Implant insights – the implant impression
In the latest implant insights, Nilesh Parmar explores the different methods of taking impressions and the different materials available.
Impression techniques in dentistry remained unchanged for several years, before the use of intraoral scanners.
Prior to this, impressions were taken in conventional or special trays with a silicone compound usually the material of choice.
Over time the materials have developed and refined. They’re now easier to manipulate, snap-set with improved hydrophilic properties.
Implant impressions require the insertion of an impression coping into the implant and a mould taken over it.
There are currently two main methods of implant impression taking used today.
Close tray impressions
This is where the clinician inserts an impression coping with a distinctive ‘head’ into the implant.
A conventional silicone impression (either one stage or two stage techniques) is taken, with the impression coping remaining within the implant when the mould is removed. The coping is then removed from the implant and inserted into the impression itself.
Various designs of impression coping are available for this purpose. Many clinicians prefer this method because it is close to conventional crown and bridge impression techniques with no alteration to impression trays needed.
If the clinician places the implant deep below the bone crest, then less of the coping ‘head’ is available above the gum line. This makes a firm seating of the impression coping in the resulting material difficult.
It was long thought that this technique is less accurate than open tray copings. However, research would disagree with this statement.
Gallucci et al found that the closed tray method was as accurate as the open tray method for single unit restorations involving dental implants.
Open tray impressions
This technique is where the clinician screws an impression coping into the implant with a long purchase screw. This screw needs to extend out into and past the impression tray.
The impression tray would have holes drilled into it, to allow the coping screw to be accessible once the impression material has set.
Standard practice is to place wax over the hole made in the impression tray in order to allow the screw to penetrate. This will allow the clinician to loosen it once the material has set.
This technique is tricky to perfect initially. The fear is that the impression is locked in the mouth if access to the impression coping screw is not available. There is some thought that multiple implants, requiring multiple holes in a stock impression tray can lead to distortion in the final impression. Therefore, in these cases, clinicians prefer a special tray with impression chimneys.
It is the method preferred by lab technicians and research does suggest it is a superior method of impression taking when restoring multiple implants when compared with closed tray options (Papaspyridakos et al, 2014).
The open tray technique requires a rigid setting impression material, with Impregum (3M ESPE) as the common material to use.
Many patients do not like the chemical taste of this. The rigid set does require blocking out of undercuts using wax or vaseline to ensure you can remove the tray successfully.
The open tray impression technique is the universal method of choice for implant impressions. However, we must not forget the digital option.
The provision and development of intraoral scanners has dramatically changed the dental environment. The ability to digitally scan restorations and implant reconstructions have resulted in faster, less invasive and reproducible protocols for the manufacture of implant crowns and bridges.
There is a cost that goes with these methods. However, the long-term savings are considerable, coupled with patient desirability.
Conventional impression techniques still have a big part to play in implant impression techniques. But the digital replacements are quickly becoming common place.
Catch up with previous implant insights column
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Papaspyridakos P, Chen CJ, Gallucci GO, Doukoudakis A, Weber HP and Chronopoulos V (2014) Accuracy of implant impressions for partially and completely edentulous patients: a systematic review. Int J Oral Maxillofac Implants 29(4): 836-45