Influence of composite inlay/onlay thickness on hardening of dual-cured resin cements.

This investigation evaluated the effect of resin composite inlay/onlay thickness on the hardness of a group of eight dual-cure resin-based cements. Fourteen disc specimens measuring 6 mm in diameter and 2.5 mm thick were prepared from each of eight dual-cure cements: Adherence, Choice, Duolink, Enforce, Lute-It, Nexus, Resinomer and Variolink. Two specimens from each material were directly light-cured while the remainder of the specimens were light-cured through resin composite spacers varying in thickness from 1 mm to 6 mm. Curing through the spacers always resulted in a decrease in the Knoop hardness number. For some cements, hardness values were reduced by 50% or more when the resin composite spacer thickness was 4 mm or greater even when measurements were made one week after dual-curing. Low hardness values indicate the presence of a weak chemical-curing mechanism that may compromise cement quality in areas of the cavity not readily accessible to the curing light.

Hardening of new resin cements cured through a ceramic inlay.

This study investigated the degree of hardening achieved through self-curing only and through dual-curing a group of eight new resin-based cements. In addition, the effect of ceramic inlay thickness on cement hardness was determined. Disk specimens measuring 6 mm in diameter and 2.5 mm thick were prepared from eight cements: Adherence, Choice, Duolink, Enforce, Lute-It, Nexus, Resinomer, and Variolink. Eight specimens were prepared from each material; half were self-cured, while the remainder were dual-cured. Knoop hardness measurements were then made at 1-hour, 1-day, and 1-week intervals. In addition 12 specimens of the same dimensions were prepared from each cement and were dual-cured through ceramic spacers of varying thickness (1-6 mm). Hardness measurements were made as above. ANOVA showed significant differences in hardness of self-cured versus dual-cured specimens for all cements (P < 0.0001). Significant differences were also found in the hardness of specimens dual-cured through ceramic spacers 2-3 mm in thickness or more compared with those that were dual-cured without spacer. It is concluded that for some materials self-curing alone was not adequate to achieve sufficient hardening; cement hardness was significantly reduced when ceramic inlay thickness was 2-3 mm or more.

Posterior resin-bonded fixed partial denture with a modified retentive design: a clinical report.

A uncomplicated clinical procedure to enhance the retention of posterior resin-bonded fixed partial dentures was described. This method involves some modifications to the preparation and casting design and requires slightly more time and attention at the cementation stage of the prosthetic treatment.

Retention of metal ceramic crowns cemented with resin cements: effects of preparation taper and height.

This study investigated the effect of varying crown preparation taper and height on the retention of metal ceramic crowns cemented with resin cements. In part 1, 32 extracted molars were divided into four groups. All groups received complete-veneer crown preparations, two with a 12-degree wall angle and two with a 35-degree wall angle. Crowns were cemented with zinc phosphate and dentin-bonded resin cements. After 24 hour water storage at 37 degrees C, the crowns were separated in tension. Crowns cemented with zinc phosphate were cleaned and recemented with another dentin-bonded resin cement and after similar storage, the crowns were separated. Mean separation forces of resin-cemented crowns were higher than those of crowns cemented with zinc phosphate cement. Mean separation force of 35-degree crowns cemented with one dentin-bonded resin cement was found to be significantly higher than for the 12-degree crowns cemented with zinc phosphate cement. In part II, 32 extracted molars were divided into two groups of complete-veneer crown preparations (group I preparations were 3 mm high and group II were 5 mm high). Crowns for all teeth were cast and cemented with zinc phosphate. After 48 hours' water storage at 37 degrees C, the crowns were separated in tension. Crowns and teeth were then cleaned and each group was subdivided into 2 equal subgroups. In one subgroup of teeth with 3 mm and in one with 5 mm high preparations, the crowns were recemented with one of the two resin cement systems and the other two subgroups were recemented with the other resin cement. After 48-hour water storage, the crowns were separated. Mean separation forces of the resin-cemented crowns were greater than those of crowns cemented with zinc phosphate. Also, mean separation force of 3 mm crowns cemented with one resin cement was significantly greater than the 5 mm crowns cemented with zinc phosphate. Cohesive dentin fracture on separation was observed with some resin-cemented crowns but never with crowns cemented with zinc phosphate.

Posterior acid-etch bridge--a modified retentive design.

This article reviews the recent literature on clinical trial studies of acid-etch bridges. Due to long-term retention problems, acid-etch bridges are not as yet considered among the permanent types of bridge prosthesis. When failure of acid-etch bridges occurs, the weak link seems to be the enamel-resin junction rather than the metal-resin junction. A modified posterior acid-etch bridge design is described in detail. This design, which incorporates a major retention-promoting component, is meant to improve long-term retention of posterior acid-etch bridges. Cast projections are added to the bridge design. These fit loosely into slot cavities made in the abutment teeth. Following bridge cementation, the slot cavities are restored with posterior composite. As a result, the cast metal projections are locked into the abutment teeth. This added retention device will maximize the long-term retention of posterior acid-etch bridges.

Chemical versus dual curing of resin inlay cements.

Dual-cure inlay resin cements polymerize both chemically and through light activation; however, clinically some aspects of the cement are not readily accessible to the light source. This study investigated the degree of cement hardening achieved through chemical curing only versus dual curing and the effect of inlay thickness on cement hardness. Disks 6 x 2.5 mm were prepared from seven commercially available cements. Eight specimens were prepared from each material; half of the specimens were cured chemically only, and the remainder were dual-cured. Knoop hardness measurements were then recorded at 1-hour, 1-day, and 1-week intervals. In addition, 24 specimens of the same dimensions were prepared from each cement. Twelve specimens were dual-cured through resin composite spacers of varying thicknesses (1 to 6 mm), and the others were cured through similar ceramic spacers, and hardness measurements were recorded. Multivariate analysis of variance revealed significant differences in hardness of chemically cured versus dual-cured specimens at the 5% level of significance for all examined cements. Significant differences were also found in the hardness of specimens dual cured through ceramic or resin composite spacers 2 to 3 mm in thickness or more versus those that were dual cured without spacer regardless of the spacer material. It is concluded that chemical curing alone was not sufficient to achieve maximum hardening of the examined cements. Cement hardness was significantly reduced when inlay thickness was 2 to 3 mm or more.

Retention of paraposts cemented with dentin-bonded resin cements.

The main purpose of this study was to assess the retention of Paraposts cemented with dentin-bonded resin cements in single-rooted teeth with elliptical canals. Forty-two mandibular premolars and canines were used in this study. The crowns of these teeth were removed 1 mm above the cementoenamel junction and the root canals instrumented to a depth of 8 mm to receive size 5 Paraposts. Prepared teeth were divided into six equal groups. Each group was assigned to a different cementation system at random. The six cementation systems used were: Fleck's Cement, Universal Post Cementation Kit, Prisma Universal Bond 3/Biomer, Scotchbond 2/Resiment, All-Bond 2/All-Bond C & B Cement, and Scotchbond Multi-Purpose/Resiment. Following post cementation, the teeth were stored in water at 37 degrees C for 24 hours, after which the posts were subjected to uniaxial tensile force on a testing machine until post separation occurred. Paraposts cemented with Prisma Universal Bone 3/Biomer or with Scotchbond Multi-Purpose/Resiment had significantly greater separation forces than posts cemented with any of the other cementation systems. Adhesive failure of the posts occurred in all of the specimens of the resin cement groups, whereas cohesive failure of the cement occurred in the majority of the specimens of the zinc phosphate cement group. The effects of thermocycling and post length (5 mm versus 8 mm) on the retention of Paraposts cemented with dentin-bonded resin cements were also investigated. Neither thermocycling nor post length had a significant effect on post separation force.

Restorative decision making by Ontario dentists.

One-half of the dentists in general practice in Ontario were randomly selected for a survey in June 1992 to determine their practices and decision-making regarding some aspects of restorative dentistry. Using patient scenarios to describe clinical situations, respondents stated the threshold at which a restoration should be placed in various tooth surfaces of persons of different ages, according to the severity of the carious lesion and the usual restorative procedure for different case situations. A total of 1,276 (52 per cent) dentists responded to a detailed mail questionnaire. Data were entered into a personal computer (PC) and analyzed using frequencies and chi-square with the SPSS/PC+ statistical package. Multivariate analyses were undertaken to examine what characteristics of dentists independently explained variations in their usual restorative procedures for approximal and occlusal caries. With approximal lesions, as seen on bitewing radiographs, 60 per cent of the dentists indicated that they would place a restoration in a 12 year old with an enamel lesion that had not reached the dentino-enamel junction, whereas with 30- and 55-year-old patients, 28 and 20 per cent, respectively, would do so. At each patient age, there was a tendency for significantly younger dentists to restore enamel-only lesions more often than other dentists (p < .01). Variations in proposed treatment for an adult patient with above average oral hygiene, but with a small (1-1.5 mm diameter) occlusal cavity that had penetrated through the dentino-enamel junction, were also observed. In this case, 23 per cent of the dentists would prepare a conventional cavity extending to include all fissures, and restore the tooth with amalgam or composite; 45 per cent would prepare a cavity just larger than the outline of the lesion and restore it in the same way; 32 per cent would prepare a small cavity and place a preventive resin restoration. Significant differences in cavity design were also observed between graduates of the University of Toronto and the University of Western Ontario with respect to restoring approximal carious lesions. A number of the dentists' characteristics were significantly associated (p < .01) with these variations in procedures. These included the dentists' gender, year and university education and type of practice. The documentation and explanation of these large variations in restorative practices have important implications for continuing dental education.

Meta-analysis on long-term clinical performance of posterior composite restorations.

Meta-analysis is a formalized method of combining results of different studies to provide conclusions about the effectiveness of a treatment modality. The aims of this study were to use meta-analysis to determine the clinical performance of posterior composite restorations using the assessment criteria of the USPHS guidelines by combining data from selected multiple studies and to estimate the overall survival rates of posterior composite restorations over time. A computer-aided search of the literature revealed 97 publications on clinical trials of posterior composites in the last 10 years. Following specific selection criteria, which included the year and language of publication, duration of study, class of cavities restored and type of resin composite material used and clinical characteristics assessed; 16 studies were found to be suitable for, and included in a meta-analysis. These involved eight different resin composite materials. Assessment criteria data were extracted from each selected study and tabulated on the basis of years of follow-up and materials. The criteria were coded as binary variables. Homogeneity amongst studies was assessed using Woolf's statistic prior to combining the data. Weighted average proportions and standard errors were determined for each of the assessment criteria. Using Kaplan-Meier estimates, survival analyses of individual assessment criteria (outcomes) for two posterior composite materials were conducted and the resultant survival curves for these outcomes for the two materials are presented. Considering the limited number of studies of variable length available for meta-analysis, the results indicate generally high clinical performance of the various posterior composites for the number of outcomes analysed.

Radiopacity of resin-based inlay luting cements.

The radiopacity of a group of seven resin-based inlay luting cements in addition to two porcelain veneer cements was determined. Four inlay cements (Indirect Porcelain System Dentist Bonding Kit, Dicor MGC, Duo, and Twinlook) were found to have radiopacity values significantly greater than the radiopacity of enamel. Two other inlay cements (Dual and Porcelite Dual Cure) were found to have radiopacity values not significantly different from that of enamel, while All-Bond Crown and Bridge cement had a radiopacity value significantly lower than the radiopacity of dentin. For the two porcelain veneer cements examined, one material (Mirage Bond) had a radiopacity value significantly greater than that of enamel, while the other (G-Cera Porcelain Veneer Bonding System) had a radiopacity value significantly lower than that of dentin. It was concluded that only the materials that have radiopacity values greater than or equivalent to the radiopacity of enamel are suitable for use as inlay cements. For porcelain veneers, a relatively high radiopacity value of the cement is advantageous.

Fracture strength and fracture patterns of maxillary premolars with approximal slot cavities.

The fracture strength and fracture patterns of maxillary premolar teeth prepared with two different cavity designs and tested with and without amalgam restorations were determined. No significant difference in the fracture strength of intact premolars and premolars prepared with conventional MOD cavities or slot MO/DO cavities was found whether or not the teeth were restored with amalgam. However, fracture pattern analysis revealed significant differences among the groups of teeth. Premolars with MO/DO slot cavities or slot amalgam restorations exhibited minimal fractures that involved enamel only in the majority of the cases. This behavior was similar to what was observed with intact premolars. In contrast, premolars with conventional MOD cavities or MOD amalgam restorations exhibited severe fractures that involved enamel, dentin, and the root in the majority of the cases. These severe fractures were statistically significantly different from those of premolars with slot cavities or slot restorations as well as from those of intact premolars. It is concluded that when maxillary premolars require restoration of carious lesions on both approximal sides without occlusal involvement, MO/DO slot amalgam restorations will be expected to result in teeth that are unlikely to undergo severe cuspal fracture compared to conventional MOD amalgam restorations.

Radiopacity of direct ceramic inlay restoratives.

The radiopacity of two new ceramic restorative materials (Dicor MGC and Cerec Vita Blocks) manufactured for use in producing direct inlays with the 'Cerec CAD-CAM' system was determined. Dicor MGC had a radiopacity significantly greater than that of enamel, while Cerec Vita Blocks had a radiopacity significantly less than that of dentine. It is concluded that Dicor MGC has a radiopacity suitable for its use as an intracoronal restorative for posterior teeth. The low radiopacity of Cerec Vita Blocks means that the use of radiopaque luting cement is essential to permit detection of secondary caries around restorations of this material. Furthermore, marginal overhangs around restorations made from Cerec Vita Blocks will be difficult to detect radiographically.

An in vivo study on the release of fluoride from glass-ionomer cement.

Fluoride release from a glass-ionomer cement was measured in vivo during an 8-day period. Maxillary acrylic resin appliances, each carrying four glass-ionomer cement specimens, were worn by four subjects at night. Unstimulated whole saliva samples were collected before the subjects retired to bed (pretreatment samples) and in the morning (posttreatment samples). The average fluoride concentration and saliva flow of pretreatment samples were 31 +/- 4 ng/mL and 0.5 +/- 0.1 mL/min, respectively. In all subjects there was a statistically significant posttreatment increase (.005 less than P less than .025) in salivary fluoride concentration. The release of fluoride was nearly constant during the test period.

Temperature-dependence of compressive properties of human dentin.

The effect of temperature on compressive stress/strain behavior of human dentin obtained from recently extracted permanent lower molar teeth has been determined over the range 0-80 degrees C. Dentin specimens were loaded uni-axially in a direction perpendicular to the tubule orientation. A statistically significant, linear regression relationship was found between modulus (E) and temperature (T): E (GPa) = 15.55-0.0734 . (T degrees C). The observed temperature coefficient of the modulus is in close agreement with that observed for cortical bone. Proportional limit, compressive strength, and resilience were also found to undergo a linear decrease with increasing temperature. Mechanical failure of specimens generally occurred along lines determined by maximum shear stresses, approximately 45 degrees to the axial load direction.

Fracture toughness of human dentin.

Plane strain fracture toughness (KIC) was determined for coronal dentin using compact tension test-pieces obtained from recently extracted permanent lower molar teeth. Specimens were prepared and tested such that the moving crack front was aligned parallel to the tubule orientation. The fracture toughness (KIC) was temperature-invariant in the range 0-60 degrees C, and the mean value obtained was 3.08 MN.(m)-1.5 (SD: 0.33). The critical strain energy release rate (GIC) of dentin was also calculated utilizing modulus data and was found to increase slightly with temperature, possibly as a consequence of significant temperature-dependence of the modulus: GIC = 2.31 (T degree C) + 653.2 (J .m-2) The fracture toughness of dentin is midway in the range (0.23 - 6.56) observed for cortical bone and is a factor of two greater than that exhibited by most current restorative materials.