FR2794639A1 - Hip prosthesis femoral implant shank has at least one proximal section groove with discontinuous lengthwise surface - Google Patents

Abstract

The implant shank (1), designed for fixing with or without cement, has at least one groove (3) with a discontinuous lengthwise surface in its proximal section (2). Each groove lies parallel to the axis (D) of the shank and is made with two or more portions (5) of similar length but with different depths, varying between 0.1 and 1 mm. The distal end of each groove portion is rounded and projects relative to the proximal end of the adjacent portion.

Description

The present invention relates to a femoral stem for total hip prosthesis, intended to be implanted with or without cement in a femoral shaft.

The invention finds a particularly advantageous application in the field of internal prostheses of the hip joint.

Implantation of a total hip prosthesis is justified only in cases where the entire joint is deficient. On the other hand, when the natural acetabulum is still healthy, it will be preferable to use a so-called intermediate prosthesis, that is to say constituted by a simple femoral stem. The use of this type of implant is particularly indicated in the case of a femoral neck fracture, a problem occurring mainly in older women.

In any case, the implantation of a femoral stem inside a medullary femur duct can be performed mainly according to two different techniques, depending on whether or not a connecting cement is used.

In the case of a rod without cement, the fixation is carried out in two stages: a primary fixation by mechanical locking of the implant directly in partial contact with the internal surface of the canal, and a secondary fixation by bone regrowth at the level of the 'interface.

The use of an intrinsically rehabitable material, such as a titanium alloy, for example, allows the bone regrowth to intimately marry the relief of the stem, and thus to reinforce the immobilization of the implant even if its surface state is smooth. The combination of primary and secondary fasteners ensures the durability of the blockage.

An equivalent result can be obtained with an implant covered with a coating that significantly increases the specific surface of the stem, and thus promotes its anchoring relative to the bone during bone regrowth. In addition to a rehabitable material, it is also possible to use as a coating a simply biocompatible material, such as a chromium-cobalt alloy or stainless steel with a high nitrogen content. In the case of a hydroxylapatite coating, the rehabitation is due to the intrinsic biocompatibility properties of the material. The increase of the specific surface can be carried out in different ways depending on whether the coating in question is in the form of beads, fibermesh, pores, etc.

The second technique for immobilizing a femoral implant in a medullary canal consists of using a bonding cement, for example a PMMA acrylic cement for orthopedic use. The fixation is in this case carried out from the outset inoperative. It is imperative however that the cemented rod be extractable in the event of a possible recovery.

In the case of an intermediate prosthesis, it is useful to have an implant that can be placed both cemented and without cement. However, if the use of a rehabitable material such as a titanium alloy is a satisfactory solution in the context of implantation without cement, it is not desirable in the case of a cemented implantation. Indeed, the bond between the cement and the surface of the stem is often imperfect. It is thus common that the cement mantle is not perfectly homogeneous, thus resulting in the risk of fractures and / or micromoüvements. However, as the titanium alloy is relatively sensitive to the fretting corrosion phenomenon, these micromovements will quickly lead to a deterioration of the surface of the rod and thus an irreversible separation of the implant more or less long term.

Whatever the material used, the use of a porous or bead-like surface state is also not desirable in the case of a cemented implantation, since this kind of particularly adherent conformation then makes the rod femoral inextractible.

Finally, the absence of relief on an implant of biocompatible material other than a titanium alloy does not allow to obtain good stability of the stem if it is implanted without cement. In fact, micro-movements often occur which, in addition to pain, lead to bone resorption which can cause loosening of the implant.

Also, the technical problem to be solved by the object of the present invention is to propose a femoral stem intended to be implanted in the medullary cavity of a femur, which would make it possible to avoid the problems of the state of the art by offering increased stability and longevity of implantation, while remaining perfectly extractable if necessary, and this regardless of the implantation technique retained, that is to say with or without connecting cement.

The solution to the technical problem posed consists, according to the present invention, in that the femoral stem comprises at least one groove having a discontinuous longitudinal profile.

The invention as thus defined has the advantage of providing a self-locking relief able to immobilize the rod in axial rotation and translation inside the medullary cavity of the femur, whether directly with respect to the internal surface of said cavity in the case of a cementless prosthesis, or via a bonding cement coat in the case of a cemented rod.

According to a feature of the invention, each groove is formed on the proximal portion of the femoral stem since it is at this precise location that the main part of the maintenance is carried out. On the other hand, the grooves can be positioned indifferently on the anterior face and / or the posterior face and / or the lateral faces of the proximal portion.

According to another feature of the invention, each groove is formed substantially parallel to the axis of the femoral stem, that is to say to the axis extending longitudinally through the distal portion. Thus, each of the grooves is directed substantially in the direction of insertion of the prosthesis in the medullary canal. It is therefore able to effectively block any rotation of the stem along the centromedullary axis of the bone. The discontinuities present along each groove are themselves oriented substantially perpendicular to the direction of insertion. This arrangement makes it possible to immobilize the femoral stem in translation along the centromedullary axis of the bone, in particular in the direction of a depression of said stem in the medullary cavity.

The locking in rotation and in axial translation, which is provided by the discontinuous profile grooves, gives the femoral stem a greater stability than those of these homologues of the technical state, and this stem is cemented or implanted without cement. This results in a longer life of increased use, particularly advantageous if we consider that the patients likely to receive this type of prosthesis are elderly, by definition unwilling to endure long and frequent surgery.

According to another particularity of the invention, each groove is composed of at least two elementary portions offset in height. The passage from one elementary portion to another constitutes a discontinuity whose existence gives the groove a broken line longitudinal profile.

In a particularly advantageous manner, the distal end of each elementary portion is overhanging with respect to the proximal end of the directly adjacent elementary portion. All discontinuities are thus oriented in a common direction, which also corresponds to the direction of insertion of the femoral stem into the medullary cavity of the bone. Each groove therefore has a broken line profile strictly decreasing as one moves away from its proximal end to go to its distal end. This formal specificity advantageously confers on the femoral stem the capacity to withstand its sinking under the effect of excessive compressive stresses, without forbidding its extraction, whether in the case of a cemented or non-cemented implant.

According to another particularity of the invention, the height difference between the different elementary portions is substantially constant. Thus, the discontinuities have relatively similar dimensional characteristics. They therefore benefit from a substantially identical locking capacity in translation when said discontinuities come into contact with the inner wall of the medullary cavity.

Preferably, the length of the different elementary portions is substantially constant so that each groove has a regular slope. As a result, all the discontinuities are aligned in the same direction, which allows them all to cooperate by contact with the intramedullary canal of the femur, assuming of course that it is linear.

It should be noted that the fact that the height of the offsets is constant, or that the lengths are identical, concerns both the elementary portions of the same groove as those belonging to different grooves.

According to another feature of the invention, each elementary portion comprises a linear longitudinal profile so that the longitudinal profile of the groove is in the form of a succession of line segments connected by discontinuity portions.

According to a particularly advantageous embodiment, the distal end of each elementary portion extends substantially perpendicularly to the axis of the femoral stem. When the latter is implanted, each discontinuity is therefore positioned orthogonally to the direction of insertion, thus allowing the relief to cooperate more effectively by blocking with the inner wall of the medullary cavity to avoid any risk of depression.

Particularly advantageously, the distal end of each elemental portion has a rounded longitudinal profile which gives a blunt shape to the overhanging ridge. This conformation is intended to prevent a large part of the mechanical stress is focused precisely at this point and then breaks the bonding cement, assuming of course that the stem is cemented.

The following description with reference to the accompanying drawings, given as non-limiting examples, will make it clear what the invention consists of and how it can be achieved.

Figure 1 is a front view of a femoral stem according to the invention.

FIG. 2 represents in detail the proximal portion of the femoral stem illustrated in FIG. 1. FIG. 3 is a perspective view of the proximal portion shown in FIG. 2. For the sake of clarity, the same elements have been designated by FIG. identical references. Likewise, only the essential elements for understanding the invention have been represented, and this without regard to the scale and in a schematic manner.

As can be seen in Figure 1, the femoral stem 1 biocompatible material has at the front face of its proximal portion 2, a series of five flutes 3 parallel to each other. The latter are also arranged along the axis D of the rod 1 so that they extend parallel to the insertion direction of the prosthesis in the medullary cavity of the femur. The axis D is obviously that which materializes the direction in which extends the distal portion 4 of the femoral stem 1. In addition, it should be noted that the non-visible rear face of the rod 1 is also provided with five grooves 3, arranged substantially symmetrically with respect to their counterparts of the anterior face.

In this embodiment, the discontinuous longitudinal profile of each groove 3 corresponds to a regular succession of elementary portions 5 of substantially identical dimensions. These elementary portions 5 are offset longitudinally relative to each other by a constant height. They also have a linear longitudinal profile giving them a cross section which is also substantially constant.

FIGS. 2 and 3 show more precisely the general conformation of the self-locking relief, and in particular the combination of the longitudinal grooves 3 with the transverse discontinuities 6 which are materialized by the distal ends 7 of the different elementary portions 5. In particular, there is a distinction here clearly the parallelism of the five splines 3, and the regular transverse alignment of the elementary portions 5 component.

Apart from a few minor formal differences in its circumference, the self-locking relief has a homogeneous structure. Only the peripheral peripheral portions have different dimensional characteristics due primarily to the particular shape of the proximal portion 2 supporting them, a shape that is defined to best fit the medullary cavity of the femur and is therefore not regular.

As can be seen in Figures 2 and 3, the elementary portions 5 are arranged in some sort of staircase within a same groove 3; the different steps of identical length having a constant height. Thus all the distal ends 7 forming discontinuities 6 are perfectly aligned, so that they are able to cooperate together by contact with the wall of the intramedullary canal, in the hypothesis that the latter is linear as it is theoretically the case after the preparation of the femur. The proximal ends 8 of the elementary portions 5 are in turn set back from the distal ends 7, but they are also aligned since the height of the offsets is constant.

In the particular embodiment illustrated in Figures 1 to 3, the elementary portions 5 of the different flutes 3 are perfectly aligned transversely and are integral with each other in a continuous manner. Thus, the elementary portions 5 of the same row form a surface of substantially sinusoidal section. This characteristic profile is found here throughout the elementary portions 5, to their respective distal ends 7. As a result, the ridges 9 separating two contiguous splines 3 are not linear in this embodiment, but they also have a discontinuous longitudinal profile, which is moreover broken line. Consequently, the different discontinuities 6 of the same row of elementary portion 5 form a continuous surface capable of cooperating entirely by contact with the inner wall of the medullary cavity. The blocking can then be carried out both at the top and at the bottom of the grooves 3.

Thus shaped and arranged, the grooves 3 are able to effectively fight against the axial rotation movements that can animate the femoral stem, under the effect of significant shearing stresses inherent in the operation of this type of implant. The multiple transverse discontinuities 6 are able to significantly limit the depression of the rod 1 inside the medullary canal of the femur, a translational movement consecutive to the quasi-permanent compressive stresses undergone by said rod 1. The stability in axial rotation and translation thus obtained advantageously contributes to increasing the longevity of this type of implant. The grooves 3 are shallow in order to advantageously avoid any excessive segmentation or fragmentation of the cement mantle, in the case of a cemented prosthesis of course. In the example shown in Figures 1 to 3, the depth of each groove 3 is 0.5 mm, but it can of course vary between appropriate values, determinable by the skilled person. Preferably, the depth is between 0.1 and 1 mm.

Similarly, the height of the offset between two consecutive elementary portions 5 of a same groove 3 is of the order of 0.3mm, knowing that it can preferably vary from 0.1 to 0.5mm.

Finally, a typical elemental portion has a length and width of approximately 5 and 4 mm, respectively.

Naturally, the invention is in no way limited by the features that have been specified in the foregoing, nor by the details of the particular embodiment chosen to illustrate said invention. All kinds of variants within the reach of the skilled person can be made without departing from the scope of the invention.

The particular structure of the self-locking relief can thus be adapted to both an intermediate rod and a total hip prosthesis rod. This relief can be provided on different types of materials such as a chromium-cobalt alloy or a steel alloy with a high nitrogen content.

The different grooves may also have dissimilar dimensions, be irregularly spaced, or extend to the distal portion.

Claims (13)

<B> CLAIMS </ B> 1. femoral stem (1) intended to be implanted in the medullary cavity of a femur, characterized in that it comprises at least one groove (3) having a discontinuous longitudinal profile. 2. femoral stem (1) according to claim 1, characterized in that each groove (3) is formed on the proximal portion (2) of said femoral stem <B> M. </ B> 3. femoral stem (1) according to one of claims 1 or 2, characterized in that each groove (3) is formed substantially parallel to the axis (D) of said femoral stem (1). 4. femoral stem (1) according to any one of claims 1 to 3, characterized in that each groove (3) is composed of at least two elementary portions (5) offset in height. 5. femoral stem (1) according to claim 4, characterized in that -the distal end (7) of each elementary portion (5) is overhanging with respect to the proximal end (8) of the elementary portion (5). ) directly adjacent. 6. femoral stem (1) according to one of claims 4 or 5, characterized in that the height difference between the different elementary portions (5) is substantially constant. 7. femoral stem (1) according to any one of claims 4 to 6, characterized in that the length of the different elementary portions (5) is substantially constant. 8. femoral stem (1) according to any one of claims 4 to 7, characterized in that each elementary portion (5) has a linear longitudinal profile. 9. femoral stem (1) according to any one of claims 4 to 8, characterized in that the distal end (7) of each elementary portion (5) extends substantially perpendicularly to the axis (D) of said femoral stem (1). 10. femoral stem (1) according to any one of claims 4 to 9, characterized in that the distal end (7) of each elementary portion (5) has a rounded longitudinal profile. 11. femoral stem (1) according to any one of claims 4 to 10, characterized in that the elementary portions (5) of the different grooves (3) are aligned transversely. 12. femoral stem (1) according to any one of claims 4 to 11, characterized in that the height of the offset between two consecutive elementary portions (5) of a same groove (3) is between 0.1 and 0.5mm. 13. femoral stem (1) according to any one of claims 1 to 12, characterized in that the depth of each groove (3) is between 0.1 and 1mm.