A New Era for Indirect Restorations With CAD/CAM and 3D Printing Technologies

Macarena Rivera, DMD, MSc; and Markus B. Blatz, DMD, PhD

Designing and fabricating dental restorations via the indirect method emerged as a strategic response to the loss of tooth coronal structure that prompted the need for different treatment options depending on the extent of the damage (eg, partial-coverage restorations such as inlays/onlays and laminate veneers, or full-coverage restorations such as crowns). The complexities of the oral environment and patient management in the dental chair also factored into the development of indirect restorations. Indirect restorations involve manufacturing them away from the oral cavity, avoiding some of the difficulties of direct techniques, such as polymerization contraction and marginal adaptation, as polymerization shrinkage stresses only affect the cement layer, reducing the impact on the cavity walls.

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Indirect restorations typically involve tooth preparation to create space for the restorative material. The amount of space needed is determined by the type of restoration and the specific material chosen for it; usually, a minimum thickness of 0.6 mm is needed for laminate veneers and 1.5 mm to 2 mm is necessary for posterior and/or full-coverage restorations.¹ This approach unlocks new possibilities for creating precise, high-quality dental restorations using a wide range of indirect materials.

The introduction of computer-aided design/computer-aided manufacturing (CAD/CAM) systems began in the 1950s, but it took nearly 30 years before these technologies were introduced in dentistry. When Werner Mörmann eventually developed the first commercial CAD/CAM system known as CEREC (Dentsply Sirona), which stands for "chairside economical restoration of esthetic ceramics," it marked a revolutionary moment as the system allowed for the design and manufacture of indirect ceramic restorations through computer-assisted methods, all within a single day at the dental office.^2,3

Today, oral structures can be directly scanned using a handheld intraoral scanner (IOS). Intraoral scanning is widely used because it is time-saving and patient-friendly. IOSs have been improved considerably during the past decade, and most currently available studies concur that the accuracy of intraoral scanning is comparable with that of conventional polyvinyl siloxane impression-taking and that intraoral scanning is suitable for the fabrication of single-tooth restorations and short-span fixed dental prostheses. Nevertheless, studies show that this modality does not capture subgingival margins well, and even less so if covered with blood, saliva, or tissue. Undercuts also present a challenge for an IOS. Conventional impressions allow the impression material to flow into the undercuts and show advantages regarding reproducing subgingival finishing lines and, therefore, are often used for these types of preparations.^4,5

Still, digital dentistry has transformed the delivery of indirect restorations. Traditional methods rely greatly on the dental technician's manual skills and expertise, while the digital workflow offers improved accuracy, consistency, predictability, and cost-efficiency in fewer clinical steps. Although digital dentistry provides many advantages, it also requires a significant learning curve and investment. Dentists must master each step, as any errors along the way may impact the restoration's clinical performance.^6,7

Computer-aided dental applications enable the creation of the final restoration using two different manufacturing techniques: subtractive manufacturing or additive manufacturing. Currently, the subtractive method remains the "gold standard" in the CAM process. However, additive CAM methods, such as 3D printing, are gaining attention due to their potential to reproduce more complex geometries than subtractive methods like milling.¹ Furthermore, the burs and tools used in subtractive manufacturing wear after repeated use, which can lead to inaccuracies and cracks in the restorations produced. Additive manufacturing methods produce the fabrication of items by layering material and incorporate a variety of technologies, such as powder bed fusion, fused deposition modeling, and light-curing techniques. The most commonly used method for polymer printing in dentistry is stereolithography, which utilizes a laser to cure light-activated polymers in thin, cross-sectional layers, gradually building up a 3D structure.^2,4

First introduced about 30 years ago, 3D printing technology today is expanding rapidly in dentistry thanks to the expiration of several patents. It has revolutionized workflows by facilitating the digital production of temporary crowns, removable dentures, and appliances like occlusal guards, orthodontic aligners, implant surgical guides, custom impression trays, and master models for treatment planning, teaching, and continuing education.⁸ These innovations have reduced manufacturing times and costs while increasing the versatility of designs. Nevertheless, further improvements in materials and techniques are required for these methods to reach their full potential, as inconsistent results have been observed regarding the mechanical properties, dimensional accuracy, and marginal fit of long-term restorations.² Other factors like material storage conditions and surface treatments also contribute to variations in results.⁹ Important clinical aspects such as scanning accuracy and restoration misfits and their effect on technical or biological complications can only be studied effectively in longitudinal in vivo studies, but most identified studies are in vitro, which limits their clinical significance.

A systematic review conducted by Sarafidou et al on the evaluation of the marginal and internal fit of fixed dental prostheses after applying digital, conventional, and a combination of impression methods highlighted the clinically acceptable fit of all materials and impression techniques.¹⁰ Still, the need for gingival retraction poses a challenge for all impression techniques, and factors related to the fabrication process significantly influence the outcome.

A study by Patzelt et al noted the superiority of 3D-printed dental models over milled models.¹¹ Another study compared three printing technologies (digital light processing, photopolymer jetting, and binder jetting) and conventional production regarding model accuracy and found no superior method.¹²

With the implementation of these technology systems, novel materials specifically suited for CAD/CAM processing have been developed. Consequently, defining and establishing clear criteria and adequate indications for each of them has become increasingly challenging for practitioners. Ceramic-based materials are dentists' preferred choices for indirect restorations, with lithium disilicate and monolithic or layered zirconia being the most widely used options. Many dental laboratories today, however, can fabricate CAD/CAM full- and partial-coverage restorations with other materials beyond zirconia and lithium disilicate, such as polymethyl methacrylate, composites, and hybrid ceramics.

A crucial factor influencing the choice of material for an indirect restoration's quality and longevity is marginal fit. Whether the restoration is fabricated through additive or subtractive manufacturing, ensuring high dimensional accuracy is essential for achieving an optimal fit of the intaglio surface. Marginal discrepancies may result in microleakage, leading to cement degradation, secondary caries, and periodontal inflammation.¹³ Despite this, the trueness and precision of indirect restorations, particularly onlays, in relation to varying CAM techniques and restorative materials such as composite resin, leucite-reinforced glass-ceramic, and lithium-disilicate glass-ceramic have not been extensively investigated in the current literature.

Multiple resin systems have been developed specifically for dental 3D printing. VarseoSmile Crown Plus (Bego, bego.com) was the first system to create permanent restorations using a ceramic-infiltrated hybrid composite resin. While both additive and subtractive manufacturing can be used to produce restorations with adequate marginal fit, errors can arise depending on the CAD system used and the clinician's expertise. Mechanical properties such as bond strength, microhardness, and wear resistance are heavily influenced by the resin's composition, production parameters, and post-production processes, including cleaning and curing. Some studies suggest that heating polymer-based resins during or after curing enhances their physical strength.⁴ Another study that compared the marginal and internal fit accuracy of lithium-disilicate glass-ceramic inlays fabricated with conventional, milled, and 3D-printed wax patterns found that lithium-disilicate glass-ceramic inlays produced from digital scans and subtractive methods resulted in better marginal and internal fit accuracy than either conventional impression/fabrication or additive 3D manufacturing.¹⁴

For years, several companies have focused on printing silica- and zirconia-based ceramic materials, employing technologies such as nano-particle jetting.¹⁵ However, limitations such as long production times and lower physical properties of the resulting restorations need to be resolved before this approach can compete with the subtractive manufacturing methods of today.

The integration of CAD/CAM technology into routine dental practice is becoming increasingly essential, as it offers a more efficient treatment planning process and can reduce chairside time. The accuracy and clinical outcomes of CAD/CAM methods, however, do not outperform traditional techniques.⁵

Dentistry is currently witnessing the beginnings of the additive age, where promising processes are developing in parallel. It is still uncertain which of these processes will ultimately prevail. Besides additive manufacturing of various ceramic materials, future advances in dentistry should focus on developing polymeric materials with low polymerization stress and high strength, optimizing surface quality, and improving marginal adaptation to fully validate 3D printing's potential for long-term restorations.

Macarena Rivera, DMD, MSc
Assistant Professor, Department of Prosthodontics, University of Chile, Santiago, Chile; Adjunct Professor, Department of Preventive and Restorative Sciences, University of Pennsylvania School of Dental Medicine, Philadelphia, Pennsylvania; Private Practice, Santiago, Chile

Markus B. Blatz, DMD, PhD
Professor of Restorative Dentistry, Chair, Department of Preventive and Restorative Sciences, and Assistant Dean, Digital Innovation and Professional Development, University of Pennsylvania School of Dental Medicine, Philadelphia, Pennsylvania