Retention and surface changes of zirconia primary crowns with secondary crowns of different materials.

OBJECTIVES: To evaluate zirconia as a substitute for gold alloy in primary crowns facing secondary crowns manufactured with different materials, in terms of long-term retention force changes, wear, and phase transformation was aimed. MATERIALS AND METHODS: A total of 12 groups, each containing six samples, consisting of gold alloy primary crown-electroformed gold secondary crowns (AA), zirconia primary crown-electroformed gold secondary crowns (ZA) and zirconia primary crown-casted non-precious alloy secondary crowns (ZC) with conus angles of 0°, 2°, 4°, and 6° were evaluated. Samples were subjected to 10,000 insertion-separation cycles in artificial saliva and retention force was measured. X-ray diffraction and scanning electron microscope analysis were performed on the sample surfaces. RESULTS: The highest retention forces were obtained from ZC-0° group (72.09-71.26 N) and the lowest were obtained from ZA-4° (12.73-19.44 N) and ZA-6° (5.36-19.73 N) groups in the beginning and after 10,000 cycles, respectively. Retention force increased as the conus angle decreased. The monoclinic phase ratio of the zirconia primary crowns decreased after the experiments. No wear was observed in zirconia primary crowns except for the ZC-0° and ZC-2° groups. The use of zirconia primary crowns resulted in a less excursive retention force. CONCLUSIONS: A more predictable and less excursive retention force can be obtained using a hard and rigid primary crown material like zirconia. CLINICAL RELEVANCE: Despite a lack of knowledge about the aging of zirconia without a veneer layer in the oral environment, zirconia primary crowns are more advantageous in terms of retention force development and wear.

Flexural properties of repaired heat-polymerising acrylic resin after wetting with monomer and acetone.

OBJECTIVES: Repair strength can be improved by treating fractured surfaces of a denture. BACKGROUND: This study investigated flexural properties of heat-polymerised acrylic resin specimens repaired with auto-polymerising and visible light curing (VLC) resins after the repair surfaces were wetted with monomers or acetone. MATERIALS AND METHODS: Fifty-four specimens (65 x 10 x 2.5 mm) were prepared and 48 of them were sectioned to simulate denture fracture. Butt-joint designed repair surfaces were wetted with heat-, auto-polymerising monomers and acetone for 180 s and repaired with auto-polymerising and VLC resins. After repairs, specimens were subjected to three-point bending test and flexural strength, strain, fracture load, modulus of elasticity and deflection values were recorded. Data were analysed with Student t and LSD tests (p < or = 0.05). RESULTS: Overall flexural strength, strain, fracture load and deflection values of specimens repaired with VLC resin were significantly higher than the specimens repaired with auto-polymerising resin for all types of wetting agent (p < 0.05). Within the wetting agents, heat- and auto-polymerising monomers produced the best mechanical properties, while wetting with acetone did not provide superior effect over both monomers. CONCLUSION: In clinical use, wetting the repair surfaces may result in stronger repairs. The use of bonding agent in VLC resin repairs in combination with wetting agent results in improved flexural properties.