A prerequisite for any long-lasting indirect restoration is a perfect marginal fit, which requires a precise impression. A perfect impression should capture, in an un-deformed manner, the entire configuration of the prepared tooth as well as some of the pristine tooth structure beyond the finishing line.
The elastomeric impression materials used for accurate impression are most commonly polyvinyl siloxanes (PVS) and polyethers (PE). Due to their optimal resistance to deformation, accuracy, good dimensional stability and tear resistance, PVS are still the material of choice for most dentists. 1
The double mix (putty and light or heavy and light) technique and the traditional two-step putty/wash technique will be described together with proper selection and use of retraction cords for soft tissue manipulation.
Gingival manipulation and cords selection
Regardless of the technique or material used, a major factor in the success of impression-making is gingival manipulation. The intra-sulcular finishing line, which corresponds to the majority of clinical situations, demands proper gingival deflection in order to provide space for the impression material.
An impression will be taken only in the conditions of complete health of the soft tissues. A routine mapping of the sulcus and a proper selection of cords is mandatory. According to the position of the preparation margins, a decision regarding the use of a single- or double-cord technique should be made. Keep in mind that for juxtagingival preparation, a single cord is sufficient to ensure a slight apicalization of the gingival margins for sufficient exposure of the finishing line to the impression material. Cord insertion is recommended in order to deflect the tissue laterally and retract them apically.
Retraction cords - general characteristics
Retraction cords can be categorised according to:
- material: cotton or PTFE (polytetrafluoroethylene - Teflon)
- configuration: twisted, braided and knitted
- chemical composition: impregnated (sulfate salts-aluminum, zinc) and non-impregnated
Non-impregnated cords can be used together with hemostatic/astringent solutions or gels (Hemodent - Gingival retraction cord, Premier Dental Products, U.S.A) just before their use.
If the cord is impregnated with a hemostatic/astringent solution, it should be kept in the sulcus no longer than 15 minutes in order to avoid any damage to the periodontal tissues. For this reason, when several abutments are involved, rinsing the cords after this time is recommended to prevent the soft tissue damage. 2, 3 For an easy and atraumatic insertion of the cords, very thin spatulas should be used.
The first apically-located cord is generally a smaller diameter, and will remain in place during the entire procedure. The purpose of the first cord is to retract the tissues in an apical direction and to block thin material remnants toured from the set impression from penetrating and remaining in the sulcus. It is recommended to start rolling the first cord from the interproximal area where the depth of the sulcus is larger and the tissue (papilla versus free gingival margin) is stiffer, which allows the cord to be fixed easier.
The second superficial cord is impregnated, in most cases, with hemostatic/astringent solution and placed at the level of the finishing line so that its superficial part is always visible after insertion. This superficial cord is typically thicker than the first cord and is aimed to laterally deflect the gingiva and to create enough space for the impression material to capture the preparation margin.
Once the cords are inserted, the clinician can proceed for a one-stage or two-stage impression technique. The main difference between the two is where and when the light body material will be injected.
Clinical steps - two-step, putty/wash technique
In cases in which several abutments are to be imprinted, a definitive path of insertion and removal of the putty is mandatory in order to facilitate reinsertion of the tray with no distortions of the putty material. For this, a proper parallelism between the prepared teeth is preferred for this technique and no undercuts present. In order to expose the margins of the preparation, a smaller size first cord was chosen (000 Ultrapack, Ultradent Products, Inc. USA).
After the first cord was placed, an initial impression (putty material, Zhermack Spa Italy) of the prepared teeth was taken with a rigid stock tray.
The diameter of the second cord should be chosen according to the amount of deflection needed, taking into account the gingival biotype. Due to the difference in the positioning of the finishing line into the sulcus, two different sizes for the second cords were chosen. Tooth #8 (11) being prepared for a full crown would need more deflection of the soft tissues in order to expose the preparation margin, so a #0 cord in size (Ultrapack, Ultradent Products, Inc. USA) was chosen. Tooth #9 (21) has been more minimally prepared for a veneer, so a thinner cord #00 would ensure the necessary deflection.
Modifications should then be made to the previously set putty. The modifications include cutting out all of the interproximal undercuts to ensure an easy and one-way insertion pattern of the set putty, which is crucial to avoid distortions, pressure and tearing of the putty material. Then channels are carved in order to allow for the excess of the wash material to be vented away later. This carving is performed using a Deta-Cut (Detax Dental GmbH &CO.KG) core removal knife. The channels are placed both externally to and within the indentation of the abutments in the axial and occlusal walls.
While doing so, special attention is required so as not to damage the finishing line captured previously in the set putty. The second cord is gently removed, and at the same time, the low-viscosity PVS material is injected into the tray.
The loaded tray is placed over the prepared teeth to a full seat while the first, deeper cords remain in place. Sometimes the first cord is removed along with the impression, which should not cause any problems for the technician as long as it is attached to the material and remains beyond the margins of the prepared teeth.
The double mix technique - putty/light, heavy body/light
This technique has the advantage of being less time-consuming but, at the same time, more technically sensitive. With this technique, the putty or heavy body and the light body will set at the same time, and the light body material will be injected around the prepared teeth simultaneous with the removal of the second cord, in the case of double cord technique being used.
- Fradeani M, Barducci G. “Esthetic rehabilitation in fixed prosthodontics “ Quintessence 2008, vol 2
- Kopac I, Cvetko E, Pavlica Z, Marion L “Gingival tissue inflammatory response following treatment with chemical retraction agents in Beagle dogs,” Pflugers Arch.2001;442(6 suppl 1):R145-6
- Dragoo MR, Williams GB.’’Periodontal tissue reactions to restorative procedures’’ int J Periodontics Restorative Dent, 1981 Apr;1(1):8-23