The International Journal of Prosthodontics, 06/1993

Computer color matching is an excellent method for reproducing certain colors of various objects using a spectrophotometer and a computer. In this study the authors attempted to reproduce the color of layered porcelain samples according to the computer color-matching formulation, wherein a computer color-matched sample is made and compared with the target sample. The color differences between the computer color-matched samples (opaque/dentin and opaque/dentin/enamel) and the target samples approximated 1.0, and the samples were indistinguishable by visual observation. The spectral reflectance curves were also very similar.

It may be desirable to extend the working time of poly(vinyl siloxane) impression materials when making a multiple-unit impression. Alteration of the base-catalyst ratio is not possible with automix systems, but chilling the impression material provides an alternative. This study evaluated the dimensional accuracy of chilled impression material and the effect on accuracy of reheating the impression to mouth temperature prior to pouring. Two brands of poly(vinyl siloxane) impression material were evaluated both after refrigeration and at room temperature. Impressions were poured at room temperature and reheated to mouth temperature prior to pouring. Gypsum dies were recovered and measured. Refrigeration had no statistically significant effect on dimensional accuracy. Reheating the impression had no clinically significant effect on accuracy.

This study evaluated the reproducibility of die position using two removable die systems and two die stones. Poly(vinyl siloxane) impressions were made of a stainless steel, U-shaped arch with four evenly spaced abutments. Six groups were evaluated: Zeiser system/Fuji Rock; Zeiser system/Die Keen; solid cast/Fuji Rock; solid cast/Die Keen; Fuji Rock/Pindex; and Die Keen/Pindex. An optical comparator was used to measure the height of each abutment, the distance between the anterior abutments, and the distance between the posterior abutments. The Zeiser system with either Fuji Rock or Die Keen yielded the greatest accuracy. Die Keen exhibited more linear expansion than Fuji Rock, and solid casts had less distortion than the Pindex system.

Long-term denture wearers undergo facial changes as a result of occlusal wear of the dentures and residual ridge resorption. This study examined 12 such patients, presenting with a clinical appearance of Angle's Class III occlusal relationships, to evaluate a clinical method of determining the correct mandibular position. Complete dentures were fabricated for each patient using standard clinical procedures, but acrylic resin blocks were left on the mandibular posterior bases instead of denture teeth. Ensuing changes in maxillomandibular relationships were recorded. After the stabilization of the mandibular relationship, the dentures were completed using mandibular posterior teeth. Patients were evaluated using cephalometric analysis before and after treatment. Results show that a careful procedure of denture fabrication allows the determination of the correct mandibular relationship, even in patients who have experienced extreme facial collapse.

The fracture toughnesses of two high alumina core ceramics were determined by using the single edge notched specimen method. Measurements were made in air, water, and artificial saliva. Fracture toughnesses of 1.89 and 1.75 MPa.m1/2 were obtained for Cerestore and Vitadur N samples, respectively, tested in air. Values obtained from Vitadur N samples that were tested in deionized water were significantly lower than those that were measured in air. Values for samples of both ceramics that were tested in artificial saliva were not significantly different from those that were tested in air. The fracture mode was a combination of intergranular and transgranular for Cerestore and predominantly transgranular for Vitadur N.

This study investigated the early erosion of glass ionomer-cement that had been used to lute complete cast crowns. Restorations had margins ending on either enamel or cementum and had various sizes of marginal openings. These restorations were compared to those using a standard zinc phosphate cement. Results showed that leaving the band of excess cement, expressed during the seating of the crown, undisturbed for 10 minutes prevented significant erosion in a wet field. A resin coat was not necessary to provide further protection after removal of the band. Zinc phosphate cement also showed significant early erosion when exposed to moisture.

The effect of in vitro dissolution in acidulated phosphate-fluoride gel on the color of metal ceramic samples having surface metal oxide colorants was determined. Three different colorants were applied using two techniques at two different firing temperatures. The color difference was most apparent when the colorant and glaze were combined with only one firing cycle. All colorants increased in value, but changes in color difference were most evident with the blue colorant. It was concluded that dissolution in acidulated phosphate-fluoride gel does affect the color of extrinsic metallic oxide colorants.

A two-dimensional, finite element method was used to examine the influence of material selection and suprabony exposure of the implant coating on thermal and mechanical stress distribution. Hydroxyapatite coating reduced the heat conduction to the surrounding tissue because of its low thermal conductivity. However, thermal stress resulted from thermal expansion of the hydroxyapatite and titanium core. This might influence the success of hydroxyapatite-coated implants because the biomechanical properties of ceramics are so poor for tensile and shearing stress. In addition, this tendency becomes more pronounced when the hydroxyapatite coating surface extends beyond the cortical bone.

A laser light-sectioning technique for measuring the three-dimensional shape of the face has been developed. Multi-slit lasers were used to minimize the recording time, and the accumulating technique of multiple images was used to reduce the memory size. The system can scan the entire human face within 6 seconds and develop a three-dimensional image in approximately 4 minutes. The accuracy of this system was found to be within a 0.4% relative error. The method has the advantages of being contact-free, rapid, automatic, and useful for three dimensional clinical assessment.

Resin cements are the preferred luting media for ceramic restorations, but manufacturers have traditionally made no recommendations concerning the use of dentin bonding agents in the luting process. This in vitro study evaluated the microleakage associated with several luting agents for ceramic inlays. One hundred twenty Class V inlays (occlusal margins in enamel and gingival margins in dentin) were luted in extracted teeth using zinc phosphate cement, two resin cements without a bonding agent, and two resin cements with three dentin bonding agents. This study suggests that the use of a dentin bonding agent with a resin cement will reduce microleakage in cast glass-ceramic restorations.

Triturated luting cements provide convenient clinical handling. This study clinically evaluated the influence of two trituration capsule cement systems on the marginal fit of inner copings for telescoping crowns. Using a randomized parallel design, one coping was cemented using zinc phosphate cement (Phosphacap) and one using a glass-ionomer cement (Ketac-Cem Maxicap) for each of 12 patients. A blind examination of the marginal fit of the restorations was made before and after cementation using the replica technique and a scanning electron microscope. The mean marginal discrepancies for all cast copings were approximately 30 microns (median 23 microns) before cementation. They increased significantly after cementation to 86 microns (median 63 microns) in the zinc phosphate cement copings but to only to 47 microns (median 44 microns) in the glass-ionomer cement copings.