Zirconia Crown Lecture (PDF)

# Dental Zirconia ## Different Terms * **Zirconium:** Mineral with silver colour, not stable. (found in the periodic table) * **Zirconium Oxide or Zirconia:** Since Zirconium is not stable, when it reacts with oxygen it forms zirconium oxide powder used to make discs, which will be milled to make...

# Dental Zirconia ## Different Terms * **Zirconium:** Mineral with silver colour, not stable. (found in the periodic table) * **Zirconium Oxide or Zirconia:** Since Zirconium is not stable, when it reacts with oxygen it forms zirconium oxide powder used to make discs, which will be milled to make a restoration. **Zirconia can be in the form of a disc, a block, or a blank**. These blocks are put in the milling machine to perform milling for a restoration (using CAD-CAM for designing). ## Microstructure of Dental Zirconia * It's a poly crystalline material with 99% Zirconia. (No glass phase) So, it has the highest strength. * It has other elements: * Yttria * Alumina * Hafnium * Metal Oxide (optional) ## Forms of Zirconia * **Monoclinic:** 1170°C * **Tetragonal:** 2370°C * **Cubic:** 2680°C → Liquid **We need it to be in the tetragonal phase at room temperature.** To have Zirconia in the tetragonal phase at room temperature, we use **Yttria**. Yttria makes stabilization for crystals to keep its shape at room temperature (stabilizing agent). Used by 3%. * After milling Zirconia, it is introduced into a furnace for sintering to compact particles with each other, which increases density, which in turn increases strength. * We need to distribute the heat over the whole restoration uniformly to have uniform sintering. Done by Alumina. **Extra Note:** On milling the Zirconia restoration, we always do oversize milling (around 1.25). After sintering, the part that shrinks will be equal to the size of enlargement and this is called the "enlargement factor." **Example:** If we need a crown with size 10, we will mill 12.5. After sintering, the part shrinks back to its size of 10. (Enlargement factor = 1.25). **So, the restoration is milled larger than its final desired size.** **We use metal oxide with Zirconia to give the shade** * In Vita Classic we have **16 shades** **Examples of metal oxides:** * Cerium Oxide * Iron Oxide * Copper Oxide * Bismuth Oxide **Hafnium** binds the powder particles together. It evaporates when Zirconia is introduced into the furnace. ## Partially stabilized zirconia (3y-TZP) * **3y:** Percentage of yttria * **TZP:** Tetragonal Zirconia polycrystals * It’s full contour, full anatomical, monolithic * Yttria keeps Zirconia in the tetragonal phase. The monoclinic phase wants to break and transform into the tetragonal phase in Zirconia. When the external load is applied to Zirconia (monoclinic phase) it will absorb the load, it will try to break, at that point it will transform into the tetragonal phase. The crack in the tetragonal phase is bigger than the crystalline site which is called the "transformation toughening process." **Since Zirconia has no glass phase, it's chemically not a ceramic material. It is treated as a ceramic material for its physical properties and translucency.** ** Zirconia is chemically Metal Oxide.** We can use Zirconia in crowns, bridges or **pallor bridge** **If we have 2 abutments, the bridge can withstand only 2 pontics.** **Expect, in case of lower bridge from L. Canine to L. Canine and in rare cases from upper Canine to upper Canine.** **If we make a bridge from 43 to 47, missing 4, 5, 6, it will be broken or flexing off the abutments.** ## Partially Stabilized Zirconia (4y-PSZ) * **4y:** percentage of yttria. * **PSZ:** Partially Stabilized Zirconia. * It has a port non tetragonal phase Cubic phase. * This Cubic phase is weak, but it is translucent so, we increase esthetical properties and decrease mechanical properties. This Cubic phase decreases strength as it doesn't have transformation toughening property and has weak crystals. * On stress, the cubic phase will not change and the remaining part with tetragonal phase will have transformation toughening. * **Can't be used in full arch bridge** * **Can be used in: ** * Single crowns * Anterior 3-unit bridge * Posterior 3-unit bridge **Note:** * **Circon rings > 25% Cubic:** (very translucent but weak) on hammering, converted to powder... * **While diamond rings are strong and are not-affected by hammering** ## Fully Stabilized Zirconia (5y-FSZ) * **5y:** percentage of yttria * It has only Cubic Phase * **No transformation toughening** * *When we apply load, it won't transform.* * **Highly esthetic** (High translucency) * **Lowest mechanical properties** * **Used in:** * Single Crown * Anterior 3-unit bridge * **Can't be used in posterior bridge**. ## Feldspatic porcelain (Glassy Phase) * Has low mechanical properties and high esthetics. * **Can be used in:** * Anterior crowns * Veneers * **Can't be used in posterior crowns as it's weak.** * **To strengthen it, add crystals.** **Leucite crystals** increase mechanical properties, but decrease esthetics. * **Leucite crystals** make scattering to light and according to the size of crystals, translucency is controlled. This porcelain is called **"leucite reinforced glass ceramic" or "Empress."** * **Can be used in:** * Veneers * Anterior crowns and posterior crowns * **Can't be used in Bridges** There are cases where we don't need a material with high translucency. A lot of times we are concerned about the shade. If the patient has an opaque shade, and you use a material with high translucency then the material will be clear and you will have to make the material thicker for the translucency reduction, This is bad because a thicker material will have lower translucency. So, sometimes it's better to use material with lower translucency. Other brands use **lithium disilicate** instead of leucite, which is **Emax (Glass Ceramic)**. * **Emax is less than leucite esthetically.** * **Used in:** * Veneers * Anterior crowns * Posterior bridge from canine to (5) 3-unit bridge (missing (4)) *** **From highest esthetics (translucency):** * Feldspathic porcelain → leucite reinforced → Emax **From highest mechanical properties (strength):** * Emax → leucite reinforced → Feldspathic Porcelain **We can use Zirconia for the bridge from (4) to (6), replace missing (5).** **Zirconia can't be used for intracoronal restorations as inlays, or onlays. Due to the high modulus of elasticity, it may lead to tooth fracture.** **We use glass ceramic for intracoronal restorations.** **We need to have a restoration with the same properties of the tooth. So, we make hybrid ceramics.** ## Hybrid Ceramics * **Mix between resin and ceramics** (glass + crystals) * **Resin** → for resilience and elasticity so, can be used in intracoronal restoration **without wedging or fracture of teeth.** * **Used in:** * Single unit restorations * Veneers * Crowns (Ant. or post) * Inlays or onlays. **The ratio between resin and glass ceramic is variable, according to the percentage of each of them,** the indications differ..