Dental Ceramics

all porcelain is ceramic but not all ceramic is porcelain

kaolin - this is a clay hydrated aluminium silicate that is opaque which is important for appearance of final product

dental ceramics need to be translucent so kaolin is removed & replaced with feldspar & silica

potassium & sodium alumina silicate acts as a flux lowers fusion & softening temp of glass is lowest fusing component & flows during firing forming a solid mass around the other components

feldspathic ceramics form leucite when heated to 1150-1500o leucite = potassium aluminium silicate this forms glass phase of ceramic & gives powder of known physical & thermal properties no further chemical reaction is required during fabrication of restoration powder melts together to form crown

heating leads to sintering ceramic particles begin to fuse into a single mass during this material contracts by about 20%

colour stable smooth surface less staining compared to others optical properties

reflectance - how much light is reflected from a surface / optical element translucency - allows light to pass diffusely opacity - does not allow light to pass diffusely transparency - allows light fully through so objects behind can be distinctly seen opalescence - colourless / white like an opal or changing colour like an opal

very stable unaffected by wide range of pH in mouth do not take up staining from food / drink good biocompatibility

similar to tooth substance TEC similar to dentine so results in low stresses to restoration in mouth thermal diffusivity is low (protective of remaining tooth)

once fully fired it is very stable during fabrication shrinkage can be a problem (20%)

high compressive strength high hardness - must be glazed to prevent abrasion of opposing teeth low tensile strength low flexural strength low fracture toughness these lead to failure on loading

static fatigue - time dependent decrease in strength even in absence of applied load (due to hydrolysis of SiO) surface microcracks - can occur during manufacture, finishing or occlusal wear slow crack growth - cyclic fatigue under occlusal forces in wet environment over time

low stress areas i.e. - only anterior crowns - not in all ptx - too brittle for use elsewhere

• produce a strong coping resistant to fracture & cover in conventional porcelain i.e. porcelain fused alloys • cast or press a block of harder ceramic • mill a laboratory prepared block of ceramic

reinforces feldspathic ceramics prevent cracks propagating through the material causing fracture aluminous porcelain is opaque & can only be used as a core material so is veneered with conventional feldspathic porcelain to produce final crown not strong enough for posterior use

zirconia powder does not sinter unless heated over 1600o pure zirconia can crack on cooling

increase yttria increases translucency but decreases physical properties if crack begins yttria presence can cause slight expansion of material to close it; due to zirconia = monoclinic crystal but yttria = tetragonal crystal structure this means it is hard, strong (1000MPa flexural strength) and tough so can be used a bridge framework

imp taken of prep and sent to lab where model is cast and scanned digitally zirconia crown then veneered with feldspathic porcelain to produce final restoration

expensive equipment required potential for veneering crown to debond from core inert fitting surface so cannot etch or bond BUT once you have equipment they are cheaper to make fit is generally excellent

zirconia lithium disilicate precious metal non precious metal titanium composite ceramics all have surface sintered layer for best aesthetics

scan in mouth design on CAD machine mill polish cement

more like casting a metal restoration waxed up invested cast from heated ingot of ceramic (1100o) no sintering occurs as ceramic ingot is already fully condensed prior to fitting once devested & cleaned restoration is heated to improve crystal structure producing crack inhibiting crystals process called ceraming cast crown can be stained often cut back labially & veneered with appropriate feldspathic porcelains

lithium disilicate glass leucite reinforced glass