Non aqueous impression materials

(elastomers)

Polysulfide Rubber

• First elastomeric impression material ,it is still used in many countries because it is inexpensive and has plenty of working

time.

• Supplied as : • Base paste (white)

• Accelerator (brown/grey)

• Available in 3 Viscosities:

Light bodied, Medium bodied, Heavy bodied

• PROPERTIES :

1. Unpleasant order and colour

2. Material is extremely viscous and sticky and exhibits pseudoplasticity.

3. Long setting time of 12.5min

4. High tear strength

5. Good flexibility

6. Excellent reproduction of surface detail.

7. Hydrophobic- tissue should be thoroughly dried before making the impression.

• Clinical considerations when used

1. Used with custom trays .

2. Allow 2mm thickness.

3. Use tray adhesive .

4. Setting time 10 min.

5. Have very good tear resistance.

Indications:

1. RPD impression

2. Crown and bridge impression

3.Secondary impression for edentulous ridge

Advantages:

1. Long working time.

2. Proven accuracy.

3. High tear resistance.

4. Inexpensive to use .

5. Less hydrophobic.

6. Longer shelf life.

Disadvantages:

1. Must be poured with dental stone immediately.

2. Potential for significant distortion.

3. Odor is offensive.

4. Messy & stains clothes.

5. Second pour is less accurate.

S i l i c o n R u b b e r I m p r e s s i o n M a t e r i a l

Types :Condensation Silicones

Addition Silicones

Condensation silicones (C-silicone)/ Conventional silicones

It was the first type of silicone impression material.

Available in: three viscosities namely light, medium bodied and heavy.

Properties:

1) Setting time – 8-9 min.

2) Mixing time – 45 sec.

3) Excellent detail reproduction.

4) Dimensional stability – high curing shrinkage.

5) Permanent deformation ( 1-3 % )

6) Tear strength – 3000gm/cm .

7) Staffer & harder than polysulfide material.

8) Hydrophobic .

Advantages:

1. Adequate working & setting timer.

2. Pleasant odor & no staining.

3. Adequate tear strength.

4. Better elastic properties on removal.

5. Less distortion on removal.

Disadvantages:

1. Adequate accuracy if poured immediately.

2. Poor dimensional stability.

3. Potential for significant distortion.

4. Slightly more expensive.

When should be used?

• Silicone, being a more elastic material, allows for more uses for dentists than alginate.

• With regard to impressions, for example, silicone is used in cases of root canals, soft structures and bone tissues, in

addition to implants and functional impressions of full dentures.

• Other uses for silicones include: reproductions of fractured teeth, through impression taking procedures, it also serves as

a recording material for occlusal relationships due to its precision and dimensional stability.

• Finally, it plays an important role in the total impressions of jaws, with or without teeth, or partial impressions for some

parts of the arches in preparations for inscriptions, crowns and bridges.

ADITION SILICONES/ (poly vinyl siloxane)PVS

Introduced after condensation silicones .

Have better properties than condensation silicones.

Properties:

1. Setting time: - 5.9 min. Mixing time - 45 sec .

2. Excellent surface detail reproduction.

3. Highest dimensional stability amongst elastomers .

4. Lowest curing shrinkage – 0.17 % .

5. Good tear strength – 3000gm/cm.

6. Extremely hydrophobic.

7. Low flexibility.

8. Electroplated with copper & silver.

Advantages:

1. Short setting time

2. Adequate tear strength.

3. Extremely high accuracy.

4. Minimal distortion on removal.

5. Dimensionally stable even after 1 week.

6. If hydrophilic, good compatibility with gypsum

Disadvantages:

1.Hydrogen gas may cause dimensional change.

Hydrophobic & hence requires a very dry field.

Expensive.

Polyether Impression Materials

Good mechanical and compressive strength comparing other elastomers.

Available in three viscosities. These materials are available in low, medium, and high viscosities,and can be used as a

single-phase material or with a syringe-and-tray technique.

The most popular method of dispensing this material is via a motorized mixing unit.

Consist of base paste and catalyst.

Properties:

1. Setting time: 8.3 min. Mixing time – 30 sec.

2. Dimensional stability is good.

3. Decreased flexibility – 3%.

4. Hydrophilic .

5. Activator paste can produce allergy if handled frequently.

6. Tear strength – 3000gm/cm

Uses:

used to record impressions for crowns, bridges, inlays, onlays, partial and complete dentures

Advantages :

1. Short working & setting time.

2. Proven accuracy .

3. Adequate tear strength .

4. Hydrophilic.

5. Long shelf life.

6. Less distortion on removal.

7. Good dimensional stability.

Disadvantages:

1.Stiffness requires blocking of undercuts

2. Slightly more expensive

3. Multiple casts can’t be poured due to stiffness of the material.

Mixing systems for Elastomer

Static mixing system

Available for all 4 types

Auto mixing system

Silicone (both types)

Poly ether

Dynamic mechanical mixing system

Poly ether and additional silicone

DIGITAL IMPRESSIONS :

The Mid-twentieth century saw a rapid movement in digital technology sweeping across different industries

worldwide from the military to aviation and ultimately to the health care field. Most of modern dentistry is already

immersed in digital dentistry, and it is likely that most practitioners use some aspect of digital dentistry in their

dental offices

Advantages of Digital impression technique:

1. Real-time visualization and evaluation .

2. Easy to correct, manipulate, or recapture images .

3. Archival digitally, therefore no need to store physical casts

4. No wastage of impression material and therefore environmentally friendly

5. Economical, considering no use of impression trays, adhesives, or gypsum

6. Do not need to disinfect before sending information to the laboratory

7. No damage or wear and tear of the stone casts .

8. Communication with the laboratory via the Internet

9. Self-assessment for tooth preparations

10.Increased patient satisfaction

11.Some systems have color scanning, shade selection, and still photograph image-taking capabilities

Disadvantages:

1. Initial cost of equipment and software maintenance fees.

2. Difficult to capture occlusion information for complex prosthodontics treatments.

3. Cannot capture sub gingival margins if obscured with blood, saliva, or tissue .

4. Unable to accurately capture images of difficult cases with edentulous arches.

5. Scanning patterns need to be followed as per manufacturer’s recommendations

Method of dental impression

There are several methods for classifying the impression and each method depend on

certain criteria.

i. The most practical and easiest one depends on the amount of pressure exerted during

impression taking,wich can be classified into three categories

1. non-pressure technique(mucostatic) .

2. Selective pressure techniques

3. pressure techniques

1. Non-pressure Technique (Mucostatic):

Attempts made to record the tissue at rest. It require minimal pressure be applied to the oral tissue during

the seating of the impression tray and set of the impression material. it eliminate all distortion of the oral

tissue and thus create a denture base that models the unloaded tissue.

Requirement of (Mucostatic) technique:

a) The impression is made with the oral mucous membrane and the jaws in a normal,

relaxed condition.

Border moulding is not done here.

b) The impression is made with an oversized tray. Spaced tray

c) It requires a material of high fluidity (low viscosity) .Impression material of choice

is impression are plaster and alginate.

d) Retention is mainly due to interfacial surface tension.

The mucostatic technique results in a denture, which is closely adapted to the mucosa of

the denture-bearing area but has poor peripheral seal.

2. Pressure technique (Mucocompressive):

In this technique the impression is subjected to pressure during taking, this pressures either applied by :

dentist finger

by the teeth of the patient and in this case it is called functional impression

Functional impression techniques/ Pressure technique /(Mucocompressive) cosiderations:

a) Uses in complete denture that is delivered ,relieved on its internal aspects and filled with the slow-seating

impression material(tissue conditioning material)

b) The patient wears prosthesis for several days, allowing the tissue to be recorded in function.

c) The complete impression then is converted to hard reline material via laboratory processing

d) This seeks to create a denture base that models the functional loaded tissue.

e) Records the oral tissues in a functional and displaced form.

f) The materials used for this technique include impression compound, waxes and soft liners.

g) f) The oral soft tissues are tend to return to their anatomical position once the forces are relieved.

3.Selective Pressure Techniques:

• Uses accustom tray constructed with less relief in the primary denture stress bearing area

and greater relief in the non bearing areas.

• Variation in relief result in theoretically in greater amount of pressure being applied directly

to the primary bearing area ,which are biologically and biomechanically more capable of

supporting and distributing the load.

• Low pressure is applied through greater relief for impression materials to the non bearing

areas

• This technique seeks to create a denture base that selecting loads to the oral tissue during

function of the prostheses, thus optimizing the stability and retention of the prostheses.

ii. Depending On The Technique:

Open mouth impressions :

The open mouth impression is built in a tray which carries the impression material of choice into the desired

contact with the supporting tissues and into an approximate relation to the peripheral tissues when the mouth is

opened and without applied pressure.

The rationale behind this method is that the dentures do not dislodge when subjected to biting force.

The open mouth methods provide clearance for the tissues that are pulled over the edges of the dentures as in

function of speech.

It develops a contour of impression surface which is in harmony with the relaxed supporting tissues, and which

may be out of perfect adaptation with these tissues when the denture is subjected to occlusal loading.

Close mouth impression technique :

These require wax occlusal rims to be fabricated on the preliminary cast. The patient is

made to close on these rims and a generous clearance is made for the various frenula so

that the patient can manipulate his tissues by closing, grimacing, sucking and swallowing

to form peripheral borders.

iii. Impression can be classified depend on the sequence of the impression into:

1.Primary impression.

2. Secondary (final) impression).