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It is customary to distinguish direct computer-aided design/computer aided manufacturing (CAD/CAM), in which all the stages (optical impression, CAD, CAM) are undertaken in the practice, from the semi-direct method, in which a digital impression is sent via the Internet to the dental technician for fabrication. When crowns and bridges are produced using semi-direct CAD/CAM, this usually includes the use of a temporary prosthesis. The temporary bridge is retained while the definitive prosthesis is fabricated in the laboratory. This helps in terms of maintaining aesthetic and functional factors, and in the event of adaptations to vital teeth, it protects pulp vitality and reduces inter session sensitivity. The therapeutic goals of temporary CAD/CAM readjustments are similar to those of conventional fabrication techniques.  The transitory fixed prosthesis is designed to enhance aesthetics, stabilisation, and/or function for a limited period, after which it is replaced by a definitive prosthesis.
Often, such prostheses are used to assist in the determination of the therapeutic effectiveness of a specific treatment plan or the form and function of the planned definitive prosthesis. The following requirements need to be met:
The materials used in CAD/CAM are more fracture-resistant than the resins used chairside; [6, 7] they are extremely aesthetic in aspect and afford repair or modification directly at chairside.  The ability to produce temporary prostheses with optimal, more predictable adjustment margins is of the greatest interest from the aspects of plaque control, gingival health and protection of residual dental structures. The respect of limits, aesthetics and the choice of sufficiently resistant restorative materials are important factors to take into account, especially during mid- or long-term temporary aesthetic restoration. 
In conventional techniques, a crown or a bridge can be prepared, that is, undertaken before the teeth are made, based on a wax-up or self-casting, using an impression taken prior to preparation. Transposed to CAD/CAM, this demands 3-D digital recording of the initial clinical situation with the aid of an intraoral scanner. This article puts forward a modern, routine development in the management of CAD/CAM-produced temporary teeth based on three clinical case studies: one using a subtraction method (milling) and the others using an additive method (3-D printing).
In the milling method (subtraction), the volume is reduced (block or disc) until the shape required is achieved. Three-dimensional printing is an additive method by the deposit of successive layers until the final shape is achieved. Stereolithography (SLA) was invented in the 1980s and involves the hardening of liquid light-cured resin placed in a tray by photopolymerisation, using a laser that builds the object up layer by layer. A similar 3-D printing technology used for the fabrication of temporary dental crowns is known as digital light processing (DLP), which uses a projector instead of the laser. The digital projection of the 3-D shape of the tooth on the liquid resin allows superimposition of successive layers by light-curing the resin. It is one of the most precise 3-D printing methods. 
For the past few years, 3-D printing has been used in various areas of dentistry: 
Quite recently, the materials used in the 3-D printing of temporary crowns and bridges have been marketed and are now being used successfully alongside intraoral scanners.  The purpose of this article is to demonstrate the usefulness of rapid prototyping for the realisation of temporary dental restorations from preliminary optical impressions gained using intra-oral scanners. We implemented this method using two scanners: a last-generation (2016) scanner (TRIOS 3 color, 3Shape) for Case 1, and a previous generation (2008) scanner (CEREC 3-D Redcam, Sirona) for Cases 2 and 3 (Fig. 1).
We describe the workflow to obtain an open STL file format of the final restorative design from proprietary format files, CDT for the CEREC 3-D scanner and DCM for the TRIOS scanner. We show that these files can be either milled or 3-D printed to produce the final prosthesis.
After the presentation and comparison of the two fabrication techniques based on three clinical case studies, we will discuss which materials are available for 3-D printing, in which type of printer they can be used, and the advantages we might draw from both case studies by manufacturing provisional prostheses prior to the preparation of the teeth.
Conventional methods for fabrication
Practitioners habitually use several methods to fabricate provisional prostheses: 
Materials used for provisional restorations produced by conventional methods involve various polymers: polymethyl methacrylate (PMMA), poly(ethyl methacrylate) (PEMA), and bis-GMA light-polymerised urethane dimethacrylate (UDMA).
Direct methods are often very quick to implement, but a number of studies have shown better quality of restoration by indirect methods:
For 3-D printing, the final CAD files of the restoration must be in open STL format to be printed. In our report, we describe that the workflow is similar regardless of the intraoral scanner used (new or old generation).
Case 1: Temporary 3-D printed posterior crown (on a Perfactory Vida high-resolution printer, EnvisionTEC) based on a preliminary impression with a TRIOS 3 color intraoral scanner.
Initial situation and preliminary treatment
In a 37-year-old patient, necrosis under a composite led to loss of the vitality of tooth #45 owing to caries. The missing tooth structure did not affect the entire morphology of the tooth, and we wanted to realise a zirconia core structure layered with aesthetic porcelain, a viable option for opacity in cases of stained, devitalised teeth. This restoration would be sealed just after the intervention and during the necessary laboratory phase.
We took a preoperative scan (pre-scan) of the initial situation with TRIOS 3 color to produce a temporary crown using a 3D printer (Fig. 2). The impression (in DCM format) was transferred to the cloud via 3Shape Communicate to be converted into STL format. The pre-prosthetic treatment involved a root canal therapy and the setting of a fibre reinforced composite post under a rubber dam (Fig. 3).
Fabrication of the provisional prosthesis with a 3-D printer
A provisional prosthesis was made according to the shape of the tooth in the pre-scan, using the provisional eggshell module of exocad software (exocad; Fig. 4). The temporary crown was fabricated with the Perfactory Vida printer (Fig. 5) and fixed in the mouth with temporary cement (Fig. 6). After the base had been removed, the temporary crown was relined and adapted in the mouth on the day of preparation.
Fabrication of the final restoration with CAD
An optical impression was taken on the day of measurement and positioning of the temporary impression. This enabled the fabrication of a zirconia coping and the stratifi- cation of the aesthetic ceramic on the printed model (Fig. 7a) and the coping was then set in the mouth (Fig. 7b).
Case 2: Temporary bridge milled in a fabrication centre from a CEREC 3-D Redcam optical impression
The aim of CEREC in the past-decade technology was to produce chairside ceramic restorations typically as single unit. The slowness of both the optical impression and the milling machine made it difficult to produce even a definitive three-unit fixed dental prosthesis (FDP) in a single appointment. The method of fabricating a provisional by pre-scan and eggshell CAD can help to temporise during the laboratory phase.
A 19-year-old patient came for a consultation after a fall of several meters on to a rock, which had caused fracture of the jaw and was followed by surgical implantation of an osteosynthesis titanium plate. Fractured tooth #31 had to be removed owing to this operation, causing unitary edentulism. Owing to the presence of the osteosynthesis plate, the option of an implant was rejected after a scan (Fig. 8). Six months later, the bone growth and maturation reached the terminal point and the bone became eligible for bridge application.
Manufacturing of a three-unit provisional bridge
A preliminary optical impression was taken with a CEREC 3D camera to record the initial situation and to create a dental morphological base for the manufacture of the FDP. The digital CDT file was then opened with the mainstream 3-D open-source Blender software (Version 2.63.33; www.blender.org) equipped with the add-on Open Dental CAD developed by Patrick Moore for the export of the file in STL so that it can be read by exocad. The digital impression was optimised by free open-source CAD software (www.freecadweb.org) by trimming the edges to remove any artefacts created by the lower lip, which obstructs visibility. The quality of the meshing and the automatic repair functions were implemented one by one if necessary. The file was then ready to be imported into the dental exocad dental CAD software.
The client file with the patient’s name was completed, indicating the type of restoration (temporary bridge on teeth #41 to 32) and the type of materials chosen (resin). As the patient was satisfied with the aspect of the existing teeth, their morphology just had to be copied for the fabrication of the provisional prosthesis without having to present an aesthetic mock-up. The design software enabled the virtual replacement of the missing tooth. The margins of the future bridge were set out, represented on screen by movable yellow dots to create a black line. The intermediary bridge sat naturally between the two existing teeth. The bridge insertion angle was set according to the future dental preparation of teeth #41 and 32 (Fig. 9).
At this stage, the thickness of the walls of the temporary FDP, as well as the shape of the margin fit, could be programmed. The shape of the pontic and its connections were programmed to match durability and hygiene requirements.
The file was exported from exocad in STL format and then sent via the Internet to a milling centre for external fabrication (dentallgroup.eu). The shade, the type of composite resin material, and the date of delivery were selected directly on the client back office panel of the website. The fabrication and delivery time of the temporary FDP was 48 hours (Fig.10), but it could have been reduced by in-office milling.
Figure 11a shows the clinical situation with abutment teeth. The bridge constructed using the files sent to the laboratory was delivered to the practice (Fig. 11). The prosthesis was tried and adapted in the patient’s mouth on the day of preparation. This stage demanded far fewer adjustments than with conventional construction methods. Relining was necessary, but the margins adapted well.
The intraoral optical impression of the preparations was taken with the 3D CEREC camera. The file was sent by the free-access website at www.wetransfer.com to the laboratory that was equipped with the Sirona inLab 4.3 software (Dentsply Sirona). The temporary FDP was then set in the mouth with temporary cement (DentoTemp, Itena; Fig. 12).
The thickness and shape of the intermediary bond guarantee the durability of the future prosthesis. The bridge was milled in a lithium disilicate CAD block (IPS e.max, Ivoclar Vivadent) in the laboratory and then made up according to the digital photograph sent by e-mail. The final bridge was fixed according to a conventional cementing protocol with an adhesive resin. The IPS e.max FDP was fixed with a resin cement (NX3 Nexus, Kerr) and made up; the total time taken for shading and construction was one week and was to the patient’s satisfaction (Figs. 13a & b).
The main limitation of the past decade in clinical so-called “chairside” CAD/CAM dentistry was the impossibility of producing an aesthetic zirconia crown, first because the proprietary format of the files could not be converted for milling at the laboratory; second, because most of the laboratories were not able to produce a 3-D printed model; and third, because the optical impression files could not be transferred quickly and correctly to the laboratory. Currently, the new digital workflows at our disposal offer us these possibilities.
Cases of aesthetic restoration of the smile are sometimes more difficult to comprehend when one is not reconstructing the entirety of the smile, but only one or two units. Communication with the patient is essential. Three-dimensional printing can help to better understand aesthetic desires by printing different tooth shapes.
A 32-year-old patient came to the practice because of an unsatisfactory restoration of her devitalised tooth #22 several years before using a composite material that now had taken on a dark, unattractive colour (Fig. 14). To optimise the aesthetic result, we decided to place a zirconia crown (made in the laboratory with the CAD/CAM method for the substructure and manual method for the stratification of the aesthetic ceramic) on a pressed corono-radicular restoration with a fibre-reinforced composite post.
A preliminary optical impression (CEREC) was taken to record the shape before preparation and to serve as a working base for the morphology of the provisional prosthesis. The CDT file was opened with the Blender software to be transformed into STL (Fig. 15a) and processed with FreeCAD (www.freecadweb.org) to eliminate any artefacts. Then, as previously, the provisional prosthesis was designed with exocad. The preparation of tooth #22 was done after cementing a fibre-reinforced composite post under rubber dam isolation.
Fabrication of a 3-D printed provisional prosthesis
A provisional prosthesis was made by 3-D printing on a PICO2 printer (Asiga; Fig. 15b) with NextDent Crown & Bridge A2 (Class IIa biocompatible; NextDent), validated for a 30-day in-use time frame in the mouth.
The CAD was made with exocad and several designs were made for the patient with the aim of closing, if possible, the diastema between teeth #22 and 23 in relation to the initial situation. Two provisional restorations were made for the patient (Fig. 16), and the preferred temporary tooth was polished and sealed (Figs. 17a & b).
Fabrication of a definitive crown
The final crown was made using an optical impression that enabled both the fabrication of the zirconia coping and the 3-D printed models, which enabled the stratification of the ceramic in the laboratory (Figs. 18a & b).
The pre-scan method can be useful in situations in which temporary restorations are required even if in some cases practitioners would prefer to make a temporary crown directly after preparation. We can ponder the question of which workflow to use (in the clinic or in the laboratory), and why it would be better to print than to mill. We can also enquire which printer and materials to use. Three-dimensional printing is currently in full development in the dental field, and the print speed is increasing steadily. Next, we will discuss what we can expect in the future for daily use in the dental clinic.
Advantages and disadvantages of the pre-scan method for temporary crowns
The principal advantage of the pre-scan method to make temporary restorations is the principle of copy and paste for the shape of the tooth. Another advantage is that, after preparation, blood can mask the limits and make optical impressions difficult. It is appealing for a single optical impression and for only minor modification of the shape of the desired tooth without modification of the occlusion.
The principal disadvantage of the pre-scan method is the necessity to keep the same occlusal schema (before/ after) even if minor correction of the antagonist can be discussed and done before final restorations. Another problem is visualising and anticipating the position of the limits of the preparation, specifically in the proximal view. The curvature of the vestibulo-lingual edge in the interdental papillary zone can be difficult to anticipate when the limit is drawn on the software, especially in automatic mode. However, rebasing of the temporary restoration can correct it. We did not observe any problems of adhesion between the bis acryl resin layer and the 3-D printed resin.
However, if the gingival finishing lines of the preparation are under the gingiva, rebasing becomes compulsory. The STL conversion (whether from CDT or DCM) yields a monochromatic model, and the limits of the gingiva and tooth can thus be difficult to visualise.
Which workflow to use?
The method of prototyping that we have described from a pre-scan offers several possibilities for producing temporary restorations, if they do not have to be made in the session:
It depends on the inter-session time and the investment to be made.
The import of digital scan files into open software such as exocad offers the possibility of using an older scanner such as the CEREC 3-D Redcam after conversion of a native CDT to STL format and overcomes the limited functions of the software whether for the realisation of a provisional prosthesis by pre-scan or a bridge.
The latest version of Sirona inLab CAD (SW 16.0) is (to our knowledge) not yet able to create temporaries from pre-scans. The add-On Open Dental CAD of Blender allows transformation to CDT (Sirona) format only into STL. The Datakit CrossManager software can transform not only CDT files but also DCM (3Shape) files into STL.
Now, TRIOS intraoral scanner users can export STL file digital impressions directly from their TRIOS Dental Desktop software. Digital provisional crown design can also be done in 3Shape Dental System but with annual licence fees. A free and original open-source method is to use STL files, as well as the Christian Brenes tooth library in Meshmixer (Autodesk, Fig. 19), which turns this free programme into dental CAD software able to perform after a few manipulations the same operation as described on a preoperative STL scan model.25 Other interesting manipulations on the initial numerical impression model, such as the Mirror function, can be performed for aesthetic simulation, numerical wax-up, mock-up or provisional restorations. A numerical dental model from an optical impression can be matched with high-quality 3D face scanning (Bellus3D) for good aesthetic integration.
Advantages of CAD/CAM provisional restorations and additive manufacturing
Temporary restorations can now be made in the dental practice by methods employing subtraction or addition; of course, we wonder which technique we should use. Is 3-D printing the best solution?
At this time, no study has compared 3-D printed resin material to CAM resin milling, but CAD/CAM-fabricated provisional crowns have demonstrated superior fit and better strength than have direct provisional crowns (Protemp, 3M ESPE). CAD-Temp (VITA Zahnfabrik) and Telio CAD (Ivoclar Vivadent) are more often used in-office for milling provisional restorations.26 CAD/CAM-milled polymers offer a wider range of translucency and chameleon effectthan manually polymerised temporary materials do  and have shown more favourable mechanical properties. CAD/ CAM-fabricated FDPs exhibit a higher mechanical strength than directly fabricated FDPs do, when manufactured from the same material. 
Advantages of additive manufacturing
There are many advantages to the techniques of addition compared with those of subtraction:
Disadvantages of additive manufacturing
Disadvantages in comparison with the techniques of subtraction should be equally considered:
Three-dimensional printing and accuracy for temporary restorations
Resolution is one of the most important factors to take into account when considering different 3D printers to obtain satisfactory clinical results, particularly for fixed protheses. The resolution of a 3-D printer partly depends firstly on the technology used:
On fused deposition modelling (FDM) printers (the process of which entails melting plastic material and extruding it in a pattern, layer by layer, on a variable-height build platform), which is not described in our article, the machines are equipped with a nozzle measuring about 250 μm at its end.
When considering 3-D printer resolution, we also have to take all three dimensions into consideration: resolution on the planar dimensions (x- and y-axes) and the vertical resolution (z-axis) determines the thickness of a layer. On FDM machines, the smallest practical layer height is generally 0.1 mm or 100 μm, and for DLP SLA machines, 0.025 mm or 25 μm.
Recent studies presented in the literature have shown the accuracy and quality of 3-D printed temporary prostheses (Table I).34–37 However, accuracy must be placed in the context of our study, which entails producing only a preform intended to be rebased in the mouth.
Which materials for which printers?
Temporary materials must be used in the DLP (LED/UV) printers with a light spectrum of 378–388 μm. The factory-preset programmes included in the 3-D printer to perform the polymerisation and the material tested and validated by the manufacturer must be respected. The post-treatment protocols must be scrupulously respected too.
At present, several manufacturers offer biocompatible resin materials that can be used in dentistry:
Each of these brands has developed printers for laboratories producing many components simultaneously and the fabrication of more voluminous objects requiring greater precision (models and removable prosthetic frame). Dental practices can, as indicated in the table, use them less frequently or for the fabrication of provisional or aesthetic mock-ups of less voluminous and therefore less costly printers (Table II).
Actual and future developments
Speed is a parameter that has greatly improved recently for 3-D printers. During the 2017 IDS, 3D Systems with NextDent presented a printer prototype that can produce 20 crowns in nine minutes.38
A dedicated printer for dental practices (DFAB), also presented at the 2017 IDS, allows one to define the shade to be printed (Photoshade technology) and the position of the different graduated shades (Fig. 22) to be printed and could guarantee a relatively fast, 20-minute, building time for a five-element bridge compared with a similar task in a milling machine. According to the manufacturer, the new EnvisionTEC Micro Plus cDLM printer can print eight to ten crowns in 15 minutes. With DLP technology, print time is not affected by the number of crowns printed.
All shades presented on screen are included in a single cartridge (Temporis). With this material, we can produce temporary bridges of up to five components.
The method for prefabrication of bridges is a new alternative using CAD/CAM and provides an answer to the demand for total dematerialisation of the digital chain and predictability of the aesthetic result, because it circumvents the need to take physical impressions. This technique also reduces adjustments if the initial project matches the preparation and simply requires temporary cementing without the use of a relining resin.
The transformation of the proprietary format to an open format means that the file can be used in the software of one’s choice (in this case, exocad) by allowing the design of a temporary bridge, as well as its transfer to a milling centre. Hence, we can benefit from all the progress made in the dental industry in terms of milling precision and the choice of materials.
The pre-management of temporary prostheses is recommended especially in simple cases (unitary restoration, small bridges without any change in the morphology or the vertical dimension between the initial and final situation). Moreover, the initial impression can be kept and reimported into the CAD software and be used as a digital base to undertake restoration. Because 3-D printing will soon be readily available, it will be possible to produce temporary bridges in-session while the practitioner prepares the teeth, thereby significantly reducing the number of sessions or the time in the chair (Fig. 23).
Hence, we suppose that 3-D printing will substitute milling for permanent prostheses in some cases and especially for the fabrication of mock-ups. A digital wax-up from an initial impression of photographs or a 3-D scan of the face and then a mock-up is much more predictable and faster to achieve, but before the technique can be used efficiently in-chair, particularly in a single session, the time required for printing and post-treatment of the prosthesis must still be reduced.
For the time being, the interest in prefabrication of CAD temporary crowns and bridges makes complete sense in terms of reducing the time taken in-session by optimising precision, relining and adjustments. To further the subject of this study, a comparison of the properties of the different temporary resin materials available for the 3-D printer should be performed in an in vitro study. Future studies will be necessary to evaluate better whether this material fulfils the stated objectives for restorations (biological, mechanical and aesthetic).
It would also be interesting to compare the mechanical and optical properties of 3D printing materials and CAD/ CAM materials available for temporary restoration. The utility of either a 3-D DLP printer or a 3-D SLA printer for fabrication of temporary restorations (comparison at the level of printing time and accuracy) may also be considered in another study.
Editorial note: A list of references is available from the publisher. This article was published in CAD/CAM - international magazine of digital dentistry No. 01/2019.