METHOD FOR IMPROVING THE STRENGTH CHARACTERISTICS OF ORTHOPEDIC PROSTHESES

Abstract

A method for improving the strength characteristics of orthopedic prostheses, such as femur necks, femoral stems, cotyloid elements, systems of the spinal column, including the step of: starting from a prosthesis made of metal, subjecting the prosthesis to a peening treatment to improve the surface hardness of the prosthesis and eliminate micro-scoring caused by the machining of machine tools, the method further including a subsequent step of performing a surface finishing treatment by vibration finishing.

Description

TECHNICAL FIELD

The present invention relates to a method for improving the strength characteristics of orthopedic prostheses, such as cotyloid elements, femur necks, femoral stems, systems of the spinal column and the like.

BACKGROUND ART

As is known, orthopedic prostheses, such as femur necks, femoral stems, cotyloid elements and systems of the spinal column, as well as other prostheses, are currently made of metal, for example titanium, so as to be very light and tough.

However, such prostheses are often subject to breakage because they have parts that can be particularly stressed as a consequence of their implantation.

Moreover, such prostheses are provided by using machine tools that use cutting tools for machining and such cutting tools leave micro-scoring on the surface of the prosthesis which is apparently not visible to the naked eye but constitutes initiation points of fatigue failure cracks.

In particular, the micro-scoring that is present on the surface of such prostheses constitutes weak points that produce cracks that ultimately lead to failure of the prosthesis, which may occur at any time in an apparently inexplicable manner.

In order to improve the fatigue resistance of the prosthesis, it is possible to subject it to a peening treatment that allows compaction of the material, increasing its fatigue resistance, and elimination or in any case substantial reduction of the “dents” produced by machining, decreasing the risk of cracks and corrosion.

However, it should be noted that the peening treatment, although effective in itself, may entail an increase in roughness due to the protrusions and hollows that it generates.

Moreover, the peening treatment causes a variation of a few microns in the geometric dimensions of the treated part.

Moreover, the peening itself might create cracks if the parameters of the process are not calculated well as a function of the material used.

For example, titanium, the material usually used for prostheses, is subject to crack forming.

DISCLOSURE OF THE INVENTION

The aim of the present invention is to provide a method for improving the strength characteristics of prostheses and the like, substantially eliminating the likelihood that the micro-scoring caused by machining with machine tools might generate cracks that lead to the failure of the prosthesis.

Within this aim, an object of the present invention is to provide a method for improving the strength of prostheses and the like that can be used after the standard production step of the prosthesis, thus without changing substantially its production cycle.

Another object of the present invention is to provide a method for improving the strength characteristics of prostheses and the like that is highly reliable, relatively simple to provide and has competitive costs.

This aim and these and other objects that will become better apparent hereinafter are achieved by a method for improving the strength characteristics of orthopedic prostheses, such as femur necks, femoral stems, cotyloid elements, system of the spinal column and the like, comprising the step of:

starting from a prosthesis made of metal, subjecting said prosthesis to a peening treatment to improve the surface hardness of the prosthesis and eliminate micro-scoring caused by the machining of machine tools,

characterized in that it comprises a subsequent step of performing a surface finishing treatment by vibration finishing.

Further characteristics and advantages of the invention will become better apparent from the description of a preferred but not exclusive embodiment of the method according to the present invention.

The method according to the invention provides, starting from an orthopedic prosthesis, such as a femur neck, a femoral stein, a cotyloid element, a system of the spinal column or a similar prosthesis, for the application of a treatment adapted to eliminate substantially the micro-scoring that is present on the surface of the prosthesis due to machining performed with machine tools.

This treatment is thus aimed at eliminating the micro-scoring that might give rise to cracks that lead to failure of the prosthesis and at increasing the surface hardness of the metal.

The method provides for subjecting the prosthesis to a peening treatment, i.e., to a method by means of which the metallic surface of the prosthesis is struck (hammered), with a specific intensity, in order to harden the surface layer and at the same time eliminate the traces of the machining of cutting tools.

The peening treatment entails the use of shot that can be made of metal, ceramics, glass or other material. The size of the shot varies and in general its diameter is a fraction of a millimeter. The intensity that defines the force with which the shot “hammers” the surface to be treated is also variable and is defined according to the curvature of specimens of known thicknesses.

Conveniently, the diameters of the shot are comprised between 0.21 mm and 0.30 mm

The treatment is performed for example with a 200% coverage of the surface to be treated.

The intensity of the treatment is comprised for example between 4 and 6 Almen.

Therefore, the peening treatment to which orthopedic prostheses are subjected allows hardening of the metallic surface of the prosthesis and at the same time elimination of the micro-scoring that is the cause of cracks and thus failures of the prosthesis.

Moreover, the hardening of the external surface decreases “fretting corrosion”, a factor that also causes the initiation of cracks.

Moreover, the peening treatment, besides eliminating the machining traces of the tool and increasing the surface hardness of the prosthesis, allows the creation of a prestressed surface layer that makes it possible to reduce part of the traction stresses, the highest values of which are, as is known, concentrated at the surface. By reducing surface traction stresses one reduces the likelihood of initiation of fracture cracks and thus the likelihood of failure of the prosthesis.

The peening treatment may be performed both on titanium prostheses and on prostheses made of other materials.

After the peening treatment, the prosthesis is subjected to a surface finishing treatment provided by vibration finishing.

In particular, the treatment cited above provides for causing the collision, at different speeds, of the prostheses being worked with an abrasive body (generally defined as “medium”) and for repeating this type of collision for a sufficient number of times, so that the result obtained locally by the single collision event can become a general characteristic of the entire surface of the prosthesis.

The vibration finishing treatment is performed in appropriate tanks in which the prostheses to be treated and a necessary quantity of medium are placed. The quantities of medium are selected so as to optimize the number of collisions that will define the surface of the finished prosthesis.

In particular, the prostheses are arranged in a circular vibration machine with conical and double wedge-shaped polyester media that contain suitable abrasive fillers.

The machining has the goal of eliminating material from the surface of the prosthesis in order to reach the desired and necessary roughness value. Surface finishing thus makes it possible to buff the prosthesis previously subjected to peening, eliminating all the remaining roughnesses and indentations and at the same time also the residues of the craters generated by peening.

The surface finishing work is performed by a wet method, with a continuous input of water and of a suitable detergent. The duration of the process is about 16/19 hours.

Subsequently, the prostheses are subjected to a polishing treatment, by means of a circular vibration machine with porcelain media (nonabrasive) of spherical shape and having a diameter of for example 4 mm.

The purpose of the polishing step is to modify the roughness parameters of the surface of the prosthesis. This step, too, is performed by a wet method with the addition of a polishing product. The duration of this last step is about 2/3 hours.

In practice it has been found that the method according to the present invention fully achieves the intended aim and objects, since it allows improving the strength characteristics of prostheses of the known type with a treatment that is simple, economic and effective and at the same time it allows obtaining a prosthesis surface with optimum finishing.

The method thus conceived is susceptible of numerous modifications and variations, all of which are within the scope of the appended claims; all the details may further be replaced with other technically equivalent elements.

The disclosures in Italian Patent Application No. MI2010A001762 from which this application claims priority are incorporated herein by reference.

Claims

1-8. (canceled)

9. A method for improving the strength characteristics of orthopedic prostheses, such as femur necks, femoral stems, cotyloid elements, systems of the spinal column and the like, comprising the step of:

starting from a prosthesis made of metal, subjecting said prosthesis to a peening treatment to improve the surface hardness of the prosthesis and eliminate micro-scoring caused by the machining of machine tools,

further comprising a subsequent step of performing a surface finishing treatment by vibration finishing.

10. The method according to claim 9, wherein said vibration finishing treatment is performed by subjecting said prosthesis to repeated collisions with abrasive means.

11. The method according to claim 10, wherein said abrasive means are polyester means.

12. The method according to claim 9, further comprising a subsequent polishing step.

13. The method according to claim 12, wherein said polishing step is provided by way of nonabrasive porcelain means.

14. The method according to claim 12, wherein said polishing step is provided by means of a circular vibration machine.

15. The method according to claim 9, wherein said prosthesis is made of titanium.

16. The method according to claim 9, wherein said peening treatment is provided by means of shot selected from metal, ceramics, glass or other material.