The influence of different ph of saliva and thermal cycling on the adaptation of different denture base materials

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1 The influence of different ph of saliva and thermal cycling on the adaptation of different denture base materials Yasir A. Hussein, B.D.S. (1) Shatha S. Al-Ameer, B.D.S., M.Sc. (2) ABSTRACT Background: The difference in the adaptation of denture bases processed from heat cure acrylic, light cure acrylic and nylon materials and the effect of artificial accelerated aging (thermal cycling and ph) on the adaptation was studied. Materials and methods: One hundred eighty stone cast were prepared and assigned into 3 main groups according to the denture base material that processed on them(heat cure, light cure, nylon), each denture base and its cast was sectioned at posterior palatal border and the gap between the denture base and the cast was measured using digital microscope. Half of each group exposed to thermal cycling (2000 cycle) between (5 C-55 C ) 30 second in each bath then the adaptation was evaluated again. All denture bases were stored in artificial saliva of different ph (5.7, 7,8.3) at 37 C for 30 days, then the adaptation was measured again. Results and discussions: The results showed that the adaptation of heat cure acrylic was better than light cure and nylon before and after aging process. The nylon showed the poorest adaptation. Thermal cycling decreases the adaptation of all denture bases. Variation of ph effected the adaptation of heat cure and light cure denture bases and did not affect the adaptation of nylon. Keywords: adaptation, light cured resin, nylon, thermal cycling and the ph of saliva. (J Bagh Coll Dentistry 2012;24(3):47-53). INTRODUCTION Adaptation in prosthetic dentistry means the degree of fitness between the prosthesis and supporting structure (1). The adaptation is important because the retention of complete denture depends on the proper fitness (2) and to ensure that the denture should maintain its shape over a period of time (3). There are several methods that are used in the measurement of the adaptation accuracy of upper complete denture like microscopic measurement of the gap space between the denture base and the cast or weighing an elastomeric materials impressed under the denture base (4). Adaptation of the denture is affected by several factors as the previous studies showed which are 1-palatal shape which was found that the better adaptation was on shallow and medium palatal depth than the deeper one (5) 2-liner shrinkage which was found that Linear shrinkage should be less than 2% for the denture to be clinically acceptable and to have minimal effect on adaptation and cuspal integration (6). 3-curing method It was concluded that all types of resin showed processing contraction and the best fitting type were prepared from auto polymerizing and microwave resins while the poorest fitting was for the conventional heat activated resin (7). (1) M.Sc. Student, Department of Prosthetics Dentistry. (2) Professor, Department of Prosthetics Dentistry, College of Dentistry, University of Baghdad 4-polymerization procedure as the studies showed that the use of tension system and immediate post pressing time improved the adaptation of the denture (8) 5- thickness of the denture which showed statistical significant effect on the adaptation (9) 6-thermal shrinkage as the shrinkage of conventional heat cure resin is mainly thermal and this demonstrate the advantage of low temperature cycle in reducing (10) thermal shrinkage 7-seperating medium produces shrinkage at the thick section and this lead to dimensional changes of the denture (11) 8- water sorption which partially compensates the polymerization shrinkage and improve the denture adaptation (12) 9- type of stone as the studies showed the use of type IV dental stone improved fitness of the denture (13) 10-internal stress as poor fitness of the denture may arise from faulty processing as immediate insertion in boiling water results in high internal stress, also rapid cooling tends to increase the stress in the denture which are released later (14) Aging process could also affect the adaptation, as the denture while in use is submitted to rigours clinical conditions as alteration in ph, salivary flow and temperature, therefore in an attempt to simulate this condition, several methods are used like thermal cycling and storage in distilled water and exposure to UV light and water spray (15). PH changes may contribute to dimensional changes. The ph of saliva could be more acidic due to certain types of food like orange juice, sugar like candy, pastries, smoking or disease Restorative Dentistry 47

2 which could cause the saliva to be acidic like Sjögren's Syndrome and chemotherapy (16). Saliva ph could be more alkaline due to food like amaranth or due to disease like problem in digestive functions, including enzyme production and pancreas secretions, and eliminative functions, especially the liver and lymphatic's (16). The purpose of this study was to evaluate the effect of aging process on the adaptation of the denture bases made from the three materials (heat cure acrylic, light cure acrylic, nylon). MATERIALS AND METHODS The total number of samples maxillary denture base with their corresponding casts" were 180 samples. They were divided into three main groups according to the type of materials of the denture base (Heat cure acrylic, light cured resin and nylon denture base materials). Each main group of denture bases was subdivided into two groups (with thermo cycling and without thermo cycling). Each sub group consisted of 30 samples. Each sub group was subdivided into three smaller groups each one contained 10 samples of denture bases stored in artificial saliva of different ph (acid, base and neutral).all the denture base samples were of 2mm thickness Cast preparation Silicon mold of upper edentulous arch and type 3 dental stone were used in preparation of 180 cast following usual conventional procedure. Hot cure denture base preparation 60 denture bases were prepared using biostar machine and biostar plate of 2mm thickness, the biostar plate was trimmed and sealed on the cast with wax in usual manner. Flasking The cast with the corresponding biostar sheet was flasked in the lower half of flask with plaster. The upper half of flask was secured to the lower half and 2 nd layer of plaster was poured and allowed to set for 1 h. Then wax was eliminated with the use of boiling water and the biostar plate was removed. One ml of separating medium was applied to the surface of cast and mold and distributed evenly by brush. Packing and curing The powder and liquid was mixed at a ratio of 3:1 by volume and left until it reached the dough stage then placed over the cast and trial closure was made then final closure was made under pressure of 20 bar for 5 min. The flask was immersed in water bath until it reached at 100 o C and kept for 30 min. The flask was removed from the bath and allowed to cool for 3 h then deflasked. The cast with corresponding denture base was sectioned at 5 mm away from posterior palatal end. Light cure denture base preparation 60 record bases were prepared using the same biostar machine but the procedure was done in two steps, step one produces spacer of 2 mm thickness. The second step produces the biostar plate to be adapted over the light cure material keeping edges of 2 nd sheet on the periphery of the cast to act as stopper. Sheet of visible light cure resin was adapted to the cast and the biostar sheet was adapted over them until the stopper touched the border of the cast. The cast with it's corresponding resin sheet and biostar plate was placed in light cure unit and cured for 12 min then the cast was removed and the light cure sheet & the biostar plate was placed again in the curing unit in the reverse direction and cured for another 12 min according to manufacture instruction. The cast with it's denture base was sectioned in the same manner as for heat cure acrylic. Nylon denture base preparation The biostar plates were prepared in the same manner as for heat cure acrylic. The cast with it's corresponding biostar plate was placed in the lower half of the flask. The mixture of water and plaster was poured in the lower half. Sprue (one major 6mm diameter and 3 minor of 4 mm in diameter) were prepared to form passage way for injected nylon, the upper half of the flask was placed and plaster was poured and allowed to set then wax was eliminated and mold was left overnight to get rid of moisture. The nylon was injected inside the flask at 287 o C after 12 min of heating the nylon capsule. Then the flask was allowed to cool for 15 min. The flask was opened and the sprues were cut then the cast denture base set was sectioned in the same manner. Thermal cycling Half of the samples in each group (heat cure, light cure, nylon) were thermal cycled between (5 o C to 55 o C ± 2 o C) for 2000 cycle with an immersion time of 30 seconds at each temperature and a total cycle time of one minute. Artificial saliva preparation The artificial saliva was prepared in three different ph 5.7,7 and 8.3. An electrolyte composition similar to that of human saliva was used in this study which includes (17) : Na2HPO g/l NaCl 0.700g/l KSCN 0.330g/l KH2PO g/l Restorative Dentistry 48

3 NaHCO3 KCl 1.200g/l 1.500g/l Buffer solution was prepared first from KH2PO4 and Na2HPO4. By dissolving each salt in 1 liter of de ionized distilled water. The acidic solution (5.7 ± 0.01) was prepared by placing 500 ml of KH2P04 in graduated flask and Na2 HPO4 solution was added gradually until it reached to the required ph, then the remaining salts of artificial saliva were added and the volume was completed to 1 liter by de ionized distilled water. The basic saliva (8.3 ± 0.01) was prepared by placing 500ml of Na2HPO4 in graduated flask and KH2PO4 solution was gradually added until it reached to the required ph then following previous procedure The neutral saliva solution was prepared as previously and the amount of Na2HPO4 solution was added until it reached ph 7 as it was recorded by the digital ph meter placed in the graduated flask. All the samples were stored in artificial saliva inside the incubator at 37 o C for 16 hours and for 8 hours inside the distilled water which simulated the daily usage of denture by the patient. This cycle was repeated for each sample for 30 days Adaptation measurement. The digital micro scope (Dino lite) was used in measuring the space between the cast and the denture base margin at 3 points which are the depth of vestibule A, the crest of the ridge B, and the midline C as in (fig.1). Measurements were made immediately after sectioning and for the half of the samples after thermal cycling and for all the samples after 30 days of storage in artificial saliva of different ph. Table 1: The mean distribution for the space(mm) for heat cure acrylic group points Mean S. D S. E A B C N =60 Table 2: The mean distribution for the space (mm) for light cure acrylic group points Mean S.D S.E A B C N =60 Table 3: The mean distribution for the space (mm) for nylon group points Mean S.D S.E A B C N =60 Comparing the adaptation of different materials; the light cure material had a larger gap space than the heat cure in (figure 2) this may be due to difference in material composition and technique used for polymerization as no pressure is applied during curing of light cure while the pressure is ordinary used in flasking and curing of heat cure, this result is in agreement with Emre et al (21). The nylon material showed the greatest gap space which is 4 times greater than heat cure at point C and 3 times greater than light cure material at point C as shown in(fig.2) this may be due to the difference in chemical composition between the materials, Nylon shows high mold shrinkage as the result of their crystallinity (22). Figure 1: The position of three points (A,B,C) RESULTS AND DISCUSSION The effect of the material before aging process The result showed that the greatest gap space was at point C for the three materials as in tables (1,2,3). This result is in agreement with Mohamed Tahir (18),Abdul Khalik (19) and Abdul Razzaq (20). Figure 2: The mean space (mm) for the heat cure, light cure and nylon at the three points (A,B,C) Restorative Dentistry 49

4 The effect of thermal cycling on the material Heat cure acrylic. There was significant increase in the mean space in the three points (A,B,C) as shown in (figure 3). This may be due to contraction of denture base due to further polymerization that occur due to exposure to high temperature with reduction in the spaces between the chains of the polymer. This result is in agreement with Ogawa and Hasegawa (23). Light cure resin. Mean gap space was increased significantly at point A,B and non significant effect on point C. This can be explained as point C is in the center of the sample and will receive more light intensities from two sides of the curing chamber more than points A and B as they are located in position that receive less intense light, which might be less effective in curing therefore after thermal cycling there might be more polymerization and more shrinkage at these points A&B. while point C was not affected as it had already been more polymerized before thermal cycling as shown in (figure 3). Nylon. The mean gap space for the three points was increased. This might be due to the reaction between amide and amine terminal groups by heating (24) such reaction could lead to the contraction of denture base and increase in the gap space, or due to exposure of polyamides to heat and oxygen which may cause changes in physical and chemical characteristics due to thermal oxidative degradation (25). There is highly significant difference between heat cure &nylon and light cure &nylon at the three points. There is significant difference at points B and C between heat cure and light cure as shown in (figure 3). Figure 3: The effect of thermal cycling on the three materials (heat cure, light cure, nylon) at the three points (A,B,C) be due to erosion (26) or due to cationic reaction (27) as shown in (figure 4). Base. The increase in the mean gap space of denture base after immersion in basic saliva might be either due to the erosion by basic media or due to hydrolysis of the polymer that will lead to more hydrogen bonding between the chains and more contraction will occur. Point C showed significant difference due to topographic effect (18) as shown in (figure 4). Neutral. The storage of heat cure acrylic denture base for 30 days in neutral saliva did not produce any significant change in the space which means no significant expansion or contraction as shown in (figure 4) Figure 4: The effect of ph changes on heat cure acrylic groups Light cure resin Light cure acrylic Acid. There was significant increase in the gap space for the three points after storage 30 days in saliva which might be due to erosion (26) as shown in (figure 5). Base. There was increase in the mean space for the 3 points due to erosion by basic media but it was only significant at point B which is due to strain release of maxillary record base which tends to draw the flange inward, and the resulting premature contact of the denture with the mold in these regions cause the denture to be elevated (28) as shown in (figure 5). Neutral. There was significant increase at point B& C due to strain release and premature contact of the flange (28) or could be due to water solubility of light cure (29) as shown in figure 5. The effect of ph changes Heat cure acrylic. Acid. There was significant increase in the mean gap space for the three point (A,B,C) after storage in saliva for 30 days as shown in graph this might Restorative Dentistry 50

5 Figure 5: The effect of ph changes on light cure acrylic groups Nylon. In acid, base and neutral saliva there was significant decrease in the mean gap space. This might be due to the polar properties of resin macromolecules through a diffusion process (15) or might be Due to presence of functional groups such as amine, amide and carbonyl in the structure of nylon polymer, therefore water absorption is likely to be higher than PMMA (24) as shown in (fig.6). Base. Significant decrease in the gap space for the three points (A,B,C)might be due to that thermal cycling might increase the degree of polymerization of heat cure acrylic by which the acrylic might become more resistant to the attack by basic saliva therefore effect of water absorption was more pronounced that antagonize the effect of erosion so the space was decreased because of expansion. Also the effect of acidic saliva was greater than the effect of basic saliva which might support the hypothesis of the negligible effect of basic saliva on heat cure acrylic after thermal cycling as shown in figure 7. Neutral. The decrease in the gap space for the three points after storage in neutral saliva for 30 days might be due to stress relaxation so the better adaptation to the cast and reduction in the space as shown in figure 7. Figure 7: the effect of aging process (thermal cycling + ph(acid,base,neutral)) on heat cure acrylic groups Figure 6: The effect of ph changes on nylon groups The effect of ph changes and thermal cycling together Heat cure acrylic Acid. The mean distribution showed significant increase in the gap space at point C this may be due to increase the degree of polymerization by the thermal cycling and therefore the erosion effect by acidic media was significant at point C only as this point is the most frequent point for change due to topographic factor (18) as shown in (figure 7). Light cure resin Acid. There was non significant effect for the three point after thermal cycling and storage in acidic saliva this might be due to the thermal cycling might increase the degree of polymerization which make the resin more resistant to erosion effect of acidic media. This may explain the non significant difference between after thermal cycling and after immersion in acidic saliva as shown in figure 8). Base. Storage in basic saliva after thermal cycling showed significant decrease in the mean gap space for the three points because thermal cycling increased the degree of polymerization this might cause more resistant to the effect of basic saliva as its effect is weaker than the effect of acidic saliva and so the effect of water absorption was more pronounced which lead to expansion and reduction in the space. As polymerized networks which contain polar groups such as hydroxyl, carbamate (urethane), amide, ester and/or ether linkage are hydrophilic in nature and tends to be Restorative Dentistry 51

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