FR2685192A1 - Metal acetabulum for coxofemoral joint prosthesis - Google Patents

Abstract

Metal acetabulum for coxofemoral joint prosthesis, including a hollow shell made of titanium alloy in a hemispherical general shape, with a diameter lying between 45 and 65 mm, the inner face of which is intended to receive a prosthetic acetabulum, and characterised in that the outer face of the said shell (2) is lined with several layers of titanium mesh (20) fixed on to the said shell by rivets (21), and in that it has two holes (23) passing entirely through it, both located at an angle lying between 35@ and 40@ with respect to the pole of the said hemispherical shell, the distance between whose centres lies between 10 mm and 35 mm along the diameter of the said shell, the said two holes being intended to allow fixation of the said acetabulum (1) into the ilium using screws.

Description

Metal cup for coxo-femoral joint prosthesis
The present invention relates to a metallic cup for a hip joint prosthesis.

 In general, the fixation of osteoarticular implants with a cement of the polymethyl methacrylate type, the only cement available today, poses a certain number of problems: - Exothermic polymerization which leads to more or less significant tissue necrosis.

- Release of the monomer in the cardiovascular system generating pressure drops.

- Release of cement particles causing tissue reactions.

- Resistance to fatigue quite low causing ruptures of the cement case with a risk of loosening.

- Difficulties in removing this cement during a reintervention.

 All of these problems have led surgeons and researchers to develop solutions for fixation by bone regrowth on the implant. These solutions imply perfect mechanical blockage of the implant during the first post-operative weeks. To achieve this result, it has already been proposed to implement: - Implants with simple corrugations of variable, longitudinal, circular shapes, in which the bone is supposed to regrow.

- Implants with external macropores, obtained by microfusion technology.

- Implants on which surface treatments have been carried out, for example sanding, in order to obtain a high surface roughness and therefore bone bonding.

- Implants onto which plasma or titanium, cobalt-chromium beads or particles have been sprayed, which create either a porous structure or a surface roughness, depending on the thickness deposited.

- Implants that have been coated with a plasma torch with calcium phosphates, such as hydroxyapatite.

- Implants on which metallic textures have been fixed by mechanical-thermal treatments.

 All these solutions pose significant problems with regard to the long-term behavior of the metal alloy of the implant: - The macro and microporosities obtained by microfusion or sandblasting can lead to fracture primers by corrosion and fatigue-corrosion.

- The coatings by plasma torch of titanium, cobaltchrome, calcium phosphates, have only a purely mechanical adhesion. They can be the object of flaking causing a release of particles with its multiple consequences at the level of friction components, tissue reactions bringing about osteolysis of the periprosthetic bone.

- The fixing of metallic textures by mechanothermal connection can bring about modifications of the crystal structure of the metallic alloy of the implant and a reduction of its mechanical characteristics.

 In all these solutions, the porous surfaces have a relatively small thickness which makes it possible to obtain only a simple apposition of bone. The interface thus created does not meet the specifications of the cementless fixing, and is not suitable for solving the problem of the behavior of the implant at the various mechanical stresses which it undergoes in vivo.

 In the case of the present invention aimed very particularly at coxofemoral articulation prostheses, it is recalled that such a prosthesis comprises a prosthetic femoral head and a prosthetic acetabulum, the latter being fixed in a metallic acetabular cup or a metal alloy.

 The object of the present invention is to produce a metal acetabulum intended to be fixed in the acetabular cavity and creating with it an interface with high mechanical performance.

 The present invention relates to a metal cup for coxofemoral joint prosthesis comprising a hollow shell of titanium alloy of generally hemispherical shape, of diameter between 45 and 65 mm, the internal face of which is intended to receive a prosthetic cup , and characterized by the fact that the external face of said shell is coated with several layers of titanium lattice fixed to said shell by rivets, and by the fact that it has two holes passing right through it, located every two at an angle between 35 and 40e with respect to the pole of said hemispherical shell, the distance of the centers of which is between 10 mm and 35 mm depending on the diameter of said shell, said two holes being intended to allow said acetabulum to be fixed in the ilium via screws.

 The fixing is carried out using titanium screws.

The angular arrangement of the holes allows easy placement of the screws which will play an important role in the mechanical locking of the acetabulum during the operation and during the short-term post-operative period. The blockage is then relayed by the bone regrowth in the titanium lattice covering the external face of the shell.

 It can be seen that the two aforementioned screws are sufficient to avoid any displacement of the rotating acetabulum.

 The presence of only two holes intended to receive screws makes it possible: - to obtain a larger operational surface area of the lattice and thereby a greater bone regrowth, - to increase the outside diameter of the two rivets associated with these holes, d 'where better fixing of the mesh under these rivets, - to avoid mechanical embrittlement of the shell itself. Indeed, the other holes which will not receive the screws may have a much smaller diameter, - to use screws with an outside diameter of the order of 6.5 mm which are particularly well suited to make the holding more reliable in the cancellous bone.

 More holes would necessarily have a smaller diameter, therefore associated with weaker screws; we would therefore obtain poorer resistance to the bone-screw interface and poorer mechanical resistance to the screw head.

 The use of rivets to achieve mechanical fixing avoids any deterioration of the shell, unlike connections based on thermomechanical treatments.

 The titanium alloy of said shell is preferably the TiA16V4 alloy, an alloy which allows excellent mechanical resistance of the acetabulum.

 The titanium of the lattice is preferably an unalloyed titanium. This mesh has pores of 250 to 500 microns, with interconnections between them, and a porosity of the order of 70% to 80%. The layers of mesh form a coating with a thickness of approximately 2 mm on the external face of the shell.

 All of the above characteristics allow an all-round vascularization along the entire thickness of the coating.

 Other characteristics and advantages of the present invention will appear during the following description of an embodiment given by way of illustration but in no way limitative. In the accompanying drawing: - Figure 1 is a diagram of an iliac bone on which is fixed a metal cup according to the invention.

- Figure 2 is a top view of the cup of Figure 1.

- Figure 3 is a sectional view of the previous cup, along the line A-A of Figure 2.

 FIG. 1 shows a metal cup 1 according to the invention fixed with the aid of two screws 3 and 4.

 The implant is fixed in the acetabular cavity with an inclination of 40 relative to the horizontal plane. The two screws are housed in the ilium 11 and will allow excellent primary fixation.

 As shown in Figures 1 and 2 the cup 1 comprises a hollow shell 2 of substantially hemispherical titanium alloy. At its equatorial diameter 11, it has notches 12 intended to receive the anti-rotation studs of a polyethylene core not shown. Different sizes are provided for the shell 2 (between 45 and 65 mm approximately); its thickness is of the order of 3 mm.

The external face of the shell 2 is covered with several layers of titanium mesh 20 constituting a coating with a thickness of approximately 2 mm. It is a flexible trellis, which forms diamonds with diagonals of approximately 4 mm and 2 mm; the trellises can be arranged parallel to each other or crossed at 90 degrees. The porosity of the coating obtained is between 70% to 80%, with a structure of interconnected pores with a diameter between 250 and 500 microns
The trellis layers 20 are immobilized by a set of rivets 21, associated with holes 22; one is arranged in the pale of the shell 2, as can be seen in FIGS. 1 and 2 (the polar axis is referenced 13). A lateral metal strip 30 immobilizes the trellis along the equatorial diameter 11.

 According to the invention, two holes 23 are provided with rivets 24 to allow passage for the screws 3 and 4 (see FIG. 1), of preferred diameter 6.5 mm.

 The axes of the holes 23 referenced 25 are located at 400 relative to the polar axis 13; the spacing of their center is of the order of 10 mm for small acetabular cups (diameter 46 mm) and can range up to 20 to 25 mm for larger ones (diameter 64 mm).

 The two rivets 24 also contribute to the fixing of the mesh sheets 20.

 The overall diameter of the cup 1 is preferably produced by 0.2 mm to 0.5 mm greater than the nominal diameter. Given the use by the surgeon of a cutter with a diameter less than 2 mm than the diameter of the acetabulum chosen, the latter allows excellent intraoperative blockage.

 Of course, the invention is not limited to the embodiment which has just been described. Without going beyond the ambit of the invention, it will be possible to replace any means with equivalent means.

Claims (7)

1 / Metal cup for coxofemoral joint prosthesis comprising a hollow titanium alloy shell of generally hemispherical shape, with a diameter between 45 and 65 mm, the internal face of which is intended to receive a prosthetic cup, and characterized in that the outer face of said shell is coated with several layers of titanium lattice fixed to said shell by rivets, and by the fact that it has two holes passing right through it, both located at an angle between 35 and 40 relative to the pole of said hemispherical shell, the distance of the centers of which is between 10 mm and 35 mm depending on the diameter of said shell, said two holes being intended to allow the fixation of said acetabulum in the ilium via quote. 2 / metallic cup according to claim 1, characterized in that the two holes are associated with rivets also contributing to the immobilization of said mesh. 3 / metal cup according to one of claims 1 and 2, characterized in that said shell is made of titanium alloy type TiA16V4. 4 / metallic cup according to one of claims 1 to 3, characterized in that the titanium of said lattice is an unalloyed titanium. 5 / metallic cup according to one of the preceding claims, characterized in that said layers of mesh form a coating with a thickness of approximately 2 mm. 6 / metallic cup according to claim 5, characterized in that said coating has pores of 250 to 500 microns, with interconnections between them, and a porosity of the order of 70% to 80%. 7 / metal cup according to claim 2, characterized in that the rivets associated with said two holes are capable of receiving screws whose diameter is of the order of 6.5 mm.