Successfully reported this slideshow.
We use your LinkedIn profile and activity data to personalize ads and to show you more relevant ads. You can change your ad preferences anytime.

Cad cam and cad-cim in restorative dentistry


Published on


Published in: Health & Medicine

Cad cam and cad-cim in restorative dentistry

  1. 1. CAD-CAM AND CAD-CIM IN RESTORATIVE DENTISTRY Presented By- Dr. Nidhi Shrivastava Pg Student Dept. of Conservative Dentistry & Endodontics
  2. 2. CONTENTS  Introduction  History  The CAD/ CAM process  CAD/CAM Systems  Restorative Materials for CAD/CAM  Marginal Integrity of CAD/CAM Restorations  CAD- CIM IN RESTORATIVE DENTISTRY  Advantages/Disadvantages of CAD/CIM  Technical innovations  Conclusion  References
  3. 3. INTRODUCTION  The technological changes taking place are truly revolutionizing the way dentistry is practiced and the manner in which laboratories are fabricating restorations.  The advent of CAD/CAM has enabled the dentists and laboratories to harness the power of computers to design and fabricate esthetic and durable restorations. Sneha S.Mantri ,Abhilasha S. Bhasin. CAD/CAM in dental restorations: an overview. J. of Annals and Essences of Dentistry 2010; 2(3): 123-128.
  4. 4.  CAD/CAM technology was introduced to the dental community in the early 1980s. Since then , the technology has evolved in two directions :- A - One is the intra-operatory application for one appointment restoration fabrication. B - In parallel, CAD/CAM systems for commercial production centers and dental laboratories emerged, expanding the range of materials that could be used and the restoration types that could be produced.
  5. 5.  Advances in CAD/CAM provide a new horizon for dentistry, creating an alternative technique for producing dental restorations. It is possible to create dental restorations that are automatically produced and meet the requirements for fit and occlusion.  The flexibility, speed, precision and efficiency of these systems have made them useful for a wide range of application.
  6. 6. HISTORY  The major developments of dental CAD/CAM systems occurred in the 1980s. Dr. Duret was the first to develop dental CAD/CAM.  From 1971, he began to fabricate crowns with an optical impression of abutment followed by designing and milling. Later he developed Sopha system.  Dr. Mormann (1985)developed CEREC System, an innovative approach to fabricate same day restorations (ceramic inlays )at the chair side in the dental office.
  7. 7. Prof Werner Mörmann (left)and Dr Marco Brandestini in 1985 with the CEREC 1 prototype. (zurich university,switzerland)
  8. 8.  Dr.Anderson (1994)developed Procera System . He attempted to fabricate titanium copings by spark erosion and introduced CAD/CAM technology into the process of composite veneered restorations(1996) .  This system later developed ( 1998 onwards) as a processing center networked with satellite digitizers around the world for the fabrication of all ceramic frameworks .
  9. 9.  During the last 2 decades, exciting new developments have led to the success of contemporary dental CAD/CAM technology.  Several methods have been used to collect 3- dimensional data of the prepared tooth using optical cameras, contact digitization, and laser scanning.  Replacement of conventional milling discs with a variety of diamond burs has resulted in major improvements in milling technology.  Another vital factor has been the development of alumina (aluminum oxide) and zirconia (zirconium oxide) ceramic materials, which possess excellent machinability and physical strength
  10. 10. THE CAD/ CAM PROCESS  A CAD/CAM system utilizes a process chain consisting of scanning, designing and milling phases.  The scanning device converts the shape of the prepared teeth into three dimensional (3-D) units of information (voxels). The computer translates this information into a 3-D map (point cloud). The operator designs a restoration shape using the computer which generates a tool path, which is used by the milling device to create the shape from a restorative material.
  11. 11. CAD/CAM SYSTEMS  Based on their production methods these systems can be divided into the following groups. In office system : Most widely and commercially used in Cerec System. This system can scan the tooth preparation intraorally and by selecting appropriate materials, the dentist can fabricate the restorations and seat it within a single appointment. Anuvanice KJ (Ed) Phillips RW. Phillips Science of Dental Materials. 11th Edition
  12. 12. CAD/CAMS – Dental laboratory models: The indirect systems scan a stone cast or die of the prepared tooth, in the dental lab (eg Cerec-in lab). Many of this system produce copings which require the dental technician to add esthetic porcelain for individualization and characterization of the restoration.
  13. 13. CAD/CAM for outsourcing dental lab work using networks: since the design and fabrication of the framework for high strength ceramics is technique sensitive, new technologies using CAD/CAM with network machining center that is outsourcing the framework fabrication using an internet have been introduced.
  14. 14. COMMON CAD/CAM SYSTEMS Cerec : An acronym for chair side economic reconstruction of esthetic ceramic Cerec introduced in 1980s, improved cerec 2 introduced in 1996 and the advanced 3-D Cerec 3 in 2000. With Cerec 1 and Cerec 2, an optical scanner is used to scan the prepared tooth or impression and a 3-D image is generated on monitor. A milling unit is used to prepare the restoration. With newer Cerec 3-D, the operator records multiple images within seconds, enabling clinician to prepare multiple teeth in same quadrant and create a virtual cast for the entire quadrant.
  15. 15. Designed restoration is transmitted to a remote milling unit for fabrication. Cerec in-lab is a lab system in which dies are laser scanned and image displayed on screen. After designing VITA In-cream blocks are used for milling. The coping is glass infiltrated and veneer porcelain added . In vitro evaluation of marginal adaptation of crown of cerec 3-D (47.5 µm+-19.5 µm) was better compared with cerec 2 (97.0 +- 33.8 µm).(Ellingsen et al,2002) (left) Simulated digitized image, (right) partially milled feldspathic ceramic (VITABLOCS Triluxe Forte) processed by the Sirona inLab CAD-CAM system. (Left) CAD-CAM ceramic block before milling. (Center) An intermediate stage of milling. (Right) After removal of the inlay from the mounting stub.
  16. 16. DCS Precident: Comprises of a Preciscan laser Scanner and Precimill CAM multitool milling center. The DCS software automatically suggests, connector sizes and pontic forms for bridges. It can scan 14 dies simultaneously and mill up to 30 frameworks unit in one fully automated operation. It is one of the few systems that can mill titanium and fully dense sintered zirconia. An in vitro study showed that marginal discrepancies of alumina and ziroconia based posterior fixed partial denture machined by the DCS system was between 60 µm to 70µm.(tinschert et al 2001)
  17. 17. Cercon: commonly referred to as a CAM system, it does not have a CAD component. The system scans the wax pattern and mills a zirconia bridge coping from presintered zirconia blanks, which is sintered at 1350 C for 6-8 hrs. Veneering is done with a low fusing, leucite free cercon Ceram to provide esthetic contour. Marginal adaptation for cercon all ceramic crowns and fixed partial dentures was reported 31.3 µm and 29.3 µm respectively (Ariko et al 2003)
  18. 18. Procera All Ceram System – Introduced in 1994, it is the first system which provided outsourced fabrication using a network connection. Once the master die is scanned the 3-D images is transferred through an internet link to processing center where an enlarged die is milled by a computer controlled milling machines.
  19. 19. This enlargement compensates for sintering shrinkage. Aluminum oxide powder is compacted on the die and coping is milled before sintering at a very high temp (>1550°C). The coping is sent back to the lab for porcelain veneering. According to research data average marginal gap for Procera all Ceram restoration ranges from 54 µm to 64 µm.( May et al 1998)
  20. 20. CICERO system (computer integrated crown Reconstruction) – Introduced by Denison et al in 1999, it includes optical scanning, metal and Ceramic sintering and computer assisted milling to obtain restoration. Basic reconstruction includes layered life like ceramic for natural esthetics, a precision milled occlusal surface and a machined high strength ceramic core.
  21. 21. The aim of CICERO is to mass produce ceramic restoration at one integrated site. It includes rapid custom fabrication of high strength alumina coping and semi finished crowns to be delivered to dental laboratories for porcelain layering / finishing.
  22. 22. Lava CAD/CAM System – Introduced in 2002, used for fabrication of zirconia framework for all ceramic restorations. This system uses yttria stabilized tetragonal zirconia poly crystals (Y-TZP) which have greater fracture resistance than conventional ceramics. Lava system uses a laser optical system to digitize information. The Lava CAD software automatically finds the margin and suggests a pontic. CAM produces an enlarged framework to compensate shrinkage.
  23. 23. A partially sintered ziroconia block is selected for milling. Milled framework undergoes sintering to attain final dimensions, density and strength. Studies on marginal adaptation of Y-TZP bridges processed with Lava system for 2 milling times (75 mins Vs 56 mins) did not affect the marginal adaptation (61+-25 µm Vs 59+-21 µm ). (Hertlein et al 2003)
  24. 24. The combination of materials that can be used and restoration types that can be produced vary with different systems. Some CAD/CAM systems can fabricate a final restoration with some materials with acceptable strength and esthetics while others require subsequent veneering to achieve acceptable esthetics
  25. 25. RESTORATIVE MATERIALS FOR CAD/CAM CAD/CAM systems based on machining of presintered alumina or zirconia blocks in combination with specially designed veneer ceramics satisfy the demand for all-ceramic posterior crowns ,inlays and onlays . Many ceramic materials are available for use as CAD/CAM restorations. Common ceramic materials used in earlier dental CAD/CAM restorations have been machinable glass ceramics such as Dicor-k or Vita Mark II.
  26. 26. Although mono-chromatic, these ceramic materials offer excellent esthetics, biocompatibility, great color stability, low thermal conductivity, and excellent wear resistance. They have been successfully used as inlays, onlays ,veneers , and crowns. However, Dicor and Vita Mark II are not strong enough to sustain occlusal loading when used for posterior crowns. For this reason, alumina and zirconia materials are now being widely used as dental restorative materials.
  27. 27. These ceramic agents may not be cost- effective without the aid of CAD/CAM technology. In-Ceram l, first described by Degrange and Sadoun, has been shown to have good flexural strength and good clinical performance. However, the manufacture of conventional In-Ceram restoration takes up to 14 hours. By milling copings from presintered alumina or zirconia blocks within a 20 minute period and reducing the glass infiltration time from 4 hours to 40 minutes, CEREC in Lab decreases fabrication time by 90%.
  28. 28. Zirconia is strong and has high biocompatibility. Fully sintered zirconia materials can be difficult to mill, taking 3 hours for a single unit. Compared with fully sintered zirconia, milling restorations from presintered or partially sintered solid blocks is easier and less time-consuming, creates less tool loading and wear, and provides higher precision. After milling, In-Ceram spinell, alumina, and zirconia blocks are glass infiltrated to fill fine porosities. Other machinable presintered ceramic materials are sintered to full density, eliminating the need for extensive use of diamond tools.
  29. 29. Under stress the stable tetragonal phase may be transformed to the monoclinic phase with a 3% to 4% volume increase. This dimensional change creates compressive stresses that inhibit crack propagation. This phenomenon, called “transformation toughening,” actively opposes cracking and gives zirconia its reputation as the “smart ceramic.” The quality of transformation toughness and its affect on other properties is unknown.
  30. 30. Zirconia copings are laminated with low fusing porcelain to provide esthetics and to reduce wear of the opposing dentition. If the abutment lacks adequate reduction the restoration may look opaque. Because they normally are not etchable or bondable, abutments require good retention and resistance form. Alumina and zirconia restorations may be cemented with either conventional methods or adhesive bonding techniques. Conventional conditioning required by leucite ceramics (eg, hydrofluoric acid etch) is not needed.
  31. 31. Microetching with Al2O3 particles on cementation surfaces removes contamination and promotes retention for pure aluminum oxide ceramic. A resin composite containing an adhesive phosphate monomer in combination with a silane coupling/bonding agent can achieve superior long- term shear bond strength to the intaglio surface of Procera AllCeram and Procera AllZirkon restorations.
  32. 32. CAD/CAM systems also can be applied to restorations requiring metal and are used to fabricate implant abutments and implant-retained overdenture bars. The DCS system can fabricate crown copings from titanium alloy with excellent precision .
  33. 33. MARGINAL INTEGRITY OF CAD/CAM RESTORATIONS One of the most important criteria in evaluating fixed restorations is marginal integrity. Evaluating inlay restorations, Leinfelder and colleagues (1993) reported that marginal discrepancies larger than 100 µm resulted in extensive loss of the luting agent. O’Neal and colleagues (1993)reported the possibility of wear resulting from contact of food particles with cement when gap dimension exceeded 100 µm.
  34. 34. Essig and colleagues (1999) conducted a 5-year evaluation of margin gap wear and reported that vertical wear is half of the horizontal gap. The wear of the gap increased dramatically in the first year, becoming stable after the second year. McLean and Von Fraunhofer (1971)proposed that an acceptable marginal discrepancy for full coverage restorations should be less than 120 µm.
  35. 35. Christensen (1966) suggested a clinical goal of 25 µm to 40 µm for the marginal adaptation of cemented restorations. However, most clinicians agree that the marginal gap should be no greater than 50 µm to 100 µm. Current research data indicate that most dental CAD/CAM systems are now able to produce restorations with acceptable marginal adaptation of less than 100 µm. Perng-Ru Liu. A Panorama of Dental CAD/CAM Restorative Systems. J. of Compendium 2005; 26(7):507-512.
  36. 36. CAD- CIM IN RESTORATIVE DENTISTRY  The problem of aesthetic restorations of hard dental tissues has long been present in dental medicine, not only in the replacement of dental tissue destroyed by caries but also in the treatment of traumatic injuries, endogenic and exogenic discoloration, hypoplastic defects of hard dental tissue, disorders in the contour and size of teeth and other malformations.  For this purpose, apart from the classical composite restorations, onlays and inlays, laboratory produced composites and ceramic inlays and veneers are also used.
  37. 37.  CAD/CAM (Computer Aided Design / Computer Aided Manufacture) system first appeared in dental medicine in 1989 with the device CEREC (CEramic REConstruction) for the fabrication of inlays, onlays and labial veneers during one appointment in the dental surgery.  An optical “impression” is used instead of the classical impression procedure, and the dentist’s own evaluation of the contour and size of the inlay, onlay or veneer.  The development of the technological method enabled complete integration of all phases of fabrication, and this is offered by the CAD/CIM system (Computer Aided Design / Computer Integrated Manufacturing).
  38. 38. ADVANTAGES OF CAD/CIM- The advantages of the method are its simplicity: the laboratory is not required, there is no classical impression procedure (the method can be repeated as necessary), rapid fabrication (it is possible to fabricate several veneers during one appointment), acceptable cost (no laboratory costs, time saving), and the fabricated restoration is of the same or higher quality than the laboratory fabricated restoration.
  39. 39. DISADVANTAGE- High costs of the CEREC apparatus and mastering the technique.
  40. 40.  Ceramic inlays, onlays, and veneers have increasingly become part of the clinical routine in dental offices. The adhesive technique permits preparations that preserve the dental hard tissues and widens the range of ceramic restorations.  The key features of dental ceramics are excellent biocompatibility, good machinability, high abrasion resistance, and durable color stability, as well as enamel-like modulus of elasticity and thermal conductivity.
  41. 41.  To date computer-aided design-computer- integrated manufacturing (CAD-CIM) restorations have shown more than 6 years of good clinical performance and have thus gained scientific acceptance.  For computer-generated inlays, practically pore-free industrial ceramics that do not require glazing are used, and the inlays offer excellent marginal seals at both the enamel and dentinal interfaces when cemented with resin cements.
  42. 42.  The Cerec unit (Siemens) is based on the process developed by Mormann and Brandestini' and is the result of constant development yia different generations of Cerec units
  43. 43. TECHNICAL INNOVATIONS The Cerec 2 camera: Optical impression The Cerec 2 camera has been given a new design and is easy to handle. To maintain good accessibility to the oral cavity, the size of the intraoral frontal part of the camera has not been modified. An important hygienic feature is the detachable cover, which can be sterilized by dry heat in case of exposure to a higher risk of infection.
  44. 44. Usually, the cover is removed, a plug is inserted in its opening, and it is then washed in a thermal disinfector. The camera can also be wiped clean with a dispensable cloth moistened with a liquid disinfectant.
  45. 45. The further development of the intraoral three dimensional camera has been carried out in accordance with the original Cerec process. Pixel size(picture element) has been reduced from 54x54um to 25 X 29 um. Thus, in the pixel image system, the voxel (volume element) pattern has come to 25 X 25 X 29 um in the pixel image system. Because of the optimization of the optical beam path by means of symmetric beam geometry, major measurement errors in the measuring volume of a typical inlay have been brought down to less than +- 25 um.
  46. 46. A more accurate control of the projected measuring pattern and the particularly low-noise level processing of the video signal has resulted in a distinct reduction of the spurious components in the measuring data. Because of the smaller pixel size and the higher accuracy in the depth measuring, the resolution of the optical impression has been doubled compared to that of the Cerec 1 unit.
  47. 47. IMAGE PROCESSING Data representation of a typical mesio-occlusodistal inlay in the image memory has been increased from 4 million voxel in Cerec 1 to 32 million voxel in Cerec 2. Accordingly, the amount of data to be processed has grown by the factor 8, As a result of the six times more efficient computing capacity, the surface operations can be carried out in about the same time and the line operations are about three times faster. Main memory (ID 4) overloads no longer occur.
  48. 48. USER INTERFACE On the 14 x 17-cm color monitor, the preparation is represented at xl2 magnification. Thus, compared to the x8 magnification in Cerec 1, the accurate drawing of the construction lines has been facilitated. Proven elements, such as selecting the tooth to be treated by clicking at it in the tooth arch, confirmation of the procedure steps by clicking at the icons, and alternative options in the interactive windows, have been maintained, extended, and at the same time simplified by Automation .Accordingly, camera calibration and the adjust procedure of the depth profile data are now fully automated.
  49. 49. In conjunction with the automatic adjust, the measuring range has been extended to the complete range of depth of field of 10 mm. Thus, even very deep cavities no longer present clinical limitations of any kind. Another useful element is the automatic proposition of the proximal contact lines. Although, for the time being, these still have to be aligned to the approximal surfaces of the adjacent teeth by editing to establish the desired close contacts, input errors in defining the starting and end points of the proximal contact lines have been eliminated.
  50. 50. User elements The monitor can be swiveled and tilted, thus facilitating visual control of the video image in the search mode during the taking of the optical impression. Furthermore, this arrangement allows the patient to watch the design and the program procedure. During the taking of the optical impression, the camera is activated by a newly arranged foot switch. Unlike the usual operational technique, the foot switch is activated not by pressing but by lifting the foot. This setup has hygienic and engineering advantages, for which reason this solution has been chosen. Operators soon get accustomed to it.
  51. 51. Extrapolation occlusion-Simultaneous grinding – Three different programs for design are available: Extrapolation, Correlation, and Veneer. There are three choices in designing the occlusion; (/) anatomically adapted (Extrapolation), (2) Correlated to a functionally generated path (Correlation),and (3) Buccolingually flat(linear)
  52. 52. for practical use, the interactive design technique, in conjunction with the extrapolation program for inlays and onlays, is the method of choice. An essential point of this method is the tracing of the mesiodistal main fissure line.
  53. 53. Fissure depth is determined in inlays by the height of the adjacent cavosurface margin line, and in onlays also by the marginal ridge-cusp line. For every supporting point of the fissure line, the next point on the cavosurface margin line or on the marginal ridge-cusp line, respectively, is searched for in buccolingual direction. To ensure sufficient porcelain thickness of at least 1.0 mm, a sufficient preparation depth in the fissure section has to be secured.
  54. 54. Through triangulation of the occlusal surface, triangles are calculated, each being defined by its edge points and two inclination vectors per triangle side." The triangle surfaces are interpolated and form a continuous surface. Grinding is performed by a cylindrical diamond (diameter of 2mm, particle size of 64 pm, 77,000 rpm, and cutting speed of about 8 m/s), working simultaneously with the radial infeed grinding of the grinding disk (64 um particle size, 18,000 rpm, and cutting speed of about 38 m/s)
  55. 55. CEREC ONLAYS: EXTENDED MACHINING With the extended machining option of the Cerec 2 system, complex floor shapes in inlays and onlays can be ground with the cylindrical diamonds. Cuspal coverage, circular margins, and buccal margins with different levels are possible. Occlusal floor sections with differing levels can also be designed; the grinding disk-dependent orientation of pulpo-axial box walls no longer exists. However, to be feasible, the preparation shapes have to be located within the 2-mm resolution range of the cylindrical diamonds. Generally, this does not present any limitations in practical use.
  56. 56. Domagoj Glavina Ilija SkrinjariÊ . A New Method for Fabricating Ceramic Inlays: the CAD/CIM System Technology for the 21stCentury. Acta Stomatol Croat 2001;35(1):53-58.
  57. 57. An anatomically adapted occlusion is created and, with little additional work, can be morphologically finished with a 40 um contouring diamond and an 8um finishing diamond and polished with flexible disks.
  58. 58. VENEERS: INCISAL EDGE COVERAGE The Cerec system is the only method in dentistry to permit the direct machining of ceramic yeneers. In the United States, where the standards of esthetic requirements are high, more than 20% of Cerec users regularly produce veneers in one appointment and place them directly with an adhesive technique. With the Cerec 1 unit and the extended computer software. Veneer 1.0, veneers and onlays can easily be designed and manufactured. Cerec 2 software, COS 4,20, permits custom veneer preparations with any kind of anatomic reduction, as well as the easy design of the veneer and its direct manufacturing.
  59. 59. Class IV situations combined with incisai edge coverage maybe designed directly, without using a wax template for the optical impression.With the extended machining option, any three-dimensional shape can be manufactured directly.
  60. 60. GRINDING PRECISION AND ACCURACY OF FIT Grinding precision, the final element in the CAD/CIM manufacturing process, is crucial to an accurate fit. The result to be achieved should be influenced as little as possible by the state of abrasion of the grinding tools, by the type of material used, and the amount of material to be removed. The optimized calibration process compensates for wear of the tools. The reduction of the bearing tolerances and the reinforcement of the axes have increased the rigidity of the Cerec 2 grinding unit as a whole and of the grinding drive in particular.
  61. 61. These modifications also reduced the deformations occurring during the service life of a grinding disk as a result of increasing contact pressure.
  62. 62. The Windows-based CEREC 3 system was introduced in 2000. While these first three models were based on 2D technology, 3D software introduced in 2003 allowed dentists to construct restorations based on virtual three- dimensional models using the computer. While for some time it was only possible to attach all-ceramic crowns adhesively, the increased precision of the new generation of milling machine, MC XL, which was launched in 2007, made it possible to attach crowns using dental cement. In 2009, Sirona switched to a new imaging technology, the CEREC Bluecam, which is based on short-wave blue light, thus significantly increasing the level of precision in comparison to the previous 3D camera. Since 2010, the use of Biogeneric has made it possible to individually reconstruct the occlusal surfaces of damaged or missing teeth, while achieving a natural look.
  63. 63. The latest development is the CEREC Omnicam intraoral camera, which was launched on the market in 2012 and facilitates powder-free digital impressions in natural colors.
  64. 64. CONCLUSION CAD/CAM systems offer automation of fabrication procedures with standardized quality in a shorter period of time. They have the potential to minimize inaccuracies in technique and reduce hazards of infectious cross contamination. It allows application of newer high strength materials with outstanding biocompatibility combined with adequate mechanical strength, provisions for esthetic designs, excellent precision of fit and longetivity. However, these advantages must be balanced against the high initial cost of CAD/CAM systems and the need for additional training.
  65. 65. Patient’s expectations, financial constrain, operator’s preference, as well as availability of CAD/CAM systems will dictate the suitability of this type of restorations on an individual basis in the future. Innovations will continue to affect and challenge dentistry.
  66. 66. REFERENCES  Mormann W.H. The origin of the cerec method: a personal review of the first 5 years. Int J Comput Dent. 2004; 7(1): 11-24.  Ellingsen LA, Fasbinder DJ. An in vitro evaluation CAD/CAM ceramic crowns. J. Dent Res 2002; 81:331.  Sneha S.Mantri ,Abhilasha S. Bhasin. CAD/CAM in dental restorations: an overview. J. of Annals and Essences of Dentistry 2010; 2(3): 123-128.  Anuvanice KJ (Ed) Phillips RW. Phillips Science of Dental Materials. 11th Edition, WB Saunders Company, Pennsylvania, USA, Chapter 21, pg 692.
  67. 67.  Domagoj Glavina Ilija SkrinjariÊ . A New Method for Fabricating Ceramic Inlays: the CAD/CIM System Technology for the 21stCentury. Acta Stomatol Croat 2001;35(1):53-58.  Perng-Ru Liu. A Panorama of Dental CAD/CAM Restorative Systems. J. of Compendium 2005; 26(7):507-512.  Florian Beuer, Josef Schweiger CDT and Daniel Edelhoff . CAD/CAM in Dentistry: New Materials and Technologies. Dentistry 2010 ;2(4).  Mormann W.H. ,The evolution of the cerec system, J Am Dent Assoc ,2006 Sep;137 Suppl:7S- 13S