Joint prosthesis made from a titanium alloy

Abstract

The invention relates to a joint prosthesis having a shaft made from a titanium alloy, in which at least the shaft is investment cast and has a body-centered cubic crystal structure. A titanium alloy having this crystal structure (known as -titanium alloy) has an advantageously low modulus of elasticity which is well matched to the physiological demands of joint prostheses. Furthermore, implementation as a shaped casting allows a complex shape to be achieved. It is particularly embodied as a femoral prosthesis for an artificial hip joint, which has an elongate shaft with grooves and sawtooth-like projections for bone anchoring.

Claims

1. A joint prosthesis, comprising: a shaft consisting essentially of a -titanium alloy, at least the shaft being produced by an investment casting process and the -titanium alloy of the shaft having a body centered cubic crystal structure and having a modulus of elasticity between 59.4 kN/mm.sup.2 and 68 kN/mm.sup.2, wherein the shaft is shaped for implantation into a medullary cavity of a human femur, wherein the -titanium alloy consists of titanium and molybdenum, and wherein the -titanium alloy has a molybdenum content in the range between 12% and 16% by weight of the alloy.

2. The joint prosthesis of claim 1, wherein the joint prosthesis is configured in a shape of a femoral prosthesis.

3. The joint prosthesis of claim 1, wherein the joint prosthesis is configured in a shape of a knee prosthesis.

4. The joint prosthesis of claim 1, wherein the shaft is subjected during production to hot isostatic pressing and solution annealing.

5. The joint prosthesis of claim 4, wherein the hot isostatic pressing is performed at a temperature which is at most equal to a beta transus temperature of the -titanium alloy and is not less than a temperature that is 100 C. below the beta transus temperature.

6. The joint prosthesis of claim 4, wherein the hot isostatic pressing is performed at a temperature that is at most a beta transus temperature of the -titanium alloy and is not less than a temperature that is 40 C. below the beta transus temperature.

7. The joint prosthesis of claim 1, wherein the body centered cubic crystal structure has a mean grain size of at least 0.5 mm.

8. The joint prosthesis of claim 1, wherein the body centered cubic crystal structure has a mean grain size of at least 0.3 mm.

9. The joint prosthesis of claim 1, wherein the molybdenum content is 15% by weight of the alloy.

Description

BRIEF DESCRIPTION OF THE DRAWINGS

(1) The invention is explained in more detail below with reference to the drawing, which illustrates an advantageous exemplary embodiment and in which:

(2) FIG. 1 shows a diagrammatic view of a first exemplary embodiment of a joint prosthesis according to the invention;

(3) FIG. 2 shows a diagrammatic view of a further exemplary embodiment of a joint prosthesis according to the invention;

(4) FIG. 3 shows an image of the crystal structure immediately after the investment casting (magnified 1000 times);

(5) FIG. 4 shows an image of the crystal structure after hot isostatic pressing and solution annealing; and

(6) FIG. 5 shows a table giving mechanical properties of the prosthesis according to the invention.

DETAILED DESCRIPTION OF THE INVENTION

(7) The exemplary embodiment illustrated in FIG. 1 shows a femoral prosthesis for an artificial hip joint. The femoral prosthesis 1 consists of a n-titanium alloy, namely TiMo15. This alloy has a body-centered cubic crystal structure at room temperature.

(8) The femoral prosthesis 1 is intended for implantation at the upper end of the femur. It can interact with an acetabulum component 2 which has been implanted in the pelvic bone. The femoral prosthesis 1 has an elongate shaft 10 as bone anchoring element and a neck 11 which adjoins it at an obtuse angle. At its end remote from the shaft there is arranged a joint head 12 which, together with a bearing insert 22 of the acetabulum component 2, forms a ball joint. Implantation involves complete or partial resection of the head of the thighbone neck, opening up access to the medullary cavity of the femur. This access is used to introduce the shaft 10 of the femoral prosthesis 1 into the medullary cavity, where it is anchored. Depending on the particular embodiment, cement is provided as anchoring means or the fixing is effected without the use of cement.

(9) The femoral prosthesis 1 introduces mechanical loads acting on the hip joint, whether static loads when standing or dynamic loads when walking, into the femur. Physiologically compatible transmission of loads is important for permanent reliable anchoring of the femoral prosthesis 1 in the bone material of the femur. If the femoral prosthesis 1 is of very rigid design, it absorbs a considerable portion of the load, thereby relieving the load on the bone material in particular in the upper region of the femur. In the longer term, this leads to degeneration of the femur in this region. This leads to the risk of the femoral prosthesis 1 coming loose and ultimately of the prosthesis failing. To prevent this failure mode, it is known per se for the femoral prosthesis 1 to be of less rigid, i.e. more elastic with a physiologically favorable low modulus of elasticity design. In particular the shaft 10 of the femoral prosthesis 1 is critical in this respect. In the cortical region, the bone material of the femur has a modulus of elasticity of approx. 20 000 to 25 000 N/mm.sup.2. According to the invention, the femoral prosthesis 1 has a modulus of elasticity of approx. 60 000 N/mm.sup.2. This is a favorable modulus which is much lower than that of materials which are conventionally used, such as TiAl6V4. These materials have a modulus of elasticity of approx. 100 000 N/mm.sup.2 or even 200 000 N/mm.sup.2 in the case of cobalt-chromium alloys.

(10) The invention allows simple production of even complex shapes by investment casting. For example, the femoral prosthesis 1 has a multiplicity of recesses and sawtooth-like projections on its shaft 10. These are used to improve anchoring of the femoral prosthesis 1 in the femur, allowing cement-free implantation. A plurality of grooves 14 are provided running in the longitudinal direction of the shaft 10. They are arranged on both the anterior and posterior side of the shaft 10 but may also be provided on the lateral sides. A plurality of rows of sawtooth projections 15 are provided in the upper region of the shaft 10. Furthermore, an encircling ring 13 is provided at the transition to the neck 11. It can be designed as a separate element, but the invention means that it may also be integral with the shaft 10 and neck 11. In general, a single-piece design of the prosthesis is preferred, with the exception of exchangeable or optional attachment parts or wearing parts. Furthermore, a fixing projection 16 is provided on the shaft 10 adjacent to the ring 13 to prevent rotation. Such complex shapes of joint prostheses can conventionally only be produced from TiAl6V4. However, this material has a different, less favorable crystal structure and therefore an undesirably high modulus of elasticity.

(11) The invention can advantageously also be used for other types of joint prostheses. FIG. 2 illustrates a knee prosthesis 3 as a further exemplary embodiment. It comprises a femur component 31 and a tibia component 30. The femur component 31 has a long shaft 33 as bone anchoring element. It is designed for implantation in the medullary cavity of the femur, which has been opened up by section of the natural knee joint. As in the case of the femoral prosthesis, in this case too the problem of degeneration of the surrounding cortical structure occurs if the knee prosthesis 3, in particular its shaft 33, is made too rigid. The same applies to a shaft 32 of the tibia component 30.

(12) The joint prosthesis according to the invention can also be used for other joints, for example at the elbow or the shoulder.

(13) The text which follows describes a way of carrying out the invention.

(14) The starting material is a -titanium alloy with a molybdenum content of 15% (TiMo15). This alloy is commercially available in the form of billets (ingots).

(15) A first step involves investment casting of the parts of the hip prosthesis. A casting installation is provided for the purpose of melting and casting the TiMo15. The casting installation is preferably a cold-wall crucible vacuum induction melting and casting installation. An installation of this type can reach the high temperatures which are required for reliable melting of TiMol5 for investment casting. The melting point of TiMo15 is 1770 C. plus a supplement of approx. 60 C. for reliable investment casting. Overall, therefore, a temperature of 1830 C. needs to be reached. The investment casting of the melt is then carried out by means of processes which are known per se, for example using ceramic molds as lost mold. Investment casting techniques of this type are known for the investment casting of TiAl6V4. The result is a body-centered cubic crystal structure. An image of the microstructure is illustrated in FIG. 3.

(16) The castings, from which the casting molds have been removed after the investment casting, are subjected to a heat treatment. This involves hot isostatic pressing (HIP) at a temperature just below the -transus temperature. This temperature may be in the range from 710 C. to 760 C. and is preferably approximately 740 C. at an argon pressure of 1100 to 1200 bar. During this treatment, inter alia undesirable precipitations in inter-dendritic zones are dissolved. It is expedient first of all for a surface zone which may have formed during casting in the form of a hard, brittle layer (known as the -case) to be removed by pickling. This layer is usually approx. 0.03 mm thick.

(17) Following the hot-isostatic pressing, the castings have only a low ductility. It is assumed that this embrittlement is attributable to secondary precipitations during the hot isostatic pressing and the subsequent, generally slow cooling from the hot isostatic pressing temperature.

(18) To dissolve these disruptive precipitations, the castings are annealed in a chamber furnace under argon shielding gas atmosphere. A temperature range from approx. 780 C. to 860 C. for a duration of several hours, generally two hours, is selected for this purpose. In this context, there is a reciprocal relationship between the temperature and the duration; a shorter time is sufficient at higher temperatures, and vice versa. After the solution annealing, the castings are quenched using cold water. The resulting microstructure is illustrated in FIG. 4.

(19) The mechanical properties achieved after solution annealing are reproduced in the table shown in FIG. 5.

(20) It can be seen that the modulus of elasticity drops as the temperature rises during the solution annealing, specifically from 68 000 N/mm.sup.2 down to levels of as low as 59 400 N/mm.sup.2. The ductility values improve with decreasing strength and hardness. For example, after solution annealing for two hours at 800 C., the result is a modulus of elasticity of approx. 60 000 N/mm.sup.2 with an elongation at break of approx. 40% and a fracture strength Rm of approx. 730 N/mm.sup.2.