Insert for acetabular cup

An insert for an acetabular cup includes an insertion body having an external surface shaped to be able to be coupled with a mating seating of the acetabular cup. The insert includes at least a friction element positioned on the external surface to determine a predefined friction with the surface of the seating, so that, during use, the friction element prevents the relative rotation of the insertion body and the acetabular cup.

This application is a §371 National Stage Application of International Application No. PCT/IB2008/001289, filed on 23 May 2008, claiming the priority of Italian Patent Application No. UD2007A000088 filed on 25 May 2007.

FIELD OF THE INVENTION

The present invention concerns an insert able to be disposed inside the casing of an acetabular cup of the hip. The insert according to the present invention functions as a positioning and rotation seating for the head of a femoral prosthesis.

The invention also concerns a method for stabilizing the relative position of said insert and the relative acetabular cup in which the insert is applied.

BACKGROUND OF THE INVENTION

It is known, in the field of orthopedic prostheses, to make an insert with a semi-spherical cavity which functions as a positioning and rotation seating for the head of a femoral prosthesis.

The insert, in turn, is able to be disposed inside a mating casing or seating, shaped like a cone or truncated cone, made of osteo-compatible material, for example based on titanium or cobalt, of an acetabular cup of the hip. Both the insert and the acetabular cup are normally axial-symmetrical with respect to a common axis of symmetry.

The insert is usually made of cobalt-based metal, such as an alloy of cobalt-chrome-molybdenum, or of ceramic, or also of polyethylene, which is lighter and more economical.

In the case of polyethylene inserts, however, there may be an unwanted tendency to a relative rotation of the insert and the acetabular cup, with respect to the common axis of symmetry. This rotation between the insert and the acetabular cup is due both to the common circular geometry of the two components, and also to the difference in material of which they are made, since the acetabular cup is usually made of metal. Such rotation must absolutely be prevented, so as not to cause serious damage to the patient's articulation, such as dislocation or other.

This problem is found, however, every time there is a tendency to reciprocal rotation of the insert and the relative acetabular cup.

Purpose of the present invention is to achieve an insert that, once disposed in the casing of the acetabular cup, will maintain during normal use the position determined during the operation to insert the prosthesis and the acetabular cup, and that will not rotate with respect to the acetabular cup, and in particular will not rotate with respect to the common axis of symmetry.

SUMMARY OF THE INVENTION

In accordance with the above purpose, according to the present invention an insert for an acetabular cup, provided with a seating, comprises an insertion body having an external surface able to be coupled with said seating of the acetabular cup.

According to a characteristic feature of the present invention, said insert comprises at least a friction element positioned, advantageously positioned fixed, on said external surface of the insertion body. The friction element is able to determine a predefined friction with the seating of the acetabular cup so that, during use, the friction element prevents the relative rotation of the insertion body and the seating and, consequently, between the insert and the acetabular cup with respect to the common axis of symmetry.

Therefore, the insert according to the present invention, once disposed in the acetabular cup, keeps, during normal use, the position determined during the operation of inserting the prosthesis and the acetabular cup.

Advantageously, the friction element is made of a material with a high friction coefficient. Even more advantageously, the friction element is made of the same material at least as the seating of the acetabular cup, for example a cobalt- or titanium-based alloy. In this way the general principle is effectively exploited according to which two equal materials on contact determine a high friction force which impedes and prevents their relative movement.

One advantageous possibility is that the friction element is attached to the insertion body releasably, in the sense that the friction element can be selectively mounted on and removed from the insertion body by means of a specific manual intervention by an operator, when the acetabular cup and the insert have not yet been implanted in the patient's hip. This is advantageous because it allows to sterilize the two components separately. Moreover, it allows to carry out assembly tests before implantation in the patient.

It is clear, however, that once the acetabular cup and the insert have been definitively implanted into the hip, the friction element can no longer be released from the insertion body.

Since a seating of an acetabular cup is typically shaped so as to have a circular cross section, advantageously shaped at least partly like a truncated cone, the insertion body too has at least a part shaped to mate with the seating.

A variant provides that said external surface of the insertion body has a first part shaped like a semi-spherical cap and a second part shaped like a truncated cone, on which the friction element is attached.

Another variant of the present invention provides that said external surface of the insertion body is shaped completely like a truncated cone, mating with the seating of the acetabular cup.

Both variants are advantageous, because they allow an effective conical coupling with the seating of the acetabular cup.

The present invention provides that the friction element can be disposed, according to contingent needs, on any portion whatsoever of the external surface of the insertion body and that, as well as a single friction element, several friction elements can also be provided.

An advantageous variant of the present invention provides that the friction element consists of a single ring, of a size coherent with that of the external surface of the insertion body.

An advantageous variant provides that, in this case, the friction element is formed by an annular wall with a shape compatible with that of the second part, and by a radial edge, protruding towards the inside of the friction element, which is able to cooperate with a corresponding annular seating, radially directed towards the inside, made along the same second part of the external surface of the insertion body.

To allow the solid clamping of the friction element and the insertion body, a suitable clamping means is provided which, according to a variant, consists of said radial edge of the friction ring, in cooperation with the annular seating of the insertion body.

Furthermore, to prevent the relative rotation of the friction element and the insertion body, it is provided to make, on the insertion body, a clamping means which is able to cooperate with the friction element or with a specific portion thereof.

A variant of the present invention provides to make a plurality of rings that function as friction elements.

Alternatively, or as an integration to the solutions described above, the present invention provides to make a plurality of friction elements, independent of each other, disposed along the external surface of the insertion body, which function as friction elements.

Advantageously, to achieve an effective impediment to the rotation around the common axis of the insertion body and the acetabular cup, the disposition of the friction element or friction elements is symmetrical with respect to said common axis and is coherent, for example circular, with the shape of the external surface of the insertion body and with the shape of the seating of the acetabular cup.

Furthermore, according to another variant, in order to have the greatest friction surface possible, the disposition of the friction element or friction elements is in substantial correspondence with the section of the external surface of the insertion body which corresponds to the greatest circular surface possible of said section, that is, in a substantial center line position, opposite the pole of the semi-spherical cap of the external surface of the insertion body.

The present invention also provides a method to stabilize the relative position between an insert for an acetabular cup and the acetabular cup itself, in which the acetabular cup is provided with a seating and in which the insert has an insertion body made of polymer material with an external surface shaped so as to be able to be coupled with said seating. The method according to the present invention comprises a first step in which a friction element is positioned, advantageously positioned fixed, on the external surface of the insertion body, the friction element being able to determine a predefined friction with said seating so that, during use, the friction element prevents the relative rotation between the insertion body and the seating itself, and a second step in which said external surface of the insertion body, bearing the friction element, is inserted and fixed in the seating of the acetabular cup.

DETAILED DESCRIPTION OF A PREFERENTIAL FORM OF EMBODIMENT

With reference toFIG. 1, an insert10according to the present invention is able to be inserted in an acetabular cup12, in turn housed in a relative acetabular seating, not shown in the drawings, of the hip bone. The acetabular cup12is of the traditional type, for example made of a cobalt- or titanium-based alloy, with a seating13at least partly shaped like a truncated cone, which is able to receive the insert10.

The insert10is formed by an insertion body18, shown in detail inFIG. 6, axial-symmetrical with respect to an axis of symmetry Y and normally made of polyethylene, preferably of the UHMWPE type (Ultra High Molecular Weight Polyethylene).

It is clear that other suitable polymer materials can be used apart from UHMWPE, such as polyethylethyleneketone (PEEK).

The body18has both an external surface19, which is inserted into the seating13, and also an internal cavity40(FIGS. 1,5and6), inside which a prosthetic head, not shown in the drawings, can be inserted. An annular portion43connects the external surface19with the inside of the cavity40.

In particular, the external surface19is formed by a semi-spherical cap part34, of a shape mating with said seating13, and a truncated cone part36, provided below the cap part34and coaxial therewith (FIGS. 5 and 6).

A variant, not shown in the drawings, provides that the external surface19is shaped completely like a truncated cone.

In particular, in correspondence with the angular position of the pole of the cap part34, and coaxial to the axis Y, a centering pin22is made (FIGS. 1-6) for coupling, in a known manner, with the acetabular cup12.

The insert10comprises a ring20, shown in detail inFIG. 7, made from a sheet advantageously made of a metal, for example a cobalt- or titanium-based alloy, the same as the one used to make the acetabular cup12. In this way, the ring20is able to determine a predefined friction between the insert10and the seating13, to prevent the reciprocal rotation between these two components with respect to the axis Y. Advantageously, in fact, the choice of making the ring20of the same material as that of the acetabular cup12allows to obtain optimum friction between the two components.

The ring20can have an aperture (not shown) which can allow the ring20to deform slightly and elastically, opening and closing it, in order to position it on the truncated cone part36(FIGS. 2 and 8).

Once positioned outside on the body18, possibly forcing it to slide on the truncated cone external surface19, the ring20, thanks to its elastic properties, is disposed to closely surround the truncated cone part36.

Advantageously, the ring20has an L-shaped cross section, as can be seen inFIG. 5, formed by an annular wall27able to surround the truncated cone part36and by a radial edge30disposed at the lower part.

The edge30is inserted into a corresponding annular seating or undercut32, made along the truncated cone part36, at the lower or upper part, according to needs, so as to determine the axial retention of the ring20with respect to the body18and to define a solid combination of these two components.

Furthermore, to prevent the reciprocal rotation around the axis Y of the body18and the ring20, at least a block23,25is provided, advantageously two, on the truncated cone part36, which act as abutment elements to prevent the rotation of the ring20. In particular, the blocks23and25are able to cooperate with corresponding hollows24and26made in the ring20. The hollows24and26are advantageously provided in diametrically opposite positions and have a length substantially equal to the length of the blocks23and25, so as to clamp the rotation of the ring20.

A variant, not shown in the drawings, provides that the hollows24and26are made instead in the truncated cone part36and that the blocks23and25are made at the ends of the ring20, in an equivalent manner to that described above.

It is clear that modifications and/or additions of parts may be made to the insert for an acetabular cup as described heretofore, without departing from the field and scope of the present invention.

It is also clear that, although the present invention has been described with reference to some specific examples, a person of skill in the art shall certainly be able to achieve many other equivalent forms of insert for an acetabular cup, having the characteristics as set forth in the claims and hence all coming within the field of protection defined thereby.