Portamprente individuale în tehnologia protezei totale

Introduction

Currently, the condition of total edentulism can be considered a pathological condition, no longer considered to be the result of an involution related to senescence. We consider, first of all, the fact that chronological age no longer corresponds to biological age.

In dentate patients, the dento-maxillary system functions optimally due to the presence of teeth and adequate dento-periodontal support. In completely edentulous patients, all functions are impaired as a result of the loss of teeth, dento-periodontal support and alveolar bone. Tooth loss is accompanied by resorption of the alveolar process which necessitates double prosthetic rehabilitation; complete dentures are volumetrically larger when compared with fixed or removable prostheses^(1-6).

In recent years, dentistry has evolved significantly in terms of both materials and technologies; nevertheless, there are still numerous cases in which the only viable treatment option remains the conventional complete denture. It can be stated with certainty that there is currently genuine competition between modern implant-based prosthetic procedures and classical prosthodontic approaches, with the completely edentulous patient having far more options for prosthetic rehabilitation than in the past.

While the longevity of fixed implant-supported prosthetic restorations has increased considerably, the number of highly complex cases in which implant-prosthetic rehabilitation is not indicated has also risen. Such cases must be managed using particularly labor-intensive conventional methods, that place substantial demands on both the patient and the dental team, including the dentist and the dental technician^(1-6).

General data regarding the manufacturing process of custom impression trays

The manufacturing of the custom impression tray is a mandatory step in obtaining a faithful copy of the prosthetic field, which is made on the preliminary model. As a definition, the preliminary model is the raw positive copy of the prosthetic field, in which the support area and partially the suction area are mostly reproduced. In order to obtain custom impression tray for the manufacturing of acrylic complete dentures, several preparations are made on the preliminary model made of type 2 dental stone, as follows^(1-6):

• drawing the peripheral outline of the prosthetic field;
• filling retentive areas;
• applying a plaster-separating solution on the surface of the model to prevent the tray material to stick to the dental stone.

The custom impression tray, having a primary role in the technological flow of manufacturing a complete acrylic denture, must have the following characteristics^(1-6):

• rigidity, an essential characteristic during the impression;
• mechanical resistance, with the role of maintaining the functional impression material in contact with the prosthetic field;
• the impression tray should cover the entire surface of the prosthetic field;
• fitting on the level of the support and suction area;
• in the case of the custom mandibular impression tray, pressure buttons should be placed;
• the edges with a thickness of 1.5-2 mm should be rounded and smooth;
• the placement of the handle should be on the midline, with a size equal to the width of two upper central incisors;
• simple technology of manufacturing;
• to have a cost price as low as possible.

Practical aspects in manufacturing custom impression trays for completely edentulous patients

Next, three common methods of manufacturing custom impression trays will be presented, in the case of completely edentulous patients.

a) Manufacturing a custom impression tray from light-curing composite resin

In the first stage, the peripheral outline of the future custom impression tray is drawn on the preliminary model. A plaster-separating solution is applied on the surface of the model to prevent adhesion between tray material and dental stone (Figure 1).

The light-curing material for the impression trays, that is sold in boxes that prevent contact with light, is applied on the preliminary model and carefully pushed to the outer limits of the prosthetic field without affecting the thickness of the material (Figure 2). The excess material is trimmed with a spatula till the line drawn on the model (Figure 3).

The handle of the impression tray is created in the last stage, using the excess material removed before. The handle has generally the width of two upper central incisors and placed on the midline, almost in the same position of upper central incisors (Figure 4).

The material offers to the dental technician sufficient working time, a major advantage of light-curing materials. After modeling, the custom impression tray is light cured using an UV curing oven built specifically for dental technology. The oven is equipped with reflective surfaces and a transparent lid, which prevents ultraviolet light from escaping from the polymerization chamber (Figure 5).

The setting of the material is followed by post-processing stage. Using different types of burs, the margins of the impression tray are rounded to prevent discomfort during the impression procedure (Figure 6).

b)  Manufacturing a custom impression tray from self-curing acrylic resin

Currently, custom impression trays made from self-curing acrylic resins are preferred to other types, due to the advantages they offer: resistance, precise fitting, they do not condition the impression material, and they do not require a specific device for curing^(1-6).

The process begins with highlighting the peripheral outline with a pencil to help the dental technician visualize the limit of the future custom impression tray (Figure 7). A plaster-separating solution is applied on the model to prevent adhesion of the acrylic resin.

The acrylic resin powder and liquid are dosed and mixed until it has a dough-like consistency (Figure 8). In this form it is applied on the preliminary model and, using light pressure, it is spread over the entire surface of the model till it covers the area drawn with the pencil, slightly overlapping it (Figure 9). The excess material is removed with a spatula and reused to make the handle and the push buttons.

The handle of the tray is placed on the midline while the push buttons are placed on the most sloping areas (left and right), to stabilize the tray during the impression process. To increase the resistance of the tray, a stainless-steel wire can be inserted in the mass of the material, on the bucco-lingual center of ridge crest. After modeling, the tray is left for setting of acrylic material for about 10-15 minutes. When the material has set, the marginal areas and the accessories of the tray are finished, any sharp areas rounded (Figure 10).

c) Manufacturing custom impression tray using 3D printing resins

As a result of Francois Durret’s research, digital impression represented an alternative to conventional impression methods^(1-6). At present, there is the possibility that certain indirect restorations, namely fixed prostheses, can also be made using CAD/CAM technology. Regarding the manufacturing of a complete dentures, it is necessary to “take into account certain characteristics of the prosthetic field which favor a certain protocol”. In fact, there are two digital technologies used to obtain a complete denture^(1-6):

• additive technology (printing);
• subtractive technology (milling), the dental technician creating a virtual design (CAD) of the restoration, which will be processed through one of the previously mentioned technologies^(1-6).

In the first stage, the mandibular and maxillary models were scanned using a dental laboratory scanner to obtain virtual models (Figure 11). Using a design program, custom trays were virtually designed on the same aforementioned principles (Figure 12).

The virtual trays are exported to the program of the printing machine, the dental technician arrange them to fit the printing platform, and the program creates additional supports to prevent deformation during the printing process (Figure 13).

In addition to the data mentioned before, the type of resin and the printing resolution are set in the printer program. The program determines the number of layers required and starts the printing process (Figure 14).

After printing, the custom impression trays were cleaned of excess unpolymerized resin using isopropyl acid in a special device and subsequently further poly­merized, according to the manufacturer’s specifications (Figure 15).

The two custom impression trays are then checked for fitting on the preliminary models, and any sharp edges will be rounded off (Figure 16).

Discussion

Technological advances in dentistry have made possible the treatment of edentulous patients with various fixed prosthetic solutions^(7,8). However, not all patients can benefit from such treatments, the limitations being either financial or morphological^(9,10). Strictly speaking of the morphological aspect, there are clinical situations in which bone support is insufficient even in the case of bone additions or general conditions prevent surgical interventions^(11,12).

Traditional treatment methods remain a viable option for complete edentulous patients, with current techniques being much improved to provide predictable results. Current practitioners – both dentist and dental technicians – use established materials such as self-curing resin and light-curing resin in the manufacturing of custom trays, an extremely important step in the technological flow of complete dentures^(13,14). Each of the materials mentioned has advantages and disadvantages at the level of working procedures, but when used skillfully, they provide remarkable final results, especially felt by patients^(15,16).

However, medical teams have been able to successfully integrate new materials on the market, such as resins used for 3D printing⁽¹⁷⁾. Their use has led to increased accuracy, but also to a reduction in the actual working time of the dental technician. This results in an improvement in the quality of life for both the patient and the medical team. Being a process that is not fully automated, the experience and skill of the dental technician are amplified by high-quality materials and equipment.

Conclusions

Complete denture prosthodontics represents one of the greatest challenges faced by the dentist or the dental technician team, as total edentulism provides the fewest reliable reference data and the most unfavorable conditions for prosthetic rehabilitation. For these reasons, complete denture treatment is, more than other forms of dental therapy, both a science and an art, requiring extensive knowledge, a thorough understanding of the clinical and therapeutic stages, and their rigorous observance.

Total edentulism most often represents a state of disability for patients, as all functions of the dento-maxillary apparatus (mastication, esthetics and phonation) are lost; like any form of disability, it primarily affects the patient’s psychological well-being. Consequently, close collaboration with the patient is essential for both the dentist and the dental technician, and must be founded on understanding, respect and patience.

Technology applied in dental medicine has evolved exponentially over recent decades. This progress has been driven by the development of new dental materials and the introduction of increasingly advanced equipment. Consequently, certain treatment concepts have changed, and new techniques and therapeutic procedures have emerged. Specifically, each stage within the extensive technological process of complete denture fabrication is of great importance, and must be approached with the highest level of professionalism.

The manufacturing of the custom impression tray represents an essential step in the process of manufacturing acrylic complete dentures. This step enables the accurate recording of the patient’s oral anatomy, thereby ensuring a prosthesis that integrates rapidly into the oral cavity and provides patient comfort. The selection of appropriate materials and strict adherence to the fabrication protocol are crucial to the success of prosthetic treatment, while careful monitoring of each procedural stage leads to a satisfactory final outcome. Any error originating at this stage may ultimately result in a poorly adapted acrylic complete denture, patient discomfort and potential functional impairments.

It should not be overlooked that the success of prosthetic treatment also depends on a thorough understanding of the elements and structures that constitute the prosthetic field. The completely edentulous patient must adapt to the new complete denture both biologically and psychologically.

From a biological perspective, the acrylic complete denture comes into contact with a region that is strongly innervated by the trigeminal nerve. The presence of a foreign body within the totally edentulous oral cavity is associated with the generation of various stimuli, in which both exteroceptors and proprioceptors are influenced by the denture’s volume, position, pressure and mobility.

With regard to psychological impact, there are patients who experience difficulties adapting to a complete denture, regardless of how accurately it has been fabricated. In certain situations, particularly in the mandible, when an unfavorable completely edentulous prosthetic field is present, complex prosthetic problems may arise.  

Acknowledgement: Viorel Ștefan Perieanu and Mădălina Adriana Malița are corresponding authors (Viorel Ștefan Perianu: viorel.perieanu@umfcd.ro; Mădălina Adriana Malița: madalina.malita@umfcd.ro).

CONFLICT OF INTEREST: none declared.

FINANCIAL SUPPORT: none declared.

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