Crown and Bridge Procedures

• Crown and bridge procedures are fixed prosthodontics.
• These procedures restore or replace damaged or missing teeth.
• Common applications include veneers, onlays, inlays, post and core attachments, and implants.
• Sometimes, the final result is referred to as a cap.
• This covers and protects the tooth from further damage.

Model Preparation

• An indirect technique involving working outside of the patient's mouth is used.
• Benefits of this technique include:
  • Less chair time
  • Variety of available materials
  • Ability to work on duplicates as opposed to directly on the tooth
• This serves as a foundation for the finished prosthesis.
• Exact duplication of prepared tooth and adjacent teeth and tissues is required.
• Requirements of model preparation include:
  • Absence of bubbles along finish lines
  • All portions must be free of distortion
  • The cast must be trimmed to ensure access for carving wax patterns

Materials and Dimensions

• Die stone type 5 using talladium at 22cc per 100g is used.
• Facial lingual dimension should be 10-12mm.
• Buccal lingual base width should be 15mm.
• Taper lingual towards the base to prevent the working model from getting locked into the base.
• Keyways (indexes) are used to keep sectioned dies stable and ensure proper placement.

Model Equilibration

• Equilibration involves removing debris or distortion between the master and opposing models.
• If not equilibrated, there is a risk of the restoration being high in the mouth.
• Spot grinding is used on the working model.
• A spacer, simulating dental cement, is applied over the entire prep area 1mm above the margin.

Anatomy

• Developmental grooves are lines of union between lobes during tooth formation.
• Junctions between inclined walls of a sulcus are present on the labial, occlusal, buccal, and lingual aspects.
• Line angles are junctions of two crown surfaces.
• Point angles are junctions of three crown surfaces.
• The long axis is an imaginary line through the crown and root around which tooth substance is symmetrically distributed.

Embrasures

• Embrasures are formed at contact areas and act as spillways, diverting food away from contact areas and soft tissues.
• Lingual embrasures are wider and deeper than the buccal ones.

Height of Contour

• Height of contour permits light massage and prevents traumatic impact of food on free gingiva margins.
• Provides protection to the periodontium by preventing food impingement against soft tissue.
• Triangular ridges are on the occlusal surface of a cusp composed of a mesial and distal incline.
• Two inclines meet to form a triangular ridge that extends from the cusp tip to the central groove.

Central Fossa

• Central fossae are present on molars and mandibular second premolars.
• Stamp cusps are lingual on maxillary and buccal on mandibular teeth.
• Stamp Cusps act to maintain a constant distance between the upper and lower jaws when teeth are in centric occlusion and hit fossa or embrasures in centric occlusion.
• Shearing cusps are buccal of upper and lingual of lower teeth.
• Shearing Cusps do not maintain the vertical distance between upper and lower jaws when teeth are in centric.

Casting

• A sprue former allows escape for wax, directs molten metal from crucible into mold cavity, and provides a reservoir of molten metal for casting.
• Sprue former dimensions are 6-9mm with 18 to 4 gauge, or 8-10 gauge for crown and bridge.
• Talmi precious alloy simulates noble alloy with a density of 8.8 g/cm3.
• Preheat temperature is 700°C and melting temperature is 860-1010°C.
• Weight of pattern + sprue x gravity of metal equates to the weight of metal needed.

Investment Ring

• Investment ring requires 6-8mm of clearance from any aspect.
• The heat center is the geometric center of the ring.
• Place the pattern in the cold zone and sprue of reservoir in the heat zone.
• Casting investment holds 100g and expands upon setting, immersion in water, and high heat.
• Use talladium galaxy with 11.5ml of water to 11.5ml of special liquid.

Burnout

• Benchset for 15 minutes.
• Orient the ring hole down for wax to flow out.
• Rapid burnout= 15sec x g =0.25 x 100g = 25 mins
• Add 10 minutes for each additional ring, and set furnace to 700°C/1300°F.
• Burnout eliminates moisture from the ring, mold cavity of wax or plastic, and residue,
• Raises the temperature of the mold to properly receive molten metal and prevents damage to investment from overheating, and provides necessary expansion of investment.

Melting and Casting

• Melting and casting requires heating and casting force.
• Use a torch with oxygen and natural gas (or propane) and a reducing zone (hottest and purest).

Laws of Casting

• Attach sprues to the thickest cross-sectional area of the wax.
• Margins should face the trailing edge to take advantage of centrifugal force.
• Position wax patterns in the cold zone and reservoirs in heat center of the casting ring.
• Castings cool first, button, sprues and reservoirs then cool last, leaving porosity.
• The reservoir must have sufficient molten alloy.
• Do not cast a button with a runner bar.
• Turbulence must be minimized through smooth pathways and avoiding sharp turns or impingement on flat surfaces.
• Select a casting ring of sufficient length and diameter to accommodate patterns.
• Investment layer should be of uniform thickness to ensure mold expansion and avoid breakage.
• Reduce surface tension and increase wettability of waxes to reduce air trapped between investment and wax.

Powder-Liquid Ratio

• A precise powder-liquid ratio should be measured.
• More investment liquid results in a looser mix, and more water results in a tighter mix.
• Use a vacuum mix investment.
• Allow investment to set completely before putting in burnout.
• Use burnout temperature appropriate for the alloy chosen and time appropriate for investment.
• Adequate heat must be available to melt and cast alloy.
• Use a reducing zone.
• Provide enough force, because too much is just as bad.
• Cast towards the margins of the wax pattern, where metal will flow downward and right first.
• Do not quench immediately after casting, which can result in tensile forces that the metal is too hot to resist, causing tears.

Morphology

• The Curve of Spee is a concave curve of the buccal cusp tips of the lower posterior teeth and then convex curve of the buccal cusp tips of the upper posterior, with the lowest point in the arch.
• The Curve of Wilson is a line drawn across the buccal and lingual cusp tips, with lingual cusps shorter than buccal on lower teeth and longer on upper teeth.
• Height of contour (HOC) are on the gingival third except the linguals of mandibular posterior teeth (⅓ occlusal).
• All HOC extend 0.5 mm past the CEJ, except the lingual of mandibular posterior teeth (0.75-1).
• The first maxillary premolar has non-functional cusps (smaller lingual cusps), the mandibular has a central fossa, and max is more crown-like while the mandible is more round.

PFM (Porcelain Fused to Metal)

• PFM Crowns are copings/frameworks.
• Primary functions of metal substructure include providing fit of restoration, forming oxides that bond to porcelain, rigid foundation, and restoring emergence profile.
• Porcelain dimensions are 0.8-2mm.
• Framework thickness is between 0.3-0.5mm.
• The uniform thickness for porcelain is approx 1-2mm with 1.5mm for interproximal.
• The minimum thickness of wax must be 0.5 mm to cast.
• Metal ceramic restorations are inorganic crystalline materials that are fired at high temperatures (sintered).

Margin Designs

• Knife edge margins have minimal destruction, may have an exposed band, and a weak crown margin, but are acceptable for full metal crowns (FMC) or lingual PFM.
• Chamfer margins reduces crown strength and are preferred for veneers and FMC, PFM, or all ceramic crowns.
• Shoulder margins at 90 degrees provide maximum esthetics but are destructive to the tooth; acceptable for FMC, PFM, and all ceramic crowns.
• Bevel margins diminish marginal discrepancy and must have an exposed metal band, and to hide band, must be prepared subgingivally, leading to over-contouring at a 45-degree angle that lengthens the margin.
• Shoulder with bevel margins are destructive to the tooth and thought to provide internal support of shoulder margin; requires an exposed metal band as wide as bevel.

Metal Ceramic Junctions

• Metal ceramic junctions provide Definite finishing lines for porcelain.
• Bands are properly designed and follow correct measurements.
• The Band follows a proper emergence profile without displacement of tissue.

Lingual Band

• The lingual shelf should be at least 1 mm thick.
• The height of the band should be 1-3 mm high.

Base Alloy

• There is no maximum thickness, metal will be thicker for support so porcelain doesn't exceed 2 mm.
• The minimum is 0.3mm.
• Use 20 ml of special liquid to 3 ml of water.
• Burnout should be at 850 c / 1550 f.
• A density of 7.8 (g/cm3) is achieved.
• A higher oxygen level is a property achieved.