CAD/CAM technology was introduced to the dental world now over 20 years ago. The CEREC 1 machine with its large diamond milling wheel allowed the clinician to fabricate all-ceramic inlays and onlays from a monochromatic block of ceramic in a single-appointment. The dental CAD/CAM world took a large leap forward in the late 90s with the introduction of crown software for the CEREC 2 machine. All-ceramic inlays, onlays, veneers and full crowns could be milled out of a ceramic block using a large diamond milling wheel and a flat-end cylindrical milling diamond. The next quantum leap came with the introduction of CEREC 3 technology. CEREC 3 took full advantage of the dramatic increase in computer power and speed that the technology world was experiencing. CEREC 3 also refined milling performance by eliminating the large milling wheel and replacing it with a cone-shaped milling diamond for finer detailing of the occlusal surface of the final restoration. More recently, and possibly most significantly, the CAD/CAM world was introduced to CEREC 3D Version 3 software and a very important change to the milling diamonds.
The introduction of the step milling diamond with its 0.9mm diameter tip may prove to be one of the most brilliant changes to CEREC technology in recent history. The advent of the step milling diamond redefined tooth preparation design required for the CEREC system. The outcome is the ability to prepare teeth as conservatively as required for commercial laboratory manufactured all-ceramic restorations. Also, the latest version of CEREC software version 3 has been re-designed to create 0.9mm milling steps so that the step diamond and milling software work in unison to produce the smallest overmilling pattern in CEREC history.
A Historical Perspective
The diameter of the cutting end of the diamond tools (Fig. 1) and the milling software that drives those diamond tools demanded a specific tooth preparation (the ‘CEREC preparation’ — Figs. 2A & 2B) to ensure proper internal adaptation of the final restoration.
Previous versions of the software used 1.6mm flat-end cylinder diamonds to mill out the internal aspect of any restoration. Any tooth preparation performed without taking this into account resulted in overmilling of the ceramic at the internal aspect of the restoration (Fig. 3).
Although the voids created by overmilling invariably fill in with resin cement we have several sig- nificant problems. Many CEREC doctors simply remove more tooth structure than necessary to ultimately produce a restoration with at least 1.5mm of ceramic thickness. A ‘non-CEREC’ prep design with its concomitant overmilling pattern must have substantial tooth reduction in order to produce a restoration with a minimum of 1.5mm ceramic thickness. This of course is contrary to the modern-day principles of conservation of tooth structure.
A second problem is thickness of the resin cement in the area of the overmill. Ideally all all-ceramic restorations should be bonded with the thinnest possible amount of resin cement. This maximizes the ultimate strength of the overlying ceramic. Research done with anterior veneer preparations show that the ceramic must be at least three times the thickness of the underlying resin cement to ensure success. 1 Based on these values for anterior veneers this equates to as much as 4mm of reduction of tooth structure for each millimeter of overmilling.
A third problem which is relevant for anterior restorations is the creation of an unaesthetic ‘headlight effect’. If the resin cement is thicker in some areas on the labial surface of an anterior restoration the resin cement will show through the thin area of ceramic. This ultimately creates an unaesthetic result even if all other parameters of esthetic design have been meticulously followed.
Many CEREC doctors, in an attempt to have more flexibility in preparation design used the optional 1.2mm cylindrical diamond. Although this permitted a somewhat more conservative preparation it was fraught with difficulty. The 1.2mm diamond broke easily allowing approximately only five restorations per milling diamond. Another major difficulty was that the 1.2mm diamond flexed while milling thereby creating unwanted overmills (Fig. 4).
Another creative solution to this problem of overmilling is to mill out the restoration in Endomode. Endomode is one of the available milling pattern choices originally invented to mill out all-ceramic posts for endodontically treated teeth. Unfortunately all restorations milled in Endomode have some degree of ‘undermilling’. In some situations the amount of undermilling is insignificant and therefore clinically irrelevant. Unfortunately in most situations the amount of undermilling makes complete seating of the restoration impossible (Figs. 5-7). The clinician must then identify and reduce the internal aspect of the restoration or alternatively mark and adjust the tooth preparation. Either system is fraught with wasted time and inaccuracy.
A Brilliant Change
Recently, the CEREC system saw a change that freed the clinician nician from a specific tooth preparation defined by the size of milling diamond and the capabilities of the software. The introduction of the step diamond tool and matching milling software redefined the preparation criteria for the CEREC system making them similar to, and as conservative as, any commercial laboratory-fabricated all-ceramic restoration.
The step diamond tool incrementally decreases in size so that its cutting end is 0.9mm in diameter. The incremental size of the step diamond tool prevents flexing while milling (Fig. 8). The recently released version 3.x software directs this step diamond tool to cut in 0.9mm increments. Clinically this translates to a tooth preparation design similar to that required for a commercial laboratory- fabricated all-ceramic restoration (Fig. 9).
The Clinical Relevance
In the posterior segment these changes allow the clinician to perform truly conservative, bonded all-ceramic inlays, on-lays and crowns. Indeed, the concept of ‘defect-oriented’ dentistry prevails here. The final preparation of any posterior tooth would include all defects (fractured, worn tooth structure, caries and old restorations). Remaining sound tooth structure of adequate volume would be retained and the preparation is finalized with selective reduction to allow for proper thickness of ceramic as defined by any all-ceramic restoration. Refinement of margins completes the preparation.
In the anterior segment the above changes afford the clinician the freedom to prepare minimal thickness veneers and perform crown preparations similar to any commercial laboratory-fabricated all-ceramic crown.
A Case Report
Brian M. presented to the dental office requesting an improvement in the aesthetics of his smile. Specifically, he did not like the mismatch of colors of his maxillary six anterior teeth and wanted the spaces between the laterals and cuspids closed. PFM crowns for the maxillary lateral incisors and cuspids were originally done by the author in 1980. The central incisors had been previously restored with composite resin. The final accepted treatment plan was to fabricate all-ceramic crowns for the six anterior maxillary teeth. Due to the low smile line no treatment was accepted for an improvement to the soft tissue contour (Figs. 10-12).
The preparations of teeth #s 13; 12; 22 and 23 were pre-defined by the original PFM preparations. Due to the recent changes as described in this article to the CEREC system teeth #s 11 and 21 could be prepared ideally and as conservatively as any commercial laboratory-fabricated all-ceramic crown (Figs. 13-15).
This preparation design allows for minimal overmilling using the step bur and Version 3 software (Fig. 16A). As a comparison this same preparation exhibits moder- ate overmilling using the 1.2mm milling diamond (Fig. 16B) and severe overmilling using the
1.6mm diamond (Fig. 16C).
The restorations were completed in a single appointment. Vita Mark II TriLuxe ceramic blocks, the only polychromatic blocks available at the time of treatment, were used (Figs. 17-20). TriLuxe was the first block created for the CEREC system that was not monochromatic. This block not only has a gradation of chroma through the block but a change in opacity. The cervical area is therefore deeper in chroma with less translucency while the incisal third has less chroma and more translucency. Recently, Vita introduced TriLuxe Forte blocks with a much more subtle transition from one level of chroma/opacity to the next. Also, the TriLuxe Forte block has increased fluorescence, an ideal quality for life-like aesthetics. Ivoclar/Vivadent also has introduced the very aesthetic Multi Block. The Multi Block composition is similar to Empress with a very natural, almost infinite, gradation in chroma and opacity throughout the block. Excellent aesthetics may be expected using the Ivoclar/Vivadent Multi Block. 3M/ESPE has also introduced their Paradigm C block. This block with its low leucite, high glass composition and its high translucency and fluorescence also allows the clinician to create extremely life-like anterior restorations. All polychromatic blocks on the market today give the CEREC clinician incredible opportunity for true, life-like aesthetics. Interestingly, a tremendous amount of research and money is being invested by major dental companies to produce the ultimate aesthetic ceramic block for CEREC technology. This in itself is a strong indicator that the market direction in dentistry today is indeed CAD/CAM.
Conclusions
Today’s patients and dentists demand aesthetic dental restorations. All-ceramic restorations afford us the best opportunity to achieve this goal. Significant changes to the milling software and the size of the milling diamonds of the CEREC system coupled with the introduction of highly aesthetic polychromatic ceramic blocks allow the dentist to satisfy patient demand for aesthetics while preparing the tooth as conservatively as possible for all-ceramic restorations. These restorations, backed by over twenty years of clinical documentation, 2-5 will also satisfy the most discerning cosmetic dentist.
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Dr. Tsotsos is Past President, Canadian Academy of Computerized Dentistry; Past President, Academy of Computerized Dentistry of North America; Past Executive Member, International Society of Computerized Dentistry; Past beta tester for Sirona CEREC system. Dr. Tsotsos has used CAD/ CAM technology since 1996.
Reprinted with permission,www.cerecdoctors.com/cerecprospect owners. aspx?ow=false
References
1. The Journal of Prosthetic Dentistry Volume 81, Issue 3, Pages 327-334 (March 1999). Crack propensity of porcelain laminate veneers; A simulated operatory evaluation. Pascal Magne, Dr Med Dent, Kung-Rock Kwon, DDS, MSD, PhD, Urs Belser, Prof Dr Med Dent, James S. Hodges, PhD, William H. Douglas, BDS, MS, PhD.
2. International Journal of Computerized Dentistry 1/2006, P. 11-22. Clinical Results of Cerec Inlays in a Dental Practice over a Period of 18 Years. Dr. Bernd Reiss.
3. J Am Dent Assoc 2006; 137: 5S-6S. Dental CAD/ CAM systems: A 20-year success story E. Diane Rekow DDS PhD.
4. State of the Art of CAD/CAM Restorations: 20 years of CEREC. Mormann, Werner H. Quintessence Publishing.
5. J Am Dent Assoc 2006, 137: 22S-31S. Clinical performance of chairside CAD/CAM restorations Dennis J. Fasbinder, DDS.
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The advent of the step milling diamond redefined tooth preparation design required for the CEREC system
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The ceramic must be at least three times the thickness of the underlying resin cement to ensure success
