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Biomedical subjects

H Baharav

Publications and source records attributed to H Baharav.

At least 19 recordsLinked to original sources
PubMed1999

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.

Aluminum Oxide↗
PubMed1997

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.

Air↗
PubMed1997

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.

Bisphenol A-Glycidyl Methacrylate↗
PubMed1997

The closure of the gingival crevice following gingival retraction for impression making.

Scant attention has been paid to the effectiveness of chemomechanical displacement of the gingiva prior to impression making for fixed partial dentures. The closure of the gingival crevice following removal of medicated retraction cord was observed using a miniature video camera. Sulcular widths were measured at time intervals at the midbuccal and transitional line angle areas. The closure rate of the transitional line angle area was significantly faster than that of the mid-buccal area during the first 90 s. An average sulcular width of 0.2 mm was reached at the transitional line angle after less than 30 s.

Adult↗
PubMed1996

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.

Analysis of Variance↗
PubMed1996

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.

Aluminum Oxide↗
PubMed1995

Removable partial denture with a lateral rotational path of insertion.

A removable partial denture with a lateral path of insertion is useful when unsightly facial clasps are objectional to the patient. Rounded rest seat preparations allow extensions of the minor connector to rotate laterally into embrasure undercuts, providing retention. Guiding planes on the contralateral side ensure a different path of withdrawal from the retentive elements. A Kennedy Class IV arch with a long edentulous span is used to illustrate the denture design.

Dental Clasps↗
PubMed1994

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 manufacturer's 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.

Acrylic Resins↗
PubMed1993

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.

Analysis of Variance↗
PubMed1990

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).

Acrylic Resins↗
PubMed1988

Effect of exposure time on the depth of polymerization of a visible light-cured composite resin.

The effect of exposure time of a visible light source on the depth of polymerization and degree of hardness of a sample of Occlusin posterior composite resin was investigated. The border between cured and non-cured composite resin was identified by a change in colour and by applying pressure with a scalpel. Knoop hardness tests were performed perpendicular to the long axis of illumination. The composite resin nearer to the light source underwent more complete polymerization. Increased exposure time resulted in greater depth of cure. The rate of polymerization was greatest in the first 10 s. Maximum hardness measured up to a depth of 1 mm obtained after 80 s of exposure time. At greater depth, a decrease in Knoop hardness was observed. At exposures under 80 s, maximum hardness was not achieved even at a depth of only 1 mm.

Chemical Phenomena↗
PubMed1987

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.

Acid Etching, Dental↗