#6 The Influence of Keeper Tray Materials on Casting Precision

T.Kogiso, M.Sakane, Y.Nakamura, H.Kumano, Y.Ohno, T.Tanaka ¹, M.Okada ¹, Y.Tanaka
	Removable Prosthodontics, School of Dentistry, Aichi-Gakuin University 1Department of Dental Laboratory , Aichi - Gakuin University Dental Hospital

Introduction
A cast-bonding technique is one of two techniques use to fabricate a keeper coping of a denture with magnetic attachments. A plastic pattern is used in this technique. However, problems including a rough surface and a misrun inside the keeper tray are indicated for a keeper coping fabricated by a direct-bonding technique. Therefore, adjustment of castings is required at the lab bench and chairside (Fig. 1).


Fig.1


Objective
The purpose of the present study was to compare casting precisions between samples fabricated using a commercial ready-made pattern and a new prototype pattern.

Materials and Methods

1. Materials

GIGAUSS C 600 KB (GC) was selected as a commercial ready-made pattern sample. The casting pattern samples tested were fabricated of two different materials. The chief component of the first tray design was acrylic. The prototype second tray design was made of polyethylene plastic (Fig. 2).

commercial ready-made pattern 　 prototype pattern

Fig.2　Materials

2. Experimental items

Four different sampling categories with five testing specimens each were prepared: 1. Cast of a commercial ready-made pattern (AC), 2. Cast of a prototype pattern (PE), 3. Cast of a combined pattern of commercial pattern and dental wax (AC-WAX), and 4. Cast of a combined pattern of prototype pattern and dental wax (PE-WAX) (Fig. 3).

Fig.3　 Experimental items

3. Methods

This study was designed to prosthetically simulate an edentulous patient clinical situation. The preparation of an abutment for a keeper coping was performed using a conventional method, followed by abutment impression taking and second model fabrication (Fig. 4). Medium Inlay Wax (GC) was used for abutment and keeper tray wax pattern fabrication using conventional methods (Fig. 5). The keeper coping wax pattern was sprued with ready casting wax (R15), and placed on the investing ring pattern holding cone (Fig. 6). The pattern was conventionally invested(CRISTQUICK II GC), and cast with a Au-Ag-Pd alloy (CASTWELL MC GC).

Fig.4 preparation of abutment

Fig.5　Wax up

Fig.6 Keeper coping wax pattern

Casting was performed under the following two heating conditions shown in the following figure. The casting condition 1 shown in Fig. 7 follows the manufactures’ instruction for plastic material incineration. 1. Casting with heating at 250°C, and 2. Casting without heating at 250°C.

Fig.7 Casting condition

Casting was performed using a vacuum-pressure casting machine (Heracast RC, Heraeus). The casting was steam cleaned, and immersed in a ultrasonic cleaner with Palla-Clearn (GC) for 5 minutes, and testing was then performed (Fig. 8).

Fig.8 Finished cast

4. Evaluation
1) Cast observation using stereomicroscope
2) Quantitative evaluation of casting surface roughness

	Digital Microscope VHX-500 was used for obtaining measurements (Fig. 9). The surface of the keeper tray base was evaluated three-dimensionally to measure the vertical surface interval difference variations. These measurements results were quantitatively compared as findings of surface roughness variation.
		Fig.9 Digital Microscope VHX-500　（KEYENCE）

Results

1. Casting observations using stereomicroscope
Fig. 10 shows the observational comparisons between conventional AC and prototype PE castings with heating at 250°C. No major casting defect was observed in both samples.

Fig.10 Heating at 250°C (AC,PE)

Fig. 11 shows the observational comparisons between conventional AC and prototype PE castings without heating at 250°C. No major casting defects were observed in both samples.

Fig.11 No heating at 250°C (AC,PE)

Fig. 12 shows the observational comparisons between AC-WAX and PE-WAX castings with heating at 250°C. Casting defects were observed on the corner edges of all AC-WAX samples. No major casting defects were observed in PE-WAX samples.

Fig.12 Heating at 250°C (AC-wax,PE-wax)

Fig.13 No heating at 250°C (AC-wax,PE-wax)

2. Three-dimensional image and surface evaluation

Fig. 14 shows the AC and PE casting data. The numbers under the graph shows the mean values of 5 measurements for each sample. The measurements were 3.9 ± 0.5 μm with heating at 250 °C, 4.6 ± 0.3 mm without heating at 250 °C in AC, 4.3 ± 0.5 μm with heating at 250°C and 4.2 ± 0.3 μm without heating at 250 °C in PE.

Fig.15 Three-dimensional image and surface evaluation

3. Measurement of the casting surface roughness

No statistically significant differences were observed between AC and AC-WAX, and AC-WAX and PE-WAX with heating at 250°C. There was no statistically significant difference between PE and PE-WAX (Fig. 16). For samples without heating at 250 °C, statistically significant differences were observed between AC and AC-WAX, but not between PE and PE-WAX castings (Fig. 17). The comparison of samples depending on different conditions demonstrated a statistically significant difference in AC-WAX and PE-WAX (Fig. 18).

Fig.16 The casting surface roughness (heating at 250 °C)

Fig.17 The casting surface roughness (no heating at 250 °C)

Fig.18 Comparison of samples by different conditions

Discussion

In each tested casting condition, the differences in a surface roughness were confirmed in AC and AC-WAX using conventional keeper trays. No significant differences were observed in prototype keeper trays. It is speculated that acrylic, a chief component of the conventional pattern and wax adversely interact with each other during the heating step after investment flasking. Further studies are needed to elucidate the postulated relationships between the keeper tray materials and casting defects, and to determine the causes of casting defects.

Conclusion

Samples were fabricated using a commercial ready-made pattern and new prototype pattern, and then compared for casting precision. The following conclusions were drawn:
1. No major casting defects were observed in the conventional and prototype castings when the patterns
were individually cast.
2. The keeper coping patterns prepared by added dental 　wax demonstrated a significant unwanted
casting defects such as blow holes and burrs in the conventional type patterns.
No major casting defects were observed using the reported prototype casting pattern.
3. When dental wax was added to the original pattern to make the shaped coping, differences in surface roughness
depending on different casting conditions was observed in both the conventional and prototype keeper trays.

These results show that the prototype keeper tray design with the added dental wax technique demonstrated a better casting precision compared to the conventional type.

References

1. Tanaka,Y.: Dental Magnetic Attachment,Q&A,Ishiyaku Publishers,Inc.(Tokyo),1995.
2. Gillings,B.R.D.: Magnetic retention for complete and partial overdentures,Part.J.Prosthet.Dent.,45(5): 484-491,1981.
3. Jackson,T.R.: The application of rare earth magnetic retention toosseointegrated implant.Inc.J.Oral & Maxill.Imp.,1:81-92,1986.

Discussion Board