How to Choose an All-Ceramic Restorative
•Measuring Abrasivity and Flexural Strength
•Good News, Bad News
As dental technology improves, more dentists are becoming comfortable placing all-ceramic restorations. If ceramic materials aren't currently the mainstay of aesthetic dentistry, they soon may be.
Surveys vary as to the percentage of practitioners using this technology, with as many as 80% reporting they have placed all-ceramic crowns. Other surveys show that although the percentage of dentists using these restorations is rising, the number may be much lower.
Ceramic materials can be broken down into two categories: bonded and all-purpose. Bonded consists of ceramic materials used exclusively for all-ceramic restorations. These materials are typically reinforced porcelains developed to withstand forces of occlusion without the support of a metal substructure.
All-purpose includes ceramics primarily marketed for use in ceramo-metal restorations. Many of these porcelains vary only in color and translucency. But it's also possible to use these materials to fabricate inlays, onlays, veneers, and all-ceramic crowns.
Every brand of all-ceramic product will yield excellent results when used as indicated. Much more important than choosing the material for an all-ceramic restoration is meticulous attention to technique. With proper case selection, you can achieve an aesthetic, functional, and durable result with any current ceramic material. Without proper technique, even the best system is destined for failure.
Having a working knowledge of the strengths and weaknesses of each product gives you the flexibility to choose a porcelain system based on the difficulties unique to every clinical case. Ask these questions when choosing an all-ceramic system:
- Will the preparations be minimal reduction? If so, a stacked porcelain system will allow for thinner margins, whereas pressed systems require a deep chamfer or rounded butt-joint system.
- Is the patient a bruxer? An aluminous core porcelain yields significantly higher flexural strength. Although the flexural strength of other all-ceramic systems varies, all of the available porcelains are significantly stronger than conventional feldspathic porcelain.
- Are multiple types of restorations being fabricated? It's not uncommon to combine single-unit crowns, bridges, veneers, inlays/onlays, and porcelain-fused-to-metal restorations in the same mouth. Give consideration to one of the more versatile all-ceramic systems.
- Does the patient have high aesthetic expectations but is vague when describing the desired outcome? Provisionally cementing anterior restorations allows you to send the restoration back to the lab for modifications if the patient decides he or she isn't happy with the shade, contours, and so forth.
- Does the patient have periodontal disease? Supragingival margins allow for improved biocompatibility. Some all-ceramic systems allow for better aesthetics with exposed margins than others, and a bonded restoration yields superior aesthetics compared to all-purpose restorations where supregingival margins are concerned.
- What will the restorations oppose? Although most newer materials offer improved wear on opposing enamel compared to conventional feldspathic porcelain, all-ceramic materials still exhibit some stability.
- How much vitality does the restoration require? The bottom line is that some porcelain systems are more aesthetic than others. The hue, value, chroma, and translucency of the various porcelain powders and stains differ from system to system. Additionally, you can achieve natural fluorescence and opalescence to varying degrees, depending on the system. Currently, there is no consensus in the dental community as to which all-ceramic system offers the best aesthetics in all situations. Developing a discerning eye takes time, but experience and patient satisfaction may be the biggest factors when deciding which material offers the most natural results.
Measuring abrasivity and flexural strength
Abrasivity and flexural strength of all-ceramic restoratives can be difficult to assess. Several systems use a core material that can be stained and used as the final restoration. Alternatively, the core can be cut back, and special veneering porcelains can be layered over the core. In some systems, the core material offers the best wear rates on opposing enamel, while others have a more biocompatible veneering material.
In all cases, the core material offers the highest flexural strength. The flexural strength of all-ceramic systems is often portrayed to be an indicator of the overall clinical success of a particular system. However, flexural strength is just one of many variables that help when predicting a restoration's success. The value of flexural strength over a certain minimum amount of time may even be debatable. Certainly, no factor can be considered independently. In the fight for market share, dental manufacturers obviously want to portray their products in the best possible way. So clinicians should be aware that flexural strengths and wear rates may apply to veneering porcelain or core material.
In evaluating an all-ceramic system, keep these measures also in mind:
- The Vicker's hardness index is an important measurement of how well the porcelain holds up under occlusion forces. Remember that hardness and wear rates are unrelated.
- Marginal adaptation, measured in microns, is a good indicator of the relative fit of the porcelain system. Generally, a pressable system yields more intimate marginal adaptation compared to a stacked restoration.
- Thermal stability refers to the ability to fire the restoration multiple times without losing marginal detail. Should the restoration require reglazing, adding a contact, or staining, an all-ceramic material should be expected to maintain integrity throughout multiple firing cycles.
The more indications for a given system, the more versatile that system may be. Several systems even offer multiple-unit capabilities, while others offer the versatility of use over a metal framework.
Chemical composition varies in each system. Some systems use an aluminous porcelain core, a high-leucite porcelain, or low-fusing materials that differ in chemical make-up from conventional porcelains. The trend is toward more exotic materials and enamel-like molecular structures.
Choosing an all-ceramic restorative system may seem daunting. The good news is that a number of excellent choices exist for placing aesthetic, funtional, and durable restorations. The bad news is that the perfect system doesn't exist yet and may never be available. But knowing which material may yield the best results in a given situation is critical to performing excellent clinical dentistry.
Reference
Miller M. Reality. 13th ed. Houston, Tex: Reality Publishing Co;1999:80.