Clinical Negative Pressure Measurement after Border Molding Procedure

Introduction: Border molding of the custom tray’s edge is an important stage in the treatment of an edentulous jaw that determines the stability of prosthesis at rest and function. Solid, thermoplastic and silicon impression materials may be used in border molding. After bolder molding procedure, the negative pressure between the custom tray and the prosthetic field is created. This is an informal indication for a good impression. Aim: To compare the negative pressure created after border molding procedure with different impression materials. Materials and methods: 35 patients (17 men and 18 women) aged 51 to 87 years with a complete edentulous upper jaw were examined. New clinical method for negative pressure measurement was created. We used a special custom tray with palatal adaptor and a pump. Two groups of impression materials were tested: thermoplastic (Kerr impression compound green sticks, GC Iso functional sticks) and silicones (Detaseal function, Sta-seal f). Statistical analysis was performed using ANOVA, confirmed by the absolute value analysis used to compare negative results, and a log transformation analysis for greater precision and also for negative data comparison. Results: A statistically significant difference was found between the two thermoplastic materials – GC Iso functional sticks and Impression compound green sticks. The mean group difference between these materials was 0.049 bars. There was no statistically significant difference between the other groups of materials. Conclusion: Quantitative measurement of negative pressure, created between the custom tray and the prosthetic field is entirely possible under clinical conditions.


INTRODUCTION
Border molding of the custom tray's edge is an important stage in the treatment of an edentulous jaw that determines the stability of prosthesis at rest and function. Applying the border molding reduces the deformation of the alveolar crest and the border of the soft tissues, resulting in an increased retention and stability.
Impressions for complete dentures fabrication can be made by minimal (mucostatic), maximal (biting) and functional pressure. Minimal pressure technique does not interfere with the blood supply and tissue-fluid circulation, but the border zone cannot be impressed precisely. The technique with maximal pressure improves the border's area shaping. The functional pressure technique is a successful solution to the problems with harmful pressure and incorrect impressing of the border zones. 1 Selective pressure concept is invented by Boucher in 1950. It combines the mucosta-tic and mucocompressive techniques. The mucosa of the alveolar ridges is thicker and can withstand the pressure, whereas the mucosa, covering the palate and a midline is thin and non-resilient. 2 The greatest value was measured in the midline, followed by palate slopes and the alveolar ridge. 3 According to another study, the greatest value was in the incisive papilla, the distal part of the midline and the deepest part of the palate. 4 According to this concept, the custom trays were made with different design to relieve the stress-bearing area -with relief (vent) holes, spacers, stops in the canine and first molar zones. 5 The pressure values depend on the type of impression materials used. The pressure is not equal during the elastification of the materials. It decreases by almost 50% during the pressure of 0.5 kg. 6 In a similar study, with a pressure of 1 kg/cm 2 the decrease was more than 80%. 7 Selection of impression materials depends on the anatomical features of the prosthetic field. Hard, thermoplastic and silicon impression materials may be used for border molding. 8,9 Thermoplastic materials include impression compound, waxes and wax masses. They all have a certain operating temperature range. 10,11 According to Voronov, they can be classified as reversible and irreversible. 12 Silicon impression materials for border molding are a separate group, having prolonged manipulation time and appropriate viscosity which allows the performance of the functional tests. 13,14 The most commonly applied tests are the Herbst tests for border molding. 15 Determination of the boundary of the prosthesis is influenced by anatomical structures -muscles, muscular and soft tissue gripping. 16,17 In the distal region, the transverse fold is located in the paratuberal space to facies infratemporalis maxillae. An asymmetry between the left and right halves of upper and lower jaws has been established. 18,19 After border molding procedure, the negative pressure is created between the custom tray and the prosthetic field. This is an informal indication for a good impression.

AIM
To compare the negative pressure created after border molding procedure with different impression materials.

MATERIALS AND METHODS
Thirty-five patients (17 men and 18 women) aged 51 to 87 years with a completely edentulous upper jaw were examined. Two groups of impression materials were evaluatedthermoplastic and elastomers. We used four different brands: • Detaseal function -additive silicone for border molding • Sta-seal F -condensation silicone for border molding • GC Iso functional sticks -synthetic resin for border molding • Kerr Impression compound green sticks for border molding A new clinical method was created for negative pressure measurement after border molding procedure. For this purpose, a special custom tray from a light-curing base plate with palatal adapter was made. This was 90°, 7 mm metal adapter, which was fixed to the midline on the palatal slope. For its fixation, a 0.7-mm wire was used for the retentions in the base plate. With this adaptor, it was possible to link the custom tray and a pump for pressure. The small distance between the base plate and the adaptor was filled with hot technical silicone (Figs 1, 2).
A combined pressure pump was used to create and measure the negative pressure. The maximum value was 3  bars for positive pressure and -1 bar for negative pressure. The working part was connected with a 7-mm plastic tube which was the connection between the pump and the custom tray (Fig. 3).
For each patient, four individual impression trays were made with a palatal adapter. Following chemical disinfection of the elements via Zhivasept rapid, the composition was clinically tested. For border molding, the functional tests of Herbst were used. For all materials the following protocol was followed: 1. Applying the impression material along the edge of the individual tray; 2. Insertion, positioning and performance of the Herbst functional tests; 3. Waiting for the elasticity or hardening of the material; 4. Assembling the clinical unit for negative pressure measurement; 5. Measuring the negative pressure, which was created between the custom tray and the prosthetic field, and recording the result; 6. Releasing the individual impression tray from the patient's mouth (Fig. 4).

Detaseal function (Detax)
The tested additive silicone had an easy clinical protocolmixing and application, shaping without any unnecessary effort from the patient. The working time is 5-6 min., which is absolutely enough for border molding procedure. There were difficulties in keeping it on the edge of the custom tray, and part of the alveolar ridge was also printed. After the procedure the extra parts were precisely cut (Fig. 5).
After a clinical border molding, a minimum negative pressure value of -0.2 bar and a maximum of -0.5 bar was

Sta-seal f (Detax)
Condensation silicone due to its moderate rigidity allowed mixing both with a wide spatula on a paper block and between the fingers of the clinician. Functional tests were easily performed and in timely fashion. The material, however, due to the average viscosity flow, also covered part of the alveolar ridge. Again the extra parts were cut (Fig. 6).
The minimum set value of the negative pressure for the condensation silicone test is -0.2 bars, the maximum is -0.6 bars. The average value was -0.36 bars. Figure 6. The custom tray after border molding with condensation silicone Sta-seal f.

GC Iso functional sticks (GC)
The synthetic resin showed very good manipulative properties. After heating, it became sticky and plastic, allowing it to be easily and finely applied to the edge of the custom tray. After performing the Herbst tests, a very good shape of the edge was observed without protruding the material to the prosthetic field (Fig. 7).
After the clinical measurement of the negative pressure created by the border molding with the synthetic resin, a minimum value of -0.1 bar (on a highly resorbed alveolar ridge) and a maximum value of -0.55 bar were recorded.
The average value for this material is -0.38 bar.

Impression compound green stick (Kerr)
The impression compound showed good manipulative qualities. Unlike the synthetic resin, it was not very sticky and covered a wider layer. The greater rigidity of the material required a difficult performance of the functional tests. The impression compound was formed only on the edges of the custom tray, with a slight protrusion into the area of the alveolar ridge (Fig. 8).  After clinically shaping and measuring the resulting negative pressure, a minimum value of -0.2 bars and a maximum value of -0.5 bars were observed. The average value was -0.34 bars.
All results are summarized in Table 1 and Fig. 9.
For the statistical analysis, ANOVA was used, confirmed by an absolute value analysis used to compare the negative results, and a log transformation analysis for greater precision as well as for comparison of negative data.
A statistically significant difference was observed be- Sta-seal f -Impression compound 0.02714 No tween the two thermoplastic materials -GC Iso functional sticks and Impression compound green sticks. The mean group difference between these materials was 0.049 bars. There was no statistically significant difference between the other groups of materials. The difference ranges were from 0.007 to 0.041 bars ( Table 2).
The clinical protocol of handling the two thermoplastic materials shows some differences. The plasticizer temperature of the synthetic resin GC Iso functional sticks is lo-  wer (approximately 40°C) and its manipulative properties are retained for a longer period, which is due to the better adaptation of this material. The temperature for impression compound plasticization is higher (approximately 50°C) and under the conditions of the oral cavity it quickly passes to a solid state. These manipulative differences explain partly the resulting statistical difference. The measured mean negative pressure values created between the prosthetic field and the custom tray showed close values for each patient -with a difference of -0.05 to -0.1 bar. This showed that the anatomical features of the prosthetic field were of great importance.

DISCUSSION
The creation of the negative pressure with the pump wasn't harmful for the mucosa. 1 This new method can be used after elastification or hardening of the impression material, therefore we couldn't measure the pressure in different phases of the elastification reaction. 6,7 The anatomical features of the prosthetic field are very important for the pressure creation. According to our study the difference between the pressure, created with tested impression materials on the same patient was from -0.05 to -0.1 bars. 8,9 Herbst's tests are appropriate to shape the gingivo-buccal sulcus and to close hermetically the space between the prosthetic field and a custom tray. [15][16][17] Unfortunately, we couldn't find a similar study in the literature to compare our pressure values.

CONCLUSION
Quantitative measurement of negative pressure is entirely possible under clinical conditions. Thermoplastic materials for border molding are retained and formed only along the edge of the custom tray. Silicone impression materials spread not only on the edge of the custom tray, but also on the alveolar ridge.
We couldn't find a similar study in the literature with which to compare our results.