FR2729292A1 - Modular assembly of femoral rods for prosthetic hips - Google Patents

Abstract

The modular assembly consists of at least two series of rods (1-18), of which the length (l) and the width (d) vary simultaneously and progressively for each rod with reference to each adjacent rod. Each series of rods has a base length, which is predetermined. At least one group of removable necks (19) carry a support stem (21) designed to receive the femoral heads (22). The necks are inclined an angle (A), which is different on the range of associated bases (23), which are formed as the end part (25) of the rod modules. Each removable neck is able to be mounted on the rod to form the required cervical-diaphysial angle and the required angle of anteversion or retroversion.

Description

 The subject of the present invention is a modular set of medullary rods intended in particular for the fitting of hip prostheses, and more particularly for the fitting of so-called recovery prostheses, dimensioned to be closely adaptable to the needs of the surgeon when he is operating on a patient.

 To better situate the invention, it is recalled that a recovery prosthesis, which can sometimes be a reconstruction prosthesis, is intended for bone parts which are frequently of poor quality. Indeed the bone wall which is often pellucid, has few reliable mechanical properties due to its thinness and the demineralization of the cortex. This is the case, for example, of degenerative femurs, the degenerative character of which is variable.

 In such situations, which are frequently the subject of final interventions following the failure of one and sometimes several previous implant placement, it is necessary to make the most of the latest available assets offered by the bone support to obtain the best possible implantation of the prosthesis in the medullary canal of the support bone, and of course to obtain the best resistance over time of said prosthesis.

 Even more than with healthy bones, the stems of revision prostheses intended for the hip joint must correspond as closely as possible to the exploitable bone parts with which they must be in close contact. This is why their shape must be particularly studied to offer the best congruence with the bone face so as to obtain the best distribution of the forces exerted on the remaining healthy bone parts. However, the problem lies mainly in the difficulties in identifying the healthy bone parts of each patient and especially in locating them case by case.

 To meet the specific needs mentioned above, it is necessary to offer the surgeon the possibility of placing a prosthesis rod which, despite all the dimensional variety required by the placement of a recovery prosthesis, corresponds in all points to the femoral dimensions of the patient, so that the greatest sum of contact surfaces with healthy bone parts is obtained.

 However, it is observed that, with the stems of femoral prostheses currently known, the surgeon does not have the flexibility of adaptation desirable to the specific anatomy of each patient, so that it is frequent that the chosen femoral stem is suitable not completely to this or that particular anatomy. For example, the cross section of the medullary canal of the patient's femur may be inadequate to that of the femoral stem when the latter has the desired suitable length. Conversely, the femoral stem may have a cross section which exactly corresponds to that of the patient's spinal canal, while the length desired to reach the healthiest bone part is not sufficient.

 The difficulties of adapting the chosen rod to the dimensions of the different spinal canals of the patients are encountered both with regard to its diameter and its length, but also with regard to the internal curvature of the proximal part of the rod. It is of course the same with regard to the adaptation to the specific anatomy of each patient of the position of the neck of the femoral stem relative to the diaphyseal femoral axis delimiting the cervico-diaphyseal angle, and relative at the anteversion (or possibly retroversion) angle. It is recalled that the cervicodiaphyseal angle is the angle formed by the axis of the femoral neck and the axis of the femoral diaphysis (axis of the femur). This angle is normally close to 1300 in adults. It should also be remembered that the anteversion angle of the femoral neck is delimited in the horizontal plane by the bicondylar axis and the axis of said cervix.

 It is recalled that a removable modular collar taken in isolation, is known per se by the European patent n-0.201.407 filed on April 24, 1986 by Mr. Patrick MONTAGNE.

 Until now, the known femoral prostheses do not allow the surgeon this triple possibility of adaptation, so that the surgeon as well as the patient must be satisfied with the prosthetic material which is imposed on them, the surgeon having to choose the femoral stem capable of best suit the patient.

 It follows from these relative inadequacies that the patient's articulation cannot be reconstructed in a completely satisfactory manner. This mismatch between the femoral prosthesis and the particular anatomy of the patients can cause post-operative pain, and even progressive dysfunctions, which in some cases may require a new surgical operation.

 However, it has been observed that the smaller the cross section of the medullary canal, the shorter the length of this canal; conversely, the greater the cross section of the medullary canal, the greater the length thereof. This means that there is a certain relationship between the cross section and the length of the same spinal canal. It must therefore be the same for the femoral stem of the prosthesis.

 The invention takes advantage of the above observation, and relates to a modular set of prosthesis rods, in particular hip prostheses, allowing the surgeon a particularly advantageous adaptation to the anatomical specificities of each patient. It thus greatly improves the possibilities offered by the prosthesis rods known up to now.

According to the invention, the prosthetic modular assembly is characterized in that it comprises
- at least two series, two series of rods whose cross section and length vary simultaneously with progressive intervals for each rod relative to the neighboring rods,
- at least one group of removable necks having support parts intended to receive the femoral heads and which are inclined by different angels on associated bases which can be joined to the ends of the rods, each removable neck being able to be mounted on the rod at a cervical angle -diaphyseal and an appropriate anteversion or retroversion angle, each series having a determined base length.

 Advantageously, the various rods have in their proximal part an identical internal curvature from which the different cross sections vary. This curvature has the particularity of being congruent so as to best match the specific profile of the bone part with which the rod is in contact.

 Finally, each rod advantageously comprises a rectilinear (flat) posterior proximal face over approximately one third to half its length.

 This feature allows maximum stabilization of the implant to be obtained because the bore made compulsory for the ablation of the cement destroys the metaphyseal curve of the medullary canal of the femur by making it rectilinear on the posterior surface.

Thus the length of the implant can more easily be increased, which makes it possible to offer the surgeon a number of sets of rods greater than two, and therefore a wider choice.

 The plane of the straight face of each stem passes through the central region of the distal end of the corresponding femoral stem.

Thanks to its originality, the modular assembly of the invention opens up the following unsuspected possibilities for surgeons which are as follows:
a) choose the cross section of stem imposed by the medullary canal of the femur of each patient, so that this cross section corresponds as accurately as possible to that of the medullary canal;
b) choose the most appropriate rod length for each patient, for the cross section previously determined
c) choose the positioning of the femoral neck relative to the axis of the stem, and therefore to orient this neck according to the cervico-diaphyseal and anteversion (or retroversion) angles that the surgeon wishes to give to the prosthesis.

 According to a feature of the invention, in order to increase the flexibility of adaptation of the prosthesis, the rods can be subdivided into several groups of neighboring lengths for each of said groups, for example into two subdivisions so as to form a group of rods short and a group of long stems. Thus, between the diameter and the length of the rods, a wide variety of choice of medullary rod is made as a function of the desired cross section and length. In this case, the rods of each group have a slightly different length from each neighboring rod, while the rods of two different groups and which have the same cross section have a relatively large difference in length.

 According to another feature of the invention, it can also be implemented, no longer with two groups of rods of relatively different lengths, but with three groups of rods, thus forming a group of short rods, a group of long rods , and a group of rods of intermediate lengths, so that an even wider choice than previously is made available to the surgeon.

 Of course, according to the invention, larger subdivisions can be made but then come up as their number increases with an increasing cost problem, due not only to the manufacture but also to the storage, to the space thus than the various manipulations that the prosthetic assembly can generate.

 To remedy such a situation, which risks becoming difficult to manage, the invention extends to a process for preselecting the closest femoral stems in cross section and in length for each particular case.

 According to this process, having beforehand bone x-rays of the patient and layers representing the femoral stems with their different dimensions in length and in cross section, the approximate dimensions of the specific prosthetic stem are defined by superposition of layers and x-rays. of the patient.

 This preselection is made by the surgeon himself or his assistant, so that the minimum number of parts making up the modular assembly of the invention is to be sent to the surgeon, and presents a footprint as small as possible, as much during the 'surgical operation only during the sterilization of said parts.

 Thus, the surgeon has the closest prostheses in cross section and in length for each particular case, and can perform his installation without risking not discovering the most suitable rod.

 For this, having prior to the actual surgical operation performed the x-rays necessary to know the bone state of the femur of his patient, and having beforehand as indicated above layers representing the femoral stems with their different dimensions in length and in cross section, the surgeon can proceed as previously explained to choose the prosthetic rod suitable for his patient. He then only needs to have at his disposal the rods closest in size to the rod he has defined, this for safety in the event of a surprise as to the bone quality of the patient's femur.

 Other features and advantages of the invention will become apparent during the description which follows, given with reference to the accompanying drawings which illustrate an embodiment thereof by way of non-limiting example.

 Figure 1 is an elevational view of a modular set of femoral stems of hip prostheses, substantially to scale, whose cross sections and lengths increase regularly.

 FIG. 2 is an elevational view along arrow F of FIG. 1.

 FIGS. 3 and 4 are elevation views, similar to FIGS. 1 and 2, of a second set of femoral rods of length greater than those of FIGS. 1 and 2.

 Figure 5 is an elevational view on a reduced scale, of a femoral stem and the corresponding femoral neck, intended to receive a spherical articulation piece on the patient's pelvis.

 FIG. 6 is an elevational view on an enlarged scale of the neck of the femoral stem of FIG. 5.

 Figure 7 is an elevational view of the neck of Figure 6 in a plane perpendicular thereto.

 Figure 8 is a top view of the proximal end of a femoral stem and the associated neck, its support portion of a femoral head (not shown) and its base define a determined angle.

 The modular set of femoral stems shown in FIGS. 1 to 4 is made up of two series of stems, namely nine short stems 1 to 9 in FIGS. 1 and 2 and 9 long stems 10 to 18 illustrated in FIGS. 3 and 4.

 Of course, this number can be different. The cross sections d, dl ... of the stems grow steadily from the shortest stalk 1 to the longest stalk 9, the base length 1 of the shortest stalk 1 also increasing steadily up to the longest rod 9. By way of nonlimiting example, the interval of the cross-sections d-dl ... between two consecutive rods can be 1 mm.

 I1 is the same for the second group of femoral stems 10-18, the shortest rod 10 has a base length significantly greater than that of the rod 1 of the first group. The external contours of the different rods 1-18 vary regularly.

 Each femoral stem (1 ... 18) has a rectilinear posterior proximal face such as 30 for the stem 9 and 31 for the stem 10. This planar face extends over approximately 1/3 to half (length L1 for the face 30 and L2 for the face 31) of its total length. In addition, the plane of each flat face (30, 31 ...) of the femoral stems passes through the central zone of the distal end of each rod, for example the central zone 34 of the distal end of the rod 9. This layout has the advantages mentioned above.

 In the above example (Figures 1 to 4) consisting of two groups of rods of different lengths, the difference between the group of short rods and the group of long rods is about 40 millimeters, and the difference in length between the rods of the same group is 3 to 5 millimeters. Of course, this example is not limiting. I1 however corresponds best to what is desired since the subdivision into two groups of rod lengths allows to cover a maximum of possibilities at an acceptable cost.

 The distance separating the end of one of the rods from its neighboring rods can be different from the distance described in the previous case (generally greater), so as to cover a relatively large field of possibilities.

 The invention applies to the production of a modular assembly composed of several groups of lengths of rods, for example three groups. By thus composing a group of short rods, a group of long rods, and a group of intermediate rods, the range of possibilities offered to surgeons is wider, and ensures a better fit with the anatomical particularities of each patient, to a somewhat higher cost, however.

 The invention can of course extend to the constitution of groups of lengths which are more numerous than two or three, but their expensive nature then makes this possibility more difficult to envisage.

 To remedy this handicap, but also regardless of the number of groups of rod lengths, it is possible to preselect the rod that the surgeon wishes to place on his patient according to the method described above. This allows the surgeon to have to ask only this rod and the rods of similar dimensions, so as not to be caught off guard.

 The assembly targeted by the invention also comprises at least one group of modular collars such as 19 (FIGS. 5 and 8), for example two groups of collars, namely a first group of short collars and a second group of long collars, suitable to rods of corresponding dimensions.

Each neck such as 19 is composed of an elongated part 21 forming a support for a spherical femoral head 22 into which it can be inserted, and of a frustoconical base 23 extended by a diametrical boss 24.

The base 23 and the boss 24 can be received in a conjugate housing 20 of the proximal part 25 of the rod.

In a manner known per se, the boss 24 is housed in an imprint formed at the bottom of the housing 20, and which determines the angular position of the neck 19 relative to the proximal end 25.

 Each part 21 consists of a conical end 21a adapted to receive the femoral head 22, an intermediate part 21t and a flared end part 21c. The latter is connected to the conical part 23 and dimensioned to cover the end of the proximal part 25 by forming with the surface thereof a continuous surface.

For each neck, the axis XX of the base 23 and the axis
YY of part 21 can either be confused, or form between them an angle A of variable value. For example, each short neck and each long neck may have four angulations A: 0 ", 10, 15, 20, the necks 19 thus forming crutches with determined angulations A.

 For each neck of given length, there are therefore four possible angulations. By way of nonlimiting numerical example, the difference in length between a short neck and a long neck can be of the order of 5 to 7 mm.

 This creates a modular neck that is very flexible for a surgeon and allows him to adapt it closely to the anatomy of each patient. Indeed, for a crutched collar of given total length, the surgeon has the choice between several angulations A, for example four between 0 and 20 as indicated above. The neck can therefore be fitted into the corresponding end 25 in four possible positions, each of these four positions corresponding to a given cervico-diaphyseal angle (not shown in the drawings), and an anteversion angle B (FIG. 8). The first position of the cervix is in valgus, the second position in antevert, the third position in varus, and the fourth position in retroversion.

 For an angle A of 10, in the first position (valgus), the cervico-diaphyseal angle is increased by 10, while the anteversion angle B remains constant (generally 15).

 In the second position (anteverted), the cervicodiaphyseal angle remains constant (generally 1300 for adults), while the anteversion angle B is increased by 5th and therefore becomes 20C (Figure 8).

 In the third position (varus), the cervicodiaphyseal angle decreases by 10 while the anteversion angle B remains constant, and finally in the fourth position (anteverse), the cervico-diaphyseal angle remains constant while the angle of anteversion B decreases by 5 and therefore goes to 10.

 Thus, the surgeon has a large number of possible combinations in the choice and orientation of the cervix to take into account the particular anatomy of each patient and the angles of anteversion and cervico-diaphyseal which he wishes to give to the prosthesis.

 In the embodiment shown, the proximal ends 25 of the rods 1-18 have an identical and constant internal curvature C whose radius is between 150 and 170 mm approximately. This curvature was determined by a search for the best congruence between the prosthesis and the femoral bone. Its advantage lies in the fact that it allows the most uniform possible distribution of the forces exerted by the femoral stem 1-18 on the femur. It is from this common curvature that the different cross sections of the rods vary.

 Thanks to the improvement in the adaptation between each femoral stem and the diaphysis, enabled by the modularity of the femoral prosthetic assembly according to the invention, the risk of play between each femoral stem and the wall of the medullary canal of the femur is considerably reduced. It has been observed that an increase of one millimeter in diameter between two consecutive rods is particularly suitable.

 The modular set of femoral stems targeted by the invention therefore has a very great flexibility of use and anatomical adaptation for the surgeon, who thus sees his choice greatly facilitated to obtain a pose of femoral stem best suited to the specific characteristics. patient's anatomy or correction needs.

 The invention also provides an appreciable reduction in the cost price of prostheses, because not only does it make it possible to have an appreciable number of rods of variable diameter and length, but also to have a very large number of combinations using only two collars of two different sizes and several angularities A for each. This allows the surgeon to orient the selected femoral neck appropriately to obtain the aforementioned angles which he desires for each patient.

 Ultimately, the invention offers the triple possibility mentioned above (virtual elimination of the clearance between the femoral stem and the wall of the medullary canal, precise orientation of the neck in order to obtain the desired anteversion and cervico-diaphyseal angles), which was not possible with the known prostheses, and became desirable in front of the increasing requirement of the surgeons.

 The invention is susceptible of various variant embodiments, in particular as regards the number of femoral stems of the modular assembly and the angularity of the crutched necks, the above numerical indications being provided only by way of example . Thus the angles A can obviously vary.

 Although the invention has a preferred application for medullary femoral stems intended for producing prostheses for recovery, it may also be capable of having applications for prostheses of first intention. Likewise, it can find an application in shoulder prostheses,

Claims (10)

 1. Modular assembly of medullary rods intended in particular for the installation of hip prostheses, and more particularly for the installation of prostheses of recovery, comprising  a) at least two series of rods (1-18) whose length (1) and cross section (d) vary simultaneously in progressive intervals for each rod relative to the neighboring rods, each series having a determined base length,  b) at least one group of removable necks (19) having support parts (21) intended to receive the femoral heads (22) and which are inclined at different angles (A) on associated bases (23) which can be secured to the ends (25) rods, each removable neck being able to be mounted on the rod at a cervico-diaphyseal angle and an appropriate anteversion (B) or retroversion angle.  2. Modular assembly according to claim 1, characterized in that the different rods have in their proximal end (25) an internal curvature (6) identical and congruent which is between 150 and 170 millimeters approximately, from which varies the section transverse of the stems.  3. Modular assembly according to claim 1 or 2, characterized in that the external contours of the different rods (1-18) vary regularly.  4. Modular assembly according to claim 3, characterized in that the cross section (d) of each rod (1-18) varies by a value of the order of 1 mm relative to the neighboring rod, and in that simultaneously the length of this same rod varies from a few millimeters.  5. Modular assembly according to one of claims 1 to 4, characterized in that it comprises two groups of rods namely a first group of short rods (1-9) for example 9 in number, having lengths (1 ) and progressively variable cross sections (d), and a second group (10-18) of long stems, for example nine in number, having progressively variable lengths and cross sections.  6. Modular assembly according to any one of claims 1 to 5, characterized in that each rod (1-9; 10,18) has a posterior proximal face (30) rectilinear over about a third to half its length.  7. Modular assembly according to one of claims 1 to 6, characterized in that it comprises at least one group of removable necks (19), each group having a determined neck length.  8. Modular assembly according to one of claims 1 to 7, characterized in that each group of removable necks (19) is composed of several necks, for example three necks, including the axis of the support part (21) of the femoral heads (22) forms with the axis of the corresponding base (23) of each neck an angle (A) variable from one neck to another, in particular an angle of 10, 15 and 20 degrees.  9. Modular assembly according to claim 8, characterized in that each group of necks (19) comprises a complementary neck whose axis of the femoral heads (22) and the axis of the corresponding base (23) are combined.  10. A method of selecting rods of a modular assembly according to any one of claims 1 to 9, with respect to a precise length and cross section in the range available for its implantation on a determined patient, characterized in that that, having beforehand bone x-rays of the patient and layers representing the femoral stems (118) with their different dimensions in length (1) and in cross section, the dimensions of the stem are defined by superposition of the layers and x-rays prosthetic likely to suit said patient as well as rods of similar dimensions.