An accurate and detailed impression of the prepared tooth is an important step in creating
superior crowns.
Impression materials are classified as elastic or non elastic. Non elastics are not used due to
their inability to record undercuts (waxes, compound, plaster). Elastic materials are classed as
hydrocolloids irreversible (alginate) or reversible (agar)(4) or elastomers.
For crown impressions we use elastomers (polyethers or addition cured silicones) due to poor
dimensional stability and low tear resistance of hydrocolloids.
Elastomers
Polysuphides have long working times, useful in impressions of multiple impressions and
possess good tear resistance. They are avoided, due to shrinkage of material, long setting
times, objectable odour and difficult application.
Polyethers are popular due to good impression accuracy( 1, 2), fast setting time, good tear
resistance, good dimensional stability. They set rigid due to high elastic modulus, so preclude
their use in severe undercuts. Polyethers are hydrophilic. If they are not stored well, they
swell from water absorption and thus should be utilised within 48 hours (10).
Type 1 silicones (condensation- cured) produce ethyl alcohol as by-product causing
shrinkage during setting and dimensional changes. Silicones type 2 (addition cured) have
good dimensional stability with no by-product, allowing a model to be poured after a number
of days (9,14). Type 2’s have the best elastic properties with good recovery strain and tear
resistance, coupled with the fact that they are non toxic, odourless and tasteless make them
the most popular choice of material (8). They have showed to be as good if not better than
polyethers (11,15).
Impression tray
Flexible plastic trays are avoided as they cause discrepancies (23)
Custom trays are more accurate than stock trays (25) but are needed when stock trays are a
poor fit.
Metal/rigid plastic impression tray showed the least discrepancy (23)
Poor bond to trays with impression material cause discrepancies in the accuracy of the
models made. Using rim-lock trays avoids this.
Perforated trays are avoided as material flows through vents rather than allowing material to
be forced around dentition, providing poorer adaptation. (29) Perforated trays produce a better
bond to the material than just adhesive (30).
The most important factor, rather than tray or impression material is technique (6, 7).
Techniques include:
1 stage impression- putty and wash record the preparation simultaneously.
2 stage impression- putty records the preparation first then allowed to set then relined with
the wash for a second impression.
2 stage spaced- same for the 2 stage but space is created for the wash.
Due to recoil and difficulty reseating the putty, 2 stage techniques are not recommended (32,
33)
Dual arch trays record the preparations, the opposing arch and the registration all in one. It
has been reported to be a good technique providing reasonable accuracy (24).
CadCam uses computer technology to scan the preparation and then mills the restoration
from ceramic blocks . It has been found accurate (46), but has limitations on cost (47).
Material is hand mixed or syringe mixed. Syringe mixing reduces air bubbles, more
homogenous mix as correct proportions and reduces mixing time.
Gingival Retraction
Equigingival/Subgingival margins need some type of soft tissue retraction for good
impressions to allow material to capture the finishing line detail.
Retraction cord technique- Placing single cord within the gingival sulcus for retraction.
The 2 cord technique- Placing thin cord, then thicker cord. After retraction, the thicker cord is
removed and impression taken leaving the thinner cord. Trauma and recession can occur (31),
but allows sufficient bulk of impression material and controls haemorrhage.
Chemicals- Soaking cord with chemicals, can increase retraction. Concerns are changes in
respiratory and heart rate occurs (34) (epinephrine), tissue necrosis(35) (aluminium chloride)
and gingival stains (ferric sulphate).
Cordless chemical retraction- using aluminium chloride is Expasyl. Causes retraction and
haemostasis without tissue damage as seen with cord (36) but induces sensitivity reactions (37).
Expasyl is pushed into sulcus and allowed time for effect, then washed away leaving
retracted gingivae.
Curettage uses burs, electrosurgery and lasers, to create a trough around the finishing
margins of the preparation when subgingival, causing pain and bleeding thus requires a
haemostatic agent (ferric sulphate).
Studies show no histological change between rotary and cord (39). Electrosurgery is modified
cautery, using electricity through a cutting electrode to create the trough. Studies showed no
difference between rotary and electrosurgery(38) but it is unpredictable (40), causes tissue
damage and avoided in patients with pacemakers. Lasers can be used with little or no
anaesthetic and have faster gingival healing rates with less bleeding (41, 42, 43).
The copper band technique- A copper band is placed around tooth margin, this retracts soft
tissue. Impression material is then filled into the band (26, 27, 28). They are cheap, accurate,
easily available but can traumatise the gingivae.
General Considerations
Poor manual dexterity, Patients with severe gag contribute to poor impressions.
Patients with severe undercuts (gingival or tooth preparation) cause recoil of the material and
distortion.
Others considerations :-
Uninhibited Material
1 Gloves containing sulphur to mix or handling retraction cord (45).
2 Temporary crown residues that come in contact with type 2 silicones which affect setting
and detail of the impression (12).
3 Poor material mix (13).
Poor Detail
1 Material stored in too high/low temperature (16,17,18).
2 Heavy-body with little flow displacing
light body from preparation.
3 Disinfection by materials which affect surface detail (21).
4 Debris/saliva/blood present during impression.
Drags
1 Poor material mix by machine/hand (13).
2 Area excessively moist causing hydrophobic materials to be repelled away
3 Saliva/debris contamination.
4 Poorly adapted tray.
5 Light body setting before the seating tray.
Margin voids
1 Air trapped in syringe tips (44).
2 Air trapped while filling material in tray.
3 Inadequate coverage of light body.
4 Blood/saliva around prep.
5 Exceeding working time so poor flow.
6 Impression material stored at elevated temperature.
Delamination
Exceeding the working time(19, 20).
Tray to tooth contact
1 Improper tray placement.
2 Incorrect tray size/adaptation.
3 Inadequate material amount.
Tearing at preparation margin
1 Inadequate/unset material mix.
2 Material out of date.
3 Insufficient gingival retraction.
4 Retracton cord with ph <5, or contains sulphur.
References
1. Impression Accuracy when Recording Impressions of Moist Surfaces
J. MCCABE, and T. CARRICK, University of Newcastle upon Tyne, England, Uk
2. Detail Reproduction of Impression Materials on a Wet Surface
R. PERRY, G. KUGEL, E. APPELIN, and B. GREEN, Tufts University, Boston,
MA, USA
3.Ensuring Accuracy and Predictability With Double-Arch Impressions
Gordon J. Christensen, DDS, MSD, PhD
4. Clinical evaluation of agar alginate combined impression
P. Manorika Ratnaweera
J Med Dent Sci 2003
5.Accuracy of elastomeric impressions disinfected by immersion
Authors: G H Johnson, D G Drennon, G L Powell
Journal of the American Dental Association
6. Influence of the type of impression material, impression tray and making impression
technology on the dimensional accuracy and depth of impression material penetration into
"gingival sulcus". In vitro study
Authors: A N Riakhovskiĭ, M A Muradov
Journal: Stomatologiia
7. Wettability and accuracy of reproduction of impression materials
Authors: F Bader, J Setz Journal: Deutsche zahnärztliche Zeitschrift
8. Journal: Australian Dental Journal
review of modern impression materials
Authors: J K Harcourt
9 Evaluation of accuracy and the time-dependent dimensional stability of silicone based
impression materials
Authors: H Yavuzyilmaz, C Dinçer, L Nalbant
Journal: Ankara Üniversitesi Diş Hekimliği Fakültesi dergisi = The Journal of the Dental
Faculty of Ankara University
10. Dimensional accuracy of a new polyether impression material
Authors: Tatsuo Endo, Werner J Finger
Impact factor: 0.73, Cited half life: >10.0, Immediacy index: 0.06
Journal: Quintessence international
11. Accuracy of alginate and elastomeric impression materials
Authors: A Peutzfeldt, E Asmussen
Journal: Scandinavian journal of dental research
12. Polyvinyl siloxane impression materials: a review of properties and techniques
Authors: W W Chee, T E Donovan
Impact factor: 1.01, Cited half life: >10.0, Immediacy index: 0.07
Journal: Journal of Prosthetic Dentistry
13. Dimensional stability of two impression materials
Authors: C L Chew
Impact factor: 0.73, Cited half life: 7.2, Immediacy index: 0.32
Journal: Annals of the Academy of Medicine, Singapore
14. Clinically oriented evaluation of the accuracy of commonly used impression materials
Authors: A H Tjan, S B Whang, A H Tjan, R Sarkissian
Impact factor: 1.01, Cited half life: >10.0, Immediacy index: 0.07
Journal: Journal of Prosthetic Dentistry
15. Factors affecting the accuracy of elastometric impression materials
Authors: S Y Chen, W M Liang, F N Chen
Impact factor: 2, Cited half life: 7.8, Immediacy index: 0.22
Journal: Journal of Dentistry
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polyether impression materials
Authors: M Corso, A Abanomy, J Di Canzio, D Zurakowski, S M Morgano
Impact factor: 1.01, Cited half life: >10.0, Immediacy index: 0.07
Journal: Journal of Prosthetic Dentistry
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impression materials during setting
Authors: John C Berg, Glen H Johnson, Xavier Lepe, Sergio Adán-Plaza
Impact factor: 1.01, Cited half life: >10.0, Immediacy index: 0.07
Journal: Journal of Prosthetic Dentistry
18. Dimensional changes of dental impression materials by thermal changes
Authors: K M Kim, J S Lee, K N Kim, S W Shin
Journal: Journal of Biomedical Materials Research
19. Working time of elastomeric impression materials: relevance of rheological tests
Authors: Markus Balkenhol, Masafumi Kanehira, Werner J Finger, Bernd Wöstmann
Impact factor: 1.28, Cited half life: 7.1, Immediacy index: 0.18
Journal: American journal of dentistry
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Authors: R J McConnell, L N Johnson, D R Gratton
Journal: Journal (Canadian Dental Association
21. Three-dimensional investigation of the accuracy of impression materials after disinfection
Authors: M Kern, R M Rathmer, J R Strub
Impact factor: 1.01, Cited half life: >10.0, Immediacy index: 0.07
Journal: Journal of Prosthetic Dentistry
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Authors: E M Langenwalter, S A Aquilino, K A Turner
Impact factor: 1.01, Cited half life: >10.0, Immediacy index: 0.07
Journal: Journal of Prosthetic Dentistry
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