in the amalgam surface. The bond strength to the spherical alloy was significantly reduced by polishing with a finishing bur or Prophy-Jet. Therefore when bonding to a large amalgam restoration with such a phosphonated anaerobic adhesive, no polishing of the amalgam should be undertaken.

Adhesiveness↗
PubMed1991

Current status and rationale for composite inlays and onlays.

Composite inlay systems, now available to the dental profession, are classified and evaluated. Principle motives for their deployment are the aviodance or reduction of in situ polymerisation shrinkage and the optimisation of restoration contour and adaptation. The criteria for placement of direct or indirect composite inlay restorations are critically reviewed in relation to alternative materials and operative procedures. Dimensions of typical molar and premolar inlay cavities are presented and interim guidelines for inlay cavity design are discussed.

Composite Resins↗
PubMed1991

Determination of polymerization shrinkage kinetics in visible-light-cured materials: methods development.

An instrument for the reproducible measurement of polymerization shrinkage kinetics is described, constructed around a disc-shaped specimen sandwiched between two glass plates. Test specimens of light-sensitive dental restorative materials were irradiated through the lower, rigid plate. The upper, non-rigid plate was readily deflected by an increase of the adhesive stress from the polymerizing and shrinking sample. Deflection was measured by an LVDT transducer and computer-recorded. Dimensional changes were confined to the specimen disc-thickness dimension, such that the fractional linear shrinkage approximated the volumetric shrinkage. Shrinkage data are reported for representative materials: unfilled and resin composites, base-lining materials, and an impression material. Equilibrium shrinkage magnitudes ranged from 0.65%, for the impression material, to 7.9% for the unfilled resin. The kinetic behavior was approximately characterized by an overall time constant, ranging from 12.5 to 280 s, associated with an exponential growth curve, although the initial shrinkage was near-linear in time, for many materials, due to non-steady-state concentrations of polymer free-radicals. The test-specimen geometry facilitates rapid and essentially uniform cure and hence the determination of minimum possible time-constants at each ambient temperature and incident light-intensity. Study of hybrid glass-ionomer materials, without spurious dehydration effects, was also achieved.

Acrylic Resins↗
PubMed1991

Light cured direct bonding--is it necessary to use a primer?

A study was undertaken to assess the influence of a low viscosity unfilled 'primer' resin upon the shear bond strength of a bracket adhesive combination to etched enamel and at the separate interfaces of the bonding system, namely the bracket/adhesive and the enamel/adhesive interfaces. No significant differences were detected between the bond strengths when an unfilled resin phase was utilized (P greater than 0.05).

Acid Etching, Dental↗