The role of impression in the technological flow of removable dentures

Introduction

The functional and esthetic rehabilitation of a complete or partially edentulous patient includes a wide range of treatment modalities with various degrees of invasiveness and reversibility, but also with risks and benefits that appear with the establishment and choice of the treatment plan. In the past, patients had low expectations for the esthetic and functional results of conventional removable dentures.

However, nowadays these expectations have changed, which has led to an increase in the number of patients who want a better esthetic appearance of their future dentures. Nowadays, dentistry has evolved a lot, but along with it, the expectations of the modern man regarding the appearance of his own smile have also increased. Wearing dentures, especially removable ones, can be a difficult thing to accept, despite the fact that the patient’s dental and clinical situations do not allow another treatment option^(1-5).

Thus, manufacturing a functional and esthetic removable denture requires a joint effort both from the dental team (dentist, dental technician) and from the patients.

General data

The correct impression in complete or partial prosthodontics is an essential stage in the manufacturing of functional dentures. Through an impression, all the necessary information is sent form the dental office to the dental laboratory, for creating the individual impression tray, with the help of which a much more accurate impression can be obtained comparing to a standard impression tray. Using the functional impression made with the help of the individual impression tray, the functional model is obtained, on which the occlusal registration rims and later on the trial denture will be made^(1-5).

Before actually moving on to case presentation, the main notions that will be approached in this material must be explained very briefly.

The preliminary impression is a negative copy of the support area of the prosthetic field and partially the suction area. The maxillary complete denture is suppor­ted on the surface of the palatal vault, on the edentulous ridges and tuberosities. By contrast, the surface of the mandibular prosthetic field is much smaller, the denture resting on the edentulous ridges and pyriform tubercles. All these elements make up support areas and also provide horizontal stabilization of the denture^(1-5).
 

The preliminary model represents the positive copy of the prosthetic field, which is accurately reproduced, while the suction area is rendered approximately. It is made of dental stone, preferably type II (dental plaster for models) and type III, to complete the examination of the prosthetic field (provides a clear perspective and all its component elements) and to make the individual impression tray, a necessary and indispensable element to obtain a faithful copy of the prosthetic field^(1-5).
 

The individual tray represents the rigid support of the functional impression material. It corresponds to the individual characteristics of each prosthetic field, and it is used only for a single patient. The individual trays are used in the technological flow of complete or partial dentures. The materials used to make individual impression trays can be: self- or heat-curing acrylic resins, light-curing plates, and shellac base plates^(1-5).

The functional impression, also known as the final impression, presents with maximum fidelity the area of the prosthetic field in its entirety: the support area and the suction area. It can fully value all factors that participate in the maintenance and stability of the denture on the patient’s prosthetic field. The final imprint provides the necessary conditions for the distribution and balance of masticatory pressures over the entire surface of the prosthetic field, the exercise of phonetic function, and the restoration of physiognomy. These necessary aspects are achieved by final impression if special materials are used, supported by the individual tray^(1-5).

The functional model is an accurate copy of the edentulous prosthetic field. It faithfully renders the surface of the supporting area covered by the fixed mucosa and the suction area covered by the neutral mucosa. The functional model is made of hard materials, resistant to the processes that will be performed during the laboratory phases in which the complete denture is made^(1-5).

Starting from all these previously mentioned details, the following cases will emphasize the importance of the preliminary impression, as well as the definitive impression in the manufacturing of partial and complete dentures.

Case presentations

Clinical case 1. Imprinting of the maxillary fully edentulous prosthetic field

A 55-year-old patient presented to the dental office following the loss of all masticatory units, reaching the stage of maxillary total edentulousness, and the masticatory efficiency being minimal. For this purpose, the doctor decided as a treatment plan a complete flexible denture made of Bio Dentaplast. The maxillary preliminary impression was made in a standard impression tray made of plastic material, with an alginic impression material. Using this impression, the preliminary model will be cast and used to create the maxillary individual impression tray.
 

At the level of the maxillary preliminary impression, the following aspects were observed:

• Areas where the standard impression tray shows through.

• The impression material is not evenly distributed across the standard impression tray.

• There is insufficient impression material in the area of the left buccal vestibule.

• Presence of air bubbles on the surface of impression material.

• The impression tray seems off-centered, being placed more to the left side of the prosthetic field.

The maxillary preliminary model was cast using type III dental model.

After trimming, the dimensions of the maxillary preliminary model were reduced, but the labial and buccal vestibule areas were protected. A light-curing resin plate was applied over the preliminary maxillary model, which was then adapted to the prosthetic field in order to make the individual impression tray and, from the excess material, the handle of the individual tray was also made. The excess of light-cured material was removed, respecting the limits of the prosthetic field. Subsequently, the individual impression tray was polymerized and processed so that it respected a uniform thickness, and its edges did not harm the patient when inserted into the oral cavity, when functional imprinting the prosthetic field.
 

In the next stage, in the dental office, the maxillary functional impression was made, using a condensation silicone material of light body consistency.
 

Following the analysis of the maxillary functional impression, the next aspects can be noticed:

• The presence of air bubbles indicates the incorrect homogenization of the silicone impression material.

• Tears in the impression material are observed.

• There are portions through which the individual tray shows through, the silicone layer being thin; this fact reduces the accuracy of the functional impression.

• The vestibular areas of the tray have a thin layer of light body silicone, indicating a labial and buccal extension of the individual tray near the limit of the prosthetic field.

The maxillary functional model was cast using type IV dental stone. The model was trimmed, keeping a part of the base higher than the labial and buccal vestibule, for a better reproduction of the prosthetic field. This configuration of the maxillary functional model contributes to the external closure of the denture, which is provided by the cheeks and lips, being a significant factor in maintaining the denture.
 

The analysis of the prosthetic field based on the maxillary functional model can be described as follows:

• A normal shape of the arch.

• Symmetrical arch.

• The residual alveolar ridges are well represented, having medium width and show retentiveness at the level of the labial vestibule area.

• The palatal vault has medium depth and no palatal torus.

• The maxillary tuberosities are neutral and do not show retentiveness.

• The incisive papilla is placed in the middle of the ridge, which indicates a significant horizontal and vertical resorption, involving the labial segment of the alveolar ridge in the frontal area.

Clinical case 2. Imprinting of the maxillary subtotal edentulous prosthetic field (Costa classification system)

A 59-year-old patient presented in the dental office to prosthetically restore the maxillary subtotal edentulous prosthetic field, resulting from a periodontal disease. Only the right second premolar and the left canine are still present on the maxillary arch. In order to solve this case, the dentist designed a treatment plan, which consists in manufacturing a flexible maxillary partial denture. The maxillary preliminary impression was made in a standard metal tray using irreversible hydrocolloid (alginate).
 

Following the analysis of the preliminary maxillary impression, the next aspects were observed:

• Uneven distribution of impression material.

• Irreversible hydrocolloid porosity indicates the incorrect mixing of the impression material.

• Presence of air bubbles at the surface of the impression material.

• Irregularities in the structure of the impression material in the distal area – the patient did not rinse his mouth before making the preliminary impression.

The maxillary preliminary model was cast using type III dental stone.
 

The impression was removed from the casted model after the setting of dental stone, and a good reproduction of the prosthetic field was observed. After trimming, the dimensions of the base of the model were reduced. On this preliminary model, the individual maxillary impression tray will be made for the functional impression. Before applying the light-curing resin plate on the preliminary maxillary model, it required a block out process. All undercut areas, labial vestibule and labial face of remaining teeth were covered using wax for an easy insertion of the tray.
 

The main goals in the processing stage of the individual tray were to obtain a uniform thickness and rounded edges that would not interfere with the final impression procedure of the prosthetic field. Later, the individual tray was tested for fitting in the oral cavity and, after the necessary adjustments, it was moved to the next stage, namely the final impression. Using condensation silicone material in two consistencies, light body and super light body, the maxillary prosthetic field was imprinted.
 

When analyzing the maxillary functional fingerprint, the following aspects were found:

• To increase accuracy, two types of silicone impression materials, light body and super light body, were used.

• The thickness of the impression material layer at the level of the labial and buccal edges was quite thin, which indicated too little amount of impression material used to cover these areas.

• The impression material had an optimal consistency, without bubbles or streaks.

• At the level of the vibrating line, there were some areas where the material had no continuity, but also areas where the material was extended beyond the limit of the prosthetic field.

• The remaining teeth were correctly imprinted.

The maxillary functional model was cast from type IV dental stone, and the trimming process preserved the configuration of the labial and buccal vestibule visible.
 

Following the study of the maxillary functional model, the analysis of the prosthetic field highlighted the next aspects:

• The shape of the dental arch was a tapered.

• The arch was not symmetrical.

• The edentulous ridges were well represented in both height and thickness, showing vestibular retentiveness.

• The depth of the palatal vault was normal; it did not present a palatal torus.

• The left maxillary tuberosity was well expressed, showing a recent extraction; the right maxillary tuberosity was obliterated, not being useful for prosthetics.

Discussion

Completing and following the work steps are essential in the manufacturing of total or partial dentures, in order to obtain results that restore the functions lost by the patient due to missing teeth.

The first steps in the technological flow have the main purpose of imprinting the mucosal and bone support as accurately as possible, so that the forces developed during the masticatory process are distributed over a surface as large as possible. Thus, the pressure on the supporting structures is lower, which creates comfort for the patient and at the same time decreases the possibility of bone changes that may occur^(6,7).

For these reasons, the preliminary impression must record as much detail as possible of the prosthetic field, maxillary or mandibular, within the limits of using standard trays in the impression process. In certain situations, corrections can be made at this stage^(8,9). It is important that the material for the preliminary impression is correctly chosen and especially correctly prepared. Any change in consistency can lead to incomplete imprinting, which will affect the subsequent treatment steps^(7,10).

The preliminary or diagnostic model is used to make an individual tray, adapted for each clinical case and which eliminates the inconveniences produced by the standard trays^(11,12). That is why it is necessary for the preliminary impression to be correctly analyzed in the office, in order to mark the transition zones between the fixed and mobile mucosa, essential elements for the stability of the prosthesis^(13-15).

The design of the individual tray, shape and components has the role of facilitating the imprinting of a surface as extensive as possible from the fixed mucosa on which the future prosthesis will rest, but also to properly avoid extension in areas of mobile mucosa, a fact that can create discomfort to the patient during the impression stage^(16,17). Thus, the information transmitted from the preliminary impression phase must be strictly followed for the best possible results. The correctness of the design of the individual tray is analyzed by trying it in the oral cavity and confirmed by the quality of the functional impression^(18,19). The impression material must cover the entire internal surface of the individual tray, without exposed areas, thus ensuring a correct and effective impression of the prosthetic field for the edentulous patient^(20,21).

Conclusions

Following the reading of this material, several aspects can be concluded, as follows:

• The essential stages in the manufacture of removable dentures are the impression and casting of the preliminary model, on which the individual impression tray will be made.

• The impression materials can be found in several forms of presentation – silicone, alginic, thermoplastic; each has its own disadvantages and advantages, and are characteristic only of a certain stage of imprinting.

• Preliminary and functional impressions must be analyzed to see if the surface of the prosthetic field has been completely recorded.

• Preliminary impressions accurately record the support zone and approximately the marginal closure zone. A good preliminary impression is essential for making the most accurate impression tray.

• Drawing the boundaries of the future individual impression tray at the periphery of the prosthetic field is an important stage that must be carried out carefully. The model is examined at the level of the entire surface and the contour of the periphery of the prosthetic field that was printed from the impression, this contour being accentuated by thickening with the help of the chemical pencil.

• Functional impressions accurately record both the support and marginal closure areas, using the individual tray, specific materials and impression techniques that shape the peripheral area.

• For economic reasons, the preliminary model is obtained from type III dental stone, while the functional model, which requires increased accuracy, is made from type IV dental stone.

• The functional impression in the case of a complete edentulous patients plays an essential role in the bio­dynamics of the denture. The accurate reproduction of the prosthetic field on the mucosal surface of the prosthesis base ensures:

  • Maintenance – by adhesion and suction.

  • Optimal support – through a good distribution of forces on the surface of the prosthetic field.

  • Stabilization – by recording all the details of the prosthetic field, and especially the correct impression of the height of the alveolar ridge.   n

Corresponding author: Viorel Ştefan Perieanu, e-mail: viorelperieanu@yahoo.com, Bogdan Alexandru Dimitriu, e-mail: bodgan.dimitriu@umfcd.ro

CONFLICT OF INTEREST: none declared.

FINANCIAL SUPPORT: none declared.

This work is permanently accessible online free of charge and published under the CC-BY.