The International Journal of Prosthodontics, 05/1992

Effective radial setting expansion of wax patterns in a gypsum-bonded investment is influenced by many factors, especially by the diameter of the cylindrical casting ring. The hypothesis tested was that the ratio of the volume of the casting rings to the volume of the ring liner is a primary determinant of radial setting expansion. Mesio-occlusodistal inlay wax patterns were invested individually in plastic casting rings of different lengths and diameters and imaged using xeroradiography. Effective setting expansion was determined in 34-mm-long, 28-mm-diameter rings and used as a predictor of expansion in rings of different sizes. Experimental setting expansion results did not differ significantly from the predicted values.

This study investigated the compressive strength of all-ceramic crowns manufactured using two recently introduced systems. The mean forces of fracture were 964 N for In-Ceram crowns, 814 N for paint-on IPS Empress crowns, and 750 N for layered IPS Empress crowns, compared with 1,494 N for metal ceramic crowns veneered on a nickel-chromium coping. The results indicate that clinical testing of these all-ceramic crowns is reasonable. Controlled, clinical, long-term evaluation is necessary to assess the safe application of these new crown systems.

A longitudinal electromyographic study of the anterior temporal and masseter activity during habitual chewing of apple was performed on 21 subjects provided with immediate complete dentures. At the pretreatment stage, when the patients having an anterior residual dentition were chewing, the peak mean voltages showed low values (range, 48 to 58 microV). After denture placement, there was a further decrease of the temporal activity that persisted to the 6-month stage. After the dentures were relined, a significant increase in chewing force of the temporal muscles was noted at the 1-year stage. During the second year, when no corrections of the dentures were made, the temporal activity decreased below the pretreatment level. The masseter muscle chewing activity generally showed no significant changes during the 2-year observation period. The mean durations of the chewing strokes and chewing cycles were longer than values reported in dentate samples and generally displayed no significant changes during the 2-year period.

Five different investing techniques were used to invest lost-wax casting patterns. The use of an investing technique that subjects the invested casting ring to air pressure (40 lb per square inch) during setting was found to produce less air bubble/cast nodule incidence than did the other four techniques investigated. Not all cast nodules can be seen by the unaided eye; magnification is needed. More cast nodules than air bubbles were seen at the interface of pattern and investment, showing that there may be many air bubbles below the investment surface that are entered by the incoming alloy during casting. The effect of a surface-tension reducing (STR) agent on the air bubble incidence between pattern and investment depends on the investing technique used, although all techniques used showed a reduction in cast nodule incidence when an STR agent was used. Sprue direction was also found to influence the incidence of casting nodules.

The effect of 1.23% acidulated phosphate fluoride, 0.40% stannous fluoride gels, and 2.00% citric acid solution on 150.00 overglazed and 150 autoglazed porcelain surfaces was measured using a profilometer. Application of 1.23% acidulated phosphate fluoride for periods of 16 and 32 minutes caused etching in both groups, but the autoglazed group was significantly more effected. In both groups increase in application time did not affect the amount of etching. No etching was detected in either group after the application of 0.40% stannous fluoride for 6 and 12 hours and 2.00% citric acid for 4 and 8 hours.

Porcelain laminate veneers can be used to increase incisal-edge length. The purpose of this study was to determine the fracture resistance of porcelain veneer restorations on Cymel 1077 mandibular incisors that were incisally reduced 0.0, 0.5, 1.0, and 2.0 mm. Variables studied were incisal-edge length and the angle of applied force. Sixty-one porcelain laminate veneers were made to restore incisal-edge length and bonded to the prepared teeth. Samples were fractured at force angles of 130 degrees and 137 degrees. No significant difference was found between varying incisal-edge lengths (P > .05). However, the lower applied angle required greater average force to fracture (P = .005) as tested by ANOVA.

The objective of this study was to make a clinical quality evaluation of resin-bonded ceramic veneers placed in general practice. Ceramic veneers were evaluated by four independent examiners according to the California Dental Association guidelines for evaluation of dental care. In addition, bleeding and margin index and mode of tooth preparation were recorded. None of the veneers had been replaced and only two had fractured. Almost all veneers received a satisfactory rating regarding surface, color, and margin integrity. More than half of the veneers had a satisfactory anatomical form. The bleeding index was comparable to that reported for other types of crowns. An equigingival or subgingival margin was most common.

Guidelines for developing a dental laboratory infection-control protocol are discussed. A detailed outline of infection-control protocol regarding universal precautions, personal protective equipment, hepatitis B vaccination, environmental and surface cleaning and disinfection, and personnel training is presented.

The accumulation of food debris in the buccal vestibule surrounding a mandibular denture flange was studied. This investigation focused on the effect of the thickness, physiologic molding, and slope of the buccal flange of denture base on the patient's ability to expel test-food particles from the area. Experimental denture bases were placed in the mandibular buccal vestibule of subjects having a complete natural dentition. Only the slope of the polished surface of buccal flange was found to have a significant positive correlation with the ability to expel test food from the buccal vestibule.

Research has shown that external venting improves the marginal fit of cast crowns by decreasing hydrostatic pressure during seating. In turn, improving marginal fit has been shown to increase the strength of castable glass and other porcelain systems. This study evaluated the effect on the compressive strength of crowns with vent holes placed during and after fabrication. Forty-five artificial crowns were made from each material and divided into three equal groups: (1) crowns without vent holes, (2) crowns with a vent hole placed before casting, and (3) crowns with a vent hole placed using a rotary diamond instrument after casting. Standardized crowns were acid etched, silane treated, and filled with epoxy resin to provide a support base for testing. The crowns were then loaded to failure. One-way analysis of variance showed a significant difference among groups. Scheffe's Multiple Comparison Test was used for discrimination. The Dicor unaltered and fabricated vent group and the Hi-Ceram unaltered group had significantly higher resistance to fracture.

The shear bond strength to human dentin and enamel was evaluated for four glass-ionomer cements: an experimental stainless steel-reinforced glass-ionomer cement, two commercially available silver-reinforced cements, and a conventional glass-ionomer cement. Bonded specimens were stored in distilled water for 7 days during which time they were subjected to thermocycling in water baths at 5 degrees C and 55 degrees C for a 1-minute dwell time per bath and 1500 cycles. Specimens were then shear tested. The experimental stainless steel-reinforced glass-ionomer cement had a significantly higher bond strength to enamel (P .01) and dentin (P .05) than did the commercially available cements.

The purpose of this research was to study fracture initiation in a glaze-strengthened magnesia ceramic substance used as a core material for all-ceramic crowns. The fracture surfaces of modulus of rupture bars were studied to locate the fracture initiation point. These points were located at the center of hackles that are oriented parallel to the direction of the crack advance. The distance from the fracture initiation point to the surface of the test bars was measured. A direct relationship was found between the flexure strength and the depth of the fracture initiation point. This is consistent with the theory that residual compressive stresses at the glaze-body interface inhibit the spread of flaws and are responsible for the increases in strengths observed.