AU7202500A - Prosthetic fixture or osteosynthesis fixture and method for producing such a fixture - Google Patents

Description

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AUSTRALIA

PATENTS ACT 1990 COMPLETE SPECIFICATION FOR A STANDARD PATENT

ORIGINAL

Name of Applicant: Marie-Corinne Nesme-Gauthier Actual Inventor: Marie-Corinne Nesme-Gauthier Address for Service: BALDWIN SHELSTON WATERS 60 MARGARET STREET SYDNEY NSW 2000 Invention Title: 'PROSTHETIC FIXTURE OR OSTEOSYNTHESIS FIXTURE AND METHOD FOR PRODUCING SUCH A FIXTURE' The following statement is a full description of this invention, including the best method of performing it known to me/us:- File: 30195AUP00 1A The invention relates to a prosthetic fixture or osteosynthesis fixture comprising a rigid component, in particular of metal, covered by a layer of a bioreactive material, and to a method for producing such a fixture.

In the field of prostheses, in particular hip prostheses, it has long been known to cover the stem of a femoral prosthesis, usually made of titanium alloy, with a bioreactive layer, the most commonly used materials being calcium hydroxyapatite or a mixture of hydroxyapatite and tricalcium phosphate. This is because covering the metal fixture with bioactive material permits rapid and reliable anchoring of the prosthesis to the bone.

Furthermore, it is known, in particular from FR-A-2,706,280 or EP-A-0,532,421, to produce an osteosynthesis fixture starting from a metal component which is covered by a biocompatible material, such as calcium hydroxyapatite or a calcium phosphate. The use 20 of such a biocompatible material permits rapid mechanical anchoring between the fixture and the various bone elements to be joined.

After bone fusion, a very stable fixation between bone and osteosynthesis fixture has the following disadvantages: the "mechanical shunt" thereby obtained disturbs the bone structure; the term "spongialization" has been used. Moreover, if the osteosynthesis fixture has to be removed, the difficulty of withdrawing it without damaging the bone is considerable.

In the field of prostheses, for example with regard to the femoral stem of total hip prostheses, it is accepted that the proximal part of the stem must be securely fixed to the spongy bone of the upper end of the femur. It has been demonstrated that rigid fixation of the distal part of the stem on the diaphyseal cortical bone leads later to secondary movements.

The secondary disadvantages which may be mentioned include painful and hypertrophic reactions at 2 the end of the stem. The disadvantages of later onset which may be mentioned include osteolysis of the proximal femur, in particular of the calcar femorale.

According to a conventional technique, it is known to apply calcium hydroxyapatite by means of a plasma torch. The outer surface of the hydroxyapatite layer deposited in this way is irregular, which does not cause any problem when fitting a prosthetic element by wedging two frustoconical or widened bearing surfaces one against the other. Likewise, an osteosynthesis plate covered by rough hydroxyapatite is easily applied to the outer surface of a bone.

By contrast, in the case of a fixture of generally cylindrical or parallelepipedal shape having to be introduced into a generally cylindrical or parallelepipedal seat or canal, the frictional forces between the outer surface of the layer of bioreactive material and the inner surface of the canal or seat are such that this layer of bioreactive material is 20 detached or dislodged from the rigid component by shearing. It is then no longer possible to obtain rapid anchoring of the fixture with respect to the adjacent bone structure, the bioreactive material then being ineffective. If the inner dimensions of the seat are 25 increased to prevent this detachment, the bioreactive material is no longer in tight contact with the bone and no longer plays its part: anchoring is no longer possible.

It is these disadvantages in particular which the invention is intended to remedy by providing a fixture which is capable of being introduced into a seat, of a shape corresponding to that of this fixture, without the layer of bioreactive material being detached from the rigid component.

With this end in mind, the invention relates to a prosthetic fixture or osteosynthesis fixture comprising a rigid component, in particular of metal, at least partially covered by a layer of bioreactive material, in particular based on calcium hydroxyapatite 3 or tricalcium phosphate, wherein the outer surface of this layer is at least partially smoothed, in such a way that it has no roughness extending beyond an envelope surface, and wherein this outer surface is provided with micro-cavities formed within this envelope surface.

By virtue of the invention, the smoothing of the outer surface of the layer of bioreactive material prevents roughnesses from interacting with the adjacent bones to the point of generating frictions capable of dislodging the layer of bioreactive material from the rigid component during the movement of penetration of the fixture into the bone. The micro-cavities which remain after this smoothing permit effective anchoring of the bioreactive material in the adjacent bone :structure. In other words, the smoothed character of S: the layer of bioreactive material ensures that it does not interfere with the introduction of the fixture, while the micro-cavities which it has inside the 20 envelope surface allow it to accelerate fixation of the fixture in the bone structure.

SAccording to advantageous features of the invention, the fixture incorporates one or more of the following characteristics: 25 the micro-cavities have a maximum length of less than 400 micrometers, preferably of the order of 200 micrometers, with a minimum width of greater than micrometers, preferably of the order of *aa.

micrometers; the surface of the rigid component covered by bioreactive material is smooth. In particular, the micro-cavities of this surface have dimensions less than or equal to those mentioned above.

According to alternative embodiments of the invention, the rigid component is generally cylindrical or at least partially plane.

The invention also relates to a method for producing a fixture as described above, and, more precisely, a method comprising a stage of application 4 of a layer of bioreactive material on a rigid component, in particular of metal, said method consisting in at least part of the outer surface of the layer of bioreactive material being subjected to a smoothing treatment, by machining, levelling ,or grinding of its roughnesses, thus creating an envelope surface within which micro-cavities remain in this surface.

According to an advantageous feature of the invention, the method also consists in subjecting the surface of the rigid component to a smoothing treatment prior to application of the layer of bioreactive material. This makes it possible to control the level of adhesion of the layer of bioreactive material to the rigid component. In particular, it is possible to leave, within the outer surface of the rigid component, :micro-cavities of length and width less than or equal to those of the micro-cavities of the outer surface of the layer of bioreactive material. These interface micro-cavities preferably have at least one dimension of less than 20 micrometers, in order to prevent or Slimit osseous regrowth after disappearance of the bioreactive layer.

The invention lastly relates to a prosthetic

S.

S. 25 fixture or osteosynthesis fixture manufactured according to the method described above.

The invention will be better understood and other advantages thereof will become more apparent from ethe following description of two embodiments of a fixture according to the invention, given solely by way of example and with reference to the attached drawings, in which: Figure 1 is a side view of an osteosynthesis pin according to the invention; Figure 2 is a view of detail II in Figure i, on a larger scale, during a first stage of production of the pin in Figure 1; Figure 3 is a view similar to Figure 2 in terms of production of the pin; Figure 4 is a cross section along the line IV-IV in Figure 3; Figure 5 is a side view of an implanted femoral prosthesis, according to a second embodiment of the invention, and Figure 6 is a partial cross section, on a larger scale, along the line VI-VI in Figure The pin 1 represented in Figure 1 comprises a core 2 made of titanium and forming a generally cylindrical rod 2a and a head 2b. The rod 2a is covered by a layer 3 of calcium hydroxyapatite deposited by means of a plasma torch.

As will be seen more clearly from Figure 2, after deposition of the layer 3, the outer surface 3a thereof comprises roughnesses 3b and micro-cavities 3c.

This surface 3a is thus irregular.

In practice, the layer 3 has a thickness between a minimum value el of the order of oo micrometers and a maximum value e 2 of the order of 200 20 micrometers.

According to the invention, the outer surface ooo.

3a of the layer 3 is smoothed in such a way that it does not extend beyond a cylindrical surface S centered on the axis X-X' of the rod 2a, which has the radius R.

25 The surface 3a can be smoothed by machining, in I particular by grinding. Thus, the roughnesses 3b are leveled off to the extent that their parts extending beyond the outside of the surface S are eliminated.

By contrast, the micro-cavities 3c are left in their parts situated within the surface S with respect to the rod 2a, that is to say inside the envelope formed by this surface. Thus, the surface 3a is calibrated to the side of the surface S leaving the micro-cavities 3c.

On completion of the machining operation, the layer 3 has the profile seen in Figures 3 and 4, in which only the micro-cavities 3c constitute reliefs, hollowed out, in the surface 3a.

-6- The length 1 of the micro-cavities 3c is less than 400 micrometers, while their width is greater than micrometers, such that their total surface area is suitable for attachment of the layer 3 to the bone. The length of the micro-cavities can be chosen in the order of 200 micrometers and their width can be chosen in the order of 20 micrometers.

The micro-cavities 3c occupy less than 20% of the surface 3a, such that the surfaces 3a and S coincide over their greater parts.

According to an advantageous feature of the invention, the outer surface 2c of the rod 2a is smoothed before application of the layer 3. In practice, this smoothing, which is done by machining or physico-chemical surface treatment, leaves microcavities 2d of length and width smaller than or equal to those of the micro-cavities 3c. This therefore go prevents a rigid secondary anchoring between the bone and the metal, which fact promotes the withdrawal of 20 the core 2 on completion of osteosynthesis. By virtue of the invention, the low adhesion force between the ".layer 3 and the rod 2a does not cause any problem during introduction, for example impaction, of the pin 1 into a cylindrical seat 4 of radius substantially 25 equal to the radius R, formed in a bone mass 5. By contrast, this low adhesion force permits removal of the osteosynthesis fixture.

The micro-cavities 2d are formed in greater number than the micro-cavities 3c, whereas they have smaller dimensions. In practice, these micro-cavities 2d are the vestiges of surface irregularities of the rod 2a, these irregularities being attenuated by the smoothing treatment. The rod 2a can have a "fissured" or "fibrous" outer appearance originating from the nature of its surface layer, which is of titanium.

The method of production and smoothing of the rod 2 is chosen in such a way that the width of the micro-cavities 2d is less than 20 micrometers in order to prevent or limit osseous regrowth in the rod after 7 disappearance of the layer 3. This makes it possible subsequently to remove the pin 1 without destroying the bone.

In the second embodiment of the invention shown in Figure 5, a femoral prosthesis 51 comprises a stem 52 intended to be introduced into the medullary canal 54 of a femur 55. A layer 53 of tricalcium phosphate is applied over the entire surface of the stem 52. This layer is kept irregular on the widened proximal portion 52a of the stem 52, in accordance with normal practice.

By contrast, and in accordance with the invention, the outer surface 53a of this layer 53 is smoothed by a mechanical process, in particular by grinding or machining, around a cylindrical distal part 52b of the stem 52, in order to remove the roughnesses resulting from the application of the tricalcium phosphate on the part 52a of the stem 52. The surface 53a is corrected to the geometry of an envelope surface S which, in this example, has the shape of a cylinder with circular 20 base. Micro-cavities 53c remain on the surface 53a, within the surface S, and permit rapid anchoring of the prosthesis 51 in the femur. The micro-cavities have dimensions of the same order of size as the cavities 3c of the first embodiment. As before, they occupy less 25 than 20% of the surface 53a.

eo As before, the outer surface 52c of the part 52b of the stem 52 is itself smoothed before oo application of the layer 53. Micro-cavities 52d are formed, in greater number and with smaller dimensions than those of the micro-cavities 53c, on the surface 52c. These micro-cavities have a width of less than micrometers.

As the micro-cavities 52d of this surface 52c have dimensions less than or equal to those of the cavities 53c, the distal part of the stem 52 can be fitted without detachment of the layer 53 and permits anchoring between the bone and the bioreactive material. Secondarily, and at a later stage, the smoothing of the metal prevents definitive rigid -8anchoring of bone and metal after at least partial disappearance of the layer 53. The problems of mechanical shunt encountered in the prior art do not therefore apply.

According to an alternative embodiment of the invention which is not illustrated, the surface of the rigid component of the prosthetic element or osteosynthesis element on which the layer of bioreactive material is applied can be generally plane, or can even assume any shape appropriate to the required function. The shape of the envelope surface, that is to say the final outer surface of the layer of bioreactive material, is adapted in consequence.

Irrespective of the embodiment in question, the fact that the outer surface of the layer of calcium hydroxyapatite or of an equivalent material is calibrated and smoothed makes it easy to fit the prosthetic fixture or osteosynthesis fixture without danger of detachment of this layer, while the smooth 20 character of the surface of the rigid component prevents excessive long-lasting stresses between the rigid component and the bone, in particular during removal of an osteosynthesis fixture or in order to prevent a mechanical shunt being effected by part of 25 the rigid component in the case of a prosthesis.

The invention is applicable irrespective of the embodiment of the layer of bioreactive material on the rigid component. The invention is applicable independently of the exact nature of the material constituting the rigid component. However, its main application is with metal components, in particular those made of titanium or a titanium-based alloy.

Claims (12)

1. A prosthetic fixture or an osteosynthesis fixture comprising a rigid component, in particular of metal, at least partially covered by a layer of a bioreactive material, in particular based on calcium hydroxyapatite or tricalcium phosphate, wherein the outer surface of said layer is at least partially smoothed, in such a way that it has no roughness extending beyond an envelope surface, and wherein said outer surface is provided with micro-cavities formed within said envelope surface.

2. The fixture as claimed in claim 1, wherein said micro-cavities have a maximum length of less than 400 micrometers, preferably of the order of 200 micrometers, with a minimum width of greater than 10 micrometers, preferably of the order of S•micrometers.

3. The fixture as claimed in any of claims 1 or 2, wherein the surface of said rigid component covered by the layer of bioreactive material is smooth.

4. The fixture as claimed in claim 3, wherein said surface of said rigid component has micro-cavities of length and width less than or equal to those of the micro-cavities of the outer surface of the layer of bioreactive material.

The fixture as claimed in any of the preceeding claims, wherein said rigid component is generally cylindrical.

6. The fixture as claimed in any of the preceeding claims, wherein said rigid component is at least partially plane.

7. A method for producing a prosthetic fixture or an osteosynthesis fixture, comprising a stage of application of a layer of bioreactive material on a rigid component, in particular of metal, said method consisting in at least part of the outer surface of said layer of bioreactive material being subjected to a smoothing treatment, by machining, levelling or grinding of its roughnesses, thus creating an envelope surface within which micro-cavities remain in said surface.

8. The method as claimed in claim 7, said method consisting in subjecting the surface of said rigid component to a smoothing treatment prior to application of said layer of bioreactive material.

9. The method as claimed in claim 8, wherein said smoothing treatment leaves, within said surface of said rigid component, micro-cavities of length and width less than or equal to those of the micro-cavities of the outer surface of said layer of bioreactive material.

10. The fixture as claimed in claim 4 or the method as claimed in claim 9, wherein said micro-cavities of said surface of said rigid component have at least one dimension of less than 20 micrometers. 6.

11. A prosthetic fixture or an osteosynthesis fixture substantially as herein described with reference to any one of the embodiments of the invention illustrated in the 15 accompanying drawings.

12. A method for producing a prosthetic fixture or an osteosynthesis fixture 9* substantially as herein described with reference to any one of the embodiments of the invention illustrated in the accompanying drawings. DATED this 4th day of December 2000 MARIE-CORINNE NESME-GAUTHIER Attorney: CAROLINE M. BOMMER Fellow Institute of Patent and Trade Mark Attorneys of Australia of BALDWIN SHELSTON WATERS