ZINC OXIDE EUGENOL CEMENT

ZINC OXIDE EUGENOL

Zinc oxide eugenol cement was developed by Dr. J. Foster Flagg in 1875 According to ADA specification no. 30, they are classified into 4 types based on their uses

1. Type I Temporary cementation – compressive strength <35 MPa

2. Type II Permanent cementation- compressive strength >35 MPa

3. Type III Temporary restoration- compressive strength >25 MPa

4. Type IV cavity lining- compressive strength >5 MPa

COMPOSITION:-

POWDER

Zinc oxide (69%) - Primary reactive ingredient

White rosin (29.3%) - Reduces brittleness, increases strength

Zinc acetate (0.7%)/ Zn stearate(1%)/ Zinc sulphate/ CaCl2 - Accelerator

MgO - Reacts similar to ZnO

LIQUID

Eugenol (85%) - Primary reactive ingredient

Olive oil (5-15%)or cotton seed oil - To control viscosity, acts as plasticizer, masks the taste of eugenol

Alcohol or acetic acid (1%) -  Accelerate the reaction

MANIPULATION:

1. (POWDER LIQUID SYSTEM)

Armamentarium:

Measuring scoop

Dropper for dispensing liquid 

Parchment paper

Large mixing spatula

Paper towel or 2”/2” gauge piece for clean up.

Powder should be fluffed in the bottle to ensure uniform density of the particles. Measuring scoop is filled with excess powder without packing it. Scoop is leveled to remove excess powder using the flat edge of the spatula. 2 scoops of powder are dispensed onto the centre of the mixing pad and the third scoop at the right corner of the pad for future use. The powder is subdivided into two equal portions. One of the portions is further subdivided into two smaller portions. Liquid container is swirled to ensure uniformity of composition. Dropper is held perpendicular to ensure uniformity of the drop size and 2 drops of liquid are dispensed adjacent to the powder. The mixing should begin by incorporating the largest portion of the powder using folding action into the liquid. This is carried on until all of the powder is wetted by liquid. Then each of the two remaining increments is added to the liquid. The zinc oxide is slowly wetted by eugenol; therefore, the material is stropped by using heavy blade of the spatula to obtain homogeneity and to provide a lower viscosity. More the powder incorporated, stronger the cement. A considerable amount of powder can be incorporated into the liquid by heavy spatulation with a stiff spatula. This should take about a minute. Spatula should then be cleaned and dipped in the powder and the material be rolled into a rope. Small portions are cut from the rope and used.

NOTE: 

Thin mixes having a low powder/liquid ratio, should be avoided since they produce inferior properties- Lower strength and higher solubility.

SETTING REACTION:

Two molecules of eugenol react with zinc oxide to form the salt. The ionic salt bonds are formed between zinc and phenolic oxygen of each molecule of eugenol. Two further co- ordinate bonds are formed by donation of pairs of electrons from the methoxy oxygen to zinc. The two aromatic rings occupy perpendicular planes. The structure can therefore, be visualized as a central zinc atom held by two eugenol ‘claws’. Compounds with type of structure are referred to as ‘chelate’ compounds. (Chelae- pair of pincer-like claws of anterior limbs of crab, lobster or scorpion) The setting reaction is ionic in nature and requires an ionic medium. The ionic medium is increased by the presence of water and certain ionizable salts which act as accelerators. A trace of water is needed to initiate the reaction, but, once started, it is a by product of the setting reaction. Therefore this reaction is called autocatalytic reaction Set mass contains residual zinc oxide particles bonded by matrix of zinc eugenolate and some free eugenol. The reaction is reversible because the zinc eugenolate is easily hydrolyzed by moisture to eugenol and zinc hydroxide.

Setting reaction: 

Involves chelation of two eugenol molecules with one zinc ion to form zinc eugenolate. This reaction proceeds very slowly in absence of moisture. When the mixed material comes in contact with water, setting is often completed within a few seconds.

ZnO + H2O (hydrolysis) → Zn(OH)2

Zn (OH)2+ 2HE → ZnE2 + 2H2O

(Base) (Acid) (Zinc Eugenolate)

Setting time: 

Usually 4-10 min

This material is available as a slow setting (root canal sealants) or a fast setting cement (periodontal dressing). The slow setting cement takes 24 hours to set hard, with fast setting cement taking as little as 5 minutes. Although this depends on the nature of the powder, its particle size and the addition of accelerators such as zinc acetate and acetic acid.

Factors affecting setting time

Setting time is decreased by:

1. Manufacture : 

The powder manufactured by decomposing Zinc hydroxide or Zinc carbonate at 3000C are most active and set faster

2. P/L ratio: 

Increased P/L ratio accelerates the reaction and reduces setting time.

3. Particle size : 

If the particle size of powder is reduced, it reduces the setting time 

4. Accelerators like Zinc acetate, Zinc stearate, Alcohol, glacial acetic acid, and small amounts of water  accelerate the reaction. Addition of accelerators like MgCl2, CaCl2 before mixing can hasten the setting  reaction thus decreasing setting time.

5. Rise in temperature increases the rate of reaction.

Setting time is increased by: 

1. Glycol, glycerin retards the setting reaction.

2. A cool glass lab slows the reaction. 

3. Saliva, blood and borax retards the reaction

 Properties: 

1. Strength: 

Compressive strength is 15MPa. 

-A maximum value of 35MPa is required for cements intended for temporary cementation. 

-A minimum of 35MPa is required for cements intended for permanent cementation and 

-25MPa for filling materials and bases. 

-Lining materials are required to have a minimum compressive strength of 5MPa 

2. Film thickness: 

40 m 

(Note: 

The film thickness should not be more than 25m for cements used for permanent cementation and not  more than 40m for cements used for temporary cementation.) 

3. pH: 

6-8 

4. Adhesion : 

No chemical bond formed with tooth. Only mechanical interlocking 

5. Solubility and disintegration: 

Solubility is high, about 1.5% by weight in distilled water after 24 hours. Eugenol is extracted from the set  cement by the hydrolytic decomposition of the Zinc eugenolate. The cement disintegrates rapidly when  exposed to oral conditions.

6. Biologic effect

a. Eugenol has anodyne as well as obtundent effect on pulp and has a bacteriostatic property. The release of  eugenol by hydrolysis of zinc eugenolate matrix relieves pain in the pulp in deep cavities.

b. These are non acidic in nature (pH 6.8-7) and hence can be easily placed at the base of the cavity near  pulp. Zinc oxide eugenol cement protects the pulp from penetration of acid ions form restorative materials. 

c. The sealing capacity and antibacterial action appear to facilitate pulpal healing. Hence, these materials are  used as pulp capping materials and are used to reduce post operative sensitivity. However, reparative  dentine formation in exposed pulp is variable.d. When exposed directly to oral conditions, the material maintains good sealing but, when in direct contact with connective tissue, the material is an irritant. Eugenol is a potential allergen.

Advantages

1. Obtundent effect on pulp

2. Good sealing ability and resistance to marginal leakage

3. Neutral pH

Disadvantages

1. Low strength and abrasion resistance

2. Solubility and disintegration in oral fluids

3. Eugenol inhibits the polymerization of resins, and sometimes causing discoloration so eugenol containing cements cannot be used in conjunction with resin based restorative materials.

Applications :

1. Zinc oxide /Eugenol cements may be used as linings in deep cavities without causing harm to the pulp. Unconsumed eugenol is able to leach from the set material and although this substance has been shown to be irritant under certain conditions, it appears to have an obtundent effect on the pulp.

2. The main uses of these cements are for lining under amalgam restorations, either used alone or as a sub-lining overlaid with a zinc phosphate material. The materials form an effective thermal barrier under metallic restorations having a value of thermal diffusivity similar to that for dentine.

3. The ease with which eugenol gains egress from the material is responsible for its relatively high solubility. Leached eugenol is replaced by water, leading to hydrolysis of the zinc eugenolate and disintegration of the cement structure. Therefore, they can only be used in temporary applications as temporary luting cements and as temporary restorative materials.