Impression Taking
Impression taking over implants requires greater accuracy than over natural teeth. Teeth are surrounded by a periodontal membrane which can adapt to minimal casting inaccuracies. Osseointegrated implants do not have a periodontal ligament and are not mobile. Any loads or stresses posed by discrepancies in the castings will be transmitted to the surrounding bone and can cause bone necrosis and/or loss of integration.
Impressions for implants usually involve either:
- Preliminary Impressions
- Custom Final Open Tray Impressions
- Custom Final Direct Impression (Closed Tray Impressions)
Preliminary impressions for study models, for surgical and radiographic stents, or for fabrication of custom final impression trays, should be taken and poured up in dental stone.
Custom Final impressions can be taken either of the implant body itself or of the abutment attached to the implant. This choice is dependent upon the type of prosthesis being made: crown, bridge, metal bar or overdenture. In either case, metal analogues must be used in the impression when pouring up the model.
Custom Final Impression taking over dental implants can be accomplished with either the Custom Final Open Tray technique or the Custom Final Direct Impression Technique.
The Open Tray technique is more accurate and recommended for all implants, if the impression is taken over multiple (more than 3) implants or if the implants diverge from each other by more than 15 degrees. In this technique, the impression coping is retained in the impression during its removal from the mouth. However, impressions taking over one or two implants which are within 15 degrees of each other can safely be accomplished using the Direct Closed Tray Impression Technique.
If using the Open Top Tray Technique, the impression coping consists of two parts, the coping itself and the impression coping screw which holds the coping onto the underlying abutment or implant.
The Direct Closed Tray Technique can be used for all cases or those with multiple implants that diverge from each other by less than 15 degrees.
The technique is most commonly performed by:
- Torquing solid abutments to the fixtures and adding transfer copings directly onto the abutments. These copings then remain in the impression and lab analogues are then attached chairside or in our laboratory.
- Closed tray impression copings are torqued onto the fixtures and a standard crown and bridge impression is taken. The impression is removed, the copings are unscrewed and then carefully repositioned inside the impression. Lab analogues can be added chairside or in our laboratory.
Clinical Tip
Zirconia based crowns are intrinsically more opaque than all other all ceramic systems. Aesthetic outcomes can only be achieved when sufficient space for the veneering ceramic has been provided by the clinician.
Solid Straumann 4mm abutments were torqued to 35 Ncm, basket and transfer copings are attached and secured with Duralay resin to avoid movement during lab analogue attachment. This closed technique is very simple and ideal in cases of limited opening and occlusal height as it avoids the need for impression copings with associated guide pins to be inserted. A bridge is generally cast as a single casting, but ectioned and soldered where appropriate.
Clinical Tip
When employing the solid abutment closed tray technique, it is imperative that the abutments are completely torqued into their final position before taking the impression. Any change of position or retorquing after the impression has been taken will have a catastrophic impact upon the accuracy of the final restoration.
