An Overview of Computer Aided Design/Computer Aided Manufacturing (CAD/CAM) in Restorative Dentistry

Introduction

During the last quarter of the 20^th century, there was a breakthrough in dentistry regarding introducing a new technology, i.e. the Computer-Aided Design/Computer-Aided Manufacturing (CAD/CAM) system (1-4). A new age started with the appearance of this technology (5). The notion of using this recent technology in dentistry, particularly restorative dentistry, commenced with diverse research approaches. For instance, in the 1970s Francois Duret in France introduced the Duret system (3, 6-12); which was named after him and supported by “Hennson International”(13). Using this system, Duret pioneered optical impression from which design and milling of an abutment tooth were possible (1). However, due to the complexity, large size, inadequate digitizing and high cost, this system did not take off in dental markets (7). Later another system launched into the market named “Sopha System”, which was supported by “Sopha Bioconcept” (1, 3, 6, 7). Then, in the 1977 Young and Atlschuler (6, 7, 13-15) suggested an optical impression method, which depended on laser holography.

Subsequently, in the 1980s at the University of Zurich in Switzerland, Dr. Werner Mormann and Marco Brandestini introduced the first commercial CAD/CAM system to a dental clinic (8, 10, 16). This technology made major changes in dental practice by permitting dentists to finish multiple works for ceramic restorations such as full crowns, inlays, onlays and veneers in a clinic or laboratory (16, 17).

The first chair-side ceramic inlay with the CEREC1 CAD/CAM system (Sirona “Siemens old name of Sirona” Dental System, Charlotte, NC.) was fabricated and used in 1985  (14, 16, 18-20). However, a study by Culp and Touchstone showed that this system was introduced earlier in 1982 (21). In addition, Freedman in a literature review argued that this system was introduced into the market in the 1987 (22). 

Following this period, Dr. Anderson attempted to introduce the Procera System (1, 3). This system enables the machining of titanium copings by spark erosion. Using this system, Anderson tried to carry out a process of composite veneered restoration (1). Eventually, it becomes a popular system in the worldwide to fabricate all-ceramic frameworks through a centralized network process with satellite digitizers (1).

Apart from these systems, several other groups have tried to introduce others from 1985 to 1990s such as the Aoki group in Japan, Diane Rekow’s in Minnesota and Reggie’s at Alabama University (6). The most developed amongst the various systems was that of Diane Rekow, which was supported by the Bego company under a particular name, the so-called the DentiCAD or Minnesota system (6). This system provided 3D measurement on the teeth surface via stereophotogrammetry. However, it was complicated to operate. Therefore, it did not remain in use for long, only until 1994 (6). Then, the technological revolution in dentistry further developed. Hence, there were certain changes, and various other systems were developed.  For instance, the DCP analogue system, the Celay system, the DFE (Krrup) system and the Erosonic system (ESPE) (13). Since then, several varieties of commercial CAD/CAM systems were developed.

In the past, the CAD/CAM system was used either at the laboratory side or in-office “chair-side” (23). However, recently it has become feasible to merge the concepts. Hence, both dental professionals and experienced technicians have benefited (23). Therefore, CAD/CAM systems have branched into other dental fields such as oral surgery, periodontology, orthodontic treatment, implantology and maxillofacial surgery; it is no longer found only in the restorative field. These are signs for a significant role and ongoing evolution of this technology, from simple fabricated machine to recently developed one (24). This paper aims to show a short glance relating to CAD/CAM system and focusing on certain other aspects.

CAD/CAM Components

CAD/CAM systems are composed of three major parts: First, a data acquisition unit, which collects the data from the area of the preparation, adjacent and opposing structures. Then converts them to virtual impressions (25) through intraoral scanners (in-office CAD/CAM or in-office CAD or image acquisition systems) or indirectly using a stone model generated through making a conventional impression. Second, the software used for designing virtual restorations on a virtual working cast and then computing the milling parameters. Third, a computerized milling device used for manufacturing the restoration from a solid block of restorative material or additive manufacturing.

General classification of CAD/CAM systems

The CAD/CAM systems are classified into laboratory systems and chairside systems. The laboratory system is further classified into laboratory CAD/CAM, in which the company has its own scanner and milling units such as (Amann Girbach, 3M ESPE, Sirona Dental Systems, Zirkon Zahn, vhf camfacture AG, Weiland Dental, Pou-Yuen and U-Best Dental, Planmeca, KaVo Dental, Dentsply Prosthetics). The CAD unit in which the company has only the scanner (e.g. D2000, 3 Shape; Dental Wings 7 series, Dental Wings; IScan D104, Imetric 3D SA; Ceramill Map, Amann Girrbach; Activity 850 3D, Smart Optics). The CAM unit in which the company retains the milling machine unit such as (DWX-50, Roland DGA Corporation; inLab MC X5, Sirona; M5, Zirkonzahn; Tizian Cut 5 Smart, Schütz Dental; S2 Model, vhf camfacture AG; Ceramill Motion 2, Amann Girrbach).

The chairside CAD/CAM system is further classified into: first, chairside CAD/CAM system in which the company has its own scanner and milling units (Sirona and Planmeca). Second,  image acquisition system in which the company has only a scanner without designing capabilities (e.g. True Definition Scanner, 3M ESPE; iTero, Align Technology, Inc; Trios, 3Shape; Apollo DI, Sirona; CS 3500, Carestream Dental LLC). These, in turn, must be connected to an open laboratory scanner for designing of the restoration.

CAD/CAM can be further classified into open and closed systems (26) according to data sharing. Closed systems offer all CAD/CAM procedures, including data acquisition, virtual design, and restoration manufacturing by the same company. Further, all the steps are integrated into one system, and there is no interchangeability between different systems from other companies. Open systems allow the adoption of the original digital data by CAD software and CAM devices from various companies (26).

The laboratory CAD systems must always be an open system because after acquiring the data and designing the restoration, the data has to be stored in an STL file “Stereolithography or Standard Tessellation Language. However, many manufacturers use their own specific data formats, with the result that data for the construction programs will not be compatible with each other” (27), and then sent to an open laboratory CAM system, which accepts that type of STL file from that laboratory CAD system where the restoration will be fabricated. Additionally, the image acquisition unit is always an open system, and the STL file of a certain restoration can be accepted by an open laboratory CAD system for the restoration to be designed and then sent to an open CAM system for the restoration or model to be fabricated.

When complex restorations are intended to be fabricated such as an implant bar or attachments, the model can be scanned through open laboratory CAD/CAM or laboratory CAD systems and the STL file sent to an outsource production center such as (InfiniDent, Sirona; Procera, Nobel Biocare; Lava, 3M ESPE; TurboDent, Pou-Yuen and U-Best Dental; Ceram M-center, Amann Girrbich; PlanEasyMillTM, Planmeca) for restoration designing and fabrication. In addition, when a  digital  model  is  intended  to  be fabricated  through  scanning  of  the  teeth   intraorally,   the STL file of the image acquisition unit or open chairside CAD/ CAM system can be sent to  an  outsource  production  center for the digital model to be fabricated through milling or additive technology. General classification of CAD/CAM system is summerized in Table 1 (5, 22).

Table 1. General classification of CAD/CAM system (5, 22)

Development of CEREC systems and comparison to former generations

CEREC; is the abbreviation of “Chairside Economic Reconstruction of Esthetic Ceramic” (1); however, in certain literature, this acronym is just explained by the terms “Ceramic Reconstruction” (19, 28). This system is more applicable than other available systems (29). The 1^st version was launched onto the market in 1985 (16, 30), whereas Akbar et al. argued that that CEREC 1 was introduced in 1984 (29). Then, after technical improvements, the next generation of CEREC 2 was fabricated and introduced to dental clinics in 1996 (1).

Recently, Akbar et al have examined CEREC 3 (29) in a similar way and found that this system was identical to the second system. However, other studies showed that CEREC 3 is better than CEREC 2 regarding marginal integrity (1,7). Previous literatures have established that CEREC 2 can be utilized to fabricate partial crowns, inlays, onlays and overlays (31). However, CEREC 3 can be used for three-unit bridge, veneers, onlays, inlays, full and/or partial crowns and copings (31). For specific crown designs such as veneers, inlays, onlays and temporary bridges CEREC 4 and 4.5 are the most appropriate systems.

Different restorative materials can be used with the CEREC system including; VITA Blocs Spinell, Zirconia, Alumina, Zirconia Mark II, YZ (VITA Zahnfabric, Bad Säckingen, Germany). Lucite reinforced ProCad (Ivoclar vivadent, Schann Liechtenstein) (31, 32).

Compare CEREC system properties to earlier versions systems

There are certain properties presented with the previous systems; the infra-red intraoral camera, and an optical image can be obtained with CEREC 2, CEREC 3, and CEREC 4. However, the two last versions improved in such a way that blue ray enhanced better properties in comparison to previous generations.  In addition, the design of models can be fabricated (Extrapolation, Function and Correlation).

The CEREC 3 and later versions have the ability to complete all the work in one appointment without the need for a second visit. In addition, it does not require to take of impressions or making a temporary restoration, i.e. preparation, designing and milling process can be completed in one visit (8, 28).

Due to significant progress in both hard and software computer programs, varieties of laboratory CAD/CAM systems have been launched into the market. In 2002, CEREC inLab was launched into the market. With the CEREC InLab, a die can be either scanned with a laser scanner automatically or separately through inEos system.

Then, a 3D image has been achieved by computer program the restoration is designed (28, 33). The subtracting process through milling chamber can be carried out automatically (7).

Recently, both the InEos and CEREC inLab system can be used as a combined unit that can conduct each scanning and milling independently. The former system was previously used only for milling or scanning purpose. However, both processes can be carried out together in combination.

With CEREC, two types of laser mode scanning are present. The overview can be used for multiple works such as crowns, inlays, onlays and bridges. It has the ability to scan upper and lower arches for occlusal restoration. The  rotational scanning mode can be also used for taking eight scanning pictures, as indicated for a single die or unit (28)   

The modern versions of the CEREC software 4 and 4.5 (Bensheim, Germany) were also released in last few years into the market by Sirona. Major steps in CEREC system development are shown in Table 2 (19).

Table 2. Major steps in the development of CEREC CAD/CAM system (19)

Strengths and weaknesses of CEREC CAD/CAM systems

The system lies on the triangulation technique which requires a uniform reflective surface since different materials such as dentin, amalgam, resin, gum reflect light differently. Therefore, it is necessary to coat the teeth with reflective powders before the scanning to provide uniformity in the reflectivity of the surfaces to be scanned precisely. Earlier versions of CEREC^® employed an acquisition camera with an infrared laser light source. The Bluecam version employs blue light-emitting diodes (LEDs); the intense blue light with a shorter wavelength projected by the blue LEDs allows for greater precision of the produced virtual model (34). Even at the periphery, the images are free from distortion, so multiple images such as a complete quadrant can be stitched together with high accuracy (34).

The CEREC^® AC Bluecam offers image stabilization systems. This means that the practitioner does not have to rest the camera wand on a tooth to get a steady focus. The camera automatically captures an image when the wand is motionless, avoiding the need for a foot pedal as the previous model required. Recently, with further development of CEREC software, it is possible to scan full arches. Earlier versions of the device made a single image from one perspective. At the end of the scanning stage, the preparation is shown on the monitor and can be viewed from every angle to focus or magnify areas of the preparation (35).

Restorative materials for CAD/CAM system

With using CAD/CAM systems, operators can fabricate restorations from an array of materials. These include ceramics, metal alloys and various composites. The ceramics currently being used for restorations are predominantly alumina including those subsequently infiltrated with glass, zirconia and porcelain based ceramics (36).

CAD/CAM systems based on machining of
pre-sintered alumina or zirconia blocks in combination with specially designed veneer ceramics satisfy the demand for all-ceramic posterior crowns and fixed partial dentures. Many restorative materials are available for use as CAD/CAM restorations as shown in Table 3 (37). Common ceramic materials used in earlier dental.
CAD/ CAM restorations have been machinable glass ceramics such as Dicor (Dentsply Caulk, Milford, DE 19963) or Vita Mark II (Vident, Bera, CA 92821). Although monochromatic, these ceramic materials offer excellent esthetics, biocompatibility, excellent color stability, low thermal conductivity, and excellent wear resistance (38). They have been successfully used as inlays, onlays (39), veneers and crowns (40). However, Dicor and Vita Mark II are not strong enough to sustain occlusal loading when used for posterior crowns (41). Therefore, alumina and zirconia materials are now being widely accepted as dental restorative materials. These ceramic agents may not be cost-effective without the aid of CAD/CAM technology (42). For instance, In-Ceram l, first was described by Sadoun and Degrange, has been shown to have the acceptable flexural strength and clinical performance (43). However, the manufacture of conventional In-Ceram restoration takes up to 14 hours. A milling copings from presintered alumina or zirconia blocks within a 20 minutes period and reducing the glass infiltration time from 4 hours to 40 minutes, CEREC inLab decreases fabrication time by 90% (44).

Zirconia is strong enough and has high biocompatibility (45). Fully sintered zirconia materials can be difficult to mill, taking 3 hours for a single unit. Compared with fully sintered zirconia, milling restorations from pre-sintered or partially sintered solid blocks is easier and less time consuming, creates less tool loading and wear, and provides higher precision (46).

The dimensional change of zirconia material creates compressive stresses that reduce crack propagation. This phenomenon, called “transformation toughening”, actively opposes cracking and gives zirconia its reputation as the “smart ceramic”(47). The quality of transformation toughness and its effect on other properties is unknown. 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 (48). Conventional conditioning needed by leucite ceramics, such as hydrofluoric acid-etch, is not required. Microetching with Al₂O₃ particles on cementation surfaces removes contamination and promotes retention of pure aluminium oxide ceramics (49).

Table 3. Common Restorative Materials for Dental CAD/CAM Systems (37)