FR2629707A1 - Femoral piece and equipment for making a hip prosthesis - Google Patents

Abstract

Femoral piece comprising a femoral rod 2 and a neck 3 terminated by a frustoconical portion 40 intended to receive a spherical head. The cross-section of the rod (planes P1 to P5) has a developing shape which ensures its correct fitting to the medullary cavity.

Description

Femoral part and equipment for the production of a hip prosthesis.

The invention relates to prosthetic hip surgery.

Hip prostheses have been produced for a little over twenty years. They essentially consist in fixing to the femur, after having eliminated the neck and the head, a prosthetic femoral piece comprising a femoral stem which is introduced into the medullary canal and a prosthetic neck which protrudes out of this canal, the neck being oriented. laterally inward of the patient's body relative to the rod. The femoral part may also include, at the end of the neck, a prosthetic head. Alternatively, the end of the neck may have means such as a Morse taper allowing its connection with an attached head.

The prosthetic head has a spherical shape which, in some cases, is of large diameter to cooperate with the natural acetabulum. However, most often, the prosthetic head is intended to cooperate with a prosthetic acetabular cup implanted in the pelvis. The diameter of the prosthetic acetabulum and the associated head is generally between approximately 22 millimeters and approximately 32 millimeters, the most common value being 22.2 millimeters.

The connection between the rod and the bone is ensured either by a cement interposed between them, or by direct embedding, the surface of the rod then being rough to allow rehabitation by the bone tissue.

Taking into account the date on which the first hip prostheses were produced, and the fact that less and less aged subjects are operated on, surgeons are more and more often able to perform restorations of prostheses put previously in place. During such recoveries, especially on prostheses sealed with cement, the inventors have encountered cases of detachment of the rod relative to the femur, leading to a limited but painful relative movement. They realized that the stems of these defective prostheses were housed in too large femoral cavities filled with a large volume of cement. This excessive volume of cement probably plays a major role in the loosening of the stems, due to the elasticity characteristics of the cement different from those of the stem material and bone tissue.

The inventors have also found, during resumptions, cracks in the upper part of the femur, extending from the cavity housing the prosthetic rod towards the plane of symmetry of the patient's body.

To avoid the drawbacks observed, the inventors have sought to develop a prosthetic part and equipment for producing prostheses ensuring that the femoral cavity is filled as completely as possible with the prosthetic rod, this cavity being obtained by very reduced enlargement of the natural channel, while limiting the mechanical stresses exerted by the rod on the bone.

For this purpose, the femoral part according to the invention, comprising a femoral stem intended to be introduced into the medullary canal of the femur and a neck intended to project out of the medullary canal by carrying at its end a femoral head, the neck being oriented laterally inward of the patient's body with respect to the rod when the latter is engaged in the medullary canal, is characterized in that the cross section of the rod presented an evolving form from one end to the other with a first substantially circular shape in the vicinity of the end opposite the neck, a second shape elongated in the lateral direction, narrowing symmetrically inward and outward, in the middle region, and a third shape elongated in the direction lateral, widening from the inside to the outside, near the neck.

It was found that the shape thus defined corresponds substantially to that of the medullary canal in its upper part, used to house the femoral stem, which can thus be correctly adjusted in a housing obtained by a very slight enlargement of the canal, as will be described further. In addition, the narrowing inward - that is to say to the left of the patient's body in the case of a right hip prosthesis, or to the right in the case of a hip prosthesis -left- of the upper part of the stem avoids subjecting the bone tissue, in this region, to mechanical stresses likely to lead to cracks.

Preferably, the first circular shape of the section of the rod is amputated by a segment inward, as known per se, to also limit the mechanical stresses imposed on the bone tissue.

As for the second and third shapes of the section, they can in particular be respectively oval and substantially trapezoidal.

It is advantageous for the cross section of the rod to evolve so that it can be extracted, without deformation, from a femoral cavity which it completely filled. This facilitates the replacement of the femoral part, if necessary, during a recovery.

Preferably, the neck is terminated by a frustoconical portion suitable for being capped by a separable head having a frustoconical recess. This arrangement makes it possible to produce the femoral part and the head in different materials, the first being chosen for its elasticity characteristics similar to those of bone tissue, its biocompatibility, its low density and its high breaking strength, and the second for its resistance to wear and its low coefficient of friction, the two materials should not form an electrochemical couple between them. As an example, a suitable material for the femoral part is a titanium-based alloy, in particular an alloy composed of 90% titanium, 6% aluminum and 4% vanadium, associated with a femoral head made of chromium alloy. -cobalt-nickel.

The equipment according to the invention comprises at least one series of femoral parts as described above, differing from one another by the length of the rod and / or the dimensions of its cross sections and / or the length of the neck.

During an intervention, the appropriate femoral part is chosen in the equipment according to the geometric characteristics of the femur to be treated.

Where appropriate, the equipment also comprises at least one prosthetic head which can be adapted either to the frustoconical end of the neck of each femoral component, and preferably a series of such prosthetic heads which differ from one another by their external diameter and / or the opening diameter of the frustoconical recess.

For the preparation of the femoral cavity, it is planned to include in the equipment a series of rasps each comprising a body bristling with abrasive projections, with each femoral piece being associated a rasp whose body has a shape and dimensions it allowing to provide in a femur a cavity suitable for accommodating the rod with minimal and substantially uniform play.

Advantageously, the length of the body of each rasp is equal to the depth of the cavity to be formed in the femur, the coincidence between the upper end of the body of the rasp and that of the femur indicating that the cavity is ready.

The equipment may further include a series of necks and test heads adaptable to the rasps to simulate, when the rasp is engaged in the cavity of the femur, the position of the head of the prosthesis relative to the latter, each neck and test head assembly corresponding to the length of the neck of a femoral component and to a type of prosthetic head.

 Other characteristics and advantages of the invention will emerge from the detailed description below and the appended drawings, in which - Figure 1 is an elevational view of a femoral part according to the invention; - Figures 2a to 2e show cross sections of the rod of the part illustrated in Figure 1, at different heights; - Figure 3 is a view similar to Figure 1, showing a slightly modified femoral part and associated with a femoral head shown in section; - Figure 4 shows, in elevation, a series of femoral parts belonging to equipment according to the invention; and - Figure 5 is an elevational view of a rasp corresponding to the femoral part of Figure 3.

The femoral part 1 shown in Figure 1, made of titanium-aluminum-vanadium alloy, is formed of a rod 2 intended to be housed in the cavity of a femur, and a neck 3 intended to protrude from the femur. The rod and the neck are connected in a plane 4 perpendicular to the axis 5 of the neck, which has a shape of revolution, and oblique with respect to the longitudinal direction of the rod. The entire part has a longitudinal plane of symmetry parallel to the plane of Figure 1, the neck 3 moving away from the rod 2 to the left of the figure.

We now describe the outline of the rod 2 in the plane of symmetry. From an upper point 6 belonging to the plane 4, the right side of this contour first comprises a convex portion 7 in the form of an arc of a circle of radius R7, then a rectilinear portion 8, which extends over the major part of the length of the rod, tangent to the portion 7 at their common end 9. The portion 8 is extended by a convex rounded end 10 with a small radius of curvature, to which another straight portion 11 is connected making a small angle with the portion 8, and of a short length compared to the total length of the rod. The portion 11, which belongs to the left side of the contour, is connected at an obtuse angle to another straight portion 12 also making, with the portion 8, a small angle, equal in the example shown to approximately 60. The portion 12 is followed by a concave portion 13 in the form of an arc of a circle of radius
R13 greater than R7, which is tangent to the portion 12 at their common end 14, closer to the end 10 than point 9. Finally, a concave portion 15 in the form of an arc of a circle of small radius, tangent to the portion 13 at their common end 16, joins the latter at point 17 diametrically opposite point 6 in the plane 4. The rectilinear portion 8, the direction of which represents substantially the longitudinal direction of the rod, made with the axis 5 of the neck 3 an angle of about 1300.

The cross sections of the rod shown in FIGS. 2a to 2e will now be described, the contours of which are formed by rectilinear portions and convex portions in arcs of a circle, these portions connecting tangentially to each other in the sections represented in FIGS. 2a to 2d . The section of Figure 2a is taken in the vicinity of the upper end of the rod, by the plane P1 parallel to the plane 4 and passing through the point 16. This section S1 has the shape of a trapezoid or a curvilinear triangle and is of course symmetrical with respect to a straight line D1 contained in the plane of symmetry of the femoral part. Its contour is formed by six portions, namely: a portion 20 in an arc of a circle of radius r20 having its center on the axis D1 of symmetry and limiting the section to the left of the figure, that is to say on the side facing the inside of the patient's body when the part is implanted; an opposite portion 22 in an arc also having its center on the right
D1, and of radius r22 greater than r20; two rectilinear portions 21 symmetrical to one another with respect to Dl, connecting to the portion 20 and diverging therefrom; and two portions 23 in arcs of a circle of radius r23 less than r20, connecting the portions il and the portion 22. The section S1 is entirely contained inside the circle C1 of which the portion 22 is a part. generally by narrowing from the right to the left of the figure, that is to say from the outside to the inside of the patient's body.

Section S2 illustrated in FIG. 2b, with an axis of symmetry
D2, is taken along a plane P2 perpendicular to the plane of symmetry of the part 1 and passing through the point 9 and through the center 18 of the circular arc 13. Its outline is composed of eight portions, including two arc portions of circle 24 and 25 limiting the section respectively to the left and to the right and of respective radii r24 and r25, r24 being less than r20 and r25 being substantially equal to r22. On each side of the straight line D2, there is successively, from the portion 24 to the portion 25: a portion 26 in an arc of a circle of radius r26 substantially equal to r25; a rectilinear portion 27 parallel to D2; and an arc portion 28 of radius r28 less than r24 and substantially equal to r23. In its left half, section S2 narrows towards the left.

Section S3, shown in Figure 2c, has an elongated oval shape in the left-right direction, or interior-exterior; with two axes of symmetry D3 contained in the plane of symmetry of the part and D'3 perpendicular to it.

Its outline is formed by four portions in an arc of a circle of two different radii, the smallest radius r29 being less than r24 and the largest r30 being less than r22 and r25. Section S3 is taken along the plane P3 perpendicular to the plane of symmetry of the part and passing through points 14 and 18.

The section S4 in FIG. 2d has a configuration similar to that of the section S3, with a small radius r31 and a large radius r32 smaller than the radii r29 and r30 respectively.

Finally, the section S5 of FIG. 2e is limited by an arc of a circle 33 of an extent greater than a half-circumference, whose radius r33 is substantially equal to r31, completed on the left by a line segment 34. This segment 34 and the segment 11 of the longitudinal profile of the rod belong to a plane according to which the end part of the rod is chamfered to prevent the end of the rod from exerting pressure on the bone tissue inwards. The edges connecting this chamfer to the curvilinear surface of the rod are preferably blunt to avoid injuring the tissues. Sections S4 and S5 are taken along planes perpendicular to the straight line 8 defining the longitudinal direction of the rod, the plane P5 of section S5 passing between the free end of the rod and the point 19 of connection of segments 11 and 12, and the plane P4 of section S4 being approximately equidistant from plane P5 and point 14. sections S2 to S5, that is to say its size in the direction of the plane of symmetry of the rod, and its width, that is to say its size in the direction perpendicular to the previous one, are less respectively the length and width of the previous section, with the exception of the width of section S3 which can be equal to that of section S2.

 Besides an excellent adaptation to the femoral canal, the shape of the stem as it has just been described allows, in the event of recovery, its extraction without any deformation either of the stem or of the bone.

The neck 3 ends with a very smooth frusto-conical part 40, of the "Morse taper" type, defined by a small and very precise apex angle, equal to 60. This part 40 is connected to the plane 4, where the neck is connected with the rod, by an intermediate part constituting the neck proper, whose generatrix is an arc 41 whose concavity is turned away from the axis of revolution 5, and which is tangent to the generatrix of the truncated cone 40 at their connection point 42. At this point 42, the diameter of the neck is substantially equal to 12 millimeters, which ensures both good mechanical resistance and good movement of the joint.

The femoral part 1 ′ shown in FIG. 3 differs from the part 1 in FIG. 1 only by the removal of the chamfer from the end of the rod, the section S5 being replaced by an entirely circular section. On the frustoconical part 40 of the neck is threaded a prosthetic femoral head 42 of chromium-cobalt-nickel alloy, in the form of a truncated sphere. The head 42 has a recess 43 in trunk of Morse cone in which the part 40 adjusts to achieve a very stable connection of the two parts. Another broken line 42 ′ is shown in dashed lines, similar externally to the head 42 and capable of being threaded in place of the latter.

The head 42 'differs from the head 42 by a smaller opening diameter and depth of the recess, so that a shorter length of the part 40 penetrates there. Thus, the center 44 'of the sphere formed by the head 42', which constitutes the center of the prosthetic joint, is more distant from the femoral stem than would be the center 44 of the head 42.

Figure 4 shows a series of seven femoral pieces of the type of that of Figure 3, designated by the letters A to G, differing from each other by the length or height of the rod, the dimensions of the cross sections of this and the length of the intermediate part of the neck, the frustoconical part thereof and the inclination of the neck relative to the rod being identical for all the parts.

The length of the rod, measured in the direction of the line segment 8 between point 6 and the free end of the rod, takes a first value hl equal to approximately 120 millimeters for parts A and B, a second value h2 equal at approximately 130 millimeters for part C, a third value h3 equal to approximately 140 millimeters for part D and a fourth value h4 equal to approximately 150 millimeters for parts E to G. The dimensions of the homogeneous cross sections of the rods, including including the diameter of the junction of the rod and the neck in the plane 4, gradually vary from part A to part G. In particular, the dimension L1 of section 51 in the direction D1 varies from 17 to 26 millimeters approximately , and the dimension Q1 in the perpendicular direction from 13 to 19 millimeters. The corresponding dimensions L3 and # 23 of section S3 vary respectively from 10 to 20 millimeters approximately and from 8 to 12 millimeters approximately, and the radius r33 varies from 3 to 4.5 millimeters approximately. As for the length of the part intermediate of the neck, it is such that the distance between the articulation center and the plane 4 is approximately 23 millimeters for the part A, 28 millimeters for the part B, 31 millimeters for the parts C and D, 35 millimeters for the part E and 39 millimeters for the parts F and G, with the same head 42, these distances being increased by a constant value of approximately 4 millimeters with another head 42 '.

This series of parts, supplemented by two heads 42 and 42 ′, preferably with a diameter of 22.2 millimeters, constitute equipment making it possible to produce femoral prostheses adapted to all cases that may arise in the first intervention. This equipment is advantageously supplemented by femoral parts of recovery such as those marked rD and rF in FIG. 4, which differ from parts D and
F respectively by the removal of the upper part of the rod, the remaining part of the rod being connected by an intermediate portion of elongated neck to a frustoconical end part identical to that of the first intervention parts. Such recovery parts are useful when it is necessary to remove the upper part of the femur during a recovery. The inventors have found that it was in practice sufficient to provide these two recovery parts corresponding to parts D and F, which frame the part E most commonly used in first intervention.

5 shows a rasp 50 intended for the preparation of the femoral cavity for the housing of the rod of the part 1 'shown in FIG. 3. The rasp 50 is made of steel. It is constituted by a body 51 having the same general shape as the rod 2 ', as well as by an axis 52 and a nipple 53. The body 51 is bristling, over the entire lateral surface and up to its end, with roughness abrasive 54 allowing, by the progressive introduction of the body into the medullary canal of the femur, to very slightly widen the latter so as to be able to accommodate the rod 2 'with a small, almost uniform clearance. The body 51 is limited by a planar end surface 55 whose plane corresponds to the plane 4 of junction between the rod and the body of the part L '. The cylindrical axis 52 projects in the middle position perpendicular to the surface 55, the stud 53 also extending perpendicular to the surface 55 from a marginal region thereof.

The axis 52 and the stud 53 make it possible to attach to the rasp 50 a handle, not shown, for the introduction of the rasp into the femur and its extraction.

The equipment according to the invention preferably comprises a series of rasps associated with the femoral parts of the series described above, respectively, as well as a single handle adaptable equally to all rasps. The equipment according to the invention can also include test parts, not shown, defining necks and spherical test heads suitable for being placed on the rasps, via the axis 52 and the stud. 53 of these, to simulate the position of the neck and head of the prosthesis, for a purpose which will appear later. These test pieces may include a series of trial necks of different lengths corresponding to the different neck lengths of the femoral parts of the equipment, as well as of the test heads adaptable indifferently to the various test necks and corresponding respectively to the two prosthetic heads. Alternatively, it is possible to use a series of mixed pieces forming both a neck and a test head, eight such pieces being necessary to make the eight neck lengths resulting from the combination of the seven femoral pieces.
A to G and two heads 42 and 42 '.

We will now describe, in broad outline, the course of a first intervention carried out with equipment comprising the following elements as described above: the femoral parts A to G and possibly H, the heads 42 and 42 ', the graters and the corresponding test pieces, and a handle for the graters. It is first determined, from radiographs of the femur to be treated, which of the femoral pieces of the existing series should be most suitable, the stem of this piece having to be housed in the medullary canal with minimal enlargement of the latter, and the neck should allow, associated with one of the prosthetic heads, to correctly position the center of articulation relative to the femur. We saw the femur flush with the natural neck, according to a plane with which the junction plane 4 between the stem and the neck of the femoral part should be merged. We mount the handle on the rasp of the size immediately below that corresponding to the piece provided, or on the even smaller grater. If, as is normal, the selected rasp penetrates almost freely in the medullary canal, we pass to a larger rasp until obtaining by enlargement of the channel a cavity housing the body of the rasp in a stable manner. The final rasp being pressed until the end face 55 of its body is flush with the saw plane of the femur, the handle is separated therefrom which is replaced by the test piece or pieces corresponding to the neck length of the prosthesis provided. It is then possible to fit the test head into the acetabulum to verify that the configuration of the joint is correct. If the position of the articulation center is not suitable, a correction is possible by changing the prosthetic head from that planned, or by using a femoral piece having a neck length different from that planned.

After removing the rasp, a plug is placed in the medullary canal, immediately below the cavity to be filled by the stem of the femoral component, to prevent the flow of cement. The latter is then introduced into the cavity, after which the rod of the prosthetic part is inserted which fills the cavity and places itself in the correct position, the cement distributing evenly in the interval between the rod and the wall of the cavity. We then put in place the prosthetic head which fits into the acetabulum.

The femoral part according to the invention, like the known femoral parts, can comprise or be associated with heads of different diameters intended to cooperate either with prosthetic cups, or with natural cups. In the case of a prosthetic cup, it is preferably made of high density polyethylene, offering a reduced coefficient of friction with the head of chrome-cobalt-nickel alloy.

The acetabulum can also comprise a metallic layer, for example made of titanium, on the side of the pelvis. This metal layer may include roughness or porosity allowing bone rehabitation.

Although having been described more particularly in relation to the production of sealed prostheses, the invention can also be used advantageously in the case of unsealed prostheses. Of course, the number of femoral parts making up the equipment according to the invention, their dimensions and the details of their shapes have been given for information only and can be modified without departing from the invention. The same applies to the materials used. In particular, ceramic heads and femoral parts made of carbon-based composite material having elasticity characteristics even closer to those of bone than those of titanium alloys are contemplated. The stem of the femoral parts can have a coating promoting adhesion of the cement.

 The prosthetic parts described are equally suitable for the right and left femurs. The natural articulation center not being strictly in the plane of symmetry of the medullary canal, it is possible, to further improve the geometric characteristics of the prosthesis, to modify the prosthetic parts by removing the axis of revolution from the neck of the plane stem symmetry. It is therefore necessary to provide right and left prosthetic parts, which doubles the number of parts in the series.

Claims (14)

Claims 1.- Femoral component (1) for producing a hip prosthesis, comprising a femoral stem (2) intended to be introduced into the medullary canal of the femur and a neck (3) intended to protrude out of the medullary canal by carrying at its end a femoral head (42), the neck being oriented laterally towards the inside of the patient's body with respect to the rod when the latter is engaged in the medullary canal, characterized in that the cross section of the stem has an evolutive shape from one end to the other with a first shape (S5) substantially circular in the vicinity of the end opposite the neck, a second shape (S3, S4) elongated in the lateral direction, shrinking symmetrically inward and outward, in the middle region, and a third shape (S1) elongated in the lateral direction, widening from the inside outward, in the vicinity of the neck. 2.- femoral component according to claim 1, characterized in that the first circular shape of the section of the rod is amputated by a segment inward. 3.- femoral piece according to one of claims 1 and 2, characterized in that the second shape of the section of the rod is oval. 4.- femoral part according to one of the preceding claims, characterized in that the third shape of the section of the rod is substantially trapezoidal. 5. Femoral part according to claim 4, characterized in that said trapezoidal section has lateral sides (21) substantially rectilinear and convex curvilinear bases (20,22,23), there small base (20) being substantially in arc of circle tangent to the lateral sides. 6. Femoral part according to one of the preceding claims, characterized in that the development of the section of the rod is such that it can be extracted, without deformation, from a femoral cavity which it completely filled. 7.- femoral component according to one of the preceding claims, characterized in that the neck is terminated by a frustoconical portion (40) suitable for being capped by a separable head (42) having for this purpose a frustoconical recess (43). 8.- Femoral part according to claim 7, characterized in that it is made of a titanium-based alloy. 9.- Equipment for producing a hip prosthesis, comprising a series of femoral pieces (AG) according to one of the preceding claims, differing from each other by the length of the rod (hl-h4) and / or the dimensions of its cross sections (S1-S5) and / or the length of the neck. 10.- Equipment according to claim 9, characterized in that the femoral parts conform to one of claims 6 and 7 and in that it further comprises at least one prosthetic head adaptable equally on the frustoconical end of the neck of each of them. 11.- Equipment according to claim 9, characterized in that it comprises a series of such prosthetic heads (42,42 ') differing from each other by their outside diameter and / or the opening diameter of the frustoconical recess . 12.- Equipment according to one of claims 9 to 11, characterized in that it further comprises a series of graters (50) each comprising a body (51) bristling with abrasive projections (54), with each femoral part being associated with a rasp whose body has a shape and dimensions allowing it to provide in a femur a cavity suitable for accommodating the rod with minimal and substantially uniform play.  13.- Equipment according to claim 12, characterized in that the length of the body of each grater is equal to the depth of the cavity to be formed in the femur. 14.- Equipment according to one of claims 12 and 13, characterized in that it further comprises a series of necks and test heads adaptable to the rasps to simulate, when the rasp is engaged in the cavity of the femur , the position of the prosthesis head relative to the latter, each neck and test head assembly corresponding to the length of the neck of a femoral component and to a type of prosthetic head.