CAD/CAM Technologies Allowing Restorative Dentistry to “Push the Envelope”
Daniel Alter, MSc, MDT, CDT
Driven mainly by the digitization of processes and protocols utilizing computer-aided design/computer-aided manufacturing (CAD/CAM) to design and manufacture the end-product, a paradigm shift is altering the practice of dentistry in terms of both treatment methodologies and restorative solutions. CAD/CAM is a comprehensive term often used to indicate digitization, computer-controlled design, and machine manufacturing, and it deploys several modalities in the process with many new and exciting innovations.
Although the dental profession is a relatively late adopter of this advanced technology, other industries such as aerospace and automotive have been using it for decades with considerable success. The numerous benefits of computerized technology include a high level of precision, simpler fabrication protocols, repeatability, and consistency, while human intervention is minimized, resulting in higher quality assurance.1
The only entry point into the CAD/CAM processes is through digitally capturing the oral environment to attain a 3-dimensional (3D) rendering. This can be accomplished intraorally with an intraoral digital scanner, or with a laboratory scanner, which requires conventional impressioning that either can be scanned in the laboratory scanner or requires pouring of gypsum to create a stone model to scan. Dentistry's adoption of digital impression technology has enabled the profession to experience tremendous change in the past several years.2 The market for intraoral scanners, also known as acquisition units, and 3D imaging has grown rapidly; in 2015 it was estimated to be worth $189.1 million and is expected to grow to approximately $310.4 million by 2020.3,4
Intraoral scanners have progressed significantly since being introduced in 1971 with the first entrant in dentistry called Sopha, developed by Dr. François Duret, and the first CAD/CAM restoration manufactured in 1983.5 CEREC I,6 developed in the mid-1980s, was considered an in-practice scan, design, and milling system. A dental practice would scan a patient, CAD design the restoration, and transmit the design file to the mill to produce a single restoration.2 However, the quality and esthetics produced, especially for anterior restorations, were not always optimal and in many cases required more manual intervention than anticipated. Most dental practices preferred sending their restorative needs to their dental laboratory while still maintaining the benefits of CAD/CAM in the office. Because laboratories had more fully embraced these technologies, dental practitioners usually opted to only scan the oral environment and send the file to their trusted laboratory to design and complete the restoration. This created an entirely new category of intraoral digital impression scanners.2 Today's digital scanners can scan in color for multiple indications, take high-definition photographs for basic shade information, and deploy useful tools for measuring precise clearance, undercuts, and more.4
The first generation of intraoral scanners had limitations and worked on the principle of “triangulation of light,” which required tooth surfaces to be coated with powder to achieve successful scans.7 The scans were taken in small geometric sequences and layered on top of each other, a process called stitching that is still used, to produce a 3D digital representation of the oral environment. It was, however, limiting in that the scanner produced a proprietary file that could be read only by a compatible system owned by the same company. This process was not conducive to meeting the needs of practice owners; subsequently, the second generation of scanners was able to scan without powder.
These scanners used video scanning or similar new technologies and produced a non-proprietary geometric file language known as a .STL file (Standard Tessellation Language or Standard Triangulated Language), which was created and could be read by any other open architecture CAD/CAM equipment. STL is an open architecture geometric file that provides a 3D texture map of a surface through a mesh of triangles.4 The closer or more compact the triangles, the higher the detail and precision of the surface, while large or curved surfaces allow for reduced detail and precision.
Offering a higher level of precision, digital impressions are not subjected to material discrepancies, both clinically and in the laboratory, as can be found in conventional impression techniques. With conventional impressioning, the mix (proportionately speaking), temperature, and setting times all introduce variables that affect the dimensional stability of the material and, ultimately, the precision of the restoration. This prompted a need for a protocol to be developed to eliminate potential errors.7 A study conducted at the University of Zurich concluded that the in vivo precision of guided scanning procedures exceeds conventional impression techniques and may be highly promising for clinical applications, including those involving orthodontic workflows.8,9 However, the same care must be adhered to in digital impression techniques as in conventional techniques when taking an impression. The management of blood and saliva, proper cord retraction, and patient operator competency and compliance4 are all key factors to attain the highest level of precision and success.
New Restorative Options
The adoption of the CAD/CAM process and manufacturing protocols opened up new and exciting restorative options in dentistry. Traditionally, metal substructures were the staple for restoring dentition in load-bearing and heavy masticatory areas. These restorations yielded less-than-optimal esthetics, and as patient awareness and education grew, patients began to prefer non-metal restorative options. CAD/CAM was able to provide that solution in multiple ways, originally with silica and leucite glass-ceramics, which have a long clinical track record, and then through the introduction of new restorative choices offering versatility and higher strength, including resin-ceramic hybrids, the popular ceramic material lithium disilicate, and the well-liked zirconia. Zirconia offers varying levels of hardness and translucency. Some zirconia materials are hue specific, while others are value specific. Recently launched multi-layered zirconia materials produce highly esthetic results.
Currently, lithium disilicate and zirconia differ in composition and mechanical properties, including in vitro performance and strength, fracture toughness, machinability, grinding effects, and strongly different moduli of elasticity or coefficients of thermal expansion.10 Furthermore, monolithic restorations can be fabricated from either lithium disilicate or zirconia ceramic, as well as other materials, which completely eliminates the use of or need for veneering such as is the case with lower-fusing ceramics. These materials have been researched both in processing and clinical aspects. In a study that reported on 2-year results for two different types of CAD/CAM ceramic crowns placed in 60 adult patients in dental practices, both lithium disilicate and zirconia single restorations yielded no occurrence of technical failures, and the number of biological complications did not differ significantly between the two types of crowns.11
A prime advantage of using CAD/CAM technology with new and well-researched restorative options is that the process is validated by the manufacturer. Material manufacturers spend substantial resources to ensure their materials deliver on their promise, and by allowing a validated manufacturing process to fabricate the restorations or abutments they can ensure that patients receive a restoration as it was intended, giving practitioners the backing of the material manufacturer and the peace of mind that comes with that.
Dental CAD/CAM technologies are genuinely changing the way dentistry is practiced and undoubtedly will continue to allow future innovation, enabling dentists to “push the envelope.” The digital process enhances the precision by which a patient's oral environment is captured and elevates the level of restorative options and materials. Digital impression systems can capture high-definition pictures for shade assessment, as well as pertinent patient-specific information to help the clinician perform comprehensive oral examinations. Because all of these digital files can be stored easily, a practitioner can readily attain a history of patient scans and examine them over time to assess any abnormalities or changes that otherwise may have previously been unrecognized. Adopting innovative digital technologies such as CAD/CAM can help clinicians produce positive—and rewarding—results.
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