Dental Advisor: All-Ceramic Restorations - 2008

This issue of THE DENTAL ADVISOR (October 2008, Vol. 25:8) reports on the performance of many of these restorations over time and provides the clinician with insight as to what works and why. Of special interest is the fracture rate of nine all-ceramic materials.

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Over the past 25 years, nearly 6,000 all-ceramic restorations have been placed and monitored by THE DENTAL ADVISOR. Dr. Farah placed restorations during the first 14 years; while during the most recent 11 years, Drs. Farah, Brown, Anderson, and their associates have added more than 4,000 restorations. Table 1 shows the number of restorations placed over that period of time.

All-Ceramic Restorations
Preparation Guidelines for All-Ceramic Restorations

• Open contacts with a #2 round (S.S. White Burs, Inc.) or #330 fissure carbide (S.S. White Burs, Inc.) bur.
• Reduce the interproximal with a cylinder bur (beveled cylinder 1812.8C NEODIAMOND/MICROCOPY).
• Reduce tooth circumferentially by 1.0-1.5 mm with a diamond bur (pointed taper 1718.8c NEODIAMOND/MICROCOPY) (Figure 1 - Dr. Farah's preferred bur).
• Reduce the occlusal 1.5-2.0 mm using a chamfer bur (bell C2C/Strauss Diamonds, chamfer C3C or C3F/Strauss Diamonds) (Figure 2 - Dr. Farah's preferred bur).
• Reduce the lingual of anterior restoration with a football diamond (1923.c, 1923.m or 1923.f NEODIAMOND/MICROCOPY).

• Uniform reduction results in optimal ceramic strength.
• Reduce the tooth uniformly circumferentially. This results in optimal stress distribution and minimizes potential fracture.
• Adequate reduction gives the laboratory more space to produce an esthetic restoration.
• Sharp edges result in high stress concentration and increase the potential for fracture of the ceramic.

Flaws in the restoration such as internal cracks, weakening of the material because of heating and cooling cycles, improper layering of the ceramic, and contour and anatomy are reasons an all-ceramic restoration can fail. Using the required firing and cooling temperatures of the material and the proper layering technique of the ceramic will strengthen the restoration.Internal cracks in the ceramic can develop during the firing and cooling process and in building the external layers of ceramic. Use of a trans-illuminating light (Microlux/AdDent, Inc.) on the intaglio side of the restoration will show internal flaws and cracks that are not always visible to the naked eye. If cracks appear, return the restoration to the laboratory. Patient occlusion can also contribute to the failure of an all-ceramic restoration. Patients that are heavy bruxers or have restorations with improper occlusion can put tremendous force on a restoration causing catastrophic failure.

High-leucite all-ceramic restorations should be bonded using a bonding agent and resin cement (see Table 2). Lithium disilicate and alumina restorations can be bonded or cemented with self-adhesive resin cement in selected cases. All-ceramic products are listed in Table 3.

Performance of All-Ceramic Restorations Over 25 Years

Fracture rates ranged from 2.1% at one year to 7.1% at 25 years. Over time, restorations will weaken, regardless of the material used. Figure 3 illustrates the fracture rates of all-ceramic versus PFM restorations. Many dentists and laboratories believe that metal is stronger than all-ceramic. However, our data show the failure rate of all-ceramic restorations at 10 years to be slightly less than the failure rate of PFM. In recent years, the cost of all-ceramic restorations has aligned with the cost of PFM due to an increase in the cost of precious metals.

There are many reasons why all-ceramic restorations fracture or chip.

Top Five Reasons for Fracture

• Improper tooth preparation
• Improper bonding/ cementation
• Pre-existing internal cracks
• High occlusal forces/bruxism
• Age of restoration

Marginal staining is noted by the graying margins of anterior and posterior crowns and veneers. Marginal staining is rarely noted for inlays and onlays, but occurs in 5.5% of other all-ceramic restorations at 10 years. The graying is unattractive to patients and is the most frustrating factor pertaining to cosmetic restorations, often occurring within days of cementation. Using a zirconia coping under a ceramic overlay masks the graying.

Possible causes of marginal staining are:

• Improper etching/bonding of tooth and/or ceramic.
• Bruxism resulting in high stress at the cervical margin, followed by debonding.
• Improper isolation/bleeding at cementation.
• 5th-generation bonding agents seem to be associated with a higher rate of marginal staining than 4th-generation bonding agents.

• About 3.6% of all-ceramic restorations (onlays, crowns and veneers) debond from 1 to 10 years of service.
• The debonding of all-ceramic inlays cemented with resin cement is quite low (< 1.0%).
• The failure of the bond could result from weakening due to thermal and loading cycling.

It is not always easy to determine if the bond failure is at the tooth/resin or resin/ceramic interface. Most often it seems to be related to both interfaces. Inlays have a much lower debonding rate, less than 1%.

• The wear of the ceramic and opposing dentition has not been a major problem in the past 10 years.
• With the introduction of low-fusing ceramics, wear of the opposing dentition has decreased markedly.

All-ceramic restorations are designed with extraordinary aesthetics in mind. Factors such as tooth staining and dark posts or cores can dramatically affect the overall final appearance and shade of the restoration. In recent years, aesthetic post systems and tooth-colored core materials have been developed to assist dentists in attaining excellent aesthetic results.

The dentist and the laboratory must have open communication and awareness of the limits of the material. These are the items you should send to the laboratory with each all-ceramic restoration request. Your preparation and clear communication with your laboratory will result in excellent aesthetics.

• Diagnostic wax-up
• Pre-op study models and digital photographs
• Custom shade or shade tabs in photo
• Helpful Hint – An easy way to take a preparation color is to use the VITA shade guide with the neck of the shade tab near the tooth. This is the dentin shade and will give an accurate shade to the laboratory.
• Preparation color
• Impression of patient approved temporaries
• Helpful Hint – The patient approved temporaries will provide the laboratory with the template for creating the ideal restorations.
• Bite registration

Trends in all-ceramic restorations are moving to exciting and new areas. Digital workflow and chairside CAD/CAM fabrication allow instant feedback for the dentist and laboratory. Our November 2008 issue of THE DENTAL ADVISOR will focus on the basic principles of CAD/CAM, the introduction of zirconia restorations, and the integration of digital dentistry into daily practice for laboratories and dentists.

Requests for reprints should be directed to The Dental Advisor.