Effect of traction direction and pressure load on the palatal plate on retentive force

The following four types of test samples were used in this study: cream-type denture adhesive (NP; New Poligrip® Sa; Glaxo Smith Kline, Tokyo, Japan), gel-type denture adhesive for dry mouth (DM; Pitatto Kaiteki Gel®; Nippon Shika Yakuhin Co., Ltd., Shimonoseki, Japan), gel-type oral moisturizer (BT; Biotene Oral Balance Jell®; T&K, Tokyo, Japan), and cream-type denture moisturizer (DW; Denture Wet®; DentCare Pvt., Ltd., Kerala, India).

A palatal plate was fabricated using a 3.0-mm sheet of thermoplastic resin (Erkodul®; Ercodent, Yokohama, Japan) from a working cast made by taking an impression of an upper dentulous jaw model. The plate was shaped such that it was 1.0 mm from the tooth cervices, and the rear edge was distal to the left and right second maxillary molars. A traction ring was made from 0.9-mm diameter Co-Cr alloy wire (SUN-COBALT CLASP-WIRE®; Dentsply Sirona, Tokyo, Japan). This was placed in the center of the palatal plate (on the intersection of the midline and the straight line connecting the central fossa of the bilateral first molars) (Fig. 1).

A digital force gauge (Digital Force Gauge RZ-5®; AIKOHENGINEERING, Tokyo, Japan) used as the measuring device (Fig. 2). We applied 2.5 g of the test sample to the inner surface of the palatal plate. Then, the plate was pressure perpendicular to the occlusal plane to the model for 10 s while the load was defined on a weighing instrument. A device indicating the direction of traction was mounted on the occlusal surface of the left first and second molar prosthesis of the model, and traction was exerted in a specified direction at a speed of 0.5 N/s using a digital force gauge. The force at which the palatal plate detached from the model was measured as the retentive force. The subsequent two experiments were conducted to investigate the influence of the direction of traction and load on the retentive force of the palatal plate. Only one palatal floor and one model should be prepared to avoid inconsistencies in the model's shape and other conditions for each measurement. The palatal plate and the model were washed after each measurement, and the following test sample was applied and measured without remnants from the previous sample.

The palatal plate coated with the test sample was mounted onto the model, and a pressure load of 2.5 kgf was applied to the centre of the palatal plate on a weighing instrument (Fig. 3). Then, the digital force gauge was used to measure the retentive force by pulling in 45°, 60°, and 90° to the occlusal plane (Fig. 4). This was done six times (from pressure contact through traction to detachment) in each of the three directions. The first measurement was excluded, and the results from the second to the sixth measurements were used. The first measurement was excluded because it was not stable [18], and the average of the second through sixth measurements was obtained.

After six measurements, the sample was rinsed completely. Six consecutive measurements were taken three times, and three averages were obtained for the three sets of measurement. Finally, the average of the three averages was calculated.

The palatal plate with the test sample applied was mounted on the model, and pressure loads of 1.0 kgf, 2.5 kgf, 4.0 kgf, and 5.5 kgf were applied to the center of the palatal floor, on a weighing instrument. Then, a digital force gauge was used to measure the retentive force by pulling perpendicularly to the occlusal plane. The retentive force was measured for each of the four conditions of pressure load.

SPSS Statistics 27.0 (IBM Corp, Armonk, NY, USA) was used for statistical analysis, and Tukey's method was used after one-way analysis of variance for multiple comparisons. All significance levels were set at 5%.

There was no significant difference in retentive force between traction in the 45° and 60° directions for any of the test samples (p < 0.05; Table 1). The 90° traction direction showed the highest retention force with DM. When BT was used, the traction direction of 90° showed significantly higher retentive force than directions of 45° (p < 0.05) (Fig. 5).

In all cases, the higher the pressure load, the higher was the retentive force (Table 2). When DM was used, there was no significant difference between 4.0 and 5.5 kgf, while the retention force was significantly higher (p < 0.05) at higher pressure loads among the other loading conditions. Similar results were obtained when NP and DW were used. When BT was used, significantly higher retention was observed at a greater load (p < 0.05) (Fig. 6).

As in the previous study [18], there were four types of test samples: a cream-type denture adhesive, a gel-type denture adhesive for dry mouth whose main ingredient was water-soluble, an oral moisturizer, and a cream-type denture moisturizer with an oil-based ingredient. Yamagaki et al. reported that high-viscosity oral moisturizers have the same retentive force as denture adhesives [14]. In this study, BT, with high viscosity, was selected among oral moisturizers. DW is a substance applied to the denture surface to reduce friction against the oral mucosa during wearing.

The retentive force can be measured stably by filling the space between the model and the denture base’s mucosal surface with the test sample [18]. In preliminary experiments, the amount of sample applied that could fill the inner surface of the palatal plate without excess or deficiency was studied, and consequently 2.5 g was used as the set amount in this study.

The palatal plate was designed to be 1 mm inward from the tooth cervices so that the floor area would be as wide as possible, without interfering with the prosthesis during traction. In addition, if the traction ring was higher than necessary, the backward detachment force during traction becomes large, and the retentive force may not be measured accurately. Therefore, we fabricated it with a height of 4 mm to match the tip of the retentive force measuring device.

Yamagaki et al. reported that a stable retentive force can be measured by setting the traction speed to 0.5 N/s when measuring the retentive force using a digital force gauge [14]. In this study, traction was also applied at 0.5 N/s. The denture adhesive for dry mouth and the oral moisturizer showed lower retentive force at 45° than at 90°, possibly due to the detachment from the back of the base caused by horizontal force arising from oblique traction. The cream-type denture adhesive and denture moisturizer, which have relatively low retentive force, were not affected by the direction of traction. Thus, the effect of traction direction on the retentive force depends on the material. Based on the above, an angle of 60° is considered to be optimal for actual intraoral measurements of retentive force, as this allows traction from outside the oral cavity, while minimizing the influence of the backward withdrawal force.

Bandai et al. examined the relationship between saliva viscosity and palatal plate retentive force and reported that 10 s of pressure contact stabilized the measured retentive force [19]. In the present study, we also measured the retentive force after 10 s of applying pressure. Kano et al. measured the thickness and adhesive force of denture adhesive between two resin plates and reported that the adhesive force increased with decreasing thickness [20]. In the present study, the higher the pressure load, the thinner was the thickness of the test sample between the plate and the model, and thus the higher was the retentive force. The magnitude of the pressure load affects the retention force. Therefore, if the pressure load is not specified in the oral cavity, the measured value of the retentive force may not be stable.

It has been suggested that the direction of traction and the pressure load should be specified when measuring retentive force of dentures after applying denture adhesives or moisturizers. However, while it is easy to define in an experiment using a model, it is difficult to do so in the oral cavity. Therefore, it is necessary to devise a method to define the direction of traction and pressure load for use in such experiments in the oral cavity. In future, we plan to measure the retentive force of a palatal plate in the presence of denture adhesives and oral moisturizers in the mouths of dentulous individuals, using these defined measurement conditions.