All ETDs from UAB

Advisory Committee Chair

John Burgess

Advisory Committee Members

Amjad Javed

Keith Kinderknecht

Nathaniel Lawson

Jack Lemons

Document Type

Thesis

Date of Award

2019

Degree Name by School

Master of Science in Dentistry (MScD) School of Dentistry

Abstract

ABSTRACT Introduction: Zirconia is classified by the mole percentage of yttria contained in the zirconia. The most strong and opaque material contains 3 mol % yttria, intermediate strength and opacity contain 4 mol %, and whereas 5 mol % is the weakest but most translucent zirconia. Refined zirconia powder is pressed into a mold to form a disc which is then milled to form a restoration. Post milling of sintered zirconia restorations is necessary to obtain final mechanical and physical properties of the restoration. Traditionally conventional sintering cycles required 6-8 hours to sinter. Whereas, fast sintering cycles have been have been experimented bringing down sintering time to as low as 20 minutes, which does not seem to affect strength of translucent zirconia. However little independent data has been collected to examine wear of opposing enamel antagonists against translucent zirconia and translucency of this newer material after fast sintering cycle. It is also important to study the wear of fast sintered zirconia against different all-ceramic materials. Contrast ratio has been used to measure the translucency of ceramic materials. However with zirconia the contrast ratio approaches 1.00 for an opaque material. Therefore the contrast ratio is not discerning enough to distinguish differences in translucency between different ceramic materials of the same composition. Hence, direct light transmission is a better representation of translucency of materials that are more than 50% opaque. Objective: The objective of this in vitro project is to measure the direct light transmission and contrast ratio of two commercially available translucent 5mol % yttria containing zirconia materials with conventional and fast sintering cycles and compare them with high and low translucency lithium disilicate materials. Second objective is to measure wear of enamel opposing conventionally and fast sintered translucent zirconia materials and the third objective is to measure wear of two all-ceramic restorative materials against conventionally and fast sintered translucent zirconia. Methods: Zirconia discs were milled from two translucent zirconia materials and sintered and polished to final dimensions: 1±0.02-mm thick and 26±0.2-mm in diameter. Specimens were conventionally sintered per manufacturers’ instructions with the hold time of 2 hours at high temperature, with 10°C/min temperature increase. For fast sintering, the furnace was pre-heated to 400°C, with an increase to 1570°C over 30 minutes, followed by a 30 minute hold time and cooling to 1200°C in 15 minutes. After 30 minutes, the furnace was opened at 400°C, and allowed to bench cooled to room temperature. Transmission and contrast ratio (n=10) were measured in a Color-i7 spectrophotometer. Wear: Enamel cusps were created from freshly extracted human molar teeth and were mounted on screws. Lava Esthetic cusp shaped specimens were milled out of a zirconia disc and were sintered with conventional and fast sintering cycles mentioned above and mounted on screws. For flat specimens, Lava Esthetic specimens, 4 mm in thickness were milled, sintered (with conventional and fast sintering cycle) and mounted in brass holders. All-ceramic materials, 4 mm in thickness were sectioned and mounted in brass holders. The specimens were then load cycled in the UAB wear testing device for 400,000 cycles at 60 Hz frequency with 20N load submerged in 33% glycerine solution. Specimens were scanned after wear and volumetric wear (mm3) was calculated. Data were analyzed by analysis of variance (ANOVA) and post-hoc tests. Results: The transmission and contrast ratio of light measured through zirconia was compared to high and low translucent glass ceramic or lithium disilicate materials. The mean (standard deviation) transmission (Tt%) for Lava Esthetic conventionally and fast sintered was 31.46 (2.56) and 32.03 (2.02), respectively. The mean (s.d.) transmission for Katana STML conventionally and fast sintered was 33.52 (0.88) and 30.14 (2.11), respectively. The mean (s.d.) enamel cusp wear (mm3) against conventional and fast sintered Lava Esthetic was 0.28 (0.07) and 0.28 (0.09), respectively. The mean (s.d.) flat surface wear (mm3) of Celtra Duo against conventional and fast sintered Lava Esthetic cusp was 4.01 (0.74) and 3.94 (0.83), respectively. The mean (s.d.) flat surface wear (mm3) of e.max CAD LT against conventional and fast sintered Lava Esthetic cusp was 1.76 (0.23) and 2.23 (0.68), respectively. Conclusions: Within the limitations of this study we can conclude that there is no difference in light transmission and enamel cusp wear between conventionally and fast

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