Materials for Indirect Restorations (2023)

In 2003, the ADA Council on Scientific Affairs classified dental restorative materials into two broad groups based on whether they needed lab work (sometimes in the office) or an additional visit to complete the restoration. Direct restorations can usually be completed in one visit, while indirect restorations are done in the laboratory based on impressions of the patient's tooth and typically require multiple visits to shape, fabricate, and finally seat the restoration.¹Although technological advances (particularly CAD-CAM) have blurred the distinction between direct and indirect materials since 2003, this oral health topic generally follows the 2003 classification (see ourOral health topic in direct restorative dental materials). A variety of indirect restorative materials are available, offering a range of strengths and durability, as well as aesthetic and cost considerations. Indirect restorations can be conventionally cemented or, depending on material properties and the clinical setting, require adhesive fixation to the tooth. A range of water-based and resin-based cements are available, further expanding the range of material combinations for the final restoration.^{1, 2}

Indirect restorations generally consist of five categories of materials: precious metal alloys, base metal alloys, ceramics, composite resins, and metal-ceramics.¹Metals have historically been common in indirect restorations because of their durability and strength, but the desire for tooth-colored materials has led to an increase in ceramic options. However, ceramics are prone to cracking and chipping, but bonded to metal they provide durability and strength. Technological advances, particularly in the use of CAD/CAM systems, have expanded the possibilities of all-ceramic restorations and have rapidly gained popularity due to their ever-increasing appearance and durability.³Metal use continues to decline as the Internet raises concerns about toxicity.³See Biocompatibility and Exposure Concerns section below for more information.

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

test table below

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

Table 2. Classification of ADA dental alloys (ADA Council on Scientific Affairs, 2003).This content is currently archived and is for informational purposes only.

precious metal alloys

Noble alloys, particularly gold, have been used the longest in the history of dentistry and are often cited as the standard by which other dental materials are judged.^{1, 4-6}Normally, the metals considered noble for dental applications are gold and platinum group metals (platinum, palladium, iridium, rhodium, osmium and ruthenium).^7-9Noble metals are comparatively thermodynamically stable and therefore inert in humid environments, making them ideal for use as dental materials (although titanium and CrCo alloys offer a kinetic barrier to oxidation; see below).^{9, 10}As dental materials, precious metals typically need to be mixed with additional elements to produce alloys with increased strength that are useful as indirect restorations.⁸Because gold is so soft and malleable, it must be tempered with copper, silver, platinum, or some other hard, durable metal.^{4, 8}For example, adding 10 wt% copper to gold increases the tensile strength from 104 MPa to 395 MPa.⁸

ANSI/ADA Specification #134*/ISO 22674:2016¹¹classifies metallic material requirements for fixed and removable prostheses and appliances:^{8, 12, 13}

Table 3. ANSI/ADA Standard No. 134/ISO 22674:2016 Requirements for Dental Casting Alloys.

Noble alloys can be used for a variety of restorative purposes, typically from tooth-supported type 1 inlays (126 MPa) to lower ductility type 2 inlays (146 – 221 MPa), but can also be used for high strength type 3 . High-strength crowns and onlays (207 MPa soft / 276 MPa hard) and Type 4 heavy-duty bridges and partial denture frameworks (350 / 607 MPa).¹²The use of lower grade precious metal alloys (≥ 25%) is more limited to Type 3 (248-309 MPa soft / 310-648 MPa hard) and Type 4 (420-460 MPa soft / 530-700 MPa) applications hard).¹²

base metal alloys

In the 1980s, the rise in the price of gold led to the development and increased use of base metals.^{1, 12}However, as mentioned above, unlike noble metal alloys which derive their corrosion resistance from their relative inertness in the oral environment, base metals used for dental applications may attribute their corrosion resistance to the presence of passive oxide layers. . These oxide layers, such as titanium oxide and chromium oxide, reduce the rate of corrosion to extremely low levels under typical oral conditions. The hardness of base alloys compared to gold makes adjustments difficult,¹and 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 various base elements can be added, including aluminum, molybdenum, manganese, and silicon to increase strength, castability, and/or corrosion resistance.^{9, 12, 15}Nickel-chromium alloys are commonly used for crowns and fixed partial dentures.^{4, 12}More elastic cobalt-chromium alloys have yield strengths of about 240 MPa to 650 MPa,^{12, 15}and are mainly used for removable partial dentures.¹²

titanium and titanium alloys

Titanium is popular in the medical and dental fields because of its low strength-to-weight ratio, corrosion resistance, and biocompatibility.^{8, 12, 16}Unlike the thermodynamic stability of precious metals, titanium's reaction with the environment is limited by a tough oxide layer (titanium oxide) that controls the rate of corrosion, reducing it to extremely low rates in typical oral conditions.¹⁰Titanium can be used as a restorative material in its unalloyed form, commercially pure titanium, with yield strength ranging from 240 MPa to 550 MPa, depending on the grade.^{8, 12}Titanium can be alloyed with aluminum and niobium (Ti-6Al-7Nb, 795 MPa) or vanadium (Ti-6AL-4V, 860 MPa) for added strength,^{12, 16}although there are some biocompatibility concerns regarding the possible release of toxic vanadium.¹²Titanium and its alloys can be used for crowns, implants and partial 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 proven to be effective, durable and long-lasting, but the desire for aesthetic and tooth-colored restorations has made the use of ceramic materials more popular in modern dentistry. The fragile nature of pottery - "can crack without warning if excessively bent"¹²–
and the possibility of its hardness causing wear damage to opposing teeth raised concerns about longevity.^{1, 17, 18}But the advantages of ceramics for dental restorations - esthetics, chemical inertness and wear resistance - have made ceramics a rapidly evolving area of ​​restorative dental science.¹²

The ISO and ANSI/ADA standards for dental ceramics classify both ceramics according to their intended clinical use (or function). They use 5 grades based on meeting 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.¹²Feldspar porcelains were the first all-ceramic restorative materials, but despite their high translucency they are inherently brittle.^{12, 20-22}with low bending strength (50 – 100 MPa). Beginning in the 1950s, feldspar porcelain was fused to metal to reinforce the restoration (see metal-ceramic section below).⁶The discovery of leucite in feldspar porcelain in the 1960s allowed the strengthening of porcelain dispersion, as well as the modification of its coefficient of thermal expansion.²³The 1980s saw the start of the development of high-strength glass-ceramics that could be made from pressed ingots rather than powder-liquid mixtures. Around the same time, improvements in computer-aided design software, the emergence and proliferation of milling machines and wax 3D printers, and improvements in dental zirconia and glass-ceramics spurred the digitization of laboratory procedures for dental ceramics.¹²Several classes of ceramic materials are currently widely used for CAD/CAM processing: zirconia, glass-ceramic, and resin-ceramic composites.

zirconium oxide ceramic

Zirconia ceramic has a naturally white appearance and has 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 make it more resistant.¹²Tetragonal zirconia can be subjected to a process known as transformation hardening, which allows the material to stop the progression of an incipient crack.¹²Increasing the yttria content further stabilizes the more translucent cubic phase, and zirconia restorative materials are generally characterized by the amount of incorporated yttria.²⁴Zirconia has been shown to be highly biocompatible (used as an orthopedic biomaterial since the 1970s),¹²and provides resistance to bacterial adhesion.²¹

zirconia frameEFully anatomical zirconium oxideare viable alternatives to PFM and all-metal restorations with high flexural strength (1000-1400 MPa). The zirconia framework, usually composed of tetragonal zirconia polycrystals (3Y-TZP) stabilized with 3 mol% yttria, is commonly used in anterior and posterior multi-unit bridges and is coated with feldspar porcelain or glass-ceramic for a natural appearance tooth due to its opacity.²⁵Full-contour zirconia, which is also commonly made from 3Y-TZP, has similar flexural strength and fracture toughness, but better translucency due to its lower alumina content, which allows it to be used as a monolithic restoration.²²Polished zirconia surfaces have been shown to be more resistant to wear on the opposing tooth structure than the feldspar porcelain used in metal-ceramic crowns.²²

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

A high-translucency zirconia stabilized with 5 mol% yttria (5Y-ZP) is more translucent than previous generations of zirconia due to the increased content of optically isotropic cubic phase and is less prone to degradation at low temperatures.²²However, it is more brittle and has lower bending strength (500-700 MPa).²⁷A recent review (2018)²⁷found no significant difference between 5Y-ZP and other ceramic materials tested in terms of enamel wear and bond strength to 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 bridges (61% and 57%, respectively), and as custom implant abutments ( 51%).²⁸Zirconia was used much less frequently for onlays, veneers and inlays (12%, 12% and 6% respectively).²⁸Please see ourACE Panel Report on Zirconia Restorationsfor more information.

glass based systems

Glass ceramics based on leucitehave a translucency almost similar to feldspar porcelain, but may have a higher strength (above 100 MPa) due to the increased leucite content.¹²However, the use of leucite-based ceramics is limited to veneers and aesthetic bonded crowns in the anterior region.ceramic-lithium disilicate(LDS) with higher flexural strength (250 – 400 MPa) and availability in low, medium and high translucency molds allow for a wide range of anterior indications.^{12, 22}There are some problems with wear compared to zirconia,^{12, 27}and with roughness in milled LDS, but it is more resistant than other glass-based ceramics and more translucent than any zirconia.¹²Lithium silicate (LS) and zirconia-reinforced lithium silicate (ZRS) are available alternatives with similar properties and indications; ZRS contains 10% dissolved zirconium oxide.²⁹

resin matrix composites

Resin matrix materials such as indirect restorations have the advantage of being easy to manipulate.^{12, 30, 31}Resin matrix composites are capable of a greater degree of loading and polymerization than direct composites, and because they cure outside the mouth, polymerization shrinkage does not occur as with direct resin matrix composite restorations.¹²CAD/CAM resin matrix composite blocks for indirect restorations may be more biocompatible than direct composites, often made from alternative, non-toxic resins, and more resistant to degradation and leakage (see Biocompatibility Issues section below) .³⁰They usually consist of a matrix of urethane dimethacrylate (UDMA), triethylene glycol dimethacrylate (TEGMDA) and/or bisphenol-A-glycidyl methacrylate (Bis-GMA) 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 present greater flexibility to masticatory loads, with less abrasiveness compared to opposing teeth, but lower flexural strength (100 - 200 MPa) and fracture toughness (0.8 - 1.2 MPa^1/2) than typical CAD/CAM blocks. Due to their lower resistance, they are mainly indicated as an alternative to inlays, onlays and single crowns.¹²

"Porcelain fused to metal (PFM)", also known as metal-ceramic prosthesis,¹²it was the most common type of indirect restoration before the advent of CAD/CAM-based ceramics.^{6, 21}PFMs combine the strength and durability of an alloy core with the natural tooth aesthetic appearance of a porcelain exterior.

Requirements for alloys used in PFM are addressed in ANSI/ADA No. 134/ISO, similar to other metallic materials suitable for the fabrication of dental restorations and appliances. With these norms^{13, 32}The fabrication of coupon sections requires that metallic materials supposedly recommended for use with ceramic veneers have their coupons tested after applying a simulated ceramic firing schedule. The standards further require that linear thermal expansion be measured for materials said 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 when is heated will be calculated and reported.^{13, 32}Ceramic veneer material performance and metal-ceramic bonding are covered in Part 1 of ISO 9693 (ANSI/ADA No. 38).³³

Therefore, the alloys used in PFM include the same types of noble, noble, titanium and base metals described in Table 2, which also meet the requirements specified in ANSI/ADA No. 134/ISO 22674 for alloys intended for use with Ceramic veneer is recommended. For a good metal-ceramic restoration system, the CTE. For a good metal-ceramic restorative system, the CTE of the alloy should be in the same range or slightly higher than^{8, 34}coating ceramics.^{12, 34}

Palladium's low CTE compared to other precious metals makes it highly compatible with a wide variety of ceramics and is a common element in precious metal alloys used in PFM systems.¹²Gold-platinum-palladium (Au-Pt-Pd) alloys were the first alloys used to melt porcelain. other combinations of gold, palladium, and silver, sometimes with gallium or copper, complete the elements added to high noble and noble alloys used for PFMs.^{8, 12, 35}Indium, tin and iron can also be added to high and noble alloys to improve bonding, while rhenium improves ruthenium's granularity and smelting.⁸

Although titanium has known biocompatibility in dentures as a PFM alloy, results have been mixed, with casting issues and some evidence of poor bond strength.^{8, 12}Surface treatments of titanium alloy prior to bonding to porcelain have been shown to improve performance.^{8, 12, 35}Likewise, base metal alloys are more sensitive to engineering due to increased hardness and stiffness compared to other alloys, although they are considered to be more fusible.^{12, 35}

Restorations made of metal and metal alloys, especially gold, have long been considered the most durable and durable.^{1, 36-39}and are reported to have an average lifespan of 18-20 years,³⁷with some reports going back over 40 years.⁴⁰A 2017 follow-up study of 25 later gold crowns showed no failure after 50 years.⁴¹In a 2010 review of the longevity of indirect and direct posterior restorations, cast gold inlays and onlays had the lowest annual failure rate at 1.4%.³⁶Secondary caries and failure of retention are among the most common reasons for failure of gold restorations.^{5, 42}However, many factors are responsible for the ultimate failure of a restoration, not just the materials used, and several studies have found a variety of failure and survival rates for a variety of materials and applications (see Table 5).

Table 5. Annual failure rates and survival rates from recent studies.

Porcelain has a natural tendency to fracture, and crown fracture has been described as the most common complication in all-ceramic materials.¹⁷A fracture rate of 1.6% per year has been reported in all materials, with central fractures accounting for 1.5% but veneers accounting for only 0.6% per year.¹⁸Lateral teeth have a significantly higher fracture rate than front teeth, especially molars.¹⁸On the other hand, a 2013 study showed that only 0.2% of PFM crowns resulted in failure due to fracture in an average of 13 years in service.⁴⁶while another study showed that 2.6% of GFPs ended within 5 years.⁴⁵Most failures in MAPs are the result of oral pathologies.⁴⁶

Fixed all-ceramic (FDP) prostheses were compared with metal-ceramic prostheses in a two-part series of systematic reviews in 2015.^{45, 53, 54}All-ceramic single crowns proved to be significantly less reliable than metal-ceramic ones with a 5-year survival rate of 90.7% and 94.7%, respectively.⁴⁵Similarly, in multi-unit FDPs, reinforced glass-ceramic FDPs had a significantly lower 5-year survival rate, 85.9%, than metal-ceramic, 94.4%.^{53, 54}Furthermore, a 2016 report by the Canadian Agency for Medicines and Health Technologies compared the effectiveness of all-ceramic versus metal-ceramic crowns and found similar results: a survival rate of 84-100% for all-ceramic and 92-96% for PFMs versus 8 years.⁵⁵Other studies have reported a 97% 10-year survival rate for metal-ceramic crowns,^{5, 56}with most failures being due to masticatory force and trauma to the anterior region.⁵⁶

Veneer chipping is a common complication with both PFMs and all-ceramic crowns; A 2015 systematic review reports a 5-year rate of 2.6% for GFPs.⁴⁵In general, among all-ceramic restorations, zirconia and alumina-based restorations have a higher frequency of veneer chipping.^{12, 45}although the 2013 report found that 3.3% of lithium disilicate crowns chipped during a 9-year follow-up study.⁴⁹

A 2016 systematic review of feldspar porcelain and glass-ceramic veneers found an overall survival rate of 89% at an average of 9 years.⁴⁸Porcelain veneers had an 87% cumulative survival rate at an average of 8 years, while glass-ceramic veneers had a 94% cumulative survival rate at 7 years.⁴⁸Chipping was the most commonly reported complication at a rate of 4%; while debonding, discoloration and endodontics had a complication rate of 2%.⁴⁸Usually feldspar porcelain⁴¹and densely sintered alumina¹⁷reported higher failure rates in anterior teeth.

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

Zirconia is still evolving as a restorative material, and while the potential for high strength and translucency is promising, evidence for long-term viability is limited. Short-term survival rates (up to 5 years) are in 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 supported teeth and 97.1% for implant-supported zirconia crowns, while a 2018 retrospective cohort study reported 5-year survival rates for implant-supported zirconia crowns. up to 98.5%, but that dropped after 10 years to only 39.3% in the region of the lateral teeth (molars).⁵⁰As with porcelain, the most common complications of zirconia-based restorations are chipping and fractures; a 5-year fracture rate of 1.09% for monolithic zirconia restorations (all types),⁶⁰and a fracture rate of 3.31% for all types of layered zirconia⁶¹have been reported (see Table 6). A 2021 ADA ACE Panel report found that among responding dentists (N= 277), 52% indicated that restoration dislodgement was the most common concern with zirconia restorations, while wear on opposing teeth (31%) and restoration fracture (23%) were also common concerns.²⁸Please see ourACE Panel Report on Zirconia Restorationsfor more information.

Table 6. Fracture rates in zirconia-based restorations.

ANSI/ADA Standard #41 and ISO 7405 provide guidelines and methods for evaluating the biocompatibility of dental materials. The US Food and Drug Administration (FDA) regulates and monitors trade in non-exempt medical and dental devices under a classification system based on risk level.^{12, 62}The 20-part ISO 10993 standard specifies the biological evaluation of medical devices and can also be used to ensure that there are no toxic, carcinogenic or other significant local or systemic health effects arising from contact with dental materials.^{12, 63}

The incidence of an adverse reaction to a dental material is reported to be as low as 0.14% in the general population,^{12, 64}and 0.33% in a population of prosthetic patients.¹²Base metals are responsible for most reactions to indirect dentures.¹²The release of metal ions as a result of corrosion from metallic materials used in restorative dentistry has been linked to causing irritation or allergic reactions.^{65, 66}The components of an alloy can leak out of the material during corrosion, which is influenced by the temperature and pH of the oral cavity.^{66, 67}The most common symptoms of hypersensitivity or an allergic reaction 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}

For information on the biocompatibility of resin materials, visit ourOral health topics page on direct restorative materials, and Bisphenol A Concerns on this page.

patient exposure

Precious metals are very resistant to corrosion but can cause adverse reactions when bonded with base metals.¹²For example, nickel is known to be a common contact allergen.¹and has the highest rate of side effects.⁶⁶Between 10% and 20% of the general population is sensitive to nickel, although its oral manifestation is less common and less severe.^{1, 8, 15, 66}Because nickel can be a naturally occurring impurity in an alloy's components, ANSI/ADA and ISO standards require manufacturers wishing to claim that their alloy is “nickel-free” to display the following marking on their packaging: “nickel-free; contains less than 0.1% nickel”.^{13, 32}

Cobalt and chromium are also known to cause allergic reactions in about 8% of the general population;⁶⁶other metals such as copper, tin, mercury and zinc, even gold, palladium and titanium have reported allergic reactions, but the prevalence is unclear.^{12, 66, 68}

Cross-reactivity can be responsible for a number of side effects, 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 it can cause inflammation, allergic reactions or other adverse effects, especially when mixed with nickel and chromium.¹²ISO 22674 and ANSI/ADA Standard #134 designate beryllium as a hazardous element, and to meet the requirements of the standard, metallic dental materials must not contain more than 0.2% (by weight) of beryllium.^{13, 32}Cobalt has also been linked to a potentially fatal heart condition called cobalt cardiomyopathy.⁶⁹Allergic reactions to highly biocompatible titanium have been reported,^{66, 70, 71}but it may be the result of cross-reactivity, particularly when alloyed with beryllium or other base metals.⁶⁸

Dental ceramics are highly biocompatible and exhibit low rates of surface degradation, although extremely acidic environments can increase the release of silicon ions.¹²Zirconia has not been associated with sensitivity or allergic reactions, and it is generally believed that minor adverse reactions reported with ceramics result from surface irritation.¹²

professional exhibition

Improper safety precautions and handling techniques can lead to occupational exposure to dental materials in the form of inhaling particles released during laboratory processing of metals and ceramics. Chronic inhalation of beryllium vapor has been linked to pneumoconiosis, along with other illnesses.^{12, 72, 73}although ANSI/ADA Standard 134 and ISO 22674 require less than 0.02% (by weight) beryllium in dental alloys.^{11, 13}A 2018 CDC report emphasized the importance of respiratory protection when working with these materials.⁷⁴According to the Occupational Safety and Health Administration (29 CFR 1910.134) Employers must provide adequate respiratory protection if workers are likely to be exposed to contaminated air in the workplace.⁷⁵

Scientific evaluation of dental restorative materials(Trad.2003:387)

Determined that although the safety and efficacy of dental restorative materials have been extensively researched, the Association will continue to actively promote such research in accordance with its research agenda to ensure that the profession and the public have the most up-to-date and scientifically valid information on which decisions to make To attend dental treatments that require restorative materials, even more so

Decided that the Association would use its existing communication tools to educate opinion leaders and policy makers about the scientific methods used to evaluate the safety and effectiveness of dental restorative materials, and be even more

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

American Dental Association
Adopted in 2003; Classified 2017

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ADA ACE Dashboard Reports:

zirconia restorations

Fixation of crowns and bridges with composite cement

adhesion promoter

Bioactive Materials

Composite resin restorations in the posterior region

Healthy ADA Mouth:

Dental Filling Options

• composite resins
• dental amalgam
• gold fillings

Bisphenol A (BPA)