Materials for Indirect Restorations

In 2003, the ADA Council on Scientific Affairs classified dental restorative materials into two broad groups distinguished according to whether laboratory work (sometimes in-office) or an additional visit was required to complete the restoration. Direct restoratives may generally be completed within one visit, while indirect restorations are fabricated in a laboratory based on impressions from a patient’s tooth, and usually require several visits to mold, fabricate, and finally place the restoration.¹ Although advances in technologies (particularly CAD-CAM) since 2003 have blurred the division between direct and indirect materials, this Oral Health Topic follows the 2003 classification generally (see our Oral Health Topic on Direct Restorative Dental Materials). A variety of indirect restorative materials are available, providing a range of strength and durability, as well as cosmetic and cost considerations. Indirect restorations can be conventionally cemented or may require adhesive bonding to the tooth depending upon the material properties and clinical scenario. A range of water-based and resin-based cements are available, further expanding the array of material combinations for the completed restoration.^{1, 2}

Indirect restorations generally consist of five categories of materials: noble metal alloys, base metal alloys, ceramics, resin-based composites, and metal-ceramics.¹ Metals had been common in indirect restorations throughout history due to their durability and strength, but the desire for tooth-colored materials has led to a proliferation of ceramic options. Ceramics, however, have a susceptibility to fracture and chipping, but bonded to metal provide durability and strength. Advances in technology, particularly in the use of CAD/CAM systems, have increased the options of all-ceramic restorations, and have rapidly gained popularity due to appearance and increasing durability.³ The use of metal is further decreasing because of increasing internet-fueled concerns regarding toxicity.³ See the Biocompatibility and Exposure Concerns section, below, for more information.

Table 1. General characteristics of classes of indirect dental materials.

Test table below

In 2003, the ADA Council on Scientific Affairs classified alloys according to their noble metal content:

Table 2. ADA dental alloy classification (ADA Council on Scientific Affairs, 2003.This content is currently archived and is for informational purposes only.

Noble Metal Alloys

Noble alloys, specifically gold, have had the longest use in dental history, and are often referred to as the standard by which other dental materials are judged.^{1, 4-6} Typically, for dental applications, the metals that are considered to be noble are gold and the platinum-group metals (platinum, palladium, iridium, rhodium, osmium, and ruthenium).^7-9 Noble metals are comparatively thermodynamically stable and thus inert in a moist environment, making them ideal for use as dental material (although titanium and CrCo alloys provide a kinetic barrier to oxidation; see below).^{9, 10} As dental materials, noble metals must typically be mixed with additional elements to make alloys with increased strength that are useable as indirect restorations.⁸ Because gold is so soft and malleable, it must be hardened with copper, silver, platinum, or another hard, durable metal.^{4, 8} Adding 10% by weight of copper to gold, for instance, increases tensile strength from 104 MPa to 395 MPa.⁸

ANSI/ADA Specification No. 134*/ISO 22674:2016¹¹ classifies the requirements for metallic materials for fixed and removable restorations and appliances:^{8, 12, 13}

Table 3. ANSI/ADA Standard No. 134/ISO 22674:2016 Dental Casting Alloy requirements.

High noble alloys can be used for the range of restorative purposes, typically from Type 1 tooth-supported soft inlays (126 MPa), to Type 2 inlays with lower ductility (146 – 221 MPa), but also may be used for Type 3 high-stress crowns and onlays (soft 207 MPa / hard 276 MPa), and Type 4 high-stress bridges and partial denture frameworks (350 / 607 MPa).¹² Lower-gold (≥ 25%) noble alloy use is more limited, to type 3 (248 – 309 MPa soft / 310 – 648 MPa hard) and type 4 (420 – 460 MPa soft / 530 – 700 MPa hard) applications.¹²

Base Metal Alloys

By 1980 the increasing price of gold led to the development and increasing use of base metals.^{1, 12} However, as noted above, unlike the noble metal alloys that get their corrosion resistance from their relative inertness in the oral environment, base metals used for dental applications can attribute their corrosion resistance to the existence of passive oxide layers. These oxide layers, such as titanium oxide and chromium oxide, reduce the rate of corrosion to extremely low values under typical oral conditions. The hardness of base alloys compared to gold complicates adjustments,¹ and base metals are more likely to have biocompatibility issues (see Biocompatibility section, below).^{1, 14}

Nickel-Chromium and Cobalt-Chromium are the most common base alloys, although a number of base elements may be added, including aluminum, molybdenum, manganese, and silicon, to increase strength, castability, and/or resistance to corrosion.^{9, 12, 15} Nickel-Chromium alloys are generally used for crowns and fixed partial dentures.^{4, 12} More elastic Cobalt-Chromium alloys have yield strengths from around 240 MPa to 650 MPa,^{12, 15} and are used primarily for removable partial dentures.¹²

Titanium and Titanium Alloys

Titanium has been popular in the medical and dental fields due to its low weight-to-strength, corrosion resistance, and biocompatibility.^{8, 12, 16} In contrast to the thermodynamic stability of noble metals, the reaction of titanium with its environment is limited by a tenacious oxide layer (titanium oxide) that controls the rate of corrosion, reducing it to extremely low rates under typical oral conditions.¹⁰ Titanium can be used as a restorative material in its unalloyed form, commercially pure titanium, with yield strengths ranging from 240 MPa to 550 MPa depending on grade.^{8, 12} Titanium can be alloyed for higher strengths with aluminum and niobium (Ti-6Al-7Nb, 795 MPa) or vanadium (Ti-6AL-4V, 860 MPa),^{12, 16} although there are some biocompatibility concerns with the potential release of toxic vanadium.¹² Titanium and its alloys may be used for crowns, implants, and partial dental frameworks.^{8, 12, 16}

*ANSI/ADA Standard No 134 supersedes ANSI/ADA Standard No. 5 for Dental Casting Alloys and ANSI/ADA Standard No. 14 for Dental Base Metal Casting Alloys.

Metal alloys have had proven effectiveness, durability, and longevity, but the desire for aesthetic, tooth-colored restorations has made the use of ceramic materials more popular in contemporary dentistry. The brittle nature of ceramics–they “may fracture without warning when flexed excessively”¹²–
and the potential for its hardness to cause wear damage to opposing teeth have led to concerns about longevity.^{1, 17, 18} But the benefits of ceramics for dental restorations–aesthetics, chemical inertness, and wear-resistance–have made ceramics a quickly evolving field of dental restorative science.¹²

The ISO and ANSI/ADA standards for dental ceramics both classify ceramics according to their intended clinical use (or function). They use 5 classes based on matching the recommended clinical indications with minimum mechanical strength and chemical solubility requirements (Table 4).¹⁹

Table 4: ANSI/ADA Standard No. 69 (ISO 6872)¹⁹

Silicate glasses, porcelains, glass-ceramics, and polycrystalline ceramics are all types of ceramics used in dentistry.¹² Feldspathic porcelains were the first all-ceramic restoration material, but despite their high translucency, they are inherently brittle,^{12, 20-22} with low flexural strength (50 – 100MPa). Beginning in the 1950s feldspathic porcelain was fused to metal to strengthen the restoration (see Metal-Ceramics section, below).⁶ The discovery of leucite within feldspathic porcelain in the 1960s allowed dispersion strengthening of the porcelain as well as modification of its coefficient of thermal expansion.²³ By the 1980s development began of high strength glass-ceramics that could be fabricated from pressed ingots rather than powder-liquid mixtures. Around this same time, improvements in computer-aided design software, the advent and proliferation of milling devices and 3D wax printers, and improvements in dental zirconia and glass ceramics have propelled the digitization of laboratory procedures for dental ceramics.¹² Several classes of ceramic materials are currently widely utilized for CAD/CAM processing: zirconia, glass ceramics, and resin-ceramic composites.

Zirconia Ceramics

Zirconia ceramics have a natural white-colored appearance and reportedly high flexural strength (≥900 MPa) and fracture toughness (~9-13 MPa m^1/2).^{12, 21, 22} Zirconia is metastable for three possible atomic arrangements, monoclinic, tetragonal, and cubic phase. Yttria is added to zirconia to stabilize the tetragonal phase of zirconia at room temperature and therefore toughen it.¹² Tetragonal zirconia may undergo a process known as transformation toughening which allows the material stop the progression of a forming crack.¹² Increasing yttria content further will stabilize the more translucent cubic phase, and zirconia restorative materials are usually characterized by the amount of yttria introduced.²⁴ Zirconia has been shown to be highly biocompatible (having been in use as an orthopedic biomaterial since the 1970s),¹² and provides resistance to bacterial adhesion.²¹

Framework zirconia and Full-contour zirconia are viable alternatives to PFM and full metal restorations, with high flexural strength (1000-1400 MPa). Framework zirconia, usually composed of 3 mol% yttria-stabilized tetragonal zirconia polycrystals (3Y-TZP), is often used in anterior and posterior multi-unit bridges, and veneered with feldspathic porcelain or glass-ceramics for a natural tooth-like appearance due to its opacity.²⁵ Full-contour zirconia, also commonly consisting of 3Y-TZP, has similar flexural strength and fracture resistance, but better translucency due to its lower alumina content, allowing it to be used as a monolithic restoration.²² Polished zirconia surfaces have been shown to be more wear resistant to opposing tooth structure than the feldspathic porcelain used on metal ceramic crowns.²²

A recent study (2020) has reported lower fracture toughness than previously published figures, averaging 5.64 MPa m^1/2 for 3Y-TZP when using focused ion beam (FIB) milling rather than saw blade-notched specimens.²⁶

A 5 mol% yttria stabilized high-translucency zirconia (5Y-ZP), is more translucent than previous generations of zirconia due to the increased content of the optically isotropic cubic phase and is less susceptible to low temperature degradation.²² However, it is more brittle and has lower flexural strength (500 – 700 MPa).²⁷ A recent analysis (2018)²⁷ found no significant difference between 5Y-ZP and other tested ceramic materials in opposing tooth enamel wear and bond strength to the adhesive cement.²⁷

A 2021 ADA ACE Panel survey found that, among respondents (n = 277), the most common uses of zirconia for fixed restorations were posterior crowns and bridges (98% and 78%, respectively), followed by anterior crowns and bridge (61% and 57%), and as custom implant abutments (51%).²⁸ Zirconia was much less frequently used for onlays, veneers, and inlays (12%, 12%, and 6%, respectively).²⁸ Please see our ACE Panel Report on Zirconia restorations for more information.

Glass-based Systems

Leucite-based glass-ceramics have nearly similar translucency as feldspathic porcelain but can have higher strength (over 100 MPa) because of increased levels of leucite.¹² Use of leucite-based ceramics is limited to aesthetic anterior bonded veneers and crowns, but lithium disilicate ceramics (LDS), with higher flexural strength (250 – 400 MPa) and availability in both low, medium, and high translucency forms, allow a wider range of anterior indications.^{12, 22} There are some issues with wear compared to zirconia,^{12, 27} and with roughness in milled LDS, but it is stronger than other glass-based ceramics and more translucent than any zirconia.¹² Lithium silicate (LS) and zirconia-reinforced lithium silicate (ZRS) are available alternatives with similar properties and indications; ZRS contains 10% dissolved zirconia.²⁹

Resin-matrix Composites

Resin-matrix materials as indirect restorations have the advantage of being easy to manipulate.^{12, 30, 31} Resin-matrix composites are capable of higher degrees of filler loading and polymerization than direct composites and, because they are cured outside the mouth, polymerization shrinkage does not occur as it does in direct resin-matrix composite restorations.¹² CAD/CAM resin-matrix composite blocks for indirect restorations can be more biocompatible than direct composite counterparts, often made with alternative, non-toxic resins and more resistance to degradation and leakage (see Biocompatibility Concerns section, below).³⁰ They generally consist of a urethane dimethacrylate (UDMA), triethylene glycol dimethacrylate (TEGMDA), and/or bisphenol A-glycidyl methacrylate (Bis-GMA) matrix with silica, silica-based glasses, glass-ceramics, zirconia, and/or zirconia-silica ceramic fillers.^{30, 31} Resin-matrix composites in the form of composite blocks have more flexibility to masticatory stress, with lower abrasivity to opposing teeth, but lower flexural strength (100 - 200 MPa) and fracture toughness (0.8 - 1.2 MPam^1/2) than typical CAD/CAM blocks. Due to their lower strength, they are primarily indicated as an alternative for inlays, onlays, and single unit crowns.¹²

“Porcelain-fused-to-metal (PFM)”, also called metal-ceramic prostheses,¹² had been the most common type of indirect restoration before the rise of CAD/CAM-based ceramics.^{6, 21} PFMs combine the strength and durability of the metal alloy core with the aesthetic natural tooth appearance of a porcelain exterior.

The requirements for alloys used in PFM are addressed in ANSI/ADA No. 134 /ISO similar to other metallic materials that are suitable for the fabrication of dental restorations and appliances. In these standards,^{13, 32} the preparation of test specimens sections require that metallic materials that claim to be recommended for use with a ceramic veneer have their specimens tested after a simulated ceramic-firing schedule has been applied. The standards further require that the linear thermal expansion be measured for materials that claim to be recommended for use with a ceramic veneer, and from these measurements, the coefficient of linear thermal expansion (CTE), which is a measure of the thermal expansion of a material as it is heated, be calculated and reported.^{13, 32} The performance of the ceramic veneer material and the metal-ceramic bond are addressed in part 1 of ISO 9693 (ANSI/ADA No.38).³³

Therefore, alloys used in PFM include the same type of High Noble, Noble, Titanium and Base Metals as described in Table 2, that also meet the requirements specified in ANSI/ADA No. 134 /ISO 22674 for alloys recommended for use with a ceramic veneer. For a good metal-ceramic restorative system, the CTE. For a good metal-ceramic restorative system, the CTE of the alloy must be near the same range as, or slightly higher than,^{8, 34} the veneering ceramic.^{12, 34}

The low CTE of palladium relative to other noble metals makes it highly compatible with a variety of ceramics, and is a common element in noble alloys used in PFM systems.¹² Gold-platinum-palladium (Au-Pt-Pd) alloys were the earliest alloy used for fusing to porcelain; other combinations of gold, palladium, and silver, sometimes with gallium or copper, round out the elements added to High Noble and Noble alloys utilized for PFMs.^{8, 12, 35} Indium, tin, and iron may also be added to High Noble and Noble alloys to increase bonding, while rhenium improves granularity and ruthenium castability.⁸

Although titanium has well-known biocompatibility in dental prostheses, results as a PFM alloy have been mixed, with casting issues and some evidence of low bond strengths.^{8, 12} Surface treatments of the titanium alloy preceding bonding to the porcelain have been shown to improve performance.^{8, 12, 35} Similarly, base metal alloys are more technique-sensitive because of increased hardness and stiffness compared to other alloys, although they are considered more castable.^{12, 35}

Metal and metal alloy restorations, particularly gold, have long been thought of as the most durable and long-lasting^{1, 36-39} and have been reported to have an average longevity of 18-20 years,³⁷ with some reported over 40 years.⁴⁰ A 2017 follow-up study of 25 posterior gold crowns found no failures after 50 years.⁴¹ In a 2010 review of the longevity of indirect and direct restorations in the posterior permanent teeth, cast gold inlays and onlays had the lowest annual failure rate, 1.4%.³⁶ Among the most common reasons for failures of gold restorations have been secondary caries and retention failure.^{5, 42} However, many factors are responsible for the ultimate failure of a restoration, not only the materials used, and a number of studies have found a range of failure and survival rates of a variety of materials and applications (see Table 5).

Table 5. Annual Failure Rates and Survival Rates from recent studies.

Porcelain does have a natural tendency to fracture, and with all-ceramic materials, crown fracture has been described as the most frequent complication.¹⁷ A fracture rate of 1.6% per year has been reported across all materials, with core fractures showing 1.5% but veneers showing only 0.6% per year.¹⁸ The posterior teeth indicate a significantly higher rate of fracture than the anterior teeth, particularly in the molars.¹⁸ Conversely, a 2013 study showed that only 0.2% of PFM crowns resulted in a failure due to fracture, with an average of 13 service-years,⁴⁶ while another study indicated that 2.6% of PFMs had chipped within 5 years.⁴⁵ Most failures in PFMs are the result of oral pathology.⁴⁶

All-ceramic fixed dental prostheses (FDPs) were compared to metal-ceramic prostheses in a two-part series of systematic reviews in 2015.^{45, 53, 54} All-ceramic single crowns were found to be significantly less reliable than metal-ceramic, with a 5-year survival rate of 90.7% and 94.7%, respectively.⁴⁵ Similarly for multiple-unit FDPs, reinforced glass-ceramic FDPs had a significantly lower 5-year survival rate, 85.9%, than metal-ceramics, at 94.4%.^{53, 54} Further, a 2016 report from the Canadian Agency for Drugs and Technologies in Health compared the effectiveness of all-ceramic against metal-ceramic crowns and found similar results: a survival rate of 84-100% for all-ceramic and 92-96% for PFMs after 8 years.⁵⁵ Other studies have reported a ten-year survival rate of 97% for metal-ceramic crowns,^{5, 56} with the majority of failures occurring as a result of masticatory force and trauma in the anterior region.⁵⁶

Veneer chipping is a common complication in both PFMs as well as all-ceramic crowns; a 2015 systematic review reports a 5-year rate of 2.6% among PFMs.⁴⁵ Among all-ceramic restorations, generally zirconia- and alumina-based restorations have higher reported veneer chipping frequencies,^{12, 45} although 2013 report found that 3.3% of lithium-disilicate crowns had chipped during a 9-year follow-up study.⁴⁹

A 2016 systematic review of feldspathic porcelain and glass-ceramic laminate veneers calculated an overall survival rate of 89%, after an average of 9 years.⁴⁸ Porcelain veneers had a cumulative survival rate of 87% after an average of 8 years, while the glass-ceramic veneers had a cumulative survival rate of 94% after 7 years.⁴⁸ Chipping was the most frequent reported complication, with a rate of 4%; while debonding, discoloration, and endodontic shared a complication rate of 2%.⁴⁸ Generally, feldspathic porcelain⁴¹ and densely sintered alumina¹⁷ have reported higher failure rates in anterior teeth.

A 2012 prospective study of 82 anterior and 22 posterior lithium-disilicate glass-ceramic framework crowns in 41 patients found a survival rate of 97.4% after 5 years and 94.8% after 8 years (replacement of restoration is considered failure).⁴⁹ Similarly, a 2017 critical review found a survival rate of 97.6% in lithium disilicate crowns.¹⁷ Again, the most common complications of all-ceramic crowns was fracturing and chipping; there was no significant difference in failure rate between anteriorly- and posteriorly-placed lithium disilicate crowns.¹⁷

Zirconia is still developing as a restorative material, and although the potential for high strength and translucency is promising, evidence for long-term survivability is limited. Short-term survival rates (up to 5 years) have been noted to be within the range of traditional ceramic and metal-ceramic restorations.^{45, 53, 54, 57-59} A 2014 systematic review found a 5-year survival rate of 95.9% for tooth-supported and 97.1% for implant-supported zirconia-based crowns, while a 2018 retrospective cohort study found 5-year survival rates as high as 98.5%, but which dropped to as low as 39.3% in posterior region (molars) after 10 years.⁵⁰ As with porcelains, the most common complications with zirconia-based restorations are chipping and fracture; a 5-year fracture rate of 1.09% for monolithic zirconia restorations (all types),⁶⁰ and a fracture rate of 3.31% for all types of layered zirconia⁶¹ have been reported (see Table 6). A 2021 ADA ACE Panel report found that, among responding dentists (n = 277), 52% found restoration debonding the most common problem with zirconia restorations, while wear from opposing teeth (31%) and restoration fracture (23%) were also common concerns.²⁸ Please see our ACE Panel Report on Zirconia restorations for more information.

Table 6. Zirconia-based restoration fracture rates.

ANSI/ADA Standard No. 41 and ISO 7405 provide guidelines and methodologies for evaluating the biocompatibility of dental materials. The US Food and Drug Administration (FDA) regulates and monitors commerce of non-exempt medical and dental devices according to a class system based on risk level.^{12, 62} ISO Standard 10993, which consists of 20 parts, specifies the biologic evaluation of medical devices, and can also be used to help ensure that no significant toxic, carcinogenic, or other local or systemic health effects result from contact with dental materials.^{12, 63}

The incidence of an adverse reaction to a dental material is reportedly as low as 0.14% in the general population,^{12, 64} and 0.33% in a prosthetic patient population.¹² Base metals are responsible for the majority of reactions to indirect dental restorations.¹² Metal ion release as a result of corrosion of metallic materials used in restorative dentistry has been associated with producing irritation or allergic responses.^{65, 66} An alloy’s constituent elements may leach from the material during corrosion, affected by temperature and pH of the oral cavity.^{66, 67} The most common symptoms of a sensitivity or allergic response to a dental material are rash (allergic contact dermatitis), cheilitis, oral lichenoid lesions, inflammation (stomatitis), and burning, tingling and itching of the oral mucosa or face.^{12, 66, 68}

Information regarding biocompatibility of resin materials may be found on our Oral Health Topics page on Direct Restorative Materials, and concerns about Bisphenol A on this page.

Patient Exposure

Noble metals are highly corrosion-resistant, but may cause adverse reactions when alloyed with base metals.¹² For example, nickel is known to be a common contact allergen¹ and has the highest rate of adverse response.⁶⁶ Between 10% and 20% of the general population has a nickel sensitivity, although its oral manifestation is rarer and less severe.^{1, 8, 15, 66} Since nickel can be a natural impurity in the components of an alloy, ANSI/ADA and ISO standards require manufacturers that would like to claim that their alloy is “nickel free” to include the following labelling on their packaging: “nickel free; contains less than 0.1% nickel”.^{13, 32}

Cobalt and chromium are also known to have allergic responses in approximately 8% of the general population;⁶⁶ other metals including copper, tin, mercury, and zinc, even gold, palladium, and titanium have reported allergic reactions but prevalence is not clear.^{12, 66, 68}

Cross-reactivity may be responsible for a number of adverse responses, and palladium is often associated with allergic reactions when mixed with nickel, chromium, and/or cobalt.^{12, 66} Beryllium, a known carcinogen, is sometimes added to base metal alloys to improve castability, but may induce inflammation, allergic reactions, or other negative consequences, particularly when alloyed with nickel and chromium.¹² ISO 22674 and ANSI/ADA Standard No. 134 designate beryllium as a hazardous element, and to meet the standard requirements, metallic dental materials cannot contain more than 0.2% (mass fraction) of beryllium.^{13, 32} Cobalt has further been associated with a potentially fatal heart condition referred to as cobalt cardiomyopathy.⁶⁹ Allergic responses to highly-biocompatible titanium have been reported,^{66, 70, 71} but may be the result of cross-reactivity, particularly if alloyed with beryllium or other base metals.⁶⁸

Dental ceramics are highly biocompatible and have low rates of surface degradation, although extremely highly acidic environments may increase release of silicon ions.¹² Zirconia has not been linked to any sensitivity or allergic responses, and reported minor adverse reactions among ceramics are generally regarded as resulting from surface irritation.¹²

Occupational Exposure

Improper safety precautions and handling techniques may lead to occupational exposure to dental materials in the form of inhalation of the particulate matter released during laboratory processing of metals and ceramics. Chronic inhalation of beryllium vapor has been associated with pneumoconiosis along with other illnesses,^{12, 72, 73} although ANSI/ADA Standard 134 and ISO 22674 require less than 0.02% (mass fraction) beryllium in dental alloys.^{11, 13} A 2018 report from the CDC stressed the importance of respiratory protection when working with these materials.⁷⁴ According to the Occupational Health and Safety Administration (29 CFR 1910.134) employers must provide appropriate respiratory protection when employees are expected to have occupational exposure to contaminated air.⁷⁵

Scientific Assessment of Dental Restorative Materials (Trans.2003:387)

Resolved, that although the safety and efficacy of dental restorative materials has been extensively researched, the Association, consistent with its Research Agenda, will continue to actively promote such research to ensure that the profession and the public have the most current, scientifically valid information on which to make choices about dental treatment requiring restorative materials, and be it further

Resolved, that the Association use its existing communications vehicles to educate opinion leaders and policy makers about the scientific methods used to assess the safety and efficacy of dental restorative materials, and be it further

Resolved, that the Association continue to promptly inform the public and the profession of any new scientific information that contributes significantly to the current understanding of dental restorative materials.

American Dental Association
Adopted 2003; Reviewed 2017

ADA ACE Panel Reports:

Zirconia Restorations

Bonding crowns and bridges with resin cement

Bonding Agents

Bioactive Materials

Posterior Composite Restorations

ADA MouthHealthy:

Dental Filling Options

• Composite resins
• Dental amalgam
• Gold fillings

Bisphenol A (BPA)