Impression Techniques
When classifying impression techniques it’s important to distinguish between the therms ‘step’ and ‘phase’. Put simply, steps are the number of times the impression tray is inserted into the oral cavity, followed by the number of phases which are the viscosities of material used where necessary.
One Step Techniques
The impression is taken in one step after preparation. The impression can be one viscosity (monophase) or two different viscosities (two-phase/blaminar/tray and wash).
The materials recommended for this technique are polyether or monophase viscosity addition silicone. A custom impression tray is recommended to reduce wastage on material.
Steps:
- The tray is filled with impression material first
- When polyether is used, an elastomer syringe is filled with impression material to inject the wash. If addition silicone is used the wash is dispersed from a cartridge with a micro-tip to help inject material into the gingival sulcus.
- The oral cavity and preparation site are air dried and the wash material is injected around the preparation margins until the prepared teeth are completely covered. Care must be taken that the tip does not exit the mass of impression material to avoid air entrapment.
- When the preparations are completely covered with wash, the occlusal surfaces of the entire dental arch are covered with wash material
- The loaded tray is placed vertically on the dental arch. The previous steps must be finished during the working time of the impression material
- The tray must stay in place a minute longer than the setting time of the material. This is to increase resistance to deformation
- The tray is pulled vertically in one swift motion to prevent deformation of the impression material
This is carried out using putty and light or heavy and light consistency addition silicone. If putty is used, a stainless steel tray is used with adhesive added before the preparation. This is because putty will deform aluminum or plastic trays while being pushed downward then rebound on removal of the tray, resulting in smaller dies and poorly fitting restorations. If heavy viscosity silicone will be used to load the tray then a custom impression tray will reduce wastage of the material.
Steps
- The tray material is mixed while the oral cavity is being dried. In the case of putty, it is hand mixed with non-latex gloves. In the case of heavy viscosity putty, it is dispensed into the tray using a wide-bore tip with caution that the tip does not exit the mass of impression material. This avoids air entrapment.
- As the tray is almost completely loaded, the wash material is injected around the preparation margins until the prepared teeth are completely covered. Care must be taken that the tip does not exit the mass of impression material to avoid air entrapment.
- The loaded tray is placed vertically on the dental arch. The previous steps must be finished during the working time of the impression material
- The tray must stay in place a minute longer than the setting time of the material. This is to increase resistance to deformation
- The tray is pulled vertically in one swift motion to prevent deformation of the impression materia
Two Step Impression Techniques
An impression is taken before preparations then a second wash impression is taken with the same tray. The method can be done spaced or unspaced.
Steps:
- A perforated stainless steel tray is loaded with putty and an impression is taken of the dental arch before preparations are carried out
- A lancet is used to scrape 3mm of space from the fit surface of the impression. Interproximal putty is removed and escape channels are carved into the putty
- After the preparation the oral cavity is dried and wash material is injected around the preparation margins until the prepared teeth are completely covered, then the rest of the teeth are quickly covered in wash material
- The tray with the trimmed putty is sully seated on the prepared teeth. Care must be taken that the tray is fully seated to avoid a stepped impression that can cause occlusal discrepancies
- One minute after setting time is complete, the tray is removed in one swift motion.
This is carried out using putty and light or heavy and light consistency addition silicone. If putty is used, a stainless steel tray is used with adhesive added before the preparation. This is because putty will deform aluminum or plastic trays while being pushed downward then rebound on removal of the tray, resulting in smaller dies and poorly fitting restorations. If heavy viscosity silicone will be used to load the tray then a custom impression tray will reduce wastage of the material.
Steps
- A perforated stainless steel tray is loaded with putty. A polyethene spacer sheet is placed on the putty and an impression is taken of the dental arch before preparations are carried out. An even space should be created around the structures of the dental arch due to the spacer
- After the preparation the oral cavity is dried and wash material is injected around the preparation margins until the prepared teeth are completely covered, then the rest of the teeth are quickly covered in wash material
- The tray with the trimmed putty is sully seated on the prepared teeth. Care must be taken that the tray is fully seated to avoid a stepped impression that can cause occlusal discrepancies
- One minute after setting time is complete, the tray is removed in one swift motion.
Evaluation of an impression
A quality control routine for impressions must be incorporated into clinical work. The following points must be verified
- Proper mixing of the material. The impression must have no streaks of base and catalyst
- There must be no points where the impression tray shows through
- There must be no signs of rotation due to inaccurate seating of the tray.
- This can cause the tray to contact the teeth and cause critical errors.
- The impression must be free of air bubbles, voids and drags
- There must bean even, uninterrupted extension of impression material beyond the margins of the prepared teeth
- The impression material must remain attached to the tray
Digital Impressions
Digital impressions in the clinic are taken using an intra-oral scanner.
Previously, scanners needed the surfaces of the dental arch to be powdered with titanium dioxide to give a uniform surface topology. The scanner can take either a series of pictures or video.
Scanning is technique sensitive. Gingival retraction or its alternatives are required for all current intra-oral scanners. Different systems require different scanning protocols, so there is a definite learning curve.
Studies show that scanners are highly accurate when scanning prepared teeth and implants in local regions. Current case studies are showing similar reliability in full arch preparations and implants. Current scanners with colour scans can aid the discrimination of the tooth-coloured margin from the surrounding soft tissues. In the future Ultrasonic 3D imaging options can
penetrate soft tissue, negating the need for gingival retraction, though such devices are highly experimental at present.
