Haim Baharav

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.

Effect of retention grooves on tooth-denture base bond

The bond strength of acrylic resin teeth with and without retention grooves processed onto standard and high-impact denture base resin was investigated. A shear compressive force was applied at an angle of 130 degrees to the lingual surface of the teeth until fracture occurred. The values were subjected to a two-way analysis of variance with repeated measures. Canine teeth bonded better than central or lateral incisors (p less than 0.01). A significantly greater force was required to fracture teeth from high-impact resin (p less than 0.01). Vertical retention grooves enhanced bond strength (p less than 0.01).

The effect of marginal thickness on the distortion of different impression materials

An impression of the margins of a prepared tooth and adjacent gingival sulcus must be of sufficient thickness to withstand distortion and tearing when the impression is removed from the mouth. The purpose of this study was to compare the dimensional accuracy of Elite, Examix, and Express polyvinyl siloxanes; Permadyne polyether; and Permlastic polysulfide elastomeric impression materials. These materials were used to make impressions of a metal model that simulated prepared abutments with gingival sulci of various widths. A traveling microscope was used to measure the abutments and impression widths, and the number of defects in each impression was recorded. Between 70% and 100% of the abutment impressions with sulcular widths of 0.05 mm exhibited defects, which prevented accurate measurements in this group. Express material demonstrated a high number of defects in the 0.10 and 0.16 mm sulcular width groups. No great difference in average maximum distortion values or coefficients of variation (CV) were detected among the materials used to make impressions of abutments with sulcular width groups of 0.2 to 0.4 mm. For the sulci of 0.16 mm and less, Examix and Permiastic materials exhibited distortion and a CV comparable to the impressions of the wider sulcular groups, whereas Elite and Permadyne showed greater distortions and CVs. The differences were not statistically significant (analysis of variance) because of the larger CV among the groups. The larger coefficient of variation in the 0.1 and 0.16 mm sulcular width groups demonstrated inconsistencies in obtaining good impressions of abutments with such narrow sulcular widths.

Effect of glaze thickness on the fracture toughness and hardness of alumina-reinforced porcelain☆☆☆

STATEMENT OF PROBLEM Although porcelain is the most esthetic restorative material available, it is subject to fracture during function. Glazing reduces the size of flaws in the surface of the porcelain and increases its resistance to crack propagation, but the optimum thickness of this glazed layer has not been determined. PURPOSE This study compared the fracture toughness (Kc ) and Vickers hardness number (VHN) of an alumina-reinforced porcelain at different thicknesses of glaze. MATERIAL AND METHODS Disks of feldspathic porcelain reinforced with 2% aluminum oxide were prepared and glazed for 0, 30, 60, 90, and 120 seconds. Fracture toughness and Vickers microhardness were determined with a microindentation technique. Thickness of the glazed layers was measured with a SEM. RESULTS Mean Vickers hardness number and fracture toughness values both decreased significantly with glazing times from 0 to 60 seconds, and increased significantly with glazing times from 60 to 120 seconds (P <.001, ANOVA). SEM examination revealed an increase in glazed layer thickness with increased glazing time. CONCLUSIONS Minimal and maximum thicknesses of glaze layers on alumina-reinforced porcelain resulted in a surface that was harder and more resistant to fracture than moderate glaze thicknesses.

Effect of irradiation time on tensile properties of stiffness and strength of composites

PURPOSE We compared the effect of irradiation time on the diametral tensile strength and stiffness of three visible light-cured composites. The sensitivity of the two mechanical properties to differentiate between the materials was also investigated. MATERIAL AND METHODS Cylindrical specimens of three composites were cured for 20, 40, 80, and 120 seconds (n = 10) and loaded up to failure in a diametral tensile strength test. Stiffness and diametral tensile strength values were recorded. RESULTS Irradiation time significantly influenced diametral tensile strength (p = 0.0017) and stiffness (p = 0.0002). With the same irradiation times the three tested materials demonstrated no significant difference when diametral tensile strength was studied (p = 0.31). However, stiffness values did show a significant difference (p = 0.0002). CONCLUSIONS This study found that stiffness is more sensitive in disclosing differences in tensile properties between the materials than diametral tensile strength.

THE INFLUENCE OF SURFACE LOADING AND IRRADIATION TIME DURING CURING ON MECHANICAL PROPERTIES OF A COMPOSITE

PURPOSE The aim of this study was to determine the influence of different surface loadings during curing with various irradiation times on hardness and diametral tensile strength of a light-cured composite. MATERIAL AND METHODS A mold was fabricated to allow loading during curing of cylindrical specimens of a composite. Four surface loadings of 0, 0.35, 0.87, and 1.73 MPa and four irradiation times of 20, 40, 60, and 180 seconds were used (n = 15). Each specimen was subjected to a microhardness test and to a diametral tensile strength test. RESULTS Surface loading during curing affected both hardness and strength properties, whereas irradiation time influenced only the hardness of the material. Both parameters gained between 15% and 20% improvement when the material was loaded with 0.87 MPa surface pressure and cured by 60-second irradiation time. Higher loading or longer irradiation times did not improve these properties. CONCLUSION Loading composite during curing improves its mechanical properties, probably through decreasing flaws and air voids of the material.

Effect of different cooling rates on fracture toughness and microhardness of a glazed alumina reinforced porcelain

Dental porcelain has superior esthetics but may be subject to fracture during mastication. Residual compressive stresses on the porcelain surface after cooling enhance resistance of porcelain to crack initiation, as quantified by its fracture toughness (Kc). The effect of different cooling rates on Kc and hardness of a glazed porcelain reinforced with approximately 2% aluminum oxide was examined in 45 porcelain disks that were divided into three groups. After final glaze firing, one group was cooled rapidly, the second was cooled at a medium rate, and the third was cooled slowly. Fracture toughness was determined with a microindentation procedure. The mean Kc recorded for rapidly cooled porcelain (1.74 +/- 0.09 MN/m3/2), for medium-cooled porcelain (1.41 +/- 0.07 MN/m3/2), and for slow-cooled porcelain (1.29 +/- 0.07 MN/m3/2) was statistically different (p < 0.001, analysis of variance and Bonferroni post hoc test). No statistically significant differences in Vickers hardness values were recorded when porcelain was cooled at different rates (530 to 540) (analysis of variance). The faster cooling rate of a glazed alumina reinforced porcelain resulted in greater fracture toughness but had no effect on hardness.

The effect of irradiation time on the shear strength of composites

OBJECTIVES The purpose of this study was to determine the shear strength of composites at a specific depth by a double shear test and to relate it to irradiation time. METHODS Aluminum molds filled with three different composites were irradiated for the manufacturers recommended time, as well as for three longer and two shorter times. Fifteen cylindrical specimens were prepared for each combination of material and exposure time and subjected to a double shear test based on the principle of rivets connecting various structural members. The shear planes were symmetrically located 1.5 mm from both outer surfaces. RESULTS Shear strength vs. irradiation time was directly but nonlinearly related. All test materials attained a maximum shear strength value which was not exceeded by a further increase of the exposure time. Maximum values obtained were 42.5 MPa for Heliomolar (Vivadent) and Durafill (Kulzer) and 66.9 MPa for P-50 (3M). Only the shear strengths using the shortest light activation time were significantly different for each product. SIGNIFICANCE Shear strength of composites at a specific depth is a function of the combination of light energy and the attenuating power of the specific material. A double shear test might serve as an adjunct to microhardness in determining the extent of cure of composites at a specific depth. The advantage of this testing is the extra information to be gathered regarding the ability of composites to withstand internal stresses at a predetermined distance.

The continuous brushing acid-etch technique

The continuous brushing of an acid-etching agent onto the enamel surface of an extracted tooth resulted in a more debris-free surface. SEM photomicrographs showed dissolution of some of the apatite crystals and a reduction in size of the remaining crystals, thus increasing the potential space between them for retention of unfilled composite.

Comparison between incisal embrasures of natural and prosthetically restored maxillary anterior teeth.

STATEMENT OF PROBLEM The loss of one or more of the incisal embrasures may pose a problem in restoring missing tooth structure. Despite its acknowledged importance in dental esthetics, few researchers have investigated the esthetic impact of incisal embrasures. PURPOSE The purpose of this study was to compare parameters of the incisal embrasures of natural maxillary anterior teeth (ND) with those of prosthetically restored maxillary anterior teeth (PRD) and to compare the resemblance of the prosthetic restoration to natural form. MATERIAL AND METHODS Twenty gypsum casts of the maxillary anterior teeth (MAT) of patients made at the completion of their orthodontic treatment comprised the control group. The experimental group consisted of 20 gypsum casts of prosthetically restored maxillary anterior segments. Five incisal embrasures of each cast were digitally photographed under uniform, standardized conditions: the incisal embrasures between the canine and lateral incisor on each side, between the lateral and central incisors on each side, and between the 2 central incisors. The photographs were measured by image analyzing software (Bioquant Nova Prime). The parameters compared were: (1) the areas of the incisal embrasures in the natural and prosthetic casts (measured in mm(2)); (2) the areas of homological incisal embrasures in each of the groups; and (3) the degree of lateral symmetry of the natural casts as compared to the prosthetic casts. The results were subjected to 2-way ANOVA and Student t tests (alpha =.05). RESULTS The total area of incisal embrasures of the PRD casts was smaller than the total area of those of the natural dentition casts (P=.005). The incisal embrasures of the PRD casts were smaller than the homological embrasures of the natural dentition casts (P<.01). Both groups demonstrated a lack of lateral symmetry (P<.001). The natural dentition casts demonstrated a greater asymmetry than the restored dentition casts (canine/lateral P<.045 and lateral/central P<.035). There was no significant difference in the sizes of the central incisal embrasures of the 2 groups. The corresponding incisal embrasure areas (ND versus PRD) were significantly different, except for the 2 central embrasures. CONCLUSIONS Prosthetically restored dentition casts differed from natural dentition casts in 2 parameters: the area of the incisal embrasures and the degree of lateral symmetry.