Michael A. Mansueto

Evaluation of mechanical and thermal properties of commonly used denture base resins

PURPOSE The purpose of this investigation was to evaluate and compare the mechanical and thermal properties of 6 commonly used polymethyl methacrylate denture base resins. MATERIALS AND METHODS Sorption, solubility, color stability, adaptation, flexural stiffness, and hardness were assessed to determine compliance with ADA Specification No. 12. Thermal assessments were performed using differential scanning calorimetry and dynamic mechanical analysis. Results were assessed using statistical and observational analyses. RESULTS All materials satisfied ADA requirements for sorption, solubility, and color stability. Adaptation testing indicated that microwave-activated systems provided better adaptation to associated casts than conventional heat-activated resins. According to flexural testing results, microwaveable resins were relatively stiff, while rubber-modified resins were more flexible. Differential scanning calorimetry indicated that microwave-activated systems were more completely polymerized than conventional heat-activated materials. CONCLUSION The microwaveable resins displayed better adaptation, greater stiffness, and greater surface hardness than other denture base resins included in this investigation. Elastomeric toughening agents yielded decreased stiffness, decreased surface hardness, and decreased glass transition temperatures.

Marginal Gap of Milled versus Cast Gold Restorations.

PURPOSE This in vitro study evaluated and compared the vertical marginal gap of cast and milled full coverage gold copings using two margin designs (chamfer and chamfer bevel) before and after fitting adjustments. MATERIALS AND METHODS Ten impressions were made of two metal master dies (one chamfer margin, one chamfer-bevel margin) and poured twice in Type IV stone. The 20 subsequent casts with 40 dies were split into four groups (n = 10); cast gold bevel, cast gold chamfer, milled gold bevel, and milled gold chamfer groups. The cast specimens received approximately 40 μm die relief no closer than 1 mm from the finish line. Cast copings were hand waxed, cast in a high noble gold alloy, chemically divested, and the sprues were removed. For milled gold copings, casts were scanned and copings designed using 3shape D900 scanner and software. Parameters were set to approximate analog fabrication (cement gap = 0.01 mm; extra cement gap = 0.04 mm, drill radius = 0.65 mm). Copings were milled from the same high noble alloy. All copings were seated on their respective master die in a custom scanning jig and measured using a measuring microscope at 90× (60 measurements per specimen, 15 per surface). Following initial measurements, all copings were adjusted on stone dies. The number of adjustment cycles was recorded and post-adjustment measurements were made using the same method. Data were analyzed using independent and paired t-tests. RESULTS Milled gold copings with a beveled margin (11.7 ± 20.4 μm) had a significantly (p < 0.05) smaller marginal gap than cast gold copings with a beveled margin (43.6 ± 46.8 μm) after adjustment. Cast gold copings with a chamfer margin (22.7 ± 24.7 μm) had a significantly (p < 0.05) smaller marginal gap than milled gold copings with a chamfer margin (27.9 ± 31.6 μm) following adjustments. Adjustments significantly decreased marginal gap for both cast groups (p < 0.05) and the milled chamfer bevel group (p < 0.05) but had no significant effect on the milled chamfer group. CONCLUSIONS Within the limitations of this study, results indicate that gold restorations milled with the tested parameters provide a vertical marginal gap that is an acceptable alternative to traditional gold crown casting techniques.

Marginal discrepancy of noble metal–ceramic fixed dental prosthesis frameworks fabricated by conventional and digital technologies

Statement of problem. Studies evaluating the marginal adaptation of available computer‐aided design and computer‐aided manufacturing (CAD‐CAM) noble alloys for metal‐ceramic prostheses are lacking. Purpose. The purpose of this in vitro study was to evaluate the vertical marginal adaptation of cast, milled, and direct metal laser sintered (DMLS) noble metal‐ceramic 3‐unit fixed partial denture (FDP) frameworks before and after fit adjustments. Material and methods. Two typodont teeth were prepared for metal‐ceramic FDP abutments. An acrylic resin pattern of the prepared teeth was fabricated and cast in nickel‐chromium (Ni‐Cr) alloy. Each specimen group (cast, milled, DMLS) was composed of 12 casts made from 12 impressions (n=12). A single design for the FDP substructure was created on a laboratory scanner and used for designing the specimens in the 3 groups. Each specimen was fitted to its corresponding cast by using up to 5 adjustment cycles, and marginal discrepancies were measured on the master Ni‐Cr model before and after laboratory fit adjustments. Results. The milled and DMLS groups had smaller marginal discrepancy measurements than those of the cast group (P<.001). Significant differences were found in the number of adjustments among the groups, with the milled group requiring the minimum number of adjustments, followed by the DMLS and cast groups (F=30.643, P<.001). Conclusions. Metal–ceramic noble alloy frameworks fabricated by using a CAD‐CAM workflow had significantly smaller marginal discrepancies compared with those with a traditional cast workflow, with the milled group demonstrating the best marginal fit among the 3 test groups. Manual refining significantly enhanced the marginal fit of all groups. All 3 groups demonstrated marginal discrepancies within the range of clinical acceptability.