GB2160892A

GB2160892A - Fe-Cr-A l type implant alloy for medical treatment

Info

Publication number: GB2160892A
Application number: GB08514297A
Authority: GB
Inventors: Eiji Tsuji; Hirobumi Okabe; Hideo Ike; Fumihiro Uratani; Takashi Ota; Hiroshi Kaneda; Yuko Hanatate; Tadashi Hamada; Hironobu Oonishi
Original assignee: Kyocera Corp; Osaka Prefecture
Current assignee: Kyocera Corp; Osaka Prefecture
Priority date: 1984-06-08
Filing date: 1985-06-06
Publication date: 1986-01-02
Also published as: GB2160892B; DE3520473A1; DE3520473C2; FR2565603A1; JPS60262943A; JPS6214213B2; FR2565603B1; GB8514297D0

Abstract

The alloy consists essentially of, by weight, 20 - 32% chromium, 0.5 - 5.0% aluminium, 0.5 - 4.0% molybdenum, 0.05 - 0.5% M (wherein M represents zirconium and/or hafnium), with the balance being substantially iron, wherein the alloy includes an oxide film on the surface, the film being composed substantially of alpha -A l 2O3. The oxide film is formed by heat treating the alloy in air or in oxygen at a temperature of 1100 DEG C - 1300 DEG C.

Description

SPECIFICATION Fe-Cr-At type implant alloy for medical treatment This invention relates to an Fe-Cr-At type implant alloy for medical tratment. The alloy of the invention is useful as an implant material for medical treatment such as orthopaedic surgery, dental surgery.

An implant material for orthopaedic surgery is required to have the following properties.

(1) the implant material should be resistant to dissolution and absorption; (2) the material should have a high corrosion resistance; (3) the material should exhibit high mechanical strength properties which should remain stable over extended periods of time; (4) the material should be non-poisonous and non-irritative; (5) the material should have good biocompatibility with the tissue of a living body around the material.

Namely, the material should be excellent in adaptability with the tissue or in affinity with the living body.

If the implant material is low in biocompatibility with the tissue, a fibrous tissue which blocks and isolates the material from coming into contact with the living body develops in the living body in contact with the implant material with the result that the connection between the material and the living body becomes loosened giving rise to various problems.

A Fe-Ni-Cr type austenite stainless steel is being used as a conventional implant material. This stainless steel is excellent in mechanical property but offers problems yet to be solved such as affinity with a living body. In addition, the stainless steel is not always sufficient in stress corrosion cracking resistance, pitting corrosion resistance, crevice corrosion resistance, and other corrosion resisting properties, so that the use of the stainless steel is limited to a short period of time. In addition, the harmful effects on the human body of the dissolved metallic ions, especially the nickel ion are a problem.

In recent years, trial use has been made of a ceramic material. This has many advantages in that it is excellent in corrosion resistance, stable in a living body over a long time, non-poisonous to the body and high in biocompatibility therewith. But it has the great disadvantage that it is deficient in mechanical strength, especially deflection resistance.

In view of the circumstances described above, the inventors have made extended researches and have finally completed a novel implant alloy having, in combination, the superior mechanical properties of a stainless steel material, excellent biocompatibility of a ceramic material with the human body, and high resistance to corrosion, and have completed a method of making the implant alloy.

That is to say, the alloy according to the invention relates to an Fe-Cr-At type implant alloy for use in medical treatment, the alloy consisting essentially of 20 - 32% by weight of chromium, 0.5 -5.0% by weight of aluminium, 0.5 -4.0% by weight of molybdenum, 0.05 - 0.5% by weight of M (wherein M represents zirconium and/or hafnium) and iron which forms the rest of the constituents.

Since the implant alloy of the invention contains aluminium and M in suitable amounts, it forms on the surface an oxide film composed substantially of (x- At203 dense and excellent in adherence by heating in the air or oxygen. This oxide film has a property excellent in biocompatibility with a living body and has excellent corrosion resistance. Furthermore, the implant alloy of the invention is not inferior in practical use to AISI 316 L conventionally used as an implant material for a living body nor to Fe-30 Cr-Mo type alloy, which is a basic alloy of the alloy of the invention, and has sufficient strength to be used as an implant material.

The invention will now be further described with reference to the accompanying drawing which is a graphic representation of the relation of the mass increased by oxidation and the thickness of oxide film with respect to heat treatment of the alloy of the invention.

The content and functions of the elements contained in the alloy of this invention are as follows; (1) Chromium : 20 -32% by weight Chromium is an indispensable element for improving the corrosion resistance of an iron-based alloy as a constituent for forming a passive film. With an increase in the amount of chromium, corrosion resistance increases. But because the alloy is synergistically embrittled in the combined presence of elements such as aluminium, molybdenum, the upper limit of chromium content is 32% by weight of the implant alloy. The alloy of the invention may find its application as an implant in vivo whose use extends over a short period of about two to toree months.In this case, the criterion of the alloy requisite for corrosion resistance is moderated in a certain degree, but corrosion resistance higher than that of high purity ferrite type stainless steel of the order of at least 18% chromium is required, so that the lower limit of the chromium content of the alloy is 20% by weight.

(2) aluminium: 0.5 -5.0% by weight Aluminium is an important element in forming an oxide film in the invention, but for improvement in the corrosion resistance of the alloy of the invention, it is necessary to add at ieast 1.5%, preferably about 3% by weight of molybdenum. And for this addition of molybdenum the content of aluminium is controlled to a maximum of 5.0% by weight On the other hand, when the aluminium content is small, a-A20a produced by heat treatment is decreased and the amount of Cr203 produced is increased. If the proportion of Cr2Q is increased, the boundary between two kinds of oxide is increased, with the result that the brittleness of oxide film on the surface of the alloy deteriorates to reduce the function of the alloy material.Accordingly, when heat treatment is performed in the atmosphere or under oxygen, a content of more than about 2% inclusive by weight of aluminium is preferred. Also, when heat treatment of the material is performed by regulating the atmosphere to a pressure lower than atmospheric pressure, it is possible to get the lower limit of aluminium content at 0.5% by weight. When the upper limit of aluminium content exceeds 5%, the alloy deteriorates in toughness and workability.

(3) Molybdenum : 0.5 -4.0% by weight Molybdenum has a marked effect in the improvement of corrosion resistance, especially pitting corrosion resistance, and crevice corrosion resistance, and an amount of more than 0.5% inclusive by weight of molybdenum is necessary for achieving this effect. On the other hand, an increase in the amount of molybdenum tends to lower the toughness and, in addition, promotes deterioration particularly in workability in the combined presence of chromium and aluminium. For this reason, the upper limit of molybdenum content is set at 4.0% by weight.

(4) M (wherein M represents zirconium and/or hafnium) : 0.05 -0.5% by weight M, in the implant material of the invention, infiltrates into the oxide film composed substantially of a At203, and imparts high toughness to the oxide film which is originally very brittle, and is somewhat higher in affinity with oxygen than aluminium, and accordingly the film is internally oxidixed to form fine oxide particles and thus improves adherence of the surface oxidized film to the alloy matrix. But when the amount of M content is increased, the degree of M mixed into the film is increased such that not only does the film deteriorate in density but also adverse effects on the corrosion resistance of the alloy matrix, cold and hot workability, and toughness of the alloy are produced.Accordingly, the upper limit of M content is 0.5% by weight On the other hand, when the M content is less than 0.05% by weight the alloy obtained does not exhibit any substantial improvement in the toughness of the film or in the adherence thereof.

(5) Others Silicon causes embrittlement of alloy and, when heat treatment is performed silicon is oxidized to SiO2 which is incorporated in the oc-At203 film, with the result that it is desirable to reduce the silicon content to less than 0.3% inclusive by weight. Carbon and nitrogen readily react with chromium under heat treatment to form a chromic compound. This chromic compound has the strong tendency of being formed in the grain boundary of the alloy, and brings about reduction of chromium density in the neighbourhood of the boundary and induces corrosion of the grain boundary. Furthermore, carbon also forms carbon monoxide and carbon dioxide gases which break the a-At203 film. From the above, it is clearly desirable to set the carbon content of the alloy at less than 0.008% inclusive by weight and nitrogen content at less than 0.015% inclusive by weight. Because phosphorus and sulphur also impair the toughness of steel, it is desirable to set those at less than 0.025% inclusive by weight respectively.

(6) The balance is substantially iron The alloy in the above range of compositions maintains a ferrite structure even under heating (1,100 1,300"C) which will be described later.

The method of making the implant alloy of the invention requires no particular restrictive steps but allows manufacture of the alloy in the range of compositions described above by conventional methods, namely by vacuum melting and as the case may be, by melting in a non-oxiding atmosphere.

The alloy of the invention, after being subjected to heat treatment is used as an implant material. This heat treatment provides an oxide film, the film being composed substantially of a-Ae,4 dense in structure and excellent in adherence to an alloy matrix. The heat treatment is carried out in the air or under oxygen normally at atmospheric pressure at a temperature of 1,100 to 1,300 C. But when the At content of the alloy is of the order of 0.5-2% by weight, it is desirable to carry out heat treatment at a pressure lower than atmospheric pressure.

The duration of heat treatment say from 0.5 to 30 hours is selected in accordance with the required thickness of the oxide film.

By the way, the composition of the oxide film is typically, for example, 90 mol % of oi-Ae,O,, 5 mol % of ZrO2, 3 mol % of Fe2O3 and 2 mol % of Cr2O3.

The implant alloy of the invention is excellent in biocompatibility with the living body and high in corrosion resistance because of the oxide film produced by heat treatment. Furthermore, the alloy matrix itself also shows superior corrosion resistance. The implant alloy of the invention is sufficient also in mechanical property for practical use as an implant material. Accordingly, the alloy of the invention satisfies the requisites for the implant material and can be used effectively.

The implant alloy of the invention will now be described by way of the following Examples.

Example 1 The Fe-Cr-At type implant alloy of composition shown in Table 1 was produced by vacuum melting.

TABLE 1 Elements Cr Ae Mo Zr Si C N O Fe % by weight 30 3.2 2.0 0.2 0.15 0.004 0.007 0.001 Bal The alloy was heat treated in oxygen to form an oxide film. The accompanying drawing shows graphically the relation of the mass increased by oxidation and the thickness of film produced with respect to heat treatment. A study of the sectional structure of the oxide film showed that the boundary between the alloy matrix and surface oxide film ran complicatedly into the matrix and the layer to provide excellent adherence. In the heat treatment in the air, the same result was obtained.

(i) Mechanical property Table 2 shows the mechanical properties which the alloy of the invention has before heat treatment.

For comparison, the mechanical properties of AISI 316 L used conventionally as an implant material in vivo is also shown in the table. Tensile strength and elongation were tested in accordance with JIS-Z2201 and hardness was tested in accordance with JlS-Z-2244.

TABLE 2 Example 1 AISI 316 L Density (g/cm2) 7.3 8.0 Magnetic property ferromagnetic nonmagnetic Tensile strength (Kg/cm2) 65 63 Hardness (HV) 250 170 Elongation (sic) 15 49 It emerges from the result shown in Table 2 that the alloy of the invention has the same order of mechanical strength as conventional products.

Shown in Table 3 is change in mechanical properties of the alloy of the invention when heat treated at a temperature of 1,250 C in the air.

TABLE 3 Heating time Tensile strength Hardness Elongation (hr) {kgiom2) (HV) (%) 1 60 230 15 3 58 220 10 5 55 220 10 20 51 210 10 From the result shown in Table 3, it emerges that prolongation of heating time makes little change in mechanical property of the alloy and provides no interference in practical use.

(ii) Corrosion resistance test It is apparent from experiment that the alloy has good corrosion resistance in the state of the alloy being coated with oxide film by heat treatment. Namely, there was no corrosion or wear observed at all by a corrosion resistance test on the items shown in Tables 4 and 5 to be later shown.

Consider the case wherein the oxide film of the alloy of the invention is damaged by mechanical impact or screw fastening and the alloy matrix is exposed into the corrosion environment of living body. A study was made of the corrosion of the alloy matrix under such conditions.

First, a test (of the crevice corrosion resistance) was conducted on the alloy in Example 1 before heat treatment. Namely, a flat test piece was immersed in an aqueous solution of 10% FeCt3 with a glass rod 5 mm in diameter placed on the test piece, and the corrosion of the test piece under the glass rod was observed after 24 hours.

The result is shown in Table 4.

TABLE 4 Liquid temperature rc) 20 40 50 60 Example 1 0 O A x AIS1316 L A x x x O: No crevice corrosion was observed x: Heavy crevice corrosion occurred A: Traces of crevice corrosion were observed Shown in Table 5 is the result of the test conducted on alkali resistance, grain boundary corrosion resistance, acid resistance, and stress corrosion resistance. The test was conducted in accordance with J IS-G-0573.

TABLE 5 Example 1 AISI 316L Alkali resistance ', 0.05 gim2 hr 3.39 g/m2 hr Grain boundary 2} 0.36 gim2 hr 0.24 g/m2 hr corrosion resistance Acid resistance 0.005 g/m2 hr 18.0 gim2 hr Stress corrosion 4i over 100 hrs below 10 hrs crack resistance (1) The amount of weight loss after immersion of the test piece in a boiling aqueous solution of 50% NaOH + 6% NaC for 48 hours.

(2) The amount of weight loss after immersion of the test piece in a boiling aqueous solution of 65% NHO2 for 48 hours.

(3) The amount of weight loss after immersion of the test piece in a boiling aqueous solution of 1% HC for 48 hours.

(4) A period of time required for the initiation of stress corrosion cracking after the immersion of a Ubend sample into a boiling aqueous solution of 48% MgCt2.

Furthermore, a test was conducted on the elution of the test piece into a physiological saline solution (3% NaCt) to find that the elution of each of Fe, Cr and At was less than 1 ppm/200 cm2/litre for 12 days at 20"C and that when the test piece was immersed in a boiling liquid for 5 hours, Fe was 2 ppm and the others were less than 1 ppm.

From the result above, it is apparent that the invention alloy was superior in corrosion resistance to conventional alloys even when it was placed in the state of being kept from forming an oxide film by heat treatment.

It was also found that the corrosion resistance of the alloy matrix after heat treatment is as much the same as before heat treatment. For example, after the alloy in Example 1 was heated at 1,250"C for 5 hours in the open air, the oxide film was completely removed by grinding and then tests were conducted on the items in Tables 4 and 5. No difference was noticed between the results obtained and those shown in Tables 4 and 5.

Example 2 The same test as that in Table 1 of Example 1 was conducted on the alloy in which zirconium was replaced with hafnium.

The mass increased by oxidation when heat treatment of the alloy in Example 2 was performed was smaller than that in Example 1. For example, when the alloy was heated at 1,200"C for 20 hours, the mass increased by oxidation was about 1 mg/cm2. The adherence of this oxide film was excellent to the same degree as that in Example 1.

A test was further conducted on the mechanical property and corrosion resistance of the alloy in the same manner as that in Example 1, to obtain much the same results as for the Hf containing alloy in Example 1. (Animal experiment result) Experiment plates each having a size 20 mm long, 7 mm wide and 1 mm thick were prepared from the alloy of the invention and an alloy of contrast example AlSI 316 L. The plates were transplanted into the shinbone surface of full- grown rabbits. The result shows that, in the oxide formed alloy of the invention, a connective tissue membrane intervening between the plate and the bone becomes thinner with the lapse of time such that it becomes hard to notice by the optical microoscope standard in four to six weeks. also, development of an osteoid substance is noticed around the transplanted plate with surface oxide. To the surface of the plate are fixed cartilage and osteoid tissue, and the amount of tissue fixed also increases and becomes dense in proportion to the length of transplantation time. The amount of the tissue fixed to the surface of AISI 316 L is less than that of bare surface of the invention alloy. The alloy of the invention after heat treatment is entirely free from detrimental dissolution of metallic ions in a living body and yet offers no problem from the viewpoint of strength of materials.

Claims

1. An Fe-Cr-At type implant alloy for medical treatment, said alloy composition of matter consisting essentially of, by weight: 20 -32% chromium, 0.5-5.0% aluminium, 0.5-4.0% molybdenum, 0.05-0.5% M (wherein M represents zirconium and/or hafnium), with the balance being substantially iron, wherein said alloy includes an oxide film on the surface, said film being composed substantially of a- At2O31.

2. An Fe-Cr-At type implant alloy for medical treatment, said alloy composition of matter consisting essentially of, by weight: 20 -32% chromium, 0.5-5.0% aluminium, 0.5-4.0% molybdenum, 0.05-0.5% M (wherein M represents zirconium and/or hafnium), ~0.3% silicon, =0.008% carbon, 0.015% nitrogen, with the balance being substantially iron, wherein said alloy includes an oxide film on the surface, said film being composed substantially of a-At2O2.

3. An Fe-Cr-At type implant alloy for medical treatment according to claim 2 wherein said alloy consists essentially of, by weight: 50.025% phosphorus, and 50.025% sulphur

4. A method of making an Fe-Cr-Ae implant alloy, said method comprising steps of: making an alloy consisting essentially of, by weight, 20 -32% chromium, 0.5 -5.0% aluminium, 0.5-4.0% molybdenum, 0.05 -0.5% M (wherein M represents zirconium and/or hafnium) with the balance being substantially iron; and heat treating said alloy in the air or in oxygen at a temperature of 1,100C -1,300 C to form an oxide film on the surface of the alloy, said film being composed substantially of cAt2Os.

5. A method according to claim 4 wherein, when said alloy contains 0.5 -2.0% aluminium by weight, melting is made at a pressure lower than atmospheric pressure.

6. A method according to claim 4 or 5 wherein said heating time is in the range of 0.5 -30 hours.

7. An Fe-Cr-At type implant alloy according to either one of the Examples hereinbefore.