Introduction
Consequences of tooth loss can be manifold and irreversible. From an anatomical viewpoint, apart from tooth loss and changes in the position of the remaining teeth, it comes to alveolar ridge resorption of the upper and lower jaw and changes of soft tissues, which are manifested in a reduction of the attached gingival surface and an increase of displaceable mucosal surface. From a functional viewpoint, it comes to occlusal and speech disorders in patients, as well as changes of their appearance, which may result in psychosocial decompensation. Treatment of completely edentulous patients or patients with a small number of remaining teeth is most often carried out with complete dentures or overdentures, which replace the lost teeth and the alveolar ridge. With the help of adhesive effect of saliva, dentures should accurately fit to the mucosal surface of the denture-bearing area and be adjusted to the surrounding masticatory and facial muscles, in order to achieve good retention and stability (1-5).
Despite the progress in education on oral health and preventive measures, due to a rising share of elderly people in the general population, and thereby of patients with a small number of remaining teeth and edentulous patients, there is an increased need for complete denture and overdenture treatment. The objective of complete denture and overdenture treatment is to enable recovery of occlusal, masticatory, esthetic, physiognomic, phonetic and psychosocial functions of patients. Since more than 10% of patients do not wear their complete dentures, the remaining teeth have an important role for denture retention and stability. They are used as abutment teeth for retention elements (conical and telescopic crowns, ball attachments, bars, locators) (5-8).
Recovery of patients’ functions is possible only by regular and accurate performing of all clinical and laboratory procedures, one of the most important being functional impression taking (5, 9-11). Functional impression shows the size and shape of the denture-bearing area, registers mucosal resiliency and forms functional margins of a future denture. It enables individual fit of a future denture base in the denture-bearing area, with uniform transfer of masticatory pressure to supportive tissues and widening of the denture base up to displaceable mucosa, i.e. nondisplaceable mucosa when in function. This creates a valve effect and good denture retention and stability (1, 5, 8, 12). Functional impression for overdentures also determines relationships between retention attachments on the remaining teeth or implants, and therefore it is called fixation functional impression (5, 11-19).
The objective of this paper was to provide a review of procedures of functional impression taking in fabrication of complete dentures and overdentures, using standardized techniques and materials.
Impression materials
The literature reports about many different techniques of anatomical and functional impressions, as well as about different types of impression materials – from dental stones, zinc oxide eugenol pastes and thermoplastic materials to reversible hydrocolloids and elastomers (20-24).
The analysis of the type of materials that are most often used for impression taking in fabrication of complete dentures and overdentures indicates that reversible hydrocolloids and alginates (Image Dust Free, Dux Dental, Germany) are commonly used for taking the first or anatomical impression; thermoplastic materials of the latest generation in the form of sticks (Bite Compound, GC, Japan) are used for formation of functional margins; aluminum wax (Alminax, Whip Mix, USA) is used for relining of the vibrating line and zinc oxide eugenol pastes (Luralite, Kerr, Germany) and several types of elastomers, condensation silicones (Coltex Extra Fine, Coltene, Switzerland), vinyl polysiloxane (Dimension or Express, 3M ESPE, Germany) and polyethers (Impregum Soft, 3M ESPE, Germany), are used for taking functional, i.e. fixation impression (5, 12, 18-22).
Techniques of functional impression taking
In fabrication of complete dentures and overdentures, impressions show hard and soft structures of the denture-bearing area, their biological and physical properties and the relationship between displaceable and non-displaceable mucosa for the purpose of achieving the best possible valve effect. With respect to the types and techniques of impression taking, in the literature there is a distinction between open and closed mouth techniques, techniques without pressure, techniques with selective pressure of the operator, mucostatic and mucodynamic techniques and techniques with active (patient) and passive (operator) functional movements. Techniques of functional impression taking with simultaneous determination of interjaw relationships are also described (20-35).
Today the most frequently used technique for taking anatomical impression is mucostatic impression with open mouth and without pressure. Mucodynamic impression technique with open mouth of the patient and selective pressure of the operator and the patient’s active performance of functional movements is mainly used for functional impressions. Open mouth enables the operator to hold the custom tray with impression material on the denture-bearing area and to control the pressure while taking impression. The pressure should be low in case of extensively resorbed alveolar ridges and displaceable mucosa, and a higher pressure is applied for impressions of the vibrating line. Open mouth also ensures performance of the patient’s active movements when impressing functional margins. Active functional movements for upper jaw impressions include mouth narrowing and widening (labial and anterior buccal valve), lower jaw movements toward left and right (posterior buccal valve – width of paratuberal space), mouth opening (dorsal border) and pronunciation of the letter A (pharyngeal valve). Active functional movements for lower jaw impressions include mouth narrowing and widening (labial and anterior buccal valve), mouth opening (posterior buccal valve and dorsal border), saliva swallowing (posterior sublingual valve) and licking the upper lip (medial and anterior sublingual valve). Performance of passive movements during functional impression taking is limited only to cases of recording frenulum and plica insertion and patients who are not able to perform active movements (1, 5, 12).
Complete dentures
Complete dentures are prosthetic replacements fabricated for completely edentulous patients. There are two impression taking procedures for the purpose of fabrication of master cast for complete dentures (1, 5, 12):
• Procedure 1 – Anatomical and functional impression - conventional procedure
Anatomical or situational impression is taken with a stock tray for edentulous jaws and with alginate as impression material. There are many known systems of stock impression trays for edentulous jaws, among which the most popular is the system according to Gutowski and Schreinemakers (12, 13).
Firstly, the proper size for the tray is selected, at least 3-5 mm wider than the denture-bearing area of the edentulous jaw. Its fit to the denture-bearing area is individualized by means of spacers made from a putty silicone or light curing acrylic resin, which create space between the tray and the denture-bearing area for loading impression material. Alginate is the impression material of choice for anatomical impressions, since its physical and chemical properties enable situational recording of all structures of the denture-bearing area at rest. After alginate is set, the border between displaceable and non-displaceable mucosa is marked on the impression, and it is also marked on the anatomical cast made from soft stone and used for fabrication of a custom tray. Such custom tray has no functional margins, but it extends up to the marked border of displaceable and non-displaceable mucosa, and it is used for functional impression taking.
Functional impression is made after static and dynamic checking of fit of the custom tray and its possible corrections. Firstly, 0.5-1 mm thick spacers made from a thermoplastic material or light curing acrylic resin are placed in the middle of the alveolar ridge of both the anterior and posterior area. In fabrication of spacers it is important to avoid areas of displaceable mucosa, in order to avoid consequential decubitus. The next step is taking impression of functional margins with a thermoplastic material, whereby the patient performs active, and the operator passive functional movements, for recording of frenulum and plica insertion. After their impression the custom tray should have a valve effect. Adhesive coating for elastomers is then applied to the whole surface of the tray (e.g. Tray Adhesive, Coltene, Switzerland), which enables perfect bonding to the tray. After the adhesive is dried, impression is made with a low viscosity elastomer, whereby condensation silicone Coltex Extra Fine (Coltene, Switzerland) is most frequently used. Functional impression with a low viscosity elastomer is used for precise marking of the denture-bearing area, as well as for registration of mucosal resiliency and correction of functional margins of a future complete denture. During elastomeric impression the patient performs active movements. Functional impression is finished when elastomer is set, except in case of functional impression of the upper edentulous jaw, where an additional 5 mm wide layer of aluminum wax is applied in the area of the vibrating line, in order to close the pharyngeal valve. The master cast showing functional margins is poured from hard stone in a dental laboratory.
• Procedure 2 – First and second functional impression
Insufficient precision of anatomical impression, especially in the area of functional margins, i.e. in the border area between displaceable and non-displaceable mucosa, often results from an imprecisely made custom tray. The consequences are necessary extensive corrections of the custom tray or application of a larger amount of material for taking functional impression, which can cause reduced valve effect of complete dentures.
Therefore, in modern complete denture treatment two functional impressions are made. The first functional impression is taken with stock tray for edentulous jaws. After selecting the correct size of the tray and fabrication of spacers for impression material, the material of choice is a high viscosity condensation silicone (Sta-Seal F, Detax, Germany). Its consistency and longer setting time enable the operator to perform functional movements during impression taking, which is not possible when using alginate, as in the conventional procedure. First the patient performs active movements, and then the operator performs passive movements. After the silicone is set, the impression is removed from the mouth, and labial and buccal functional margins are trimmed in width, since they are most often excessively extended due to high viscosity of condensation silicone. Then a low viscosity condensation silicone (Coltex Extra Fine, Coltene, Switzerland) is mixed and applied onto it, and impression is taken under the patient’s performance of active functional movements. The first functional impression is finished when the material is set, except in case of an upper jaw impression, where it is necessary to reline the vibrating line by means of aluminum wax (Figure 1). Based on this first functional impression, the first functional cast is poured from soft stone, and it is used for fabrication of the custom tray, which already has functional margins and a valve effect on the denture-bearing area (Figure 2). This is an important advantage compared to the conventional procedure, because the subsequent second functional impression enables more precise recording of the denture-bearing area and functional margins, using less material. More time is also left for the patient’s performance of functional movements, and the whole impression taking procedure is better controlled.
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Before the second functional impression taking, fit of the custom tray is checked by static and dynamic tests and possible minimal corrections are made. A 0.5-1 mm thick spacer is then made from a thermoplastic material or light curing acrylic resin. Functional margins are formed with a thermoplastic material under the patient’s active movements (Figure 3). A silicone adhesive layer is then applied on the custom tray and functional margins, and impression of the denture-bearing area, recording of mucosal resiliency and correction of functional margins are performed by means of a low viscosity condensation silicone. Finally, a 5 mm wide layer of aluminum wax is applied in the vibrating line area in case of the impression of an upper edentulous jaw (Figure 4). The second functional impression is used for pouring of the master cast from hard stone.
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Overdentures
Overdentures are fabricated in patients with a small number of remaining teeth, when a combined fixed-removable prosthetic replacement is not possible. The remaining teeth are prepared and used as abutment teeth for retention elements for overdentures. The most commonly used retention elements are conical or telescopic crowns, and bars and ball attachments are also often used. In today’s daily clinical practice overdentures are often supported by implants as abutment teeth for retention elements (5, 12-19).
In fabrication of overdentures the most frequently used procedure for making master casts (12, 18) is anatomical and functional fixation impression taking.
• Procedure 1 – Anatomical and functional fixation impression
In fabrication of overdentures, the first impression is an anatomical impression, which is taken with a stock tray for dentate or partially dentate jaws and alginate as the impression material (Figure 5). First, a tray of proper size is selected and spacers are fabricated, and then a mucostatic impression with alginate is taken (Figure 6). After alginate setting, the borders between displaceable and non-displaceable mucosa are marked and anatomical impression is poured from the soft stone. An open or closed custom tray is fabricated on the anatomical impression without functional margins, and it is used for taking a functional fixation impression.
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Functional impression in fabrication of overdentures is also called fixation impression, since it determines the position of retention elements on the remaining teeth or implants. It can be made with an open or closed custom tray. An open tray is most often used with conical or telescopic crowns and custom made bars on teeth. It covers the whole edentulous denture-bearing area to the border of displaceable and non-displaceable mucosa, and it also covers the remaining teeth and retention elements, except the vestibular side (Figure 7). This ensures a good control of fit of retention elements during impression taking. Impression taking with a closed tray is applied for all types of retention elements on teeth or implants, but their fit cannot be controlled during impression taking, and therefore displacements are possible (Figure 8).
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The procedure of functional fixation impression taking with an open tray starts with fabrication of spacers from a thermoplastic material or light curing acrylic resin on edentulous ridges of the denture-bearing area. The impression of functional margins is taken with a thermoplastic material, by means of the patient’s active movements and controlled passive movements of the operator. The impression of the denture-bearing area and correction of overdenture margins are made with zinc oxide eugenol paste or a low viscosity condensation silicone. The excess material in the areas of vestibular opening is removed in order to control the fit of retention elements to teeth. After the material is set and the fit of retention elements is checked, vestibular openings are closed by soft stone. After hardening of soft stone, a cover impression is taken with a stock or custom tray using elastomer, by means of which the functional fixation impression and vestibular fixation are removed from the mouth (Figure 9). The master cast is made from hard stone with fixed position of retention elements and visible functional margins.
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Impression taken with a closed custom tray covers the whole edentulous denture-bearing area and retention elements on teeth or implants to the border of displaceable and non-displaceable mucosa. Spacers are made after fitting of the tray and its possible corrections, after which functional margins are impressed with a thermoplastic material. As distinguished from impressions taken with an open tray, polyethers of middle or low viscosity are mainly used with closed trays, due to their physical properties (Figure 10). Namely, due to their hardness after setting, the retention elements stay fixed in the impression after its removal from the mouth. Therefore, after the master cast is made from hard stone with fixed retention elements and functional margins, it is necessary to fabricate a control key from a chemically curing acrylic resin (Pattern Resin, GC, Germany), which is used for control of uniformity and precision of the relationship and fit of retention elements between the master cast and mouth. This is a precondition for making precise overdentures.
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Conclusion
Functional impression taking is one of the most important clinical stages in fabrication of complete dentures and overdentures. It records the denture-bearing area of the denture base, registers mucosal resiliency and shows functional margins of the denture. Although there are references in the literature regarding the lack of scientific evidence for the importance of functional impressions for the success of complete denture and overdenture treatment (36, 37), this paper argues that standardization of their procedures and use of proven techniques and materials ensures good retention and stability of complete dentures and overdentures. By means of such dentures, the patients’ lost occlusal, masticatory, esthetic, physiognomic, phonetic and psychosocial functions can be restored.
