Evaluation of Distortion in the Posterior Palatal Seal Area of Maxillary Cast by Different Adaptation Techniques of Denture Bases

ABSTRACT

Aim:

To evaluate and compare the amount of distortion in the posterior palatal seal area in V-shaped and U-shaped palatal form of maxillary cast by different adaptation techniques of maxillary denture bases.

Materials and Methods:

A total of 60 stone casts were made, out of which 30 identical stone casts of U-shaped palatal form and 30 identical stone casts of V-shaped palatal form of maxillary casts were prepared. The 30 stone casts of U-shaped palatal form and 30 stone casts of V-shaped palatal forms were divided into three groups for measurements to be done. Data were statistically analyzed by using two-way analysis of variance (ANOVA) without replication.

Results:

Discrepancies of fit of dentures on U-shaped casts and V-shaped casts of Groups 1, 2, and 3 in transverse and sagittal sections were measured, and comparative statistical analysis between Group 1&2, Group 1&3, and Group 2&3 was done for statistical significant values.

Conclusion:

Discrepancy in the central region was maximum, and the anchoring methods Group 2 and Group 3 produced a significantly better adaptation than the conventional method (Group 1). The adaptations for V-shaped palate were better when compared with U-shaped palate in the central region.

INTRODUCTION

In general, the warping of the maxillary complete dentures follows a pattern: The flanges draw inward on the lateral aspects of the tuberosities, and, as a result, the denture base is “raised” from the palate, and a lack of adaptation, mostly in the posterior palatal seal area, occurs.[1] Internal strain is introduced as the coefficient of linear expansion of the resin is much greater than that of the gypsum products.[2] The capillary forces of the salivary film are at maximum when the distance between denture surface and the basal seat is at minimum. The better the denture base conforms to the supporting tissues better the retention.[3] The oral tissues have been shown to possess remarkable properties of adaptation, but this does not mean that they are healthy and normal under conditions that have distortion of 1 mm or more.[4] Therefore, the study was conducted to compare and evaluate the effectiveness of improving the adaptation of denture bases by anchoring them on stone casts. The primary aim of the study was to evaluate and compare the amount of distortion in the posterior palatal seal area in V-shaped and U-shaped palatal form of maxillary cast by different adaptation techniques of maxillary denture bases.

MATERIALS AND METHODS

Elastic rubber mould was used to fabricate an edentulous non-undercut maxillary cast of U-shaped palatal form that had a 5 mm border, whereas a V-shaped silicone rubber mould was made from a cast that was chosen from a V-shaped palatal form of an edentulous patient who had no undercuts. Dental stone was measured and mixed with the recommended amount of water. A total of 60 stone casts were made, out of which 30 identical stone casts of U-shaped palatal form and 30 identical stone casts of V-shaped palatal form of maxillary casts were prepared. The 30 stone casts of U-shaped palatal form and 30 stone casts of V-shaped palatal forms were divided into three groups:

The conventional method for Group 1 was used without any modification [Figure 1a and b]. The anchorage method for Group 2 is by drilling six holes on the stone cast to a depth of 10 mm and a diameter of 2 mm. The anchorage method for Group 3 is by extending a layer of base plate wax onto the posterior aspect of the cast and extending it 8 mm over the back edge of the cast. The width of the posterior extension was 25 mm.

Figure 1.

(a) Transverse section. (b) Sagittal section

The silicone duplicating material was mixed to a thin consistency and was poured into duplicating flask. The waxed-up denture was retrieved from the silicone material in which the occlusal index was embedded. With the same size of acrylic resin teeth placed on the occlusal index, the denture base wax-ups were duplicated by melting and pouring wax into the mould that held the acrylic resin teeth and stone casts in correct relationship. A traveling microscope was used to measure the distance between the cast and its denture base.

Formula for the calculation of distance was:

TR = MSR+ (CVD* LC)

TR = Total reading

MSR = Main scale reading

CVD = Coinciding Vernier division

LC = Least count

Least count at .001 cm was fixed for the microscope. Seven and four locations were selected in the transverse and midline sections, respectively, to determine the discrepancies between dentures and stone casts. The measurements were repeated, and the average values were taken as the discrepancy in each location. Data were statistically analyzed by using two-way analysis of variance (ANOVA) without replication.

RESULTS

This study was carried out on 30 U-shaped and 30 V-shaped palatal casts to determine the vertical discrepancy incorporated into the posterior aspect of maxillary denture bases by conventional method and by two anchoring methods on the stone casts. Data was statistically analyzed by using two-way analysis of variance (ANOVA) without replication. The results for sagittal sections revealed a range of marginal gaps from 0.001 to 0.065 cm in four different locations. A significantly greater degree of discrepancy was observed with the conventional method in posterior location S for U-shaped and V-shaped palate. No significant difference was identified among the anterior locations P, Q, and R in the midline sections in U-shaped and V-shaped groups [Table 1]. The range of discrepancy in the transverse section measured at seven different locations across the posterior border of maxillary complete dentures was 0.001 (minimum discrepancy) to 0.084 cm (maximum discrepancy). The greatest mean discrepancy (0.056 cm) among both (U-shaped and V-shaped) resulted from dentures processed with the conventional method (Group 1 of U shape) in location C (midline). In the transverse section for the U-shaped casts, the distortion in the conventional processing method showed the greatest discrepancy in the midpalate region (location C) and on either side of the tuberosities when compared with Groups 2 and 3 [Graph 1]. Result from the two-way ANOVA test without replication of the transverse sections indicated that a highly significant difference existed between the dentures processed by anchoring them on stone casts with the incorporation of holes (Group 2) and the conventional method (Group 1) in location C (midline) for U-shaped form [Graph 2].

Table 1.

Comparison of fit of dentures on U- and V-shaped casts with their respective groups in transverse section

Location	R1	R2	R3	L1	L2	L3	C
Group 1
U shape	0.017	0.026	0.019	0.017	0.025	0.018	0.056
V shape	0.024	0.024	0.018	0.030	0.030	0.025	0.037
Group 2
U shape	0.013	0.014	0.014	0.018	0.014	0.011	0.025
V shape	0.013	0.012	0.011	0.013	0.010	0.012	0.018
Group 3
U shape	0.011	0.014	0.014	0.014	0.010	0.011	0.030
V shape	0.013	0.015	0.009	0.015	0.012	0.015	0.020

Graph 1.

Location wise distortion in three groups

Graph 2.

Location wise distortion in two groups

DISCUSSION

Considering these factors, the present study undertook to investigate the distortion by the conventional methods and the two anchoring methods and their influence in decreasing shrinkage. It also compared the accuracy levels with U-shaped and V-shaped palates with these methods. A highly significant difference existed between the dentures processed on the stone casts by anchoring them on stone casts by extending a layer of base plate wax on the posterior aspect of the cast (Group 3) and the conventional method (Group 1) in location C (midline) for U-shaped form.

These results were exhibited due to the anchoring of the acrylic resin onto the casts in the posterior aspect of the palate. This is in accordance with the study proclaiming its usefulness in decreasing the distortion. This suggested that the shape of the palate influenced the distortional changes irrespective of the processing methods used, thus signifying the importance of the anatomical form of the palates. This is in acceptance with the previous study. These results showed that V-shaped palates were conducive for the good adaptation of the denture bases to the tissues. Also, the anchoring methods used during the processing of Group 2 suggested enhancing the accuracy of the fit of the denture base in the transverse section.

The results reiterated the earlier observation that maxillary dentures show the greatest discrepancy at the central portion of the posterior border. It was also evident that Group 2 and Group 3 were superior to conventional method Group 1 in improving adaptation at the posterior palatal midpoint and on either side of the midline.

Retention in denture is evaluated by its resistance to withdrawal in direction opposite to insertion.[5] The principal physical factors of denture retention are the capillary forces of the liquid film between the oral tissues and the denture bases. Denture retention is greatest when the underlying film thickness is minimal, i.e., the fit of the denture should be as close as possible with the underlying oral tissues.[2]

Often these variations have been introduced with a claim of superior properties, improved accuracy of fit, and greater dimensional stability in service.[6]

The dimensional stability of acrylic resin was studied, and it was stated that “the accuracy of fitting dentures or the accuracy of duplicating the contours of the final impression is difficult to determine with any degree of accuracy.” They also concluded that the most practical method of evaluating the fit of dentures is to place them on a master cast and measure the fit at the borders.[7]

The coefficient of linear expansion of the resin is much greater than that of the gypsum products in which it is formed, thus introducing internal stress. In addition, the temperature of the curing cycle affects the polymerization shrinkage.[8] The pressure applied and maintained during flasking tends to improve the accuracy of the fit of the denture bases.[9] The anatomic form of the palate also shares the responsibility of maintaining positive contact with the tissue of the palate.[10]

The manifestation of these dimensional changes on the posterior palatal edge of the prosthesis must be compensated by the creation of a postpalatal seal. A positive mucosal contact helps to maintain the integrity of the peripheral border seal, enhances retention, reduces gagging tendencies, and permits thinning of the posterior border, which makes it less noticeable to the tongue.[11] Experiments have been described to demonstrate that no perceptible displacement occurs in an anteroposterior direction in the mandibular denture base during processing. A very definite forward tooth shift was consistently observed in maxillary dentures. In the maxillary arch, the labial flange is anchored against the labial aspect of the maxillary alveolar ridge. There are no reciprocal stops in the palatal or posterior ridge areas; hence, all shrinkage occurs posteriorly and moves anteriorly, shifting the posterior tooth in the anterior direction. Mandibular dentures show no similar phenomenon due to the labial aspect of the alveolar process being neutralized by the lingual flange. Thus, in this study, the base of the maxillary prosthesis was tested for discrepancies.

Processing changes that occur in complete dentures during curing of the acrylic resin have been observed to result in overall contraction requiring both base and occlusal adjustments. Clinical signs of such contraction in maxillary prostheses can usually be seen as “pulling away” from the midpalate with corresponding pressures on the lateral tuberosities.

CONCLUSION

Based on the results and conditions of this study, the following conclusions were drawn:

1. Maximum amount of discrepancy was seen in the central region.

2. The anchoring methods Group 2 and Group 3 produced a significantly better adaptation than the conventional method (Group 1).

3. The adaptations for V-shaped palate were better when compared with U-shaped palate in the central region.

Financial support and sponsorship

Nil.

Conflicts of interest

There are no conflicts of interest.