Materials Transactions, Vol. 43, No. 12 (2002) pp. 2936 to 2942Special Issue on Biomaterials and Bioengineeringc©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 numerousapplications that require high mechanical properties, however this alloy contains vanadium, which has been proven to be cytotoxic. Twotypes of vanadium-free titanium alloys were developed and applied to artificial hip joints. As for the cemented type, Ti–15Mo–5Zr–3Al alloywas adopted because of its high fatigue strength, and its low elastic modulus, which approaches bone elasticity. As for the non-cementedtype, Ti–6Al–2Nb–1Ta–0.8Mo alloy was adopted because of its less decrease of fatigue strength through heat treatments up to 1270 K, whichis necessary to create the porous surface to activate the reaction between the implant and the bone. In addition, new coatings and bioactivemethods were applied to the newly developed non-cemented type of prostheses. These hip joints are now successfully being used with excellentclinical 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, theirmetallic parts were made of stainless steel. But by the indica-tion that nickel, found in stainless steel, is carcinogenic, thematerial preferences have gradually shifted to titanium and itsalloys. 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 beenreported that metallic vanadium is strongly toxic to cells bySteinemann1) and Okazaki.2) Naturally, the development andapplication of vanadium-free titanium alloys to surgical im-plants has created great interest among researchers around theworld,3–5) but there are few clinical application cases.

In this paper, two types of vanadium-free titanium alloysare introduced. In addition, it is described that these alloysare applied to newly developed artificial hip joints, which aredesigned in consideration of suitability to Japanese physiquesand to Japanese lifestyles, and which have shown excellentclinical results.6)

2. Composition of Artificial Hip Joint

Artificial hip joints are implanted with a surgical opera-tion, when the function of the femoral head and/or the pelvicacetabulum becomes damaged or pathologically abnormal.Figure 1 shows artificial hip joints, in which the stem andthe metal back are made of the newly developed vanadium-free titanium alloys. The acetabulum is superseded by a cup,which is made of an ultra high molecular weight polyethylene(UHMWPE). The stem is inserted into the medullary cavityof the femur, and the neck taper fits into a zirconia ceramic oralumina ceramic head.

Artificial hip joints can be classified into two types accord-ing to the bone fixing method. The cemented type is fixed

∗This Paper was Presented at the Fourth Pacific Rim InternationalConference on Advanced Materials and Processing (PRICM4), held inHonolulu, December 11–15, 2001.

in place with acrylic bone cement. The basic design of thistype was developed by Sir John Charnley in the UK in the1960s.7) On the other hand, the non-cemented type joint isfixed by directly bonding the bone and the implant withoutthe use of bone cement. Development of this type of joint wasprogressed rapidly in the 1980s, when the secondary effectsof acrylic bone cement were proven to be negative. Currently,each type is considered to have both advantages and disad-vantages. Doctors usually decide which type to use accordingto the patient’s age and other pathological factors.

As shown on the left side of Fig. 1, the shape of thenewly developed cemented type prostheses is a modificationof Charnley’s design, but the parallel shape was added on thedistal part of the stem so that the prostheses can be easilyguided into the cavity. Nine stem variations with a distal di-ameter ranging from 8 to 13 mm, and a total length rangingfrom 139 to 160 mm were created. Two types of head diame-ters, 22 mm and 26 mm, were prepared, and eight UHMWPEcup sizes with even diameters ranging from 40 to 54 mm canbe supplied.

For bonding between the non-cemented type prosthesesand bone, it is common to coat the bone surface with anosteo-conductible material such as hydroxyapatite and/or to

Fig. 1 Artificial hip joints: cemented type (left) and non-cemented type(right).

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 materialsfor which the fatigue strength is not decreased by the heattreatment process. Table 2 shows the evaluation results forvanadium-free titanium alloys. Investigation showed that theβ transus of the alloy must be over 1270 K. Ti–6Al–2Nb–1Ta–0.8Mo9) is the only material, which has both a higherβ transus and a better formability than other vanadium-freealloys. 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 acicularα matrix are observed at 1323 K′, an equiaxed α are observedat 1223 K′ and the latter matrix are desirable to enhance lowfrequency fatigue strength. As shown in Fig. 4, the fatiguestrength of Ti–6Al–2Nb–1Ta–0.8Mo does not change muchafter heat treatment at 1223 K and the value is higher thanTi–15Mo–5Zr–3Al.

Figure 5 shows the effect of treating temperature on thetensile strength of the plasma sprayed porous layer. The truetensile strength of the porous layer was measured by form-ing a porous layer on the separated substrates. The tensilestrength of porous layers treated at 1123 K and at 1253 K werethree or four times higher than that of non-treated layers. Byapplying Ti–6Al–2Nb–1Ta–0.8Mo, it came to be realized tomanufacture the prostheses, which is free from anxiety of theomission and the exfoliation of the porous layer by such high

Fig. 2 Porous surface of a non-cemented prosthesis.

3. Properties of Vanadium Free Titanium Alloy

3.1 Mechanical propertiesIn the selection of materials for implants, the mechanical

properties of potential materials in the living body at normaltemperatures and in normal environments are very important.As shown in Table 1, the fatigue strength and ductility of Ti–15Mo–5Zr–3Al8) are higher than those for Ti–6Al–4V. Theelongation and reduction of area of both alloys are about thesame. As for the cemented type prostheses, Ti–15Mo–5Zr–3Al was chosen because of its high fatigue strength and itslow elastic modulus, which approaches bone elasticity.

As mentioned in Section 2, the newly developed non-cemented type implants were manufactured with a porous sur-face structure. To prevent the omission and the exfoliation ofthe porous layer, a 1220–1270 K vacuum diffusion heat treat-ment is required. This heat treatment is also required for the

make a porous layer for growing and anchoring the new bone.In the present study, a porous layer with the pore size of200–400 µm and the porosity of 45–50% was created by aplasma spray of pure titanium powder. Figure 2 shows theporous structure. The right side of Fig. 1 shows the newly de-veloped non-cemented type prostheses, which have a porouslayer only on the proximal area of the stem. It is expected forthis shape to create a smooth stress distribution on the femoralcavity, and to avoid the effect of stress shielding, which cancause bone atrophy. Eight stem variations with a distal diam-eter ranging from 7 to 14 mm, and a total length ranging from138 to 173 mm were created. Two types of head diameters,22 mm and 26 mm, were prepared, and eight metal backedUHMWPE cup sizes with even diameters ranging from 40 to54 mm can be supplied.

Table 1 Mechanical properties of titanium alloys for surgical implants.

MaterialTensile strength Yield strength Fatigue strength Elongation Reduction of area Elactic modulus

/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

Table 2 Evaluation list for vanadium-free titanium alloys.

Alloysβ transus

Formability/K (over 1270 K)

Ti–3Al–2.5V 1193–1223 × �Ti–5Al–2.5Sn 1313–1363 © �

Ti–6Al–2Nb–1Ta–0.8Mo 1273–1303 © ©Ti–6Al–2Sn–4Zr–2Mo 1253–1283 � ©

Ti–8Al–1Mo–1V –1313 © �Ti–5Al–2Cr–1Fe 1243 × �

Ti–6Al–2Sn–4Zr–6Mo 1208 × ©Ti–6Al–4V 1268 × �

Ti–6Al–6V–2Sn 1203–1233 × ©Ti–5Al–2Sn–2Zr–4Mo–4Cr 1153–1173 × ©

Ti–10V–2Fe–3Al 1063–1078 × �Ti–13V–11Cr–3Al –973 × �

Ti–15V–3Al–3Cr–3Sn 1023–1043 × �Ti–15Mo–5Zr–3Al 1058 × �

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).

that the corrosion resistance of these two types of vanadium-free titanium alloys is as high as commercial pure titaniumand Ti–6Al–4V. Consequently, such vanadium-free alloys areconsidered to be able to remain stable in the living body aswell.

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

4.1 Program for obtaining approval from MHWIt is necessary to acquire approval from the Ministry of

Health and Welfare (from 2000 onward, the Ministry ofHealth, Labor and Welfare) for manufacturing, importing,selling and repairing medical devices and tools. Figure 8shows the classification of examinations necessary for the ap-proval of medical devices.12) Since Ti–15Mo–5Zr–3Al and

temperature diffusion process.

3.2 Bio-compatibilityFigure 6 shows the results of colony formation testing us-

ing V79 cells.10) No colony formed in the presence of metal-lic vanadium, whereas good colony formation was observedin other specimens including Ti–6Al–4V. Though the tox-icity of titanium alloys, which contain vanadium are not soclear, the vanadium-free titanium alloys are clearly regardedas desirable for prostheses, which is implanted in the humanbodies for the long period.

3.3 Corrosion resistanceFigure 7 shows the results of anode polarization testing

which measures corrosive properties.11) The data indicates

Fig. 5 Effect of treating temperature on tensile strength.

Fig. 4 Comparison of fatigue strength of titanium alloy for surgical im-plants.

Application of Vanadium-Free Titanium Alloys to Artificial Hip Joints 2939

Ti–6Al–2Nb–1Ta–0.8Mo are new for surgical implants, artifi-cial hip joints made of these metallic materials were classifiedas “Medical devices with a new structure” , and examined assuch by the MHW.

To tender the approval of manufacturing artificial hip jointsmade 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, implantationinto muscles and implantation into bone marrows as shownin Table 3, were performed. These tests were conducted inaccordance with the “Japanese guideline for basic biologicaltests of medical materials and devices” . Both these alloys sat-isfied the required safety values. Newly formed bone and anabsence of inflammatory reaction were observed around thenew alloys, which were implanted in bone marrow. These re-sults prove the excellent biocompatibility of both new alloys.

4.2 Cemented type prosthesesCemented type prostheses called the SS Hip System, were

first implanted in 1991 in clinical trials.13) They have been

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

Fig. 7 Results of anodic polarization testing in physiological saline at310 K.

Fig. 8 Classification of examinations for the approval of medical devices in Japan.

2940 K. Maehara, K. Doi, T. Matsushita and Y. Sasaki

commercialized since 1995, when approval was obtained.The left side of Fig. 9 shows an X-ray photograph of sucha joint.

Since Ti–15Mo–5Zr–3Al has roughly a 20% higher fatiguestrength than Ti–6Al–4V, the outer diameter of the stem necktaper can be reduced to 9 mm, compared to 10–12 mm in otherproducts made of Ti–6Al–4V. As shown in Fig. 10, this al-lows for a wide oscillation angle of the hip joint and preventscontact between the stem neck and the UHMWPE cup. Inmodern artificial hip joints, UHMWPE wear has become oneof the biggest problems because it leads not only to incom-

plete joint function, but also to unusual bone cell reactions,which are related to bone dissolution (osteolysis). In order toreduce wear, alumina ceramics were applied to the head, andthen the surface was polished to 0.02 µm or less in Ra.

Table 3 Data list for approving application.

Fig. 9 Implanted prostheses: cemented type (a) and non-cemented type(b).

Fig. 12 Comparison of interface shear strength for various coating meth-ods.

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

Fig. 10 Wide oscillation angle, which is realized by reducing the neck di-ameter.

Application of Vanadium-Free Titanium Alloys to Artificial Hip Joints 2941

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

tween the ABC Hip System and the Q Hip System is in theshape of the stem.

The porous layer of these products is manufactured througha vacuum plasma spray coating process. The most suitablepore size for the growth of neogenetic bone is 200–400 µm.To obtain the optimum pore size, coarse and fine raw materialpowders are mixed in moderate proportion to increase parti-cle size distribution. Optimum spraying conditions were alsodeveloped.16)

Furthermore, as shown in Fig. 11, AW glass ceramic17, 18)

with excellent osteo-conductivity characteristics was coatedonto the bottom of the porous layer only. The heat treatmentprocess was also optimized for the crystallization and stabi-lization of the AW glass ceramic. It is expected that rapidbony ingrowth into the porous layer will lead to early me-chanical anchoring between the bone and the porous layer. Inorder to confirm this function, the interface shear strength wasmeasured and compared.19) Test pieces with various kinds ofsurfaces were pulled from the tibia condyle of dogs. As indi-cated in Fig. 12, the test piece coated with both plasma sprayand AW glass ceramic showed the best values in the earlystages after implantation.

This effect was also confirmed by clinical application.Figure 13 shows a stem which was taken seven months af-ter the implant procedure, and a cross sectional view of it.The figure clearly shows that the bone has grown fully intothe bottom part of the porous layer.

4.3 Non-cemented type prosthesesThe clinical trials of this type of prostheses called the ABC

Hip System, were started in 1992 and the results were satis-factorily good.14, 15) The right side of Fig. 9 shows an X-rayphotograph of such a joint. Of course, the exclusion of toxicelements, wear reduction and wide oscillation angle designfactors were incorporated in this joint and a new follow-upproduct called the Q Hip System. The only difference be-

5. Conclusions

New vanadium-free titanium alloys for artificial hip jointswere developed. Their mechanical properties and biocom-patibility are superior to the conventional Ti–6Al–4V alloy.It was demonstrated that both the cemented and the non-cemented type prostheses, which made of newly developedtitanium alloys, have performed reliably in 5000 or more clin-ical cases.

New vanadium free titanium alloys, including Ti–15Mo–5Zr–3Al and Ti–6Al–2Nb–1Ta–0.8Mo, for medical use wereinvestigated by the JIS20) (Japanese Industrial Standards)Committee and recognized as biomaterials for medical de-vices by the MHLW and the METI (Ministry of Economy,Trade and Industry).

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