Dental Materials Journal 2010; 29(4): 411–417

Comparative adaptation accuracy of acrylic denture bases evaluated by two different methodsChung-Jae LEE1,2, Sung-Bem BOK1, Ji-Young BAE1 and Hae-Hyoung LEE1,3

1Department of Biomaterials Science, Dankook University, School of Dentistry, San-29 Anseo, Cheonan 330-716, Korea2Department of Dental Technology, Shinheung College, 117 Howon, Uijeongbu 480-701, Korea3Institute of Tissue Regeneration Engineering, Dankook University, San-29 Anseo, Cheonan 330-714, KoreaCorresponding author, Hae-Hyoung LEE; E-mail: haelee@dku.edu

This study examined the adaptation accuracy of acrylic denture base processed using fluid-resin (PERform), injection-moldings (SR-Ivocap, Success, Mak Press), and two compression-molding techniques. The adaptation accuracy was measured primarily by the posterior border gaps at the mid-palatal area using a microscope and subsequently by weighing of the weight of the impression material between the denture base and master cast using hand-mixed and automixed silicone. The correlation between the data measured using these two test methods was examined. The PERform and Mak Press produced significantly smaller maximum palatal gap dimensions than the other groups (p<0.05). Mak Press also showed a significantly smaller weight of automixed silicone material than the other groups (p<0.05), while SR-Ivocap and Success showed similar adaptation accuracy to the compression-molding denture. The correlationship between the magnitude of the posterior border gap and the weight of the silicone impression materials was affected by either the material or mixing variables.

Keywords: Acrylic denture base, Adaptation accuracy, Correlationship

INTRODUCTION

Among the polymer materials introduced in prosthetic dentistry, Polymethyl methcrylate (PMMA) is the only proven material for a successful denture base on account of its optimal physical properties and excellent esthetics with relatively low toxicity compared to other plastic denture bases1-3). A long-established method for denture processing for acrylic polymer is a closed-flask compressing molding with heat activation in a water bath for resin polymerization2). However, polymerization shrinkage of the resin and distortion of the denture base due to thermal stress is virtually unavoidable during the processing of dentures. These adverse effects cause movement of the artificial teeth position and increase the gap between the denture base and underlying mucosa, resulting in an ill-fitting denture4-6). It has been suggested that an accurate fitting is a key factor in the physical mechanisms of complete denture retention7,8).

Several alternative methods to conventional compression-molding processing for denture base have been developed to increase the adaptation of the denture base, such as injection-molding9) and fluid-resin10) techniques. Recently, many manufacturers have introduced newer denture processing systems using light-curable and microwave-curing resins2). Therefore, the adaptation accuracy or dimensional changes in the denture bases has become a focus of many studies in removal prosthodontics.

A variety of methods have been used to evaluate the dimensional changes and/or adaption accuracy of a denture base, there is still no gold standard. Microscopic measurements of the posterior border

gap11-16) and weighing a elastomeric material impressed under a denture base11,17-19) are one of the most frequently used methods to evaluate the denture adaptation to a master cast due to their simplicity and feasibility. The magnitude of the palatal border gap can express the ability of a border seal, and the weight of an impression material indicates the overall volume of internal space under denture base, which are all important factors in complete denture retention8). A reasonable correlationship between the results of both parameters can be expected for a well-fitted denture base. However, much of previous results regarding the adaptation accuracy of a denture base with variations in the test method are inconsistent20).

This study compared the adaptation accuracy of a maxillary complete acrylic denture base processed using fluid-resin, injection-molding, and conventional compression-molding techniques. Methods of direct measurements of the posterior palatal border gap and weighing the impression material between the denture and master cast were used to evaluate the adaption accuracy of denture base specimens. A correlation between the data measured using these two different test methods was investigated in this study.

MATERIALS AND METHODS

Preparation of denture base sampleThe adaptation accuracy of denture bases conducting should be measured using specimens with the same size and shape for comparison. In order to fabricate uniform dentures, thirty-six master casts were made from a silicone negative replica (U-402, Nissin, Kyoto, Japan) for maxillary complete edentulous arch using a

Received Oct 23, 2009: Accepted Mar 12, 2010doi:10.4012/dmj.2009-105 JOI JST.JSTAGE/dmj/2009-105

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high-strength stone (Fujirock, GC, Belgium) (Fig. 1). The impression was taken from each master cast using a resin individual tray (Quicky, Nissin Dental, Japan) and polyether impression material (Impregum, 3M ESPE, Germany). A yellow stone (Neo Plumestone, Mutsumi, Japan) was poured into the impression to make the respective working cast for denture processing. One complete wax denture was formed on the master cast using base plate wax. The palatal base was adapted using one layer of baseplate wax (Hard wax, Daedong, Korea) at a uniform thickness of 1.7 mm.

It was reported that the adaptation accuracy of the upper denture base can be influenced significantly by the existence of denture teeth4,5,18). Therefore, anatomical artificial teeth (#264 for anterior, 32M for posterior, Biotone, Dentsply, USA) were arranged in the wax denture. The wax denture-cast set was impressed with a silicon putty impression material (Exafine, GC, Tokyo, Japan) to make the index mold for reproducing the wax denture. The putty mold with denture teeth positioned in the indexed places was carefully seated on each working cast. The wax denture was completely reproduced by injecting molten baseplate wax into the mold space through two holes prepared on both alveolar ridge ends (Fig. 2).

Total thirty-six wax dentures on working casts were prepared for this study. The wax dentures with their respective master casts were divided into six groups of different denture processing systems. One group of denture bases was processed using the fluid-resin technique (PERform-Inkovac-system, Hedent, Germany). Three groups were processed by their respective injection-molding techniques; SR Ivocap Injection system using High impact resin (Ivoclar, Liechtenstein), Success injection system with Luciton 199 (Dentsply, USA), and Mak Press (Saitama, Japan) with Vertex RS resin (Vertex, Zeist, Netherlands). Another two groups were processed by compression-molding using the silicon-gypsum technique at two different curing cycles in a bronze flask. The wax denture was covered with silicone putty (Platinum95, RO, Italy) without the cusp tips of resin teeth. The overall procedure was similar to that used by Becker et al.21). The dentures were polymerized with Vertex RS resin at 74°C for 9 hours (slow curing cycle) or by boiling in water for 30 minutes (fast curing cycle). Six maxillary complete dentures for each group were prepared using the respective special equipment according to the manufacturer’s instructions. Table 1 provides detailed information on the denture base materials and processing techniques.

After polymerization, the curing flasks were bench-cooled to room temperature, and the denture samples were retrieved from the working cast. All injection spruces and small flashes were removed carefully and the denture bases were polished with a wet rag wheel and pumice. All dentures were cleaned and stored in distilled water at 37°C for 14 days before the measurement.

Fig. 1 A silicone negative replica for maxillary edentulous arch and high-strength stone master cast.

Fig. 2 Silicone putty mold with denture teeth positioned in the indexed places (a) and the reproduced wax denture (b).

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Measurement of adaptation accuracyTo observe the overall gap formation of the denture base, one denture placed on the respective master cast for each group was scanned by computerized tomography (CT). The CT images were obtained at a scanning interval of 0.6 mm from the anterior to posterior areas using 64 channels MD-CT (Light Speed VCT, GE, USA). The frontally-sectioned images of the denture-cast set and sagittal images reformed at the palatal midline were obtained from the CT data.

For the posterior palate border gap measurements, the denture base placed on the respective master cast was trimmed to a horizontal line 5-mm away from the posterior end using a vertical trimmer (MT2, Renfert, Germany) with a diamond disc under water cooling. The cut surfaces were polished using P# 1,000 SiC abrasive paper to allow a visual distinction. Digital images were then captured at the area of mid-palatal line and buccal vestibules using a zoom stereomicroscope (SMZ-1500, Nikon, Tokyo, Japan) with 3.0 megapixel CCD camera (Moticam 2300, Motic, Hong Kong) at a ×125 PC-monitor magnification. Whilst capturing the images, the denture bases were positioned on their master casts by fixing with two clothespins exerting a 10-N force. Calibrated image software (Motic Images Plus 2.0) was used to measure the maximum border gap (Pmax) within a 5-mm range at the posterior palatal midline and deepest vestibule

gap (V) in micrometer resolution (Fig. 3). For consistency, two examiners checked all measurements simultaneously. The dimension, V, was determined from the mean of both vestibule gaps (V=(V1+V2)/2). The gaps at the top of alveolar ridges and palatal slope were not measured due to difficulties in obtaining accurate readings22).

Product/Denture resin Group Batch no.

Powder/Liquid Processing type Powder/Liquidratio

Polymerization procedure

PERform-Inkovac-System (Hedent, Germany)/Inkovac pouring resin

PER FB07681/JI19736

Fluid-resin Chemical activation

30 g/21 ml Pouring in vacuum, Pressurized bath at 6-bar/45°C

SR-Ivocap Injection System (Ivoclar, Liechtenstein)/High impact standard kit

IVO K44278/K09925

Injection-molding Heat activation

20 g/30 ml Air-pressing at 6-bar, boiling in water for 30 min

Success® Injection System (Dentsply, USA)/Luciton 199 (Dentsply)

SUC 051208/060207

Injection-molding Heat activation

21 g/10 ml Air-pressing at 6 bar, heat curing at 73°C for 9 h

Mak Press (Toho Inc., Japan)/Vertex RS (Vertex dental, Netherlands)

MAK YN452P02/YK482L01

Injection-molding Heat activation

23 g/10 ml Air-pressing at 8 bar, heat curing at induction range for 30 min

Conventional Heat-curing/Vertex RS (Vertex dental, Netherlands)

CMS YN212P02/YK211L06

Compression-moldingSlow heat activation

23 g/10 ml Pack and press, heat curing at 74°C for 9 h at Hanau bath

Conventional Heat-curing/Vertex RS (Vertex dental, Netherlands)

CMF YN212P02/YK211L06

Compression-moldingFast heat activation

23 g/10 ml Pack and press, boiling in water for 30 min

Table 1 Fabrication systems for denture base and processing techniques

Fig. 3 Measurement of the maximum palatal border gap (Pmax) and vestibule gap (V1 and V2) between the denture base and master cast.

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As a secondary evaluation of the adaption accuracy, the weight of the low viscosity silicon materials impressed on the respective master cast was measured for all dentures. First, white silicone for fit checking (Fitchecker II, GC, Tokyo, Japan) was hand-mixed thoroughly at equal lengths of the base and catalyst for 20 seconds, and placed on the tissue side of the denture. The denture base with the impression paste then seated on the master cast and positioned on a constant loading apparatus (A-001, MECC, Tokyo, Japan) with a load of 49-N from the master cast. After 5 minutes, the load was removed and the excess material beyond the denture base was trimmed from the entire denture periphery with a scalpel. Then the set silicone material was carefully peeled away from the denture base and weighed immediately on an analytical balance (Voyager, Ohaus, NJ, USA) to a level of 0.1 mg. The procedure was repeated twice for each denture and the average weight of the impression material is expressed as the adaptation accuracy of the denture base.

After two days storage of the dentures in water, the weighing procedure of the impression material was repeated using an automixed polyvinyl siloxane light body (Perfect-F, Handae, Korea) with an auto-mixing gun to prevent possible error from imperfect mixing or an inconsistent mixing ratio of the silicone material. However, the weight of the automixed silicon impressed was measured only once.

The adaptation accuracy for the sample groups was examined by an analysis of variance (ANOVA) with a Tukey’s multiple comparisons test at p<0.05 level. The correlationship of data between the gap measurement and weight of silicone for all thirty-six dentures was examined using Pearson’s correlation test.

RESULTS

Figure 4 shows the CT images reformed at the mid-sagittal line of denture bases on the respective master cast. For all dentures, gap formation between the tissue surface of the denture base and master cast increased from the anterior to posterior side of plate and also from the alveolar ridge to the mid palatal line.

However, the gap distance or volume could not be measured from the CT images due to the low resolution.

Table 2 summarizes the posterior border gaps at the area of the mid-palatal line (Pmax) and buccal vestibule (V), as well as the weights of the impression materials. One-way ANOVA revealed a significance difference between the groups at each of the evaluation results. The Tukey’s post-hoc multiple comparisons test showed significantly smaller Pmax values for the fluid-resin group PER and an injection-molding group MAK than those of the other groups (p<0.05). Similarly the V values of the PER and MAK groups were smaller than the other groups. Group MAK produced a significantly smaller mean weight of automixed silicone impression material (AS). There was no significant difference in the adaptation accuracy between the compression-

Fig. 4 Computerized tomography images for each denture at the mid-sagittal line.

Denture groupPosterior border gap (µm) Weight of impression materials (g)Pmax V FC AS

PER 210 (112)a 284 (66)a 2.01 (0.09)ab 2.15 (0.17)b

IVO 427 (116)b 584 (85)b 2.00 (0.11)ab 2.35 (0.10)b

SUC 406 (145)b 527 (121)b 1.82 (0.13)a 2.19 (0.28)b

MAK 234 (53)a 375 (71)ab 1.78 (0.02)a 1.50 (0.20)a

CMS 419 (57)b 547 (189)b 2.15 (0.18)b 2.61 (0.22)c

CMF 375 (91)b 580 (198)b 2.18 (0.26)b 2.11 (0.12)b

Standard deviation in parentheses. Mean values superscripted by the same letter at each parameter are not significantly different (p>0.05).

Table 2 Mean values and standard deviations for the maximum posterior border gap (Pmax) and vestibule gap (V), and the weight of hand-mixed silicone (FC) and auto-mixed silicone (AS) impression materials

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molding groups (CMS and CMF), except for the weight of AS.

Figures 5-7 show the correlation plots of Pmax as a function of the vestibule gap (V), weight of the hand-mixed silicone (FC), and automixed silicone (AS) with their respective Pearson’s correlation coefficient (PC). The Pmax value revealed a moderate but significant correlation with the V value (PC=0.633, p<0.01) and weight of AS (PC=0.434, p<0.01). However, there was little correlation between the Pmax value and the weight of FC (PC=0.167, p>0.05).

DISCUSSION

Gap formation between the denture base and cast are generally attributed to polymerization shrinkage of the resin material and a tendency of cooling shrinkage toward the central area of the denture base, as well as to subsequent distortion caused by confinement of the surface topography of the alveolar ridge23). Therefore, the greatest gap will generally be observed in the central portion of the posterior border and buccal vestibule area, while the contact state around the ridge crest remains stable. The CT images of individual denture base-cast set did show this tendency of gap formation in medial-lateral and anterior-posterior areas (Fig. 4). These findings are also predictable with the results reported by Consani et al.15), who compared the posterior border gap of the denture base-cast sets sectioned transversally at each area of the canine, molar and posterior ends. Moreover, the magnitude of the posterior border gap generally increased medially along the palatal vault reaching a maximum at the midline of the plate11-13). These results suggest that the overall internal adaptation of the denture base can be evaluated simply from the maximum border gap (Pmax) at the mid line of the posterior plate23-26). The magnitude of the vestibular gap (V) showed a somewhat stronger correlation with that of Pmax

11,12), as shown in this study (Table 2, Fig. 5).

Two injection-molding denture groups (IVO and SUC) had a similar degree of Pmax and V gaps to the compression-molding groups (CMS and CMF), and also showed a similar weight of the impression materials. These results are in general agreement with previous results4,5,22,27) reporting no significant difference in internal adaptation between the denture bases processed using injection-molding (IM) and compression-molding (CM) techniques. The IM denture has known to produce a less increase in vertical dimension of occlusion than CM denture28-30), which caused by movement of artificial teeth during processing. However, a minor change in the vertical dimension is probably not related directly to adaptation accuracy of a denture base, because their horizontal dimension changes (anterior-posterior or medio-lateral) are not different between the IM denture and CM denture29,30).

However, one injection-molding denture, MAK, showed a better accuracy of fit than the other injection-

Fig. 5 Correlation between the maximum palatal border gap (Pmax) and vestibule gap (V).

Fig. 6 Correlation between the maximum palatal border gap (Pmax) and weight of hand-mixed silicone (FC).

Fig. 7 Correlation between the maximum palatal border gap (Pmax) and weight of auto-mixed silicone (AS).

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molding and conventional groups. Moreover, MAK produced the least mean weight of both impression materials showing excellent internal adaption of the denture base. Only Matsumura et al.24) reported the adaptation accuracy of the MAK by measuring the posterior palatal gap. Although they used different resin base materials to this study, the posterior palatal gap (227±38 µm after 14 days storage in water) at the palatal midline of MAK was similar to the present result (234±53 µm), which showed better adaptation than some compression-molded denture bases. The MAK denture is polymerized in the induction-heating range and formed by injection molding through a gateway formed by three forked wax sprues attached vertically to the denture base of the palatal area (Fig. 8), which was originally introduced by Prior9). Unlike other injection molding systems, vertical injection against the denture base is probably attributable to the higher adaptation accuracy of the MAK denture.

The fluid-resin processing system with an autopolymerizing resin (PER) produced significantly smaller Pmax and V dimensions than those of the other groups. It has been reported that autopolymerizing resins, including PER, produce a denture base with a more accurate fit7,11,31). However, the weight of the impression materials (FC and AS) of PER in this study was not lower than those of other groups. This might be explained by the relatively lower ability of a detailed reproduction by fluid-resin processing32), possibly due to the lack of resin packing pressure.

In previous data27), the magnitude of mean total volume of space between the denture base and master cast (as measured by the moiré topography) well correlated with the respective mean weight of the impression materials for four different denture systems, although the correlation between these data for individual samples was unknown in the study. However, in the other report11) on the adaptation

accuracy of six different dentures measured by weighing an impression material and subsequent optical measurements, the rank order of mean values for the denture groups was not the same for both measurement methods.

In the results of present study, there was a statistically significant positive correlationship between the maximum border gap, Pmax, and the weight of an automixed silicone (AS) for all denture specimens in the six groups (Figs. 6 and 7, n=36). However, the strength of correlation (PC=0.434) was moderate, suggesting that there may be uncertain factors in comparing both methods for an evaluation of the adaption accuracy. Moreover, a poor correlationship was observed between the weight of hand-mixed silicone (FC) and Pmax. This result might be due to the difference in the mixing procedure and/or in viscous changes after applying the load of the impression materials. Although a standardized process in the weighing the impression material for evaluating denture adaptation has emphasized11,18), further variables, such as the mixing ratio, type of material, viscoelastic behavior with time and temperature, and applied load should also be considered33).

The previous results reported an increase in the weight of the impression material trapped under the denture base after storage in water for 14-30 days19,34), while the maximum palatal gap of several denture bases decreased after storage in water for more than 14 days24). Therefore, a repeated measurement method for weighing impression materials of the denture base may produce an incorrect measure of the adaptation accuracy. In our preliminary study, a considerable increase in Pmax was observed when it was measured after measuring the impression material (data not shown). It is recommended that the measurement of the gap dimensions of the denture base should be conducted primarily before weighing the impression material, as in this study.

The clinical significance of the adaptation inaccuracy measured in a denture base-cast set is controversial22). Moreover, consideration of factors other than the adaption accuracy have been advocated in the selection of denture processing system21,22). However, newer denture materials and processing techniques are being introduced continuously because the contemporary technique for denture fabrication are still unsatisfactory35). A relatively higher fit of the denture base may be helpful in reducing the initial adaptation period of the denture wearer. A further standardized method for comparing the adaptation accuracy of the denture base with well-controlled clinical comparison is needed. Micro-CT was successfully used to measure the marginal fit of a dental ceramic crown36). This nondestructive examination can be more constructive method to evaluate the adaptation accuracy of denture base.

Fig. 8 Mak Press denture base with three forked sprues after deflasking.

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ACKNOWLEDGMENTS

This study was supported by Priority Research Centers Program through the NRF of Korea funded by the Ministry of Education, Science and Technology (No. 2009-0093829).

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