How Has the Recent Interest in High Strength Ceramics Influenced the Development of Chairside CAD/CAM Clinical Applications?
Dennis J. Fasbinder, DDS | Daniel J. Poticny, DDS | Russell Giordano II, DMD, DMSc, FADM, FADI
Dennis J. Fasbinder, DDS, is a clinical professor in the Department of Cariology, Restorative Sciences, and Endodontics at the University of Michigan School of Dentistry.
Daniel J. Poticny, DDS, is an adjunct clinical associate professor in the Department of Cariology, Restorative Sciences, and Endodontics at the University of Michigan School of Dentistry and maintains a private practice in Grand Prairie, Texas.
Russell Giordano II, DMD, DMSc, FADM, FADI, is an associate professor and the director of biomaterials in the Department of Restorative Sciences and Biomaterials at Boston University’s Goldman School of Dental Medicine.
Dennis J. Fasbinder, DDS: Chairside CAD/CAM systems (eg, CEREC, Dentsply Sirona) have been embracing the clinical application of high strength ceramics for single tooth restorations for some time. With excellent published clinical results relative to both strength and esthetics, lithium disilicate (eg, IPS e.max® CAD, Ivoclar Vivadent) has been the most popular chairside CAD/CAM ceramic block material for more than 7 years. Full contour zirconia restorations have been a more recent introduction to the chairside CAD/CAM material line. In 2016, Dentsply Sirona introduced the CEREC SpeedFire, which was the first induction oven for rapid zirconia sintering. This enabled full contour zirconia to be sintered in less than 20 minutes, creating the opportunity for fabrication and delivery in a single dental appointment.
The success of high strength ceramics has also enabled the development of new chairside CAD/CAM clinical applications. In January 2014, the CEREC software introduced the ability to design and fabricate implant-supported restorations. This included screw-retained crowns fabricated from lithium disilicate, hybrid ceramics, and provisional materials as well as custom abutments made from full contour zirconia, lithium disilicate, or hybrid ceramics.
High strength ceramics have also been used to fabricate short-span fixed partial dentures with good clinical success. Both lithium disilicate and full contour zirconia may be used in 3- to 4-unit fixed partial dentures. The main limiting factor to their use is the connector size.
The majority of chairside CAD/CAM materials require delivery by adhesive bonding to maximize their physical properties and ensure clinical longevity. However, the introduction of full contour zirconia as a single appointment alternative offers clinicians the opportunity to conventionally cement the restoration rather than adhesively bond it. Conventional cementation tends to be a preferred delivery technique for most clinicians because generally, it is a more consistent and efficient process. This preference can be expected to contribute to the increased use of full contour zirconia as a chairside CAD/CAM material.
Daniel J. Poticny, DDS: Having produced chairside restorations since 1995-along with mentoring, teaching, and advocating for their use-I have been intrigued by how high strength ceramic materials have influenced both the decision to incorporate chairside CAD/CAM into a practice as well as the treatment decision-making process.
In the early years of chairside CAD/CAM, the profession's perception was that the original adhesively bonded silicate glass ceramics were not strong enough to make chairside CAD/CAM worth the investment despite overwhelming evidence to the contrary. Material strength likely influences material selection more than any other parameter. Clinical failures (eg, breakage, fracture) are expensive to a practice; thus, it's easy to understand how clinicians adopt the mantra of "stronger is better."
When newer materials were afforded to chairside dentists around 2008, sales increased for chairside systems due to the stronger is better perception. Application-wise, the newer materials reversed the trend established by chairside dentists from that of inlays, onlays, and cusp replacements using a "defect-oriented approach" to that of full coverage/full contour crowns predominately made from these high strength materials. This occurred despite the fact that full coverage/full contour crowns generally require more chairside effort by the dentist and offer less potential benefit to the patient if the conservative preservation of tooth structure is valued.
Although no argument can be made to fault the performance of high strength ceramics, nearly the same can be said of their lower strength high glass content counterparts. Chairside CAD/CAM affords unique opportunities to leverage esthetic materials with tooth sparing preparations to quickly and reliably deliver restorations without the extra effort required for high strength ceramics. High strength materials have their place, but not as a universal solution where chairside applications are routinely employed. This mindset is tantamount to adapting every patient to a material, not the other way around.
Russell Giordano II, DMD, DMSC, FADM, FADI: As advances in CAD/CAM have made the technology much easier to use, it has become the dominant method of fabricating indirect restorations. In turn, this has led to ceramics becoming the dominant indirect restorative material. There are a variety of ceramic materials available that possess a wide range of translucencies and mechanical and physical properties. These materials can meet most clinical requirements.
One mechanical property that many dentists and manufacturers focus on is strength. However, strength can be deceiving when one only looks at the numbers. In fact, the flexural strength of natural teeth is about 120 to 160 MPa, which is in the range of feldspathic block materials (eg, Vita Mark II, Carestream Dental; IPS Empress CAD, Ivoclar Vivadent). So how do they survive? The natural tissues have an excellent ability to resist fracture, and the interaction between the dentin and crystalline enamel helps prevent catastrophic failure.
Another important property is fracture resistance. The types of zirconia that exhibit transformation toughening (ie, white and intermediate translucency) have the ability to change crystal size under stress to help prevent crack propagation and catastrophic failure. Materials such as glass ceramics leverage a high crystal content to prevent cracks. Interpenetrating phase materials have an interconnected structure that inhibits crack growth, which can remain intact in the planes above and below the fracture.
The load-bearing capacity of a material is another criterion. All of the materials used for indirect restorations have minimum thickness requirements that should be accomodated by tooth preparation. Even when a ceramic possesses high strength, if the minimum thickness for a given clinical indication is not respected, then the restoration is likely to fail. This is similar to building a house; larger beams are used to support high load-bearing areas. From clinical experience, we know that if lower strength materials are used with sufficient thickness, they survive just as well as zirconia.
One advantage to materials with high fracture resistance is that they facilitate the use of thinner crowns, thus preserving tooth structure and enabling restorations in cases where interocclusal space is limited. Thickness recommendations range from 2.0 mm for feldspathic materials to 1.0/1.5 mm for glass ceramics, 1.0 mm for interpenetrating phase materials, and as little as 0.8 mm for low translucency "transforming" zirconia. When conservative treatment is desired, the degree of restoration thickness that can be achieved in each case might be the ultimate determining factor in material selection, particularly for teeth under high stress such as molars and canines.