Influence of carboxylic anhydrides on selected mechanical properties of heat-cured resin composites

Resin composites are still in need of improved abrasion resistance for them to be ideal restorative materials for use in large occlusal cavities. The present study proposes a concept for additional cross-linking of dental monomers, by which mechanical properties and possibly the resistance to abrasion of the resin composites are increased. Cyclic acid anhydrides were added as cross-linking agents to different monomer mixtures, which were then loaded with filler. The monomer mixtures were varied with respect to type and ratio of monomer and anhydride. For measurement of diametral tensile strength, flexural strength, modulus of elasticity, and modulus of resilience, specimens were initially cured by light and then post-cured for one h at 150 degrees C. Resin composites based on UEDMA and HEMA were found to be superior to BISGMA- and TEGDMA-based composites. Increases in mechanical properties were highest when unsaturated anhydrides were used. An optimal effect of anhydride addition was found in resin composites also containing methacrylamide. Such materials resulted in a 20% increase in the mechanical properties investigated.

Marginal adaptation of a restorative resin polymerized at reduced rate

The aim of this study was to investigate the effect of reduced rate of polymerization on the marginal adaptation of a composite resin inserted in dentin cavities treated with a simplified Gluma system. The effect on bonding strength to dentin and compressive strength was also investigated. The light intensity of the polymerization unit was lowered by the use of a transformer, and thus the rate of polymerization of the composite resin. When the resin was irradiated for 30 s at 110 V followed by 30 s at 220 V, the marginal adaptation was significantly improved. This condition resulted in acceptable values of bonding strength to dentin and diametral compressive strength. It is suggested that the reduced rate of polymerization may allow for increased flow of the material, decreasing the contraction stress in the filling.

Effect on bonding of curing through dentin

The aim of this study was to investigate the effect of the thickness of dentin (1, 2, or 3 mm) placed between the light guide and the composite resin on the shear bond strength to dentin treated with a simplified Gluma system. The effects of the thickness of composite resin (2 or 3 mm), irradiation time (20, 40, or 60 sec), and shade (universal or brown) were also examined. The results showed that the thickness of dentin influenced bonding and interacted with the other three variables. It is suggested that dentin located between the light guide and composite resin may attenuate the light aimed at the bonding interface in the same manner as a layer of composite resin.

Effect of 2-hydroxyethyl methacrylate in an Al2Ox3/glycine solution used as primer for dentin bonding

The aim of this study was to investigate the effect of 2-hydroxyethyl methacrylate (HEMA) in an Al2Ox3/glycine solution used as primer for dentin bonding in combination with an experimental BISGMA-HEMA-based adhesive resin. With various amounts of HEMA in the Al2Ox3/glycine solution, a shear bond test showed a maximum strength at 50 wt% of HEMA. In this case, scanning electron microscopy examination showed a HEMA-infiltrated surface layer of the dentin along the interface between dentin and the adhesive resin. It is suggested that HEMA in Al2Ox3/glycine solutions may facilitate penetration of HEMA into etched or primed dentin.

Influence of the solubility parameter of intermediary resin on the effectiveness of the gluma bonding system

The aim of the present study was to investigate the effect of the solubility parameter of the intermediary resin in the Gluma system on the bonding to dentin. The solubility parameter of the resins was varied between 18.8 x 10(3) and 21.1 x 10(3) J(1/2)/m3/2 by varying the composition of the resin. The efficacy of the bonding system was determined by measurements of marginal gaps formed by polymerization contraction of a restorative resin in dentin cavities treated with the bonding system. The bonding system had maximum efficacy at a solubility parameter of the intermediary resin of delta = 20.0 x 10(3) J(1/2)/m3/2. This finding corroborates a concept of bonding to dentin that involves a mechanical interlocking by interpenetrating resins.

Mechanical properties of three composite resins for the inlay/onlay technique

The present study measured the diametral compressive strength, flexural strength, and modulus of elasticity of composite resins used in inlay/onlay systems. The effect of additional curing also was determined. SR-Isosit material had the highest diametral compressive strength, and SR-Isosit-Dentin material had the lowest flexural strength. The SR-Isosit composite resins had lowest elastic modulus and Estilux posterior C VS resin the highest. A negative correlation was found between diametral compressive strength and elastic modulus of the materials. It was concluded that the additional curing of Brilliant resin did not result in improved mechanical properties. For Estilux resin, additional curing increased flexural strength and modulus of elasticity.

[Retention of composite inlays in enamel-dentin cavities]

The retention of composite inlays depends on acid-etching of marginal enamel of the preparation. In many cases, only little marginal enamel is available, making loss of retention a liability. The present study evaluated the retention of three brands of composite inlays under various conditions. Inlays were fabricated and cemented in standardized enamel/dentin cavities prepared in extracted human teeth. The force necessary to extract a cemented inlay was used to express the retention of the inlay. The effects of thermocycling and choice of dentin-bonding agent on inlay retention were also determined. Inlays made of Estilux posterior C VS were more retentive than inlays of either Brilliant Dentin or SR-Isosit. The latter two products were found to provide similar retentive strengths. The retention of Estilux posterior C VS and SR-isosit inlays declined when samples were thermocycled. Treatment with Gluma increased retention of inlays, resulting in retentive strengths of the same magnitude for all three inlay systems. The choice of dentin-bonding agent was found to affect composite inlay retention to a greater extent than the choice of either composite brand, mode of inlay curing, or effect of thermocycling.

Distortion of impression materials used in the double-mix technique

According to the so-called double-mix technique for impression taking, a material of high and one of low viscosity set against each other. We found that a distortion of the joined materials occurred which may compromise the precision of the impression. The distortion of such combinations of materials of high and low viscosity was measured on beam-like specimens. Significant differences between materials were observed. The phenomenon may be explained by a swelling of the material of low viscosity. The swelling may be associated with a diffusion of unreacted compounds from the material of high viscosity into the material of low viscosity. Measurements of the swelling of specimens immersed in the catalyst component of unset material showed an increase in length of 2-5%.

Retrograde root filling with composite and a dentin-bonding agent. 1

A method is described, by which retrograde root filling with a composite resin can be performed. The cavity design is a slightly concave dissection of the apical part of the root, which is treated with the bonding agent Gluma followed by an application of Retroplast. Retroplast is a chemically curable composite containing silver for radiopacity and aerosil to obtain a suitable consistency. Endodontically treated teeth with a eugenol-containing root canal sealer did not affect the strength of the bond between Retroplast and apical dentin. A tight seal between the composite and the cavity surface was observed by light and SEM microscopy, and histology of tissue surrounding fillings placed in monkeys revealed absence of inflammatory cells around the filling and a close contact between filling and fibroblasts with collagenous fibers. In some cases, cementum and Sharpey's fibers formed in contact with the filling. Fillings placed in humans performed successfully in most cases, and the main causes of failure were either inadequate hemostasis during filling, or root fracture unnoticed by the time of filling. The retrograde technique promises a new treatment principle, with a root canal effectively sealed and the periapical ligament restored after apicoectomy.

[Color stability of three composite resins used in the inlay/onlay technique]

The color stability of three composite resins used in the inlay/onlay technique was determined by means of irradiation with UV-light and by water storage. Generally, water storage resulted in greater changes in color than did UV-light, but no correlation was found between the results of the two chromogenic procedures. The microfilled composite resin was the least color stable, but none of the resins had alarmingly low color stability.

[A comparison of accuracy in seating and gap formation for three inlay/onlay techniques]

The accuracy (fit) of MOD inlays of three brands of composite resin was determined by measuring the axial discrepancy (marginal opening in the approximal area). The axial discrepancy varied between 17 and 121 microns. Directly manufactured inlays were more accurate (axial discrepancy: 17-26 microns) than indirectly manufactured inlays (axial discrepancy: 40-121 microns). Inlays of a microfilled resin, SR-Isosit, were less accurate than inlays of two hybrid materials, Brilliant Dentin and Estilux posterior C VS (axial discrepancy: 121 microns vs 44 and 41 microns respectively). There was a tendency for 24-hour-old inlays to be less accurate than 10-min-old inlays. The formation of marginal gaps due to contraction of the resin cement was assessed in a light microscope on cemented single-surfaced inlays. Gaps were not formed when enamel margins were etched or dentin margins treated with a dentin-bonding agent of high efficacy. Gaps (2.4-5.6 microns) were formed, however, at untreated dentin margins and at margins treated with dentin-bonding agents of low efficacy.

[In-depth polymerization of a light-activated resin cement]

The potential for in-depth cure of the light-activated resin cement TULUX-CEM was investigated. Disks of TULUX-CEM were irradiated through a layer of inlay/onlay material of varying thickness. After the irradiation, the hardness of the disks was measured. It was found that the resin cement became softer as the thickness of overlying inlay/onlay material increased from 2.0 to 3.5 mm. At a thickness of 4.0 mm, the resin cement remained unpolymerized. It was concluded that in certain situations the in-depth cure of resin cements activated only by light may not be sufficient.

Retention of composite inlays in enamel dentin cavities

The retention of composite inlays depends on acid-etching of marginal enamel of the preparation. In many cases, only little marginal enamel is available, making loss of retention a liability. The present study evaluated the retention of three brands of composite inlays under various conditions. Inlays were fabricated and cemented in standardized enamel/dentin cavities prepared in extracted human teeth. The force necessary to extract a cemented inlay was used to express the retention of the inlay. The effects of thermocycling and choice of dentin-bonding agent on inlay retention were also determined. Inlays made of Estilux posterior C VS were more retentive than inlays of either Brilliant or SR-Isosit. The latter two products were found to provide similar retentive strengths. The retention of Estilux posterior C VS and SR-Isosit inlays declined when samples were thermocycled. Treatment with Gluma increased retention of inlays, resulting in retentive strengths of the same magnitude for all three inlay systems. The choice of dentin-bonding agent was found to affect composite inlay retention to a greater extent than the choice of either composite brand, mode of inlay curing, or effect of thermocycling.

[Glass ionomer cement and dentin: effect of pretreatment with polyacrylic acid]

This investigation measured the effect of polyacrylic acid treatment of dentin on adhesion of glass ionomer cement. The dentin was ground on paper No. 220 or No. 500 and treated with a polyacrylic acid solution of 10% or 25% for either 10 s or 30 s. The differences observed in the scanning electron microscope between the differently treated dentin surfaces were not reflected in the bond strength results. Only differences in surface texture due to different coarseness of grinding influenced the adhesion. As to specimens ground on paper No. 220, pretreatment with polyacrylic acid enhanced adhesion of glass ionomer cement. No effect of polyacrylic acid was found on bonding between glass ionomer cement and dentin ground on paper No. 500.

Mechanical properties of heat treated restorative resins for use in the inlay/onlay technique

Inlay/onlay materials based on composite resins have recently been introduced. It was the aim of the present study to investigate the effect of post-curing temperature on selected mechanical properties. Four different composite resins were produced. The composite resins had identical filler content, but varied with respect to the content of the monomers BISGMA, TEGDMA, UEDMA, and HEMA. After initial curing by light, the materials were post-cured at 37, 100, 125, 150, 175, or 200 degrees C for 1 h. We then measured diametral tensile strength, flexural strength, and modulus of elasticity. These properties were found to vary with the composition of the material. Composites containing UEDMA or UEDMA/HEMA were stronger and stiffer than BISGMA/TEGDMA formulations. The optimum post-curing temperature was 150 degrees C, but the increase in strength and stiffness was moderate, only about 9%.

Flexural and fatigue strengths of root canal posts

This study measured the flexural and fatigue strengths of root canal posts of different alloy, design, and diameter. Fixed posts were loaded up to a preset force and the number of loadings endured until fracture was recorded at each loading force. The flexural strength was defined as the maximum force a post could endure before fracture when loaded once. The fatigue strength was determined as the maximum force a post could endure when loaded 10,000 times. Both modes of strength were found to increase with increasing diameter. Aluminum posts had lower flexural and fatigue strengths than posts of titanium, stainless steel, and a Ag-Pd alloy, all of which had strength values of about the same magnitude. Among the investigated brands, Triax and Radix were stronger than the others at comparable diameters. The high strength of Triax may be explained by the design of the post, whereas a superior alloy may account for the high strength of Radix.

Marginal adaptation of resin in relation to application technique and use of a calcium hydroxide liner

The marginal adaptation of a restorative resin was examined in cone-shaped cavities prepared in extracted human teeth. The use of a thin layer of a Ca(OH)2 liner in the bottom of the cavities did not influence the wall-to-wall (wtw) polymerization contraction and it did not affect the gap-reducing efficacy of a dentin-bonding agent. The wtw contraction was markedly increased when the restorative resin was applied in two layers parallel to the free surface of the cavity; the reason for this is supposed to be that the ratio between the volume of the filling (V) and the area of the cavity wall (A) is increased for the second layer compared to the V/A ratio found in cavities filled with one increment.

Effect of polyacrylic acid treatment of dentin on adhesion of glass ionomer cement

This investigation measured the effect of polyacrylic acid treatment of dentin on adhesion of glass ionomer cement. The dentin was ground on paper no. 220 or no. 500 and treated with a polyacrylic acid solution of 10% or 25% for either 10 or 30 sec. The differences observed in the scanning electron microscope between the differently treated dentin surfaces were not reflected in the bond strength results. Only variations in surface texture due to different coarseness of grinding influenced the adhesion. For specimens ground on paper no. 220, pretreatment with polyacrylic acid enhanced adhesion of glass ionomer cement. Polyacrylic acid had no effect on bonding between glass ionomer cement and dentin ground on paper no. 500.

In vivo fractures of endodontically treated posterior teeth restored with enamel-bonded resin

The cumulative survival rate of 190 endodontically treated posterior teeth were assessed in a retrospective study; all teeth had an MO/DO or an MOD cavity restored with a composite resin without cuspal overlays after previous acid-etching of the enamel. In contrast to our previous study on endontically treated posterior teeth restored with amalgam, the survival rate of the MOD resin-restored teeth was equal to that of MO/DO teeth. Teeth restored with a light-activated resin had a much lower survival rate than teeth restored with a chemically-activated material, the cause presumably being that the light-activated resins were insufficiently irradiated. Nearly 25% of the teeth had been restored with a microfilled resin for anterior use and these teeth had a lower survival rate than had teeth restored with a macrofilled or hybrid resin. It was also found that a beveling technique did not decrease the fracture rate while the use of an intermediate layer of low-viscosity resin resulted in a significant improvement.

Effect of dentin adhesives on contraction of restorative resins in cavities surrounded by acid-etched enamel

Enamel-surrounded cavities prepared in extracted human teeth were treated with an enamel-bonding agent, one of two commercial dentin adhesives (Scotchbond 2 and Tenure), or one of two recent modifications of the Gluma system. In all cases the enamel margins were subjected to some form of acid etching. The cavities were filled with one of two different light-cured posterior resins, and the wall-to-wall polymerization contraction was assessed in the enamel surface level and, after grinding, measured and presented as linear shrinkage in the dentin surface level. The linear shrinkage varied from 0.66% in the control group, in which only composite resin was applied, to 0.28% in a group in which the cavities were pretreated with dentin adhesive. No difference could be demonstrated between the dentin adhesives or between the dentin adhesives and the enamel-bonding agent.

Storage stability of solutions of pyruvic acid/glycine used in adhesive bonding

The rate of deterioration of pyruvic acid/glycine solutions used as common pretreatment of enamel and dentin in the Gluma bonding technique was investigated. Bond strengths between composite resin and enamel/dentin were measured. The bonds were mediated by pyruvic acid/glycine solutions, applied from 0 to 6 months after the time of manufacture. Bond strengths to enamel varied between 8.6 and 13.5 MPa and to dentin between 6.2 and 18.0 MPa. The pH of the solutions rose slightly during the experimental period, but no statistically significant change in bond strengths was observed.