October 2012, Volume 8, Issue 10
Published by AEGIS Communications
Evolution of Chairside CAD/CAM Dentistry
Dentists who master the techniques and adapt their practices to this technology can produce chairside restorations to a high standard.
The use of computer-aided manufacturing computer-aided design (CAD/CAM) systems in dentistry, which were introduced in the mid-1980s, has increased dramatically during the last decade. The first generation of CAD/CAM was designed to fabricate immediate chairside inlay and onlay ceramic restorations.1 The initial CAD/CAM technology results looked very promising, but they required an excessive amount of time for fabrication. This first generation of computer hardware and software offered a limited 2-dimensional (2-D) view of the scanned images. The hard drive capacity was unable to store the large volume of data required for a 3-dimensional (3-D) view. The evolution of supportivecomputer technology over time has resulted in the chairside design and milling of.complete crowns and multiple-unit ceramic restorations to a high standard. As a result, CAD/CAM scanning and milling systems have be a practical clinical reality, which makes it possible for the dental professional to produce chairside restorations.2
The traditional method of making a dental impression with an elastic impression material can be alternatively performed with an intraoral digital scanner. This process is called the “optical impression.”3 Using either an optical laser or video digital technology, an intraoral digital impression-scanning wand is used to capture complete detail of the teeth and supporting soft-tissue structures. A specialized 3-D rendering program permits the images of intraorally scanned optical impressions to be visualized in 3-D on the computer monitor in real time. The dental restoration design software offered by D4D Technologies (E4D Dentist System, D4D Technologies, www.e4dsky.com) and Sirona Dental Systems (CEREC® AC, Sirona Dental Systems,www.sirona.com) is more intuitive and user friendly for the dental professional. These software programs come with features that allow dentists to mark the margins, digitally design virtual wax-up proposals of the restoration, place accurate occlusal contacts, and refine the proximal contact areas with the adjacent teeth. All of these tasks can be completed in minutes using the chairside design center before sending the final data to the computer-controlled milling unit. The following steps summarize the workflow: tooth preparation, intraoral scan, restoration design, milling of the ceramic monoblock, restoration finishing (coloring, glazing, polishing), and adhesive luting.
Simultaneously, there have occurred continual innovations in esthetic restorative materials. Monoblock ceramics can now withstand the stress of masticatory function as well as the damage introduced during the milling. The first-generation monoblocks made of feldspathic ceramic material have largely been replaced by reinforced ceramic with silica (feldspar, leucite, and lithium disilicate), non-silica (alumina and zirconia), and accombination of resin-ceramic–based materials resulting in a 3- to 11-fold increase in flexural strength.4,5 Factorial analysis of the variables influencing stress based on a computer simulation model showed that single thick monolithic all-ceramic crown materials performed better under stress compared to ceramic core material with veneering porcelain, aside from other influences.6,7 Furthermore, the coefficient of thermal expansion mismatch between core and veneer materials may initiate the internal stress that causes delaminating or internal cracking of porcelain.
The marginal fit of the milled ceramic restoration is an essential criterion for evaluating clinical success. Several investigators have evaluated the marginal fit of crown restorations fabricated with CAD systems. They reported an average marginal fit range from 25 µm to 113.88 µm.8-10 The authors’ study showed that a user experienced with scanning who had completed three full training sessions produced restorations with significantly better-fitting margins than an inexperienced user who had completed a half-day training session and had no prior experience with the CAD system.11 Furthermore, the marginal/internal crown fit of laboratory-fabricated all-ceramic crowns showed the same accuracy as the CAD/CAM chairside systems.12,13 A marginal gap up to 120 µm is considered to be clinically acceptable with a resin-bonded luting agent, and a marginal gap up to 160 µm might be acceptable with regards to longevity14-16 although, theoretically, requirements of cementation films should be between 25 µm to 40 µm.17
One concern about the ceramic block has been its monochromatic appearance. The early ceramic blocks for chairside milling were only available in limited shades. Dental professionals had to overcome this deficiency with external staining procedures. However, with the new advances in manufacturing technology, a greater selection of the blocks with esthetic qualities is available in the marketplace.
The polychromatic leucite-reinforced ceramic block (IPS Empress® Multi-block CAD, Ivoclar Vivadent, www.ivoclarvivadent.com) for E4D and CEREC systems has portioned cervical, body, and incisal segments. This block is incrementally graduated by chroma and value. This is done in an attempt to mimic the polychromatic effect of the natural dentition. The block’s three ceramic segments can be portioned in the milled restoration by the design software in the restoration proposal stage.
The lithium-disilicate glass-ceramic block (IPS e.max® CAD, Ivoclar Vivadent) for the E4D and CEREC systems is now available in more values and shades, with nine high- and low-translucency blocks. Ivoclar has recently introduced its new lithium-disilicate blocks called Impulse. These blocks are available in three brightness values—V1, V2, and V3—along with two opalescent shades—Opal 1 and 2. The Opal blocks are designed mainly to create thin veneers and partial and single crowns.
The feldspar fine-particle ceramic blocks also come with two new products: Vitablocs® Triluxe Forte and Vitablocs® RealLife (Vita Zahnfabrik, www.vita-zahnfabrik.com). Currently, these two new ceramic blocks only can be used in the CEREC system. The Vitablocs Triluxe Forte contains a graded variation in color saturation—with the middle layer (body) having a regular chroma; the top layer (enamel) having a low, less intense chroma with high translucency; and the lower layer (cervical) having the highest chroma and lowest translucency. This refined color gradation provides a smoother transition of color between layers that makes it possible to match the optical characteristics of natural tooth color, including translucency and color intensity.
The Vitablocs RealLife blocks have been created to mimic the tooth’s natural enamel-layered-over-dentin design. They are especially appropriate for the restoration of anterior teeth, to make them look as much as possible like natural teeth. These blocks are designed to reproduce the shade effect in regard to translucency, chroma, and lightness by positioning the restoration to be milled within the spherical dome of dentin, which is surrounded by more translucent enamel.
A reinforced resin-ceramic block has also been recently introduced to the market. The Lava™ Ultimate CAD/CAM (3M ESPE, www.3mespe.com) for the CEREC system is a unique new resin-nano-ceramic material for which the company is claiming long-lasting esthetics and performance. The advantage of this block is that post-milling oven firing is not necessary. However, data on material wear properties are not yet available at this time.
Limited clinical data using these new innovative esthetic ceramic-reinforced blocks has been reported in the literature. Herrguth et al evaluated two types of crowns made by layered-ceramic crown and monolithic CAD/CAM techniques on single anterior crown restorations. Both crowns were stained and glazed and evaluated by three independent examiners to assess the esthetic appearance. A scale of 1 to 6 was used, with 1 representing excellent characteristics and 3.5 marking the threshold of clinical acceptability. Regardless of the fabrication method, the crowns were esthetically acceptable in all 14 patients with no statistical difference between groups.18
These rapid advances in ceramic monoblock technology have radically changed the performance and perceived esthetics of the restorations milled by the CAD/CAM chairside system. There are four chairside CAD systems currently available in the market for dental professionals—CEREC® AC, E4D DentistTM, LavaTM C.O.S., and the Cadent iTeroTM (Cadent, www.cadentinc.com). Only two of these CAD systems, CEREC AC and E4D Dentist, have the linked CAM system unit that can mill the restorations in 7 to 30 minutes depending on the size and complexity of the restoration. The material selection to be used is decided on a case-by-case basis. For esthetic reasons, dental professionals may choose a fine feldspar-particle glass ceramic or a leucite-reinforced glass ceramic. As mentioned above, they are available in layered blocks with improved esthetic options. There are reinforced resin-ceramics, which may be chosen for their low modulus property and potential to decrease wear. These blocks do not require an additional firing process, but the leucite material does gain strength with oven firing during the stain and glaze cycles. The lithium-disilicate ceramic might be able to better withstand posterior mastication forces.
CEREC AC by Sirona is the newest version of CEREC; the earliest versions have been available since the mid-1980s. The system not only has the ability to mill a ceramic single-unit chairside restoration, but it can also mill a temporary three-unit bridge out of an acrylic block. There is also a block that functions as a wax casting (burn-out block) for a cast-metal crown. Through Sirona’s digital dental network, CEREC Connect, the optical impression can be sent out digitally by e-mail to the dental laboratory for fabrication of models, multiple units, bridges, implant abutments, and zirconium or metal crowns. It can also be integrated with Sirona’s Galileos system to construct surgical guides for implant placement. Sirona’s CEREC Biogeneric software can analyze the individual patient’s occlusion and the anatomy of the adjacent teeth so that the restoration can be designed to be patient-specific. The recently released software version, 4.0, is more intuitive and user friendly.
E4D, made by D4D Technologies, has been available since December 2007. Clinically, the system does not require the application of a contrast agent (an aerosolized spray opaque powder) on the teeth to be scanned, and the scanning wand can make contact with the target. E4D Compass integrates 3-D data from a leading cone-beam digital system that is the corollary of the Galileos system for implant surgical planning. With the release of version 2.0, E4D Dentist shares many of the above-mentioned features of the CEREC AC. The most significant shared feature is that both of these systems will be able to export their digital files in STL format, which is common to the stereolithography CAD data supported by many other 3D software packages, which are widely use in for rapid prototyping and CAM.
Based on the current information from Sirona, there are more than 11,000 CEREC users in the United States and 34,000 CEREC users internationally. This does not include the E4D systems. In addition, approximately 50 dental school in the United States use or have the CEREC or D4D CAD/CAM systems. Several dental schools have integrated the CAD/CAM technology into their pre-doctoral clinical curriculums. It has been estimated that by 2015, the number of CAD/CAM restorations—which includes crowns, bridges, veneers, and inlays—will be greater than 25% of the total units produced.19
A healthy 85-year-old woman presented to the dental office as an emergency, with a coronal fracture of tooth No. 7. After an examination of the remaining tooth structure, it was determined that tooth No. 7 required a crown restoration with a supported post-and-core restoration. The tooth was endodontically treated, followed by a prefabricated post-and-core build-up with composite at the same visit (Figure 1 and Figure 2). The color of the crown restoration was predetermined with the VITA Classical Shade Guide (Vident, www.vident.com), and a digital photograph was taken for tooth surface morphology. A conventional impression was taken with PVS impression material in addition to an optical impression with the E4D CAD/CAM system in the office. Digitally, a virtual-design wax-up of tooth No. 7 was designed following the margin markings (Figure 3). The sprue location was determined and the numerical crown design was sent to the computer-controlled milling machine. A temporary crown was milled and delivered to the patient due to the lengthy procedure in the first visit. The final restoration was restored with Impulse mono ceramic blocks, which come in five shades; D3-V1 block was the authors’ choice for this case. After milling, the crown was customized using laboratory burs (Figure 4) and was stained and glazed on the stone die (Figure 5). The patient was asked for her feedback about the esthetics prior to cementation on the second visit. She was extremely pleased with the final result. The crown was tried in, with limited adjustments needed, after which it was permanently cemented using accomposite luting cement (Figure 6).
A healthy 35-year-old woman was diagnosed with idiopathic internal resorption unresolved by endodontic treatment on tooth No. 9. The tooth was extracted and a socket preservation procedure was performed. The implant was placed 4 months later using a closed-flap procedure. After bone integration and healing, an optical impression was made with an iTero scanner to record the position of the implant fixture head with a scan body. The STL file was transmitted to the laboratory and the working cast and abutment were milled (Figure 7). The cast with the abutment replica was scanned with the E4D Dentist system, and the crown was designed and milled using an IPS Empress® CAD Multi Block (Ivoclar Vivadent). After milling and try-in (Figure 8) at the same appointment, the crown was stained and fired to enhance the blend color with the adjacent teeth (Figure 9).
It should be kept in mind that regardless of the technology used, the final restoration depends on the clinical skill of the dentist and his or her ability to employ CAD/CAM technology for the optimal clinical outcome. The practicing dentist must be willing to make the necessary practice changes as well as to take the time to learn the subtleties of preparation and the software. Everything starts with the proper diagnosis and tooth preparation, followed by the proper utilization of the available CAD/CAM technology to assist in the final restoration.
Drs. Kugel, Simon, and Stein are lecturers on CAD/CAM dentistry supported by Henry Schein and Sirona Dental Systems.
The authors would like to thank Lee Culp, CDT, for his assistance in the dental laboratory work in one of the cases reported.
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For product information about Chairside CAD/CAM, visit: dentalaegis.com/go/id427
About the Authors
Masly Harsono, DMD
Department of Prosthodontics and Operative Dentistry
Tufts University School of Dental Medicine
James F. Simon, DDS, MEd
Department of Restorative Dentistry
University of Tennessee College of Dentistry
James M. Stein, DMD
Assistant Clinical Professor
Department of Prosthodontics and Operative Dentistry
Tufts University School of Dental Medicine
Gerard Kugel, DMD, MS, PhD
Dean for Research
Department of Prosthodontics and Operative Dentistry
Tufts University School of Dental Medicine