The Rationale for the Vented-crown Technique and Its Application in Today's Dental Practice

Abstract

It would be safe to assume that the integrity of a full coverage restoration is directly related to its marginal fit. Complete, unimpaired seating of a full coverage restoration can be significantly affected by hydraulic pressure from the cement.1–2 Hydraulic pressure, created by seating a restoration, especially a well-fitting one, can result in a final marginal gap that is clinically unacceptable. Incomplete seating due to hydraulic pressure is a greater problem with posterior teeth. The large, flat occlusal surface of this type of preparation creates a large well or platform for cement to pool and prevent seating. Also, preparations with long, parallel walls and a large occlusal surface are even more of a concern. Anterior preparations tend to be more conical in shape and have no or minimal occlusal table. They may be less affected with the problem of hydraulic pressure.3Improving the marginal fit of restorations can help to prevent the problems of open margins, cement washout, recurrent decay, poor gingival health and disrupted occlusion. The seating of a full coverage restoration can be improved significantly by using external venting,1–24–6 a die spacer7–9 and internal venting.310–12 When the literature is consulted, it is consistently found that, when no relief is used, the marginal gap can vary from 200–100 μ, based on the theory of hydraulic pressure.1–610–14 Using a die spacer will reduce the marginal gap to 60 μ.13–14 The best seating, close to 20 μ, was achieved when an external vent with die spacer was used.1–24–6 Different cement types do not seem to play a significant role in the marginal fit if a die spacer and external venting are used.13–15 Marginal design also does not seem to affect marginal fit, if a die spacer and/or vents are used.16 Actually, the better the casting fits at the margin, the more there is need for relief during seating, because hydraulic pressure has the potential to be a greater problem. An oversized, poor fitting casting cannot build-up as much hydraulic pressure.Venting to reduce internal hydraulic pressure may also have additional benefits with porcelain fused-to-metal crowns. Research indicates that internal hydraulic pressure can deform a crown and possibly contribute to premature porcelain fractures.17–18 The escape provided by the vent is a great way to reduce these internal hydraulic pressures and the potential for lateral deformation during seating.The use of a single internal vent channel placed on the wall of a preparation has also been shown to improve the seating of a full coverage restoration.310–12 The escape channel needs to be placed as close as possible to the margin and facilitate the release of excess cement. If the escape channel is under extended, in the range of 2 mm or greater from the margin, the benefit of this technique is lost. This technique has shown results in the 40–50 μ range for marginal gap sizes. It is a simpler technique and does not require a vent hole to be repaired.The goal of obtaining the best marginal fit seems to lie with using the die spacer and an external vent. The ease of using a die spacer alone will provide consistent margins of 60 μ, which may very well be clinically acceptable. However, when the extra step is taken with an external vent, the following benefits are achieved:This article reviews a simplified external venting technique first described by Basset2 and later by Seberg.22 This technique will be extrapolated to porcelain fused-to-metal crowns. The technique demonstrates a simple, yet effective, way to seal the external vent hole. Once the vent hole is sealed with a gold pin and polished, it is practically invisible. An example will also be shown and briefly discussed with a case using an internal vent.Special thanks go to my lab support: Penny Marazzo, Stoneybrook Noble Gold, 242 Stoneybrook Rd, Newtown, PA 18990, USA, (215) 860–3936 and Art Nelson, Bjorn Dental Lab, 16 Polly Drummond Hill Rd, Suite 7, Newark, DE 19711, USA, (302) 454-8245.