Application of Vanadium-Free Titanium Alloys to Artificial Hip Joints

Materials Transactions, Vol. 43, No. 12 (2002) pp. 2936 to 2942 Special Issue on Biomaterials and Bioengineering c 2002 The Japan Institute of Metals

Application of Vanadium-Free Titanium Alloys to Artiﬁcial Hip Joints ∗

Katsuhiko Maehara, Kenji Doi, Tomiharu Matsushita and Yoshio Sasaki

Medical Implants & Materials Department, Kobe Steel, Ltd., Kobe 651-8585, Japan

The application of titanium and its alloys to surgical implants has created much interest recently. Ti–6Al–4V has been used for numerous applications that require high mechanical properties, however this alloy contains vanadium, which has been proven to be cytotoxic. Two types of vanadium-free titanium alloys were developed and applied to artiﬁcial hip joints. As for the cemented type, Ti–15Mo–5Zr–3Al alloy was adopted because of its high fatigue strength, and its low elastic modulus, which approaches bone elasticity. As for the non-cemented type, Ti–6Al–2Nb–1Ta–0.8Mo alloy was adopted because of its less decrease of fatigue strength through heat treatments up to 1270 K, which is necessary to create the porous surface to activate the reaction between the implant and the bone. In addition, new coatings and bioactive methods were applied to the newly developed non-cemented type of prostheses. These hip joints are now successfully being used with excellent clinical results.

(Received June 3, 2002; Accepted July 12, 2002) Keywords: titanium alloys, vanadium-free, surgical implants, cytotoxicity, fatigue strength

1. Introduction

In the early development stages of artificial hip joints, their metallic parts were made of stainless steel. But by the indica- tion that nickel, found in stainless steel, is carcinogenic, the material preferences have gradually shifted to titanium and its alloys. Such alloys are widely applied in surgical implants, because of their excellent corrosion resistance, good mechanical properties and superior biocompatibility. However, the most generally used titanium alloy for artificial hip joints, Ti–6Al–4V contains vanadium. It has been reported that metallic vanadium is strongly toxic to cells by Steinemann1) and Okazaki.2) Naturally, the development and application of vanadium-free titanium alloys to surgical implants has created great interest among researchers around the Fig. 1 Artificial hip joints: cemented type (left) and non-cemented type world,3–5) but there are few clinical application cases. (right). In this paper, two types of vanadium-free titanium alloys are introduced. In addition, it is described that these alloys are applied to newly developed artificial hip joints, which are in place with acrylic bone cement. The basic design of this designed in consideration of suitability to Japanese physiques type was developed by Sir John Charnley in the UK in the and to Japanese lifestyles, and which have shown excellent 1960s.7) On the other hand, the non-cemented type joint is clinical results.6) fixed by directly bonding the bone and the implant without the use of bone cement. Development of this type of joint was 2. Composition of Artificial Hip Joint progressed rapidly in the 1980s, when the secondary effects of acrylic bone cement were proven to be negative. Currently, Artificial hip joints are implanted with a surgical opera- each type is considered to have both advantages and disad- tion, when the function of the femoral head and/or the pelvic vantages. Doctors usually decide which type to use according acetabulum becomes damaged or pathologically abnormal. to the patient’s age and other pathological factors. Figure 1 shows artificial hip joints, in which the stem and As shown on the left side of Fig. 1, the shape of the the metal back are made of the newly developed vanadium- newly developed cemented type prostheses is a modification free titanium alloys. The acetabulum is superseded by a cup, of Charnley’s design, but the parallel shape was added on the which is made of an ultra high molecular weight polyethylene distal part of the stem so that the prostheses can be easily (UHMWPE). The stem is inserted into the medullary cavity guided into the cavity. Nine stem variations with a distal di- of the femur, and the neck taper fits into a zirconia ceramic or ameter ranging from 8 to 13 mm, and a total length ranging alumina ceramic head. from 139 to 160 mm were created. Two types of head diame- Artificial hip joints can be classified into two types accord- ters, 22 mm and 26 mm, were prepared, and eight UHMWPE ing to the bone fixing method. The cemented type is fixed cup sizes with even diameters ranging from 40 to 54 mm can be supplied. ∗ For bonding between the non-cemented type prostheses This Paper was Presented at the Fourth Pacific Rim International Conference on Advanced Materials and Processing (PRICM4), held in and bone, it is common to coat the bone surface with an Honolulu, December 11–15, 2001. osteo-conductible material such as hydroxyapatite and/or to Application of Vanadium-Free Titanium Alloys to Artificial Hip Joints 2937

successive coating of AW glass ceramic, which induce osteo- conductivity. Therefore, it is important to select materials for which the fatigue strength is not decreased by the heat treatment process. Table 2 shows the evaluation results for vanadium-free titanium alloys. Investigation showed that the β transus of the alloy must be over 1270 K. Ti–6Al–2Nb– 1Ta–0.8Mo9) is the only material, which has both a higher β transus and a better formability than other vanadium-free alloys. Moreover, the fatigue strength of Ti–6Al–2Nb–1Ta– 0.8Mo at normal temperatures is higher than that for Ti–6Al– 4V, as shown in Table 1. Figure 3 shows the optical microstructure of Ti–6Al–2Nb– 1Ta–0.8Mo treated at 1323 K and 1223 K. While an acicular α matrix are observed at 1323 K, an equiaxed α are observed at 1223 K and the latter matrix are desirable to enhance low Fig. 2 Porous surface of a non-cemented prosthesis. frequency fatigue strength. As shown in Fig. 4, the fatigue strength of Ti–6Al–2Nb–1Ta–0.8Mo does not change much after heat treatment at 1223 K and the value is higher than make a porous layer for growing and anchoring the new bone. Ti–15Mo–5Zr–3Al. In the present study, a porous layer with the pore size of Figure 5 shows the effect of treating temperature on the 200–400 µm and the porosity of 45–50% was created by a tensile strength of the plasma sprayed porous layer. The true plasma spray of pure titanium powder. Figure 2 shows the tensile strength of the porous layer was measured by form- porous structure. The right side of Fig. 1 shows the newly de- ing a porous layer on the separated substrates. The tensile veloped non-cemented type prostheses, which have a porous strength of porous layers treated at 1123 K and at 1253 K were layer only on the proximal area of the stem. It is expected for three or four times higher than that of non-treated layers. By this shape to create a smooth stress distribution on the femoral applying Ti–6Al–2Nb–1Ta–0.8Mo, it came to be realized to cavity, and to avoid the effect of stress shielding, which can manufacture the prostheses, which is free from anxiety of the cause bone atrophy. Eight stem variations with a distal diam- omission and the exfoliation of the porous layer by such high eter ranging from 7 to 14 mm, and a total length ranging from 138 to 173 mm were created. Two types of head diameters, Table 2 Evaluation list for vanadium-free titanium alloys. 22 mm and 26 mm, were prepared, and eight metal backed UHMWPE cup sizes with even diameters ranging from 40 to β transus Alloys Formability 54 mm can be supplied. /K (over 1270 K) Ti–3Al–2.5V 1193–1223 × 3. Properties of Vanadium Free Titanium Alloy Ti–5Al–2.5Sn 1313–1363 3.1 Mechanical properties Ti–6Al–2Nb–1Ta–0.8Mo 1273–1303 In the selection of materials for implants, the mechanical Ti–6Al–2Sn–4Zr–2Mo 1253–1283 properties of potential materials in the living body at normal Ti–8Al–1Mo–1V –1313 temperatures and in normal environments are very important. As shown in Table 1, the fatigue strength and ductility of Ti– Ti–5Al–2Cr–1Fe 1243 × 15Mo–5Zr–3Al8) are higher than those for Ti–6Al–4V. The Ti–6Al–2Sn–4Zr–6Mo 1208 × elongation and reduction of area of both alloys are about the Ti–6Al–4V 1268 × same. As for the cemented type prostheses, Ti–15Mo–5Zr– Ti–6Al–6V–2Sn 1203–1233 × 3Al was chosen because of its high fatigue strength and its low elastic modulus, which approaches bone elasticity. Ti–5Al–2Sn–2Zr–4Mo–4Cr 1153–1173 × As mentioned in Section 2, the newly developed non- Ti–10V–2Fe–3Al 1063–1078 × cemented type implants were manufactured with a porous sur- Ti–13V–11Cr–3Al –973 × face structure. To prevent the omission and the exfoliation of Ti–15V–3Al–3Cr–3Sn 1023–1043 × the porous layer, a 1220–1270 K vacuum diffusion heat treatment is required. This heat treatment is also required for the Ti–15Mo–5Zr–3Al 1058 ×

Table 1 Mechanical properties of titanium alloys for surgical implants.

Tensile strength Yield strength Fatigue strength Elongation Reduction of area Elactic modulus Material /MPa /MPa /MPa (%) (%) /GPa Ti–6Al–4V 860 780 410 10 25 110 Ti–15Mo–5Zr–3Al 940 900 580 12 25 80 Ti–6Al–2Nb–1Ta–0.8Mo 860 780 490 12 28 110 2938 K. Maehara, K. Doi, T. Matsushita and Y. Sasaki

Fig. 3 Optical microstructures of Ti–6Al–2Nb–1Ta–0.8Mo treated at 1323 K (a)(b) and 1223 K (c)(d).

temperature diffusion process.

3.2 Bio-compatibility Figure 6 shows the results of colony formation testing using V79 cells.10) No colony formed in the presence of metallic vanadium, whereas good colony formation was observed in other specimens including Ti–6Al–4V. Though the toxicity of titanium alloys, which contain vanadium are not so clear, the vanadium-free titanium alloys are clearly regarded as desirable for prostheses, which is implanted in the human bodies for the long period.

Fig. 4 Comparison of fatigue strength of titanium alloy for surgical im- 3.3 Corrosion resistance plants. Figure 7 shows the results of anode polarization testing which measures corrosive properties.11) The data indicates that the corrosion resistance of these two types of vanadium- free titanium alloys is as high as commercial pure titanium and Ti–6Al–4V. Consequently, such vanadium-free alloys are considered to be able to remain stable in the living body as well.

4. Application of New Titanium Alloys in Clinical Situa- tions

4.1 Program for obtaining approval from MHW It is necessary to acquire approval from the Ministry of Health and Welfare (from 2000 onward, the Ministry of Health, Labor and Welfare) for manufacturing, importing, selling and repairing medical devices and tools. Figure 8 Fig. 5 Effect of treating temperature on tensile strength. shows the classiﬁcation of examinations necessary for the approval of medical devices.12) Since Ti–15Mo–5Zr–3Al and Application of Vanadium-Free Titanium Alloys to Artiﬁcial Hip Joints 2939

Fig. 6 Results of colony formation testing using V79 cells.

Ti–6Al–2Nb–1Ta–0.8Mo are new for surgical implants, artificial hip joints made of these metallic materials were classified as “Medical devices with a new structure”, and examined as such by the MHW. To tender the approval of manufacturing artificial hip joints made of Ti–6Al–2Nb–1Ta–0.8Mo and Ti–15Mo–5Zr–3Al, various tests such as heavy metal detection, acute toxicity, in- tracutaneous reactivity, pyrogenicity, hemolysis, implantation into muscles and implantation into bone marrows as shown in Table 3, were performed. These tests were conducted in accordance with the “Japanese guideline for basic biological tests of medical materials and devices”. Both these alloys sat- isfied the required safety values. Newly formed bone and an absence of inflammatory reaction were observed around the new alloys, which were implanted in bone marrow. These results prove the excellent biocompatibility of both new alloys.

4.2 Cemented type prostheses Fig. 7 Results of anodic polarization testing in physiological saline at 310 K. Cemented type prostheses called the SS Hip System, were ﬁrst implanted in 1991 in clinical trials.13) They have been

Fig. 8 Classiﬁcation of examinations for the approval of medical devices in Japan. 2940 K. Maehara, K. Doi, T. Matsushita and Y. Sasaki

Table 3 Data list for approving application. plete joint function, but also to unusual bone cell reactions, which are related to bone dissolution (osteolysis). In order to reduce wear, alumina ceramics were applied to the head, and then the surface was polished to 0.02 µm or less in Ra.

Fig. 10 Wide oscillation angle, which is realized by reducing the neck diameter.

commercialized since 1995, when approval was obtained. The left side of Fig. 9 shows an X-ray photograph of such a joint. Since Ti–15Mo–5Zr–3Al has roughly a 20% higher fatigue strength than Ti–6Al–4V, the outer diameter of the stem neck taper can be reduced to 9 mm, compared to 10–12 mm in other products made of Ti–6Al–4V. As shown in Fig. 10, this al- lows for a wide oscillation angle of the hip joint and prevents contact between the stem neck and the UHMWPE cup. In modern artiﬁcial hip joints, UHMWPE wear has become one of the biggest problems because it leads not only to incom-

Fig. 11 Cross sectional view of the porous layer, on which AW glass ceramic has been coated onto the bottom only.

Fig. 9 Implanted prostheses: cemented type (a) and non-cemented type Fig. 12 Comparison of interface shear strength for various coating meth- (b). ods. Application of Vanadium-Free Titanium Alloys to Artiﬁcial Hip Joints 2941

Fig. 13 Appearance (a) and cross sectional view (b) of a retrieved prosthesis.

4.3 Non-cemented type prostheses 5. Conclusions The clinical trials of this type of prostheses called the ABC Hip System, were started in 1992 and the results were satis- New vanadium-free titanium alloys for artificial hip joints factorily good.14,15) The right side of Fig. 9 shows an X-ray were developed. Their mechanical properties and biocom- photograph of such a joint. Of course, the exclusion of toxic patibility are superior to the conventional Ti–6Al–4V alloy. elements, wear reduction and wide oscillation angle design It was demonstrated that both the cemented and the non- factors were incorporated in this joint and a new follow-up cemented type prostheses, which made of newly developed product called the Q Hip System. The only difference be- titanium alloys, have performed reliably in 5000 or more clin- tween the ABC Hip System and the Q Hip System is in the ical cases. shape of the stem. New vanadium free titanium alloys, including Ti–15Mo– The porous layer of these products is manufactured through 5Zr–3Al and Ti–6Al–2Nb–1Ta–0.8Mo, for medical use were a vacuum plasma spray coating process. The most suitable investigated by the JIS20) (Japanese Industrial Standards) pore size for the growth of neogenetic bone is 200–400 µm. Committee and recognized as biomaterials for medical de- To obtain the optimum pore size, coarse and fine raw material vices by the MHLW and the METI (Ministry of Economy, powders are mixed in moderate proportion to increase parti- Trade and Industry). cle size distribution. Optimum spraying conditions were also developed.16) REFERENCES Furthermore, as shown in Fig. 11, AW glass ceramic17,18) with excellent osteo-conductivity characteristics was coated 1) S. G. Steinemann: Evaluation of Biomaterials, ed. by G. D. Winter, J. L. Leray and K. de Goot (John Wiley & Sons Inc., 1980) p. 1. onto the bottom of the porous layer only. The heat treatment 2) Y.Okazaki and E. Nishimura: Mater. Trans., JIM 41 (2000) 1247–1255. process was also optimized for the crystallization and stabi- 3) M. F. Semlitsch, H. Weber, R. M. Streicher and R. Schon: Biomaterials lization of the AW glass ceramic. It is expected that rapid 13 (1992) 781–788. bony ingrowth into the porous layer will lead to early me- 4) S. M. Perren, V.Geret, M. Tepic and B. A. Rahn: Biological and Biome- chanical Performance of Biomaterials, ed. by P. Christel et al. (Elsevier chanical anchoring between the bone and the porous layer. In Science Publishers B. V., Amsterdam, 1986) pp. 397–402. order to confirm this function, the interface shear strength was 5) K. Wang, L. Gustavson and J. Dumbleton: Beta Titanium Alloys in the measured and compared.19) Test pieces with various kinds of 1990’s, ed by D. Eylon, R. R. Boyer and D. A. Koss (The Minerals, surfaces were pulled from the tibia condyle of dogs. As indi- Metals & Materials Society, 1993) pp. 49–60. 6) T. Matsushita, K. Doi, Y. Sasaki, K. Maehara and T. Suda: Materia cated in Fig. 12, the test piece coated with both plasma spray Japan 38 (1999) 239–241. and AW glass ceramic showed the best values in the early 7) J. Charnley: Lancet 1 (1961) 1129–1135. stages after implantation. 8) Materials Properties Handbook: Titanium alloys, ed. by R. Boyer, G. This effect was also confirmed by clinical application. Welsch and E. W. Collings (ASM International, 1994) pp. 949–956. 9) Materials Properties Handbook: Titanium alloys, ed. by R. Boyer, G. Figure 13 shows a stem which was taken seven months af- Welsch and E. W. Collings (ASM International, 1994) pp. 321–336. ter the implant procedure, and a cross sectional view of it. 10) Y. Kotoura, M. Oka, Y. Nakayama, T. Yamamuro, T. Shinke and Y. The figure clearly shows that the bone has grown fully into Sasaki: Proc. of Japanese Society for Biomaterials 9 (1987) 67. the bottom part of the porous layer. 11) Y. Nakayama, T. Yamamuro, Y. Kotoura and M. Oka: Biomaterial 10 (1989) 420–424. 2942 K. Maehara, K. Doi, T. Matsushita and Y. Sasaki

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