Biomedical Applications of Titanium and Its Alloys

Biological Materials Science Overview Biomedical Applications of Titanium and its Alloys

C.N. Elias, J.H.C. Lima, R. Valiev, and M.A. Meyers

Titanium alloys are considered to be toxic effect resulting from released va- nium is associated with its oxides. C.B. the most attractive metallic materials nadium and aluminum. For this reason, Johansson1 demonstrated in in-vivo for biomedical applications. Ti-6Al-4V vanadium- and aluminum-free alloys animal model studies that the titanium has long been favored for biomedical have been introduced for implant appli- oxide may differ from metallic bioma- applications. However, for permanent cations, based on the Ti-6Al-4V im- terials such as Ti-6Al-4V, CoCr alloys, implant applications the alloy has a plants. These new alloys include Ti- and stainless steel 316 LVM. The inter- possible toxic effect resulting from re- 6Al-7Nb (ASTM F1295), Ti-13Nb- face between the titanium implant and leased vanadium and aluminum. For 13Zr (ASTM F1713), and Ti-12Mo- the bone is a thin proteoglycans layer. this reason, vanadium- and aluminum- 6Zr (ASTM F1813). Commercially pure titanium (Cp Ti) free alloys have been introduced for A great number of in-vivo and in-vi- is considered to be the best biocompat- implant applications. tro titanium experiments have been ible metallic material because its sur- done at universities and industries face properties result in the spontane- INTRODUCTION throughout the world for the last 50 ous build-up of a stable and inert oxide Materials used for biomedical appli- years. These experiments found that layer. The main physical properties of cations cover a wide spectrum and must the excellent biocompatibility of tita- titanium responsible for the biocom- exhibit specific properties. The most patibility are: low level of electronic important property of materials used conductivity, high corrosion resistance, for fabricating implants is biocompati- How would you… thermodynamic state at physiological bility, followed by corrosion resistance. …describe the overall significance pH values, low ion-formation tendency of this paper? The main metallic biomaterials are in aqueous environments, and an iso- This article enumerates some stainless steels, cobalt alloy, and titani- materials used for biomedical electric point of the oxide of 5–6. In ad- um and titanium alloys. applications, emphasizing the use dition, the passive-film-covered surface Stainless steel was the first metallic of commercially pure titanium for is only slightly negatively charged at biomaterial used successfully as an im- dental implants, and explains the physiological pH, and titanium has a importance of titanium chemical plant. In 1932, the cobalt-based alloy composition on osseointegration. dielectric constant comparable to that named Vitallium was developed for of water with the consequence that the medical applications. Titanium is the …describe this work to a Coulomb interaction of charged spe- materials science and engineering newest metallic biomaterial. In both professional with no experience in cies is similar to that in water. medical and dental fields, titanium and your technical specialty? DENTISTRY APPLICATIONS its alloys have demonstrated success as Ultrafine grain titanium should biomedical devices. have adequate biocompatibility Titanium and its alloys are also used and higher mechanical properties for dentistry devices such as implants, MEDICAL APPLICATIONS than commercially pure titanium. crowns, bridges, overdentures, and AND BIOCOMPATIBILITY Dental implants were inserted in a rabbit and no statistical dental implant prosthesis components Titanium alloys are now the most at- difference was observed between (screw and abutment). Commercially the osseointegration of cp Ti tractive metallic materials for biomedi- and ultrafine grain titanium. pure titanium is used preferentially for cal applications. In medicine, they are endosseous dental implant applica- used for implant devices replacing …describe this work to a tions. There are currently four cp Ti layperson? failed hard tissue. Examples include ar- grades and one titanium alloy specially In medicine titanium alloys are used tificial hip joints, artificial knee joints, for implant devices replacing failed made for dental implant applications. bone plates, screws for fracture fixa- hard tissue. This article compares These metals are specified according to tion, cardiac valve prostheses, pace- biocompatibility properties of ASTM as grades 1 to 5. Grades 1 to 4 makers, and artificial hearts. Ti-6Al-4V different biomaterials and shows are unalloyed, while grade 5, with 6% that ultrafine grain titanium has has long been a main medical titanium adequate biocompatibility for aluminum and 4% vanadium, is the alloy. However, for permanent implant dental implant use. strongest. According to ASTM F67 applications the alloy has a possible and F136, the titanium bar mechanical

46 www.tms.org/jom.html JOM • March 2008 osseointegrated dental implant, one Table I. Selected Mechanical Requirements Properties of Titanium Bar for Implant* must ﬁrst be knowledgeable of the im- ASTM Grade plant parts, as shown in Figure 2. Al- Property 12345though each available implant system Yield Strength (MPa) 170 275 380 483 795 has a different shape, the parts are the Ultimate Tensile Strength (MPa) 240 345 450 550 860 same. The implant is the main compo- Elongation (%) 24 20 18 15 10 nent that actually has bone contact. To Elastic Modulus (GPa) 103–107 103–107 103–107 103–107 114–120 improve the biological response to tita-

*Adapted from ASTM F67 (Grade 1 to 4) and F136 (Grade 5). nium, different implant surface modifi- cations have been introduced. Tissue reactions following implantation are properties of grades 1 to 5 are summa- influenced by physiochemical proper- DENTAL IMPLANTS rized in Table I. ties of the implant surface. Figure 3 Titanium grade 1 has the highest pu- There are three types of dental im- shows an implant with good wettability rity, lowest strength, and best room- plant: osseointegrated, mini-implant surface during the surgery. The second temperature ductility of the four ASTM for orthodontic anchorage, and zygo- component is the abutment, which titanium unalloyed grades. Grade 2 ti- matic. Each group needs different me- gives the connection between the im- tanium is the main cp Ti used for indus- chanical properties and must be made plant and the prosthesis and makes trial dental implant applications. The of cp Ti or a titanium alloy. contact with soft tissue. Usually the guaranteed minimum yield strength of abutment is connected to the implant Osseointegrated Implant 275 MPa for grade 2 is comparable to with a screw, or it can be cemented. those of annealed austenitic stainless The osseointegration of dental im- The third part of the implant structure steels. plants was initially defined by P.-I. is the prosthesis, which can be attached Titanium grade 3 has 0.30 maximum Branemark et al.3 as a direct bone-to- to the abutment with a screw or cement. iron content, which is lower than grade implant contact and later on defined on The implant is made with cp Ti or a ti- 4 (0.50 maximum). Grade 4 has the a more functional basis as a direct tanium alloy, the abutment with a tita- highest strength of the unalloyed bone-to-implant contact under load. nium alloy, and the abutment screw ASTM grades. Grade 5, an ASTM tita- In the past, osseointegrated endosse- with a titanium or gold alloy. nium alloy (Ti-6Al-4V), is the most ous dental implants have been made in Some designs of titanium dental im- widely used titanium alloy in medical a variety of shapes, including hollow plants and their prosthesis components implants but not common in dental im- baskets, blades, tripods, needles, disks, have a small diameter and thickness plants. The alloy is most commonly truncated cones, cylinders, and screws. wall, especially with internal abutment used in the annealed state. Currently the most commonly used fixation (see the last five implants in Titanium and Ti-6Al-4V present low dental implant has a screw shape and is Figure 1). In these cases the implant shear strength and low wear resistance made of cp Ti or Ti-6Al-4V, as shown must be made of Ti-6Al-4V to prevent when used in an orthopedic prosthesis. in Figure 1. The dental implants are fracture. However, when titanium al- Also important is the mismatch of available with diameters from 3.3 mm loys are implanted, higher levels of the Young’s modulus between the titanium to 6.0 mm and lengths from 6 mm to 16 component elements can be detected in implant (103–120 GPa) and bone (10– mm. tissues locally and systemically.4 30 GPa), which is unfavorable for bone To understand the importance of the L. Morais et al.4 analyzed the vana- healing and remodeling. Some research material properties and function of an dium ion release during the implant has been done to resolve these prob- lems and many new titanium alloys have been developed for biomedical applications. However, there is a con- tradiction between the elastic modulus and other mechanical properties. When the elastic modulus is reduced, the strength of the titanium alloy is also decreased. Conversely, when the strength is enhanced, the elastic modulus is also increased. Several studies have compared cp Ti to Ti-6Al-4V implants inserted in rabbit bones. It has been shown that when twisted, the cp Ti implant has higher removal torque values than Ti-6Al-4V Figure 1. Examples of commercial dental implant designs. (Courtesy of Conexão Sistema screws and significantly higher bone e Prótese, Brazil.) contacts.2

2008 March • JOM www.tms.org/jom.html 47 I. Semenova et al.6 showed that ul- Table II. Dental Implant Removal Torque trafine grain cp Ti presents ultimate (N·cm) after 8 Weeks tensile strength as high as 1,240 MPa Inserted in Rabbit Tibia while retaining a ductility of 11%. The present work analyzed the pos- Average Deviation sibility of using ultrafine grain titanium Ti ASTM Grade 2 17.0 4.2 in dentistry. Screw-shaped dental im- Ultrafine Grain Ti Grade 2 18.9 1.9 plants with pitch-height of 0.5 mm, outer diameter of 3.3 mm, length of 8.0 Mini-Implants for Orthodontic mm, a square head, and inner threaded Anchorage hole of 2.0 mm were turned from titanium rods. Two types of dental implant Another dentistry implant is a tem- screws were used: cp Ti ASTM grade 2 porary orthodontic mini-implant used and ultrafine grain titanium grade 2. generally to secure anchorage in con- The implants were inserted in the temporary orthodontic treatments tibiae of New Zealand white rabbits. (Figure 4). This implant has a small Bone tissue responses were evaluated diameter (1.2 mm to 2.0 mm) and the by removal torque tests that were un- orthodontic load can deform the mini- dertaken after 8 weeks. Table II shows implant. Consequently, the orthodontic Figure 2. Dental implant components. the removal torque results. Descriptive implants are made with Ti-6Al-4V in- statistical parameters were calculated stead of cp Ti due to the alloy’s supe- and a one-way analysis of variance rior strength. However, the Ti-6Al-4V healing process. Titanium alloy im- with Tukey’s test was used to evaluate corrosion resistance is lower than that plants were inserted in the tibiae of rab- the removal torque. No statistical dif- of cp Ti, allowing for metal ion release. bits. After 1, 4, and 12 weeks, they ference was observed between cp Ti This implant does not result in osseoin- were submitted to removal torque test- and ultrafine grain titanium. tegration. ing. The kidney, liver, and lung were extracted and analyzed by atomic ab- sorption spectrometry. In comparison with the control values, the content of vanadium increased slightly after 1 week and significantly after 4 weeks, and decreased slightly after 12 weeks, without reaching the 1 week values. To avoid ion release, it is necessary to develop new titanium alloy processing or increase the mechanical properties of cp Ti. One solution is nanocrys- talline materials, which can offer very high strength, toughness, and fatigue resistance. Processing of nanomateri- als to improve both strength and ductil- Figure 3. A dental implant with good wettability. ity is of primary importance for fatigue strength and fracture toughness. R.Z. Valiev et al.5 refined the microstructure of bulk billets using severe plastic deformation and increased the mechanical properties of titanium grade 2. Ultrafine Grain Titanium in Dental Implants As described, some dental implants made with Ti-6Al-4V can release ions to tissues locally and systemically. For stronger implants, though, alloyed titanium is preferred over unalloyed. However, biomechanically the cp Ti Figure 4. An example of an orthodontic mini-implant for anchorage implants have significantly higher re- application. (Courtesy of Flavia Rabello.) moval torque than the alloy implants.2

48 www.tms.org/jom.html JOM • March 2008 sequently, a modification to the form of the implants is necessary, making the implants longer than conventional dental implants. Normally, the zygomatic implant has a diameter equal to 4–5 mm and 30–53 mm length. It penetrates the maxilla at the second premolar region as close to the alveolar crest as possible.8 References Figure 5. A zigomatic implant. (Courtesy of 1. C.B. Johansson, “On Tissue Reactions to Metal P. Saad.) Implants” (PhD thesis, Dept. of Biomaterials/Handicap Research, University of Göteborg, Sweden, 1991). 2. C.B Johansson et al., “Quantitative Comparison of Machined Commercially Pure Ti and Ti-6Al-4V Implant 4 in Rabbit,” J. Oral Maxillofac. Implants, 13 (1998), p. Morais et al. inserted mini-implant increase the titanium alloy mechanical 315. orthodontics in two groups of rabbits. properties or use cp Ti with ultra-fine 3. P.-I. Brånemark et al., “Osseointegrated Titanium One group was loaded immediately grains. Fixtures in the Treatment of Edentulousness,” Biomaterials, 4 (1983), pp. 25–28. after the surgery and the second group Zygomatic Fixture 4. Liliane S. Morais et al., “Titanium Alloy Mini-Implants was not loaded during the healing time. for Orthodontic Anchorage: Immediate Loading and When a stress analysis on the mini- The third dentistry implant group Metal Ion Release,” Acta Biomaterialia, 3 (3) (2007), pp. 331–339. implant was carried out, the torque at is the zygomatic implants, which are 5. R.Z. Valiev, R.K. Islamgaliev, and I.V. Alexandrov, which cp Ti and Ti-6Al-4V deform made of cp Ti (Figure 5). “Bulk Nanostructured Materials from Severe Plastic plastically and the shear strength of the There are some technical approaches Deformation,” Prog. Mater. Sci., 45 (2000), p. 103. 6. I. Semenova et al., “Superplasticity in Ultrafine- interface mini-implant-bone was calcu- to the treatment of the atrophic maxilla Grained Titanium: Observations and Properties Studies” lated. No increase was observed in the involving a series of clinical consider- (Presentation at the Structural Materials Division removal torque value between 1 and 4 ations and producing different results. Symposium: Mechanical Behavior of Nanostructured Materials, in Honor of Carl Koch: Poster Session: weeks of healing, regardless of the load. The development of the zygomatic im- Mechanical Properties of Nanostructured Materials, Nevertheless, after 12 weeks, a signifi- plant (Nobel Biocare, Göteborg, Swe- TMS 2007 Annual Meeting & Exhibition, Orlando, FL, cant improvement was observed in both den) represents an excellent alternative February 25–March 1, 2007). 7. S.M. Parel et al., “Remote Implant Anchorage for the groups, with the highest removal torque for these situations. The zygomatic Rehabilitation of Maxillary Defects,” J. Prosthet. Dent., value for the unloaded group. The implant developed by P.-I. Brånemark7 86 (2001), p. 377. stress analysis reveals that the removal has been used as posterior anchorage 8. L.R. Duarte et al., “The Establishment of a Protocol for the Total Rehabilitation of Atrophic torques for cp Ti dangerously approach for implant-supported prostheses in Maxillae Employing Four Zygomatic Fixtures in an its yield stress. The results of this rab- patients with atrophic maxillae since Immediate Loading System—A 30-Month Clinical and bit model study indicate that titanium 1990. It was initially conceived as a Radiographic Follow-Up,” Clinical Implant Dentistry and Related Research, 9 (4) (2007), p. 186. alloy mini-implants can be loaded im- treatment for the victims of traumas or mediately with no compromise in their tumor resection where there was con- C.N. Elias and J.H.C. Lima are with the Biomaterials Laboratory, Instituto Militar de Engenharia, Rio de stability. The detected concentration of siderable loss of maxillary structure. Janeiro, Brazil. R. Valiev is with UFA State Aviation vanadium did not reach toxic levels in Following maxillectomy, many pa- Technical University, Ufa, Russia. M.A. Meyers is the animal model. Consequently, to im- tients retain anchorage regions only in with the Department of Mechanical and Aerospace Engineering, University of California at San Diego, prove the mini-implant orthodontic for the body of the zygoma or in the frontal San Diego, California. Dr. Elias can be reached at anchorage behavior it is important to extension of the zygomatic bone. Con- [email protected].

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