FR2751527A1 - Femoral rod for cement type hip prosthesis - Google Patents

Abstract

The rod (1) comprises a centring end piece (9) at its distal end (2) for centring the rod in the medullar channel (6) of a femur (7). This end piece forms a distance piece between the distal end and a cortical wall of the femur. The end piece comprises a solid block spider having four identical fins (11) perpendicular to each other. The distal end of the rod comprises four open slots (13) which are perpendicular to each other, with two in the sagittal plane and the other two in the frontal plane. Theses slots receive the corresponding fins of the spider.

Description

 The subject of the present invention is a femoral stem of a hip prosthesis of the type called a cementing stem, that is to say, bonded over its entire surface to the cortical wall of the femur by an orthopedic cement which completely fills the space situated between the bone wall and the outer surface of the prosthesis.

 The invention relates more specifically to a centering device of the femoral stem to be cemented which is placed at the end of a tapered portion intended for its fixation in the femur, by axial insertion into the medullary canal. Such a device makes it possible to block the transverse movements of the rod inside the medullary canal, in particular its end because of the amplitude that is given to it at this free end portion. These movements originate from those due to the hip itself and which can lead to unwanted movements of the latter in varus, that is to say in tightening of the cervico-diaphyseal angle formed by the axis of the cervix. of the femur and the axis of the femoral diaphysis, or else in valgus, that is to say in enlargement of this same cervico-diaphyseal angle. In addition there are parasitic or mixed movements which are uncontrollable and which hinder the good behavior over time of the prosthesis stem.

 In order for a hip prosthesis centering device to perform its function satisfactorily, it is essential that it be able to minimize the movements of the shaft and that the shaft can be totally immobilized at its end and thereby also blocked. on the rest of its length. For this, the device must have the lowest mechanical moment possible, but in addition it must be made in such a way that it does not oppose the axial flow of the orthopedic cement and that it can be distributed along of the entire stem even in its distal zone, without creating any interruption.

 In addition, such a device must be of a simple embodiment so inexpensive, it must be easy to install so advantageously do not have a preferred orientation to install at the end of the rod, and mechanically effective so resistant enough at various mechanical stresses.

 The known state of the art corresponds to US Pat. Nos. 4,770,660 (EP-A-325,407) and 5,108,439 (EP-A-483,057).

 US Pat. No. 4,770,660 discloses a hip prosthesis whose femoral stem end is placed in a receiving socket, the outside diameter of which is equal to the inside diameter of the medullary canal. This sleeve is a locking seat of the stem end to maintain it and simultaneously give free rein to the rod itself, retaining the flexibility it must have to absorb the different movements of valgus and / or of varus mentioned above. This prosthesis is not of the type to be cemented: the stem is not in fact embedded in orthopedic cement, and the receiving sleeve of the rod end is arranged to be removed, if necessary.

 If it were to be embedded in cement, it could no longer be flexible against mechanical stresses, and it would be an impassable obstacle to the flow of cement in its distal part.

 US Pat. No. 5,108,439, on the other hand, describes a hip prosthesis to be cemented, thus of the type of the invention that is the subject of the application. Its centering tip allows the flow of orthopedic cement in the distal part, but it is placed in the extension of the axis of the stem, which causes an increased mechanical moment, weakening the device, or requires a reduction of the possible length of the rod, thus reducing the contact area with the cement. In addition, the production of such a tip is expensive because of its shape to be machined and when placed by the surgeon, this tip must be placed in a precise direction of installation, significantly complicating the work of the surgeon in space operative that he spared on his patient.

 The object of the invention is to propose a femoral stem equipped with a centering tip arranged so as to eliminate the above disadvantages, in particular by guaranteeing better mechanical strength and by eliminating any mechanical moment between the centraliser and the rod itself. .

 According to the invention, the tip consists of a monobloc crosspiece with four identical fins perpendicular to each other, in the distal end of the rod are arranged four open slots perpendicular to each other, two of the slots being made in a sagittal plane. and the other two in a frontal plane of the stem; these slots are adapted to receive the four corresponding wings of the spider.

 The invention also relates to a method of manufacturing the tip, characterized in that this tip is made by successive cuts of a profile out of a die, each cut to deliver a corresponding tip, and this without creating the slightest rejection.

 Other features and advantages of the invention will become apparent from the description which follows, with reference to the accompanying drawings which illustrate two embodiments as non-limiting examples.

 FIG. 1 is a longitudinal elevational view of a hip prosthesis femoral stem, the distal end of which is equipped with a centering device according to one embodiment of the invention and is inserted into the medullary canal of a femur.

 FIG. 2 is a view from above of the femoral stem centralizing device of FIG. 1.

 Figure 3 is an elevational view of the centering device of Figures 1 and 2.

 Figure 4 is an end view of the distal end of the shaft of Figure 1.

 Figure 5 is a view similar to Figure 3 of an alternative embodiment of the centering cross.

 The femoral stem 1 of hip prosthesis illustrated in Figure 1 comprises a tapered distal portion 2, a proximal portion 3, extended by a neck 4 adapted to be capped by a ball 5 intended to be housed in a cup not shown.

 The stem 1 is of the type to be cemented, that is to say that it is intended to be inserted into the medullary canal 6 of a femur 7 filled with a cement 8.

 Since the medullary femur ducts are more or less close to cylinders depending on the individual, the function of such a device is, before cementing, to center the stem in relation to the internal cortical wall of the medullary canal, in order to allow obtaining a cement sheath as homogeneous as possible. In fact, the setting quality of the cement depends on the homogeneity of the thickness of the cement sheath, to which the quality of the fitting of the prosthesis is also linked, that is to say the obtaining of a good maximum strength. in time without loosening.

 The centering nozzle 9 is constituted by a monobloc crosspiece with four identical fins 11 perpendicular to each other. In the distal end 2 of the rod 1 are arranged four open slots 13 perpendicular to each other, two of the slots 13 being made in the sagittal plane and the other two in the frontal plane of the rod 1. The slots 13 open longitudinally on the the tip of the distal end 2, and radially thereto, and are adapted to receive the corresponding four fins 11. They have a length 1 equal to the height of the fins 11, so that when the latter are fully inserted into the longitudinal slots 13, the tip 9 does not increase the length of the femoral stem 1.

 The fins 11, of identical widths, have vertices forming rounded leaves 12, which facilitate the penetration of the tip 9 and the rod 1 into the medullary canal 6.

 According to an alternative embodiment (Figure 5) the spider 14 has fins 16 whose vertices 15 form right angles.

 The tip (9; 14) is manufactured by a method consisting in executing successive cuts of a profile out of a extrusion die, each cutout for delivering a tip. The braces 14 then obtained can be used, rough cut, on femoral stems 1 with their peaks 15 at a sharp angle, or after machining rounded 12.

 The braces 9, 14 are monobloc and can be introduced into the slots 13 without having to respect a particular direction of introduction.

 The fins 11, 16 are dimensioned so as to project radially from the surface of the rod 1, over a width such that, once the rod 1 provided with the nozzle 9 or 14, pressed into the medullary canal 6, the edges longitudinal outer fins lla fins are substantially supported on the cortical wall of the bone. The fins 11 or 16 thus form a spacer which is in contact with the cortical wall, and have a small thickness e, preferably identical for the four fins, so as not to hinder the passage of the cement 8 in which the centering tip 9 is drowned.

 The spider (9; 14) effectively counteracts the effects of both varus and valgus thanks to its diametrically opposed wings.

As non-limiting numerical examples, the end pieces (9, 14) may have the following dimensions

<tb> Diameter <SEP> Height <SEP> (h) <SEP> Radius <SEP> of <SEP> Thickness <SEP> of
<tb><SEP> rounded <SEP> 12 <SEP> fins <SEP> (e)
<tb> 12.5 <SEP> mm <SEP> 12 <SEP> mm <SEP> 4 <SEP> mm <SEP> 1.6 <SEP> mm
<tb> 14.5 <SEP> mm <SEP> 12 <SEP> mm <SEP> 4 <SEP> mm <SEP> 1.6 <SEP> mm
<tb> 16.5 <SEP> mm <SEP> 12 <SEP> mm <SEP> 4 <SEP> mm <SEP> 1.6 <SEP> mm
<Tb>
Being made from the same profile, the end pieces (9, 14) have an identical thickness, between about 1.5 and 2mm, and preferably 1.6mm. The height h may be between about 10 and 15mm.

Any material that is biocompatible and has good mechanical strength for this height may be suitable for braces (9, 14), in particular polymethyl methacrylate.

This material must also be non-absorbable.

 In the various embodiments, the thickness and width of the brace vanes are preferably the same. After the setting of the cement 8, the centering device no longer fulfills an effective function since its role is, as already indicated, to center the rod 1 to obtain a mantle or sleeve of cement of uniform thickness, before taking the cement.

 The centering tip 9 equipping the rod according to the invention is very simple and low manufacturing cost. I1 has a better reliability than the known centering devices, as well as an absence of mechanical moment relative to the rod 1 thanks to the fact that it is completely integrated in the tip of the distal end 2 thereof, unlike the centering device of EP-A-0 483 057.

Claims (7)

 1. femoral stem (1) of hip prosthesis, in particular of the type to be cemented, whose distal end (2) is equipped with a centering end (9) of the stem in the medullary canal (6) of a femur (7), this tip being adapted to constitute a spacer between said distal end and a cortical wall of the femur, characterized in that the tip (9) consists of a one-piece cross-brace with four identical fins (11) perpendicular between them, in that in the distal end (2) of the rod (1) are arranged four open slots (13) perpendicular to each other, two of the slots being made in a sagittal plane and the other two in a frontal plane of the rod, and in that these slots are adapted to receive the four corresponding fins of the spider.  2. femoral stem according to claim 1, characterized in that the tip (9) is dimensioned so that its fins (11) project radially from the surface of the distal end (2).  3. femoral stem according to claim 2, characterized in that the fins (11) have vertices at right angles (15) or rounded (12).  4. femoral stem according to any one of the preceding claims, characterized in that the slots (13) for housing the associated fins (11) have a length (1) equal to the height (h) of the fins and in that this height is of the order of 10 to 15 mm.  5. femoral stem according to claim 4, characterized in that the fins (11) have identical widths between 1 and 2 mm, and preferably 1.6 mm.  6. Femoral stem according to one of claims 1 to 5, characterized in that the tip (9) is made of a non-absorbable material such as polymethyl methacrylate.  7. A method of manufacturing the tip (9) for the femoral stem (1) according to any one of claims 1 to 5, characterized in that said tip is made by successive cuts of a profile out of a extrusion die, each cutout for delivering a tip.