Written by Gary Kaye, DDS
Restorative digital dentistry (RDD) has changed the way dentistry is practiced today, as well as how our patients will receive restorative care in the future. The evolution of RDD has been driven by advances that have taken place in general technology, material science, and in parallel with developments in adhesive dentistry.
We are living in a world where the pace of technological developments is ever more rapidly changing the way we live our lives. The manner in which we perform daily tasks and how we interact with the world around us are changing. These advancements have had far-reaching consequences on society in a similar way to how RDD is affecting the landscape of restorative dentistry. In this series of articles, an attempt will be made to provide an original, insightful look at the topics that general dentists encounter in their journey from traditional to digital dentistry. The author will draw from the almost 20 years of RDD clinical experience in identifying RDD workflows. The best practices developed at the New York Center for Digital Dentistry (NYCDD) on how to manage the integration of new technology into a practice will be shared, looking at the pitfalls and success factors. Lastly, an examination of how RDD fits into the contemporary paradigm of delivering better outcomes at lower costs will be presented.
The RDD workflows are divided into 2 groups. Those that involve a physical impression are partially digital, and those that have no physical impression involved in the workflow are totally digital. Figures 1 and 2 show the different RDD workflows in comparison to the traditional restorative workflow.
The clinical protocol for the traditional workflow and partially digital is the same: an impression is taken and sent to the laboratory with a prescription, and the lab team returns the restoration for chairside seating. The laboratory portion of the workflow incorporates digital processes including scanning, printing, designing, and milling.
The totally digital workflow eliminates the physical impression by utilizing an intraoral scanner to create a digital impression, then transmitting the data via a software program that can be used to design a restoration. An easy way to envision a digital scan is to imagine a virtual impression. It is notably different from a physical impression because the latter essentially captures a negative image of the dentition, whereas a virtual impression captures an exact digital replica of the dentition. That digital replica is then used as a virtual master cast on which restorative components are created. The totally digital workflow eliminates inaccuracies inherently associated with the use of impression materials and the associated issues with consistency in die stone setting.
Scanning and Image Capture
There are numerous scanners available today that have different features and functionality. Until recently, we could have divided the scanners into those that are bundled with and designed specifically to work with a dedicated milling unit (such as complete chairside CAD/CAM systems like PlanScan [Planmeca] and CEREC [Dentsply Sirona]); or scanners whose functionality mirrors taking an impression and whose sole purpose is to be able to capture and transmit information about dental morphology to a dental laboratory (such as scan-only systems like TRIOS [3Shape], True Definition Scanner [3M], and iTero [Align Technology]).
The lines are quickly being blurred as more mills have become available (such as TS150 [Glidewell Laboratories] and PlanMill [Planmeca]) that can be tethered to existing stand-alone scanners. The scanners in chairside CAD/CAM systems can function as scan-only devices as well. The promise that the different systems can interact with one another fits into the broad category of interoperability, or what are called open systems or closed systems. The concept of whether a particular scanner from a manufacturer can seamlessly work with a design system and/or mill from another manufacturer is the logical progression that we should expect as we move forward with this technology.
Like any procedure in dentistry, there are nuances and learning curves when familiarizing oneself with these technologies. At the NYCDD, we have found that the learning curve can vary from scanning unit to scanning unit and from operator to operator. We have also found that, for most offices and systems, it is highly recommended to have an assistant present during the process. This is not to say that an efficient and accurate scan cannot be done by an adept operator on particular unit; rather, it is recommended to have a trained assistant on hand until a level of proficiency is reached.
Scanners come on dedicated carts, laptops, or can be integrated into a workstation in the operatory. The monitor should be placed behind or on the opposite side of the patient so that the operator, whether seated or standing, is able to manipulate the scanner in the patient’s mouth while effortlessly shifting gaze from the scanner to the screen as it is manipulated within the oral cavity. The operator would task his or her attention between watching the virtual (or digital) model being generated on the screen and using tactile and visual cues to reposition the scanner in order to capture the data that will build that virtual model. These digital models are built by moving the scanner around the oral cavity, capturing the surfaces of the hard and soft tissue so that an exact virtual replica is generated (Figures 3 to 5). Particular attention is paid to keeping the extraneous structures (such as the tongue and buccal tissues) out of the view of the scanner.
This is where the assistant can be very helpful, since to become fully accustomed to this technique can take time. In addition, the assistant can help with intraoral moisture control.
Lastly, manufacturers usually provide recommendations for specific scan patterns, such as where to start the scan and how to move it around until all the data has been captured. When we look at scan patterns, we can differentiate between quadrant scanning or full-arch scanning (Figures 6 and 7).
In quadrant scanning, it is important to note that the operatory needs to capture at least 2 teeth, in addition to the details of the prepared tooth, and must also ensure that the proximal surfaces of the adjacent teeth are clearly defined. With quadrant scanning, it is generally advised to scan the occlusal surface, and then to move on to scan the buccal and lingual surfaces. The opposing teeth that will affect the occlusion of the restoration and the corresponding buccal surfaces need to be scanned so that there is sufficient occlusal information for virtual articulation. It is not always necessary to capture all of the lingual information on the opposing arch, because the virtual bite registration utilizes a scan of the buccal surfaces of the teeth in occlusion.
It should be noted that the scanning patterns for full-arch scanning are manufacturer-specific. If they are not followed, the accuracy of the scan may be affected (Figure 8).
A standard requirement in successful prosthetic rehabilitation of the dentition is that the underlying tooth structure is prepared in a way that takes into account the mechanical properties of the restorative material being used. RDD has spurred the development of numerous materials that can be fabricated by reductive techniques using multiaxis milling machines that fall into the CAD/CAM space. The technology also dictates how the restorative materials are formed and, in particular, the finite limitations that arise from the shape and size of the milling burs.
We have to pay particular attention when preparing the teeth so that the line angles and surfaces that are created will not inhibit a precise fit of the CAD/CAM restoration. Sharp line angles can interfere with seating the restoration and create stress points that may cause premature failure of the restoration. The tooth structure should also be reduced in accordance with preparation guidelines for the particular material that will be used. Guidelines for lithium disilicate are 1.5- to 2.0-mm occlusal reduction, 1.0- to 1.5-mm axial walls, with a 0.5-mm (minimum) chamfer margin. Polymer-ceramic materials—such as Enamic (VITA), Lava Ultimate (3M), or CERASMART (GC America)—are 2.0-mm occlusal reduction, 1.0- to 1.5-mm axial walls, with a 0.5-mm (minimum) chamfer margin (Figures 9 and 10).
For proper function, an all-ceramic restoration is generally designed to have a passive fit, although this may not be the case with the high-strength zirconia restorations. In addition to a passive fit, it is very important that the marginal gap or “cement thickness space” is uniform as a minimal thickness.
Before a tooth can be scanned, the margins must be prepared. This should be done with meticulous attention to detail as described above. Conventional impression materials can displace soft tissues while keeping to the margins and any submarginal surfaces needed to replicate the tooth structure. However, IO digital scanners are unable to displace tissue, so adequate retraction must be achieved prior to scanning. This can be accomplished with retraction cords using a single-cord technique, or a double-cord technique in which the second cord is removed immediately prior to scanning. Using a soft-tissue diode laser or electrosurgery can also be useful in removing tissue that may inhibit both the scanning and the seating of a restoration. The author has found that retraction paste, such as Traxodent (Premier Dental Products), can also be used as an aid in the process. The retraction paste is placed for 2 minutes in the sulcus prior to scanning. We have found that Traxodent, in addition to acting as a hemostatic agent, will expand on setting, “pushing” the tissue away and opening the sulcus. Care should be taken to rinse away all of the material prior to scanning.
The first stage in designing a restoration is to inspect that the preparation meets the requirements for the specific material to be used, as well as having clearly defined margins. In most cases, the design phase can be commenced while the patient is in the chair. Because the technology allows the clinician to do this, the design phase is considered to be a part of the preparation phase; any adjustments that the in-office designer sees can be communicated immediately to the dentist. In the case of the dentist doing this, the scans are inspected and any necessary refinements to the preparation or retraction process are made in order to improve the outcome.
In the design phase, the operator has control over all the elements that affect how the restoration will be milled (Figures 11 to 15). We have learned that the there is almost no variability in the designed elements, such as morphology contact strength and occlusal points, from the proposed design to the milled product (Figures 16 to 20).
Fabrication and Finishing
Once the restoration is designed, that information is sent to a mill for fabrication. The restoration is molded out of a monolithic block, which ensures consistency in the material from one restoration to the next. The milling time can vary from less than 10 minutes to almost an hour, depending upon the restorative material selected.
In general, a polymer-ceramic restoration (such as Enamic, Lava Ultimate, or CERASMART) requires the shortest milling time. Finishing is done with rubber wheels and a suitable polishing paste. These materials do not require any firing time in an oven so the overall appointment time is the shortest.
The lithium disilicate (such as IPS e.max CAD [Ivoclar Vivadent]) and Obsidian (Glidewell Laboratories) will have relatively fast milling times but require time for sintering in an oven (Figures 21 to 23). The sintering process, via heat in a sintering oven, significantly increases the strength and surface characteristics of the material.
Zirconia requires much more milling time because of its inherent hardness and often an extended time in a sintering oven. We have successfully produced chairside zirconia crowns using BruxZir NOW (Glidewell Laboratories) blocks with a TS150 Mill. The milling time is less than an hour, and these restorations can either be hand-polished or placed in an oven for a very short glazing and staining cycle.
It is worth noting that another chairside zirconia product was just introduced in 2016, the CEREC Zirconia (Dentsply Sirona). CEREC Zirconia features a dry milling technique that reduces the sintering times to 15 minutes for crowns and 25 minutes for bridges.
Next: The best practices developed at NYCDD on how to manage the integration of new technology into a practice while examining the pitfalls and success factors.
- Kaye G. The restorative digital dentistry puzzle: including all of the necessary pieces. Dent Econ. November 20, 2014. dentaleconomics.com/articles/print/volume-104/issue-11/features/the-restorative-digital-dentistry-puzzle-including-all-of-the-necessary-pieces.html. Accessed June 28, 2016.
- Poticny DJ, Klim J. CAD/CAM in-office technology: innovations after 25 years for predictable, esthetic outcomes. J Am Dent Assoc. 2010;141(suppl 2):5S-9S.
- Battersby J. CAD/CAM—The end for dental labs or a new beginning? Dentistry iQ. May 6, 2014. dentistryiq.com/articles/2014/05/cad-cam-the-end-for-dental-labs-or-a-new-beginning.html. Accessed June 28, 2016.
Dr. Kaye completed his graduate dental school training at the Columbia School of Oral Medicine in New York City. He maintains a private practice in New York City and is the founder and principal of the New York Center for Digital Dentistry. He is a graduate of the Dawson Center for Advanced Dental Training and the Sirona Speakers’ Academy, and he is on the guest faculty for Planmeca University in Richardson, Tex. His lecture topics include ceramics, occlusion, and digital dentistry. He can be reached via email at email@example.com.
Disclosure: Dr. Kaye is a consultant to Henry Schein, is owner of the New York Center for Digital Dentistry and Kaye Dentistry, and maintains an ownership interest in the New York Center for Digital Restorative Solutions.