Biomedical subjects

R W Wassell

Publications and source records attributed to R W Wassell.

At least 37 records · Page 2Linked to original sources

PubMed1998

Effects of adhesive fixed prosthesis retainer design on resultant resin luting agent thickness.

STATEMENT OF PROBLEM: Resin cement thickness may influence the success or otherwise of adhesive fixed prostheses and needs to be quantified for future stress analyses. PURPOSE: This study evaluated the effect of different complexities of retainer preparation design on resultant resin lute thickness. MATERIAL AND METHODS: Five retainer designs were evaluated for resin luting thickness (Panavia Ex) after cementation to appropriately selected natural teeth as follows: (1) flat plate retainer; mandibular canine; (2) occlusal rests, mandibular premolar; (3) occlusal rests, proximal grooves, mandibular premolar; (4) modified three-quarter crown, mandibular premolar; and (5) occlusal strut, maxillary premolar. Five samples of each design were prepared in vitro. A mesiodistal and a buccolingual section were obtained from each sample with a diamond saw. Measurements were carried out on the fluorescent cement lute with a confocal microscope. Mean resin thickness and cervical resin thickness were analyzed with analysis of variance and Tukey tests. RESULTS: Resin thickness (micron) for each of the retainer designs were as follows (mean +/- SD): design 1, 58 +/- 40; design 2, 67 +/- 42; design 3, 80 +/- 28; design 4, 87 +/- 15; and design 5, 85 +/- 29. There were no significant differences among the designs (mean global value 75.4). The three-quarter crown (design 4) had a significantly wider cervical resin thickness (P < or = .01) than designs 1, 2, and 3.

Analysis of Variance↗

PubMed1998

Considerations when planning occlusal rehabilitation: a review of the literature.

As one of the most demanding tasks facing the restorative dentist, planning and executing an occlusal rehabilitation should not be undertaken lightly. The stakes are high and failure is costly. Treatment planning decisions should be undertaken on the basis of scientific evidence, where this is available, or on the basis of documented experience where it is not. This review article identifies the major biological and clinical considerations used when planning an occlusal rehabilitation. These include the indications for reorganising the occlusion, the choice of condylar position and occlusal scheme, the implications of and indications for increasing the vertical dimension, replacing missing teeth and the choice of materials. Finally, the literature surrounding the controversial issue of occlusal rehabilitation as a means to treat temporo-mandibular disorders is also reviewed.

Decision Making↗

PubMed1997

Wear rates of regular and tempered composites.

OBJECTIVES: One of the reasons for using composite inlays is that restorations can be tempered. Composites tempered with heat and light have, at least initially, an enhanced degree of cure which, it is claimed, improves their physical properties, including wear resistance. The aim of this study was to compare the wear rates of a selection of composites in their regular and tempered conditions. METHODS: Two hybrid composites (Brilliant Dentin, Coltène; Occlusin, ICI) and a microfilled material (Heliomolar, Ivoclar-Vivadent) were chosen to provide a variety of filler particle and resin systems. Regular composites were cured normally with light while tempered composites were given an additional 8 min secondary cure at 120 degrees C in the Coltène DI500 heat and light oven. Specimens of an amalgam (Dispersalloy, Johnson & Johnson) and a heat/pressure cured composite (Isosit, Ivoclar-Vivadent) were used for comparison. The specimens were stored for either 7 days or 1 year to determine if prolonged water storage affected wear rate. Wear testing was carried out in a two-body wear machine using a steatite abrader as an enamel substitute. ANOVA was used to test the effects on wear rate of tempering, storage time and type of material. RESULTS: Significant differences were found in wear rate between materials (p < 0.001), but not for tempering or prolonged storage time. CONCLUSION: These findings agree with clinical studies which have shown that tempering has little or no influence on the occlusal wear of composite inlays.

Acrylic Resins↗

PubMed1996

A survey of the methods of disinfection of dental impressions used in dental hospitals in the United Kingdom.

The potential for cross-infection from microbial contaminated dental impressions has long been recognised. This study set out to investigate impression decontamination procedures currently used in UK dental hospitals (1995) and to see how these may have changed since a previous survey, carried out in 1988. A variety of disinfection solutions and regimes were highlighted both within and between dental hospitals. Several of the disinfecting solutions currently being used have not been specifically tested for efficacy with impression materials. The laboratories were asked to highlight any adverse reactions. Five laboratories reported that some alginates resulted in casts with poor surface properties when immersed in hypochlorite (0.1 and 1%), sodium dichloroisocyanurate, and 2% glutaraldehyde solutions. This paper highlights that there is no universally recognised impression disinfection/sterilisation protocol. It is recommended that all impressions should at least undergo a disinfecting procedure by immersion in 1% sodium hypochlorite for a minimum of 10 minutes.

Dental Impression Materials↗

PubMed1996

Sublingual ulceration associated with cementation of adhesive bridges.

The use of adhesive bridges is becoming more popular as designs are refined and more effective agents are developed for cementation. However, the fitting of such bridges is very technique-sensitive; an ill-fitting bridge will cause discomfort and difficulty with oral hygiene. It is important that practitioners take the problems illustrated in this case report into account when fitting adhesive bridges.

Adult↗

PubMed1995

Direct composite inlays versus conventional composite restorations: three-year clinical results.

A clinical trial of direct composite inlays versus conventionally (incrementally) placed restorations made from the same material was started in January 1989. Restorations were randomly allocated to matched pairs of cavities. This study reports the 3-year performance of 71 of the 100 pairs of restorations placed over a 2-year period and followed-up every 6 months. Clinical assessments were made using USPHS criteria (indirect measurements of occlusal wear were made using Ivoclar standard dies) and annual bite wing radiographs. Direct inlays showed significantly less occlusal wear than conventional restorations, but the difference was small. The clinical performance of both types of restoration was similar and compared favourably with studies of other materials. No secondary decay was diagnosed. The direct inlays, however, took longer to place and did not reduce postoperative sensitivity or failure rate (8% failure of inlays and 4% of conventional composites over 3 years). Contouring of proximal and occlusal aspects was not facilitated with direct inlays but may be easier with indirect inlays on removable dies.

Adult↗

PubMed1995

Radiographic detection of overhangs formed by resin composite luting agents.

OBJECTIVES: An in vitro model was used to assess the ability of standard radiographic techniques to detect marginal overhangs of resin composite luting agents beside porcelain and resin composite inlays. METHODS: The radiodensity of five commercially available luting resins was determined using ISO 4049 methodology. For four of the luting agents, artificial overhangs (0.5 x 0.5 x 2 mm) were created at the cervical margin of standard resin composite and porcelain inlays. Radiographs were recorded, using wax as a tissue equivalent, and the overhangs reduced incrementally in depth by 0.5 mm with serial images at each depth. The images were assessed in random sequence by three observers. RESULTS: There were significant differences between the radiopacity of the luting resins. These correlated well with the ability of the observers to detect marginal overhangs adjacent to resin composite inlays. CONCLUSIONS: Even with the most radiopaque luting material, a substantial marginal ledge could not be detected in association with a radiopaque resin composite inlay. The threshold for detection of the overhang was lower when using a radiolucent porcelain inlay.

Analysis of Variance↗

PubMed1995

Impregnation by dentine bonding agents into instrumented root-face dentine.

OBJECTIVES: To examine the penetration of unfilled resin into instrumented root-face dentine, facilitated by five commercially available dentine bonding agents. METHODS: Upper third molar teeth were decoronated, and a given dentine bonding agent and resin composite applied to the instrumented root face, according to the manufacturer's instructions. A control specimen was similarly prepared but not dentine primer was applied prior to resin composite. The teeth were sectioned longitudinally and the sectioned surface demineralized and deproteinated. Specimens were examined by scanning electron microscopy (SEM) for resin tag length and density in different areas of the root face. RESULTS: Specimens treated with dentine bonding agent systems exhibited minimum resin penetration of peripheral root-face dentine and much longer penetration of resin tags in dentine above the pulp chamber. A possible 'hybrid' layer of resin-impregnated dentine was observed in all specimens where a dentine bonding agent system had been used but was not observed in control specimens. Resin tags were also more sparsely observed in control specimens. CONCLUSION: Resin tag penetration is superior in both density and length of penetration in dentine overlying and adjacent to the pulp chamber, with shorter and sparser penetration exhibited in peripheral root-face dentine. The possible clinical implications of these observations are discussed.

Adhesiveness↗

PubMed1995

A comparative study of three dental adhesives.

The shear bond strengths of three dental adhesives to dentine and a NiCrBe alloy were determined. Forty human third permanent molar teeth were ground to expose superficial dentine, to 20 of which P50 composite was bonded, using All-Bond 2 (n = 10) or Scotchbond 2 (n = 10). Alloy discs were bonded to the remaining 20 specimens, using either All-Bond 2 (n = 10) or Panavia Ex (n = 10). All specimens were stored at 37 degrees C. 100% RH, for 24 h. Testing was carried out using a shear jig at a cross-head speed of 1 mm min-1, following which a 0.5 mm increment was removed to expose middle dentine for further bond tests (n = 10 for each adhesive). Removal of a further 0.5 mm increment exposed 'deep' dentine for final dentine bond strength tests. Results were then aggregated for each substrate/adhesive combination (n = 30). Alloy/alloy bond strengths were determined after bonding alloy discs together using Panavia Ex (n = 30) or All-Bond 2 (n = 30). Results were analysed using ANOVA and Weibull analysis. All-Bond 2 produced significantly greater and more reliable bond strengths to dentine than did either Scotchbond 2 or Panavia Ex. The bond strength of Scotchbond 2 and Panavia Ex to dentine was dependent upon the dentine depth whereas the bond strength of All-Bond 2 to dentine was independent of dentine depth. Alloy/alloy bond strengths were significantly greater when using Panavia Ex compared with All-Bond 2.

Acid Etching, Dental↗

PubMed1995

The use of cantilever bridges.

Replacement of missing teeth with fixed bridgework often involves producing full crown retainers on teeth on both sides of an edentulous space. Unfortunately, this approach can result in the destruction of much healthy tooth tissue, and the clinician must balance the benefits of replacing missing teeth with the amount of tooth preparation required. Current thinking in restorative dentistry places the preservation of tooth tissue at a premium, and most practitioners are happy to use techniques that embrace this philosophy. Because of this, cantilever bridges have an increasing role in dental practice, where the replacement for a missing tooth or teeth has one or more abutments on only one side of the edentulous space, being unsupported at the other. Cantilever bridges fall into several types, depending on the number of abutments and types of retainers. This article describes the various cantilever bridge designs, considers the biomechanics of these restorations, and provides guidelines for their clinical use.

Biomechanical Phenomena↗

PubMed1994

The use of cantilevered, adhesively retained bridges with enhanced rigidity.

Adhesively retained bridges replacing missing posterior teeth are reported to be less successful than those used to replace anterior teeth. This is due in part to the increased occlusal loading in the posterior region and inadequate rigidity of the retainers. This report describes the use of minimal preparation cast restorations incorporating an 'occlusal strut' to improve the rigidity of the retainers used to support pontics replacing upper premolars. The advantages of using intracoronal minimal preparations are discussed.

Adult↗

PubMed1994

Wear characteristics in a two-body wear test.

OBJECTIVES: A previous report compared spherical steatite (ceramic enamel substitute) abraders with those of natural enamel in a two-body wear test. The wear rates and coefficients of friction of the two abraders against various composites and an amalgam were well correlated although the wear rates were slightly higher with steatite. This report investigates the characteristics of the worn abrader and specimen surfaces. METHODS: Scanning electron microscopy and laser profilometry were used. RESULTS: Similar wear characteristics were found for the two types of abraders. Adhesive wear was evident for the amalgam, Dispersalloy (Johnson & Johnson), and the heat/pressure-cured microfill composite, Isosit (Ivoclar-Vivadent). Abrasion was seen with the hybrid composite, Occlusin (ICI), and, to a lesser extent, the microfill composite, Heliomolar (Ivoclar-Vivadent). The appearance of the worn small particle hybrid composite, Brilliant Dentin (Coltène), suggested that fatigue and delamination were involved. Laser profilometry showed that the hybrid composites caused much greater wear to the abraders than either the microfill composites or amalgam. The Ra values of the worn abraders and specimens were similar, suggesting conformal contact between them and endorsing the well controlled conditions of the wear test. SIGNIFICANCE: The results of this and other publications suggest that steatite can be used as an alternative to enamel in performing two-body wear tests on dental composites. This should help significantly in materials evaluation and development.

Acrylic Resins↗

PubMed1994

A two-body frictional wear test.

Enamel abraders used in two-body wear tests suffer the disadvantage that standardization is difficult or impossible to obtain. The aim of this study was to test whether steatite (a ceramic material with a Vickers Hardness similar to that of enamel) could be used as a satisfactory human enamel analogue. The friction and wear characteristics of 'spherical' abraders made from these two materials against composite and amalgam specimens were therefore compared. A two-body wear test for restorative materials was devised which could be carried out in a Universal Testing Machine and which allowed for continuous monitoring of the frictional forces during wear. The results showed that steatite and enamel abraders produced similar coefficients of friction (correlation coefficient, r = 0.98). A linear relationship was found between depth (and volume) of wear and number of cycles (r = 0.98 to 0.99). Wear rate was not affected by the increase in abrader facet area. The wear rate against the steatite abrader was slightly greater than that against enamel, but the two abraders were reasonably correlated (r = 0.94). Friction and wear were correlated for the steatite abrader (P < 0.05, r = 0.88) but not the enamel abrader. The hybrid composites had a high wear rate and wore both the abraders more than did the microfilled composites or amalgam. These findings suggest that steatite is a suitable substitute for enamel in this type of test.

Acrylic Resins↗

PubMed1994

Comparison of a range of addition silicone putty-wash impression materials used in the one-stage technique.

This study investigated the accuracy of one-stage polyvinylsiloxane impressions recorded in metal and plastic stock trays using regular and soft putties. Impressions were made using the one-stage technique (putty and wash simultaneously) of a full arch, copper-plated typodont with turned full crown preparations. Dies resulting from all the putty-wash impressions recorded in plastic trays had significant differential die distortion at the cervical finishing line. Although President regular putty, President soft putty, Express and Provil produced acceptable dies when used with metal trays, significant differential die distortion occurred with Extrude and Reprosil. The putty-wash/plastic tray combination is unsatisfactory. Metal trays reduce die distortion with some putty-wash systems but not with others.

Dental Impression Materials↗

PubMed1993

Displacement Rheometer: a method of measuring working time and setting time of elastic impression materials.

A new method of measuring both working time and setting time of elastomeric impression materials has been developed. A Displacement Rheometer was used to monitor the development of elasticity in setting elastomers. The results obtained with the Displacement Rheometer were compared to the results of the ISO test as well as those of the Controlled Stress Rheometer. Working time using the ISO method was calculated according to the standard method described in the specification, while for both the Controlled Stress Rheometer and the Displacement Rheometer working times were the times when initial elastic recovery was observed at 23 degrees C. With the Displacement Rheometer the setting time is the time when the mixed material reaches optimal elastic recovery at 37 degrees C. Statistical analysis with ANOVA showed that for five of the test materials there were no differences between the three methods. For three of the materials the working time determined by the ISO method was significantly longer than that determined by the other two methods. The advantages and disadvantages of each method are discussed and the Displacement Rheometer is recommended as a supplement to the standard specification method.

Alginates↗

PubMed1992

A comparative study of the fit and retention of interocclusal splints constructed from heat-cured and autopolymerized polymethylmethacrylate.

Fifty-three subjects each had a single Michigan splint constructed of either heat-cured or autopolymerized polymethylmethacrylate. The choice of material was decided randomly. A blind assessment for fit and retention was used on a three-point scale. Fit and retention were apparently unaffected by the material used. Thirteen percent of splints were unsatisfactory for clinical use (poor fit or retention) and this was also independent of material. A small number of splints contained structural faults (voids, porosity, or cracks) that may shorten the clinical life of these restorations; 9 of 12 such faults occurred in the autopolymerized group. Previous research has shown that conventional heat-cured polymethyl-methacrylate has superior properties compared with autopolymerized resin. In the absence of differences in accuracy of fit or retention, a heat-cured material is therefore advised where splints are intended for long-term clinical use.

Adult↗

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

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↗