An apparatus for measuring the rheological properties of dental materials.

An indirect extrusion capillary viscometer has been developed. This has been tested for nonsetting Newtonian fluids and was found to give results close to, but slightly lower than the actual viscosity. The same apparatus has been successfully applied to a non-Newtonian fluid to determine the dependence of viscosity on shear rate. The technique described should meet the requirements for assessing the rheological characteristics important in the mixing and setting of dental materials. The developed viscometer must be coupled with a sensitive mechanical testing machine capable of an adequate range of crosshead speeds that can be changed rapidly. By obtaining force vs time curves at different shear rates for setting materials, viscosity can be calculated as a function of time. Also, the viscosity at any given time during the setting process can be calculated as a function of shear rate. This chould be of aid in the interpretation of changes in rheological properties during setting of dental materials.

The rheological characteristics of elastomeric impression materials.

An indirect extrusion viscometer has been used to assess the shear rate dependence of viscosity of a range of pastes supplied for elastomeric impression materials. A suggested specification for these pastes has been proposed.

A new single-step technique for apical retrofilling that significantly reduces microleakage.

One of the properties of an ideal retrofilling material is little or no microleakage. A new dentin bonding agent based on polyacrylic acid (PAA) is ideally suited to the moist apical environment because of its hydrophilic nature. PAA as a dentin bonding agent for apical retrofillings was tested in an in vivo leakage study using single canal roots of extracted human teeth. Bonding was attained by pretreatment of the dentin in apical retrofilling preparations with a solution of PAA, followed by placement of a nonaqueous restorative material. Using PAA as dentin bonding agent reduced apical leakage at statistically significant levels with Super EBA as a restorative material. In vivo testing is required to validate these findings.

Early strength and adaptability of amalgam in relation to coherence time.

The minimum mixing time required to give a coherent mix of a dental amalgam ("coherence time") was determined for eleven representative materials using two different amalgamators. The effect of mixing these amalgams at various multiples of their coherence time on early compressive strength and adaptability was determined. It was found that the two amalgamators gave different coherence time values for the alloys, but that the strength data for any given alloy was similar for a given multiple of coherence time. Longer mixing tended to yield amalgams of greater early strength, but reduced the adaptability of the mixed material.

Magnetron sputtered tin coatings on unbiased ceramic substrates.

OBJECTIVES: The aim of this work was to study the application of tin oxide coatings on alumina, with a view to developing improved bonding for high-alumina dental ceramics. METHODS: By use of a magnetron sputtering technique, alumina was coated with tin which was subsequently oxidized, and the bond strength of coated surfaces to a resin-based dental cement was measured in tension. Tensile bond strength data were assessed for their degree of skewness and the homogeneity of the residual variances with a view to applying appropriate transformations prior to performing ANOVA with subsequent Tukey's analysis as necessary. The color of coatings, as a function of the amount of oxidation of the surface, was noted. Coatings were characterized by SEM and EDAX. It has been found possible to produce esthetic tin oxide coatings on alumina using a simplified magnetron sputtering apparatus in conjunction with post-deposition oxidation. RESULTS: Although mean tensile bond strength (TBS) values in excess of 15 MPa were recorded for a number of groups, no causal relationship was found between coating time and mean TBS. SIGNIFICANCE: The need exists for improved bonding of alumina-based ceramics; further development of a magnetron sputtering technique may provide the means for achieving this.

Rheological properties of vinyl polysiloxane impression pastes.

OBJECTIVES: Some vinyl polysiloxane impression (VPS) materials are claimed to be 'thixotropic' so that they will have 'non-drip' properties and not flow off an impression tray during insertion of the material into the mouth. This work was carried out to investigate rheological properties of VPS materials by determination of their flow properties under different experimental conditions relevant to their dental application. METHODS: The rheological characteristics of each component paste of two low viscosity VPS materials were assessed by a Dynamic Stress Rheometer using cone and plate geometry. RESULTS: Both pastes of one product (Imprint II) showed a yield stress of around 40 Pa in addition to some degree of true thixotropy. In contrast, both Examix pastes had no yield stress value, but the catalyst paste was markedly thixotropic, as shown by a drop in viscosity as a result of application of shear force over time. Both products had thixotropic properties. The Imprint II in addition showed a yield stress that would also contribute to the 'non-drip' properties. SIGNIFICANCE: Combination of yield stress and thixotropy will prevent the undesirable drip of the materials once it has been injected around the tooth preparation until the impression tray is loaded and seated. After seating the tray, a thixotropic material will not flow.

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.

Tin oxide coating of aluminous porcelain by reactive ion plating.

Alumina reinforced dental porcelain has been coated directly with tin oxide by reactive ion plating. Samples were prepared at different distances from the tin source in the ion plating rig. Tensile bond strengths of treated and untreated porcelain discs to a commercially available phosphate-methacrylate based dental cement were determined. Bond strengths of certain coated samples were found to be in excess of the cohesive strength of the porcelain substrates (greater than 7.8 MPa), whereas untreated porcelain achieved an average bond strength of only 3.4 MPa. The microstructures of coatings produced under conditions similar to those which yielded the maximum bond strength were examined in a scanning electron microscope and were found to be approximately 0.5 microns thick. It is believed that ion plating has great potential for rendering inert ceramic surfaces capable of direct bonding to dental cements.

Fracture resistance of five pin-retained core build-up materials on teeth with and without extracoronal preparation.

Core build-ups should provide satisfactory strength and resistance to fracture both before and after crown preparation. This paper examines the resistance to fracture of core build-ups in different materials and the fracture resistance of core build-ups when these have been reduced for full crown preparation. Standardized core build-ups were made on groups of extracted molar teeth of similar size, with 10 teeth per group. Three resin-composite (prisma APH: Dentsply, Weybridge, UK; Ti-Core, Essential Dental Systems, NJ, US and Coradent, Vivadent, Liechtenstein), one cermet (Ketac-Silver, ESPE GmbH, Seefeld, Germany) and one amalgam material (Duralloy, Degussa Ltd, Cheshire, UK). These specimens were subjected to compressive force on a universal testing machine and the force at fracture noted. Standardized full crown preparations were made on a further five groups of core build-up specimens using the same materials as above. These prepared specimens were subjected to compressive force on a universal testing machine and the force to fracture noted. The results indicated that amalgam core build-ups demonstrated higher fracture resistance than the other materials examined. There was a general decrease in the fracture strength of the specimens following crown preparation, with the teeth restored with the amalgam core build-ups showing a greater percentage reduction in fracture strength than the other materials tested. Prepared core build-ups in a hybrid composite material provided the highest fracture resistance. The cermet material used provided the lowest resistance to fracture in both the core build-up and crown preparation specimens. In terms of fracture resistance, no advantage was apparent in using the two composite materials designated as being specifically appropriate for core build-ups.

Effects of restorative and adhesive curing methods on dentin bond strengths.

PURPOSE: To evaluate the shear bond strengths of "one-bottle" adhesives to dentin when used with self-cured composite. MATERIALS AND METHODS: 100 bovine teeth were ground to expose superficial dentin, which was polished to 600-grit. The dentin was etched for 15 s with 35% phosphoric acid, rinsed, and blotted with tissue paper. The surface remained visibly moist for all specimens. The following adhesives were applied to the etched dentin according to manufacturers' directions: Scotchbond Multi-Purpose (in its visible light-cured [VLC] and self-cure modes) as a control, One-Step (VLC only), OptiBond Solo Plus (VLC only), Prime & Bond NT (VLC) and Prime & Bond NT Dual-Cure. For each adhesive, specimens were bonded with a self-cure composite, while 10 were bonded with a VLC composite as a control. After 24 hrs water storage, shear bond strengths were determined using an Instron universal testing machine. Data were analyzed using two-way ANOVA, Tukey's test, and Student's t-tests. RESULTS: Using the light-cured composite, mean bond strengths ranged from 11.0 MPa for light-cured Prime & Bond NT to 26.3 MPa for OptiBond Solo Plus. Using the self-cure composite, mean bond strengths ranged from 0.0 for light-cured Prime & Bond NT to 21.4 MPa for One-Step. Two adhesives, light-cured Prime & Bond NT and OptiBond Solo Plus, had significantly lower bond strengths when used with the self-cure composite. CLINICAL SIGNIFICANCE: Some "one-bottle" dentin adhesives may not provide acceptable bonding to tooth structure when they are used with self-cure composites.

Development of two new lining/base materials based upon calcium hydroxide/polyacrylic acid.

Two experimental adhesive calcium hydroxide cements containing polyacrylic acid were formulated and tested. These experimental materials were stronger than a proprietary calcium hydroxide formulation and were capable of adhesion to dentine. The properties of these new formulations indicated that preliminary biological testing is warranted.

Pulp response to a novel adhesive calcium hydroxide based cement.

This study compares pulp responses to 3 formulations of calcium hydroxide, namely: a) An experimental adhesive calcium hydroxide cement containing polyacrylic acid, b) Dycal (L.D> Caulk Co, Milford, Delaware) Batch Nos 176970/176990, c) "Analar" calcium hydroxide mixed with sterile distilled water. After 28 days dentine bridges were present in 77% of teeth capped with the test material, 64% of teeth treated with Dycal and in 62% of teeth capped with calcium hydroxide and water. Inflammatory infiltrates were observed in a number of teeth remote from the bridges. Bacteria were detected in these specimens. Exposed rat molar pulp responses to an experimental adhesive calcium hydroxide cement were similar to to those observed with 2 other calcium hydroxide formulations.

Present and future aspects of materials in dentistry.

The success of most forms of dental treatment is dependent on the wise choice and good manipulation of materials. A vast range of new products has become available, leading to types of treatment that were not dreamed of a generation ago. This paper reviews those products and procedures.

Development of new adhesive pulp capping materials.

The pulp capping materials currently available do not adhere to dentine but cover it and therefore extensive removal of tooth tissue is necessary to expose sufficient dentine for the adhesives used to secure the restoration (the adhesives do not adhere to the capping material). The authors describe how two new pulp capping materials have been developed, which combine the properties required for pulp capping with an ability to adhere to dentine.

Contemporary resin-based composite materials for direct placement restorations: packables, flowables and others.

Resin-based composite (RBC) materials are composed of principal and diluent monomers, fillers, coupling agents and other components. These materials have been developed in recent years to produce packable and flowable materials, and materials which may release ions under specific conditions and which may contain fibres. Materials which are organic-inorganic copolymers have also become available. Long term clinical trial data are not available for the most recently introduced materials.

The future of dental materials.

This article reviews some of the rapid developments in dental materials that have occurred during the last 25 years, especially adhesive, biocompatible and aesthetic materials. Some innovative research developments, which have exciting possibilities for future dental treatment, are outlined.

Dentine bonding systems: 1. Mode of action.

The achievement of a reliable bond to dentine has been more difficult than bonding to enamel, due principally to the anatomy of the dentine substrate. The mechanisms of bonding are now much better understood than a decade ago and clinically-viable bond strengths to dentine are now achieved with many dentine bonding systems.

Dentine bonding systems: 2. Clinical uses.

As the reliability of dentine bonding agents has improved, the clinical uses of these systems has increased and now include the bonding of directly and indirectly constructed restorations, amalgam bonding and treatment of dentinal sensitivity. Areas of potential use which require further evaluation include pulp capping and the attachment of fractured tooth fragments.

Assessing restorative dental materials: 2. Questions for a dental sales representative.

Many methodologies are used during the testing of dental materials, although different tests are suitable for different materials and clinical situations. This paper discusses factors relevant to the choice of a dental material, and suggests questions that a practitioner would find helpful to ask a dental sales representative, such as ease of use, effect on the environment and the relevance of fluoride release.

Assessing restorative dental materials: I. Test methods and assessment of results.

Many methodologies are used during the testing of dental materials. Among these are compressive, tensile and flexural strengths, and fracture toughness. However, different tests are relevant to different materials and clinical situations. This paper describes different test methodologies and discusses the substantiation of research claims in publications and advertising.