Digital Workflow for the Lava COS System
New digital system blends the lines between laboratory-based and dental office-based CAD/CAM systems to digitally record tooth preparations.
Computer-assisted design, computer-assisted machining (CAD/CAM) systems fall into two general categories. One is a laboratory-based system. This type of system requires a dentist to send a traditional final impression to the laboratory where it is poured in stone and the ensuing model is scanned to transfer the volumetric shape of the preparation to the design software. The dental laboratory uses the CAD/CAM system to create the substructure or entire restoration and it is finished on the model before returning it to the dentist for delivery. The second category is a dental office-based system. These systems allow the dentist to scan the tooth preparation intraorally to transfer the volumetric data on the preparation directly to the design software. These systems allow the dentist to fabricate the final ceramic restoration in the dental office during a single appointment.
A newer digital system, the Lava Chairside Oral Scanner (COS) (3M ESPE; www.3mespe.com), has been introduced that blends the lines between these two types of CAD/CAM systems. The dentist digitally records the tooth preparation intraorally instead of using a traditional impression. The digital data is electronically transferred to the dental laboratory for fabrication of the desired restoration. The purpose of this article is to describe the digital workflow for the Lava COS system.
Tooth Preparation
A patient presented with the need for a crown and elected to have an all-ceramic restoration for the maxillary left first molar (Figure 1). Tooth preparations for CAD/CAM ceramic restorations must take into account the features required for all-ceramic restorations as well as unique features required for milled restorations. Milled restorations adapt optimally when the tooth preparation has a smooth, flowing surface with no sharp angles (Figure 2). The more complex the preparation geometry, meaning the more sharp internal angles, the more difficult it is to mill the restoration. For this reason sharp, angular boxes or grooves are not recommended in the preparation. Rounded internal angles, smooth margins, and flowing curves facilitate the accuracy of the milling process. Tooth preparations for ceramic restorations also require adequate thickness to ensure strength of the ceramic and rounded internal angles to avoid creating stress points within the restoration.
Digital Impression
The key element in creating an accurately fitting restoration is to precisely reproduce the volumetric shape of the tooth preparation. Traditionally, this has been done with a final impression using polyvinylsiloxane or polyether materials. Digital impressions rely on a similar need for accuracy. As much as dentists may prefer it, no system at this time has the capability to image the tooth surface through soft tissues. Soft tissues must be retracted from the margins of the tooth preparation to ensure direct visual access for the camera (Figure 3). Fluid contaminants such as blood and saliva must also be isolated from the field of view of the camera, and the tongue and cheeks must be controlled from interfering in the impression as well.
The Lava COS uses an intraoral camera based on the application of 3D-in-motion technology. It records continuous 3D video images to create a volume model of the quadrant or arch in real-time on the computer monitor. The camera captures 20 3D data sets per second, or about 2,400 3D data sets per arch to create the volume model. The system uses high-speed image processing algorithms and real-time modeling software to create the digital impression.1 The amount of the dentition to be recorded is determined by the dentist and can range from a sextant to a full arch. The final model may be recorded in a single scan with the camera or by using overlapping segmental scans depending on the preference of the dentist.
No specific preparation of the tooth surface is required other than drying with an air/water syringe. The teeth and soft tissues are lightly sprayed with a titanium dioxide powder to create a reflective surface. The powder provides contrast points for scanning to enhance recording the 3D models and improve the speed of capture (Figure 4).
The prepared tooth is scanned first to ensure that the preparation is accurately recorded. The camera is positioned intraorally, activated, and automatically begins scanning as the correct focal distance is reached. The volume model is visualized on the monitor in real time throughout the scanning process. The touch-screen monitor allows the digital model to be rotated, magnified, and evaluated through video review or 3D stereographic review to assess accurate recording of the dentition.1 This provides immediate feedback on the tooth preparation and digital scan that affords the opportunity to correct deficiencies before transferring the data to the laboratory for fabrication of the restoration (Figure 5).
After the preparation has been recorded, the additional teeth in the quadrant or arch are scanned to complete the desired model. It is not required to complete the entire scanning process in a single pass of the camera. Overlapping segments of the arch or quadrant may be scanned and the segments are assembled into a single volume model. Two data scans are required to record the case; the quadrant or arch containing the tooth/teeth preparation(s) and the opposing quadrant or arch.
A critical element for the fabrication of indirect restorations is recording the occlusal relationship of opposing models. The patient is instructed to close into maximum intercuspation and hold the position (Figure 6). The centric position of occlusion is digitally recorded by scanning the maxillary and mandibular teeth and adjacent soft tissues from the facial. This scan determines the relationship of the opposing arches or quadrant scans in the digital file (Figure 7). No other bite registration materials are required. At this time, lateral or protrusive functional movements may not be digitally recorded as the scan is a static record of the centric position.
A common question about digital recording of the dentition is what is the length of time required to do this? The maximum time available to scan a maxillary or mandibular model is 7 minutes. This refers to 7 minutes of actual scanning time. Because the quadrant or arch is scanned in strips or segments, this is more time than is generally used to record the dentition. Quadrants are scanned in 2 to 3 minutes and bite registrations in less than 1 minute. There is a minimum acceptable amount of data required to create a model, that being two to three teeth. Similarly, there is a minimal amount of data required to accurately record the occlusal relationship of the opposing models. If insufficient data is recorded, the software will not relate the opposing models and the centric relationship will have to be rescanned.
Digital Workflow
After the digital scans have been recorded, the prepared tooth is identified on the digital model before submitting the case electronically to the laboratory (Figure 8). The dentist also has the option to mark the margin of the preparation. A magnification screen is available to aid in accurate placement of the margin marking points (Figure 9).
After the digital models have been approved by the dentist, the electronic laboratory prescription is completed on the system and the case is digitally transmitted to the dental laboratory. The laboratory technician reviews the data file and identifies and marks the location of the preparation margin. The laboratory technician also selects the locations to section the model to create individual dies for the preparations.
The data file is forwarded to 3M ESPE for fabrication of the model. The quadrant or arch models are made using stereolithography (SLA). Stereolithography is a rapid prototyping technology that creates models using a visible light curing (VLC) polymer resin material.2 It consists of a laser micro-curing process that creates the models from a volumetrically stable polymer resin material. An automated process is used to mount the sectioned dies and opposing models in an articulator based on the scanned digital data from the patient’s centric position. A second solid model of the tooth preparation and adjacent teeth is also manufactured as a verification model for the laboratory (Figure 10 and Figure 11).
While the SLA resin models are being fabricated, the digital impression data may be input directly into the Lava CAD/CAM software program. The software program is used to design the appropriate thickness and unique contour of the zirconium coping or substructure for the prescribed restoration. The final design file is used to direct the Lava Milling Unit to fabricate the coping or substructure from blocks of zirconia. The Lava COS digital impression is not just for Lava zirconium crowns. The SLA model can be fabricated from the digital impression and allows for the full complement of indirect restorations offered by the laboratory if Lava zirconium restorations are not the restoration of choice for the case.
Once the zirconia coping has been milled, the addition of the veneering porcelain and final processing of the restoration can be completed by the technician using the SLA model. The case is returned to the dental office for delivery of the restoration to the patient (Figure 12 and Figure 13).
Conclusion
The Lava Chairside Oral Scanner blends the lines between laboratory and chairside CAD/CAM systems. The dentist records the tooth preparation intraorally with a digital impression instead of using a traditional impression. The immediate feedback of the digital impression affords the opportunity to correct model deficiencies before the patient leaves the office. Digital impressions avoid the tedious technique of final impressions with the added benefit of a more comfortable patient experience. A number of conventional steps are avoided with the digital impression technique, such as time-consuming, conventional impression techniques, pouring models, trimming dies, and packaging, shipping, and tracking the models. The digital workflow of the Lava COS system results in an efficient and accurate technique for the fabrication of Lava zirconium crowns and fixed partial dentures.
References
1. McMaster D, Cohen B, Spitz SD: Digital workflow. Dental Economics. August 2008;30-36.
2. Dunne P. Digital dentistry and SLA technology. Lab Management Today. Nov/Dec 2008;44-45.
Disclosure
The author has received research support from 3M ESPE.
About the Author
Dennis J. Fasbinder, DDS, ABGD
Clinical Professor of Dentistry Director
Advanced Education in General Dentistry
School of Dentistry
University of Michigan
Ann Arbor, Michigan
