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.

High Noble Alloys

≥ 60% (gold and platinum group)
and gold ≥ 40%

Titanium and Titanium Alloys

≥ 25% (gold and platinum group)

Predominantly Base Alloys

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 22674:2016 11 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.

Type

Applications

Yield Strength

Elongation

Low-stress small single tooth fixed restorations.

Low-stress single-tooth fixed restorations:
one-surface inlays, veneered crowns.

Single-tooth fixed restorations:
crowns or inlays without restriction on the number of surfaces.

Multiple-unit fixed prostheses, e.g. bridges

Appliances with thin sections subject to very high forces:
removable partial dentures, clasps, etc.

High stiffness (greater than 150 GPa) and strength:
thin removable partial dentures,
parts with thin cross sections, clasps.

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

Class

Indications

Minimum Flexural Strength

(a) Monolithic ceramic for single-unit inlays, onlays, veneers,
anterior prostheses; (b) coverage of substructure.

(a) Monolithic ceramic for adhesively cemented single crowns, anterior and
posterior prostheses;
(b) substructure for anterior or posterior prostheses.

(a) Monolithic ceramic for non-adhesively cemented single crowns,
anterior or posterior fixed prostheses; and non-molar three-unit fixed prostheses.

(b) Substructure ceramic for single unit anterior or posterior prostheses
and non-molar three-unit fixed prostheses.

(a) Monolithic ceramic for three-unit prostheses with molar restoration;
(b) Substructure for three-unit fixed prostheses with molar restoration.

Substructure for multi-unit fixed prostheses

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