While dentistry has taken somewhat longer than anticipated to integrate high technology into its standard of restorative care, it appears that the new millennium has been a catalyst for change as there are now more than 20 different CAD/CAM systems being used by some portion of restorative dentistry.
Dentistry has cautiously welcomed a future that is based on technology adopted from aerospace, automotive, and even the jewelry-making industries. CAD/ CAM technology is now being accepted in dentistry due to the increased speed, accuracy, and efficiency it offers for restorative solutions without a compromise in quality, function or esthetics. Today’s CAD/CAM systems — both chairside and laboratory-based (e. g., Procera, Nobel Biocare, Yorba Linda, CA; Lava and Lava COS, 3M ESPE, St. Paul, MN; Cercon, Dentsply Ceramco, Burlington, NJ; CEREC, Sirona, Charlotte, NC; E4D Dentist, D4D Technologies, Richardson, TX) — is being used to design and manufacture metal, alumina, and zirconia frameworks, as well as metal-free (ceramic and composite) full-contour crowns, inlays, onlay and veneers that may be stronger, fit better, and are more esthetic than restorations fabricated using traditional methods. As restorative dentistry evolves into the digital world of laser image capture, computer design and milled creation of dental restorations through robotics; our perceptions, definitions and techniques of clinical dentistry must change to complement and maximize the potential of these changes in technology and material advances.
The E4D Dentist System (D4D Technologies LLC, Richardson, TX) introduced in 2008, (Fig. 1) along with its accompanying DentaLogic™ software and Autogenesis™ restoration proposal process, was the first to accurately present a true 3-D virtual model chairside that takes into consideration the occlusal affect of the opposing (antagonistic) dentition and enables the operator to design and edit multiple teeth at the same time. It essentially takes a complex occulsal scheme and its parameters and condenses the information, displays it in an intuitive format that allows dental professionals with basic knowledge of dental anatomy and occlusion to make modifications to the design, and then sends it through to a precise automated milling unit. For the dental profession, the introduction of the E4D Dentist system effectively automated many of the more mechanical and labor-intensive procedures (waxing, investing, burnout, casting, and or pressing) involved in the conventional fabrication of a dental restoration, allowing the dentist, dental assistant and/or technician to create functional dental restorations with a consistent, efficient, predictable and precise method. This process and technology has created a whole new area of professional opportunity amongst the dental team — the Chairside Dental Designer™ (D4D Technologies, Richardson, TX).
While these unique advances accelerate the restorative process, they cannot compensate for any compromise in the fundamentals of treatment planning, preparation design, soft tissue management and material selection.
The Take C. A. R. E. ™ Approach
A simple mnemonic can assist all dental professionals in the preparation design, material selection and treatment planning process.
Take C. A. R. E:
1) Conserve tooth structure
2) Advise the patient of all treatment options
3) Restore the form and function
4) Enhance esthetics
These characteristics should be considered with all treatment plan and restorative options, but most especially with metal-free CAD/CAM restorations.
In particular, conservation of tooth structure, keeping the margins away from soft tissue and relying on more partial coverage restorations are more appropriate to metal-free CAD/CAM restorations. Advising the patient on all treatment options is a professional responsibility, however additional options become available with chairside CAD/CAM Dentistry — Same Day Dentistry, Next Day Dentistry — are all possible and should be provided the patient. The restoration of form and function is the number one priority in restorative care and only then should the enhancement of esthetics be considered in treatment options. All of the Take C. A. R. E. elements are integral to the chairside CAD CAM restorative process.
Unique aspects of CAD/CAM (milled) Restorat ions
The introduction of chairside CAD/ CAM systems and advanced metal-free material options presents the dental professional with new opportunities for intra-oral preparation visualization. The optical imaging system can be used to magnify the virtual representation of his/her preparations, which enables them to enhance and improve their technique and design capabilities (Fig. 1). Features such as the ICE™ Mode (ICEverthing) with the E4D System also allow dental professionals to accurately view the enamel vs. dentin content, the presence of build up materials and to see the preparation in a visually informative method (Fig. 2).
Conventional porcelain fused to metal (PFM) and gold restorations require preparations with parallel walls and optimal resistance and retention form for macro-mechanical retention. With the advances in bonding technology and development of new luting agents that are based on the chemical bond and micro-mechanical retention to enamel and dentin, there is less need of creating this type of retentive preparation. It should be noted, however, that the restorative materials available for milled metal-free restorations also require unique preparation design criteria to optimize the performance of the materials.
The design of a preparation for a digitally designed metal-free restoration and the efficient creation of that design are based on several aspects: 1) preservation of tooth structure, 2) marginal integrity and 3) sufficient reduction for proper restorative material thickness. Also to be considered is providing enough occlusal reduction to allow the automatic occlusal anatomy software sufficient room to design a proper occlusal-anatomical proposal (Fig. 3). In addition to simply replacing lost or removed tooth structure, the restoration must preserve and reinforce the remaining tooth structure. Healthy tooth structure, which can be maintained while producing a strong, retentive restoration, should be saved. Whole surfaces of tooth should not be removed in the name of convenience or efficiency. Even though full coverage restorations are an excellent restorative option some form of partial coverage restoration should be considered (Conservation) and communicated (Advise) if warranted. Inlay-onlay, veneer and full crown restorations are all very successfully created in the digital world.
Preparation Design for Milled Restorations
Preparations for restorative materials fabricated via CAD/CAM do not differ significantly from traditional all-ceramic preparations — they in fact use the same restorative materials (e. g. IPS Empress). All metal-free preparation designs will be distinctly more tapered and have a more aggressive clearance requirement than preparations for many PFM and metal restorations. All ceramic preparations for CAD/CAM restorations may however have a slightly more tapered design than for pressed or layered ceramic fabrication, due primarily to restrictions on bur implementation vs. wax or direct application of wax or ceramic material.
In principle, for metal-free restorations, an axial reduction of at least 1.0mm and an occlusal reduction 1.5 -2.0mm is required (Fig. 4). To minimize stress concentration within the ultimate restoration and facilitate proper intra-oral laser scanning of the preparation, the use of sharp line angles, narrow boxes or tortuous grooves are contra-indicated. A medium chamfered or shoulder margin with adequate axial reduction with smooth flowing outline form and rounded internal angles is required. It is critical to round off all
surface transitions and there must be no residual sharp edges to the preparation. To facilitate appropriate tooth preparation for chairside CAD/ CAM dentistry a set of customized diamond burs is available for each type of preparation (Fig. 5). Preparation margins should be placed supragingivally wherever possible. The use of equigingival margins, if needed for aesthetic reasons, should be restricted to the labial aspect of upper anterior teeth. This will not only simplify scanning procedures but it will also help to maintain optimal periodontal health.
Even though recommended axial reduction can vary in thickness, it is important to respect proper occlusal reduction; this will allow correct reproduction of occlusal anatomy and maintain the optimal physical properties of the restorative material indicated. Adequate tooth preparation becomes more important in the case of all ceramic restorations where adequate material thickness is imperative to guarantee longevity of the restoration (Fig. 6). The preparation must be designed so that it would be possible to have an adequate bulk of material to allow the restoration to withstand the forces of occlusion. The contours of the restoration must be kept as ideal as possible to prevent either periodontal or occlusal problems. Inadequate clearance makes the restoration weaker. In addition, it leads to shallow, flat anatomy on the occlusal surface of the restoration; this will cause the restoration to be more easily perforated by intraoral finishing procedures. A very common mistake in tooth preparation design is flattening the occlusal table of posterior teeth; correct tooth preparation should follow the contours of the natural tooth. This allows optimal space to recreate or improve the original anatomy (Fig. 7).
Inadequate reduction in the axial walls leads to over-contoured restorations to compensate the material thickness or in other cases to weak restorations.
Build up materials are not required in many cases as the area prepared can be restored with the same restorative material of the crown, (e. g. IPS Empress CAD, IPS e. max CAD) maximizing the volume of the restorative material.
The placement of finish lines has a direct correlation on the ease of fabrication of a digitally created restoration, and also upon then ultimate success of the restoration. The best scanning results can be expected from margins that are as smooth as possible. When and where possible, the finish lines should be placed in an area where the margin is supra or equigingival.
Conventional margin placements with PFM restorations often placed the margin subgingival in order to “hide” the metal collar or the metal reflection near the feather edge margin. With metal-free dentistry the margin can be placed in the optimal position rather than a compromised placement for esthetic reasons. This can lead to a more conservative approach on margin placement. When margins are placed supragingival, the scanning process for modern dentistry and the impressioning technique for conventional dentistry is greatly enhanced. In addition, placement of any restorative material above the sulcus or tissue area aids in the periodontal health of the environment. Supragingival margins also facilitate the isolation and bonding process where total isolation is required.
Instrumentation For Optimal Preparations
Restorative dentists routinely use diamond rotary instruments. Many brands are available that vary significantly by manufacturing type, number of uses, diamond size, cutting efficiency and design.
Two-Striper™ Diamonds (Premier Dental) feature a patented PBS manufacturing process that fuses diamond crystals directly to one-piece hardened stainless steel shanks. The crystal exposure and the low matrix bond level sloops away the diamond particles. This allows for greater cutting edge exposure, which means the diamond instrument will cut faster and run cooler with less buildup.
PBS bonding maximizes the exposure of diamond cutting sur- faces — especially at the tips and upper circumference of the diamond instrument where most cutting occurs. Crystals placed evenly and precisely in a uniform matrix, are fused permanently bonded to a surgical-grade stainless steel shank; they are guaranteed to not fall out.
Diamond particles on electroplated burs are trapped mechanically in inconsistent plating layers that can expose either too much or too little of the particle. Some particles are attached loosely and are prone to premature dislodgement. Other particles are buried completely in the plating layer and are unavailable to cut tooth structure, creating unnecessary heat and friction. Preparation kits designed for metal free chairside systems (E4D Dentist Inlay/Onlay, Veneer and Full Coverage Kits) have been designed and optimized in conjunction with a bur manufacturer and CAD/CAM system.
Soft Tiss ue Management
With conventional impression techniques, typically a low viscosity material is injected into the sulcus or around the margins of a preparation. This physically distends or maintains space between the margin of the preparation and the adjacent or sometimes overlaying soft tissue providing a clear “read” of the margin. With optical scans, that is not the case; there is no physical interaction during the scan and the soft tissue — what you see is what you get. So it is critical that soft tissue management and clearance be a priority in preparation design when considering equigingival or subgingival margins. Creating a clear separation using cord, expansion materials (Expasyl, Kerr) or what is most commonly used, troughing with a soft tissue diode laser (e. g. Odyssey, Navigator, Ivoclar Vivadent, Biolase or VersaWave, KaVo) is imperative to optimally visualize the margin and capture it for virtual reproduction.
Fundamentals
CAD-CAM dentistry and technology is more than just a machine; it is a totally new method to create functional esthetic restorations for our patients in a more productive manner. While automation can provide new efficient methods to reach the desired goal of excellence in dentistry, the fundamentals of preparation design, material selection and proper restorative care cannot be ignored. Taking C. A. R. E. will provide your patients and your team with a systematic and successful approach to restorative dentistry, and combined with modern technological advances, will maximize the patient’s dental experience, satisfaction and long-term success.
Dr. Gary Severance Is Vice President Of Marketing And Clinical Affairs At D4D Technologies.
Dr. Lida Swan is Clinical Instructor at E4D University in Richardson, Texas.
Oral Health welcomes this original article.
———
There are now more than 20 different CAD/CAM systems being used by some portion of restorative dentistry
———
Restorative materials available for milled metal-free restorations also require unique preparation design criteria
———
When margins are placed supragingival, the scanning process for modern dentistry and the impressioning technique for conventional dentistry is greatly enhanced
