Stock impression trays can be metal or plastic. A high viscosity material is usually used with stock trays. These materials sometimes exhibit poor flow and the increase in pressure when seating the full impression tray can cause distortion of the tray if it is not sufficiently rigid. This distortion can be across the whole arch and/or in a cross sectional dimension.
Roughly only 23% of Dentists use metal trays. (McCracken et al., 2017)
The ideal impression tray should:
- Have good adhesion to the impression
- Have good dimensional stability
- Allow even thickness of impression material
- Have sufficient rigidity to resist deformation
- Not cause damage to the intra-oral tissues
Are metal impression trays better than plastic trays?
Metal Impression Trays:
1. Provides maximum support for impression material
Metal impression trays can resist distortion across the arch. Furthermore, plastic stock trays are usually perforated. If the impression material is displaced on removing the tray from the patient’s mouth, it is almost impossible to re-seat the set impression back into the tray.
Metal impression trays that are un-perforated will also allow the maximum amount of impression material to cover intra-oral tissues with less voids.
2. Rigid and doesn’t flex
Metal impression trays trays can resist distortion in across the arch and also in cross-section! Plastic trays exhibit the most distortion when used with highly viscous materials.
3. Durable and long lasting
Metal will not crack/break/bend as easily as plastic and can withstand sterilisation cycles.
4. Can be perforated if required
Although non-perforated metal impression trays are ideal for getting the most accurate impression with a stock tray, clinicians have the option of choosing perforated metal trays. Almost all stock plastic trays are perforated by default.
5. Can be used with all elastomeric materials
Increased retention to stop displacement of impression material
7. Don’t have to routinely use adhesive
Adhesive use can cause inaccuracies as it is difficult to get an even spread and rarely given enough time to set properly. The rim-locked trays will prevent displacement of the impression once set.
8. Easily sterilised and reused
In a study published in the Journal of Prosthodontics various impression trays were measured without impression material, on seating and polymerisation of the material and after removing the impression. All tested plastic trays exhibited distortion in cross section and across the arch. Metal trays displayed no distortion. The authors concluded that high viscosity impression materials should not be used with plastic stock trays. (Cho & Chee, 2004)
The use of plastic trays may be cheaper, but their cost is outweighed by the clinical loss of time and increased lab fees over a long period of time.
Images: https://www.smartpractice.com/shop/wa/style?id=S15703&m=SPD | https://www.gceurope.com/products/impressiontrays/ | http://medind.nic.in/cab/t13/i3/cabt13i3p313.htm
Cho, G. C., & Chee, W. W. L. (2004). Distortion of disposable plastic stock trays when used with putty vinyl polysiloxane impression materials. Journal of Prosthetic Dentistry, 92(4), 354–358. https://doi.org/10.1016/j.prosdent.2004.07.020
McCracken, M. S., Louis, D. R., Litaker, M. S., Minyé, H. M., Oates, T., Gordan, V. V., … Gilbert, G. H. (2017). Impression Techniques Used for Single-Unit Crowns: Findings from the National Dental