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

Abstract

Objectives: This study was to examine the effect type of flasking technique and investing stone casts were prepared, a uniform 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 investing 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 measured 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 WORDS acrylic denture base, flasking technique, maxillary complete denture, investing method, plaster, stone, teeth movement
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
ABSTRACT
Objectives: 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 WORDS
acrylic denture base, flasking technique, maxillary complete denture, investing method, plaster, stone, teeth movement
Received on January 6, 2019 and accepted on May 15, 2019
Department of Prosthodontics, University of Anbar, College of Dentistry
Iraq
Correspondence to: Ibrahim H. Alfahdawi
(e-mail:ibrahimhm7@yahoo.com)
516
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 Teeth
518
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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... After the application of separating media the wax pattern was completely invested in dental stone. 15 Heat cure acrylic resin polymer and monomer (Meadway Royale Heat Cure, MR. Dental, and UK) was mixed according to manufacturer's instructions with 2.5gm powder to 1ml monomer in a ceramic container and covered with a lid to prevent evaporation of monomer. ...
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Dentistry in general and prosthodontics in particular is evolving at greater pace, but the denture base resins poly methyl methacrylate. There has been vast development in modifying chemically and the polymerization techniques for better manipulation and enhancement of mechanical properties. One such invention was introduction of visible light cure (VLC) denture base resin. Argon ion lasers have been used extensively in dentistry, studies has shown that it can polymerize restorative composite resins. Since composite resin and VLC resin share the same photo initiator, Argon laser is tested as activator for polymerizing VLC resin. In the Phase 1 study, the VLC resin was evaluated for exposure time for optimum polymerization using argon ion laser and in Phase 2; flexural strength, impact strength, surface hardness and surface characteristics of laser cured resin was compared with light cure and conventional heat cure resin. Phase 1; In compliance with American Dental Association (ADA) specification no. 12, 80 samples were prepared with 10 each for different curing time using argon laser and evaluated for flexural strength on three point bend test. Results were compared to established performance requirement specified. Phase 2, 10 specimen for each of the mechanical properties (30 specimen) were polymerized using laser, visible light and heat and compared. Surface and fractured surface of laser, light and heat cured resins were examined under scanning electron microscope (SEM). In Phase 1, the specimen cured for 7, 8, 9 and 10 min fulfilled ADA requirement. 8 min was taken as suitable curing time for laser curing. Phase 2 the values of mechanical properties were computed and subjected to statistical analysis using one-way ANOVA and Tukey post-hoc test. The means of three independent groups showed significant differences between any two groups (P < 0.001). Triad VLC resin can be polymerized by argon ion laser with 1 W/mm(2) power and exposure time of 8 min to satisfy ADA specification. Impact strength, surface hardness of laser cure was better than light cure and heat cure resin. Flexural strength of light cure was better than laser cure and heat cure resin. The SEM study showed similar density on surface, the fractured surface of heat cure resin was dense and compact.
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Context: Most of the maxillary complete dentures do not adapt accurately to the cast because of the changes in the resin during polymerization. The amount of heat associated with processing of polymethyl methacrylate has been correlated with the adaptation of the processed denture base to its supporting tissues. Aims: This study conducted to determine the effect of different curing temperatures on the accuracy of fit of a complete maxillary denture and to compare with that of fiber-reinforced acrylic resins. Materials and Methods : An ideal maxillary rubber mould used to make an edentulous cast. Forty-eight stone casts made from the ideal rubber mould. Out of 48 identical stone casts, 44 stone casts with wax adapted processed using simple acrylic resin. The remaining four stone casts were processed using fiber-reinforced acrylic resin. However, processing time for 12 hrs was maintained constant for all the samples. The variable to investigate was the temperature. Therefore, four denture bases processed at each temperature from 60°C to 80°C with two-degree increments for a total of 44 samples using simple acrylic resin. Remaining four samples processed at 70°C using fiber-reinforced acrylic resin. A traveling microscope used to measure the discrepancy between each cast and its denture base. Statistical analysis used: Analyzed with Student′s unpaired t test. Results : Minimum distortion was observed at 70°C. Maximum distortion was observed for the denture bases processed at 80°C. Amount of distortion increases as the processing temperature increases which was highly significant. Distortion was significantly high from 60 to 68°C in the decreasing order. Conclusion: This study verified the observation that maxillary complete denture base show the greatest discrepancy at the central portion of the posterior palatal seal region as the processing temperature increases.
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Heat cured acrylic resins undergo dimensional changes during polymerization. Dimensional changes which occur in the heat cure acrylic resins are shrinkage and expansion which affects the fit of the denture and occlusal relationship. The purpose of this study was to access the linear dimensional changes of four heat cure acrylic resins before and after curing and compare the changes among four different acrylic brands. Twenty four patients irrespective of age and sex were taken and four commercially available brands were procured. After the teeth arrangement on the mandibular trial denture, two pins were fixed in central fossae of first molar on both sides and one pin in the cingulum of left central incisor. Meliodent heat cure acrylic resin was used in Group A; Trevalon heat cure acrylic resin was used in Group B; Triplex heat cure acrylic resin was used in Group C and Vertex heat cure acrylic resin was used in Group D. Linear measurements of the trial wax up before and after curing and before and after finishing and polishing were measured and compared. Collected data was analyzed with analysis of variance and 't' test at 95% level of confidence (P=0.05). The maximum percentage changes were seen in cases of Group A (Meliodent) followed by Group B, Group C and Group D (Trevalon, Triplex and Vertex). Meliodent showed the highest percentage change i.e. 1.18% and Vertex showed least percentage change of 0.37 %. Shrinkage occurred after curing and after finishing and polishing, which varies significantly with the four commercially available heat cure acrylic resins. Among the four different brands of heat cure acrylic resin Group D (Vertex) had the least linear dimensional changes after curing and after finishing and polishing, so that D (Vertex) could be the material of choice for fabrication of complete denture among the four brands.
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The purpose of this study was to investigate the displacement of teeth occurring in dentures following storage in water at a temperature of 37ºC. Ten maxillary dentures were constructed with Classico heat-cured acrylic resin using conventional metal flasks. Metallic reference pins were placed on the incisal border of the central incisors (I), buccal cusp of the first premolars (PM), and mesiobuccal cusp of the second molars (M). The acrylic resin was polymerized twelve hours after final flask closure in a water cycle at 74ºC for 9 hours. The flasks were removed from the thermo-curing unit after water cooling and then bench stored for 3 hours. After deflasking and finishing, the dentures were stored in water at a temperature of 37ºC for periods of 7, 30, and 90 days. Following deflasking and each storage period tested, the I-I (incisor to incisor), PM-PM (premolar to premolar), and M-M (molar to molar) transverse distances, and LI-LM (left incisor to left molar) and RI-RM (right incisor to right molar) anteroposterior distances were measured with a STM Olympus microscope, with accuracy of 0.0005 mm. Colleted data were submitted to ANOVA and Tukey test (5%). No statistically significant difference was observed in the displacement of teeth in the transverse and anteroposterior distances.
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Purpose: This study investigated the tooth movement of standardized simulated dentures processed by traditional closure or the new Rafael Saide (RS) tension system when cooled in the curing water itself or in curing water followed by bench storage for 3 hours. Materials and methods: Forty stone casts were formed from a mold of an edentulous maxillary arch. The wax denture record bases were made on the casts; the height of the wax rim occlusion was 20 mm in the labial sulcus of the cast and 10 mm in the posterior region. The upper stone cast was mounted on a Mondial 4000 semi-adjustable articulator with wax rim interocclusal references in relation to the lower stone cast teeth. Metallic pins were placed in the incisal border of the maxillary central incisors (I), labial cusp of the first premolars (PM), and mesiolabial cusp of the second molars (M). The incisor-to-incisor (I-I), premolar-to-premolar (PM-PM), and molar-to-molar (M-M) transversal distances and left incisor-to-left molar (LI-LM), and right incisor-to-right molar (RI-RM) anteroposterior distances were measured before and after denture polymerization with an optical microscope with a tolerance of 0.0005 mm. For traditional closure, the dentures were flasked conventionally in standard metallic flasks, which were afterward placed in spring clamps. For the new RS system closure, the flasks were pressed between the metallic plates of the tension system after the final closure. The Clássico heat-polymerizing acrylic resin dough was packed in the flasks under a final packing pressure of 1.250 kg f. Twelve hours after flask closure, the dentures were polymerized in a moist heat-polymerizing cycle for 9 hours at 74 degrees C. The denture was deflasked after cooling in the water of the polymerizing cycle (groups A and C) or in the water of the polymerizing cycle and then bench-stored for 3 hours (groups B and D). Collected data were analyzed with analysis of variance and Tukey's test (p< 0.05). Results: There was no statistically significant difference (p> 0.05) between the conventional and new RS system closure methods for the transversal distances after polymerization in all studied groups. The anteroposterior distances did not change with deflasking after water bath cooling. There were statistically significant differences (p< 0.05) in the anteroposterior distances with deflasking after water bath cooling and then bench storage for 3 hours. Conclusion: Tooth movement was similar in dentures processed by traditional closure and by the new RS tension system, with the exception of the anteroposterior distances when the flasks were cooled in their own curing water and bench-stored for 3 hours.