| Abstract|| |
The aim of this study was to evaluate the marginal gap in terms of cement film thickness associated with shoulder, shoulder with 45° bevel, shoulder with 30° bevel and chamfer, under thermo-mechanical loading. Forty human mandibular molars were prepared and restored with ceramo-metal crowns. Teeth were thermo-mechanically loaded and vertically sectioned to evaluate the cement film thickness. Shoulder with 45° bevel provided the least marginal gap as compared with all the tested finish lines.
Keywords: Finish lines, marginal adaptation, mechanical loading, thermocycling
|How to cite this article:|
Gupta R. In in-vitro evaluation of effect of different finish lines on marginal adaptation in metal-ceramic restorations under thermo-mechanical loading. Indian J Dent Res 2011;22:608-10
Marginal fit and adaptation is crucial for the prognosis of restorations including metal-ceramic crowns. Any marginal discrepancy can lead to microleakage, marginal discoloration, secondary caries and eventually failure of restoration.  'Various factors including ceramic firing effect, curvature of finish line and type of cement on the effect of the marginal adaptation of metal-ceramic crown restorations have been reported.  Regarding finish lines, there are conflicting results in the literature. Majority of authors suggests that shoulder finish lines produce less marginal discrepancy as compared to chamfer finish lines. On the other hand, several investigators have reported that the finish line design has no effect on the marginal discrepancy of conventional full crowns. , The restoration and the teeth are unavoidably subjected to thermal and mechanical stresses.  Very few studies have evaluated the effect of finish line on marginal adaptation of metal-ceramic crown restorations under thermo-mechanical loading. The aim of this study was to evaluate the marginal discrepancy in terms of cement film thickness between the restoration and the prepared tooth with different margin designs - shoulder, shoulder with 45° bevel, shoulder with 30° bevel and chamfer, under thermo-mechanical loading. Null hypothesis was kept that different margin designs will have no effect on the marginal discrepancy.
|How to cite this URL:|
Gupta R. In in-vitro evaluation of effect of different finish lines on marginal adaptation in metal-ceramic restorations under thermo-mechanical loading. Indian J Dent Res [serial online] 2011 [cited 2019 Sep 20];22:608-10. Available from: http://www.ijdr.in/text.asp?2011/22/4/608/90315
| Materials and Methods|| |
Forty freshly extracted, caries free, human permanent mandibular third molars (having approximately same crown width and length) were collected. The specimens were partially embedded in acrylic resin with a layer of poly-vinyl siloxane matrix to simulate periodontal ligament. Samples were randomly assigned to one of the four finish line design groups: 1 mm 90° shoulder, 1 mm 90° shoulder with 45° bevel, 1 mm 90° shoulder with 30° bevel, 0.5 mm chamfer [Figure 1] and [Figure 2]. A poly-vinyl siloxane matrix was made for every tooth prior to tooth preparation to serve as a guide for amount of preparation. Prepared teeth were duplicated in polyvinyl siloxane impression material and type IV stone dies (Whip Mix) were fabricated in them. Four layers of gold and silver die spacer (Nice-Fit, Shoufu Dental) were alternately applied on the occlusal and axial surfaces, avoiding the finish line. Wax pattern was made with blue inlay wax (Kerr) and castings were performed with silver-palladium alloy (Palliag M, Degussa). Copings were tried and porcelain firing was performed. The restorations were luted with dual-cured resin cement (Calibra Esthetic Resin Cement, Dentsply/Caulk). A uni-axial load of 45 kg was applied and the cement was polymerized from all directions, for 20 seconds with a light curing unit. Teeth were embedded in a cold cure resin block with an elastomeric impression material lining (to simulate the periodontal ligament). The samples were subjected to cyclic loading of 150,000 cycles at 60 N (simulating 6 months of oral masticatory stresses. Simultaneously 5000 thermal cycles (5 ± 2°C to 55 ± 2°C, dwell time 30 seconds) were performed. The specimens were vertically sectioned with a diamond disk on ultra structural analysis cutting machine (Buhler Ltd, Evanston, Ill). The cut surface was finished with 240 grit sandpaper towards the crown to eliminate flash. The cement film thickness was then measured (at 100 × magnification) to the nearest micron with an eyepiece micrometer mounted on optical microscope (MP 320; Carl Zeiss) (accuracy: 0.001 mm). All measurements were performed by an examiner who was blinded to the test groups.
|Figure 1: Schematic representation of various finish lines during the study|
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|Figure 2: (a) Shoulder finish line; (b) Shoulder with 45° bevel; (c) Shoulder with 30° bevel; (d) Chamfer|
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| Results|| |
Marginal gap was measured in each group and evaluated using one way analysis of variance (ANOVA) test [Table 1]. There were significant differences among all groups.
|Table 1: Marginal gap in all groups of tooth preparation with different finish lines during the study|
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| Discussion|| |
In the present investigation, thermo-mechanical loading was applied to simulate the clinical conditions. Overall the marginal gap was less than 100 μm, which is considered as clinically acceptable,  though there was a statistically significant difference between the marginal gaps associated with different finish lines. Shoulder with 45° bevel exhibited minimum marginal gap, and chamfer finish line exhibited maximum marginal gap. The results of this study can be attributed to the concept of reducing the marginal opening of casting by the use of a beveled finish line.  As the bevel approaches parallelism with the path of insertion of the restoration, the thickness of the space between the bevel and the restoration approaches to minimum value. Also bevel on the margin of a prepared tooth helps to reduce the inherent defects in casting and cementation, protects the enamel rods at the margins and develops circumferential retention.
| Conclusion|| |
Results of this in vitro study suggest that finish line designs may influence the marginal adaptation of ceramo-metal restorations. Clinically, shoulder with 45° bevel design facilitate marginal adaptation in comparison to shoulder or chamfer design and may be preferred for this type of crown.
| References|| |
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|2.||Beschnidt SM, Strub JR. Evaluation of the marginal accuracy of different all-ceramic crown systems after simulation in the artificial mouth. J Oral Rehabil 1999;26:582-93. |
|3.||Syu J, Byrne G, Laub LW, Land MF. Influence of finish-line geometry on the fit of crowns. Int J Prosthodont 1993;6:25-30. |
|4.||Boening KW, Wolf BH, Schmidt AE, Kästner K, Walter MH. Clinical fit of Procera AllCeram crowns. J Prosthet Dent 2000;84:419-24. |
|5.||Rosner D. Function, placement and reproduction of bevels for gold castings. J Prosthet Dent 1963;13:1160-6. |
Department of Prosthodontics, Institute of Dental Sciences and Technology, Modinagar
Source of Support: None, Conflict of Interest: None
[Figure 1], [Figure 2]