Dental Cements Flashcards
Classification of Dental cements
For luting
For pulp protection
For restoratives
Mechanical retention is obtained by
Combination of microscopically rough surface of tooth and prosthesis surface along with rigid cement that fills the gaps.
Dominating cement for luting
Zinc Phosphate
Factors on which the cement retention relies
Prepared tooth form
Height
No. Of abutments
Accuracy of superstructure fit
Distribution of cement
Occlusion
Dental prostheses that are mechanically retained
.Fixed prosthodontics made of metal, ceramic veneer or ceramic
.Temporary restorations
.pins and posts used for retention of restoration.
. implant superstructures.
What is luting traditionally
To use nonadhesive materials for retaining prostheses
What’s bonding
To use adhesive bonding materials
What happens when device is seated without a luting agent
It creates point contact between tooth and appliances
Requirement of luting a device to toooth
Cement of low viscosity and low contact angle to tooth and the appliance to form a continuous film
Porosity in powder-liquid cement is created how
Because of incorporation of air while mixing powder into liquid.
Highest porosity cement (decreasing order)
Polycarboxylate
Zinc Phosphate
GICs
Key characteristics to choose cement for attaching a fixed prosthesis
Working time..
Setting time
Consistency
Ease of removal of excess material
Adhesion
Flim thickness of luting cement should be
Less than 25um
For endodontic sealers the film thickness should be
50um
ISO flim thickness requirements
Freshly mixed cement placed between two optically flat surface and a 150N vertical load is applied.
7 mins later, thickness of film is measured
How is luting done
Luting cements are designed to dill microscopic gaps between a prosthesis and prepared tooth.
Each surface is microscopically rough, having peaks and valleys.
The areas that are not is contact must be filled by cement to prevent oral fluid flow and bacterial invasion.
Procedure for luting a single crown
1.Cement placement
2. Seating of crown
3. Excess cement removal
Cement placement for luting is done
Cement should coat entire inner surface of crown and extend slightly over the margin to ensure the space between crown and tooth is completely sealed.
Cement should fill about half of the inner cavity volume of crown and must be free of bubbles
How is seating of crown done
Use moderated fingure pressure to displace excess cement.
Which three characteristics make seating of a crown easier
- Lower viscosity cements.
- More taper of the prepared tooth
- Decreased height of prepared tooth
Removal of excess cement according to different cement
ZOE- does not adhere to the surrounding surface so the cement should set off completely before excess cement is removed.
Glass ionomer, polycarboxylate and resin cements adhere chemically and physically to surrounding surfaces and should be removed as soon as the seating is completed to prevent adhesion to the exterior of the prosthesis or surrounding teeth.
Separating medium-petrollium jelly should be applied carefully.
Zinc polycarboxylate should not be removed at rubbery stage as it may pull the cement away from marginal area or remove some cement from within the prostheses
What is film thickness
The thickness of continuous cement after setting under pressure
What is cement thickness
Thickness of cement between the tooth structure and cemented cast crown, inlay, inlay or veneer.
Cement thickness ranges from
25-120um
What the cements used for pulp protection
Cavity varnishes
Cavity liners
Bases
Increasing order of inflammation caused by chemically irritating cements
Polycarboxylate
Zinc Phosphate
GIC
Cavity varnishes
Composed - natural gums like copal, rosin or synthetic resins dissolved in organic solvent (acetone, chloroform, ether).
Varnishes have high solvent content so atleast two thin layers should be applied otherwise small pin holes may occur.
Varnish resumes infiltration of irritating fluids through marginal crevices and lessens pulpal irritationm
Prevents penetration of corrosion.
It’s not indicated in GIC or bonding agents for resin based composites
Cavity Liners
Main ingredient:- calcium hydroxide.
Because it is antimicrobial, has elevated pH and stimulates formation of secondary dentin over injured pulp to protect it over long term.
Does not posses mechanical strength or thermal insulation but can neutralize acids that migrated toward the pulp and in that process induce formation of secondary dentin.
It should not be left on the margin of prepared cavity as it is soluble in water, or else the marginal wall will not be sealed.
Used for direct and indirect pulp capping and as a dressing after vital pulpotomy procedures on primary tooth.
Cement bases
Applied in thick layer (greater than 75um) beneath restorative material to protect the pulp against thermal injury, galvanic shock and chemical irritation.
Zinc Phosphate and ZOE are commonly used
Zinc Phosphate as a base
Is effective for thermal insulation but low pH may acquire cavity liner under the cement to protect the pulp
GIC as a base
Used as a base in very deep cavities a calcium hydroxide liner should be applied first to protect the deep areas where pulp exposure is more likely to occur
Considerations for selection of bases
The design of cavity
Direct restorative material
Proximity of the pulp chamber relative to the cavity floor or wall
ZOE and non Eugenol cements
Powder-liquid and two paste systems.
Setting reaction starts with water present in tooth or Eugenol solution hydrolysing zinc oxide to form zinc hydroxide.
Zinc hydroxide and Eugenol chelates and solidify.
Reaction proceeds rapidly in a warm, humid environment.
Zinc acetate dihydrate is included in powder and reaction is faster.
Acetic acid also accelerates dissolution of zinc oxide.
Compressive strength for ZOE cements
3-55 MPA
Four clinical types of ZOE
1.Temporary cementation
2.Long-term cementation of fixed prosthesis
3.Temporary filling and thermal insulating bases
4.Intermediate restorations.
Disadvantage and advantages of ZOE
Can cause pulpal necrosis, therefore should not be used directly on pulp.
Indirect used if ZOE has a sedative effect by sealing the dentinal tubules against ingress of oral fluids until more permanent restoration can be fabricated and cemented.
To improve strength of ZOE cement what is replaced
Part of Eugenol liquid has been replaced by orthoethoxybenzoic acid and alumina.
Polymer-reinforced ZOE contains 20-40wt% fine polymer particles and zinc oxide particles treated with carboxylic acid.
Zinc Phosphate Cement’s chemistry and setting
Oldest luting cement.
Powder- more than 75% or zinc oxide and 13% magnesium oxide, sintered above 1000°C , then grounded into fine powder and blended with radio opaque powders.
Liquid- phosphoric acid(38-59%) aluminium phosphate (2-3%).
When mixed phosphoric acid dissolves zinc oxide which reacts with aluminium phosphate and forms zinc alumino-phosphate gel on remaining undissolved zinc oxide particles.
Loss of water from liquid lengthens the reaction, adding of water accelerates the reaction.
Clinical manipulation of zinc Phosphate cement
Exothermic reaction between zinc oxide and phosphoric acid.
Mixing- each increment should take about 15-20s before adding another increment.
All mixing should be completed within 1.5-2 minutes.
If a string of cement can be lifted about 12-19 mm before separating from the spatula it is considered fluid enough for cementing a prosthesis.
Removal of excess zinc Phosphate
With a knotted dental floss after completing seating and remaining excess cement should be allowed to set before removal .
Control of working time of zinc Phosphate
- P/L ratio can be reduced to produce a thinner mixture. But this will result in lower initial pH of cement affecting the mechanical properties of cement .
- Smaller portions of powder should be mixed for first few increments. It reduces acidity of liquid and retards the reaction rate with subsequent increments.
- Prolonging the Spatulation of last increment of powder.
Spatulation destroys the matrix as it forms which means extra time is needed to rebuild the bulk of matrix. - Colder mixing temperature retards the chemical reaction. But the temperature of the slab should be above the dew point.
This procedure yields lower cement viscosity at completion of mixing as indicated by a much lower height of cement at separation when lifted by spatula.
Retention of zinc Phosphate
Zinc Phosphate does not chemically adhere to teeth or prostheses, it’s bond is completely mechanical.
Mechanical and biological properties of zinc Phosphate
Compressive strength - 104 MPA
Tensile strength -5.5 MPA
Elastic modulus -13 GPA
Low solubility in water.
Phosphoric acid in liquid makes the mixture acidic therefore cytotoxic when prostheses is luted with this cement.
Cement remains acidic for 24 hours with pH rising from 3-6.
Zinc polycarboxylate cement
First dental Cement to exhibit chemical bonding to teeth.
It’s not used for restorative purpose because the cement is opaque.
Chemistry and setting of zinc polycarboxylate cement
Powder-liquid system that is set by acid-base reaction.
Composition of Zinc polycarboxylate
Liquid:- water solution of polyacrylic acid or a copolymer of acrylic acid wit. Other carboxylic acid such as itaconic acid.
Powder:- zinc oxide with some magnesia , tin oxide, bismuth oxide and/or alumina.
Stannous fluoride - increases strength, enhances the manipulative properties.
Setting reaction of zinc polycarboxylate
Setting begans by dissolution of powder by acid releases zinc, magnesium and tin, these bind and cross link with the carboxyl groups.
The result is cross-linked polycarboxylate matrix phase encapsulating th unreacted portion of the particles
Mechanism of adhesion of zinc polycarboxylate cement
The polyacrylic acid bonds to the calcium ions on the surface of enamel or dentin.
Clinical manipulation of zinc polycarboxylate
The inner surface of the crown must be cleaned to improve wettability and mechanical bond at the cement-metal interface.
The surface can be abraded with a stone or sand blasted with alumina abrasive.
Crowns should be thoroughly rinsed to remove debris and dried.
Outer surface of prosthesis should be coated with separating medium.
Mixing of zinc polycarboxylate
A long mixing time can yield cement that is too viscous for its application.
The cement must be used before it loses its glossy appearance because the glossy surface indicates free carboxylic acid groups are still present for good bonding to the tooth.
A dull looking mixture means insufficient carboxyl groups are present for bonding to calcium in the tooth.
Removal of excess zinc polycarboxylate cement
During setting, the cement passes through a rubbery stage. Excess cement should not be removed in this stage.
It can be removed as soon as the cementing process is completed or when the cement becomes hard, provided that the outer surface of prosthesis is coated with separating medium.
Control of working time of zinc polycarboxylate
Working time- 2.5-5 min.
Refrigeration of the powder is useful because it retards the reaction without raising the viscosity of the liquid.
Cooling the glass slab faus s th polyacrylic acid to thicken which hinders the mixing.
Mechanical and biological properties of zinc polycarboxylate
Compressive strength - 55MPa
Tensile strength - 6.2
Elastic modulus - 5.1.
Polycarboxylate cements produce minimal irritation to the pulp.
Larger size of polyacrylic acid compared to phosphoric acid limits acid penetration into the dentinal tubules contributing to excellent biocompatibility and lack of Postoperative sensitivity of polycarboxylate cement.
