The majority of impressions taken by orthodontists are used for pre- and post- records of dental arches.
“Restorations can be as true as your impressions”.
The underlying principle emphasizes the need for complete and accurate impression and highlights the importance of clinical impression procedures.
Purpose of impression is the formation of ‘positive’ model of the proper physical dimension, shapes and spatial relationship of these structures.
DEFINITIONS
Impression – An imprint or negative likeness of the teeth and/or edentulous areas where the teeth have been removed made in a plastic material which becomes relatively hard or set while in contact with these tissues.
-Boucher
Impression material – Any substance or Combination of substances used for making impression or negative reproduction.
-GPT 8
HISTORY
As early as the 17th century G. PURMAN of Breslau was said to have used wax for taking dental impressions.
During the late 1790s plaster was used to take impression for the famous dentures of Sir George Washington. (Dr. John Greenwood—a New York dentist)
In 1844-Westcott Dwinelle and Dunning started the use of PLASTER OF PARIS for taking impression for denture fabrication.
- Elastic Impression first introduced to the dental profession in 1925 in the form of AGAR HYDROCOLLOID called ‘Reversible Gel’. Since temperature changes its physical state.
- ALGINATE HYDROCLLOID WERE DEVELOPED JUST BEFORE WORLD WAR 2(1930).
- Since then, search for the ideal impression material has continued.
Historically, impression making was accomplished with inelastic materials for both soft and hard tissues.
Hydrocolloids ware initially introduced to make impression of hard tissues in place of inelastic materials.
After World War II, advances in polymer technology brought to the dental profession a group of synthetic rubbery materials called ‘ELASTOMERS’
They are capable of making impression of both hard and soft tissue.
In 1950s the rubber base material first in the form of Mercapton Polysulfide and later the Silicon began to be used as dental impression materials.
In 1994, Mitchell described a technique of occlusal registration for functional appliances using elastomeric bite registration materials.
The colloidal material used for making impression are either —-Agar or Algi dissolved in water –Hence the term Hydrocolloid impression materials.
Hydrocolloid impression materials can be classified as:-
- REVERSIBLE eg. Agar
- IRREVERSIBLE eg. Alginate
AGAR – REVERSIBLE HYDROCOLLOID
In 1925, Alphous Poller of Vienna was granted a British patent for a totally different type of impression material.
It was later described by Skinner as colloidal sols of emulsified type.
Sears introduced reversible hydrocolloids in 1937
MODE OF SUPPLY:
Supplied as gel in collapsible tube or as Cylinders in a jar.
COMPOSITION
INGREDIENT |
WEIGHT(%) |
FUNCTION |
Agar |
13-17 |
To provide dispersed phase |
Borate |
0.2-0.5 |
Provides strength |
Sulfate |
1-2 |
Gypsum hardener |
Wax |
0.5-1 |
Filler |
Thixotropic materials |
0.3-0.5 |
Thickener |
Water |
Balance |
Reaction medium |
The ADA specification No 11 sets standard properties required by agar hydro colloidal material.
Flow:
The material should be able to record the fine details
Gelatin temperature:-
After boiling for 8 min, the material should be fluid enough to be extruded from container. After tempering the solution should be homogenous and should set between 37 and 45 when cooled.
The ADA specification requirement for flexibility allows a range of 4% to 15% at a stress of 14.2 psi.
Elasticity and elastic recovery:
Agar hydrocolloids are highly elastic in nature and elastic recovery occurs to the extent of 98%
Accuracy and dimensional stability
Agar impressions are highly accurate at the time of removal from mouth but shrink when stored in air and expand when stored in water. The least dimensional change occurs when stored in 100% humidity but immediate pouring of cast is recommended.
Working and setting time:-
Working time ranges between 7-15 minutes and setting time is about 5 minutes. They can be controlled by regulating flow of the water through cooling tubes.
Tear and Compressive strength
Tear strength is 4 psi (psi = pound per square inch)
Compressive strength is 800 gm/cm2.
MANIPULATION:
Conditioning unit is required for manipulation of agar. The Conditioner consists of
- Boiling section
- Storage section
- Tempering section.
In boiling section, the material is kept for 10 min for liquefaction.
Then material is stored in storage section at 65deg.cel. in sol form until it is required.
Material is tempered at 45deg.cel. For 3 to 10 min so as to be tolerated by the patient.
Just before the tempering process for the tray material is completed, the syringe material is directly taken from the storage compartment and applied to teeth. The water soaked on the layer of the tray hydrocolloid is removed from the container and gauge is removed.
The tray is immediately brought into position and seated with tight pressure and held with a very light force.
Gelatin is accelerated by circulating cool water (18-21deg.cel.) through the tray for 3-5 min. After the gelatin is over, the tray is removed suddenly with a snap.
Advantages:
- Accurate dies can be prepared if handled properly.
- Good elastic property and reproduces undercuts properly
- Well tolerated by patient
- Used for duplicating models.
Disadvantages:
1) Flow is less Compared to recent elastic impression materials
2) Gelatin may be painful to the patient.
3) Tears relatively easily.
4) Extensive equipment is required
Colloids exist as sol or gel.
A sol has the appearance and many characteristics of a viscous liquid.
A gel is a jelly-like elastic semisolid and is produced from a sol by a process called gelation.
It occurs in two ways
1) By temperature changes
2) By chemical means
TEMPERATURE CHANGES
Gelation in this case is a reversible process
Eg. Agar
The fibrils are held together by secondary molecular forces— they break at slightly elevated temperature and become re-established as the hydrocolloids cool to room temperature.
The temperature at which these changes occur is GELATION TEMPERATURE.
Gelation occurs at approx 37 deg. C temperature whereas liquifaction occurs at a higher temperature, i.e. 60-70 deg C higher than the gelation temperature.
This temp lag between liqueifaction and gelation is known as hysteresis.
CHEMICAL CHANGES
- Conversion of sol into gel
- The fibrils formed are held together by primary bonds And is unaffected by temperatures.
Hence, they are called IRREVERSIBLE HYDROCOLLOIDS
eg. ALGINATE
DIMENSIONAL EFFECTS
A majority of gel volume in hydrocolloid is made of water.
Gel may lose water by:-
a)Evaporation- from the surface
b)Syneresis- Exudation of fluid on the surface.
The exudate is not pure water, may be alkaline or acid depending on the composition of gel, both these process leads to SHRINKAGE.
Gel may absorb water if placed in water by a process called IMBIBITION causing gel to swell.
Both SYNERESIS and IMBIBITION should be avoided
As former can cause SHRINKAGE and later EXPANSION.
The dimensional changes are important in dentistry as any changes in dimension of impression after it has been removed from the mouth is a source of error.
IRREVERSIBLE HYDROCOLLOID
ALGINATE (IRREVERSIBLE HYDROCOLLOIDS)
It is extracted from certain brown seaweed. The substance is called anhydro-beta-d mannuronic acid or alginic acid.
TYPES:-
1) Type I – Fast setting (1-2 min)
2) Type II- Normal setting ( 2-4.5 min)
COMPOSITION
INGREDIENT |
WEIGHT(%) |
FUNCTION |
Potassium alginate |
15% |
Dissolves in water and reacts with calcium ions |
Zinc Oxide |
4% |
Acts as filler |
Potassium titanium fluoride |
3% |
Accelerator |
Diatomaceous earth |
60% |
Filler |
Sodium phosphate |
2% |
Retarder |
Coloring and Flowering agents |
TRACES |
|
CHEMISTRY AND SETTING REACTION
The chief ingredient of irreversible hydrocolloid is one of the soluble alginates. Reaction occurs by a chemical reaction. Soluble alginates reacts with Calcium sulfate to produce insoluble calcium alginate as a gel.
The production of calcium alginate is delayed by the addition of a third soluble salt to the solution with which calcium sulfate will react in preference to the soluble alginate to form an insoluble calcium salt.
Thus, the reaction between the calcium sulfate and the soluble alginate is prevented as long as any of the added salt is left. The added salt is called retarder.
The reactions that occur are
2Na3 PO4 + 3 CaS04 à Ca3 (Po4)2 + 3 Na2 So4 When the supply of retarder ( trisodium phosphate) is exhausted, Calcium ions begins to react with potassium alginate to produce calcium alginate.
Sod. Alg + n CaSo4 + H2Oà nNa2So4 + Cal Alg
GEL STRUCTURE
The gel structure consists of a brush heap of calcium alginate fibril network enclosing unreacted sodium alginate sol, excess water, filler particles and reaction by products.
It is a cross linked structure where calcium is responsible for cross linking.
CONTROL OF SETTING TIME:
Setting time can be altered by altering the amount of water.
As the temperature increases, the setting time decreases.
For 10 rise of temperature 1 min reduction in setting time occurs.
Bowl and spatula also can be cooled to increase setting time.
MANIPULATION:
Gypsum can cause acceleration of setting of alginate
Alginate contamination can decrease strength of casts
For manipulations a clean bowl and metal spatula.
The First step in the manipulation is to prepare a proper mixture of water and powder. Water is poured in to bowl and specified weight of powder is added and powder is carefully incorporated into water with spatula.
A vigorous figure 8 motion is best with the mix being stopped against the sides of bowl with intermittent rotations of the spatula to press out air bubbles. The final mix should be smooth creamy mixture that does not drip off the spatula when it is raised. Mechanical devices are also available for mixing. Their advantages are speed convenience and eliminating human errors.
REMOVAL OF THE IMPRESSION
Alginate impression develops effective seal which can be freed by running the finger around periphery. Impression should be removed suddenly with a jerk. Torquing or twisting should be avoided. After removal impression should be washed with water to remove saliva, covered with wet gauge to prevent synerisis and cast should be poured as soon as possible.
PROPERTIES
ADA specification No. 18 for alginate sets following standards
Mixing time:
Fast setting– 45 Sec
Normal setting – 60 Se
Working time:
Fast setting – 1.2 to 2minutes
Normal setting – 2-4.5minutes
Setting time:
Fast setting – 1.2 minutes
Normal setting – 2-4.5minutes
Over mixing Causes:
Reduction in final strength as gel fibrils are destroyed
Reduction in working time.
Under mixing Causes:
Lack of homogeneity and reduced strength
STRENGTH
The compressive strength of alginate ranges from 5000-8000gm/cm2.
The tear strength ranges between 350-700gm/cm2.
Both these properties are time dependent. If removal is delayed, strength increases.
The final strength depends on
W/P ratio: Too much or too little water reduces strength
Mixing time: Over and under mixing reduces strength
DIMENSIONAL STABILITY
Alginate impression lose water by evaporation and shrink in air. If placed in water it absorbs and swells. So cast should be poured immediately
Advantages
Inexpensive
Hydrophilic
Contact Angle 37 degrees (which means they will displace blood and saliva & pour well with gypsum stones)
Easy manipulation
Minimum equipment
Low cost
Accurate if properly handled
Disadvantages
Low tear strength
Poorer reproduction of surface details
Not Dimensionally stable on storage due to syneresis
RECENT ADVANCEMENTS
Zhermack advertised that the alginate impression can be kept up to 48 hours prior to pouring in their long life Hermetically sealed bags
The Vanilla flavoured Orthoprint is Anti Nausea.
Vival NF: Dimensionally stable for up to 100 hours.
High tear strength alginates: Silicone reinforced two paste system one with alginate, other with calcium reactor. It has higher tear strength
Self disinfecting alginates containing iodophor.
Dust free alginates: Organic glycol is used to agglomerate the dust particles
LAMINATE TECHNIQUE/ALGINATE AGAR METHOD
In this technique agar in the syringe is injected on to the area to be recorded, an impression tray containing chilled alginate is placed over it.
The alginate sets by chemical reaction, where as agar gels by means of contact with cool alginate. Since agar is in contact with teeth maximum detail can be obtained.
MODIFIED ALGINATES:
Traditionally alginate is supplied in two component system, powder and water. But other forms are also available.
In the form of a solution containing water and a reactor of plaster of Paris
As a two paste system one containing solution and other calcium reactor. These materials contain silicone and are supplied in tray viscosity and syringe viscosity.
DISINFECTION
Rinse the impression
Place in zip lock bag, pour in disinfectant to cover all surfaces
Seal for 10 minutes
Remove and rinse prior to pouring stone
Chlorhexidine has also been incorporated to limit cross infection (Hydrogum + CHX- Zhermack)
ELASTOMERS
Polysulfide
Condensation polymerizing silicone
Addition polymerizing silicone
Polyether
Each type is further divided into four viscosity classes
Light body
Medium / regular body
Heavy body
Putty
GENERAL PROPERTIES
Excellent reproduction of surface details. The low viscosity is able to record fine details
They are hydrophobic. So oral tissues in the area of impression should be dry.
Co-efficient of thermal expansion of elastomers is high. So thermal contraction of set material occurs when transferred to room temperature
Tear strength is excellent. So impressions can be removed from thin areas.
The elastomeric materials can be copper/silver plated
Shelf life of elastomers is good
Dimensional stability
For visco-elastic material slow elastic recovery may control for sometime after withdrawal of the impression, producing dimensional change. In this case the dimensional change results in more accurate impression.
But many impression materials contain volatile substances either as primary components or as by products of setting reaction. Loss of such volatile materials results in shrinkage of impression material resulting in decrease in accuracy. So for majority of materials, accuracy can be maintained by pouring the cast soon after the removal of impression.
Curing shrinkage
Loss of byproducts
Polyether being hydrophilic absorbs water and loses soluble plasticizers causing change in dimension
Thermal contraction when transferred from mouth to room temperature
Incomplete elastic recovery after deformation during removal
Amount of filler
Uniform thickness of material gives more accurate impression
Time of pouring of impression. Impression should be powered after elastic recovery and before dimensional change
POLYSULFIDES
Also called as Mercaptan or Thiokol
MODE OF SUPPLY
It is supplied in tubes as base and accelerate in paste form
Available in three viscosities
Light bodied
Medium bodied
Heavy bodied
COMPOSITION
BASE PASTE |
FUNCTION |
Polysulfide polymer – 80-85% |
This is further polymerized and cross linked to form rubber |
Filler (Titanium dioxide zinc sulfate) – 16-18% |
To provide required strength |
Plasticizer (Dibutyl Phthalate) |
To provide appropriate viscosity |
Small quantity of sulphur – 0.5% |
To accelerate the reaction |
Accelerator paste
Lead dioxide – 60-68% |
To react with thiol group causing setting and give brown colour |
Filler and plasticizer |
To provide strength and to give adequate viscosity |
Oleic / stearic acid |
Act as retarder to control rate of setting reaction |
CHEMISTRY AND SETTING REACTION
The main component of polysulfide is a multifunctional mercaptan (-SH) or polysulfide polymer. These linear polymer contains pendent –SH groups
Reaction yields water as by product
PROPERTIES
Unpleasant odour and colour
It has a long Setting time of 8 to 12minutes
Mixing time is 45 seconds
Excellent reproduction of surface detail
Dimensional stability:
The curing shrinkage is high (0.45%)
Has the highest permanent deformation (3.5%)
This improves with time. So pouring of model should be delayed by half an hour
Tear strength is 4000 gms/cm2
It has good flexibility. A 2mm spacing in the tray is sufficient to make the impression.
It is hydrophobic. So impression area should be dry
Shelf life is good i.e. 2 years
SILICONE RUBBER IMPRESSION MATERIALS
Types:
Two types of silicone impression materials are available based on the type of polymerization reaction occurring during setting
Condensation silicone
Addition silicone
MODE OF SUPPLY
Available as base and catalyst in paste form. The condensation silicone catalyst can also be supplied as a liquid. The putty is supplied in jars.
Condensation silicone is available in three viscosities.
Light bodied
Medium bodied
Putty
CONDENSATION SILICONES
BASE PASTE
Hydroxyl terminated polydimethyl siloxane |
Undergoes cross linking to form rubber |
Colloidal silica |
Filler |
REACTOR PASTE
Tetraethyl orthosilicate |
Cross linking agent |
Stannous octate |
Catalyst |
CHEMISTRY AND SETTING REACTION
It is a condensation reaction
Polymerization occurs as a result of cross linkage between orthoethyl silicate and terminal hydroxyl group of dimethyl siloxane to form a three dimensional network
Stannous octate acts as a catalyst
Ethyl alcohol is by product. Its evaporation probably accounts for much of the condensation taking place in set silicone impression
PROPERTIES
Pleasant colour and odour
Setting time is 6-8 minutes, mixing time is 45 seconds
Excellent reproduction of surface details
Dimensional stability is low due to high curing shrinkage (0.4-0.6%) and shrinkage due to evaporation of ethyl alcohol. To avoid this cast should be poured immediately.
Tear strength is 3000gm/cm2
It is hydrophobic. So impression field should be dry
Can be electroplated with silver or copper
Evaporation of alcohol can lead to dimensional instability
Production of hydrogen can lead to pitting of dental stone surfaces.
ADDITION SILICONES
Superior to condensation silicones
Also called polyvinyl silioxane impression materials.
COMPOSITION
Base:
Polymethyl hydrogen siloxane
Other siloxane prepolymers
Fillers
Accelerator
Divinylpolydimethyl siloxane
Other siloxane prepolymers
Fillers
Platinum salt-catalyst
CHEMISTRY AND SETTING REACTION
It is an addition reaction
In this case polymer is terminated with vinyl groups and it is cross linked with silane (hydride group). The reaction is activated by platinum salt
No reaction byproduct develops as long as correct proportions of vinyl siloxane and silane siloxane are maintained
-Sulfur compounds retard the setting of addition silicone.
PROPERTIES
Pleasant colour and odour
Good reproduction of surface details
Setting time is 4-7 minutes and mixing time is 45 seconds
It has the best dimensional stability among all elastomers. It has low curing shrinkage (0.17%) and low permanent deformation (0.05-0.3%)
Good tear strength (3000gm/cm2)
It is hydrophobic with contact angle 80deg. – 100deg. so impression field should be dry
Can be electroplated with copper/silver
Shelf life ranges from 1-2 years
POLYETHER
Mode of Supply:
Available as base and accelerate paste in tubes
Available in three viscosities
Light bodied
Medium bodied
Heavy bodied
COMPOSITION
Base:
Polyether polymer Cross linked to form rubber
Colloidal silica Acts as filler
Glycol ether (or) phthalate Plasticizer
Accelerator Paste:
Alkyl aromatic sulfonate Initiates cross linking
Colloidal silica Filler
Phthalate Plasticizer
PROPERTIES
Pleasant odour and taste
Setting time is short 3.5minutes, mixing time is 30 seconds
Good dimensional stability
Curing shrinkage is 0.24%
Polyether absorbs water and can change in dimension. So should not be stored in contact with water or in humid climates
It is extremely stiff
Tear strength is 3000gm/cm2
Hydrophilic, has good compatibility with stone
Can be electroplated
Good shelf life i.e., more than 2 years.
IMPRESSION MAKING WITH ELASTOMERS
It involves five steps:::
- Preparing a tray
- Preparing the material
- Making impression
- Removing the impression
- Preparing stone casts and dies
PREPARING A TRAY:
Impressions are usually made with custom trays. Perforated stock trays are used for making impression in putty.
To prepare a custom tray, an alginate impression is made and stone cast is constructed
Teeth are covered with modeling wax (1 or 2 sheets approximately 3-4mm) to act as spacer and stops are provided
Custom tray is prepared with tray material
Slightly roughened surface of tray inside improves retention.
PREPARING THE MATERIAL:
Materials are provided in two paste tubes
Same length of materials should be dispensed on to mixing pad
Catalyst paste is first collected on to spatula and then distributed over the base
Mixing should be thorough, uniform in colour with no streaks of base of catalyst
If one of the components is in liquid form such as catalyst for condensation silicone, a length of base is dispensed from tube onto the mixing pad and a drop of catalyst is added for each unit length of base.
Putty consistence are dispensed by volume using equal number of scoops and kneaded.
Automatic mixing and dispensing devices are available used for light, medium and heavy body consistencies.
It consists of a double barrel gun with mixing tip. The tip consists of spiral inside. The mixing tips vary in diameter length and size of the tip opening for specific consistency.
POLYETHER IMPRESSION MATERIALS
It has the advantages over hand mixing
Less air bubbles
Reduced working time
More uniform mix
A recent development in the automatic mixing and dispensing device is a dynamic mechanical mixer. Materials are supplied in plastic bags housed in a cartridge. Device uses a motor to drive parallel plunger that force the materials into a mixing tip and the spiral inside the mixing tip rotates as the materials are extruded through the tip. With this uniform mix of even higher viscosity material can be achieved
Making of impression
Initially tray is coated with adhesive that forms bond between tray and impression material. Slightly roughened surface of tray increases adhesion.
METHODS OF MAKING IMPRESSION
Three techniques
Single mix technique
Multiple mix technique
Reline technique
Single Mix technique:
Viscosity used is regular body
Method:
The paste is mixed and part of it is loaded into tray and part into syringe. The syringe material is then injected on to the prepared area. Then tray is seated over it.
Multiple mix technique
Viscosity used is heavy body and light body
Method
The two viscosities are mixed simultaneously on separate pads. Heavy body is loaded onto the tray and light body in to syringe. The syringe material is injected into area of preparation. Tray with heavy body is seated over it.
Reline technique:
Viscosity used is putty and light body
Two approaches are present
Two stage procedure
Single stage procedure
For two stage procedure, thick putty material is placed in stock tray and a preliminary impression is made. This forms an intra oral custom tray. Space for light body or was is provided whether by cutting away some of the putty or by using thin polyethelene sheet as a spacer between putty and prepared teeth.
A mix of thin wash material is placed into putty and putty with combination tray is seated finally to make impression.
For single stage procedure, material is syringed into place and then unset putty is seated over light body but the disadvantage is that putty may displace light body.
Removal of the impression:
The material is checked for set by probing with the blunt instrument. Then impression is dislodged from the mouth with a steady pressure
APPLICATIONS:
- Impressions for fixed partial dentures
- Impressions for removal partial dentures
- Impressions for complete dentures
- polyether is used for Border moulding
- To make impressions for implant procedure
- In maxillofacial prosthesis
- To make impression in orthodontic cases for study model
REVIEW OF LITERATURE
# Andree, Alfred and Christoph evaluated the dimensional accuracy of monophase elastic impression making with addition silicone and polyether and found no significant change.
int J Prosthodont . 2002;15:168–174
# Steven O Hondrum assessed the shelf life of different elastomers like polyether, polysulfide, additive silicone and found all the materials were fairly efficacious. But addition silicone showed better value during 72 month period and polysulfide with a little change.
Journal of prosthodontics 2001 Jan;85(1):73-81.
# The influence of mixing methods and disinfectant on the physical properties of alginate impression materials.
The European Journal of Orthodontics Advance Access published May 10,2012
# Ohn Chai, Yutaka investigated modulus of elasticity, strain and tear energy of VPS, polysulfide and polyether and concluded that high strain tolerance of PVS allows their removal without distortion from undercuts. The high tear energy of polysulfides indicates their superior resistance to tear in thin sections
Volume 15, Number 2, 2002 183 The International Journal of Prosthodontics.
# Alvin G. Wee evaluated the accuracy of solid implant casts fabricated from different impression materials (Polyether, AS, CS, polysulfide) and concluded that polyther (medium) was recommended for direct implant impression
# Shirley, John H Park and Daneil E. Tira compared the accuracy of one step putty wash with two step putty wash of addition silicone impression. They found different was not significant.
J Prostho Dent 2010;103:228-239;
# Kern and Reinhold tested the influence of disinfectants (MD/520 and impresept) on the accuracy of reversible hydrocolloid and found accuracy change was not significant.
j.of prostodontic 1993 Nov;70(5):449-56
# Willium Heisler and Anthony evaluated for dimensional accuracy and bond strength of irreversible hydrocolloid with reversible hydrocolloid system and found that accuracy was suitable for clinical use and bond strength was similar to tear strength of reversible hydrocolloid.
J Prosthet Dent. DOI:10.1016/0022-3913(92)90369-L
# L.W. Carlyle evaluated compatibility of irreversible hydrocolloid with three dental stones (Die keen, Quick stone, Hemihydrate). Results showed that Die keen was most compatible.
J Prosthet Dent. 1983 Mar;49(3):434-7.
# Paul, Gardener and Steven conducted a study to evaluate the effect of storage time in a 100% humid environment on the accuracy of gypsum casts poured from reversible hydrocolloid impression and concluded that reversible hydrocolloid can be stored in 100% humidity for 60 minutes before pouring.
(J Prosthet Dent 2001;86:244-50.)
REFERENCES
- Anusavice. Dental Material, 11th edition
- Robert G. Craig. Restorative Dental Material
- Smith. The clinical handling of Dental material
- International Journal of Prosthodontics, 2002; 15: 168-174.
- Journal Prosthet Dent, 2000; 83: 161-165
- Journal Prosthet Dent, 2001; 85: 73-81.
- International Journal of Prosthodontics, 1998; 11: 219-23
- Journal Prosthet Dent, 2000; 83: 323-31.
- Journal Prosthet Dent, 2003; 90: 354-64.
- International Journal of Prosthodontics, 1992; 5: 55-8.
- Journal Prosthet Dent, 1990; 63: 12-15
- J Prosthet Dent, 1993; 70: 449-53
- BJO mayed.1998VOL25 NO.2