The International Journal of Prosthodontics, 01/1998

Purpose: This article reviews the literature on maxillary implant-supported overdentures. Materials and Methods: Retrospective studies and clinical repo rts comprise the majority of publications, with few prospective studies. Results: Maxillary implant-supported overdentures are associat ed with implant and prosthesis survival rates similar to those of fixed implant prostheses when patients are matched for bone quantity and quality. High failure rates are associated with extreme atrophy, poor bone quality, and the use of short implants, which represent situat ions in which overdentures are most commonly used. Conclusion: Further research is required to elucidate the most appropriate implant number and distribution as well as the most favorable prosthetic constructions.

Purpose: This in vitro study compared the effects of four surface treatment on the bond strength between porcelain dentu re teeth and heat-polymerized denture base acrylic with and without a ceramic primer and bonding agent and after three storage conditions. Materials and Methods: Polished porcelain specimens (120) were divided into 24 groups of five each as follows: surface treatments (none, etched, sandblasted, and air abraded), ceramic primer and bonding agent (yes and no), and storage conditions (water storage, thermocycling, and accelerated aging). All the specimens were tested to failure in tension on a testing machine. Results: Hydrofluoric acid etching, sandblasting, and air abrasion did not improve the bond strength of specimens without ceramic primer and bonding agent compared to those polished with 600-grit silicon carbide paper. Use of ceramic primer and light-activated bonding agent (Scotchbond MP) increased the bond strength. For thermocycled specimens treated with ceramic primer and bonding agent, the bond strengths ranged from 10 to 31 MPa in the following order: air abraded = etched no treatment sandblasted. Conclusions: Ceramic primer and bonding agent dramatically improved the bond strength of porcelain teeth to heat-polymerized denture resin. Sandblasting improved bond strength of bonded samples compared to those that were etched or polished only.

Purpose: The aims of this study were to assess changes in the microhardness and surface roughness of a dual-cured composite resin cement after water storage for different periods of time. Materials and Methods: Sixty specimens were divided into four groups comprising high- and low-viscosity cement specimens stored eithe dry or in water for 1 to 60 days. Microhardness and surface roughness measurements were made after 1, 7, and 60 days of storage. Results: It was found that although interactions complicated the interpretation of the results, the water-stored samples had a significantly lower microhardness compared to the dry-stored specimens for every time interval. In addition, the high-viscosity specimens had a significantly higher microhardness compared to the low-viscosity specimens. An increased microhardness was found for all groups, except for the low-vis cosity, water-stored specimens after 60 days of water storage, which was attributed to an effect of the chemical postcuring process. Although difficult to evaluate from a clinical point of view, the laser profilometry analysis revealed that a significantly increased surface roughness was found after water storage and increased storage time that was possibly caused by a deterioration of the resin-matrix surface. Conclusion: For the permanency of the interfacial luting material, a high microhardness value seems to be important.

Purpose: The objective of this study was to test the hypothesis that bulk fracture of glass-ceramic disks of variable thickness originates at the inner, resin-bonded surface and is dominant over Hertzian fracture at the lower range of thickness values. Materials and Methods: Eight groups of seven glass-ceramic disks (Dicor, Dentsply), 12 mm in diameter with thicknesses ranging from 0.4 to 2.4 mm, were cast, cerammed (to produce approximately 55 vol% of tetrasilicic fluormica cyrstals), air abraded, etched, and silane coated according to the manufacturer's instructions. The disks were bonded to an eposy die substrate (with an elastic modulus comparable to that of dentin) using a light-activated resin cement. The bonded samples were supported on a flat surface and loaded at the top center of each disk until crack initiation occurred. All disks exhibited an initial crack within the bonded surface. Three randomly selected samples for each thickness were loaded beyond the point of crack initiation until Hertzian failure occurred. Results: Although the crack-initiation force increased with increasing thickness, the failure stress approached a maximum level at a thickness of approximately 1.6 mm. These results suggest that the estimated maximum occlusal load for each patient should be used to select the minimum thickness of carmic crowns rather than using the arbitrary traditional selection of a 1.5 - mm thickness. Conclusions: The authors conclude that bulk fracture is initiated within the bonded surface of a glass-ceramic specimen (for samples 0.4 to 2.4 mm in thickness) when the glass-ceramic is supported by a substrate with an elastic modulus similar to that of dentin. Furthermore, a Hertzian failure mechanism is unlikely to cause bulk fracture for these conditions.

Purpose: Dental casting requires replication of complex shapes with high fidelity. To achieve this objective, the problem of scale formation on titanium dental castings must be overcome. Scaling occurs readily at high temperatures because of the high reactivity of molten titanium with investment materials. The purpose of this study was to examine the effectiveness of using stable oxide coatings on the mold surface to reduce the interfacial reactions. Materials and Methods: A traditional phosphate-bonded dental investment, a commercial titanium investment, and an experimental oxide coating were used for the molds. Pure titanium samples were cast, divested, and prepared for scanning electron microscopic and energy dispersive x-ray spectroscopic analyses. Results: Layers of 7- to 10-um-thick scale were formed on titanium castins during reaction with traditional mold materials. Interface reaction was reduced between the molten titanium and the investment material when yttrium oxide or zirconium oxide coatings were applied to the mold before casting. Conclusion: Less titanium-mold interfacial reactions occurred when protective coatings were used as diffusion barriers for titanium casting. Y2O3-coating oxide particles applied withotu binder were entrapped in the cast titanium surface layer. Further study of a binder system for Y2O3 coating is needed.

Purpose: The influence of tin plating on marginal seating and retention of complete cast crowns was evaluated. Materials and Methods: Thirty-six standardized gold-alloy complete crowns were made on extracted human molar teeth prepared with a modified milling machine using conventional laboratory techniques. The crowns were randomly assigned to two groups according to the treatment of the fitting surface (air abraded with 50-um aluminum oxide for 10 seconds, or tin plated with a microtin system for 3 seconds), and luted with adhesive resin cement (Panavia-EX, J. Morita). Scanning electron microscopy was used to evaluate the variables for the cast alloy. Retention was measured with an Instron testing machine after thermocycling (1,500 cycles between 5 degrees C and 55 degrees C with 1 minute dwell times). Marginal adaptation was measured with a Nikon microscope before and after cementation. Results were subjected to one-way analysis of variance and Student's t test. Results: Retention was significantly different (P 0.001); the highest retention (367 N) was reported in tin-plated crowns. Differences for marginal seating were not statistically significant. Conclusion: Tin plating after air abrasion was found to be a predictable and simple technique to roughen the casting surface, increase the surface area for bonding, and significantly improve the retention of cast crowns in vivo.

Purpose: The effect of water sorption on the flexural strength at the proportional limit (FSp) of a denture base mat erial relined with four different denture reline materials was studied. Materials and Methods: Denture base specimens were relined either after 24 hours or 30 days of water immersion. The relined specimens were tested after another period of 24 hours or 30 days of water immersion. Results: Relined denture specimens generally exhibited lower FSp with an increase4 in water immersion time and an increase in the thickness of the reline material. Conclusions: The plasticizing effect of absorbed water molecules explains the general decrease in FSp of immersed relined specimens. The absence of a significant effect that water immersion had on some relined specimens under certain immersion conditions was explained by water sorption into the denture base achieving an equilibrium after a period of water immersion, and the resistance of some reline materials to the effect of water immersion.

Purpose: This paper focues on the derivation of implant loading forces as influenced by occlusal anatomy. Vertical occlusal forces on cusp inclines produce resultant lines of force that result in lateral rather than vertical forces to the supporting bone. Materials and Methods: An analysis of resultant lines of force with different impacting occlusal surfaces was illustrated. Methods were suggested to decrease implant laoding by reducing cusp inclines, utilization of cross occlusion, and the modification of occlusal anatomy to provide a continuous 1.5-mm flat fossae, rather than the line angles of the usual cuspal anatomy. The relationship of incisal guidance to the cusp inclines on the adjustable articulator were reviewed. Modification of the incisal pin and articulator settings were suggested to produce a 1.5-mm fossae throughout teh prosthesis. Practical laboratory and intraoral occlusal adjustment techniques were suggested to provide a modified centric occlusal anatomy to help decrease implant loading. Results: Clinical examples were shown to verify the accuracy of the modified settings on the semi-adjsutable articulator and the resultant modified occlusal anatomy. Conclusion: Implant loading can be reduced by modifying the location of the impact area and the occlusal anatomy. Simple modification of the incisal pin and articulator settings can be used to produce a 1.5-mm flat fossae, which results in more vertical forces to the supporting bone. The same procedures are used to reduce cusp inclinations, which effectively lessens the torque exerted on the prosthesis, implant, and bone. A combination of all these factors can prevent implant overload

Purpose: The objective of this study was to determine radiopacity values of a group of resin-based cements and to compare them to those of enamel and dentin. Materials and Methods: Three specimens 2.5 mm thick were prepared from each of 18 cements (nine chemical-cured and nine dual-cured). Three tooth sections 2.5 mm thick were cut from extracted molars. Specimens and tooth sections were divided into three matching groups. Following standard radiographic techniques, images of each group of specimens, along with an aluminum step wedge were obtained. Optical density readings for each material were determined with a transmission densitometer. Radiopacity values were subsequently calculated as equivalents of aluminum thickness. Results: Analysis of variance indicated significant differences in radiopacity values among the materials (P 0.0001). Four dual-cured cements (Variolink, Geristore, Enforce, and Nexus) had radiopacity values significantly greater than that of enamel, while two (Choice and Adherence) had radiopacity values similar to enamel. For the remaining three dual-cured cements, Duolink had a radiopacity value significantly lower than that of enamel but higher than that of dentin, while both Lute-It and Resinom er had values similar to dentin. For the chemical-cured cements, five materials had radiopacity values significantly higher than enamel (Sealbond, Advance, Scotchbond, Cement-It, and Dyract-Cem), while two had values significantly lower than enamel but higher than dentin (Biomer and Panavia 21). The remaining two cements had radiopacity values significantly lower than dentin. Conclusion: For a group of 18 resin-based cements, some had radiopacity values similar to or significantly lower than that of dentin.

Purpose: The purpose of this study was to investigate in vitro the levels of adhesion of phenotypically switched and nonswitched Candida albicans to the smooth and rough surfaces of heat-cured acrylic resin, Molloplast B, and Novu Materials and Methods: Nonswitched and switched cells of C. albicans 3153A were preapred and adhesion assays were performed. Results: There was no difference in the level of adhesion of blastospores in their nonswitched or switched state. The adherence of hyphae to all three denture base materials was statistically significantly increased when in a switched form. There was greater adhesion to the two soft lining materials than to the acrylic resin. Conclusions: Rough surfaces promote adhesion. There was no significant difference in the adhesion of switched and nonswitched blastospores, but there was increased adhesion of hyphal cells.