base and liner 2 l Preclinical Operative MCQs

 

Dental base and liner (ZINC PHOSPHATE CEMENT, ZINC POLY CARBOXYLATE CEMENT,  ZINC OXIDE-EUGENOL CEMENT, ZINC OXIDE-EUGENOL CEMENT , CAVITY VARNISH, CALCIUM HYDROXIDE CEMENT  l Preclinical Operative MCQs for dental students

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Base and liner in preclinical operative dentistry

Material placed under restoration to act as chemical and thermal barrier for tooth structure.

ZINC PHOSPHATE CEMENT

Other names:

1. Crown and bridge cement
2. Zinc oxyphosphate cement

• It is the oldest of the luting cements and serves as a standard for comparison with new cements.

Composition of ZINC PHOSPHATE CEMENT:

Powder

1. Zinc oxide (90%):  Basic ingredient
2. Magnesium oxide(8.2%): Aids in sintering
3. Other oxides (0.2%) (Bismuth, calcium): Improves smoothness of mix
4. Silica (1.4%): Filler

Liquid

1. Phosphoric acid (38.2%): Reacts with zinc oxide
2. Water (36%): Controls rate of reaction
3. Aluminum or Zinc phosphate (16.2%): Retards reaction
4. Aluminum (2.5%)
5. Zinc (7.1%)

Setting reaction of ZINC PHOSPHATE CEMENT:

1. Phosphoric acid reacts with zinc oxide forming zinc phosphate. Aluminum of the liquid is essential for cement formation. It forms complexes with phosphoric acid to form zinc alumino phosphate gel.
2. Adding small quantity of powder into liquid in increments increases the setting and mixing time and permits more powder to be incorporated into the mix.
3. Cooling the mixing slab markedly increases the setting time. It is the most effective method of controlling the setting time.
4. Prolonged spatulation retards the setting reaction as the formed matrix is effectively destroyed and extra time is needed to rebuild the bulk of the matrix. (In dental stones, prolonged spatulation accelerates the setting reaction)
5. The acidity of the cements is quite high as the time they are inserted in the tooth. Fresh mix has a pH of 2 and then increases rapidly in 24-48 hours. Any damage to pulp occurs during the first few hours after insertion.

ZINC POLY CARBOXYLATE CEMENT

• First cement developed with a potential for adhesion to tooth structure.

Composition of ZINC POLY CARBOXYLATE CEMENT:

Powder

1. Zinc oxide: Basic ingredient
2. Magnesium oxide:  Modifier
3. Bismuth and Aluminum oxide: Improves smoothness of mix
4. Stannous fluoride: Anticariogenic property

Liquid

1. Polyacrylic acid
2. Itaconic acid, Maleic acid, Tricarbalyllic acid: decreases viscosity

Properties of ZINC POLY CARBOXYLATE CEMENT:

1. Because of the larger size of the polyacrylic acid molecules, it is less irritant to pulp than zinc phosphate cement.
2. The pH of liquid is 1.7 and that of freshly mixed cement is 3-4. After 24 hours, pH of the cement is 5-6.
3. Cement bonds chemically with the tooth structure due to chelation of the carboxyl group with calcium in the tooth structure.
4. Unlike ZnPo cement, adhesion is better to smooth surface than rough surface.
5. These cements are good thermal insulators and so can be used as base material.

ZINC OXIDE-EUGENOL CEMENT

• Consider weak cement and not indicated under amalgam due to need of force during condensation of amalgam

Classification ZINC OXIDE-EUGENOL CEMENT:

1. Type I: For temporary cementation
2. Type II: For permanent cementation
3. Type III: Temporary filling material and base
4. Type IV: Cavity liners

Composition ZINC OXIDE-EUGENOL CEMENT:

POWDER

1. Zinc oxide-69%: Principal ingredient
2. Magnesium oxide-29.3%: To reduce brittleness
3. Zinc stearate -1%: Accelerator, plasticizer
4. Zinc acetate-0.7%: Accelerator
5. Magnesium oxide: Reacts with eugenol like ZnO

LIQUID

1. Eugenol-85%: Reacts with zinc oxide
2. Olive oil-15%: Plasticizer

• NOTE: Addition of drop of water, alcohol and acetic acid accelerates the setting reaction while addition of a drop of glycerin retards the reaction.

Properties:

• Solubility is highest among dental cements.
• Least irritating among all dental cements, so called as pulp friendly cement.
• It has obtundant (or) soothing effect on the pulp in deep cavities.

Modified ZINC OXIDE-EUGENOL CEMENT:

EBA- Alumina modified cement

• Powder

1. Zinc oxide-70%
2. Alumina-30%

• Liquid

1. EBA-62.5%
2. Eugenol-37.5%

• Polymer reinforced ZOE cement.
• Cements containing vanillate esters.

CALCIUM HYDROXIDE CEMENT

• Ca (OH)2 is a relatively weak cement.

Applications of CALCIUM HYDROXIDE CEMENT:

1. For direct and indirect pulp capping.
2. In apexification technique.
3. In pulpotomy(apexogenesis)
4. As a root canal sealer
5. As a root canal disinfectant
6. As a cavity liner and base

Properties CALCIUM HYDROXIDE CEMENT:

1. The solubility in water is high. So it should not be exposed at the margins when used as a liner.
2. Solubility is higher when exposed to phosphoric acid and ether, so care should be taken during acid etching and application of varnish.
3. The cement is alkaline in nature. It has a pH of 11.
4. The high alkalinity and its consequent antibacterial and protein-lysing effect helps in formation of "Reparative dentin".
5. It is an effective antibacterial agent.

CAVITY VARNISH

• Cavity varnish is a solution of one or more resins which when applied onto the cavity walls, evaporates leaving a thin resin film, that serves as a barrier between the restoration and the dentinal tubules.

Composition of CAVITY VARNISH:

• Natural gum such as copal, resin or synthetic resin dissolved in organic solvent such as alcohol, acetone or ether.

Applications of CAVITY VARNISH:

• In amalgam restorations it decreases the micro leakage due to margin sealing.

• It also prevents discoloration by preventing the penetration of silver ions into the dentin.

• It may be used as a surface coating over GIC, silicate restorations to protect from dehydration.

• It prevents galvanic shock if applied over metallic restorations.

Properties:

• They neither possess mechanical strength nor provide thermal insulation because of its thin film. They are insoluble in oral fluids.

Contraindications:

• Varnish interferes with Composite resins polymerization.

• Varnish interferes with the GIC & Polycarboxylate potential for adhesion Dentin bonding agents have replaced the role of varnish in sealing the restorations.

Important hints in CAVITY LINERS

• A cavity liner is used like a cavity varnish to provide a barrier against the passage of irritants from cements or other restorative materials and to reduce the sensitivity of freshly cut dentin.

• The liners are volatile or aqueous suspensions of zinc oxide or calcium hydroxide that can be applied to a cavity surface in a relatively thin film.

• The suspension liners neither possess strength nor provide thermal insulation.

• Hard setting Ca(OH)2, ZOE (Type IV) and GIC liners placed in thickness of 0.5mm, have adequate strength to withstand the condensation forces.

• If removal of infected dentin does not extend deeper than 1mm. from initially prepared pulpal or axial wall→ No liner indicated.

• Very deep excavations and known pulpal exposures→ Ca(OH) liner.

• If excavation is in between above two →ZOE liner

• Calcium hydroxide (or) GIC liner should be used under composite restorations.

• ZOE liner is contraindicated as the eugenol in the cement may interfere with polymerization of composite.

• Calcium hydroxide stimulates reparative dentin formation while ZOE has an obtundant effect on pulp.

Important hints in  CEMENT BASES

• A base is a thicker layer of cement placed under permanent restoration to encourage recovery of the injured pulp and to protect the pulp from thermal, chemical and galvanic insults.
• The base serve as substitute of dentin that has been destroyed by caries or cavity preparation.
• Low strength bases - ZOE, Calcium hydroxide High strength bases - Zn PO 4 , GIC, polycarboxylate
• Base does not prevent micro leakage and acid penetration when used with amalgam or gold foil. A varnish or dentin -bonding agent is used to assist sealing of the restoration.
• With ZnP04 as a base, the sealant should be applied first followed by base.
• If the base is Ca(OH) 2, ZOE, GIC or polycarboxylate, the base should be placed first followed by the sealant.

Properties of cement bases :

1. for effective thermal protection the base should have a minimal thickness of 0.75 mm
2. The base must have sufficient strength to withstand forces of condensation and masticatory stresses.

Indication of cement bases :

1. Ca(OH)2 or ZOE under amalgam
2. ZnP04 , GIC or Polycarboxylate under Direct filling gold
3. Ca(OH)z or GIC under composite