The International Journal of Prosthodontics, 06/1996

External hexagonal implants of known dimensions were assembled with premachined cast abutments and rebroached cast abutments. The abutment screws were tightened to 20 Ncm and 30 Ncm, and the samples were loaded off axis with 133.3 N. Load application was at 1,150 cycles per minute with a sample counterclockwise rotation of 28 cycles per minute. The premachined cast abutments tightened to 20 Ncm failed with a mean of 357,162 cycles (SD - 77,981 cycles). The rebroached cast abutments were cycled to 1 million cycles without failure. At 30 Ncm, the premachined cast abutments failed at a mean of 5.0 million cycles (SD - 2.2 x 10 6 cycles). Two of five rebroached cast abutments failed at 4.3 million and 9.5 million cycles, but the remaining samples showed no evidence of screw loosening after 10 million cycles, at which time the test was terminated. The results indicate a direct correlation between rotational misfit and screw loosening. Screw joints can be made more resistive to screw loosening by the elimination of rotational misfit.

The thickness and adaptation of superplastic forming Ti-6Al-4V alloy denture frameworks were investigated using cross-sectional measurements. The frameworks for maxillary com plete dentures were fabricated from Ti-6Al-4V disks of thicknesses ranging from 0.75 to 1.30 mm. Although the thickness of each disk was reduced by superplastic forming, the completed framework had more than 78% of the original disk thickness. Differences in the thickness did not significantly affect either the thickness ratio of the framework to the disk or the gap discrepancy between the framework and the working cast. The accuracy of fit of the framework would satisfy ordinary clinical requirements.

This study investigated the resistance to deflection fatigue and surface hardness of grade 2 titanium bar samples to a protion of which a layer of cobalt-chromium alloy was cast under standard dental laboratory conditions. The uncovered part of the titanium bar was deflected, and the number of loading cycles required to cause a fatigue fracture in the titanium bar was recorded. The Vickers hardness of the titanium bar was measured, and the fracture surface was examined by scanning electron microscope and energy-dispersive spectroscopy. The number of loading cycles was considerably lower in bars with the cobalt-chromium surface cast (83 vs. 13.770 cycles) (P .001), while the surface hardness of the titanium bar with the cobalt-chromium surface cast was higher than that without the surface cast (Vicers hardness number 329 vs. 178) (P .001). These findinfgs suggest that the cobalt-chromium surface cast dramatically affects the fatigue resistance of pure titanium, which should be considered when novel implant-supported prostheses made from premanufactured titanium parts are planned.

The clinical outcome of treatment using conical crown-retained dentures was evaluated. Of the initial 25 patients provided with 26 conical crown-retained dentures, 18 patients with 18 restorations could be examined after a time ranging between 73 and 92 months. Of the eight restorations lost, four had been changed as a result of factors that might have been related to the prosthodontic care. Most of the patients were very satisfied with the restorations both functionally and esthetically and found their chewing comfort to be better after treatment with conica l crown-retained dentures. However, 50% of the patients reported speech problems related to treatment. Technical failures were not insignificant but were treatable. The survival rate after 73 to 92 months was 78.3%

The masticatory cycles of 11 men (mean age 51.5 years) with ex tensive tooth wear were investigated before and after rehabilitation with fixed partial dentures. Parameters such as the tooth wear index (IA) and masticatory mandibular movement were recorded. Before treatment, the patients were also given a questionnaire related to possible background factors of importance to tooth wear. At baseline a mean score of 48.6 (range ) to 100) for the tooth wear index (IA) was found. The clinical recall examination 3 years after prosthodontic rehabilitation displayed obvious wear of restorative material for two patients, and, in another patient, one of the fixed partial dentures had to be remade because of fracture of abutment teeth. Following increased and mandibular movement velocity decreased. The mandibular closing angle, near to occlusal contact, became steeper after prosthodontic treatment, indicating a changed mandibular movement pattern.

This investigation compared the accuracy of electroplated copper and stone dies. Sixty copper die replicas of a stainless steel master die were fabricated using different current settings, plating times, and backing materials. Die accuracy was assessed by measuring the misfit of a stainless steel ring machined to fit the master die. All the copper dies were more accurate than the 10 stone dies. The variability of the copper/stone dies was less than that of the stone dies (P = .011) and copper-resin dies (P .001). There was no significant difference between the variability of the 40-mA, 48-hour copper-resin dies and the stone dies. The variability of the copper-resin dies was related to the thickness of the electrodeposit, which was a function of plating time, current, and plated surface area.

This study evaluated the shear bond strength of Rexillium III (Jeneric Pentron, Wallingford, CT) to enamel using various resin composite luting systems. Cast alloy cylinders (3.9 mm x 6.0 mm) were bonded with each cement to human molar buccal enamel surfaces 9n = 8). The enamel was etched using a 35% phosphoric acid solution for 30 seconds. Bonded specimens were stored in distilled water for 7 days at 37 degrees C and thermocycled (1,500 cycles) in 5 degrees C and 55 degrees C water baths (1-minute dwell time). Specimens were randomly tested in shear using a crosshead speed of 0.5 mm per minute. Use of a one-way analysis of variance (P .001) and the Ryan-Einot-Gabriel-Welsh multiple range test showed significant differences between several of the resin cements. Panavia exhibited a significantly higher shear bond strength than any of the other cements tested (P .05)

This study compared the firing shrinkages, flexural strengths, and chemical solubilities of two low-fusing porcelains formulated for use with titanium (Procera, Nobel Biocare, Goteborg, Sweden; and Duceratin, Degussa, Plainfield, NJ) to those of a conventional low-fusing feldspathic porcelain (Vita VMK 68, Vident, Baldwin Park, CA). Procera demonstrated the highest mean firing shrinkage and was significantly different from the other two porcelains. There were no significant differences among the porcelains in flexural strength and chemical solubility.

Two phosphate-bonded investments that were pured into molds of a duplicating gel and a poly(vinyl siloxane) duplicating medium were examined for surface hardness. Untreated surfaces were also examined by the confocal microscope and the scanning electron microscope. The Brinell hardness of samples poured in poly(vinyl siloxane) was greater than that of the same investment poured in duplicating gel. Investments were hardest at set and were significantly softened by drying. Cast hardeners achieved some rehardening of the dried investment samples. Microscopic examination of the investment surfaces revealed greater surface irregularity of samples poured in duplicating gel than in poly(vinyl siloxane), particularly in one investment.

This in vitro study measured the fracture resistance of intact extracted molars and three types of all-ceramic crowns: feldspathic porcelain, glass-ceramic, and glass-infilt rated alumina. All crowns were of a simplified three-cusp occlusal configuration, and were placed on prepared, extracted third molars. The ceramic crowns were bonded to dentin with a resin composite cement, except for 10 of the feldspathic crowns, which were luted with zinc phosphate cement. The fracture resistance was measured using a spherical steel indenter that contacted the occlusal surface at three points. A total of 40 ceramic crowns and 50 extracted maxillary third molars were tested. The Weibull distribution was used for data analysis. Intact extracted teeth were significan tly stronger than all-ceramic crowns. Of the ceramic restorations, the glass-infiltrated alumina crowns exhibited the highest fracture resistance.