Metal Ceramic Restorations MSA PDF

‫ﺑﺳم ﷲ اﻟرﺣﻣن اﻟرﺣﯾم‬  Fixed Prosthodontics Metal Ceramic restorations Professor Name : Dr Sohaila Ali M Sahel Professor title at MSA: Lecturer of Fixed Prosthodontics LECTURE OUTLINE I- Metal ceramic Restoration A. Clinical As...

‫ﺑﺳم ﷲ اﻟرﺣﻣن اﻟرﺣﯾم‬  Fixed Prosthodontics Metal Ceramic restorations Professor Name : Dr Sohaila Ali M Sahel Professor title at MSA: Lecturer of Fixed Prosthodontics LECTURE OUTLINE I- Metal ceramic Restoration A. Clinical Aspect : i. Aim of Preparation ❑ Anterior & posterior reduction ❑ Finish line Geometry LECTURE ILOs Identify the various uses, types and geometry of finish lines used in ceramometallic preparation to ensure marginal integrity and restoration survival Display appropriate professional attitudes and behavior towards colleagues and supervisors. Emphasize on the value of the teamwork through group assignments. Definition It is a complete coverage cast metal crown veneered with a layer of fused porcelain. Full veneered Labial veneered ALLOYS USED IN CERAMO-METALLIC RESTORATION Are classified by the American Dental Association (ADA) according to their noble metal content into: I- High noble: Gold content > 40 wt % and noble content > 60 wt %. II- Noble: Noble metal content > 25 wt %. III- Base metal: Noble metal content < 25 wt %.  Dental Alloys fused to porcelain High noble Noble Base Metal alloys Metal alloys Metal alloys 60% noble metal 25%noble metal Less than 25% noble 40% gold and no gold metal. Au-Pl -Pd Pd-Ag Ni-Cr Au –Pd-Ag Pd-Cu-Ga Co –Cr Au - Pd Pd-Ga Ti Requirements of alloys used for metal-ceramic restorations: 1- The difference of coefficient of thermal expansion of porcelain and metal should be not greater than 1 10-6 0C. to avoid production of shear stress due to the difference in their cooling rates thus leading to failure of the bond between them. 8  2- The melting range temperature of the metal should be higher than the fusing temperature of the porcelain by at least 170-280 0C ………otherwise the metal coping may undergo flow or creep and deformation. 3- High yield strength to decrease coping thickness. 4- High modulus of elasticity to avoid bending in long span bridges. 5- High sag resistance to avoid thermal distortion. 9 6- A metal ceramic alloy must be able to produce surface oxides for chemical bonding with dental porcelain. Base metal alloys possess a natural tendency to oxidize when subjected to the elevated temperature of a porcelain furnace. Noble alloys, on the other hand, do not oxidize. So trace amounts of base elements are added for oxidation to take place. Indium (In), Tin (Sn), Gallium (Ga), Iron (Fe) Hardens the alloy and Provides oxides for ceramic bonding Facial view of the porcelain-bearing area of the finished substructure 11 Cleaning procedures: Air-abrasion of the veneering area. The margins have been protected by soft wax. The completed substructure ready for oxidizing 12  -- To establish the chemical bond between metal and porcelain, a controlled oxide layer must be created on the metal surface. -- The oxide layer is typically obtained by placing the substructure on a firing tray, inserting it into the muffle of a porcelain furnace, and raising the temperature to a specified level. 13 Oxidizing: Metal-ceramic substructure after cleaning and before oxidizing in the porcelain furnace. 14 Basic requirements needed for dental porcelain used as veneering material: 1. Low fusing temperatures, less than the melting range of the cast metal substrate by about 170-280 oC. 2. Should be of high viscosity high resistance to slumping to maintain their basic shapes during firing 3. Must be chemically & optically stable over a series of firing cycles. 16  3. Should resist devitrification :. if porcelain fired many times it becomes milky (cloudy) & difficult to glaze 4. Coefficient of thermal expansion should be lower than that of metal by 1 × 10 -6 0C to enhance bond strength & avoid crack formation. 17 Time is ticking away 11 12 1 10 2 9 3 8 4 7 5 6  Principles of metal substructure design  1- No sharp or acute angles: 📫 Rounded convex metal surface with no sharp angles is important to * eliminate areas of stress concentration * Distribute occlusal forces evenly * Facilitates wetting of metal with porcelain so enhancing bonding 20 ▪ The metal substructure ‘d have a distinct margin for finishing the veneer. {Rounded angles } 21 2- Adequate support to porcelain: Otherwise fracture of brittle porcelain. thick porcelain as layers away from metal is under tension subsurface porosity 22 A and C, Cross section through a metal-ceramic restoration. Ideal porcelain thickness is ensured by waxing to the full anatomic contour and cutting back. B and D, Incorrect framework design has insufficient support for the incisal porcelain. This can lead to fracture. 23 Unsupported thick porcelain. Sub-surface porosity increased with thickness of porcelain 24 3- No occlusal contacts on porcelain/metal interface: otherwise porcelain fracture * Junction ‘d be away from all centric occlusal contacts by at least 1-1.5 mm to avoid metal flow and subsequent 25 Occlusal contact away from the metal-ceramic junction. 26  4- Proximal contacts: 📫 For anterior teeth be on porcelain for better esthetics & translucency If on metal … block light transmission & poor esthetics 27 Cutback for proximal contact in porcelain. 28 Proximal contact in metal 29 5- Facilitates the wrap around effect : Metal is designed to allow porcelain to wrap around the metal to a- porcelain resistance against splitting b- better translucency & esthetics 30 Wrap around of porcelain veneer Porcelain covering part of the lingual surface 31 6- Compensate deficiencies in correct form of prepared teeth: - Deficiencies in the incisal edges or buccal or lingual cusps ‘d be compensated for with extra-metal thickness , not in porcelain thus …having even, minimal thickness of porcelain 32 ▪ Thin porcelain of uniform thickness ( which doesn’t impair esthetics ) supported by rigid metal ‘s stronger than thick porcelain. ▪ Minimum thickness of porcelain 0.7 mm ▪ Optimum thickness 1.0 mm ▪ Maximum 1.5 mm 33 7- Metal thickness to provide adequate rigidity: as … Flexing or bending during a) seating or b)under occlusal forces porcelain fracture 34 ▪ Minimal metal thickness noble metal alloys base metal alloys 0.3-0.5 mm 0.2-0.3 mm can be finished to these thin sections, withstand distortion as - Melting range - Yield strength -Modulus of elasticity 35 Porcelain and metal thickness. 36 ❑Metal finishing: The metal caliper should be used to measure the thickness of metal before any adjustments are made. Hold the gauge securely and allow the movable area to close on the casting. The thickness of metal will be indicated on the scale at the base of the gauge 37 Finishing the surface in one direction with light pressure helps avoid trapping debris between folds of the metal. 38 Some of the irregularities may retain debris that can later contaminate the porcelain 39 Finish the metal in one direction. Unidirectional finishing should leave the metal surface smooth and free of debris. 40 1) No sharp or acute angles. Convex surfaces and rounded contours of metal surfaces should be created. 2) The junction between metal and ceramic as definite (90o degree angle) and as smooth as possible. Is the occlusal contact to be in metal or porcelain? ▪ Occlusion in metal require less tooth reduction 1-1.5 mm. On the other hand ▪ 2 mm of occlusal reduction for posterior teeth and 1-1.5 mm for anterior teeth for porcelain on occluding surfaces. ▪ Metal surface can be more easily adjusted and re polished in chair side without adversely affecting the restoration.. ▪ Metal is potentially less abrasive to opposing natural teeth than dental porcelain. On the other hand removal of the glaze in the metal ceramic restoration in the intra-oral adjustment weaken the porcelain greatly. The transverse strength reduced by ½ compared with porcelain with intact glazed surface 3)No occlusal contact on porcelain metal interface contact at porcelain metal junction leads to porcelain fractures. The metal-ceramic interface must be at least 1.5 mm away from all centric occlusal contacts to avoid metal flow and subsequent ceramic fracture. Occlusal contacts against natural opposing teeth should be wherever possible in metal. The centric occlusal 1.5-2 mm from the porcelain-metal junction. When the anterior teeth contact in the incisal region, it is often necessary to consider a design with the lingual surface in porcelain to avoid functioning on or over the porcelain-metal junction. Do not design the substructure so contact occurs at the porcelain-metal junction. When the anterior teeth occlude in the gingival one half of the maxillary teeth, or when the lingual tooth reduction is less than 1 mm, it is best to design the substructure with the occlusion in metal. 4) Facilitates the porcelain wrap-around effect: as this would result in: a) increasing porcelain resistance against splitting (fracture under mastictary force). b) Better transleucency and good esthetics. 5) Metal thickness should provide adequate rigidity to prevent it distortion during porcelain firing or flexing under occlusal load resulting in porcelain fracture. A minimum thickness noble metal alloys is (03.05 mm) and 0.2-0.3 mm is for base metal. 6) Adequate support to porcelain veneer where lack of metal support results in porcelain fracture due to its brittleness and low flexure a strength. 7) The metal framework should be shaped to allow for a distinct margin. 8) Deficiencies in the incisal edge , buccal or ligual cusps should be compensated for with extra-metal thickness and should never economise on metal by replacing areas of gross destruction of tooth structure with porcelain. Minimal thickness of porcelain is0.7 mm, optimum thickness is 1 mm maximum is 1.5 mm. 9) Proximal contacts occlusal contacts Proximal contacts: should allow light transmission through the porcelain enamel in the proximal area. A natural tooth will always exhibit high tranleucency at its mesial distal and incisal margins. So the lingual metal collar should not encroach into the proximal space by more than needed for strength. Porcelain for metal veneering Dental porcelain is produced from a blend of quartz (SiO2), feldspar (potassium aluminum silicate, sodium aluminum silicate), and other oxides. During manufacture, the materials are heated to high temperature to form a glassy mass and then rapidly cooled by quenching them in water, which causes the glassy mass to fracture. The resulting product is called a frit. 55  Dental porcelain is supplied in powder form of several shades Mixing the powder with water or water-base glycerin containing liquid to form paste which is used to build-up restoration to its proper contour. Building-up using hairbrush , paste condensed onto metal through several condensation techniques ( vibration, spatulation or dry brush technique) 57 A variety of instruments from a porcelain kit includes a large whipping brush, and brushes for porcelain application, metal carving instruments, and a glass mixing rod. 58 A large ceramic slab is recommended for the buildup procedure because you have more working area to accommodate mixes of dentine, enamel, translucent porcelain, and various modifiers. 59 Condensation: To obtain a dense mass by: Vibration technique Spatulation technique Dry brush technique Purpose of porcelain condensation: 1. To remove excess water to firing shrinkage 2. To adapt the paste in adequate form. 3. To porosity. 61 The porcelain layer types: There ‘re three types of porcelain powders: 1. Opaque porcelain 2. Body or dentin porcelain 3. Enamel or incisal porcelain 62 Porcelain powders. 63 1. Opaque porcelain: First porcelain layer applied to the metal substructure. Oxides of titanium , tin or zirconium are added to original porcelain blend to mask the underlying metal color through scattering & reflection of the incident light rather High refractive than transmission index of these metal oxides 64 Role of opaque porcelain: 1. Masks the color of underlying metal. 2. Provides the basis for the total color tone of the crown as it ‘s the background of the overlying porcelain 3. provides chemical bond to the metal through it’s oxides 65 Opaque layer A unform thickness of 0.2 – 0.3 mm generally is regarded as ideal. 67 2. Body (Dentin) porcelain Fired onto the opaque layer together with the enamel porcelain. Contains oxides that aid in matching the proper color as it forms the main bulk of the build -up 68 Body layer 69 3. Enamel (incisal)porcelain Translucent porcelain that imports natural translucency to the final restoration Enamel layer 70 Porcelain Margin: With a vertical-loading porcelain furnace, work is placed on the centre of the ceramic platform and automatically raised into the vertical muffle. 72 The porcelain-metal bonding mechanisms 1.Mechanical 2. Compressive 3. Chemical 4. Van der waals forces 74 1. The mechanical bond The micro-roughness on the metal surface which are produced by finishing the metal surface with: * Non-contaminating stones or discs * Air abrasion ( sand blasting) creates some form of mechanical interlocking between opaque P. and metal. 75  Air abrasion (sand blasting) leads to : 1- Enhances the wettability of metal substrate with porcelain. 2- Mechanical interlocking. 3- surface area for chemical bonding. 76  But excessive micro-roughness leads to: a) Stress concentration at metal-ceramic interface. b) Deep interface angles no complete wetting air entrapment and voids at metal-ceramic interface. 77 2. Compressive stresses: Due to the slightly smaller coefficient of thermal expansion of the porcelain than that of the metal ( 1×10-6 o C) being placed in a state of compression during cooling to room temp. Porcelain in a state of compression at the alloy interface. 78 In a restoration with full-porcelain coverage, the metal is probably under tension (T) and ideally the porcelain is under compression (C). 79 3. Chemical bonding: Plays the major role in metal-ceramic bonding mechanisms ……formation of oxide layer on the metal surface. Oxide forming trace elements such as tin, indium or gallium are added to gold alloys They migrate to the interface where they oxidize & form covalent or ionic bond to similar oxides in the opaque porcelain. 80 On a very basic level, a metal ceramic restoration is made up of a metal substructure, porcelain veneer, and (preferably) an intermediary monomolecular oxide layer. 81  In base metal alloys all metal elements are oxidizable, chromium oxide layer is readily formed that bonds to porcelain. Elements are added in nickel-chromium alloys to control oxide layer thickness prevent premature bond failure through metal oxide or at metal-metal oxide interface. 82 4-Van der waals forces: The attraction between charged atoms that are in intimate contact yet do not actually exchange electrons is derived from van der waals forces. Ceramo-metallic failure Possible modes of failure of alloy-porcelain restorations. Thank You