CAM technology: Options for practical implementation

JPOR-319; No. of Pages 13 journal of prosthodontic research xxx (2016) xxx–xxx

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Review

Advancements in CAD/CAM technology: Options for practical implementation Tariq F. Alghazzawi BDS, MS, MSMtE, PhDa,b,* a

Department of Prosthetic Dental Sciences, College of Dentistry, Taibah University, Medina, Kingdom of Saudi Arabia Department of Materials Science and Engineering, School of Engineering, The University of Alabama at Birmingham, United States b

article info

abstract

Article history:

Purpose: The purpose of this review is to present a comprehensive review of the current

Received 26 October 2015

published literature investigating the various methods and techniques for scanning, de-

Received in revised form

signing, and fabrication of CAD/CAM generated restorations along with detailing the new

10 December 2015

classifications of CAD/CAM technology.

Accepted 16 January 2016

Study selection: I performed a review of a PubMed using the following search terms ‘‘CAD/

Available online xxx

CAM, 3D printing, scanner, digital impression, and zirconia’’. The articles were screened for further relevant investigations. The search was limited to articles written in English,

Keywords:

published from 2001 to 2015. In addition, a manual search was also conducted through

CAD/CAM

articles and reference lists retrieved from the electronic search and peer-reviewed journals.

Milling

Results: CAD/CAM technology has advantages including digital impressions and models,

3D printing

and use of virtual articulators. However, the implementation of this technology is still

Scanner

considered expensive and requires highly trained personnel. Currently, the design software

Digital impression

has more applications including complete dentures and removable partial denture frame-

Virtual articulator

works. The accuracy of restoration fabrication can be best attained with 5 axes milling units. The 3D printing technology has been incorporated into dentistry, but does not include ceramics and is limited to polymers. In the future, optical impressions will be replaced with ultrasound impressions using ultrasonic waves, which have the capability to penetrate the gingiva non-invasively without retraction cords and not be affected by fluids. Conclusion: The coming trend for most practitioners will be the use of an acquisition camera attached to a computer with the appropriate software and the capability of forwarding the image to the laboratory. # 2016 Japan Prosthodontic Society. Published by Elsevier Ltd. All rights reserved.

Contents 1. 2. 3.

Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . CAD/CAM components . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . General classification of CAD/CAM systems . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

000 000 000

* Correspondence to: P.O. Box 51209, Riyadh, 11543, Saudi Arabia. E-mail address: [email protected]. http://dx.doi.org/10.1016/j.jpor.2016.01.003 1883-1958/# 2016 Japan Prosthodontic Society. Published by Elsevier Ltd. All rights reserved.

Please cite this article in press as: Alghazzawi TF. Advancements in CAD/CAM technology: Options for practical implementation. J Prosthodont Res (2016), http://dx.doi.org/10.1016/j.jpor.2016.01.003

JPOR-319; No. of Pages 13

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4. 5. 6. 7. 8. 9. 10.

1.

Classification of scanners . . . . . . . . . . . . . . . . Protocol for scanning. . . . . . . . . . . . . . . . . . . . Virtual articulators and facebows . . . . . . . . . . Design software . . . . . . . . . . . . . . . . . . . . . . . . Digital fabrication processes . . . . . . . . . . . . . . Limitations and future CAD/CAM technology . Summary and conclusions . . . . . . . . . . . . . . . References . . . . . . . . . . . . . . . . . . . . . . . . . . . .

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Introduction

With the rapid evolution of CAD/CAM (Computer Aided Design, Computer Aided Manufacture), this has led to a dramatic impact on all disciplines of dentistry especially in the fields of prosthodontics and restorative dentistry. The integration of these technological systems with advances in biomaterials, such as zirconia high strength ceramics, has led to major alterations in education and patient care. Accordingly, the entire dental educational landscape has been and will continue to be altered relative to economics, time efficiencies and most importantly, predicting postoperative [1,2], clinical treatment and delivery. The advantages of CAD/CAM technology will be included into three main protocols including digital impressions [3–12], digital models, and virtual articulators and facebow [13–19] as illustrated in Fig. 1. Furthermore, prosthodontic care has become a complex integration of sequential techniques involving the patient, student clinician, faculty clinician, and commercial laboratories at multiple levels. Therefore, the purpose of this study is to review the current published literature investigating the various methods and techniques for scanning, designing, and fabrication of CAD/CAM generated restorations along with detailing the new classifications of CAD/CAM technology. It must be noted that there are significant and broad variations in acquisition systems, CAD design mechanisms, and CAM fabrication processes. Accordingly, it should be stated that every system may not be capable of developing the full range of restorations necessary to address individual prosthetic solutions.

2.

CAD/CAM components

CAD/CAM systems are composed of three major parts: (1) a data acquisition unit, which collects the data from the area of the preparation, adjacent and opposing structures and then converts them to virtual impressions [20] through intraoral scanners (in-office CAD/CAM or in-office CAD or image acquisition systems) or indirectly by means of a stone model generated through making a conventional impression; (2) software for designing virtual restorations on a virtual working cast and then computing the milling parameters; and (3) a computerized milling device for manufacturing the restoration from a solid block of restorative material or additive manufacturing.

3.

General classification of CAD/CAM systems

The CAD/CAM systems are classified into laboratory systems and chairside systems. The laboratory system is further

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classified into laboratory CAD/CAM in which the company has its own scanner and milling units (e.g. 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) while CAD (Computer Aided Design) systems 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, AmannGirrbach; Activity 850 3D, Smart Optics) and CAM (Computer Aided Manufacture) systems in which the company retains the milling machine unit (e.g. DWX-50, Roland DGA Corporation; inLab MC X5, Sirona; M5, Zirkonzahn; Tizian Cut 5 Smart, Schu¨tz Dental; S2 Model, vhf camfacture AG; Ceramill Motion 2, Amann Girrbach). The chairside CAD/CAM system is further classified into (1) chairside CAD/CAM system in which the company has its own scanner and milling units (Sirona and Planmeca); and (2) 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 [21] according to data sharing. Closed systems offer all CAD/CAM procedures, including data acquisition, virtual design, and restoration manufacturing by the same company. Furthermore, 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 different companies. 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 [22]) 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 (e.g., InfiniDent, Sirona; Procera, Nobel

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JPOR-319; No. of Pages 13 journal of prosthodontic research xxx (2016) xxx–xxx

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Fig. 1 – Summary for the main advantages of CAD/CAM technology.

Biocare; Lava, 3M ESPE; TurboDent, Pou-Yuen and U-Best Dental; Ceram M-center, Amann Girrbich; PlanEasyMillTM, Planmeca) for restoration designing and fabrication. Furthermore, when the 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.

4.

Laboratory scanners are classified into (1) optical scanners which use the projection of a measuring light grid onto dental structures under a definite angle causing a depth-dependent phase shift of the grid, which the camera registers on its digital sensor. The computer calculates the 3D data of the dental structure from the image of the depth modulated measuring grid; and (2) mechanical scanners, in which with the scanner (e.g., Procera Scanner from Nobel Biocare), is capable of reading a master cast mechanically line by line by means of a ruby ball in order to obtain 3D measurements.

Classification of scanners 5.

The intraoral cameras are optical scanners and can be separated into two types [22–24] as shown in Table 1: (1) single image cameras that record individual images of the dentition. The iTero (Align Technology), PlanScan (Planmeca), CS 3500 (Carestream Dental LLC), and Trios (3 shape) cameras are single image cameras which record about three teeth in a single image. To record larger areas of the dentition, a series of overlapping individual images are recorded such that the software program can assemble these into a larger three-dimensional virtual model. The camera is positioned in different angles to ensure accurate recording of data below the height of contour that would be hidden from the camera if only an occlusal view was obtained. Those areas not visualized by the camera in the overlapping images would then be extrapolated by the software program to fill in the missing data areas in the virtual mode; and (2) video cameras which are used by the True Definition scanner (newest version of the Lava Chairside Oral Scanner, COS), Apollo DI (Sirona) and OmniCam (Sirona) systems. The difference between intraoral scanners is noted in Table 1.

Protocol for scanning

Depending on the system, the clinician has two scanning options intraorally for developing the final restoration: (1) preoperative scanning which provides for incorporating the existing anatomical contour and occlusal planes into the final restoration; and, (2) postoperative scanning of the preparation only with the CAD design being extrapolated from selected data points in the acquired image, and which may be combined with an internal library of tooth anatomic designs contained within the computer data base. The patient receives a standard preparation of the abutment tooth according to Table 2 under clinical criteria [25]. The preparation margins can be exposed by a cord retraction technique (use of retraction cords with double or single cord techniques) or cordless retraction technique (Expasyl, Kerr; Racegel, Septodont; Traxodent, Premier; GingiTrac, Centrix). Once the margin has been exposed, the operator maneuvers the control to allow the scanner tip to slide over the tooth in multiple directions depending on the manufacturer sequential protocol for capturing the images because most systems need a particular scanning path to

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Table 1 – Comparisons of different in-Office CAD/CAM systems. All the cameras can get full arch scan and they can scan implants intraorally except PlanScan. The Trios, iTero, and True Definition Scanners can perform orthodontic analysis. Open/close Color Portable system matching CEREC Omnicom (Sirona)

Closed

No

No

PlanScan (Planmeca)

Open

No

Yes

Trios Color (3 Shape)

Open

Yes

iTero (Align Technology) True Definition Scanner (3M ESPE) CS 3500 (Carestream Dental LLC) Apollo DI (Sirona)

Open

a

Type of CAD/CAM

Yes

Digital imaging and in-office manufacturing Digital imaging and in-office manufacturing Image acquisition unit

No

No

Open

No

Open Closed

Acquisition Powder Color technology required image

Imaging type

White light

No

Yes

Filming (Video)

Blue Laser

No

No

Filming (Video)

Blue LED

No

Yes

Image acquisition unit

Red Laser

No

Yes

No

Image acquisition unit

Blue LED

Yes

No

Photographing (multiple images) Photographing (multiple images) Filming (Video)

No

Yes

Image acquisition unit

White LED

No

Yes

No

No

Image acquisition unit

NAa

Yes

No

Photographing (multiple images) Filming (Video)

NA = information not available.

Table 2 – Guidelines of tooth preparation for CAD/CAM generated restoration. Criteria

Recommendation

1.

Incisal/occlusal reduction

2.

Axial reduction

3.

Total convergence angle

It depends on material typea and restoration designb (it ranges from 0.5 mm to 1.5 mm) It depends on material typea and restoration designb (it ranges from 0.5 mm to 1.5 mm) It should be between 48 and 68

4.

Morphology of internal line angle

It should be rounded

5.

Morphology of gingival margin

It should be either a rounded shoulder or a deep chamfer

a b

Reasons Not enough preparation will result in fracture of the restoration Not enough preparation will result in fracture of the restoration Parallel axial walls will confuse most scanners and may prevent accurate scanning of the preparation Sharp line angles on the occluso-axial surface should be avoided because the milling bur, which has a specific diameter, will remove excessive material in trying to reproduce detailed design configuration and a sharp line angle, thereby causing an over-milled restoration which can result in structurally compromised areas and improperly fitting restoration. The 908 internal angle (sharp internal angle) is contraindicated because of the same reason previously mentioned about the sharp line angle at the occluso-axial line angle. Trough or gutter margins should be avoided because they may prevent accurate scanning of the preparation. Knife-edge or feather margins are not acceptable because they do not allow for adequate areas for porcelain build-up or enough thickness of the margin of the milled ceramic restoration.

Zirconia, glass-ceramics, metal, composite resin. Restoration design: monolithic (full contour restoration), reduced restorations, copings/frameworks.

achieve accurate scanning results. After the scan of the prepared tooth is completed, the antagonists of the opposing arch are scanned in the same exact manner. The information for the development of a CAD/CAM restoration may be also acquired extraorally from the final impression or working cast. Additionally, some scanners can record a glossy, reflective surface such as a titanium abutment while other types require an opaquing powder [26]. The transfer of the image from the tooth to the final fabrication of the restoration can be divided into four methods as illustrated in Table 3, in which Method 1 involves scanning

of the teeth and implants without any models, Method 2 involves scanning of the teeth plus fabrication of digital models, Method 3 involves a physical impression plus scanning, and Method 4 involves a physical impression plus scanning of its stone working casts. In Method 1 [27–29], the maxillary and mandibular arches, including teeth and implants, are scanned using an intraoral scanner or image acquisition unit. The virtual interocclusal record is attained through a buccal scan in which the patient is instructed to close into maximum intercuspation and the facial aspect of the opposing quadrants in this static position is

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JPOR-319; No. of Pages 13

Digital Impression Method 1

Method 2 Scan of the teeth and implants using chairside scanner or image acquisition unit and then sending the STL file to a laboratory scanner or outsource production center Physical record which is used on physical articulator Polyurethane casts using milling or 3D printing Option 1

1.

Type of impression

2.

Type of interocclusal record

Scan of the teeth and implants using chairside scanner or image acquisition unit and then sending the STL file to a laboratory scanner or outsource production center Virtual record which is used on virtual articulator

3.

Type of working casts

Virtual casts

4.

Mounting of maxillary and mandibular casts + transfer of the maxillary and mandibular casts to the laboratory scanner Indications

Virtual mounting with virtual articulator

5.

Physical Impression

Full contour restorations

Full contour restorations, and copings/frameworks

Method 3

Method 4

Scan of the full arch impression tray including teeth only using laboratory scanner

Physical record which is scanned for virtual articulator Virtual casts Virtual mounting with virtual articulator

Full contour restorations

Impression of teeth and implants

Physical record which is used on physical articulator Polyurethane casts using milling or 3D printing Option 1

Physical record which is used on physical articulator for Option 1 only Stone casts using vacuum mixing of gypsum products Option 1 Option 2

Full contour restorations, and copings/frameworks

Full contour restorations, and copings/frameworks (Option 1 only)

journal of prosthodontic research xxx (2016) xxx–xxx

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Table 3 – Methods for scanning physical casts, physical impressions and teeth intraorally.

Option 1: The maxillary cast is mounted to The physical articulator using a physical facebow, and the mandibular cast is mounted to the upper cast using a standard physical inter-occlusal record. Depending on the type of laboratory scanner used, the physical articulator is then inserted with mounted maxillary and mandibular casts into the laboratory scanner or the mounted maxillary and mandibular casts are transferred from the physical articulator via a transfer kit or plate which is then inserted into the laboratory scanner. Option 2: The maxillary and mandibular casts are inserted without an interocclusal record into the laboratory scanner, and then scanned with virtual mounting of the casts on the virtual articulator.

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scanned. This method is mainly indicated for monolithic restorations only. For implant crowns, the healing abutments with optical markers can be directly scanned or the dentist can digitally capture a scan body seated on the implant. A scan body is a plastic or metal coping with markers that provide 3D registration of the implant location [30,31]. All intraoral scanners are able to scan implant scan bodies, but the difference is the compatibility of the intraoral scanner with the different implant manufacturers. For example, the Sirona CAD/CAM system has the ability to scan for implant abutment fabrication with the chairside scanner CEREC Omnicam such as Certain1 (external connection), Astra Tech OsseoSpeed, and Frialit/Xive. Once the scan body, indicating the implant level position in the jaw, is recorded, the 3D data file can be used to design and mill the crown. Either this can be done chairside, in the office by an assistant or dental technician or the file can be sent to a CAD/CAM equipped laboratory or production center for abutment and crown fabrication which may include fabrication of a monolithic abutment crown. Alternatively, healing abutments with optical markers can be scanned with an intraoral scanner and generate implant abutments and cement-retained restorations without the use of impression materials, dental stone, or implant impression copings and analogs [32]. A study reported that the implant definitive casts fabricated from the encoded healing abutment impressions were found to be less accurate than those fabricated from an open tray with the splinted impression copings technique for restoring 2 paired (108 or 308) convergent internal connection implants with non-engaging screwretained splinted 2-unit implant restorations. Accuracy of fit was not influenced by the implant angulation or position for either impression technique or by the encoded healing abutment height for the encode impression technique [33]. In Method 2 [34,35], as noted in Table 3, this construction method allows the fabrication of polyurethane working casts. Following an intraoral scan of the maxillary and mandibular arches including teeth and implants using an image acquisition unit or CAD/CAM system, the images are electronically transmitted using an STL file to the laboratory CAD system or outsource production center. Polyurethane working casts are then fabricated either by milling or additive manufacturing. Once, the working casts are developed, the maxillary cast is mounted to the physical articulator using a physical facebow, and the mandibular cast is mounted to the upper cast using a standard physical interocclusal record. Depending on the type of laboratory scanner used, the physical articulator is then inserted with mounted maxillary and mandibular casts into the laboratory scanner or the mounted maxillary and mandibular casts are transferred from the physical articulator via a transfer kit or plate which is then inserted into the laboratory scanner. This method is considered the best because of the advantages of polyurethane casts. In Method 3 [34], the physical impressions including teeth only are scanned with a laboratory scanner. A standard interocclusal record is also obtained. This method offers a dual option. The first option is that the physical record is scanned with the impression to generate 3D virtual casts; and the second option follows the same protocol as Method 2. This method is not recommended because there is no literature to

prove that the restoration margin accuracy from scanning the impression is enhanced, and may be decreased if there is an undercut in the preparation. In Method 4 [34,36,37], the physical impressions are poured with gypsum products including teeth and implants. The maxillary and mandibular casts can be managed with two different protocols: (A) the maxillary cast is mounted to the physical articulator using a physical facebow, and the mandibular cast is mounted to the upper cast using a physical interocclusal record. Once again, depending on the type of scanner used, the physical articulator is inserted with mounted maxillary and mandibular casts into the laboratory scanner or the mounted maxillary and mandibular casts are transferred from the physical articulator via a transfer kit or plate which is then inserted into the laboratory scanner. (B) The maxillary and mandibular casts are inserted without an interocclusal record into the laboratory scanner, and then scanned with virtual mounting of the casts on the virtual articulator. This method is the most common used and is indicated for monolithic and coping/framework restorations. Flu¨gge et al. [38] reported that scanning with the intra-oral scanner (iTero) is less accurate than scanning with the laboratory scanner (D250) because of the presence of saliva, blood, movable gingiva, and translucency of the teeth. Intraoral scanning with the iTero is less accurate than model scanning with the iTero. For treatment planning and manufacturing of tooth-supported appliances, virtual models created with the iTero can be used. An extended scanning protocol can improve the scanning results in some regions [38]. Data acquired by intraoral scanning, computed tomography, cone-beam computed tomography, and extraoral surface scanning can be combined for implant treatment planning [39] to ensure the effective positioning of implants relative to anatomic structures such as mandibular canal and maxillary sinus.

6.

Virtual articulators and facebows

The facebow is used in conjunction with an articulator to relate the maxillary arch to the axes of the condylar hinge axis in the three planes of space. A facebow is a mechanical device that uses a tripod location for the two posterior references by approximating each of the TMJ’s and an anterior reference point to relate the maxillary cast vertically to the selected horizontal reference plane. This transfer is critical for extensive oral rehabilitation [13] and can be done by two methods. The CAD/CAM virtual articulator replicates a fully adjustable mechanical articulator. The first method requires that the mechanical facebow be adapted to the patient and then transferred to the mechanical articulator to mount the maxillary cast. Subsequently, the mechanical articulator is transferred to the virtual articulator by inserting the mechanical articulator with the mounted maxillary and mandibular casts (e.g., inEos X5, Sirona) or the maxillary and mandibular cast are fixed with a transfer assembly (Ceramill map400, Amann Girrbach) or plate (e.g., D2000, 3 Shape) individually depending on the type of laboratory scanner. This method cannot be used for complete dentures. Several companies have customized a virtual

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JPOR-319; No. of Pages 13 journal of prosthodontic research xxx (2016) xxx–xxx

articulator for their CAD/CAM system. Examples of companies using this practice are Amann Girrbach, Smart Optics, and Zirkonzahn. Therefore, the selection of the mechanical articulators should be based on the type of laboratory scanner in which the corresponding transfer assembly or plate specific to the articulator which will match the same type of virtual articulator for minor post-operative adjustment after fabrication of the restorations. This method is indicated for copings/ frameworks, layered restorations, or when additive technology is intended to be used. The second method is implementing a virtual facebow using optical scanning and novel methodology based on reverse engineering by scanning six points with a reference head plus transverse horizontal axes to transfer the exact position of the maxillary cast to the virtual articulator. The maxillary and mandibular arch are scanned with an optical scanner (intraoral scanner) connected to a personal computer with specific software. Three extraoral points are determined on the patient head (two points on the temporomandibular joints and one at the infraorbital point just below the left eye) in order to generate the horizontal plane. Then, articulating paper is placed on the flat metal facebow fork, which is placed on the maxillary teeth, and three intraoral points (most prominent cusps) are determined to generate the occlusal plane. The total of six points can create a cranial coordinate system with different reverse engineering software in which the cranial coordinate system of the patient is in coincidence to the cranial coordinate system of the virtual articulator. Therefore, the maxillary digital cast is transferred to the virtual articulator software (virtual mounting of maxillary cast into virtual articulator in centric occlusion). Finally, the patient is instructed to close his/her mouth in centric occlusion and the buccal scan (digital occlusal record) is performed from three different directions (right, left, front) using intraoral scan to orient the mandibular digital cast to the maxillary digital cast on the virtual articulator in centric occlusion (virtual mounting of mandibular cast to maxillary cast) [14–17]. This method is indicated for full contour restorations to be fabricated with milling only.

7.

Design software

Special software is provided by the manufacturers for the design of various kinds of dental restorations. With different software from different manufacturers, various designs can be implemented such as copings and fixed partial denture (FPD) frameworks, full anatomical crowns and FPD, inlays, onlays, veneers, table-tops and non-prepared veneers, temporaries including FPD and pontics, diagnostic wax-up including physical models, post and core, telescopes, customized abutments with positioning guides, implants FPD and bars, implant planning with surgical guides, removable partials, denture design including impression trays, splints, model builder (crown and FPD/Implants), orthodontics and appliances can be designed. The final anterior restorations can be fabricated through a copy scan of the models of temporary restorations to compensate for the anterior guidance table and silicone matrices. In these systems, multiple tooth morphologies are available in their own internal digital libraries. However, the

7

general forms of tooth morphology provided by these CAD/ CAM systems can provide only basic shapes. There is always some manual alterations and modifications required because every patient is unique, and every tooth has its own morphological features that are unique for the patient’s system [40,41]. The alternative method is to use the database of the biogeneric tooth morphology to identify and imitate the individual occlusal morphology of a patient. With the digital CAD model being visible on the computer monitor, it can be rotated in three dimensions as well as magnified to evaluate critical areas of the model prior to transmitting the file to the manufacturing process. Furthermore, the recommended die spacer thickness can be selected [42] thereby eliminating the use of manual application of die spacer with different colors.

8.

Digital fabrication processes

This is the last phase of the dental CAD/CAM process. It involves developing a restoration from a CAD model into a physical part that undergoes processing, finishing, and polishing before being inserted into the patient’s mouth [40]. The two primary methods used to fabricate these restorations may be subtractive (milling and grinding) or additive manufacturing (Rapid Prototype, RP or 3D printing) as illustrated in Table 4. Milling/machining technology is a type of restoration fabrication that utilizes subtraction manufacturing technology from large solid blocks. The technology dentists and technicians are familiar with is computer numerically controlled machining (CNC), which is based on processes in which power-driven machine tools are used with a sharp cutting tool to mechanically cut the material to achieve the desired geometry with all the steps controlled by a computer program. The selection of milling materials is based on application as illustrated in Table 5. The milling units are categorized into two classifications according to Fig. 2: (A) dry/wet/milling and grinding in which some milling materials need dry milling and others need wet milling (according to Table 5) or (B) number of axes (3 axes or 4 axes or 5 axes) in which both the 4 axes and 5 axes move linearly up and down through different axes (X, Y, Z). The main difference is the number of rotations, the block/disc can rotate around X axes only (A rotation), but in the 5 axes, the block/ disc rotates around X axes (A rotation) and the spindle rotates around Y axes (B rotation). The main difference between 4 axes and 5 axes milling units is illustrated in Table 6. Furthermore, restorations milled with a 5-axial milling unit have a greater accuracy than those milled with a 4-axial milling unit because 5-axial milling unit can mill undercuts in all directions [43]. Not all 5 axes milling units are the same because of differences in the amount of A and B rotations. A rotary cutting instrument with a smaller diameter results in a more accurate milling process [44]. The main disadvantage of milling technology is the milling procedure accuracy is dictated by the diameter of the smallest bur [45]. Therefore, any surface details less than the diameter of the milling bur will be overmilled, and it will contribute to low retention of the restoration. There is a difference between in-office and laboratory milling units in terms of the number and the types

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Table 4 – Comparison between milling and 3D printing. Subtractive technology ‘‘milling and grinding’’ Chromium Cobalt Removable Partial Framework

No

Chromium Cobalt Copings, Crowns, Bridges Complete Dentures Digital Models

Yes Yes (Weiland, AvaDent) Yes

Burnout Pattern for Copings/Frameworks, Crowns, FPD, Inlays, Onlays, Veneers, Removable Partial Framework Zirconia Restoration Glass-Ceramic Restoration Titanium Abutments Titanium Bars Wax-up Provisional Restorations Splint Custom Trays Surgical Drill Guide Advantages

Yes by wax or resin

Disadvantages

a

Yes Yes Yes Yes Yes Yes Yes No Yes It is available for all types of materials

(1) The thinnest part of the restoration is limited by the size of the bur; if the thinnest part is smaller than the smallest bur, it will result in over-milling and cause loose fit restoration, (2) expensive for using glass-ceramic blocks, and (3) require expensive CAM unit.

Additive technology ‘‘3D printing’’ Yes through Direct Metal Laser Sinteringa (DMLS) Yes through DMLS Yes (Pala, Dentca) Preferred through StereoLithogrAphya (SLA), Scan, Spin and Selectively Photocuringa (3SP), PolyJeta, Direct Light Projectiona (DLP) Yes by photopolymeric resin through DLP

No No No No Yes through DLP Yes through DLP No Yes through PolyJet Preferred through PolyJet, DLP (1) Finer detail reproduction (undercuts, better anatomy), (2) more economical than milling, (3) more mass production (greater numbers of units), (4) larger objects produced (facial prosthesis), (5) better passive production (no force application), (6) can reproduce complex shapes without requiring special cutting tool, (7) unlimited geometry options, (8) faster than milling, and (9) print exactly as designed without waste. It is not available for ceramics and titanium metals

Different techniques of additive technology.

of burs, number of axis (4 or 5 milling axis), wet/dry, milling/ grinding. Anadioti et al. [34] reported that the internal gap obtained from the lava digital impression/pressed crowns group (0.211 mm  SD 0.041) was significantly greater than that obtained from the other groups (P < .001), while no significant differences were found among silicone impression/pressed crowns (0.111 mm  SD 0.047), silicone impression/CAD/CAM fabricated crowns (0.116 mm  SD 0.02), and lava digital impression/CAD/CAM fabricated crowns (0.145 mm  SD 0.024). Additive manufacturing is defined as the process of joining materials to make objects from 3D model data, usually layer upon layer [46]. Once the CAD design is finalized, it is segmented into multislice images. For each millimeter of material, there are 5–20 layers in which the machine lays down successive layers of liquid or powder material that are fused to create the final shape. This is followed by further refinement to remove the excess material and supporting frame. The main problem with this type of manufacturing is that it can cause differences in the final model production

because of shrinkage during building, postcuring, and minimal thickness of the layers. There are several techniques that can be involved in the additive technology including Direct Metal Laser Sintering (DMLS), StereoLithogrAphy (SLA), Scan, Spin and Selectively Photocuring (3SP), PolyJet, and Direct Light Projection (DLP). The primary difference is related to developing the z-plane, which represents the vertical components of the restorations [46]. Printing a digital model is more accurate than milling [35] and more accurate than conventional plaster models [47]. However, the fabrication of an implant surgical template is fabricated more accurately using five-axis milling than rapid prototyping [48]. Printing can be used for soft tissue models for implant cases, and the socket can be prepared for the implant analog to be inserted. Furthermore, the patient name and the record number can be inscribed on the virtual model. Based on the final crown design, a preparation guide can be easily created to help the dentist to validate the shape and size of the preparation. The abutment design can be improved by visualizing the osseous structure.

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Table 5 – The milling materials can be classified according to application, cutting tool, dry/wet/milling/grinding with hard or soft protocol. Main applications

Presintered zirconia1,2,3

Cutting tool

Milling/ grinding (hard/soft)

Copings/frameworks, crowns, inlays, onlays, FPD, abutments Copings/frameworks, crowns, inlays, onlays, FPD Copings/frameworks, crowns, FPD

Diamond or carbide bur Diamond bur

Soft milling

Carbide bur

Soft or Hard milling

Titanium2,4

Implant abutments and bars

Carbide bur

Hard milling

Polymethylmethacrylate (PMMA)1,2

Provisional restorations, burnout pattern (casting, pressing, overpressing), splint, complete dentures, verification of the final prosthesis intraorally Digital models

Carbide bur

Hard milling

Carbide bur

Hard milling

Burnout pattern or diagnostic wax-up Crowns, inlays, onlays, veneers

Diamond or carbide bur Carbide bur

Soft milling

Crowns, inlays, onlays, veneers

Diamond burs

Hard grinding

Crowns, inlays, onlays, veneers

Diamond burs

Hard grinding

Copings/frameworks, crowns, inlays, onlays, anterior FPD, veneers Crowns, inlays, onlays, veneers

Diamond burs

Hard grinding

Diamond burs

Hard grinding

Crowns, inlays, onlays, veneers

Diamond burs

Hard grinding

Crowns, inlays, onlays, veneers

Diamond burs

Hard grinding

Fully sintered zirconia 1 Chromium cobalt1,2

Polyurethane (PU) 2

Wax1,2 Composite resin1,2 (e.g., Paradigm MZ 100, Ceramill Comp) Zirconia reinforced lithium silicate, ZLS1 (e.g., Celtra DUO) Zirconia reinforced lithium disilicate1 (e.g., Vita Suprinity) Lithium disilicate based glass-ceramic1 (e.g., e. max CAD) Lithium silicate based glass-ceramic1 (e.g., Obsidian) Leucite based glassceramic1 (e.g., IPS Empress CAD) Resin reinforced feldspathic ceramic1 (e.g., Vita Enemic, Lava Ultimate)

Hard milling

Hard milling

Milling/grinding (wet/dry)

Dry because wet milling will cause softening Wet because dry milling will cause cracks and fracture Dry or wet milling which depends on milling type (hard or soft) Wet milling to keep the tools from overheating/breaking It can be both but preferred dry because wet milling will cause undesired residue

It can be both but preferred dry because wet milling will cause undesired residue Dry because wet milling will cause undesired residue It can be both but preferred dry because wet milling will cause undesired residue Wet because dry milling will cause cracks and fracture from heat Wet because dry milling will cause cracks and fracture from heat Wet because dry milling will cause cracks and fracture from heat Wet because dry milling will cause cracks and fracture from heat Wet because dry milling will cause cracks and fracture from heat Wet because dry milling will cause cracks and fracture from heat

Processing materials can be fabricated in block (1), disc (2). As the strength of zirconia increases, the translucency drops so there are 3 forms of zirconia (3): very translucent (anterior copings/frameworks, crowns, FPD), translucent (posterior copings/frameworks, crowns, FPD), traditional (bruxer patients, conceal dark dentin). Titanium (4) can be present as pure for implant abutments and alloy for bars.

Regarding the removable partial denture, the framework design is drawn on the working cast and then scanned using a laboratory scanner. The framework is always fabricated by printing a photopolymeric framework and then cast with chromium cobalt, or the framework can be printed directly from chromium cobalt through Direct Metal Laser Sintering. Complete dentures can be fabricated digitally; as certain clinical procedures are performed according to the manufacturers of the digital dentures, then the complete dentures are fabricated [49–55]. Some companies can mill the denture base and then bond the prefabricated denture teeth to the recesses of the milled denture base (Weiland and AvaDent), or both the denture base and teeth are milled as one unit (AvaDent). An alternative method is 3D printing of the base and the teeth as one unit (Pala and Dentica).

After the coping and framework are fabricated through additive technology (printing of the resin copings and frameworks, and then pressed later with glass-ceramic) or milling/grinding, the veneering porcelain can be fabricated by three different methods (Fig. 3) to include traditional layering of porcelain, pressing technique (manual wax-up of the pattern or printing/milling of the resin pattern to be pressed), and the CAD-on veneering technique (the computer will calculate the interocclusal distance between the prepared tooth and opposing tooth/prepared tooth), then the computer will design both the coping/framework and veneer porcelain at the same time. The copings/framework will be milled from a zirconia disc and the veneer porcelain (CAD-on veneer) will be milled from a lithium disilicate block. The two parts will be attached to each other using fusion glass or luting cement. The

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Fig. 2 – Classifications of subtractive technology. The soft and dry milling, and grinding strategy is dictated by the material type, while the number of milling axes is dictated by the design of the dental restoration.

Table 6 – Classification and comparison of milling units according to the number of axes. Four axes milling unit

Five axes milling unit

Dry/wet Maintenance Weight Applications

It can be wet (chairside) or dry (laboratory) Low Lighter General dentistry: veneers, inlays, onlays, copings/frameworks, crowns, fixed partial dentures

Linear movement and rotations

Three spatial directions X, Y, Z and tension bridge A (rotation around X axes)

Cost Milling of sharp angles and undercuts Number of cutting tools Milling time Milling accuracy Chairside milling unit Laboratory milling unit Processing material: block Processing material: disc

Cheaper Yes (one direction which is less accurate)

Always dry and wet High Heavier In addition to general dentistry, it can mill attachments, implant abutments, telescope crowns, splints, models, bars, screw retained implant crown and FPD, surgical drill guide Three spatial directions X, Y, Z, tension bridge A (rotation around X axes) and milling spindle B (rotation around Y axes) More expensive Yes (different directions which are more accurate) More Long High No Yes Yes Yes

Less Short Low Yes Yes Yes (chairside and laboratory) Yes (laboratory only)

CAD-on veneering technique is becoming more popular because of (1) absence of voids or defects [56], (2) ensures an even thickness of the veneer porcelain (veneer to core thickness ratio), (3) no effect of cooling rate, (4) less number of firings, (5) no need for liner, (6) lower coefficient of thermal expansion, CTE (closer to the CTE of zirconia) than pressed and layered porcelain, (7) resistant to aging, and higher bond strength between the veering ceramic and zirconia as compared to layering and pressing techniques [57,58].

9. Limitations and future CAD/CAM technology The cameras are line of sight, which means that the camera can only record what is visible to the camera lens. Therefore, those structures or margins obscured by saliva, blood, or soft tissue are not visible to the camera and will not be accurately recorded [23,59,60]. The absence of glass-ceramics in a disc

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Fig. 3 – Methods for veneering copings and frameworks.

form is a deficiency. Once it becomes available in disc form, the pressing technique will most likely vanish. Furthermore, additive technology is limited to polymeric and metallic materials and thus far does not include ceramics in dentistry. One more limitation is the limited full arch accuracy of digital impressions as compared with conventional impressions [61– 64]. Furthermore, it has been noted that zirconia frameworks on teeth requiring longer curved frameworks are subjected to a greater sintering distortion than the shorter straight frameworks, which may potentially affect fit and adaptation. The zirconia frameworks exhibit accurate fit for partial arch prosthesis only [65]. In the future, ultrasound impressions will be implemented using ultrasonic waves, which have the capability to penetrate the gingiva non-invasively without retraction cords and not be affected by saliva, sulcular fluid, and blood. This will lead to decisive advancements, as detailed cleaning and drying of the oral cavity and associated tooth structure will become unnecessary, as well as reducing treatment time and increasing patient comfort compared with optical impressions [66,67]. Furthermore, the restorations will be fabricated through laser milling [68] and/or direct inject printing [69] of zirconia and glass ceramics. Additionally, in the future, ultrasound impressions will be used in concert with monolithic restorations as in Method 1 of Table 3.

10.

Summary and conclusions

Recently, several aspects of CAD/CAM systems have had significant technological improvements. These include the development and application of new materials, the introduction of virtual articulator software, and development of scanners, the availability of more efficient milling and 3D printing machines, and transfer of digitized casts to the virtual articulator. The coming trend for most practitioners will be the use of an acquisition camera attached to a laptop computer with the appropriate software and the capability of forwarding the image to the commercial laboratory. All types of restorations can be milled by an inhouse CAD/CAM center except titanium bars and complete dentures, which need to be sent to a production center. The production of digital models is still expensive as compared with stone models. Because of the capability to mill undercuts and minute anatomical details in multiple directions, a 5 axes milling unit is best used to fabricate a restoration.

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Please cite this article in press as: Alghazzawi TF. Advancements in CAD/CAM technology: Options for practical implementation. J Prosthodont Res (2016), http://dx.doi.org/10.1016/j.jpor.2016.01.003