FR2758977A1 - Centring guide for hip prosthesis intramedullary nail - Google Patents

Abstract

The guide (1) for a cemented medullary nail (11) is positioned at the distal end (12) of the nail to guide it along the centre of the medullary canal in which are previously fitted an obturator (20) and fixing cement. The guide allows passage of the connecting cement past it during insertion of the nail. The guide can have a central boss (2) with three radially extending wings (2) defining gaps between them.

Description

CENTERING DEVICE FOR CEMENTED ROD OF
TOTAL HIP PROSTHESIS
The present invention relates to a distal centering device for a femoral stem of total hip prosthesis intended to be cemented inside a femoral barrel.

 The invention finds a particularly advantageous application in the field of orthopedics, in particular to provide the guiding and centering functions during the implantation of a cemented rod within the medullary canal of a long bone.

 The centering devices known from the prior art generally consist of a metal or cement wedge, placed at the distal end of the femoral stem to be cemented, and having an external shape of which at least one section is complementary to the spinal canal with which it is intended to cooperate by contact. This conformation makes it possible above all to guarantee the placement and the maintenance of the implant relative to the femur, in a defined and reproducible position. This feature also plays an important role during the actual introduction of the prosthesis inside the femoral barrel, by guiding the distal part of the stem along the medullary canal of the bone.

 This type of device, commonly called a centering device, however has a major drawback in the specific case of cemented total hip prostheses. Indeed, the implantation of the latter requires the almost systematic use of a obturator in order to contain the bonding cement at the level of the femoral stem, and thus to prevent it from flowing lower inside the bone marrow canal. In practice, the obturator and then the cement are previously introduced into the femoral cavity, the rod provided with the wedge then being impacted in the bone. However, as the centering device has a section substantially identical to that of the medullary canal, there is almost no possibility of passage for the binding cement, a large part of which is therefore trapped between the centering device and the obturator. The bringing together of these two elements, inherent in the introduction of the femoral stem into the medullary canal, will initially compress significantly the binding cement.

Then any further movement of the centering device will have the effect of pushing the obturator into the femoral barrel by piston effect. The consequences engendered by this phenomenon then materialize by positioning very far from the obturator with respect to the distal end of the femoral stem, and consequently by the formation of a cement plug of too long length.

 If for some reason whatsoever, it is necessary to perform an implant recovery later, the elimination of said plug will undoubtedly constitute a long and very delicate operation. The extreme hardness of the cement compared to the surrounding bone tissue is more likely to significantly complicate this operation, and this all the more since the surgeon will want to avoid false routes as much as possible.

 The phenomenon of internal overpressure also renders precise positioning of the obturator relative to the distal end of the femoral stem futile. The piston effect indeed generates a maximum accumulation of cement between these two elements, and consequently increases their relative spacing in a more or less random manner compared to what could have been theoretically envisaged.

 Also the technical problem to be solved by the object of the present invention is to propose a centering device for cemented femoral stem of total hip prosthesis, intended to be positioned at the distal end of said femoral stem in order to ensure its guide as well as centering inside a femoral barrel into which a obturator and a bonding cement will have been previously introduced, centering device whose use would not cause involuntary and random movements of the obturator which could therefore be set up precisely by means of a suitable positioning device, in order to produce a cement plug of determined length.

 The solution to the technical problem posed consists, according to the present invention, in that said centering device has a permeable structure capable of allowing the bonding cement to pass during the implantation of the femoral stem in the medullary canal of the bone. .

 It is quite obvious that at this precise moment, the bonding cement can be assimilated to a viscoelastic fluid, consequently liable to flow through the specific structure of the centering device. Despite a high viscosity, resulting from relatively high friction between the molecules that make it up, the cement will initially faithfully match each part of the space available between the shutter and the centralizer. Then it will escape through the latter as the rod is pushed in, so as to advantageously avoid any increase in pressure liable to displace said shutter.

 It should be noted that the concept of permeable structure covers any form capable of being let through by the bonding cement, whatever the number of available passages, their conformation, their dimensions or their relative arrangement. These passages can therefore be constituted by bores, grooves, or any other more or less regular through holes. They can also communicate with each other and possibly form a complex flow network.

 According to a feature of the invention, the general shape of the centering device is profiled in order to offer a minimum of resistance with respect to the bonding cement and thus to promote its flow. This particular configuration of the centering device, like its permeable structure, tends to limit the piston effect encountered in the prior art as much as possible; phenomenon resulting from the introduction into the medullary canal of an element having a complementary shape on at least one section.

 Since the implantation of a cemented prosthesis provided with such a centering device does not cause displacement of the obturator, it is possible to form a cement plug of determined length by precisely placing said obturator at the inside the bone marrow canal.

This is why, in a particularly advantageous manner, the centering object of the present invention can be used in combination with a shutter implemented by means of an extremely precise positioning device.

 The part of this ancillary, intended to be inserted into the femoral cavity, comprises for this a portion whose shape is substantially identical to the proximal part of the femoral stem. This insertable part of the positioning device also has a length greater than or equal to the sum of the lengths, respectively of the insertable part of said femoral stem and of the centering device. The obturator, removably mounted at the end of the positioning device, can thus be placed in the medullary canal at a given depth, and then be separated from the centralizer by an equally determined interval, once the femoral stem is cemented . This interval, and consequently the length of the resulting cement plug, corresponds precisely to the difference between the respective lengths of the insertable parts of the positioning device on the one hand and of the rod coupled to its centralizer on the other hand.

 The description which follows with reference to the appended drawings, given by way of nonlimiting examples, will make it clear what the invention consists of and how it can be implemented.

 Figure 1 is a side view showing a centering device according to the invention.

 FIGS. 2 and 3 are views, respectively from above and in longitudinal section along the plane AA, of the centering device illustrated in FIG. 1.

 FIG. 4 represents a femoral stem provided at its distal end with a centering device similar to that of FIGS. 1 to 3.

 Figure 5 is a side view of an ancillary able to perform the preparation of the bone marrow cavity.

 Figure 6 partially illustrates a side view of a positioning device according to the invention, that is to say allowing a precise placement of a obturator in a medullary canal of the femur.

 Figure 7 is a longitudinal section illustrating the assembly of the various elements making up the ancillary device of Figure 5 on the one hand, and the positioning device of Figure 6 on the other hand.

 For reasons of clarity, the same elements have been designated by identical references. Likewise, only the elements essential for understanding the invention have been shown, and this without respecting the scale and in a schematic manner.

 According to Figures 1 and 2, the centering device 1 consists of a central element 2 of substantially cylindro-ogival shape, around which are regularly distributed three centering fins 3a, 3b, 3c. These are arranged longitudinally along the generatrices of the cylindrical proximal part 4 of the central element 2, and extend radially so that their respective ends 5a, 5b, 5c can come into contact with the medullary canal of the 'bone.

 In this preferred embodiment of the invention, the centering device 1 only has three centering fins 3a, 3b, 3c. This constitutes the ideal minimum configuration for immobilizing in two dimensions, in this case in a plane perpendicular to the medullary axis of the bone. Variations can obviously be envisaged, knowing that the use of more than three fins does not significantly improve the guiding and centering functions but disadvantageously increases the points of contact between the centering device and the bone. This results in a proportional decrease in the contact surface between the cement and the bone, a decrease detrimental to the durability of the cemented prosthesis. The use of only two fins, on the other hand, is totally to be discouraged since it only allows real immobilization in one direction, orthogonal to the medullary axis of the bone. We could then consider widening said fins, but this would be to the detriment of the dimensions of zones 6, 7, 8 reserved for the passage of the bonding cement, without however achieving the stability offered by the preferred embodiment.

 As can be seen in FIG. 2, the regular distribution of the fins 3a, 3b, 3c, and consequently that of the passage zones 6, 7, 8, makes it possible to give the centering device 1 a symmetrical shape capable of guaranteeing a certain homogeneity both in terms of the multidirectional support of the prosthesis, as well as the actual distribution of the bonding cement. The end 5a, 5b, 5c of each fin 3a, 3b, 3c is in the form of a portion of cylindrical surface substantially complementary to that of the medullary canal with which it is intended to cooperate by contact.

 According to Figure 1, each centering fin 3a, 3b, 3c further has a spindle-shaped section relative to the radial direction in which it extends. This swollen shape in the middle and tenning in a point at the two ends advantageously makes it possible to offer a minimum of resistance with regard to the flow of the bonding cement, said ends being arranged along an axis parallel to the generatrices of the cylindrical proximal part. 4 of the central element 2, that is to say substantially in the direction of flow. This direction also corresponds to the axis along which the centering device 1 is intended to be moved in the medullary canal of the bone. The ogival conformation of the distal part 9 of the central element 2 is also defined to favor the flow of the bonding cement during the introduction of the prosthesis into the femoral barrel.

3 shows a sectional view along one of the three planes of symmetry available to the centralizer illustrated in Figures 1 and 2, namely the plane
AA. We note in particular the presence of a blind bore 10, the frustoconical shape of this embodiment is able to cooperate by interlocking with the distal end 12 of the femoral stem 11, of substantially complementary shape. Furthermore, the joining of these two elements can be achieved by any known means.

 FIG. 4 illustrates a femoral stem 11 of total hip prosthesis ready for cementing inside a femoral barrel, that is to say provided at its distal end 12 with a centering device 1 according to the invention. Its implantation, however, requires prior preparation of the femoral cavity. The entirely conventional operation consists in digging this part of the bone relatively precisely, in order to produce a recess whose shape and dimensions correspond to those of the proximal part 13 of the rod 11, and thus guarantee a stable positioning of said rod 1 1 in the femoral cavity. For this, the surgeon uses a series of rasps 30 of increasing sizes until reaching the desired dimensions.

 Figure 5 shows one of these ancillaries and more precisely the last used, that is to say the one whose active part 3 1 forming a rasp most precisely corresponds to the shape of the final implant. In order to guide the movements inside the medullary canal of the bone, a guide olive 33 is mounted reversibly at the end 32 of the rasp 30.

With a diameter corresponding to that of said channel, this olive 33 can be used with all other rasps of smaller sizes.

 As we have seen previously, the implantation of a femoral rod 1 1 provided with its centering device 1 does not cause any displacement of the associated obturator 20, previously introduced with a bonding cement inside the femoral barrel. Also, it is particularly advantageous to combine the use of a centering device 1 according to the invention with the precise positioning of a shutter 20, this in order to generate the formation of a cement plug of determined length.

 Like the last rasp 30 used for the preparation of the femoral cavity, the positioning device 40 comprises a portion of shape substantially identical to the proximal part 13 of the femoral stem 11 (FIG. 6); this in order to be able to adapt precisely in the medullary canal of the bone as the implant itself will do later. Only the length of its insertable part can be greater than the value obtained by adding the corresponding length of the femoral stem 1 1 and the length of the centering device 1. This feature allows the obturator 20 to be fixed, temporarily fixed to the end of said device 40, at a lower level but determined relative to the position that said centralizer 1 will subsequently occupy in the femoral barrel. Once the prosthesis has been implanted, the obturator 20 and the centering device 1 will be separated by a space of known height, since it corresponds substantially to the difference in lengths mentioned above.

 As can be seen in FIG. 6, the positioning device 40 is composed of a substantially cylindrical shutter-holder tip 35.

This is mounted, in place of the guide olive 33, at the end 32 of the last rasp 30 used for the preparation of the femoral cavity, that is to say the one whose active part 3 1 corresponds most precisely to the shape of the final implant. Of course, the length of the shutter-carrying end piece 35 is fixed as a function of the interval which it is desired to obtain between the upper part 21 of the shutter 20 and the end of the centering device 1.

As the end piece 35 is removable, an assortment of increasing sizes can advantageously be provided in order to give the surgeon the choice of the length of the cement plug which will be generated.

 The shutter 20 is removably attached to the free end 36 of the shutter-holder end piece 35. It has a completely conventional external shape including, in this embodiment, three annular lips 22 d '' a diameter greater than that of the medullary canal, but having a certain flexibility in order to be able to adapt to it after deformation.

 FIG. 7 illustrates an assembly principle making it possible to link to the grater 30, either the obturator-holder end piece 35 or the guide olive 33. In this particular embodiment, the end of each of these parts is provided of a tapped blind bore, respectively 41, 37, 34.

Each of the last two 37, 34 is able to cooperate by screwing with one of the two threads 43, 44 carried by a connecting pin 42, the remaining thread cooperating with the bore 41 formed at the end 32 of the rasp 30 .

 The fixing of the shutter 20 is very different since it consists in substantially making a connection of the tenon-mortise type. For this, the free end 36 of the shutter-holder end piece 35 is provided with a cylinder 38 forming a tenon, which cooperates with a blind bore 23 forming a mortise, formed at the end 21 of the shutter 20. The cylinder 38 also has a longitudinal slot 39 allowing it to deform in order to come to slightly block in the blind bore 23 of frustoconical shape. The removable connection thus produced is such as to allow the obturator 20 to be held on the positioning device 40, before and during their insertion into the medullary canal of the bone. Once the positioning is completed, said obturator 20 is immobilized in the medullary canal due to the deformations caused by its slightly larger diameter. The withdrawal of the positioning device 40 will therefore leave it in place at the desired position in the femoral barrel.

It should be noted that the centering device 1 can be produced by
PMMA, metal, polyethylene or any other commonly used material. Similarly, the shutter 20 can be made up of gelatin, bone, dextrose, lactic or glycolic acids, etc.

 Naturally, and as it already clearly follows from the above, the invention is not limited to the particular embodiments which have been described by way of preferred examples, of which it encompasses all the variants remaining within the scope of the scope of the patent.

Claims (11)

1. Centering device (1) for cemented femoral stem (11) of total hip prosthesis, intended to be positioned at the distal end (12) of said femoral stem (11) in order to ensure its guidance as well as the centering inside a femoral barrel into which a obturator (20) and a bonding cement have been previously introduced, characterized in that said centering device (1) has a permeable structure capable of allowing said bonding cement to pass at the time of implantation of the femoral stem (11) in the medullary canal of the bone. 2. Centering device (1) according to claim 1, characterized in that it has a profiled shape capable of offering the minimum resistance vis-à-vis the flow of bonding cement. 3. Centering device (1) according to one of claims 1 or 2, characterized in that it consists of a central element (2), capable of being secured to the distal part (12) of the femoral stem (11) and on the surface of which are radially distributed at least two centering fins (3a, 3b, 3c). 4. Centering device (1) according to claim 3, characterized in that it comprises three centering fins (3a, 3b, 3c). 5. Centering device (1) according to one of claims 3 or 4, characterized in that the centering fins (3a, 3b, 3c) are distributed regularly around the central element (2). 6. Centering device (1) according to any one of claims 3 to 5, characterized in that each centering fin (3a, 3b, 3c) is arranged longitudinally on the central element (2) and extends radially so that its end (5a, 5b, 5c) can come into contact with the medullary canal of the bone. 7. Centering device (1) according to any one of claims 3 to 6, characterized in that each centering fin (3a, 3b, 3c) has a spindle-shaped section relative to the radial direction in which it extends, the pointed ends of this bulged shape being oriented in the axis along which said centering device (1) is intended to be moved in the medullary canal of the bone. 8. Centering device (1) according to any one of claims 3 to 7, characterized in that the end (5a, 5b, Sc) of each centering fin (3a, 3b, 3c) is in the form a portion of cylindrical surface complementary to that of the medullary canal with which it is intended to cooperate by contact. 9. Centering device (1) according to any one of claims 3 to 8, characterized in that the central element (2) has a cylindroogival shape. 10. Centering device (1) according to any one of claims 1 to 9, characterized in that it cooperates with a obturator (20) previously placed in a determined position in the medullary canal of the bone, at by means of a positioning device (40) the part of which intended to be inserted into the femoral cavity, comprises on the one hand a portion of shape substantially identical to the proximal part (13) of the femoral stem (11), and has on the other hand, a length greater than the sum of the respective lengths of the insertable part of said femoral stem 11 and of the centering device 1. 11. Centering device (1) according to claim 10, characterized in that the positioning device (40) comprises a stopper holder (35) mounted at the end (32) of the last rasp (30) used for the preparation of the femoral cavity, the obturator (20) being removably mounted at the free end (36) of said obturator-carrying end piece (35).