Raphael Pilo

EFFECT OF COMBINATIONS OF SURFACE TREATMENTS AND BONDING AGENTS ON THE BOND STRENGTH OF REPAIRED COMPOSITES

STATEMENT OF PROBLEM Enhancement of bond strength between new and old composite usually requires increasing the surface roughness to promote mechanical interlocking and coating of old composite with unfilled resin bonding agents to advance surface wetting and chemical bonding. PURPOSE The purpose of this study was to evaluate the effect of combinations of surface treatments and bonding agents on the shear bond strength between new and old composite. MATERIAL AND METHODS Six surface treatments, two bonding agents, and an untreated control comprised 18 different subgroups. RESULTS The use of unfilled resin, alone or combined with silane, was the most effective procedure to enhance the shear bond strength of the repaired composite specimens, irrespective of the surface pretreatment processes. Silanation and unfilled resin slightly but not significantly improved the repair strength compared with unfilled resin alone. CONCLUSIONS Different combinations of surface treatments and bonding agents affect shear bond strength differently. The highest shear bond strength values were achieved by grinding the surface with green Carborundum stone or sandblasting, whereas the lowest values were obtained with hydrofluoric acid as the surface treatment agent.

Effect of post design on resistance to fracture of endodontically treated teeth with complete crowns

The effect of post design on the fracture resistance of endodontically treated premolars restored with cast crowns was examined in vitro. The experimental model used cast posts and cores to test the effect of post design in a post-core system with identical rigidity. Samples loaded on an Instron testing machine until failure revealed that post design did not influence the fracture resistance of endodontically treated premolars restored with complete cast crowns. There was also no statistically significant difference between restored teeth with or without cast posts and cores.

Post-irradiation polymerization of different anterior and posterior visible light-activated resin composites

The Knoop hardness number was measured for bottom and top surfaces of 2 mm thick specimens of different anterior and posterior visible light-cured composites. The variables for this study included various exposure times and a series of time intervals from immediately after curing up to 72 h. Post-irradiation microhardness at the bottom and top surfaces increased rapidly over the first hour, was slower during 24 h and showed no further increase after 24 h. Increasing exposure time resulted in higher microhardness values at both surfaces. Magnitudes of change were larger for the bottom surface. Exposure times longer than those recommended by the manufacturers were needed, especially for the anterior microfilled composites, to achieve adequately photo-activated resin that had an optimum bottom/top surface microhardness of 80-90%. Maximum hardness values were found to be dependent on the volumetric fraction but not on the average inorganic filler size. The post-irradiation increase in hardness value was independent of composite parameters and is probably commensurate with polymerization kinetics.

A survey of output intensity and potential for depth of cure among light-curing units in clinical use

OBJECTIVES The light intensity of curing lights used in private dental offices was measured using commercial curing and heat radiometers and related to uniformity of cure depth of standardized composite specimens. METHODS The intensity of 130 curing light from 107 dental offices was measured with curing and heat radiometers. Due to analogue readings, results were recorded in steps of 25 mW cm-2 and assigned a category number. A total of 50 lights were randomly selected to polymerize standardized 3 mm thick composite cylinders. The composite was irradiated for 50 s according to the manufacturers instructions. The Knoop hardness value was measured at the top and bottom surfaces and the uniformity of cure depth was calculated from the ratio of these two values. RESULTS Light intensity measured by the curing and heat radiometers was in the range of 25-825 and 0-325 mW cm-2, respectively. Functions of maximum likelihood estimation of the top and bottom surface hardness were 57 N/N + 1.3 and 80 N/N + 17.7, respectively (N = light intensity category number). The relationship between the logarithmic transformation of the hardness ratio and light intensity was linear (R2 = 0.84 p < 0.001). CONCLUSION According to the manufacturer, a curing light is considered as unsuitable for use with a reading of < 200 mW cm-2 by the curing radiometer and > 50 mW cm-2 by the heat radiometer. Applying these criteria to the present study, 46% of the lights (without repetitions) required repair or replacement. The strong correlation found between the hardness ratio and light intensity verifies the usefulness of the curing radiometer in predicting the polymerization ability of the light activation units.

The influence of abutment angulation on strains and stresses along the implant/bone interface: Comparison between two experimental techniques

STATEMENT OF PROBLEM Preangled abutments produce different stress distribution compared to straight abutments. PURPOSE The objectives of this study were to (1) test the hypothesis that preangled abutments produce different stress distribution than straight abutments by using strain gauges attached to implants embedded in a medium simulating bone to determine strain distribution along the implant/bone interface; (2) test this hypothesis by photoelastic method; and (3) compare the two experimental techniques. MATERIALS AND METHODS Five Integral Omniloc cylindrical implants 13 x 4 mm were polished to remove the hydroxyapatite coating, then six linear miniature strain gauges were attached, three on each side of the implants surface. Two similar implants were embedded in a photoelastic material. Three abutments, straight, 15 degrees, and 25 degrees, were connected to each implant; strain versus applied compressive forces were recorded. Strain response to force parameter was defined as the slope of the strain-force curve. Isochromatic fringe patterns were also recorded. RESULTS The strain gauge measurements showed higher, threefold and 4.4-fold, compressive strain concentration in the coronal zone of the implant when 15-degree and 25-degree angulated abutments were used, respectively, compared with the straight abutment; whereas the photoelastic method showed an increase of only 11% in fringe order. Tensile strains were also measured from the coronal contralateral position on the implant, where photoelastic models did not show a change in stress type. CONCLUSIONS Data obtained from strain gauges bonded to implants embedded in a medium can represent a precise simulation of the clinical condition when analyzing stress distribution along the implant/ bone interface. Photoelasticity provides different information and therefore should be regarded as a complementary method.

The effect of porcelain color on the hardness of luting composite resin cement

Visible light- and dual-curing composite resin luting cements were cured under porcelain disks of different colors to examine the effect of porcelain color on surface hardness. Knoop microhardness measurements were made at exposure times of 48, 72, and 120 seconds. Higher levels of hardness were obtained with dual-cured resin. Visible light-cured resin irradiated through colored porcelain required more prolonged exposure times to reach a degree of hardness approaching that of dual-cure resin. From the standpoint of microhardness, dual-cured composite resin is preferred for luting porcelain restorations of > or = 2 mm thickness.

Comparison of microleakage for three one-bottle and three multiple-step dentin bonding agents.

PURPOSE This in vitro study compared the ability of several recently introduced 1-bottle adhesives to their preceding multiple-step dentin bonding agents in reducing microleakage around Class V composite restorations. MATERIAL AND METHODS Facial and lingual Class V cavities were prepared in 48 molar teeth, which were randomly divided into 6 equal groups. Three groups used the fourth-generation multiple-step systems, and 3 used the fifth-generation single-bottle adhesives. All cavities were restored by composite and subjected to thermocycling and intermittent occlusal loading. After immersion in 0.5% basic fuchsin, the teeth were cut faciolingually in 5 consecutive sections and evaluated for dye penetration using a binocular stereomicroscope. RESULTS There was no statistically significant difference in the degree of microleakage comparing the adhesives generation and manufacturer and the location of the cavity margins. A significant interaction was found between margin location and manufacturer, which implied that the sealing capacity at the enamel and cementum margins was material-specific. CONCLUSION Scotchbond Multi-Purpose and Single Bond adhesives provided the best seal for enamel margins, and One-Step and Optibond FL adhesives were best for cementum margins.

Residual dentin thickness in mandibular premolars prepared with hand and rotatory instruments

The residual dentin thickness in the coronal part of root canals of mandibular premolars after stepback preparation and flaring with Gates Glidden (GG) drills was assessed sequentially in a newly designed muffle device. Twelve extracted teeth were embedded in clear polyester resin, sectioned horizontally (1, 3, and 5 mm apically to the cementoenamel junction), and reassembled in the muffle device. The sequence of canal preparation was K-files to #40, then GG-2 and GG-4. After each procedure, the slices were separated and residual dentin thickness measured by a toolmaker microscope in four directions (buccal, lingual, distal, and mesial) and reassembled in the muffle device. Statistical analysis by three-way analysis of variance with repeated measures was undertaken. The difference in residual dentin thickness was highly significant with regard to instrument (control, K-40, GG-2, GG-4; p < 0.0001), slice (upper, middle, and lower; p < 0.0003), and direction (B, L, M, and D; p < 0.001). In each slice, the width of the mesial side was similar to the distal side, as was the buccal to the lingual sides. Reduction of residual dentin thickness in the mesiodistal direction, from the unprepared upper slice to the GG-4 prepared lower slice, was appreciably greater (35%) than in the buccolingual direction (5%).

Analysis of strength properties of light-cured resin composites

OBJECTIVES To determine and correlate the compressive and tensile strengths of resin composites, to scale their failure probability and to analyze their failure mode under combined state of stresses. METHODS Ten brands of composites were tested for compressive and diametral tensile strengths. A recently introduced device for testing of pure shear stresses was modified to adapt to smaller specimens. Uniformity of pure shear stress distribution in the significant section was verified by a photoelastic model. Loading specimens in pure shear up to failure determined their mode of fracture under combined state of stresses. RESULTS Diametral tensile strength yielded values that were 20% of their respective compressive strength. Multiple comparison test indicated that strength properties of the tensile strength test were much more sensitive in predicting differences between resin composites when compared to a compressive strength test. Pertac (Espe) had the highest compressive strength, Graft LC (GC) and Z-100 (3M) had the highest diametral tensile strength. No correlation was found between tensile and compressive strengths. The Weibull modulus disclosed differences in the liability of the materials to fracture. When combined state of stresses were applied through the pure shear test, failure of each specimen occurred at the principal tensile planes. SIGNIFICANCE Compressive strength cannot predict the ability of the resin composite to withstand tensile stresses. The importance of compressive strength is limited as failure of a brittle material occurs in tension.

Residual dentin thickness in mandibular premolars prepared with gates glidden and ParaPost drills.

STATEMENT OF PROBLEM The main factor that determines the prognosis of restored pulpless teeth is preservation of sound dentin. PURPOSE This study evaluated the residual dentinal thickness (RDT) of mandibular premolars after preparation of post space with Gates Glidden and ParaPost drills. MATERIAL AND METHODS Twelve extracted single canal mandibular premolars were embedded in clear polyester resin to the cementoenamel junction (CEJ) in a muffle device. Three horizontal sections were made 1, 3, and 5 mm apical to the CEJ. Mesiodistal (MD) and faciolingual (FL) axes were carefully marked and the RDT was measured for each slice. Each tooth slice was reassembled in the muffle device with orientation pins, then secured with stabilizing pins. This procedure was repeated after enlarging the root canal to K-40 file and preparing the coronal root canal space with Gates Glidden drills and ParaPost drills Nos. 3, 4, and 5. RESULTS Residual dentinal thickness in a MD direction was 3.77 +/- 0.51 mm in the unprepared upper slice and 2.23 +/- 0.31 mm in the No. 5 ParaPost drill prepared lower slice, for a difference of 41%. The corresponding values for the FL direction were 4.35 +/- 0.51 mm and 4.08 +/- 0.46 mm, respectively (6%). CONCLUSION The average dentinal thickness 5 mm below the CEJ in the mesial and distal directions after post space preparation approached the accepted minimal 1 mm. A conservative approach to post space preparation was advocated.