FR2675042A1 - Principle of diaphyseal modularity, with high mechanical strength, adaptable to any hip prosthesis - Google Patents

Abstract

The hip prosthesis according to the invention comprises an elongate body having a cervical part (C) supporting a spherical head (5) which is intended to engage in an acetabular cavity, a diaphyseal part (D) forming a cervical/diaphyseal angle with the cervical part, and a metaphyseal region (M) of bent shape. The diaphyseal part (D) has the shape of a rod (6) which is rigidly integral with the metaphyseal part (M) and on which there may be engaged a removable and interchangeable diaphyseal sleeve (8, 8'), the outer surface of which has a shape substantially complementary to that of the said femoral cavity. The invention makes it possible to limit the number of shapes of bodies included in one series.

Description

PRINCIPLE OF HIGHLY MECHANICAL STRENGTH MODULARITY, ADAPTABLE TO ANY HIP PROSTHESIS.

The present invention relates to a principle of diaphyseal modularity, with high mechanical strength, adaptable to any hip prosthesis.

In general, we know that a hip prosthesis consists of an elongated body having two parts, namely: a cervical part and a diaphyseal part whose axes form, at the level of a methaphyseal region, an angle cervico-diaphyseal which can for example vary from approximately 130 to 1350.

The cervical part consists of a rod usually of cylindrical shape, the end of which carries a spherical head which is intended to come to engage in a natural acetabular cavity or, possibly, of prosthesis.

To allow a good adaptation of the prosthesis head in the acetabular cavity, prostheses are already used having interchangeable removable heads having a multiplicity of diameters, the assembly of these heads being effected by engagement of the conical end of the part neck of the prosthesis body in a conical cavity of the removable heads.

Of course, in order to be able to take account of the differences in shape which the femoral bone may present, it is necessary to provide a relatively large range of prosthesis bodies and heads. However, as much the production of spherical heads of different diameters and easily industrialized at low cost, the production of prosthesis bodies which have left shapes, is difficult and expensive.

This is the reason why we tend to limit the number of body shapes available in a range, and this to the detriment of the biomechanical laws which require obtaining a maximum press-fit contact, between bone and implant, limiting the use of cement as much as possible.

To solve these problems, it has therefore been proposed to make the diaphyseal part of the body of the implant removable by providing for screwing of this diaphyseal part in the metaphyseal region.

It turns out that this solution was rejected due to its mechanical fragility. Indeed, it is exerted, mainly at the level of the diaphysometaphyseal junction, intense mechanical contacts (bending moment) which easily reach the breaking point of this junction (weakened due to the mechanical discontinuity of the assembly by screwing) .

The object of the invention is more particularly to provide a solution which has the advantages of that previously mentioned without understanding its drawbacks.

To this end, it offers a hip prosthesis of the aforementioned type but in which the body comprises a diaphyseal part being in the form of a rod, for example cylindrical, rigidly integral with the metaphyseal part and on which can come to engage and attach an interchangeable removable diaphyseal sleeve whose outer surface has a shape substantially complementary to that of the cavity of the femoral bone in which it must come to engage.

Of course, this diaphyseal sleeve can be part of a range of sleeves covering all the possible forms of femoral bone cavities. Because of their substantially frustoconical shapes, these sleeves can easily be produced industrially at reduced prices, or even made to measure.

The axial attachment of the diaphyseal sleeve to the rod may advantageously be carried out by screwing, for example using a nut of semiovoidal external shape which is screwed onto a thread produced at the end of the diaphyseal rod.

An embodiment of the invention will be described below, by way of non-limiting example, with reference to the accompanying drawings in which
Figure 1 is a schematic axial section
exploded view of an implant according to the invention;
Figures 2 and 3 are axial sections of
the diaphyseal part of the implant shown
figure 1 in the assembled state with two sleeves
different diaphysaries
Figure 4 is an exploded axial section of a
variant of the implant according to
the invention;
Figure 5 is an axial section of the part
diaphyseal of the implant shown in Figure 4.

In these examples, the hip prosthesis 1 comprises a body comprising, as previously indicated - a cervical part C consisting of a cylin rod
drique 2 whose end has a conical shape 3
intended to come forcibly engage in a cavity 4 of
conical shape of a spherical head 5 - a diaphyseal part D having the shape of a rod
cylindrical 6 whose end has a thread 7
and - a metaphyseal region M having an angled shape
which connects the cervical part 2 to the rod 6.

On the cylindrical rod 6 forming the diaphyseal part, engages a sleeve 8, 8 'having an internal bore 9 substantially at the diameter of the rod. The outer surface of this sleeve 8 has a conformation suitable for the bone cavity in which it must engage.

The sleeve 8 is fixed to the rod 6 by means of a nut 10 of oblong shape, substantially frustoconical with a rounded end, this nut 10 being integral with the sleeve 8.

The screwing can be carried out using a hexagonal key which engages in an axial cavity 11 formed in the end. Once the screwing has been carried out, this axial cavity 11 can be closed by means of a plug 11 '.

Advantageously, the sleeve 8 is part of a range of sleeves covering all the sizes usually encountered, the sleeves 8, 8 ′ showing two different sizes of this range.

The hip prosthesis represented in FIGS. 4 and 5 is similar to that previously described except that - the junction zone 12 between the metaphyseal region M
and the diaphyseal rod 6 has a conical shape, and - the cylindrical internal surface 9 of the sleeves 8 is ter
mine, on one side, by a conical portion 13 intended for
abut against the conical shape 12 of the
junction and, on the other side, by a narrowing
bore 14 of diameter substantially equal to that of
the threaded part 7 of the rod 6.

These features are very advantageous for the following reasons - the conical shape of the junction zone 12 allows
ensure continuity of form between the meta region
physical M and the diaphyseal stem 6, which increases
considerably the mechanical resistance of the implant - the action of the conical shape 13 of the sleeve 8 on that
of the junction zone, under the effect of the axial force
exerted by the nut 11 (here distinct from the sleeve 8),
provides a seal between the outside and the inside
of the sleeve 8 and a slight take-up of wear; - the conical shape 12 ensures continuity of
shape between sleeve 8 and metaphyseal region M
without a projecting edge, and this, for a
wide range of sleeve thickness 8.

 Thus, in addition to the fact that it overcomes the unfavorable prejudices which tainted the previous solutions mentioned above, the invention makes it possible to considerably reduce the costs of hip prostheses while offering the practitioner a greater choice and personalization of the implant.

Claims (8)

Claims  1. Adaptable hip prosthesis, with high mechanical resistance, of the type comprising an elongated body having a cervical part (C) carrying a spherical head (5) intended to come to engage in an acetabular cavity, a diaphyseal part (D) intended to engage in the femoral bone cavity, this diaphyseal part (D) forming a cervico-diaphyseal angle with the part the cervical part (C) and a metaphyseal region (M) of bent shape, characterized in that the diaphyseal part ( D) has the shape of a rod (6) rigidly integral with the metaphyseal part (M) and on which can come to engage and fix an interchangeable removable diaphyseal sleeve (8, 8 '), the outer surface of which has a shape substantially complementary to that of said femoral cavity.  2. Prosthesis according to claim 1, characterized in that the fixing of the diaphyseal sleeve (8, 8 ') on said rod (6) is carried out by screwing.  3. Prosthesis according to one of claims 1 and 2, characterized in that the fixing of the diaphyseal sleeve (8, 8 ') on said rod (6) is effected by means of a nut (11) coming to be screwed on a thread made at the end of the said rod (6).  4. Prosthesis according to claim 3, characterized in that said nut (10) has a semi-ovoid shape.  5. Prosthesis according to claim 4, characterized in that the above nut (10) comprises an axial cavity (11) in which can engage a screwing tool.  6. Prosthesis according to claim 5, characterized in that it comprises a plug (11 ') for closing the said axial cavity.  7. Prosthesis according to one of the preceding claims, characterized in that the junction zone between the metaphyseal region and the diaphyseal stem has a conical shape (12).  8. Prosthesis according to claim 7, characterized in that the diaphyseal sleeve (8) comprises an internal cylindrical surface (9) ending on one side by a conical portion (13) intended to come to bear against the conical shape (12) of the above junction area.