|Year : 2012 | Volume
| Issue : 4 | Page : 556
|An investigation on the influence of tin foil substitute contamination on bond strength between resin denture teeth and the denture base: An in vitro study
Sapna Bhaskaran1, RB Hallikerimath2
1 Department of Prosthodontics, Noorul Islam College of Dental Sciences, Thiruvanthapuram, Kerala, India
2 Department of Prosthodontics, Maratha Mandal's Nathaji Rao G. Halgekar Institute of Dental Sciences and Research Centre, Belgaum, Karnataka, India
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|Date of Web Publication||20-Dec-2012|
| Abstract|| |
Statement of Problem: The problem of acrylic resin denture teeth separating from their denture base remains a major problem in prosthodontic practice and is frustrating to the patients as well as the dentists.
Purpose: This study investigated the influence of tin foil substitute contamination on bond strength between acrylic resin denture teeth and their denture base.
Materials and Methods: A total of 80 modified acrylic resin maxillary left central incisors were processed to their denture base material. The 80 specimens were divided into two groups of 40 specimens each: the experimental group and the control group. The experimental group had the denture tooth ridge lap area contaminated with tin foil substitute.The 40 specimens in the control group were not contaminated. After 7 days of storage at room temperature, the bond strengths of the two groups were tested with a universal testing machine.The nature of failure was noted as adhesive (interface) or cohesive (in tooth or denture base material). The Welch test and z-test were performed to identify significant differences.
Results: The uncontaminated specimens (group I) showed superior bond strength as compared to the contaminated specimens (group II); the difference in bond strength between the groups was highly significant (P<0.0001). Also the contaminated specimens exhibited predominantly adhesive failure (89.74%), whereas the uncontaminated specimens exhibited 55% adhesive failure and 45% cohesive failure.
Conclusion: Tin foil substitute contamination of the ridge laps of acrylic resin denture teeth greatly reduced the bond strength.
Keywords: Bond strength, failure, tin foil substitute contamination
|How to cite this article:|
Bhaskaran S, Hallikerimath R B. An investigation on the influence of tin foil substitute contamination on bond strength between resin denture teeth and the denture base: An in vitro study. Indian J Dent Res 2012;23:556
|How to cite this URL:|
Bhaskaran S, Hallikerimath R B. An investigation on the influence of tin foil substitute contamination on bond strength between resin denture teeth and the denture base: An in vitro study. Indian J Dent Res [serial online] 2012 [cited 2019 Dec 16];23:556. Available from: http://www.ijdr.in/text.asp?2012/23/4/556/104975
This article received the best faculty paper award at 37th Indian Prosthodontic Society Conference held at Thrissur in November 2009.The original acrylic denture teeth introduced in 1940 were aesthetic and easy to adjust but were susceptible to wear and crazing. Denture base materials were made from acrylic resin as early as 1937. These denture bases could craze and fracture easily. Cross-linking helped to solve the problem of crazing, but it made bonding to acrylic teeth more difficult. 
Debonding of denture teeth from denture base remains a major problem in prosthodontic practice.  A study has shown that 33% of the repairs carried out are for debonded /detached teeth.  With the increased use of implants and the commensurate increase in forces applied to prosthetic components it is probable that debonding will become an even greater clinical problem in the future. 
Two processes effect are necessary for chemical bonding between resin denture teeth and the denture base. First, the polymerizing denture base resin must come into physical contact with the denture tooth resin. Second, the polymer network of the denture base resin must react chemically with the denture tooth resin polymer to form a interwoven polymer network. Mutual solubility or compatibility of the two polymers is essential for the establishment of the interwoven polymer and thus for the strength of the bond. 
The strength of the union between the denture tooth and the denture base depends on a combination of factors: adhesion of the parts to one another, the properties and dimensions of the materials that compose the parts and the mechanical connection of the parts to one another. 
Failure of tooth-denture base bond has been attributed to careless laboratory techniques, namely the faulty boil-out procedures that fail to eliminate all traces of wax from the ridge lap surfaces of the teeth and the contamination of the ridge lap surface by careless application of tin foil substitute. These foreign materials interfere with the contact between the polymerizing denture base resin and the denture tooth resin, and thereby affect the bond strength. 
The purpose of this study was to determine the effect of tin foil substitute contamination on adhesion of modified resin denture tooth to its denture base.
Objectives of the study
- To determine whether tin foil substitute contamination has a significant effect on the bond strength between acrylic resin denture teeth and the processed acrylic resin base.
- To determine whether the nature of bond failure is adhesive or cohesive.
| Materials and Methods|| |
- Dental stone type III (Goldstone)
- 'MAARC' modeling wax
- Acry Pan XL mold-G2 (upper left central incisor)
- Aquasil Soft Putty/Regular Set
- Acrylic resin material: DPI Heat Cure Acrylic-denture base polymer resin; shade: pink, veined; Product No. 242
- Plaster of paris (Everest brand)
- DPI cold mold seal
- Distilled water: Swastik (1000 ml)
This study evaluated the bond strength between modified acrylic resin denture teeth and denture base with and without tin foil substitute contamination.
- Maxillary edentulous metal master model
- Maxillary edentulous mold (Columbia Dentoform Corp.)
- Rubber bowls, plaster spatula, wax knife, wax spatula, Lacron carver, BP blade and scalpel
- Acrylic mixing jar
- Hydraulic bench press (Kavo)
- Mini vibrator (Dentarum, Germany)
- Buffalo flask and clamps
- Acrylizer (Confident)
- Universal testing machine
- Drilling machine (AC motor)
- Silicon carbide bur
- Sandpaper disc
A total of 80 test specimens were made and divided into two groups with 40 specimens in each group:
group I (control group) and
group II (experimental group).
A maxillary edentulous metal master model of columbia mold with a palatal bearing screw was used for this study.
Duplication of master model: Eighty maxillary edentulous casts were duplicated with type III dental stone in Columbia molds.
Preparation of specimens: A single sheet of modelling wax was warmed and adapted on all the duplicated casts.
In this study only one brand (upper left central incisor of Acry Pan™ XL mold - G2) of cross-linked acrylic teeth was used. In order to standardize the tooth size, only the same mold of anterior teeth were chosen for this study.
The glossy surfaces of the ridge lap areas of all the teeth were abraded using 160-grit sandpaper, in one direction and with the same frequency, till the gloss disappeared.
The modified acrylic resin teeth were attached to the wax bases, i.e., one tooth was attached onto each base in their respective position with normal incisal angulation. A labial putty index [Figure 1] was made to standardize the tooth position on the wax bases. The wax base was sealed onto the casts and then each specimen was flasked individually. The specimen were placed in a boil-out tank at 100°C for 15 minutes. They were then separated and thoroughly washed with a household detergent solution, followed by flushing with clean hot water for 5 minutes to remove all wax residue. Cold mold seal/tin foil substitute was applied to the investing surface of both halves of the flask with a camel-hair brush while the mould was still warm.
For group I (control group), a coat of tin foil substitute film was applied carefully to the investing surface, avoiding contact with modified denture teeth. For group II (experimental group), a coat of tin foil substitute film was applied to the investing surface as well as to the ridge lap surface of the modified denture teeth. The flasks were then allowed to cool at room temperature for 1 hour.
As per the manufacturer's instructions, a mixture of polymethylmethacrylate was prepared in a mixing jar. Each flask was then packed with denture base material (DPI - veined) once it reached kneadable stage. These flasks were kept under 1000 psi pressure in a hydraulic press and then clamped. Finally, they were allowed to bench cure for 24 hours.
After the 24-hour period of bench curing, the flasks were placed in the acrylizer for processing. The curing cycle was according to the manufacturer's instructions, i.e., the flasks were immersed in cold water, which was gradually heated to boiling point (over not less than 30 minutes). The specimens were left in boiling water for 30 minutes, after which they were removed and allowed to bench cool before deflasking.
Following the bench cooling, the flasks were opened and the cured specimens were carefully retrieved. Each specimen was examined to ensure that no denture base resin was in contact with the denture teeth at any location other than the ridge lap area. Each specimen was placed back on the metal master model and a hole was drilled with a silicon carbide bur in the palatal screw-bearing area. This palatal screw would prevent the lifting of the denture base when it was subjected to shear load during testing of the specimen. The excess flash was trimmed using a laboratory micromotor (AC motor) and polished. Each sample was marked as contaminated/uncontaminated (C/U) and numbered.
The specimens were stored at room temperature in two separate plastic bags containing distilled water for 7 days at room temperature before testing.
Measurement of bond strength
The bond strengths of the group I and group II specimens were measured using a universal testing machine with a 5-kg load cell at a cross-head speed of 2.025 mm/minute [Figure 2].
Each specimen was placed on the universal testing machine and a shear load was applied to the cingulum area of the tooth [Figure 2] until fracture of the denture tooth-denture base bond or cohesive fracture in the tooth or base occurred. The bond strength (B) of each specimen was determined using the formula
B = L/A
Where L = fracture load and A = area of the interface between denture teeth and denture base resin. Area was calculated using the formula
Where D=diameter of the ridge lap area of the tooth, i.e. 7 mm.
Nature of fracture
The nature of fracture was noted as either adhesive or cohesive by visual inspection of the fractured specimens. It was classified as adhesive fracture if the fracture occurred only at the interface of the acrylic resin tooth and the denture base resin [Figure 3]. It was classified as cohesive fracture if the fracture occurred entirely in the tooth [Figure 4] or denture base material. The obtained data was subjected to statistical analysis using the z test and the Welch test.
| Results and Observations|| |
[Table 1] gives the data on the bond strength of specimens of groups I and II. The maximum strength was found in group I (208.41) and the least strength was found in group II (53.31). For the control group (group I), the mean bond strength was 171.15 (SD: 15.51). For the contaminated group (group II), the mean bond strength was 114.58 (SD: 20.53). There is significant variation in the mean bond strength between the two groups [Graph 1].
There was highly significant statistical difference between group I and group II (P<0.0001) using the Welch test and the z-test [Table 2].
[Table 3] represents the data on site of fracture; it can be seen that in both groups, the greatest number of fractures occurred at the tooth/denture base interface. Group 2 had predominantly adhesive failure (89.74%); in contrast, group I had 55% adhesive failure and 45% cohesive failure [Table 4]. [Graph 2] shows that the maximum number of adhesive failures occurred in the contaminated group (group II).
| Discussion|| |
Detachment of anterior plastic teeth from maxillary dentures is an all-too-common occurrence. A survey has shown that 33% of the repairs carried out are for debonded or detached teeth.  Taking into account the significance of the problem of bond failure, the present study was undertaken to evaluate bond strength between denture teeth and denture base.
Acrylic teeth are often preferred over porcelain teeth because they chemically bond to the denture base material and are easy to adjust in close interridge space.  Studies have showed that with acrylic teeth there is a more natural appearance, less proneness to breakage, elimination of clicking, ease of grinding and polishing, and better bond between teeth and denture base. 
In the present study cross-linked acrylic teeth of Acry Pan XL Co, G2 mold were used. The plain methyl methacrylate teeth are susceptible to wear and crazing. This problem was overcome by better methods of molding, addition of cross-linking agents, and complete removal of the residual emulsifiers and stabilizers employed in the suspension polymerization of the resin used in the fabrication of teeth.  Cross-linking has been shown to improve the hardness and abrasion resistance and also to help solve the problem of crazing. However, it has been reported to make bonding to acrylic teeth more difficult. 
For bonding to occur, the monomer-polymer mix must dissolve the ridge lap portion of the tooth. However, as the degree of cross-linking increases, the polymer swells less and bonding becomes more difficult. 
The method of polymerization of the denture base affects the bond strength between teeth and denture base. Studies have shown that the strongest bond occurred between heat-cured resin and the standard acrylic resin teeth when compared to that of microwave-cured, cold-cured, and light-cured denture base.  With the heat-polymerized material, more contact time is available for the diffusion of the monomers into the tooth material to form interpenetrating polymer networks than with autopolymerized and microwave-polymerized resins. The bond strength of autopolymerized systems has improved with an increase in polymerization temperature and the bonding characteristics changed from adhesive to cohesive failure, particularly at temperatures above 50°C. 
The VLC urethane dimethacrylate resin does not appear to be capable of diffusing effectively into the denture tooth surface to ensure a satisfactory bond. The mismatch of polymer structure between tooth and base when VLC resin is used does not provide the same free monomer transfer as seen with heat-cured resins. Therefore, the lowest bond strength was measured with the light-cured resin. 
Processing errors can also adversely affect bond strength. The probable causes of poor bond strength are the presence of a thin film of grease or wax, or the presence of cold mold seal on the surface of tooth. Careless performance of laboratory procedures has been noted to be the most common cause of bond failure. 
In the present study it was found that group I had higher bond strength compared to group II. The highest value recorded for group I was 208.41 kg/cm 2 and lowest value was 145.4 kg/cm 2 . The highest value recorded for group II was 150.25 kg/cm 2 and lowest was 53.31 kg/cm 2 . For group I, the mean bond strength was 171.15 kg/cm 2 , and for group II the mean bond strength was 114.58 kg/cm 2 . We compared the difference between the two means using the Welch test and the z-test and found that the difference is highly significant (P<0.0001). Also, the maximum number of adhesive failures occurred in group II [Graph 2].
These results indicate that contamination of acrylic resin denture teeth with tin foil substitute reduces the strength of the bond of denture teeth to the denture bases. This is in agreement with an earlier study conducted to evaluate the bonding of plastic teeth to two heat-cured denture base resins.  However, the present study is in disagreement with another study conducted to investigate the adhesion of acrylic resin teeth to dentures, which concluded that there was no significant reduction in bond strength for denture teeth contaminated with tin foil substitute.  This may have been because the sample size was too small to yield a statistically significant difference between the two groups.
In both group I and group II in this study, the maximum number of fractures occurred at the tooth-denture base interface. Group II (experimental group) had 89.74% (35/39) adhesive failure. It was interesting to note that in one of the specimens in the experimental group the modified acrylic tooth did not bind to the denture base at all. This specimen was considered to have failed before testing. In comparison, group I had 55% (22/40) adhesive failure and 45% (18/40) cohesive failure. The substantially higher number of adhesive failures in the experimental group reveals that contamination with tin foil substitute does affect the denture tooth-denture base bond.
None of the heat-cured specimens crossed the minimum bond strength recommended by the ADA (i.e., 315 kg/ cm 2 ). This may be because of the variance of the testing machine or the material itself.
Studies have shown that improved bond strength can be achieved by the inclusion of vertical grooves  or by the application of methyl methacrylate monomer on the ridge lap area of the tooth.  The present study was limited to testing the effect of tin foil substitute contamination on the bond strength of acrylic resin denture teeth bonded to their denture base.
| Conclusions|| |
Within the limitations of the present study the following conclusions can be drawn:
In the present study only the influence of tin foil substitute contamination on bond strength between acrylic resin denture teeth and denture base was tested, keeping all other factors constant. Several other factors also affect the bond, including cross-linking of materials, retention grooves, type of curing (heat-, cold-, microwave- or light-curing), and available monomer during processing. So, further studies should be carried out to evaluate the effect of these variables on the bond strength of specimens.
- Uncontaminated specimens show higher bond strength than contaminated specimens.
- The contaminated specimens exhibit mostly adhesive failure, which reveals that contamination with tinfoil substitute affects the denture tooth-denture base bond. The uncontaminated specimens exhibit both adhesive and cohesive failures.
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Department of Prosthodontics, Noorul Islam College of Dental Sciences, Thiruvanthapuram, Kerala
Source of Support: None, Conflict of Interest: None
[Figure 1], [Figure 2], [Figure 3], [Figure 4]
[Table 1], [Table 2], [Table 3], [Table 4]
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