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 Journalreview 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: 0.06Journal: 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: 0.07Journal: 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: 0.32Journal: 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: 0.07Journal: 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: 0.22Journal: Journal of Dentistry

16. The effect of temperature changes on the dimensional stability of polyvinyl siloxane and 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: 0.07Journal: Journal of Prosthetic Dentistry

17. Temperature effects on the rheological properties of current polyether and polysiloxane 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: 0.07Journal: 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: 0.18Journal: American journal of dentistry

20. Working time of synthetic elastomeric impression material

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: 0.07Journal: Journal of Prosthetic Dentistry

22. The dimensional stability of elastomeric impression materials following disinfection

Authors: E M Langenwalter, S A Aquilino, K A Turner

Impact factor: 1.01, Cited half life: >10.0, Immediacy index: 0.07Journal: Journal of Prosthetic Dentistry

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influence of the impression tray on the accuracy of impressions for crown and bridge work--an investigation and review.

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Authors: J Orstavik

Journal: Journal of Prosthetic Dentistry

28. Copper-band gingival retraction to produce void-free crown and bridge impressions

Authors: H Darby, L H Darby

Journal: Journal of Prosthetic Dentistry

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Authors: Khaled Q Al Hamad, Wesam Z Azar, Hayder A Alwaeli, Khalid N SaidJournal of clinical periodontology

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