Luting Agents And Cementation Techniques

Luting Agents And Cementation Techniques

Describe the ideal requirements of luting agents and various luting agents used in crown and bridge cementing. Explain the procedure and care to be taken to lute porcelain jacket crowns.

Ideal requirements of luting agents are as follows:

• Should be tissue-compatible
• Should be resistant to disintegration in the oral cavity
• Should provide a durable bond between dissimilar materials

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• Should possess favorable compressive and tensile strengths
• Should have a sufficient fracture toughness
• Should be able to get the tooth and the restoration
• Should form a complete seal
• Should demonstrate adequate working and setting times.

Various luting agents are as follows:

• Zinc phosphate cement
• Zinc polycarboxylate cement
• Glass ionomer cement
• Resin-modified glass or hybrid ionomers
• Zinc oxide-eugenol with and without ortho ethoxy benzoic acid (EBA)
• Silico-phosphate cement
• Unfilled resins—hydroxyethyl methacrylates and 4 methacrylate-ethyl trimellitic anhydride (4-META).

Zinc phosphate cement

Zinc phosphate cement Advantages

• Prolonged success rate
• Good compressive strength (9000–20000 psi)
• Adequate tensile strength (720 psi)
• It is a basic cement for comparison
• Reasonable working time
• Excess material is removed easily.

Zinc phosphate cement Composition

Powder

• Zinc oxide and magnesium oxide in the ratio of 9–1
• The water content is 33%.

Liquid

• Phosphoric acid is buffered by aluminum (50%) and zinc salts (traces).

Zinc phosphate cement Disadvantages

• Pulp irritation
• Solubility
• Poor seal between the cement and the dentin
• Not an ideal cement when preparation is close to pulp.

Zinc phosphate cement Manipulation

• A cooled thick glass slab is used for mixing
• Small increments of powder are incorporated into the cement liquid and mixed with a wide circular motion on about one-half of the glass slab
• When the mixture follows the spatula, it is ready for use as a luting medium.

Zinc polycarboxylate cement

Zinc polycarboxylate cement Advantages

• Good biocompatibility
• Adhesive to tooth structure
• Exhibit thinning with increased shear rate (less film thickness)
• Less pulpal irritation as polyacrylic acid molecules are large and cannot penetrate the dentinal tubule
• Cleaning the surfaces of metal with an airborne abrasive increases the adhesion of polycarboxylate cement to the metal.

Zinc polycarboxylate cement Disadvantages

• Working time is short (2.5 minutes)
• Residual zinc polycarboxylate is more difficult to remove
• It provides less retention.

Zinc polycarboxylate cement Indication

• On retentive preparations (e.g. in children with large pulp chambers).

Zinc polycarboxylate cement Properties

• The compressive strength is at least one-half that of zinc phosphate cement, whereas the tensile strength is similar to zinc phosphate cement
• The solubility of carboxylate cement is about the same as zinc phosphate
• The film thickness of the polycarboxylate cements is approximately ±20 um.

Glass ionomer cement

• Invented by Wilson and Kent
• Put to clinical use by McLean and Wilson.

Glass ionomer cement Advantages

• Adhesive to enamel and dentin
• Good biocompatibility
• anticariogenic effect (releases fluoride)
• Set cement is translucent, and more esthetic when used with the porcelain labial margin technique
• The mechanical properties are almost similar to zinc phosphate.

Glass ionomer cement Disadvantage

• Is susceptible to moisture contamination.

Glass ionomer cement Composition

• Glass particles and polyacrylic or polymaleic acid—Polyhyrogel + Silica gel.

Glass ionomer cement Reaction

• When the powder and liquid are mixed, the aluminum and calcium are displaced forming an alumino-silicate network, which degrades to form a hydrated siliceous gel.

Glass ionomer cement Manipulation

• The measured powder is divided into two equal parts and mixed with a plastic spatula
• The first increment is rapidly incorporated in 10 seconds and the second increment is incorporated and mixed for a further 10 seconds
• The mixing is completed when the cement can be lifted off the slab with the spatula.

To reduce belated sensitivity following can be done:

• Rapid incorporation of powder and liquid (10–20 seconds)
• Slight hydration of the tooth before cementation by placing a drop of water on the tooth during mixing, which is gently blown off just before placing the prosthesis on the tooth
• Allowing the cement to set hard to the touch, plus 1 minute before removing the excess
• Placing glass ionomer cement varnish on the margins of the restoration after removing the excess cement.

Resin-modified glass or hybrid ionomers (Fuji II LC and Vitremer)

Differences from conventional resin and glass ionomer cement (GIC) are as follows:

• The first reaction is an acid/base setting reaction as in GIC.
• The second reaction is that the limited quantity of a monomer present will polymerize by light cure (as in resin-based luting agents).
• Polymer content is very minimal which will not interfere with the normal acid/base setting reaction and ion exchange adhesion with tooth structure as in GIC.
• A third setting reaction occurs in which the remaining monomer will still polymerize chemically.

Zinc oxide eugenol (ZOE) with and without ortho ethoxy benzoic acid (EBA)

• Quartz and alumina have also been substituted in the mix.

Zinc oxide-eugenol

Zinc oxide-eugenol Advantages

• Reinforced ZOE cement is extremely biocompatible
• Provides an excellent seal.

Zinc oxide-eugenol Disadvantages

• Physical properties (such as compressive strength, solubility, and film thickness) are inferior to other cement
• Eugenol leakage can occur
• Can be only used in restorations with good retention forms.

EBA cement

• The 2-ethoxy benzoic acid (EBA) replaces a portion of the eugenol in conventional ZOE cement, which improves compressive strength without affecting its resistance to deformation
• The EBA cement has a short working time and excess material is difficult to remove
• The compressive strength of the reinforced zinc oxide eugenol cement is approximately one-half that of zinc phosphate, whereas the tensile strength is nearly identical to zinc phosphate. The values are similar to the averages for polycarboxylate cements.

EBA cement Advantages

• Palliative effect on the dental pulp
• Ease of placement in a moist environment.

Silico-phosphate cement

It is a mixture of zinc phosphate and silicate cement.

Silico-phosphate cement Advantages

• Physical properties in the range of zinc phosphate cement
• Exhibits lower solubility
• The set material is translucent, useful with porcelain margin crowns
• Some anti-cariogenic properties.

Silico-phosphate cement Disadvantage

• Low pH and consequent potential for pulpal irritation.

Unfilled resins

• Used since1950s
• Less biocompatible than other cement.

Based on the polymerization method:

• Chemical-cure
• Light-cure or dual-cure.

Metal restorations: A chemically cured system is used

Ceramics: A light or dual-cure is used

Resin bonded: Resin cement bonded onto etched enamel surfaces.

Unfilled resins Indications

For luting:

• Porcelain veneers
• Porcelain inlays and on-lays
• Resin-bonded prostheses
• Post and core reinforcements (calcium hydroxide placed in deep preparation followed by etched glass ionomer liner, before placement of resin cement).

Unfilled resins Advantages

• Insoluble in oral fluids
• Good strength properties
• Easy to use.

Unfilled resins Disadvantages

• Bonds to tooth structure
• Is not cariostatic
• Leaks at dentinal margins
• The film thickness of resin cement is greater.

Unfilled resins Recent development

• Organophosphates, hydroxyethyl methacrylate (HEMA)
• 4 methacryl-ethyl trimellitic anhydride (4-META).

Other Cement

• Red and black copper cements: Excessively irritating to the pulp
• Zinc phosphate cement mixed by adding water: Is weaker and more soluble than conventional cement
• Cyanoacrylate cement: Just in the beginning stages.

Procedure to Lute Porcelain Jacket Crowns

Glass ionomer is the luting agent of choice. Other luting agents that can be used are resins and zinc phosphate.

Cleaning tooth surfaces

• All preparation surfaces are checked for any remaining provisional luting agent
• The luting agent shade is selected depending on the adjacent tooth shade.

Preparing restoration

• The casting is cleaned ultrasonically and any remnants of polishing agents are removed.

Preparation for cementation

• Isolate the area with cotton rolls and place the saliva evacuator.

Manipulation of glass ionomer

• Powder-liquid ratio: 1.25 part of powder to 1 part of liquid
• After shaking the powder bottle, 2 level scoops of powder and 8 drops of liquid are dispensed in a paper mixing pad
• Reduced temperature while mixing increases working time and allows the addition of more powder
• The measured powder is divided into two equal parts and mixed with a plastic spatula. The first part is rapidly incorporated in 10 seconds and the second part is incorporated and mixed for a further 10 seconds
• Mixing time: 45 seconds
• Working time: 3 minutes from the start of the mix
• Luting consistency:
  • The cement lifts off the slab with the spatula and can be pulled into a thread of 20 mm in length before snapping back onto the slab. The consistency is more viscous than zinc phosphate, but the material thins out with seating pressure.

Application of cement

• A thin coat is applied to the clean internal surface of the restoration
• Ask the patient to bite on a wooden stick
• Dry the tooth with a light blast of air and seat the restoration into place.

Seating the restoration

• Final seating is seated firmly with a rocking, dynamic seating force
• After the casting is seated, the margins are verified for complete seating.

Removing excess cement

• After the cement has set (7 minutes from the start of the mix) remove excess cement with an explorer
• Early removal of cement exposes margins to moisture, increasing its solubility
• A margin seal can be applied to prevent moisture contamination
• Dental floss can be used to remove residual cement interproximal and from the gingival sulcus.

Evaluation of occlusion

• An articulating paper or Mylar strip is used to check the occlusion once more.

Instructions to patient

• As the cements take at least 24 hours for their final set, patients are instructed not to chew on that side for a day or two
• Oral hygiene instructions are also given.

Resin luting agents

• A light- or dual-cure can be used for porcelain jacket crowns when retention form is compromised.