ASSOCIATION I REPORT

PHYSICAL AND CHEMICAL PROPERTIES

ADA COUNCIL ON SCIENTIFIC AFFAIRS

Both titaniumand titanium

alloys appearto be especially

suitable fordental implantsand prostheses.

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Background. The ADA Council on Sci­entific Affairs outlines the status oftita­nium and titanium alloys used in dentistryand provides its recommendations con­cerning their use in clinical practice.Conclusions. Titanium and titaniumalloys, based on their physical and chemicalproperties, appear to be especially suitablefor dental implants and prostheses. For theconstruction of endosseous implant devices,titanium and its alloys have become well­accepted and can be considered the mate­rials of choice.Clinical Implications. For crown andbridge prostheses, dentists can considertitanium and its alloys as viable options tomore traditional noble and base metalalloys, but careful selection of processingmethods and laboratory skill are necessaryto ensure success.

when chemically pure, is similar to thatof many dental alloys. Titanium alsocan be alloyed with other metals, suchas aluminum, vanadium or iron, tomodify its mechanical properties.

ASTM International (the AmericanSociety for Testing and Materials) rec­ognizes four grades of commerciallypure titanium, or Ti, and three titaniumalloys (Ti-6Al-4V, Ti-6Al-4V Extra LowInterstitial [low components] and Ti-Al­Nb).5 Titanium is a highly reactivemetal that readily passivates to form aprotective oxide layer, which accountsfor its high corrosion resistance. Thelow density of titanium provides forhigh-strength, lightweight prostheses.Additionally, dental porcelain can befused and bonded to titanium to pro­duce an esthetic, lifelike restoration.

Titanium's highly reactive natureprovides both advantages and disadvan­tages for its use. Titanium must bemelted in a vacuum or under inert gasto prevent oxidation and the incorpora­tion of oxygen that can lead to embrit­tlement of the cast meta1. 6 Contamina­tion with even low concentrations ofatmospheric oxygen can lead to signifi-

The use of titanium and titanium alloys formedical and dental applications has increaseddramatically in recent years. Historically,titanium has been used extensively inaerospace, aeronautical and marine applica­

tions because of its high strength and rigidity, its lowdensity and corresponding low weight, its ability towithstand high temperatures and its resistance tocorrosion. 1

Over the past three decades, the development ofnew processing methods-such as lost-wax casting,computer-aided machining and electric discharge

__________________________________ machining-has expanded titanium'suseful range of applications in biomed­ical devices.2 Today, titanium and tita­nium alloys are used for the fabricationof prosthetic joints, surgical splints,stents and fasteners, dental implants,dental crowns and partial dentureframeworks. This report outlines thecurrent status of titanium and titaniumalloys used in dentistry and presentsthe Council's recommendations con­

cerning their use in clinical practice.

Titanium applicationsin dentistry

Titanium, in the form of the oxide rutile, is abundant inthe earth's crust. Titanium ore can be refined to metallictitanium using a method called the Kroll process.3 In itsmetallic form at ambient temperature, titanium has ahexagonal, close-packed crystal lattice (ex phase), whichtransforms into a body-centered cubic form (B phase) at883 C (with a melting point of 1,680 C).4

Many of titanium's physical and mechanical proper­ties make it desirable as a material for implants andprostheses. The strength and rigidity of titanium arecomparable to those of other noble or high noble alloyscommonly used in dentistryy and titanium's ductility,

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Copyright ©2003 American Dental Association. All rights reserved.

ASSOCIATION REPORT

Equipment isavailable to cast

titanium into single­and multiple. unit­crown-and-bridge

frameworks.implant-supported

strudures and partialor full denture bases.

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using laser welding or electric spot welding.

DENTAL USES

IMPLANTS

For more than 25 years, titanium has been usedfor both endosseous and subperiosteal implants. lO

Endosseous implants have taken the form ofrods, posts and blades made of either pure tita­nium or titanium alloys. The passivating oxideon the implant surface permits close appositionof physiological fluids, proteins, and hard andsoft tissues to the metal surface. This process,whereby living tissue and an implant becomestructurally and functionally connected, is calledosseointegration. ll Titanium also has been usedsuccessfully as a biocompatible implant material,and continual improvements in both devicedesign and clinical implantation techniques haveled to well-accepted and predictable procedures.

In 1996, the ADA's Council on Scientific

Titanium has been used in cast dental prosthesessince the 1970s.4 Equipment is available to casttitanium into single-and multiple-unit-crown­and-bridge frameworks, implant-supported struc­tures and partial or full denture bases. Methodshave been developed for fusing dental porcelainto titanium for crowns and bridges, but thechoice of dental porcelain is limited by twocritical factors: the porcelain fusion temperaturemust be below 800 C to avoid the ex to Bphasetransition, and the coefficient of thermal expan­sion of the porcelain must match that of the

metal. 2.4 High fusing temperatures------------------------------------------------- also can lead to excessive oxide for-

mation, and a recent study9 showedthat porcelain fired under inertatmosphere resulted in improvedbonding. Furthermore, it is difficultto maintain consistency in tita­nium dental castings because oftheir inherently poor castability,and few laboratories are able toprovide this service. Though tita­nium is economical, biocompatible

................................................. and readily available, the technolo-gies necessary for casting,machining, welding and veneering

this metal are relatively new and more expensivethan those used for conventional dental metals.For these reasons, the use of titanium for dentalcastings has not become a prevalent laboratoryand clinical practice.

CASTING

Dental castings are made via pres­sure-vacuum or centrifugal castingmethods. 7 The metal is meltedusing an electric plasma arc orinductive heating in a meltingchamber filled with inert gas orheld in a vacuum. The moltenmetal then is transferred to therefractory mold via centrifugal orpressure-vacuum filling.

Castings of titanium commonlyare used to fabricate crowns, bridgeframeworks and full or partial denture frame­works. Several commercial machines for castingtitanium are available, but their cost is consider­ably higher than that for standard dental castingequipment. I Materials with low reactivity areused to prevent surface reaction with the moltenmetal, and materials with high setting expansionare used to compensate for the high castingshrinkage of titanium.

MACHINING

Dental implants generally are machined frombillet stock of pure metal or alloy. Dental crownsand bridge frameworks also can be machinedfrom solid metal stock via computer-aidedmachining. Abrasive machining of titanium,however, is slow and inefficient, which greatlylimits this approach. Another method for fabri­cating dental appliances is electric dischargemachining, which uses a fabricated graphite die(often reproduced from the dental working die) toerode the metal to final shape via spark erosion. 8

Multiple dental prostheses also can be connected

cant loss of ductility. The molten alloy also canreact readily with refractory investment mate­rials, requiring careful selection of compatiblematerials, removal of the surface-reacted layer ofmetal or both.

This same reactivity is responsible for many oftitanium's favorable properties. The metal oxi­dizes almost instantaneously in air to form atenacious and stable oxide layer approximately10 nanometers thick. 1.6 This oxide layer providesa highly biocompatible surface and a corrosionresistance similar to that of noble metals. Inaddition, the oxide layer allows for bonding offused porcelains, adhesive polymers or, in thecase of endosseous implants, plasma-sprayed orsurface-nucleated apatite coatings.

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Copyright ©2003 American Dental Association. All rights reserved.

ASSOCIATION REPORT

Affairs updated its position regarding the use ofendosseous implants as a treatment modality forfull or partially edentulous patients. 12 In this1996 update, the Council stated that ADA­Accepted endosseous implants, including thosemade of pure titanium or titanium alloys, can beused only to treat carefully selected patients withwhom the relative merits of benefit and risk havebeen fully discussed. Before the 1996 update, theCouncil had not recommended endosseousimplants for routine clinical practiceY Neverthe­less, the Council's report indicated that many fac­tors must be considered when deciding whether touse endosseous implants as a treatment option,and that some of these factors required furtherstudy. Some of the factors identified by theCouncil included the use of single-tooth implants,new methods of retaining prostheses, effects ofvarious surface treatments and coatings on tita­nium and titanium alloys, and oral hygieneissues. If endosseous implants are to be placed,however, titanium and titanium alloys are recom­mended due to their biocompatibility and clinicalsuccess.

CONCLUSIONS

Both titanium and titanium alloys, based on theirphysical and chemical properties, appear to beespecially suitable for dental implants and pros­theses. Processing difficulties, however, have lim­ited titanium's usefulness in fixed and removableprostheses. For the construction of endosseousimplant devices, titanium and its alloys havebecome well-accepted and can be considered thematerials of choice.

The Council recommends that practitionerscontinue to use implants in selected patients for

whom the risks and benefits have been carefullyweighed and thoroughly discussed. For crown andbridge prostheses, dentists can consider titaniumand its alloys as viable options to more traditionalnoble and base metal alloys, but careful selectionof processing methods and laboratory skill are nec­essary to ensure success. To date, however, thereare few clinical data available that compare thelong-term success of titanium restorations withthose made from more traditional metals. •

The ADA Council on Scientific Affairs thanks Dr. Frederick C. Eich­miller, Paffenbarger Research Center, Gaithersburg, Md., for his con­tribution to this report.

Address reprint requests to American Dental Association, Council onScientific Affairs, 211 E. Chicago Ave., Chicago, Ill. 60611.

1. Wang RR, Fenton A. Titanium for prosthodontic applications: areview of the literature. Quintessence Int 1996;27:401-8.

2. Walter M, Reppel PD, Boning K, Freesmeyer WE. Six-year follow­up of titanium and high-gold porcelain-fused-to-metal fixed partial den­tures. J Oral Rehabil1999;26:91-6.

3. Renner AM. The versatile use of titanium in implant prosthodon­tics. Quintessence Dent TechnoI2001;188-97.

4. Lautenschlager EP, Monaghan P. Titanium and titanium alloys asdental materials. Int Dent J 1993;43:245-53.

5. McCracken M. Dental implant materials: commercially pure tita­nium and titanium alloys. J Prosthodont 1999;8:40-3.

6. Brown D. All you wanted to know about titanium, but were afraidto ask. Br Dent J 1997;182:398-9.

7. Zinelis S. Effect of pressure of helium, argon, krypton and xenonon the porosity, microstructure, and mechanical properties of commer­cially pure titanium castings. J Prosthet Dent 2000;84:575-82.

8. Lovgren R, Andersson B, Carlsson GE, adman P. Prospective clin­ical 5-year study of ceramic-veneered titanium restorations with theProcera system. J Prosthet Dent 2000;84:514-21.

9. Atsu S, Berksun S. Bond strength of three porcelains to two formsof titanium using two firing atmospheres. J Prosthet Dent 2000;84:567-74.

10. Rizzo AA, ed. Proceedings of the consensus development confer­ence on dental implants. J Dent Educ 1988;52(special issue):677-827.

11. Albrektsson T, Zarb GA, eds. The Branemark osseointegratedimplant. Chicago: Quintessence; 1989:21-78.

12. Council on Scientific Affairs. Dental endosseous implants: anupdate. JADA 1996;129:1238-9.

13. Council on Dental Materials, Instruments and Equipment. Dentalendosseous implants. JADA 1986;113:949-50.

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ADA has placed titanium and titanium alloys, because of their ex-cellent biocompatibility, between high noble and noble alloys in the revised classi�cation system. (Code: D 2794, D 6794)