01 April 2021

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Do Different Finish Lines Determine Different Fitting of CAD-CAM Crowns?

Edoardo Mancuso

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It is well known that marginal and internal fit play a key role in the long-term success of indirect restorations. Misfit leads to plaque accumulation, secondary caries or periodontal diseases, especially in restorations with subgingival margins. The internal fit is a clinically relevant topic also because it codetermines the success, or failure affecting the retention and fracture resistance of a complete crown. To correctly define the fitting of an indirect restoration different measurements have been used in the literature over the years:  the marginal gap (the perpendicular measurement from the internal surface of the restoration to the axial wall of the preparation at the margin); the absolute marginal discrepancy (the angular combination of the marginal gap and the extension error); and the internal gap (the perpendicular measurement from the internal surface of the restoration to the axial wall of the preparation). It has been established that marginal gaps and internal gaps up to 120 mm and 150 mm might be considered clinically acceptable, however is still unclear if a finishing line design can improve or worsen the fitting of a CAD/CAM crown. 

An interesting research written by Dr. De Coster and his team, have been published recently in the Journal of Prosthetic Dentistry with the objective of evaluating the marginal fit, internal fit, and cement volume of lithium disilicate CAD-CAM crowns based on 3 different finish lines, chamfer, rounded shoulder, and feather-edge preparation. 

Materials and methods

In the study carried out at the Gent University, thirty anterior lithium disilicate complete crowns were fabricated following a digital workflow. The crowns were adhesively cemented on duplicate dies of the respective prepared typodont teeth, and the marginal gap, absolute marginal discrepancy, and internal gap were evaluated by using microcomputed tomography. The values obtained for each specimen were used to calculate the volume of the cement gap for each specimen utilizing of 3D region growing.

Results

In the study, marginal gap and absolute marginal discrepancy values were statistically significantly different between the groups. Marginal gap values were 23 ±14 mm for rounded shoulder, 54 ±28 mm for chamfer, and 96 ±36 mm for feather-edge finish lines. Absolute marginal discrepancy values were 96 ±34 mm for rounded shoulder, 124 ±37 mm for chamfer, and 157 ±34 mm for feather-edge finish lines. Internal gap values were 111 ±14 mm for feather-edge, 136 ±22 mm for chamfer, and 168 ±25 mm for rounded shoulder finish lines. The differences in cement volume between groups were instead not statistically significant.

Discussion

In the study, MG, IG, and AMD differed statistically significantly among the 3 finish line groups. From the results, Dr. De Coster speculates that the marginal gap of e.max CAD crowns was within the range of clinically acceptable values in all of the finish line analysed. However, the article continues assessing that the rounded shoulder delivered significantly inferior marginal gap and absolute marginal discrepancy values than the chamfer or feather-edge. As for the internal fit, the data collected in the research showed it was the feather-edge to deliver better results, followed by chamfer and lastly by the rounded shoulder. 

Conclusion

The article ends drawing the following conclusions:

1. The internal and marginal fit and AMD of e.max CAD complete crowns differed significantly ac- cording to the finish line configuration. 

2. e.max CAD complete crowns with a rounded shoulder finish line had the best marginal fit but the poorest internal fit. 

3. e.max CAD complete crowns with a feather-edge finish line had the best internal fit but the poorest marginal fit. 

4. The volume of the cement film was not significantly different among the 3 finish line groups. 

The precision of a CAD/CAM lithium disilicate crown is fundamental for the long-term survival of a restoration. Using a different finishing line design could enhance this precision but it is to know that many other factors are to be taken into consideration while deciding what preparation design to use. The preservation of tooth structure, the quantity of residual enamel, the presence of composite build-ups or different substrates, the cementation or luting procedures that will follow, are all factors that must be scrupulously analysed. For every tooth and every crown there could be a better finishing line design to use, and as dentists, we must learn which one to use in every different situation. 

For additional information:  Evaluation of marginal and internal fit of lithium disilicate CAD-CAM crowns with different finish lines by using a micro-CT technique