Cement Film Thickness Research Articles

Stress distribution in metal-ceramic crowns with a facial porcelain margin.

The use of metal-ceramic restorations with porcelain butt-joint facial margins has increased in the past several years. Although these crowns exhibit improved esthetics compared with metal-ceramic crowns which display a metal gingival collar or metal knife-edge margin, the effectiveness of this design in resisting intra-oral forces is not known. The objective of this study was to analyze the stress distribution induced by simulated intra-oral loads on crowns with variable coping configurations. The copings, with a thickness of either 0.1 or 0.3 mm, were modeled with a facial termination of metal at three locations: at the gingival floor, 0.9 mm above the gingival floor, and 4.2 mm above the gingival floor. The coping and crown dimensions were based on a prepared maxillary central incisor with a facial shoulder and a lingual chamfer. Both Ni-Cr and Au-Pd alloy copings were employed in the crown models. Finite element stress and analyses were performed on crowns which were subjected to several loading conditions. A cement film thickness of 0.030 mm was assumed. For all cases, the stresses which developed in porcelain and cement near the facial and lingual margins due to a vertical load of 200 N were predominantly compressive in nature. For the crowns with Ni-Cr copings, the tensile stress in porcelain ranged from 11.0 MPa (for crowns with a facial metal thickness of 0.3 mm) to 12.5 MPa (for a metal thickness of 0.1 mm). The corresponding stresses for crowns with Au-Pd alloy copings were 8.3 MPa and 8.6 MPa, respectively.(ABSTRACT TRUNCATED AT 250 WORDS)

Forces fracturing cements at die interfaces and their dependence on film thickness

Film thickness (FT) and retention of zinc phosphate, polycarboxylate and ionomer cement as a function of powder-liquid (P/L) ratio were determined for both non-vented and vented crowns and using a standard ADA/ANSI test. The effect of P/L ratio on FT was more dependent on the composition of the brand than on the type of cement. FT values obtained from the specification test usually lay between those obtained from the vented crowns and from the non-vented crowns. Higher consistency (higher P/L ratio) of the mix increased the FT. The relationship between FT of P/L ratio and retention is complex and dependent on the type of cement, brand, and on the technique used. Venting of the prepared crown lowered the FT and usually tended to increase retention.

Variables affecting bond strength of resin-bonded bridge cements

In vitro tensile bond strengths of an etched Ni-Cr-Be alloy to enamel were measured using 5 two-paste and 2 paste-primer resin cements. Bond strengths of 2 cements tested were 50–60% higher to rough than to smooth enamel. Increasing the film thickness of a paste-primer cement from complete seating to 0.51 mm decreased the bond strength by 73%. Bond strengths of the 7 cements ranged from 7.3 to 12.0 MN/m 2 and all failures occurred at the cement-enamel interface for samples completely seated to rough enamel.

Film thickness of dental luting cements.

The film thickness as a function of time after start of mixing was measured for several dental luting cements (one zinc phosphate, 2 polycarboxylate and 3 glass ionomer cements) with 2 different methods, i.e., between 2 plane plates as used to control film thickness in several specifications/standards (Method 1), and in a crown-tooth model (Method 2) simulating clinical conditions. The results showed different film thickness values for the various cements, and these values differed depending on the time after start of mixing in the 2 systems. The minimum film thickness was, in general, lower when measured using Method 2 than using Method 1. For the zinc phosphate cement a shorter time was recorded in Method 1 than in Method 2 for the time lapse from commencement of mixing until a rapid increase of the film thickness started. For the polycarboxylate cements, an opposite relation was recorded for this time-dependent change. The results for the glass ionomer cements were variable. The start of the rapid increase in film thickness was in general more distinct in Method 2 than in Method 1. The results question the validity of defining working time for dental luting cements by a specified film thickness value as measured between 2 plane plates.

Factors affecting the seating and fit of complete crowns: A computer simulation study

E ffective cementation is crucial to the successful placement of a complete crown, and poor seating results in inadequate marginal adaptation. Furthermore, an incompletely seated crown or fixed partial denture (FPD) compromises function.ls4 American Dental Association (ADA) specification No. 8 states that cement film thickness should not exceed 25 pm, but studies have demonstrated that the film thickness is frequently greater.5-” Burnishing subgingival crown margins to improve the marginal seal, particularly with ceramometal restorations, is impractical or impossible. I2 Therefore, the coping, the materials, and the crown cementation determine the quality of the seal. The effects of modifying technical and clinical procedures on seating have been repeatedly measured.5-‘4 However, clinical research is often limited by the experimental design, the technical difficulties of reproducing the unique orientation and fit of the coping to the die, or the consistency of the cement.*5,‘6 To overcome these problems, investigators have devised theoretical approaches to demonstrate fundamentals that influence the fit of crowns or crown cementation.17”9 A computer model was developed to combine the benefits of precision theoretical analysis and clinically applicable experimental design to reproduce several experimental studies.*, “r “3 21 This investigation applied the computer model in simulation of crown cementation under various clinical conditions.

A mathematical treatise on the fit of crown castings

A mathematical analysis was given to calculate the marginal cement thickness, Dm, of a cemented crown casting as a function of the minimum cement film thickness, F; the width, W, of the base of the tapered region; the width, W', at the bevel (if present) of the tooth preparation; the taper angle, theta; the marginal angle, beta; and the compensatory expansion, X, of the metal investment system: Dm = [F/sin theta - 0.5(X - 1) (W cot theta - W' cot beta)] sin beta. The elevation of the cemented casting, delta H, above the tooth preparation may also be obtained from this equation by substituting beta = 90 degrees, in which case Dm = delta H. The effect of the preparation of a chamfer margin or bevel on the marginal film thickness may be calculated from this equation. An additional analysis showed that the minimum marginal cement thickness, Dm = F, can be obtained for a shoulder preparation and X values of common metal-investment systems by applying a spacer of appropriate thickness on the entire occlusal and axial walls of the die. This renders the preparation of bevels superfluous. The thickness of the spacer should allow for the cement film thickness, roughness of the tooth and casting surfaces, dimensional inaccuracies of the die, and distortions of the wax pattern. If the spacer is thicker than required, a minimum value of Dm is still obtained for preparations without bevels. If the horizontal part of the die shoulder is covered as well with the spacer, a greater spacer thickness is required to obtain a minimum value of Dm than if the spacer extends to the end of the axial walls. In this case the spacer thickness should be greater on the occlusal die wall than on other regions of the die for values of X smaller than 1. The spacer should also be thicker in the occlusal region if restriction of the investment setting expansion by the wax pattern occurs mainly in this region.

Margins of complete crowns—Literature review

The margin is one of the components of the cast restoration most susceptible to failure, both biologically and mechanically. Most of the investigative proof shows that supragingival margins are kinder to the gingiva than are subgingival margins. However, practicality dictates that supragingival margins are not always usable. There is some indication that quality of the margin may be of as much importance to gingival health as location. Research on the configuration of margins seems to agree with the majority of authors writing technical articles. Most agree that feather edge or knife edge margins are not always the most acceptable. Apparently bulky margins with subtle internal line angles are best. Bevels are recommended where esthetics allows due to their slip-joint effect affording closer adaptation. Most cemented castings do not fit nearly as well as theoretical tests of cement film thicknesses would lead us to believe. Venting or providing space for cement through electromilling, aqua regia etching, or die spacing markedly improves the seating and, therefore, the marginal fit of castings. This seems to be a more useful approach to close fitting margins than burnishing and finishing margins, particularly for subgingival margins. Most of the commonly used fixed partial denture alloys, both precious and non-precious, are too hard to be burnished effectively. As long as fixed prosthodontics must rely on the cemented casting, the search for more knowledge about an innocuous, esthetic, indestructible margin must continue.

Effects of cement on crown retention

This study indicates that the frequent clinical technique of permanently cementing a crown without removing all traces of a temporary ZOE cement from the tooth has no adverse effect on retention. Circumferential grooving of the tooth preparation did not significantly increase retention. In contrast, cutting a shallow groove in the crown significantly improved retention. Therefore, it would appear that the grooving of the gold casting might be advantageous in conditions where the crown restoration has minimal retention. Significant improvement in retention with grooving of the restorations was accompanied by a change in the location of the cement film. The cement was retained in the crown coronal to the groove rather than on the tooth. This finding was interesting, since previous retention studies showed that adhesive failure occurred at the metal-cement interface. From this data it appears that a primary factor of crown retention is the adhesion or mechanical interlocking of the cement to the crown. This would explain why small variations in cement film thickness, such as those due to the use of die relief, have little effect on retention. In view of these findings, the current concepts on cement retention and cement adhesiveness should be reevaluated.

Effect of multiple axial grooves on the marginal adaptation of full cast-gold crowns

Twenty-seven crowns were fabricated on the standardized ivorine dies to determine the effect of multiple axial grooves. No grooves were placed on the control (group I). In groups II and III, respectively, two and four grooves were replaced. The mean value of the occlusal cement film thickness was used to appraise the vertical discrepancy (lack of seating) of the crown on the die. This value was obtained by averaging seven measurements of the occlusal cement film thickness taken at different areas. These measurements were recorded from the photomicrographs at x 100 magnification. There were significant differences between the control group and both test groups. Results of this study show that the presence of the grooves has an adverse effect on the seating of full cast-gold crowns.

Powder enrichment effects on film thickness of zinc phosphate cement

The effects of powder enrichment and subambiant mixing temperatures on the film thickness of three zinc phosphate cements were measured by two methods. A decrease in film thickness accompanied reduced mixing temperatures and standard powderliquid ratios. Increased powder-liquid ratios with reduced mixing temperatures showed a decreased film thickness when measured according to ADA Material Specification No. 8. Both Fleck's and S. S. White cement showed an increased film thickness when measured by a tapered-die system. Because of the geometry of the preparations, small increases in cement film thickness can be clinically significant when cementing complete veneer crowns.

Film thickness of cements beneath complete crowns

The results showed that Durelon, EBAC, zinc phosphate, Epoxylite CBA, and Fynal had thinner film thicknesses than EBA and Fluro-Thin. In addition, venting of a crown resulted in lower film thicknesses with all cements except Fynal and EBAC. No significant differences occurred with these two cements whether the crown was vented or not. Obviously many other factors besides film thickness, such as toxicity, strength, and solubility, have to be considered before selecting a cement for clinical use. When only film thickness is to be considered, all of the cements in the study could be recommended for the cementation of crowns with the exception of EBA and Fluoro-Thin.

The extent of slits at the interfaces between luting cements and enamel, dentin and alloy

Four different cements were used to assess the presence of slits at the cement/tooth or the cement/alloy interfaces using a tooth-crown model. The model consisted of ground sections of teeth and plane plates of silver/palladium alloy. The plates were fixed with bolts between two brass plates and with three different dimensions of the cement film between tooth and alloy, i.e. 50 micrometer, 100 micrometer and 200 micrometer. The tooth-alloy specimens were sectioned and the adaption of cements was studied with an indirect technique (replica) in a scanning electron microscope. The extent of slits was expressed as the length of all slits relative to the total length of the interface in each specimen. The results showed that the zinc phosphate cement and polycarboxylate cement exhibited a slight to moderate tendency to formation of slits at the interfaces. The EBA cement had a small extent of slits adjacent to thin cement films, but more slits were observed with increasing film thickness. The composite resin cement had a marked tendency to slit formation independent of the cement film thickness.

Measurement of the cement film thickness beneath full crown restorations.

Measurement of the cement film thickness beneath full crown restorations.

The estimation of cement film thickness by an in vivo technique.

The estimation of cement film thickness by an in vivo technique.

The effect of the surface roughness of prepared dentin on the retention of castings

Thirty-six preparations were made from extracted teeth and divided into 3 groups. The dentin surfaces of the 3 groups were made smooth, fine, and rough. Open-top gold crowns were cast for each tooth and cemented with zinc phosphate cement. Ten samples from each group were separated twice under an axial tensile force and 2 samples from each group were sectioned for study of the cement film thickness. The reliability of the testing procedure was checked by making a set of 8 ivorine dies. No significant difference between groups was found.

Zinc oxide and eugenol cements for permanent cementation

A group of zinc oxide and eugenol cements designed for permanent cementation were compared to zinc phosphate cement on the basis of compressive strength, tensile strength, solubility, and film thickness. In addition, the amount of tensile force required to remove standardized gold inlays from the prepared cavity when they were cemented with zinc phosphate cement and various zinc oxide and eugenol materials was determined. The zinc oxide and eugenol materials were inferior to zinc phosphate cement in respect to compressive strength, but were comparable when tested under tension. Only one of the zinc oxide and eugenol cements complied with the American Dental Association Specification for the film thickness of zinc phosphate cement. The solubility in both acid and water of two of the materials was comparable to zinc phosphate cement, while the remaining three materials demonstrated slightly greater disintegration under the conditions of the test. Inlays cemented with two of the zinc oxide and eugenol preparations required approximately the same tensile force to remove them as did inlays cemented with zinc phosphate cement.

The relationship between the film thickness of zinc phosphate cement and the retention of veneer crowns.

The relationship between the film thickness of zinc phosphate cement and the retention of veneer crowns.

Factors Affecting the Film Thickness of Zinc Phosphate Cements

(1960). Factors Affecting the Film Thickness of Zinc Phosphate Cements. Acta Odontologica Scandinavica: Vol. 18, No. 4, pp. 479-490.