Application of Vanadium-Free Titanium Alloys to Artificial Hip Joints
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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 mechan- ical properties and superior biocompatibility. However, the most generally used titanium alloy for arti- ficial 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 im- plants 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 treat- ment 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.