Metal-Ceramic (PFM) Crowns: Principles of Tooth Preparation PDF

Metal-ceramic (PFM) crowns INESSA SLIPAK, DDS RCC 1003 LECOM-SDM 2   Metal-ceramic restoration is also known as Porcelain-Fused-to-Metal (PFM) restoration.  A ceramic (porcelain) layer bonded to a thin cast metal coping that...

Metal-ceramic (PFM) crowns INESSA SLIPAK, DDS RCC 1003 LECOM-SDM 2   Metal-ceramic restoration is also known as Porcelain-Fused-to-Metal (PFM) restoration.  A ceramic (porcelain) layer bonded to a thin cast metal coping that fits over the tooth preparation. Introduction  Cast metal coping: the metal substructure (understructure) of the crown. Provides strength and accurate fit.  Ceramic (porcelain): cosmetic effect. 8/29/2024 3 4   Attempts to veneer metal restorations with porcelain had several problems:  Bond strength (metal to porcelain).  Appearance Success: A little   End of the 1800s: - technique to “fire” all- history porcelain jacket crowns on a platinum matrix first developed.  Mid-1950s: a dental porcelain was developed with a coefficient of thermal expansion similar to that of dental casting alloys.  1950s: The metal-ceramic restoration first became available commercially. 8/29/2024 5  6 Metal substructure (coping) + ceramic veneer = metal-ceramic restoration. A little material science The ceramic is mechanically and chemically bonded to the metal. Chemical: via firing (baking). 8/29/2024 7 8 Adequate preparation=natural looking 9 crown  Porcelain powders of varying composition  The shiny, lifelike appearance of the completed and color are applied and fired to metal-ceramic restoration results from a surface produce the desired appearance. glaze formed during an additional firing after the restoration has been shaped.  The first ceramic layer, the opaque, masks the dark metal oxide and is the primary  To be successful, a metal-ceramic crown source of color for the completed preparation requires considerable tooth reduction restoration. wherever the metal substructure is to be veneered with dental porcelain.  The opaque is covered with slightly translucent body porcelain, which is then  Only with sufficient thickness can the darker color veneered with a more translucent of the metal substructure be masked and the enamel overlay that contains only a few veneer duplicate the appearance of a natural pigments tooth.   To be successful, a metal-ceramic crown preparation requires considerable tooth reduction wherever the metal substructure is to be veneered Important with dental porcelain. principle!!!!!  Only with sufficient thickness can the darker color of the metal substructure be masked, and the veneer duplicate the appearance of a natural tooth. 8/29/2024 10  11 Which would you prefer?  12 Preparation Parameters are crucial for success and natural appearance  13 Clearance: difference between opposing tooth and your preparation. Clearance Reduction: how much tooth structure versus you actually “cut off”. Reduction Important: you may have enough visual clearance, but it is reduction that matters to the success of the restoration!!!! 8/29/2024 How does thickness impact strength? 14 Load to crack ratio formula Strength x (Thickness)² Crown Stiffness Example: 4 (4)²/4=16 4(2)²/4=4 What that means is ½ thickness = ¼ strength Source: Dr. Rick Fox Sirona Clinical Advisor   Teeth require complete coverage with esthetic  Need to contour axial demands. surfaces to correct minor mal-inclinations.  Extensive tooth destruction (caries,  Can be used to correct Indications trauma, existing restorations). the occlusal plane. for PFM  Need for superior retention and strength.  Endodontically treated tooth. 8/29/2024 15  16 Contraindications  Patients with active caries or untreated periodontal disease.  In young patients with large pulp chambers.  Whenever a more conservative restoration is feasible.  Nickel sensitivity: does not have to be an issue, if you request a nickel free restoration.  17 Dramatic gingival response to Nickel-containing PFM.  18 ADVANTAGES Combines the strength of cast metal with the esthetics of an all-ceramic crown. DISADVANTAGES The preparation for a metal-ceramic crown requires significant tooth reduction to provide sufficient space for the restorative materials.  Crown LET'S REVIEW IT AGAIN marginal configurations 8/29/2024 19  20 FEATHER BEVEL SHOULDER CHAMFER BEVELED SHOULDER SLOPED SHOULDER  21 Feather-edge (NEVER AT LECOM) Advantage: Disadvantage: ✓ Conservation of tooth ✓ Does not provide sufficient structure. bulk.  Chamfer 22 Advantages: Disadvantages: ✓Distinct margin ✓Care needed to ✓Adequate bulk avoid an ✓Easy to control unsupported lip of enamel ✓Aka J-margin 23 8/29/2024  24 chamfer Chamfer with Chamfer with metal collar cutback for porcelain finish  CROWN MARGIN DESIGNS: 25 Shoulder Advantages: Disadvantages: ✓ Bulk of restorative material ✓ Less conservation of tooth structure Shoulder Metal Cut back- This allows seamless esthetics between the tooth structure and the crown.  CROWN MARGIN DESIGNS: 26 Shoulder beveled margin Advantages: Disadvantages: ✓ Bulk of material, plus removes ✓ Less conservative unsupported enamel and allows ✓ Extends preparation apically finishing of metal margins This metal margin is recommended in all periodontally involved cases, where gingival health is important 27 8/29/2024  28 The preparation   Hybrid PFM: This PFM will be ½ PFM preparation (buccal)+ ½ FCG preparation (lingual) PREPARATION Shoulder margin: Buccal into early contact zone (1.5mm and narrowing) Chamfer margin: Mid-contact to lingual (1mm and narrowing) 8/29/2024 29 8/29/2024 30  31 Step by step preparation Preparation sequence 32 1. Guiding grooves (occlusal posterior, incisal anterior) 2. Incisal or occlusal reduction (functional cusp bevel) 3. Buccal guiding grooves, followed by buccal axial reduction (2nd plane!). Lingual guiding grooves, followed by lingual axial reduction. 4. Proximal reduction 5. Margination and Final finishing of all prepared surfaces.  33 1. Guiding grooves 1. Use the 1.5mm depth cutting bur in the areas where ceramic coverage is required (final reduction is 2mm) 2. Occlusal 34 reduction  Blend the guiding grooves  2mm reduction required for any occlusal surface that has porcelain and metal (spaced is required for both)  Include the functional cusp bevel in your reduction  Caution must be used; excessive occlusal reduction shortens the axial walls and thus is a common cause of inadequate retention and resistance form in the completed preparation (also remember the pulp!)  8/29/2024 35 8/29/2024   When completed, the reduction of the facial surface should have produced sufficient space to accommodate the metal substructure and porcelain veneer.  A minimum of 1.2 mm is necessary for the ceramist to produce a restoration with 3. BUCCAL REDUCTION satisfactory appearance (1.5 mm is preferable).  In the cervical area of small teeth, obtaining optimal reduction is not always feasible. A compromise is often made with lesser reduction in the area where the cervical shoulder margin is prepared. (anterior mandibular for example) 8/29/2024 36 37   The location and specific configuration of the facial margin depend on several factors: The facial or 1. The type of metal-ceramic restoration selected, buccal 2. 3. The cosmetic expectations of the patient, and Operator preference. margin…  From a periodontal point of view, a supra- gingival margin is always preferred. 8/29/2024 38  39  Where a subgingival margin is to be placed, careful tissue manipulation is essential; otherwise, there will be damage that leads to permanent gingival recession and subsequent exposure of the metal collar. 8/29/2024  4. Proximal reduction and 40 margination  Start the chamfer margin Mid-contact  As the lingual chamfer is developed, extend it buccally into the proximal to blend with the interproximal shoulder placed earlier.  Over taper is a common issue. Complete the Buccal and Lingual axial reduction (wrap the margin into the contact zone) to reduce amount of proximal tissue reduction required.   Smooth margin (check for J margin)  Avoid any sharp line angles  Check 2nd plane (follow tooth anatomy)  Check isthmus reduction  At LECOM-SDM: 5. Finishing  PFM will have rounded shoulder facial (1.5- 2.0 mm) and light chamfer lingual (0.5 mm) with continuous blending interproximally.  Remember, in the Sim the lingual chamfer needs to be 0.7-0.9mm 41   Anterior PFM?  A porcelain labial margin requires proper Anterior tooth support for the porcelain. A shoulder with a 90- degree cavosurface angle is recommended. consideration  Cut-back concept 8/29/2024 42 8/29/2024 43 44