|
Taking VPS Imressions for Digital Records |
|
|
|
Originally published April 2008
Taking VPS Imressions for Digital Records
With the popularization of new orthodontic techniques that require digital scans of highly accurate and stable impressions, Vinyl Polysiloxane (VPS) materials have gained a foothold in the previously exclusive territory of alginate impression materials. Remakes and retakes with alginate were no big deal other than the time that had to be reinvested by the staff member. Do-overs with silicon materials can be much more expensive due to the significantly higher cost of the impression materials. The good news is that Wehmer is now providing extremely cost-effective VPS materials for one-step, two-step, and bite registration requirements that are highly compatible with digital techniques. We have also fine-tuned some specific procedures that optimize silicon impression-taking regardless of the material you choose to use.
The simplest method for getting a silicon impression is to use a one-step method with a medium-body MonoPhase impression material. The technique is very similar to alginate, except that the material is dispensed from a 50ml two-component cartridge through a mixing tip. Estimating a volume of 20ml per typical impression, this method can cost between $6.00 and $10.00 per tray depending on the purchase price of your material. A two-step putty/ wash impression can be made for about $3.00 using Wehmer IMPRESS™ VPS. For this reason, we have highlighted this specific method in this issue.
Various studies have indicated that the thickness of the wash material in a putty/wash VPS impression can have a significant effect on the accuracy of the impression. In one such study1, wash thicknesses in excess of 2 mm caused statistically significant variations in measurements of accuracy. The following two-step impression technique is an excellent method of maintaining optimum accuracy in your impressions by controlling the thickness of the light body wash material. It is a hybrid technique gleaned from various articles available on the internet and minimizes variables in staff technique that can necessitate multiple retakes.
1. Take an alginate impression of the upper and lower arch, pour the models in orthodontic or dental stone, and trim them as if they are to be used for fabricating an Essix®-style thermoformed invisible retainer.
2. Use a vacuum- or pressure-type former to form a 1mm thick thermoplastic tray over each model arch. I have been using a soft mouthguard-type material since it is easier to remove from the model, but many clinicians (reference the excellent article by Dr. David Paquette in Orthodontic Cyber Journal2) use a rigid tray material that can later be trimmed to a training aligner or bleaching tray.
3. With the models still in their trays, trim the bulk tray material away to provide ease in handling.
4. Find the correct size plastic disposable tray to correspond to each arch.
5. Mix 1 scoop each of Wehmer IMPRESS™ A+B putty material. Mix well by hand (no latex gloves) until homogenous (30-45 seconds). Roll into cylinder and place into tray. Spread material evenly throughout tray.
6. Push corresponding model with thermoplastic tray into impression material, fully seating in tray. Make sure that the second molars are fully captured and there is sufficient material reaching to the top of the tray to form a dam for the subsequent thin-bodied wash material. Allow to set for 3 minutes.
7. Repeat steps 5 & 6 for other tray.
8. Remove the models and the thermoplastic material from the impression.If you don’t have access to a thermoforming machine, check out an excellent article in Orthodontic Products by Drs. Stephen Tracy and Craig Andreiko called “The Right Bite”. It describes using a closed-cell foam spacer to create the space required for the wash material.
You now have two custom trays that are essentially .5-1.0mm larger than the patients’ actual dentition. This creates the space for the light body wash material that will capture all detail and provide an incredibly accurate impression for scanning, as well as minimizing the amount of light body material used in each impression.
Final Impression
1. Disinfect your custom putty trays; rinse, and blow dry with compressed air so that the impression is free from moisture.
2. Set a timer for 3:30 minutes. Fill the custom tray with light body IMPRESS wash from the cartridge. With 1:30 indicated on the timer, insert the tray into the patient’s mouth and firmly hold in place.
3. At 00:00, remove impression tray. You’re done. Repeat for other arch.
1 Effects of wash bulk on the accuracy of polyvinyl siloxane putty-wash impressions; Nissan J, Gross M, Shifman A, Assif D.; J Oral Rahabil, 2002, Apr 29 (4) 357-61
|
2009 Laboratory Equipment and Products (3962)