It has been reported in several published articles that prosthodontic success is contingent on patient cooperation both during treatment and after care. As dentists we all know that one of the most difficult procedures is trying to make an impression on the edentulous patient. Some patients have extremely strong muscles and we are fighting to get the tray in the mouth, whereas other patients have such weak and flaccid muscles that the cheeks and lips collapse around the tray while we are trying to do our best to get access to making the impression. One common problem is not properly maintaining adequate cheek and tissue retraction while making the edentulous impression.
It is vital to see everything you are making an impression of before inserting the tray. The facial mask (cheeks and lips) if not retracted properly can result in irregular tissue borders and subsequent retention deficiency. The erratic movement of the lips and cheeks may rub impression material off certain areas of the tray while leaving excess on other parts, thus resulting in an inaccurate result. Most patients are generally unhappy having one impression made much less two or three. In order to minimize patient discomfort and increase cooperation it is extremely helpful to utilize tissue retractors to expose the facial mask, thereby gaining access to both the maxillary and mandibular ridges during impression making.
Training the patient to use retraction devices such as these Columbia retractors (Fig. 1 Miltex USA) will facilitate the process. The patient will guide the retractors to separate the lips and cheeks of both the maxillary and mandibular arches without creating discomfort to themselves (Figs. 2 & 3). This also gives you and your dental assistant an extra hand to deal with other tasks, thereby concentrating on making the best impression possible.
Once the ridges have been exposed adequately the practitioner can now begin to precisely evaluate and make the maxillary or mandibular impressions. The first step would be to size the tray, once the tray is appropriately sized and/or adjusted to fit the maxillary/mandibular ridge, then tissue stops are placed in the tray with a high viscosity polyvinyl siloxane (PVS, Aquasil Ultra, Dentsply Caulk).
For the maxillary impression the tray is then placed in the mouth making sure it is centered with the ridge, frena attachments, vestibular sulcus and the post palatal zone. These stops will control tissue displacement and minimize tray movement when reinserting the tray during subsequent steps. The stops will also control the volume of material and center the tray and stabilize it against the residual ridge. This step is key to making the best impression by assisting the practitioner in controlling how the tray is seated and will allow for a centered even pressure technique.
Once these PVS stops have hardened, they should be evaluated and trimmed appropriately, making sure that no excess is on the tray vestibular border (Figs. 4A & B). These trays can easily be shaped to accommodate most ridges by heat molding. Now a high viscosity PVS is added to the borders of the tray and then reseated in the mouth until the stops are tactilely felt. The border molding procedure is now initiated until completion. Once the PVS sets, the tray is taken out and adjusted, trimming 1/2mm to 1mm off the set PVS to allow for additional room for the next and final viscosity to be layered on. This high viscosity PVS (similar to border molding compound) captures the anatomical details of the frena and vestibular sulcus and extends sufficiently to capture the post palatal area (Fig. 5A). For the mandibular arch it is important to maintain all frena, vestibular sulcus, retromylohyoid space, and retromolar pads. This is done very easily with this new layering technique (Fig. 5B).
The final step is to load a light or extra light viscosity PVS material to act as the wash. The wash material is placed into the tray and spread evenly throughout and the border molding procedure is repeated as before. Once the PVS sets the tray is removed and the impression evaluated for proper horizontal and vertical border integrity and for all load-bearing areas (the basal seat) (Figs. 6A & B). This method has increased our productivity by at least 50% and has decreased those frequent irregular borders which create distortion and affect the final result. Appropriate access into the oral cavity with the cheeks and tissues retracted properly for tray placement is key. Removing the speed bumps to drive smoothly is a good analogy.
It is a fallacy to think that once we have taken an excellent impression as described here, that the final denture will have an excellent fit. It is extremely important to understand that if we are going to transfer our definitive impression it must be done in such a way as to control the casting procedure. Proper casting is vital to transfer the accuracy of the final impression. For the edentulous imprint the high volume of stone is such that without controlled measuring and mixing distortion will result, thereby affecting the final fit of the restoration.
Every detail in the edentulous impression is imperative to follow. Generally retention is dependent on the peripheral row (vestibular sulcus and frena attachments) including the post palatal zone and stability is a factor of compression (the basal seat) while chewing. The impression will take into consideration the entire peripheral row and the basal seat which comprise both suction and stability.
To preserve and protect the detail of the peripheral row it is imperative that the final impressions be boxed to adequately preserve this area (Fig. 7). It is extremely important to maintain these peripheral borders when boxing to avoid the frequent over trimming of the master cast. The boxed cast will guide the technician to precisely trim the model for ideal processing. It is necessary to select an appropriate dental stone with controlled expansion/contractive properties and follow the directions for use exactly. The figures below show the desired boxing and casts with optimal landing areas for technicians to complete their final wax up for processing (Figs. 8A & B, 9A & B).
Joseph Massad, DDS, Associate Faculty, Tufts University School of Dental Medicine, Boston, Massachusetts and Adjunct Associate Faculty, Department of Prosthodontics, University of Texas Health Science Center, San Antonio, Texas. Dr. Massad will be lecturing on this topic at the Pacific Dental Meeting in Vancouver, BC, March 2007.
Dr. Massad maintains a private practice in Tulsa, Oklahoma.
Oral Health welcomes this original article.
REFERENCES
1.Masri R, Driscoll CF, Burkhardt J, et al. Pressure generated on a s simulated oral analog by impression materials in custom trays of different design. J Prosthodont. 2002;11:155-160.
2.Ferracane, JL. Materials in Dentistry Principles and Applications. 2nd ed. Philadelphia, PA: Lippincott Williams and Wilkins; 2001.
3.Kois JC, Fan PP.Complete denture impressioning technique. Compend Contin Educ Dent. 1997;18: 699-708.
4.Drago CJ. A retrospective comparison of two definitive impression techniques and their associated postinsertion adjustments in complete denture prosthodontics. J Prosthodont. 2003;12:192-197.
5.Salinas TJ. Contemporary materials for removable prosthodontics. Pract Periodontics Aesthet Dent. 1999;11:888.
6.Massad JJ, Lobel W, Garcia L, Monarres A, Hammesfahr P. Building the edentulous impression – A layering technique. Compend Contin Educ Dent 2006;27:446-451.
