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Statement of Problem: Rigidity is a requirement of removable partial denture frameworks. Although commonly used by dentists, the U-shaped (horseshoe) maxillary major connector lacks the rigidity of other maxillary major connector designs., Purpose: The purpose of this study was to determine the effects of changing width, thickness, and shape on the rigidity of U-shaped maxillary major connectors., Material and Methods: Five nickel-chrome alloy frameworks were fabricated from the same master cast for each of 4 different U-shape removable partial denture designs. The designs were an 8-mm wide U-shaped strap with a 6-mm posterior strap (A-P strap), a 13-mm-wide U-shape strap (Wide), an 8-mm-wide U-shape strap that widened to 13 mm at the midline (Notch), and an 8-mm-wide U-shaped strap that was twice the thickness of the other straps (Thick). A fifth group of 5 frameworks was made by removing the posterior strap from the A-P strap frameworks (A strap). Two testing points were marked on each framework corresponding to the first premolar and second molar positions. The frameworks were mounted in a universal testing machine, and vertical (torsional) and horizontal (compressive) loads were applied using a 10-kN load cell at a cross-head speed of 2 mm/min until a deflection of 1 mm occurred. A force-deflection curve was generated for each test, and slope of the curves (N/mm) was compared with analysis of variance and Scheffe's F test (alpha=0.05)., Results: The Thick group was found to be significantly more rigid (P<.05) than the other frameworks when torsional loads were applied to both the premolar (22.42 N/mm) and molar (10.88 N/mm) areas, and when a compressive load was applied to the premolar (232.85 N/mm) area. The A-P strap group was significantly more rigid (P< or =.05) than the other designs when a compressive load was applied to the molar (69.56 N/mm) area. Both the Thick and A-P strap groups were significantly more rigid (P<.05) than the Notch and A strap groups in all tests., Conclusion: Doubling the thickness of the anterior strap of a U-shaped maxillary major connector improved the rigidity of the framework to torsional loads. A posterior strap became more effective in maintaining framework rigidity to compressive forces as the length of the arch increased.

Statement of Problem: Storage stability, or shelf life, is a critical property of perishable dental materials., Purpose: The purpose of this investigation was to document changes in the properties of nonaqueous elastomeric impression materials over time and on exposure to various environmental conditions., Material and Methods: Materials investigated included a polyether impression material, a polysulfide material (light, regular, and heavy consistencies), and an addition-reaction silicone material (light, regular, heavy, and putty consistencies). Tests included viscosity of individual pastes, elastic recovery, working and setting times, strain in compression, dimensional change, creep compliance, and tear energy. Means and standard deviations were calculated for each test interval. Statistical analysis included 2-way ANOVA (variables: time and storage condition) and comparison to baseline values using 1-way ANOVA (variable: time) and Scheffe's multiple comparison test (P < or = 0.05) within each material, consistency, and test., Results: Results over 72 months revealed changes in viscosity, working/setting times, elastic recovery, and creep compliance. Separation of components was seen with polysulfide and addition silicone materials. Some batches of polysulfide base paste and polyether reactor paste solidified in the tube., Conclusion: Most batches of the materials and consistencies tested remained efficacious well past their designated shelf lives under a variety of storage and use conditions. Data for the addition-reaction silicone impression material changed little during the 72-month testing period; thus, the material may be considered relatively storage stable. Creep compliance was the most precise storage-stability indicator. The storage stability of perishable materials should be addressed in American Dental Association Specifications.

Statement of Problem: Mechanical properties are considered to be important to the clinical success of dental cements. Storage time of dental luting agents (dental cements) before use and at the manufacturer may have substantial effects on their properties., Purpose: This study observed and investigated the changes in the properties of water-based dental luting agents over time and on exposure to various environmental conditions., Methods and Material: Properties of zinc phosphate, zinc polycarboxylate, and glass-ionomer luting agents were recorded at baseline and at intervals past 84 months, whereas the materials were exposed to controlled storage conditions as well as various conditions typical of clinical situations. Tests included viscosity, diametral tensile strength, ultimate compressive strength, rigidity, hardness, and working/setting times., Results: Viscosity increased over 24 to 60 months for the acidic polymer liquids (zinc polycarboxylate and glass ionomer). Tensile and compressive strengths began to decrease after 36 months for all cements, most acutely for glass ionomer., Conclusion: Changes in critical properties on aging may not be noticed by the clinician and yet may impact clinical success. These data emphasize the importance of storage stability for perishable dental materials and portend the need for standardized tests.

Statement of Problem: Titanium and its alloys are more commonly used in prosthodontics and welding has become the most common modality for their joining. Studies on the welding of titanium and its alloys have not quantified this value, though its importance has been suggested., Purpose: This study compared the strength and properties of the joint achieved at various butt joint gaps by the arc-welding of cast Ti-6Al-4V alloy tensile bars in an argon atmosphere., Material and Methods: Forty of 50 specimens were sectioned and welded at four gaps. All specimens underwent tensile testing to determine ultimate tensile strength and percentage elongation, then oxygen analysis and scanning electron microscopy., Results: As no more than 3 samples in any group of 10 actually fractured in the weld itself, a secondary analysis that involved fracture location was initiated. There were no differences in ultimate tensile strength or percentage elongation between specimens with weld gaps of 0.25, 0.50, 0.75, and 1.00 mm and the as-cast specimens. There were no differences in ultimate tensile strength between specimens fracturing in the weld and those fracturing in the gauge in welded specimens; however, as-cast specimens demonstrated a higher ultimate tensile strength than welded specimens that fractured in the weld. Specimens that fractured in the weld site demonstrated less ductility than those that fractured in the gauge in both welded and as-cast specimens, as confirmed by scanning electron microscopy examination. The weld wire showed an oxygen scavenging effect from the as-cast parent alloy., Conclusions: The effects of the joint gap were not significant, whereas the characteristics of the joint itself were, which displayed slightly lower strength and significantly lower ductility (and thus decreased toughness). The arc-welding of cast titanium alloy in argon atmosphere appears to be a reliable and efficient prosthodontic laboratory modality producing predictable results, although titanium casting and joining procedures must be closely controlled to minimize heat effects and oxygen contamination.

Statement of Problem: A secure bond between resin and an alloy framework is necessary for clinical success in removable prosthodontics., Purpose: This study compared the shear bond strength of composite to composite to metal with two commercially available chemical bonding systems: a silicoating system (Silicoater) and a nitrogenous heterocycle-acrylonitrile system (Kevloc)., Material and Methods: Fifty-two nickel-chromium cubes were cast and composite resin was bonded to the alloy surfaces according to manufacturer's directions. After storage at 35 degrees C for 15 days, and thermocycling at 5 degrees C and 55 degrees C for 1200 cycles, the bonds were fractured in shear on a universal testing machine., Results: Mean bond strength for the silicoated sample was 10.93 MPa and for the heterocycle-acrylnitrate system 11.44 MPa. An unpaired t test revealed no difference between the groups (p = 0.60). Visual inspection of the fracture surfaces revealed that failure was adhesive at the resin-to-metal surface in almost all the specimens.

Statement of Problem: Storage stability is a critical characteristic for perishable dental materials., Purpose: The purpose of this shelf-life study was to document changes in the properties of an alginate impression material on exposure to various environmental conditions for more than 78 months., Material and Methods: Properties measured included recovery from deformation, strain in compression, compressive strength, tear strength, working time, and creep compliance., Results: Results revealed increases in strength and working time and a decrease in recovery from deformation at 30 to 50 months: strength and recovery then remained constant past 6 years, whereas working time and creep compliance decreased. Only the most stressful environmental conditions (heat and humidity) caused spontaneous failure of the material to set., Conclusions: We concluded that, under most storage conditions, properties of the alginate material tested remain within ADA specification limits well past manufacturer's designated shelf life. ADA specifications should require manufacturers to verify that the shelf life of each perishable material is based on valid data. An accelerated aging test was developed to simulate real time property changes and to assist in evaluating similar materials.

Statement of Problem: Studies have suggested that coating a temporary soft denture liner with a semiset methyl methacrylate resin extends the period of resiliency of the liner., Purpose: This study determined whether coating three temporary soft denture liners with two different denture surface sealants, followed by thermocycling, affected the resiliency of the liners., Material and Methods: thirty 20 x 12.5 mm cylindrical samples each of Lynal, Coe Soft, and Visco-Gel soft lining materials were prepared and divided into three groups of 10 uncoated samples, 10 Palaseal-coated samples, and 10 mono-poly coated samples. Samples were thermocycled from 5 degrees C to 45 degrees C for 500 cycles and then compressed 10 mm on an Instron universal testing machine. Resiliency was determined by measuring the energy absorbed by the soft liners when stressed to a specific yield point., Results: Two-way ANOVA indicated a significant difference among materials (p = 0.0001) and among treatments (p = 0.0001). One-way ANOVA of each material group revealed a significant difference among Lynal groups (p = 0.0001) and a significant difference among Visco-Gel groups (p = 0.030)., Conclusions: Surface-coating Lynal samples with Palaseal or mono-poly and Visco-Gel samples with Palaseal significantly increased the resiliency of these samples in the laboratory when compared with uncoated samples.

New ceramic materials for restorative dentistry have been developed and introduced in recent years. This article reviews advantages and disadvantages of dental ceramics, concentrating on strength properties. Included are factors affecting the strength of dental ceramic materials and the most common mechanisms for increasing the strength of dental ceramics. The properties of presently available materials such as dispersion-strengthened ceramics, cast ceramics, and foil-reinforced materials are discussed. Current research efforts to improve the fracture resistance of ceramic restorative materials are reviewed. A description of methods to evaluate the strength of ceramics is included, as a caution concerning the interpretation of strength data reported in the literature.