The Maturation of CAD / CAM

Initially, it was thought that the design and processing of dental devices using CAD/CAM technology would be simpler and easier compared with the use of industrial products. However, this has not been the case for the following reasons:

1) Total cost, operatory time, and manipulation of the systems for processing dental devices using CAD/CAM technology should be equivalent to conventional systems, or superior. This is in order to replace the conventional customized restorations and ensure new systems are practical in daily laboratory work and clinical practice.

2) Morphology of the abutment teeth, adjacent teeth, and opposing teeth, as well as the interarch relationship, must be accurately digitized to allow the designed restoration to be adequately adapted to the prepared abutment teeth and dentitions. However, technicians had difficulty recognizing the delicate margins prepared by dentists using the compact digitizers available at that time. Therefore, the development of an accurate and compact digitizer and related sophisticated software was necessary for high-precision digitizing of complex and delicate targets.

3) The numeric representation of the shape of crowns and fixed partial dentures (FPDs) is complex in comparison to the typical industrial products that are produced using functional equations. In addition, because restorations not only had to be adjusted for abutment teeth but must also be harmonized with adjacent and opposing teeth, the development of sophisticated CAD software was necessary.

4) Accurate processing, including mechanical milling of sharp corners and delicate margins of crowns and FPDs, was difficult with brittle ceramic materials. Therefore, the development of a stiff processing machine and sophisticated software to control the tool path was necessary. Also, the machine size was problematic for dental laboratories or offices.¹

The use of CAD/CAM has dramatically impacted the clinical and laboratory aspects of restorative dentistry. Materials are now more biocompatible and certainly more esthetic to meet patients’ increased expectations.

Originally investigated in 1971 by Duret,² the first digital restorative technology was brought to the dental marketplace in 1987 by Moermann.³ Clinicians now had the opportunity to deliver single-appointment and “direct/indirect” single-tooth ceramic restorations. The data capture could be accomplished by a combination of camera and laser use, depending on the manufacturer’s approach.

In 2007, technology was made available that linked the operatory to the laboratory for the milling, firing, and customized characterization of single-tooth restorations without a developed master cast. This digital connection allowed quick delivery of well-matched restorations in the esthetic zone, and when appropriate, digital images for color matching were provided. The firing of the milled ceramic block in a porcelain oven also increased the material strength by as much as 170%.

The first system strictly intended for the capture of digital impressions was introduced in 2006. Patient comfort and efficiency were improved dramatically. The ability to completely manage the capture of critical data in the sulcus by stopping the capture sequence as needed to manage intracervicular fluids is a huge advantage compared to conventional impression methods. Staff involvement dramatically enhances team production, freeing the clinician to perform more critical procedures and to serve as the final inspector of the captured data prior to submission for the model fabrication.

The margin visibility, as well as axial wall and occlusal clearance, are viewed prior to final submission. The file is sent wirelessly to a separate facility for CAD/CAM milling or stereolithography (SLA) printing of the articulated models and dies. The data may also be transferred to a compatible milling software for the design and milling of a zirconia, alumina, or castable resin/wax framework while the casts are fabricated.

The introduction of the contact probe scan by Andersson⁴ in 1985 induced the first successful “off-site” CAD-generated process for the development of alumina substrates to support veneering ceramic. The laboratory scanned a working die and sent the data by modem to a central manufacturing facility for fabrication. The production process included the milling of a replica refractory die on which the alumina material was pressed and externally milled to the desired external dimensions.

High-strength ceramic materials have evolved as the core/framework material for all-ceramic restorations due to their improved esthetics and better biocompatibility compared to dental alloys. The continually improving mechanical properties are illustrated by the following products, which are listed chronically. Lithium disilicate (Empress 2®, Ivoclar Vivadent, www.ivoclarvivadent.com), glass-infiltrated alumina (In-Ceram® Alumina, Vident, https://www.vident.com), and glass-infiltrated alumina with partially stabilized zirconia (In-Ceram zirconia, Vident) were hand-fabricated. Subsequent products designed for CAD/CAM production include densely sintered high-purity alumina (Procera®, Nobel Biocare, https://www.nobelbiocare.com), yttria-stabilized tetragonal zirconia polycrystal materials (Y-TZP) (Cercon®, DENTSPLY, https://www.dentsply.com; DCS Precedent®, DCS Dental AG; and Lava®, 3M ESPE, https://www.3mespe.com),^5,6 and most recently e.max® (Ivoclar Vivadent). In the author’s opinion, the use of Y-TZP reigns as the industry standard for high-strength nonmetal crown-and-bridge frameworks.

Two types of zirconia blocks are being used. The first is fully sintered dense blocks for direct machining using a CAD/CAM system with a grinding machine for higher stiffness. The other is partially sintered blocks for CAD/CAM fabrication, followed by postsintering, to obtain a product with sufficient strength. The former has a superior fit because no shrinkage is involved but can cause wear of the milling tool. In addition, microfracture formation of the material during the milling procedure may compromise mechanical durability.^7,8 The latter has easy machinability without leading to wearing of the milling tool or chipping in thin marginal areas. The +-30% sintering shrinkage that occurs during the post-sintering process must be compensated for by the dimensional adjustment of CAD procedures involving the frameworks.^7,9

“On-site” technology has provided the laboratory the ability to completely control the process. The adaptation of SLA 3D printing technology to wax pattern development is being adopted on a broad scale in larger laboratories. These patterns can be cast as fixed frameworks for the ceramo-metal, full metal, and removable partial denture restorations.

A major milestone was the introduction of CAD/CAM-fabricated custom abutments and bar assemblies for implant restoration. The available materials are titanium, CoCr, and zirconia.

The contemporary clinician and technician are significantly challenged to stay abreast of the improvements in restorative dentistry. The broad scope of science involved in the development of materials and devices makes this increasingly difficult. The term paralysis by analysis becomes more applicable as we move along this digital pathway. This article should bring some clarity to the topic, and if nothing else, will stimulate the reader to search for more information. There is possibly a new malady—informitis.

David R. Avery, AAS, CDT
Director of Training and Education
Drake Precision Dental Laboratory
Charlotte, North Carolina