PubMed Health⌕ Search

Biomedical subjects

I Krejci

Publications and source records attributed to I Krejci.

At least 19 recordsLinked to original sources
PubMed1992

Effect of contact area size on enamel and composite wear.

The effect of contact area dimensions on the wear of composite specimens and their opposing enamel cusps was evaluated in vitro. Thirty-six standardized cylindrical composite specimens were placed into metal cavities (8 mm x 2 mm) and divided randomly into five groups. The composite used was a fine-particle hybrid and was stressed as follows: storage in 75% aqueous ethanol solution for 24 h, toothbrush/toothpaste-abrasion for 30 min, followed by 300 thermal cycles in water ranging from 5 degrees to 55 degrees C and simultaneous 120,000 occlusal chewing loads at a frequency of 1.7 Hz at 53 N maximum force. In group 1 (n = 12), the occlusal chewing loads were applied by palatal cusps of extracted human maxillary molars with natural morphology. In groups 2 to 5 (n = 6), the cusp tips had standardized contact area dimensions of 0.26, 0.38, 1.18, and 4.10 mm2, respectively. Wear of composite specimens and antagonistic enamel cusps (means +/- SD) was assessed in microns by means of a 3-D scanner. Additionally, the contact surfaces of the restorations and of the antagonistic enamel cusps were evaluated by SEM. Increases in enamel contact areas after being loaded were measured by means of a digitizer and expressed in percent of the initial size before stress exposure. The wear of the composite specimens varied from 69.8 +/- 19.9 to 9.5 +/- 3.6 microns, and that of antagonistic enamel cusps from 31.3 +/- 3.4 to 8.8 +/- 1.5 microns. The increase in contact area varied between 27.8 and 0.1%.(ABSTRACT TRUNCATED AT 250 WORDS)

Cold Temperature↗
PubMed1992

Chewing pressure vs. wear of composites and opposing enamel cusps.

The effects of various chewing pressures on the wear of composites and enamel were assessed in vitro. Standardized composite discs (8 mm in diameter, 2 mm in height) were made of a fine-particle hybrid (FPH), a coarse-particle hybrid (CPH), and a homogeneous microfilled composite (HMC). The composite specimens were chemically degraded by immersion in 75% ethanol for 24 h, brushed for 30 min, and then thermocycled 300 times (5-55-5 degrees C) while being occlusally loaded 120,000 times at 1.7 Hz, with chewing forces of 25, 50, 75, and 100 N. Standardized human enamel cusps with a uniform contact area of 0.384 mm2 served as antagonists in the chewing machine. Wear of the composites and enamel cusps, their combined wear, and the increase of the enamel contact surfaces were quantified. An increase in chewing pressure significantly enhanced the wear of both composite and enamel in all groups except for the antagonists opposing a HMC. The FPH was most wear-resistant to in vitro chewing pressures in the range of 6.58 to 19.74 MN/m2, the CPH at 26.32 MN/m2, while the HMC was the most enamel-friendly of the three composites tested. The FPH composite had the least disintegration in the occlusal contact area. The ranking of the composites generally varied at the different chewing pressures with respect to the three types of quantified wear--that is, composite wear, enamel wear, and total wear.

Acrylic Resins↗
PubMed1992

Effects of cement-curing modes on dentin bonding of inlays.

The aim of this in vitro study was to evaluate dentin adhesion after cementation of immediate direct "All Purpose" Hybrid (AP.H) composite inlays (Dentsply) and Cerec Dicor-MGC (Dentsply) inlays with the dentin adhesive Prisma Universal Bond 2 (Dentsply) and the dual-curing Dicor-MGC luting composite (Dentsply). In 24 extracted human molars, standard MOD cavities were prepared with one approximal margin located in enamel and the other one located in dentin. They were divided into four groups: (I) AP.H inlays, luting composite only, chemically cured; (II) AP.H inlays, luting composite, immediately-light-cured; (III) AP.H inlays, luting composite, initially chemically and delayed-light-cured (15 min); and (IV) MGC inlays, luting composite, initially chemically and delayed-light-cured (15 min). In vitro load cycles corresponding to five years of clinical stress followed. Initially and after specimens were loaded, the margins were analyzed quantitatively by SEM. The tooth/cement and cement/inlay interfaces were scored separately. The initial percentages of "continuous margin"--at both the tooth/luting composite and luting composite/inlay interfaces--were higher than 94% for all groups. At the end of the load cycles, the quality of the margins at the tooth/luting composite interface significantly decreased for all groups. The highest decrease was found for the cervical margins located in dentin, where only 37%-61% were scored as "continuous margin". The AP.H inlay/luting composite interface showed almost no change. At the MGC inlay/luting composite interface, the percentage of "continuous margin" decreased to 74%. After specimens were loaded, the percentage of "continuous margin" in dentin was lower than in enamel, despite the use of a dentin bonding agent (PUB 2).(ABSTRACT TRUNCATED AT 250 WORDS)

Analysis of Variance↗
PubMed1992

The importance of proximal curing in posterior composite resin restorations.

This in vitro study compared the marginal adaptation of enamel-bonded mesio-occlusodistal composite resin restorations placed using two different insertion techniques (one-step and two-step) and three different curing techniques (occlusal using opaque matrix and wedge systems and interproximal using transparent matrix systems and either transparent, non reflecting, or transparent, laterally reflecting wedges). The micromorphology of the enamel-restoration interface and the marginal seal were evaluated after the restorations were exposed to mechanical and thermal stresses. Although the difference was not statistically significant, the marginal quality of the restorations placed with the two-step technique was better than that of the restorations placed with the one-step technique. The restorations cured from the interproximal direction had significantly better marginal adaptation than did restorations cured only from the occlusal direction. The quality among the groups of interproximally cured restorations was not significantly different, because all the restorative techniques that were evaluated resulted in generally poor marginal quality.

Analysis of Variance↗
PubMed1992

Restoration quality in relation to wedge-mediated light channeling.

Thirty large mesio-occlusodistal cavities with their margins totally in enamel were restored using the three-sited light-curing technique in five different ways: group 1 = Mark I version of a light-reflecting wedge; groups 2 and 3 = interproximal curing with transparent, nonreflecting wedges; groups 4 and 5, Mark II version of the light-reflecting wedge with increased resiliency. In groups 3 and 5, the light that was not conducted by the transparent wedges was shielded by a piece of aluminum foil. It was found that, when used directly and unshielded, both Mark I and Mark II versions of the light-reflecting wedge induced significantly better marginal adaptation than the transparent, nonreflecting wedge, especially gingiovproximally. With both the nonreflecting and reflecting wedges, the shielding had no effect on the overall percentage of excellent margin. However, gingivoproximally, the shielding significantly improved the marginal quality in the nonreflecting wedge groups, leading to the conclusion that the first increment should be cured as thoroughly as possible from a gingivoproximal direction, preferably indirectly via light-conducting wedges. Direct irradiation from a lingual or buccal direction is less favorable.

Composite Resins↗
PubMed1992

In vitro evaluation of dentinal bonding agents in mixed Class V cavity preparations.

The effect of ten dentinal bonding agents on composite resin restorations was tested in mixed Class V cavity preparations. The marginal quality of the restorations was quantified under a scanning electron microscope before and after thermocycling. In addition, the marginal seal was semiquantitatively evaluated with a dye penetration technique. Marginal adaptation in dentin was poor in seven test groups. Three products showed values of excellent margin in dentin exceeding 50% after thermocycling. Furthermore, some dentinal bonding agents had a deleterious effect on marginal adaptation to the etched enamel surfaces.

Bicuspid↗
PubMed1992

[The effects of the processing technic on dentinal adhesion].

In this in vitro study, the effect of application techniques on marginal adaptation of thermal cycled class V restorations restored with an actual dentinal adhesive were quantitated using SEM and dye penetration. Under optimal application conditions, excellent results were generated: though a simple bulk insertion technique was used, no difference was found between enamel and dentin margins before and after thermal loading. On the other hand, contamination of the bonding agent with saliva before composite resin insertion dramatically decreased the percentage of "excellent margin" in dentin and in enamel. Similar effects were found after protecting the bonding agent with an insulating gel. No improvement in marginal adaptation was found after reapplication of the bonding agent. The class V inlay did not show better results than the direct filling technique. However, postcured inlays performed better than their non-postcured counterparts. Without etching enamel with a phosphoric acid gel, but by priming its surface, approximately 80% of "excellent margin" were found in enamel before thermal cycling. This percentage decreased significantly after thermal loading. Restorations totally confined to dentin had slightly lower percentages of "excellent margin" than the mixed class V fillings. The results of this study indicated, that by using dentinal adhesives, little changes and deviations from the correct procedure result in significant alterations in marginal adaptation.

Composite Resins↗
PubMed1992

[Composite inlays: clinical and scanning electron microscopic research after a 1-year functional period].

24 composite inlays placed and documented for post-graduate education purposes in 1990 were reassessed clinically and by means of SEM. The clinical examination after 1 year of clinical service gave excellent results. The quantitative SEM evaluation of the marginal adaptation in the replicable areas revealed 6.6% of marginal gaps at the tooth/cement- and 27.1% at the cement/inlay-interface. To be able to compare these findings to the initial situation, additional five inlays were placed and examined in the SEM immediately after placement. This group exhibited next to gap-free margins. The results were selectively analyzed according to fabrication technique, type and localization of the restorations. No significant difference was found among molars and premolars. On the other hand, higher percentages of marginal gaps at the cement/inlay interface were observed with multi-surfaced, indirect lab-made inlays.

Composite Resins↗
PubMed1992

[Marginal adaptation of adhesive inlays after removal of unpolymerized composite cement].

The removal of polymerized excess resin cement around adhesive inlays is very time consuming and often tooth-destructive. In this study we evaluated, if brushing away of the unset resin cement produced any marginal imperfections. It was found that this procedure caused underfilled margins especially in the occlusal portion of the inlay margins. For this reason the use of this method to remove excess resin cement is not recommended.

Cementation↗
PubMed1992

[Substance ablation with a superpulsed CO2 laser].

In this study a new, superpulsed CO2-laser was evaluated to remove enamel, dentin and several dental restorative materials. In the first part of the experiment, superpulsed mode was compared with pulsed and continuous wave using semiquantitative test methods. It was found that only superpulse was suitable for the purpose. In the following second part of the experiment, temperature rise, crater depth and crater diameter were quantitated for the superpulsed mode in a range of parameters defined in the first experimental part. It is concluded that superpulsed CO2-laser basically has the potential for a mechanical drill substitute, provided that the output power is limited to about 1 W and short pulses of 0.2-0.3 s duration are applied. In the present form the system is not adapted to clinical use. Cooling of the substrate and flexible beam delivery must be developed.

Dental Amalgam↗
PubMed1992

Computer-designed inlays after 5 years in situ: clinical performance and scanning electron microscopic evaluation.

Eight mesio-occlusodistal adhesive inlays, fabricated from ceramic blocks using a computer-aided design/machining method, were reexamined after 5 years of clinical service. The inlays wre evaluated according to modified US Public Health Service criteria. The results indicated excellent clinical performance, with eight Alfa ratings for wear, recurrent caries, and color match, and five Alfa and three Bravo for marginal discoloration and marginal integrity. Under scanning electron microscope, 81.0% of the tooth-cement interfaces and 84.1% of the cement-inlay interfaces were rated continuous at the occlusal margin. Axially, 73.6% of the tooth-cement interfaces and 87.0% of the cement-inlay interfaces were rated continuous. Although this long-term investigation revealed good clinical performance of the inlays, improvements in the ceramic structure and its properties, the cavosurface design, and the luting composite resin were recently introduced to further optimize quality.

Ceramics↗
PubMed1992

Clinical evaluation of a new pressed glass ceramic inlay material over 1.5 years.

Ten IPS/Empress pressed glass ceramic inlays were cemented in box-shaped, non-beveled, Class II, posterior cavities. They were evaluated clinically according to modified US Public Health Service criteria after 1.5 years in vivo. In addition, quantitative marginal analysis was performed immediately after placement of the inlays and at the 1.5-year recall. Clinical evaluation revealed that the inlays performed well after 1.5 years; all inlays received scores of Alfa or Beta for all criteria evaluated. Scanning electron microscopic examination indicated that the excellent initial marginal adaptation decreased significantly over 1.5 years.

Adhesiveness↗
PubMed1992

[The optimization of dentin bonding in mixed Class-V cavities].

A new functional bonding agent was tested in the placement of mixed class V in vitro restorations. Variable insertion procedures of the composite material were also evaluated. One of the procedures was the direct filling technique, the other were direct immediate composite inlays, cemented with light, chemically or dual cured composite cements. The quantitative SEM marginal analysis results showed a significant superiority (p less than 0.05) of the adhesive inlay restorative procedure over the direct filling technique. Dye penetration confirmed these findings. Among the inlay groups, postcuring and the curing method of the composite cement were crucial for the success.

Dental Bonding↗
PubMed1991

Quality and durability of marginal adaptation in bonded composite restorations.

Excellent marginal adaptation extends the longevity of restorations. Unfortunately, polymerization shrinkage of composite restorations adversely affects this quality requirement. The residual stress within the cured resin compromises the material's properties, causes marginal openings, and flexes cavity walls. In this study, the wall-to-wall contraction in MOD cavities was measured for different placement techniques. In addition, the restoration margins were quantitated before and after thermo-cycling and mechanical stressing. Factors which enhanced adaptation also optimized marginal quality and reduced the amount of residual stress. The latter was expressed by intercuspal narrowing after the restoration was completed. Both quality and stress resistance of the marginal adaptation were inversely correlated to the intercuspal narrowing caused by the polymerization contraction of bonded and excellently adapted resin restorations. The most effective factors which optimized marginal quality included: guidance of the shrinkage vectors; reducing the ratio of bonded to free, unbonded restoration surfaces; and minimizing the mass of in situ-cured composite. The latter principle was followed best in the adhesive inlay technique. In medium-sized adhesive MOD composite inlays, the volume loss induced by the polymerization contraction of the composite cement was non-destructively compensated for by an inward flexing of each cavity wall of approximately 10 microM.

Composite Resins↗
PubMed1991

Marginal adaptation of Class V restorations using different restorative techniques.

This in vitro study compares the marginal adaptation of Class V restorations with margins located half in enamel and half in dentine, which were placed using different restorative techniques. Five operative procedures were evaluated both in saucer-shaped erosion lesions and in box-shaped cavities with bevels in enamel. The five procedures included a composite inlay technique using both the chemically and the light curing versions of a resin based composite cement, a bulk placement technique using a chemically curing composite resin, an incremental technique and an incremental technique combined with a built-up base, using a light curing composite resin. A combination of Gluma/Clearfil served as the dentinal adhesive. The micromorphology of the tooth/restoration interface was analysed before and after thermal cycling; the marginal seal was analysed after thermal cycling only. In the conventional cavities, the restorations showed less leakage, and micromorphologically a better, but statistically insignificant superior marginal adaptation. The inlay technique rendered the best marginal quality in both enamel and dentine before and after thermal cycling. Due to the unique curing characteristics of the chemically cured composite resin and cement resulting in a significantly reduced rigid contraction, the inlays cemented with the chemically curing cement and the restorations placed with the chemically curing composite resin were superior to their light cured counterparts. The built-up base yielding a reduction of the composite mass did not enhance marginal adaptation because of the partial replacement of the strong adhesion to dentine mediated by the Gluma/Clearfil combination by the weaker bond promoted by the etched glass ionomer cement.

Acid Etching, Dental↗