International Medical Journal Vol. 26, No. 6, pp. 516 - 519 , December 2019

DENTISTRY

Effect Type of Flasking Technique and Investing Materials on Movements of Teeth during Complete Denture Construction

Ibrahim H. Alfahdawi

ABSTRACTObjectives: This study was to examine the effect type of flasking technique and investing stone casts were prepared, a uni-

form denture base wax pattern was made on each stone cast with a 3 mm thickness. Methods: G1: conventional flasking. G2: modified flasking. 30 specimens for each group, 15 specimens according to invest-

ing material. P/L ratio of acrylic 1:3, cured by curing cycle 90 min. at 74℃ followed by 30 min. at 100℃. The dentures were stored in water at 37℃ for 24h. Inter-molar (M-M) and inter-central incisor (CI-CI), left (CI-M) and right(CI-M) were mea-sured with electronic digital caliper, which can record changes as small as 0.01. The measurements were made before and after denture polymerization, differences between them indicated movement of teeth.

Results: movements of teeth occurred in two directions in 2 groups, (M-M), R(CI-M) and L(CI-M) were significantly greater in specimens with plaster than in other with stone. Fewer movements showed in G2(S-S), no significant differences in (CI-CI) movement. Movements in all directions in G1 was significantly greater than in G2.

Conclusion: Dental stone during investing procedure with modified flasking led to more reduction in displacements of teeth.

KEY WORDSacrylic denture base, flasking technique, maxillary complete denture, investing method, plaster, stone, teeth movement

Received on January 6, 2019 and accepted on May 15, 2019Department of Prosthodontics, University of Anbar, College of DentistryIraq Correspondence to: Ibrahim H. Alfahdawi (e-mail:ibrahimhm7@yahoo.com)

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INTRODUCTION

Dentistry as a branch of science, has witnessed a continuous search for a new materials and techniques, ever since its evolution, to improve the quality of treatment service. The practice of dentistry, in particular prosthetic dentistry is at an interface delicately balancing technology and science on one side and patient oriented service on the other1). The most common material used for denture base construction is acrylic resin2), which has good physical, mechanical, and esthetic properties, and it is easy to use with low cost equipment3,4). However, this material does not fulfill all requirements for an ideal material of denture base construction5,2). The advent of acrylic resins in 1937 is a major break-through in the annals of modern dentistry. Their recognition and accep-tance in prosthodontics is indeed incredible as they are found to be more esthetic, easy to manipulate both in the clinic and in the laboratory4). Acrylic resin has been applied in dentistry since 1946, commonly as a denture base material and mostly consisting of methacrylates, especially methyl methacrylate (MMA)6). This resin has been considered a suitable material to be used in the oral environment and has been widely applied7).

The use of pour-type (fluid) resins is increased markedly over the past decade for the fabrication of denture bases8). The dimensional change that occurs during polymerization shrinkage is the critical factor in the retention and stability of the complete denture9). This change may be partially compensated by absorption of water10) the gingival mucosa resiliency11) and the saliva film formed between the soft supporting tis-sue and the denture base12,13).The consequences of edentulous include disability to speak and eat, reduction of social contact and inability of the residual ridge and its overlying tissues to withstand masticatory forces14). Various flasking, polymerization techniques and materials have been studied in efforts to overcome undesirable processing effects15).

Rudd (1969) indicated that the use of dental stone as an investing medi-um can significantly reduce movement of teeth. Another study, when flasking complete dentures, showed that the use of a silicone investment layer resulted in the smallest changes in artificial teeth position, regard-less of polymerization technique15).

This is acknowledged in 1943 itself by Skinner and Cooper17) who found that at least two unavoidable dimensional changes which are active in every acrylic denture like shrinkage, which occurs during pro-cessing, and subsequently expansion, which occurs upon water immer-sion. Differences in thickness of denture base have also been found to lead to variation in movements of teeth18). The method of flask closure and time of post-pressing were reported to be important factors affecting displacements of teeth19). One of the successful prosthesis of the clinical criteria is its accurate adaptation the denture bearing area. Investing and processing procedures were developed, such as direct and trial pack techniques, dry and wet curing, pour techniques and injection tech-niques. However little research has been conducted on the influence on the type of dental stone used to fabricate the cast20).

The objective of this in vitro study was to examine the effect type of flasking technique and investing material on movements of teeth during complete denture construction

MATERIALS AND METHODS

Sixty identical maxillary stone casts were prepared using a silicone mold of a master edentulous maxillary cast with regular alveolar ridge surfaces. The casts were made of artificial stone (Begostone Plus, Germany) using water: powder ratio 30 ml: 100 g21). A uniform denture base plate wax pattern (Base plate wax, Shanghai - china) was made on each stone cast with a 3 mm thickness. An occlusal wax rim (height =

C 2019 Japan Health Sciences University & Japan International Cultural Exchange Foundation

Alfahdawi I.H. 517

22 mm) was created in the buccal sulcus of the cast, and the height was reduced gradually to 10 mm in the second molar area. Acrylic resin arti-ficial denture teeth (ortolux top; unidesa - odi, Madrid, Spain) were arranged on the cast. The teeth neck was grinded (for group two) from the palatal or lingual toward buccal side to allow more space for the flow of acrylic.

A wax-up was used to form the polished surfaces of the upper den-tures. Replicate dentures were made using a silicone matrix. After the placement of artificial teeth and prepared stone casts in the matrix, mol-ten pink base-plate wax was poured into the matrix and allowed to cool before removal. All sets of teeth were from the same mold. Simulated metallic reference pins were placed in the artificial teeth at the mesial aspects of the central grooves of the second molars, and upright on cin-gulae of both central incisors22), (Fig. 1).

Inter-molar (M-M) and inter-central incisor (CI-CI) transverse dis-

tances, and antero-posterior distances from the central incisors to the second molars, lateral[L(CI-M)] and right [R(CI-M)], were measured with electronic digital caliper (STAINLESSHARDENED), which can record changes as small as 0.01 mm23), (Fig. 2).

The measurements were made at the wax denture stage (pre-polym-erization) and after denture polymerization (post-polymerization). Differences between the final and initial measurements indicated move-ment of teeth. The samples were divided into two groups, Group 1: con-ventional flasking method (dough consistency). Group 2: modified flasking method (sandy consistency). The teeth neck was grinded (for group two) from the palatal or lingual toward buccal side to allow more space for the flow of acrylic. 30 specimens for each group, according to flask type, 15 specimens for each (S-S) and (P-P) according to investing material. In group one, the plaster-plaster (P-P) group, the lower part of flask was filled with dental plaster or stone (Al- Alamiya, Iraq).After

Figure 1. Simulated metallic reference pins

Figure 2. Electronic digital caliper

Figure 3. Lower part of flask was filled with dental plaster

Figure 4. Plaster covering the buccal, labial, incisal and occlusal surfaces of denture teeth

Table 1. Means and standard deviations (mm) of tooth movement in relation to the flasking type and invest-ing materials tested.

Group one Displacements of tooth

(M-M) R (CI-M) L(CI-M) CI-CI

P-P 0.23851 ± 0.02686 0.22322 ± 0.02240 0.22756 ± 0.02732 0.09102 ± 0.02303

S-S 0.08643 ± 0.01526 0.06881 ± 0.02198 0.07971 ± 0.02104 0.07995 ± 0.02125

Group two

P-P 0.01653 ± 0.011536 0.01174 ± 0.01322 0.03663 ± 0.01334 0.02215 ± 0.01126

S-S 0.01242 ± 0.01233 0.02577 ± 0.01124 0.04532 ± 0.01981 0.02942 ± 0.01263

P-P = plaster + plaster

S-S = stone + stone

R (CI-M) = right central incisor to right molar.

L(CI-M) = left central incisor to left molar.

Means followed by identical letters in each column do not differ statistically at p < .05.

Flasking Technique and Investing Materials on Movements of Teeth518

separating medium was applied to the exposed surface, the remaining portion of the flask was also filled with plaster (P-P) or stone (S-S) (Fig. 3).

The same procedure was applied to the group two (modified flask technique) exception the volume of the definitive casts was reduced as much as possible and stone or plaster covering the buccal, labial, incisal and occlusal surfaces of denture teeth (Fig. 4).

The flasks were placed in a boiling water to soften the base plate wax. The flask parts were separated, the wax removed, and the stone cleaned with boiling water and liquid detergent. Separating medium was used as a mold separator. When the flask parts have cooled, polymethyl methacrylate (Classico Dental Products, Sao Paulo, SP, and Brazil) was used with a monomer: polymer ratio of 1:3 (by volume) according to the manufacturer instructions for flask pressing. For group 2, the prepared liquid mixture was packed immediately after mixing according to the conventional packing methods (at sandy stage), while for group 1 the mixture was packed (at dough stage).

The flasks were placed in a traditional metallic clamps after final pressing was performed using a hydraulic press with a load of 1.250 kg for 5 min. The flasks were transferred to a flask carrier and immersed in water according to the post-pressing times and maintained (Cured by using short curing cycle 90 min. at 74℃ followed by 30 min. at 100 ℃). After the curing cycle, the flasks were removed and allowed to bench cooling at room temperature before deflasking, then decasting were carefully completed. The dentures were stored in water at 37℃ for 24 h. Then, the transverse and anteroposterior distances were measured again.

RESULTS

Tables 1 showed that teeth movement occurred in two directions in group one. M-M, R(CI-M) and L (CI-M) movement was significantly greater in specimens prepared with dental plaster alone (0.23851 ± 0.02686 mm, 0.22322 ± 0.02240 mm, and0.22756 ± 0.02732 mm, respectively) than in the other specimens prepared with dental stone alone, no significant difference in CI-CI movement was observed. In group two, M-M, R(CI-M) and L(CI-M) movement was significantly greater in specimens prepared with dental plaster alone (0.01653 ± 0.011536mm, 0.01174 ± 0.01322 mm, and 0.03663 ± 0.01334mm, respectively) than in the other specimens prepared with dental stone alone, no significant difference in CI-CI movement was observed. In two groups, transverse movement along M-M and CI-CI was signifi-cantly greater than antero-posterior movement in specimens prepared with dental plaster alone, no significant differences among measure-ments was observed in specimens prepared with dental stone alone. Movements of teeth in all directions in group one was significantly rath-er than in group two. Fewer movements of teeth showed in group two (S-S).

DISCUSSION

Results revealed that using modified flask technique with dental stone investing procedure resulted in less displacement of denture teeth. Various studies have proven that movement of teeth occurs during and after the processing of complete dentures14,24). An understanding of this phenomenon may permit one to construct functional complete dentures that require decrease occlusal adjustment on the articulator and in the mouth of patient22). The authors of previous studies have reported that the greatest magnitude of tooth movement occurred in the posterior teeth24), however, this study showed that maximum M-M teeth move-ment occurred only in group one for (P-P). As dimensional changes caused by water sorption cause expansion, apparently due to the entrance of water between polymethyl methacrylate molecules25), final measurements were taken after the dentures were immersed in water for 24 h in this study. Many factors influence movement of teeth during the processing of complete denture. The effects of base thickness26), geomet-ric palatal form22), and pressure of closure flask27,28) have been investigat-ed. The displacements of teeth could be adversely affected by the flask closure method and post pressing time association was in part accepted. Careful measurement has been taken to overcome denture inaccuracies, such as base distortion and displacement of artificial teeth29).

Grant (1962) stated that movement of teeth related to the setting expansion of a gypsum mold. Dental plaster (type II) has a setting expansion of 0.2-0.3%, while dental stone (type III) has a setting expan-

sion of 0.15-0.25%30,31). Moreover, setting expansion of gypsum increas-es with reduced w/p ratio32), thus, the increased setting expansion of den-tal stone relative to that of plaster may contribute by compensating the polymerization shrinkage of an acrylic resin denture base, ultimately reducing movement of teeth during decreased by confining it within a flask. Although dental stone is harder than plaster31), investment with dental stone may lead to more movements of teeth relative to the use of plaster. However, artificial teeth undercuts support the restrictive effect of the investing material. Taken together, these factors may explain the observation of reduction in movements of teeth in all groups for (S-S) than in(P-P) in this study.

The recommended amounts of water for mixing with 100 g plaster and 100 g stone are 45 0 ml and 28 0 ml, respectively33). However, only 18.6 g water reacts with the plaster or stone; the excess is distributed as free water in the set mass. When the gypsum is dried, the excess water evaporates and leaves pores in the composition, weakening it31). Thus, set plaster is weaker than dental stone because it contains more free water. Thus, the restrictive effect of investing plaster on position of teeth may be less than that of dental stone throughout the resin polymer-ization and cooling processes. Although transverse (M-M) displacement of teeth did not differ significantly between dentures invested by dental stone alone (S-S) and those invested by dental plaster (P-P) for group two in this study, but showed less movement of teeth for dental stone alone (S-S). This result may be attributed to the rigidity of stone and, which contributes to the binding of teeth together and prevents them from displacement34), and covering the buccal, labial, incisal and occlu-sal surfaces of denture teeth give more teeth support.

The results of this study are not consistent with the concept that teeth always tend to move toward the midline section of the palate9). They are also inconsistent with the findings of Carr et al, 198535) and Alaa'aM, et al36) that showed high movement of tooth occurred when stone was used as an investing material in comparison with plaster. The present findings are in agreement with those of19,34), that showed the use of dental stone contributes to the reduction of movements of teeth.

Further research is required to evaluate the effects of different flask-ing technique and investing methods on the retention, support and sta-bility of complete dentures during use. The effect of water storage on movement of teeth when different flasking technique and investing methods are used with the compression molding technique also requires further examination.

CONCLUSION

Within the limitations of this in vitro study results suggest that the flasking technique and investing method appears to be an important fac-tor in efforts to control the magnitude movements of teeth. Using dental stone (S-S) during investing procedure with modified flasking led to more reduction in displacements of teeth.

ACKNOWLEDGEMENTS

I would like to express my appreciation to all those who gave me the possibility and financial support to complete of the study.

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