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Biomedical subjects

J F McCabe

Publications and source records attributed to J F McCabe.

At least 109 records · Page 6Linked to original sources
PubMed1993

The use of Weibull statistics in mechanical testing of brittle dental materials.

The mean strength that has traditionally been taken as a measurement of the strength of a material does not reflect the true strength, and therefore it cannot be used as a design parameter. This explains why many brittle materials fail at unpredictable stress, either below or above the mean strength. By using Weibull statistics, the prediction and assessment of strength can be made more sensibly. The performance of a material can be predicted by considering a stress at a lower level of failure probability.

Dental Materials↗
PubMed1992

Subsurface deformation associated with hardness measurements of composites.

This study was prompted after a series of composites had been hardness-tested with the Vickers (pyramidal diamond indenter) and Barcol tests (flat-ended steel cone indenter). The results were found to differ markedly from the supplied Barcol/Vickers conversion chart. A sectioning and silver staining technique was therefore developed to examine the modes of subsurface deformation of four composites under the two indenters. Under the Vickers indentations, the large particle hybrid, Occlusin, showed a clear hemispherical area of subsurface disruption while the small particle hybrid, Brilliant Dentin, had a less distinct zone of disruption. The microfilled materials, Heliomolar and Isosit, showed minimal silver uptake, suggesting a less disruptive mode of plastic deformation. Under the Barcol indentations, Occlusion and Brilliant Dentin produced clear hemispherical zones of deformation, while Heliomolar and Isosit were badly crazed with distinctive patterns of subsurface flaws. These consistent patterns of failure to some extent explain the comparison of hardness measurements and may be of use in explaining the wear mechanisms of composite restorations.

Acrylic Resins↗
PubMed1992

Effect of different storage methods on the performance of dental local anaesthetic cartridges.

Self-aspirating dental local anaesthetic syringe systems rely on deformation of rubber diaphragms contained in the anaesthetic cartridges to produce the aspirating force. A test has been developed which can examine the performance of these rubber diaphragms under functional conditions. This method could be useful in the production of standards for dental local anaesthetic cartridges. The test has been used to determine the influence of storage conditions on the performance of dental local anaesthetic cartridges. The results show that, with the cartridges investigated, storage at 37 degrees C increased the amount of distortion per unit force of the aspirating diaphragms in comparison with storage at room temperature. However recovery of the diaphragms was unaffected. Storage at the higher temperature decreased the amount of force required to initiate movement of the cartridge rubber bung during injection. Soaking cartridges in alcohol did not affect the distortion of the aspirating diaphragms or the force needed to move the bung during injection, but did increase the degree of recovery of the diaphragm in the bung of self-aspirating cartridges.

Anesthesia, Dental↗
PubMed1992

Cavity convergence angles for direct composite inlays.

Direct composite inlays may sometimes be difficult to remove from the cavity following primary polymerization. The aim of this work was to find the lowest cavity convergence angle (taper) which would allow reliable inlay removal. Cavity finish and cavity size were also taken into account. Standardized mesio-occlusal-distal cavities of 6 degrees taper were cut into 10 teeth of varying mesiodistal width and finished with either 12-bladed tungsten carbide or 25 microns diamond burs. After separating medium had been applied. Coltene Brilliant Dentin composite resin was packed into the cavity and light cured. The force required to remove the inlay was measured with an Instron Universal Testing Machine. The cavities were then refinished with the alternative finish and the experiment repeated. The same sequence was used for cavities of 12 degrees and finally 18 degrees convergence angle which were cut consecutively in the same 10 teeth. Force levels related to cavity convergence angle, cavity finish and tooth size were tested by ANOVA and regression analysis. The forces required to remove some of the inlays from the 6 degrees, and to a lesser extent 12 degrees angled cavities, proved unacceptably high, while an 18 degrees convergence angle resulted in statistically significant lower forces which were unlikely to damage tooth or inlay. Cavity finish and tooth size did not influence inlay withdrawal force.

Analysis of Variance↗
PubMed1992

Dentine bonding agents--characteristic bond strength as a function of dentine depth.

The tensile bond strength to dentine of four dentine bonding agents has been measured at various depths within dentine and the results analysed using Weibull analysis. There were significant differences in the bond strength values among the four products particularly with near-surface dentine. Despite the differences in chemical composition among the materials the trend of decreasing bond strength with increasing depth of dentine was common to them all. This suggests that the underlying mechanism of adhesion may be common to all four products despite their differing chemical compositions.

Adhesiveness↗
PubMed1992

Light-activated glass polyalkenoate (ionomer) cements: the setting reaction.

The setting reaction of two light-activated glass polyalkenoate cements has been investigated using differential thermal analysis and surface hardness measurements. One material was found to have two distinct phases to its setting reaction. Light activation resulted in a rapid initial set with a large exotherm. A slower setting reaction was detectable when the cement was allowed to set in the absence of light. The surface hardness of this cement increased for some time after the cessation of light activation, indicating continuation of the chemical setting reaction within the material. The ultimate hardness of the cement was significantly higher than its hardness at the termination of light activation. Light activation of the second material increased the rate of set of the cement but no evidence of a dual setting reaction was observed. This material remained very soft and flexible for over 1 h after light activation. A 30 s exposure with a visible light source produced some immediate hardening up to 1.5-2.0 mm below the surface for both materials.

Chemical Phenomena↗
PubMed1992

The effect of using layered specimens for determination of the compressive strength of glass-ionomer cements.

Compressive strength is widely used as the criterion of strength of glass-ionomer dental cements, despite the difficulties in interpretation of the findings. With the introduction of light-cured glass-ionomer cements, which can be used only in thin layers, the question arises of how test specimens should be prepared for the measurement of compressive strength. A suggested method has been to prepare test pieces by building them up in layers, an approach which is examined critically in the current paper. Two different conventional (acid-base) glass-ionomers were studied with the use of layered and unlayered specimens of dimensions 6 mm (height) x 4 mm (diameter) and 12 mm (height) x 6 mm (diameter). While smaller samples gave the same value of compressive strength as larger specimens, layered specimens gave significantly lower values of compressive strength for both sizes. In view of these findings, and since the layered specimens are tedious to prepare, we conclude that compressive strength is unsatisfactory as a criterion of strength for light-cured glass-ionomer cements.

Acrylic Resins↗
PubMed1992

An easily removable ceramic bracket?

The use of ceramic brackets in fixed appliance orthodontics has highlighted several problems, amongst which are the problems of enamel damage and possible trauma to the patient during debonding. A ceramic bracket has recently been developed with a polycarbonate laminated base (Ceramaflex) claiming to make debond much easier, although there are no independent data to substantiate this claim. The in-vitro bond strength of these ceramic brackets to enamel was tested and compared with metal standard Edgewise brackets. The bond produced with the Ceramaflex bracket was similar in magnitude, but less reliable. No fracture of the bracket or damage to enamel was seen during debonding. Debonding of these brackets was easy to carry out, but it is possible that more debonds may occur during treatment than with the use of metal brackets.

Adhesives↗
PubMed1991

Mechanical properties of a composite inlay material following post-curing.

Compressive strength, fatigue limit, hardness and wear factors were determined for two shades of a composite inlay material both with and without post-cure annealing! The effect of the annealing procedure was to cause a small general improvement in properties. This was more noticeable in the Dentin shade of the material than in the Enamel shade. It is doubtful whether the small improvement in properties would have any significant effect on clinical durability.

Bisphenol A-Glycidyl Methacrylate↗
PubMed1991

The 5-year results of a clinical trial comparing a glass polyalkenoate (ionomer) cement restoration with an amalgam restoration.

A clinical trial comparing the efficacy of a glass polyalkenoate cement (GPC) restoration with an amalgam cement restoration (ACR) in the management of caries in the deciduous molar dentition was undertaken. Two hundred and thirty-eight restorations, that is 119 pairs, were placed in 76 patients with an age range of 5 to 11 years. The durability of these restorations was assessed during a 5-year follow-up period, using modified United States Public Health Service criteria. The glass polyalkenoate cement restorations occupied 16% of the occlusal surface of the tooth compared to 28% for the amalgam restorations, had a lower median survival time (33.4 [SE 2.26] months compared to 41.4 [SE 2.24] months) and underwent greater loss of anatomical form and marginal integrity than the paired amalgam controls.

Analysis of Variance↗
PubMed1991

Comparison of a novel photogrammetric technique and modified USPHS criteria to monitor the wear of restorations.

This paper describes monitoring the wear of restorations borne by partial dentures over a 12 months period using a novel photogrammetric technique and modified United States Public Health Service (USPHS) criteria. The performance of Class II restorations of Dispersalloy was compared with that of similar restorations of either KetacFil or Occlusin. The photogrammetric technique highlighted differences in performance not detected by the modified USPHS criteria. It is concluded that the photogrammetric technique should prove valuable in the in vivo assessment of the performance of restorative materials but that further refinement of the method is required particularly with regard to the orientation of replicas for sequential measurements.

Composite Resins↗
PubMed1991

Properties of a titanium nitride coating for dental instruments.

Adhesion of tooth-coloured restorative materials to the instruments used to place them is a clinical problem. This paper examines the low-stick properties and the durability of a 5 micron coating of titanium nitride on stainless steel when used with two composites (a hybrid and a microfilled) and a glass polyalkenoate (ionomer) cement. Titanium nitride-coated dies were compared to polished stainless steel for adhesion to the unset restorative material before and after a period of wear, and also for properties of surface hardness, contact angle with unfilled resin and frictional coefficient. The results demonstrated that, while the coated instruments were significantly harder and showed a lower coefficient of friction and a higher contact angle with resin, they were slightly 'stickier' than highly polished stainless steel, and this difference persisted even after wear. However, the material is very hard, resistant to damage, and appears to have little potential to discolour the restorative materials with which it is used. It is concluded that a titanium nitride coating confers no additional advantage as regards low-stick properties, over clean, polished stainless steel used for dental instruments.

Adhesiveness↗
PubMed1991

Bond strengths of orthodontic bonding materials: an in-vitro study.

The shear bond strength of a conventional composite resin was compared in-vitro with the bond strength of a fluoride releasing composite and a glass ionomer cement. The results obtained showed that the conventional composite had the highest mean bond strength. The results were also analysed using the Weibull analysis which examines bond reliability rather than mean bond strength. It is likely that the two composites will behave in a similar manner in the clinical situation.

Acid Etching, Dental↗
PubMed1990

Plastic dental local anaesthetic cartridges: a laboratory investigation.

The pressures applied to the bungs of glass and plastic dental local anaesthetic cartridges which were necessary to cause failure of the system were measured. Plastic cartridges did not fracture, but they failed due to leakage of solution around the bung at pressures less than those needed to break glass cartridges. At the peak injection pressures which may be obtained during intraligamentary injections, it was established that the theoretical incidence of failure was 1.4% for glass and 75.1% for plastic cartridges.

Anesthesia, Dental↗
PubMed1990

Alternative approaches to evaluating the fatigue characteristics of materials.

Two models of fatigue failure (designated type 1 and type 2) have been investigated by use of a total of seven materials. For type 1 behavior, there is a clear relationship between fatigue life and fatigue stress, whereas for type 2 behavior no such relationship exists. For materials exhibiting type 2 behavior, failure occurs at a level of stress below the ultimate strength of the material, but the values of fatigue life appear to be distributed randomly when several specimens of the same material are tested. A value of fatigue limit can be calculated for these materials which is independent of fatigue life. It is likely that, for these materials, resistance to fatigue is primarily dependent upon the presence or absence of flaws such as porosities. For the dental materials investigated in this study, it appears that brittle materials such as dental plaster and heavily filled composites are likely to exhibit type 2 behavior, whereas less brittle materials, such as more likely filled composites, are more likely to exhibit type 1 behavior.

Calcium Sulfate↗
PubMed1990

Adaptation of two different calcium hydroxide bases under a composite restoration.

A preliminary scanning electron microscope (SEM) study was carried out to investigate how the adaptation of two calcium hydroxide bases (one chemically cured, one light cured) was affected by the polymerization contraction of a supervening light-cured composite resin restoration. Occlusal cavities were prepared in 40 sound extracted human premolars, divided into two equal groups. In the first group a chemically cured calcium hydroxide (Dycal, De Trey Dentsply, Konstanz, FRG) was placed as a base. In the second group a new light-cured calcium hydroxide product (Prisma VLC Dycal, De Trey Dentsply) was used. The restorations were completed with an acid-etched, incrementally placed composite resin. The specimens were sectioned vertically and debrided. A replica was made of each half-tooth. The interfaces between composite resin/base and base/dentine were viewed and photographed in the SEM. The marginal adaptation at these two interfaces was classified into three categories according to the extent of the gaps that were observed. Prisma VLC Dycal base was found to be pulled away from the dentine floor of the cavity as a result of an apparent adhesion to the composite resin during polymerization contraction. Dycal was better adapted to the cavity floor than Prisma VLC Dycal. Disorganization of the resin-bonded Prisma VLC Dycal was minimal even after acid etching the enamel, sectioning and ultrasonic debridement. Dycal appeared to be more friable, and occasionally exhibited marked disorganization as a result of these procedures.

Acid Etching, Dental↗
PubMed1990

An investigation of test-house variability in the mechanical testing of dental materials and the statistical treatment of results.

The suitability of a test for standard specification testing depends inter alia on the ability to repeat the tests in a reproducible manner at a number of test centres. This work involved the investigation of three tests currently proposed as standard specification tests for dental materials. It was found that one test (compressive strength of cements) is inappropriate for inclusion in standards due to an unacceptable variation in test results between test centres. The treatment of results suggested in standards should place less emphasis on the mean value of a relatively small number of test specimens and the use of a simple form of probability theory in which, say, 80 per cent of specimens are required to achieve a certain pass level. In some standards there may be a need to increase the numbers of test specimens significantly in order to achieve a more meaningful and reliable result. The use of Weibull statistics can be adopted as a means of identifying tests which are suitable for inclusion in standard specifications, although it is doubtful that a test at this level of sophistication is required or desirable for inclusion in the standards themselves.

Composite Resins↗