In 2007, approximately 26% of indirect restorations from one of the largest US dental laboratories were ceramic or resin-based. By 2013, this percentage had increased dramatically to 81%.1 The adoption of digital workflows (design and manufacturing) in dental laboratories has accelerated the trend toward non-metal indirect restorations. Currently, nearly half of practitioners use intraoral scanning, which facilitates the process of digital crown fabrication.2 Additionally, the introduction of in-office milling and 3D printing has expanded material options, including filled dental polymers for both provisional and definitive restorations. As fabrication technologies have advanced, material formulations have evolved in response, requiring clinicians to stay informed about changes in their properties.
Contemporary Materials
Digitally fabricated dental materials are broadly categorized into ceramic and resin-based. Ceramic materials can be further divided into glass-ceramics (eg, lithium disilicate) and polycrystalline-ceramics (eg, zirconia). Glass-ceramics consist of a high volume of crystalline phases embedded within an amorphous glass matrix. This combination provides a balance of esthetics and mechanical strength, with the glassy phase contributing to translucency and the crystalline phase improving strength. In contrast, polycrystalline-ceramics like zirconia are composed entirely of densely packed crystals, resulting in superior strength but limited translucency due to increased light scattering. Resin-based materials, in contrast, generally offer lower strength than ceramics. However, their polymeric composition provides flexibility and toughness, which may offer advantages for certain clinical applications such as splints, dentures, and hybrid prostheses.
Glass-Ceramics
Glass-ceramic materials gained popularity because of the clinical success of lithium disilicate. Variations in the microstructure of the crystalline component of this material have led to so-called lithium-x-silicate materials. The long, rod-shaped microstructure of lithium disilicate offered resistance to crack propagation within the material. A shorter, rounder crystalline structure included in other lithium-x-silicate materials may offer advantages with regard to fabrication technique or alternative optical properties. These changes, however, negatively impact the fracture resistance of lithium disilicate.3
Polycrystalline-Ceramics
Zirconia is available in three distinct formulations that differ in their balance of strength and translucency. Zirconia stabilized with 3 mol% yttria (3Y-Z), sometimes referred to as high-strength zirconia, possesses the highest toughness of any dental ceramic but offers limited translucency. Zirconia stabilized with 5 mol% yttria (5Y-Z) contains approximately 50% cubic phase, which imparts increased translucency but also reduces the material’s ability to undergo transformation toughening. Finally, 4 mol% yttria-stabilized zirconia (4Y-Z) provides properties that are intermediate between 3Y and 5Y. In short, as the yttria content increases from 3Y to 4Y to 5Y, toughness decreases while translucency increases.
A recent innovation in zirconia restoration involves the use of multilayered zirconia discs that not only blend shades for esthetics but also vary the yttria content throughout the block. These advanced discs are engineered to provide higher-strength zirconia (lower yttria) in the cervical region of a tooth for durability and more translucent zirconia (higher yttria) in the incisal region for esthetics.4 Mechanical testing across layer boundaries indicates that the strength between the cervical and incisal regions is intermediate, suggesting effective adhesion between the layers.4
Resin-Based Materials
Resin-based materials for indirect restorations gained popularity with resin-based blocks used for in-office milling. These materials eliminated the need for heat treatment, thus expediting the service of same-day crowns. Because “ceramic” is defined as a material that is predominantly composed of ceramic, the roughly +70% ceramic filler in resin-based blocks has allowed them to be classified as permanent ceramic restorative materials. Compositionally, these materials are the same as those used for direct composite restorations, however they have undergone additional polymerization under high temperature and heat. As a result, they obtain a roughly 17% higher strength than direct composite. Printed crown materials available from in-office vat printers are currently all resin-based materials. The filler content of these materials is limited by the ability of the printer to cure through it.5 Therefore, most commercially available resins for vat printing peak at 50% to 60% filler.6,7 A new technology for 3D printing dental resins is referred to as digital press stereolithography in which a highly filled (approximately 70%) resin is pushed through a chamber, which exposes successive layers to light-based printing.
Manufacturing Technologies
Broadly, non-metal crown materials can be produced by analog (hand-layering or pressing) or digital (milling or printing) techniques.8 Hand-layered porcelain slurry is used frequently to characterize the facial aspect of anterior zirconia or lithium-disilicate restorations and less frequently for porcelain veneers produced on refractory dies.8 Pressing is a technology used by dental laboratories in which a melted ingot of a glass-ceramic is pressed into an investment. This technique offers several advantages for laboratories, including efficiency of multiple restorations in a single investment, less material waste as pressing consumes most of the ingot, and the avoidance of tool wear that occurs with milling. A clinical benefit of pressing is the ability to attain superior margins due to reduced chipping; however, both milled and pressed glass-ceramic restorations may achieve clinically acceptable margins.9,10 Pressed crowns may still undergo digital design and then be transferred to the analog realm through printing the wax pattern used for the investment.
Milling (or subtractive manufacturing) is a process by which a solid piece of a restorative material (eg, zirconia, glass-ceramic, or resin-based material) is precisely cut and shaped by rotating burs under computer control. In-office (chairside) milling machines are typically four-axis units in which the block of material can rotate and move linearly around and along the machine’s mandrel (axis 1) and the burs can move in some combination of x, y, and z directions (axis 2, 3, and 4) to cut the block. Laboratory mills are typically five-axis machines in which the discs of material can tilt in two directions (axis 1 and 2) and the burs can move in x, y, and z directions (axis 3, 4, and 5). The five-axis machines produce restorations with better marginal adaptation compared to four-axis units.11
Once ceramic restorations are milled or pressed, they require heat treatment to achieve their final properties. Glass-ceramics require crystallization, a process by which the glassy matrix undergoes nucleation and controlled crystal growth, forming a crystalline phase to enhance mechanical and optical characteristics. Although some newer glass-ceramic blocks are available fully crystallized, avoiding the need for a heat treatment step, they have lower mechanical properties than blocks that undergo heat treatment.3,12 Heat treatment not only alters crystal microstructure, but may also allow repair of machining defects through glass melting.12 Any block containing resin composite cannot be heat-treated as this will burn the resin component.
Zirconia undergoes heat treatment for sintering, a process in which the shaped ceramic is heated to high temperatures (typically 1,350°C to 1,550°C) to bond particles together and form a dense, solid structure. Conventionally, sintering is a process that requires several hours in a laboratory furnace. Fast sintering is a method for chairside zirconia that reduces sintering time to around 18 minutes, a process which has been validated to achieve similar microstructure and mechanical and optical properties as conventional sintering.13 Speed sintering, however, is only possible with specific zirconia types that are only available as blocks unlike laboratory zirconia sintering, which is designed for zirconia discs.13
The process of sintering zirconia results in a linear reduction of the restoration by around 20%. Each zirconia disc is labeled with a specific shrinkage factor to indicate the amount to scale up the restoration in the milling software (eg, a shrinkage factor of 1.250 indicates that the design should be scaled up 1.25x). An advantage of milling the restorations at a larger size is that margin and texture detail may be created more easily by the mill or hand finishing.
Printing dental restorations most commonly utilizes vat photopolymerization, in which liquid resins are selectively cured layer by layer using a light source, such as a laser (stereolithography), a digital light projector (digital light processing), or a liquid crystal display (LCD) mask (masked stereolithography), to create restorations. These restorations then require post-rinsing to remove residual uncured resin and post-curing to complete the polymerization of the resin. These steps ensure adequate mechanical and biologic properties of the materials.5
The 3D printing of zirconia and lithium disilicate is an emerging technology. Such printers (eg, CeraFab System S25, Lithoz, lithoz.com; Ceramaker 100, 3DCeram, 3dceram.com) operate by mixing ceramic particles and a binder into a slurry and printing the restoration with similar stereolithography techniques as used for dental resin vat printing. Once the restorations are printed, they undergo debinding (burning out the binder) and sintering. Although initial testing recently showed slightly lower strength and accuracy compared to milled zirconia, the values were within clinically acceptable limits.14,15 Shade can also be a limitation because these printing slurries are currently monochromatic and cannot achieve the multilayered esthetics of milled zirconia.
Conclusion
As digital workflows continue to evolve, understanding the properties and limitations of emerging materials is essential for clinical success. Whether selecting ceramics or resin-based options, clinicians must align material choice with fabrication technique, esthetic goals, and functional demands to ensure long-term performance in modern restorative dentistry.
ABOUT THE AUTHORS
Princy Thakkar, BDS
MS in Dentistry (Biomaterials) Candidate, School of Dentistry, University of Alabama at Birmingham, Birmingham, Alabama
Nitish Surathu, BDS, MDS
Course Coordinator, The ACE Institute, Hamilton, New Zealand; Private Practice, Hamilton, New Zealand
Neeraj Surathu, BDS, MS
Instructor, The ACE Institute, Hamilton, New Zealand; Private Practice in Prosthodontics, Hamilton, New Zealand
Nathaniel C. Lawson, DMD, PhD
Associate Professor, Division of Biomaterials, School of Dentistry, University of Alabama at Birmingham, Birmingham, Alabama
References
1. Christensen GJ. Is the rush to all-ceramic crowns justified? J Am Dent Assoc. 2014;145(2):192-194.
2. Revilla-Leon M, Frazier K, da Costa JB, et al; Council on Scientific Affairs. Intraoral scanners: an American Dental Association Clinical Evaluators Panel survey. J Am Dent Assoc. 2021;152(8):669-670.e2.
3. Hill TJ, Stevens CD. Fracture toughness of esthetic pressed and milled restorative materials. Int J Periodontics Restorative Dent. 2025;0(0):1-26. doi: 10.11607/prd.7739.
4. Inokoshi M, Liu H, Yoshihara K, et al. Layer characteristics in strength-gradient multilayered yttria-stabilized zirconia. Dent Mater. 2023;39(4):430-441.
5. Caussin E, Moussally C, Le Goff S, et al. Vat photopolymerization 3D printing in dentistry: a comprehensive review of actual popular technologies. Materials (Basel). 2024;17(4):950.
6. Bora PV, Sayed Ahmed A, Alford A, et al. Characterization of materials used for 3D printing dental crowns and hybrid prostheses. J Esthet Restor Dent. 2024;36(1):220-230.
7. Nejat AH, Kee E, Belanger MR, et al. Radiographic evaluation of current restorative CAD/CAM ceramics indicated for fabrication of indirect restoration. J Esthet Restor Dent. 2025;37(7):1882-1890.
8. Villa HL, Brindis M, Lawson NC. Material selection for single-unit crown anterior restorations. Compend Contin Educ Dent. 2020;41(9):477-482.
9. Papadiochou S, Pissiotis AL. Marginal adaptation and CAD-CAM technology: a systematic review of restorative material and fabrication techniques. J Prosthet Dent. 2018;119(4):545-551.
10. Mounajjed R, Layton DM, Azar B. The marginal fit of E.max Press and E.max CAD lithium disilicate restorations: a critical review. Dent Mater J. 2016;35(6):835-844.
11. Pilecco RO, Machry RV, Baldi A, et al. Influence of CAD-CAM milling strategies on the outcome of indirect restorations: a scoping review. J Prosthet Dent. 2024;131(5):811.e1-811.e10.
12. Maharishi A, McLaren EA, White SN. Characterization of lithia-based machinable glass-ceramic materials. J Esthet Restor Dent. 2025. doi: 10.1111/jerd.13489.
13. Lawson NC, Maharishi A. Strength and translucency of zirconia after high-speed sintering. J Esthet Restor Dent. 2020;32(2):219-225.
14. Pinelli LAP, Ferreira I, Cândido Dos Reis A. Accuracy and adaptation of 3D printed zirconia crowns: a review of current methodologies. J Prosthet Dent. 2025:S0022-3913(25)00068-X.
15. Pinelli LAP, Ferreira I, Reis ACD. Analysis of flexural strength and Weibull modulus of printed and milled zirconia: a systematic review. J Prosthet Dent. 2025;134(3):628.e1-628.e8.
