FR2814060A1 - Femoral shaft, for hip replacement joint, has neck at top of medullar insert axes of these two components being at angle of 125 degrees, with ball of joint being mounted on neck with displacement between center and axis of medullar insert - Google Patents

Abstract

The shaft for a hip replacement joint has a neck (16) at the top of a medullar insert (15), the axes of these two components being at an angle of 125 deg . The ball (17) of the ball and socket joint is mounted on the neck and the displacement between the center of the ball and the axis of the medullar insert is 47 mm.

Description

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 The present invention relates to a femoral stem of a hip prosthesis.

 A femoral stem conventionally has a tapered medullary part, intended to be engaged in the medullary cavity of the femur, a neck surmounting this medullary part and a spherical head engaged on the end of this neck. The mounting of the head on the neck is very often carried out by fitting the head on the end of the neck, the latter having for this purpose a conical tip with a slight slope while the head has a conical cavity of corresponding shape.

 The prosthetic head cooperates with a corresponding spherical cavity which delimits a cotyldfdienne cup attached to the pelvis.

 A hip prosthesis including such a femoral stem must best respect the articular mechanics, which implies a correct positioning of the center of the femoral head, and must best restore the anatomical angle according to which the gluteal muscles act on the femur, this which depends on the shape of the stem and the level of implantation of this stem in the femur.

 The morphotypes encountered are numerous and variable with regard to the shape and size of the femur. A hip joint is defined by the cervico-diaphyseal angle, that is to say the angle formed by the axis of the neck with the axis of the femoral diaphysis, and by "lateralization" or "offset ", that is to say the distance separating the center of the femoral head from this same axis of the femoral diaphysis.

 In approximately 80% of cases, this cervico-diaphyseal angle is of the order of 135 and the "lateralization" is of the order of 40 mm. The existing prostheses are determined with reference to these data and therefore have a cervico-diaphyseal angle of 135 and a lateralization of 40 mm.

 However, about 20% of morphotypes remain unsuitable for these implants, for which insufficient lateralization of the prosthetic stem results in medialization of the lower limb, and, so to speak, an anatomical defect. This results in a reduction in the lever arm according to which the gluteal muscles act on the femur, and therefore in a loss of efficiency of these muscles, leading to lameness and pain.

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In the case of a coxa vara, that is to say of a femur presenting a cervico-diaphyseal angle more closed than the 135 statistically most frequent, the implantation of a standard femoral stem leads to a risk of elongation of the lower limb, which can generate lameness, and also a reduction in the lever arm according to which the gluteal muscles act on the femur. In this case too, the latter lose their effectiveness.

osseuse, et sur la position de la tête prothétique sur le col, en choisissant la tête prothétique à implanter parmi un jeu de plusieurs têtes identiques comprenant des cavités d'emmanchement de différentes profondeurs. To adapt the position of the standard prosthesis in the best possible way, the practitioner plays on the level of implantation of the femoral stem in the femur, adapting accordingly the level of the cut

bone, and on the position of the prosthetic head on the neck, choosing the prosthetic head to implant from a set of several identical heads comprising fitting cavities of different depths.

 Another solution consists in providing modular necks, of different lengths and / or angulations, which can be adapted on the same rod.

 The resistance of these collars over time is however questionable, and these collars generate more or less undesirable particles under the effect of the repeated stresses which they undergo. Risks of embrittlement and corrosion can also be noted in the case of rods with modular necks.

 As a result, the rods in which the medullary part and the neck are in one piece remain of very general use.

 The present invention aims to remedy the aforementioned drawbacks. Its objective is to provide a femoral stem adapted to approximately 20% of the morphotypes for which standard implants are not optimal, and to allow a perfect restoration of the effectiveness of the gluteal muscles, without lengthening of the lower limb.

 The femoral stem which the invention relates comprises, in a manner known per se, a tapered medullary part, intended to be engaged in the medullary cavity of the femur, a neck surmounting this medullary part and a spherical head engaged on the end of this neck , and has a one-piece structure.

 According to the invention, the cervico-diaphyseal angle, that is to say the angle formed by the axis of the neck with the axis of the medullary part of the stem, is 125, and

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- la "latéralisation" de la tige, c'est-à-dire la distance séparant le centre de la tête prothétique de ce même axe de la partie médullaire, est de 47 mm.

- the "lateralization" of the rod, that is to say the distance separating the center of the prosthetic head from this same axis of the medullary part, is 47 mm.

 It follows from these two characteristics that the center of the prosthetic head is lowered by 5 mm relative to a rod of the prior art, as mentioned above.

 The femoral stem thus shaped allows, when the patient's morphotype is not compatible with the use of a standard implant, positioning of the lower limb relative to the pelvis allowing perfect restitution of the anatomical movement. In particular, the rod makes it possible to obtain an adequate tension of the gluteal muscles and a restitution of the anatomical angle according to which these muscles act on the femur, without medialization or elongation of the lower limb.

 The invention thus simultaneously provides a solution to the problem of treating an articulation with significant lateralization and an articulation with coxa vara.

 Advantageously, the rod comprises a base or console forming an overhang, arranged at the base of the neck and projecting from the internal edge of the medullary part in a direction substantially perpendicular to this neck.

 This base makes it possible to increase the section of the medullary part of the rod at the base of the neck, which makes it possible to obtain a reinforcement of the rod at this level. This reinforcement is able to perfectly withstand the significant and repeated stresses generated in this area by the reduction of the cervico-diaphyseal angle and the increase in the length of the neck.

 In addition, said base forming an overhang can rest on the calcar, that is to say on the medial area of the resected femoral cortex, which anatomically has a significant thickness. The seat of the rod in the bone is thus perfectly ensured, which in particular avoids the varying stresses on the rod. The stresses exerted on the bone, which result from the increased lever arm, are distributed over an increased surface, which avoids localized hyper-stressing of the bone.

 Preferably, the base forming an overhang dies on either side of the neck at the proximal end of this medullary part.

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 The base is advantageously connected to the neck by a rounded wall, forming a fillet, which allows an increase in the mechanical strength of the prosthesis.

 Advantageously, the proximal zone of the medullary part comprises stepped steps to ensure its support, in the longitudinal direction, against the femoral cortex, the rod being intended to be anchored by growth of bone cells.

 These steps are preferably oriented transversely to the diaphyseal axis, and preferably have a sawtooth shape, seen in profile.

 Advantageously, the diaphyseal zone of the medullary part comprises longitudinal grooves to ensure its support, in rotation around the diaphyseal axis, against the femoral cortex.

 For a good understanding, the invention is again described below with reference to the appended diagrammatic drawing representing, by way of nonlimiting example, a preferred embodiment of the femoral stem which it concerns.

 Figure 1 is a simplified view of a hip joint whose native femur anatomically has significant "lateralization", this joint having been treated with a known hip prosthesis, whose femoral stem is of standard type; the position of the native femur is shown in dashed lines while the position of the femur after placement of the prosthesis is shown in solid lines; Figure 2 is a view similar to Figure 1, in the case of a hip joint in which the native femur has a coxa vara, that is to say a reduced cervico-diaphyseal angle, this joint having also been treated with a known hip prosthesis, of which the femoral stem is of standard type; Figure 3 is a view of the femoral stem according to the invention, in solid lines, while this stem is superimposed on a known femoral stem as shown in Figures 1 and 2, this known stem being shown in broken lines; Figure 4 is a view of the femoral stem according to the invention, in a direction parallel to the axis of the neck which it comprises, and Figure 5 is a view of a hip prosthesis including the femoral stem according to the invention, after implantation.

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 FIG. 1 represents a hip joint in which the native femur 1 has a cervico-diaphyseal angle slightly more closed than the average and above all a "lateralization" or "offset" greater than the average.

 The cervico-diaphyseal angle is that formed by the axis of the femoral diaphysis 3 with the axis of the neck 4 of the femur 1; in the example shown in this figure 1, this angle is 1250.

 The "lateralization" or "offset" is the distance separating the center of the femoral head 2 from the axis of the diaphysis 3; in the example shown in Figure 1, this lateralization is of the order of 47 mm.

 To treat this joint, a conventional type prosthesis has been implanted, comprising an acetabular cup 10 and a femoral stem 11.

 The cotyldid cup 10 has a generally hemispherical shape and delimits a spherical cavity. It takes place, according to a perfectly known technique, in the acetabular cavity of the pelvis 7.

 The femoral stem 11 is intended to be engaged in the medullary canal of the femur 1, after resection of the cervix 4 and of the head 2 and adequate bore of the metaphysis and of the proximal diaphysis of the bone.

 The rod 11 has a tapered medullary part 15, intended to be engaged in the medullary cavity of the femur, a neck 16 surmounting this medullary part 15 and a spherical head 17 engaged on the end of this neck 16. As is more particularly visible in Figure 3 in broken lines, on this known rod 11, the cervico-diaphyseal angle A is 1350 and the lateralization of the head 17 is 40 mm.

 The mounting of the head 17 on the neck 16 is carried out by fitting the head 17 onto the end of this neck 16, the latter having for this purpose a conical tip 18 with a slight slope while the head 17 has a conical cavity of corresponding shape.

 The head 17 cooperates with the cavity which delimits the cup 10.

 It appears in FIG. 2 that the fitting of the prosthesis leads to a slight lowering of the femur 1 relative to the pelvis 7, resulting from the slight difference existing between the cervico-diaphyseal angle of the native femur and that of the rod 11 , and especially medialization of the lower limb. This medialization not only induces a relaxation of

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 gluteal muscles, shown schematically by line 19, but also a reduction in the lever arm according to which these muscles act on the femur.

 Figure 2 shows a hip joint with a coxa vara, that is to say a native femur 1 whose cervico-diaphyseal angle is more closed than normal; in the example shown by this figure, this angle is 1100.

 The implantation of the rod 11 then leads to an elongation B of the lower limb and to a loss of efficiency of said lever arm, detrimental to the efficiency of the gluteal muscles. Medialization of the lower limb also exists.

- l'angle cervico-diaphysaire C est de 125 , et - la "latéralisation" de la tête est de 47 mm. Referring to FIG. 3, it appears that in the femoral stem 20 according to the invention,

- the cervico-diaphyseal angle C is 125, and - the "lateralization" of the head is 47 mm.

 It follows from these two characteristics that the center of the head 17 of the rod 20 has a lateralization increased by 7 mm relative to the rod 11 and a lowering by 5 mm relative to this rod 11.

 In addition, the rod 20 according to the invention comprises a base forming an overhang 25 arranged at the base of the neck 16. This base 25 projects from the internal edge of the medullary part 15, in a direction substantially perpendicular to this neck 16. It can have different widths according to the different sizes of rods 20, and in particular go from 10 mm to 16 mm, varying in a homothetic manner.

 It appears in FIG. 4 that this base 25 dies on either side of the neck 16 at the level of the proximal end of the medullary part 15.

 The neck 16 is connected to this base 25 by a rounded wall 26, forming a fillet.

 Figure 5 shows that the rod 20 ensures, in one or other of the indications according to Figure 1 or Figure 2, a positioning of the femur 1, and therefore of the lower limb, relative to the pelvis which corresponds substantially at the original position of this femur 1 and which allows a faithful reproduction of the anatomical movement. In particular, the rod 20 makes it possible to obtain an adequate tension of the gluteal muscles 19 and a restitution of the anatomical angle according to which these muscles act on the femur 1, without medialization or elongation of the lower limb.

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 The base 25 makes it possible to increase the section of the part 15 of the rod 20 at the base of the neck 16, in order to reinforce the rod 20 at the level of the zone undergoing significant stresses. The base 25 also allows the rod 20 to bear on the calcar, to ensure perfect seating of this rod 20 in the bone 1 and a good distribution of the stresses.

 Leave 26 allows an increase in the mechanical strength of the prosthesis.

 As appears from the above, the invention provides a femoral stem for a hip prosthesis which has the essential advantages of being suitable for approximately 20% of morphotypes for which standard implants are ill-suited, and of allowing perfect restoration of the efficiency of the gluteal muscles, without lengthening the lower limb.

Claims (8)

CLAIMS 1-Femoral stem of hip prosthesis, comprising a tapered medullary part (15), intended to be engaged in the medullary cavity of the femur (1), a neck (16) surmounting this medullary part (15) and a spherical head ( 17) engaged on the end of this neck (16), and having a one-piece structure, rod (20) characterized in that: - the cervico-diaphyseal angle, that is to say the angle formed by l the axis of the neck (16) with the axis of the medullary part (15) of the rod (20) is 125, and - the "lateralization" of the rod, that is to say the distance separating the center of the prosthetic head (17) of this same axis of the medullary part (15), is 47 mm.  2-femoral stem according to claim 1, characterized in that it comprises a base or console forming an overhang (25), arranged at the base of the neck (16) and projecting from the internal edge of the medullary part (15) according to a direction substantially perpendicular to this neck (16).  3-Femoral stem according to claim 2, characterized in that the base (25) dies on either side of the neck (16) at the proximal end of this medullary part (15).  4-Femoral stem according to claim 2 or claim 3, characterized in that the base (25) is connected to the neck (16) by a rounded wall (26), forming a fillet.  5-Femoral stem according to one of claims 1 to 4, characterized in that the proximal zone of its medullary part (15) comprises stepped steps, these steps making it possible to provide support for the stem, in the longitudinal, against the femoral cortex.  6-Femoral stem according to claim 5, characterized in that the steps are oriented transversely to the diaphyseal axis.  7-Femoral stem according to claim 5 or claim 6, characterized in that the steps have a sawtooth shape, seen in profile.  8-Femoral stem according to one of claims 1 to 7, characterized in that the diaphyseal zone of its medullary part (15) <Desc / Clms Page number 9>  includes longitudinal grooves, these grooves ensuring the support of the rod, rotating around the diaphyseal axis, against the femoral cortex.