FR2598609A1 - Hip prosthesis - Google Patents

Abstract

The invention relates to a hip prosthesis of the type comprising a prosthesis tail 1 intended to be cemented into the medullary cavity of the femur, a cotyle 3, 4 intended to be fixed into the iliac bone and a joint ball 2 interposed between the tail and the cotyle. According to a characteristic of the invention, the prosthesis tail comprises a femoral rod 5 which has a diaphyseal bottom part 5a of circular cross section and a metaphyseal top part 5b of oval cross section, connected without discontinuity to the circular bottom part and flaring in the direction of the upper pivot 7.

Description

HIP PROSTHESIS
The invention relates to a hip prosthesis of the type comprising a prosthesis tail intended to be sealed in the medullary cavity of the femur, a acetabulum called to be fixed in the iliac bone, and a joint ball interposed between the tail and the acetabulum.

 Such prostheses have been known and used for a long time and many models are available to practitioners. However, the existing models have imperfections. In particular, after a certain implantation time, the current prostheses give rise to cracks and ruptures of the cement sheath which seals their tail inside the medullary cavity of the femur: the holding of the prosthesis is thereby affected, until sometimes require a new intervention.In addition, we also observe, in many prostheses, poor behavior of the acetabulum in the iliac bone, resulting either from an immediate blockage of the acetabulum, which requires a rehabilitation of the bone tissue for become effective (prostheses with acetabular cups of the "poros é ta 1" type), either from the obligation to make a very significant bone sacrifice during implantation, or even from anchoring by a very limited surface of the acetabulum. In addition, the implantation of certain prostheses (tail and acetabulum) is tricky in practice: in particular for most prostheses, it is difficult to position the tail satisfactorily or to extract it without altering the sheath cement.

 The present invention aims to provide an improved hip prosthesis, which removes certain defects from known prostheses and combines multiple advantages.

 An essential objective of the invention is in particular to ensure better grip of the prosthesis, both at the level of its femoral tail and of its acetabular element.

 Another objective is to facilitate the implantation of the prosthesis (or, if necessary, its removal).

 (To facilitate understanding and simplify the terminology, the prosthesis will be described and defined below, assuming it in its implantation position).

 The hip prosthesis targeted by the present invention comprises, on the one hand, a prosthesis tail composed of a femoral rod and an upper pivot, on the other hand, a ball joint hollowed out of a female shape adapted to said upper pivot , finally, a cup adapted to receive the patella and intended to be fixed in the iliac bone; according to a characteristic of the present invention, the femoral stem of the prosthesis has a lower diaphyseal part of approximately circular section and a high metaphyseal part of approximately oval section which is continuously connected to the circular lower part and widens in the direction of the upper pivot .

In particular, the circular lower part and the oval upper part can each extend very approximately over half the height of the rod.
femoral. The lower circular part has a diameter D such that it is capable of substantially filling the medullary cavity of the femur. The upper part of oval section extends this lower part with a large diameter (according to the frontal plane) and a small diameter (according to the anteroposterior plane) both increasing from the diameter D of the circular part to upper limits, from l '' order from 2 D to 2.6 D for the large diameter, and from 1.1 D to 1.5 D for the small diameter.

The use of the prosthesis according to the invention significantly reduces the amount of cement required for sealing compared to that used in the case of conventional prostheses. In addition, an analysis of the constraints which are exerted shows, on the one hand,
the shear stresses are greatly reduced and,
on the other hand, that their distribution is very noticeably
improved in particular in external pull-ups. This reduces the risk of cracking or rupturing the cement sheath, and ultimately limits demineralization in the region
trochanteric, retaining at this level tensile stresses.

 According to another characteristic of the invention, the upper pivot is attached to the tail by a flange comprising on its upper face a blind hole with a vertical axis adapted to allow the femoral stem to be oriented or extracted.

 This hole comprises in particular, on the one hand, a threaded cylindrical central core into which a threaded extraction tool can be screwed, on the other hand, lateral recesses, giving it a suitable shape for the introduction of a appropriately shaped orientation tool.

Thus, the extraction is easily carried out along the axis of the prosthesis tail (and not along the axis of the upper pivot) which prevents damage to the cement sheath. In addition, during implantation, the practitioner can precisely orient the femoral stem relative to the femur in order to choose the desired version year.

 It should be noted that the collar by which the pivot is connected to the upper end of the femoral stem, may or may not have an overflowing plate called to come to bear on the calcar. In the absence of a support plate, the prosthesis is used as a self-locking prosthesis.

 In addition, the upper pivot of the tail is preferably arranged so as to form an angle of 1400 (to within 2 ") relative to the diaphyseal axis of the femoral stem. Such a geometric arrangement avoids excessive lateralization and makes it possible to respect the moment of action of the gluteus medius.

 Furthermore, according to a preferred embodiment, the acetabulum is composed of an internal cup having a spherical bearing and an external acetabular part.

According to an advantageous characteristic of the invention, said cup and said self-tapping cotyloid # dienne part have frustoconical surfaces, with a half-angle at the top equal to 150 (to within 5 nz).

 This acetabular structure allows, during implantation, to minimize the bone sacrifice necessary, both at the level of the background of the acetabular cavity and at its periphery. The cup, similarly conicitb, comes to block in the external part, with a locking by studs arranged at the bottom thereof, which are housed in the orifices of the external part used to screw it.

 According to. another characteristic of the invention, the self-tapping acetabular piece carries a thread cut, over its entire height, by several notches so as to form self-tapping fins having an angle of attack of 300 (to within 10 s). such fins combine with the geometry of the acetabulum to ensure excellent penetration of this part into the iliac bone, possible even in the absence of prior tapping of the bone.

 Furthermore, in a variant, the external acetabular piece can be adapted to be anchored by means of cement in the iliac bone and for this purpose comprises on its external surface, a plurality of anchoring pins.

Other characteristics, objects and advantages of the invention will emerge from the description which follows with reference to the appended drawings, which present, by way of non-limiting examples, a preferred embodiment and a variant; on these drawings which form an integral part of the present description
FIG. 1 is a view in axial section of a hip prosthesis according to this embodiment (the various parts forming said prosthesis having been separated from one another in this figure),
FIG. 2 is an external view of the prosthesis tail,
FIG. 3 is a front view of this,
FIGS. 4a, 4b, 4c, 4e and 4f are cross sections of the femoral stem respectively by planes A, B, C, E and F,
FIG. 5 is a sectional view of the acetabulum composed of its internal cup and of its external acetabular part, with a detail of the thread of the latter on a larger scale,
FIG. 6 is a view along arrow V of these parts, with a detail of a larger scale self-tapping fin,
- Figures 7, 8 and 9 show a variant of the cup, respectively in section along a plane G, seen from below according to the arrow W and in detail section H.

 The complete hip prosthesis shown in the figures is made up of four parts: a prosthesis tail 1 made of titanium or titanium alloy, a ball joint 2 also designated by "femoral head", made of alumina (or if necessary made of stainless steel), a cup 3 in polyethylene (in particular "RCH 1000"), and a self-tapping acetabular part 4, in titanium or titanium alloy.

 The prosthesis tail 1 is composed of a femoral tick 5, surmounted by a collar 6 which is extended by a pivot 7 with a Morse taper, well known in itself.

 The axis Y of the pivot forms an angle i of 14On with the diaphyseal axis X of the rod.

 In the example, the flange 6 extends beyond the internal side to form a support 6a with the squid. However, according to a variant, this support can be eliminated.

 On its upper face, the collar 6 has a blind hole 8 with a vertical axis (parallel to the axis X), formed of a central cylindrical threaded core 8a and two lateral recesses such as 8b, which are shallower.

 The femoral stem 5 is composed of two parts, a lower diaphyseal part Sa of circular section and a high metaphyseal part 5b, of oval section. Each part affects approximately half the length of the femoral stem (155 to 160 mm approximately).

 The circular section of the lower part Sa has a diameter D adapted to fill the medullary cavity of the femur. This diameter can be around 15 mm.

 The oval section of the upper part 5b is of increasing dimensions from the ace upwards as illustrated in FIGS. 4c, 4e and 4f. It connects continuously to the circular section of the lower part Sa.

 In the example, the small diameter Dp of the oval section (according to the anteroposterior plane) increases from the value D (15 mm) to a value of the order of 1.3 D (20 mm), while the large diameter Dg (according to the frontal plane) increases from this same value D to a value of the order of 2.3 D (35 mm).

 In addition, the lower end 5c of the lower part is oblique towards the inside (along the diaphyseal axis from top to bottom) so as to exert at this end an asymmetrical pressure, avoiding placing the prosthesis too much varus.

 In addition, the femoral stem includes longitudinal ridges 9 intended to facilitate the escape of excess cement during implantation.

 The prosthesis tail described above is preferably forged from titanium so as to increase its strength and elasticity by aligning the fibers. This gives an elasticity closer to that of the cortical bone, associated with remarkable qualities of resistance.

 The shape of this tail facilitates its correct positioning in the medullary cavity during implantation. The practitioner has the option of introducing into the hole 8 a lug tool for precisely adjusting the orientation of the tail according to the desired anteversion. In addition, this tail shape considerably reduces the thicknesses of cement put in place, improves the distribution of stresses and greatly reduces shear stresses. The risks of cracking or rupturing of the cement, which exist with known prostheses (in particular at the level of the trochanteric region and the internal edge of the femur) are very reduced.

 If necessary, the prosthesis tail can be removed by screwing a tool into the threaded core of the hole 8 and by exerting a traction thereon parallel to the diaphyseal axis X.

 Furthermore, the ball joint 2, in itself conventional, is hollowed out of a frustoconical female form 2a, combined with that of the pivot 7 of the tail. This ball, preferably of alumina, forms with the polyethylene cup 3 a very favorable torque.

 Said cup 3 is provided with a spherical bearing 3a conjugate with the ball 2 and is blocked in the acetabulum 4 after placement of the latter in the iliac bone. It has a frustoconical external surface 3b of the same half anole at the top ss as that of the cotylo # dienne part (15). Its bottom is provided with two external studs such as 10 for its rotation blocking in said part. In addition, a chamfer 11 to 300 is formed on the edge of this cup in order to avoid a cam effect with the pivot of the prosthesis tail. This edge is flush with that of the acetabular part without overflow. Two small grooves such as 12 allow angular positioning of the cup in the acetabular part when it is put in place.

 The self-tapping acetabular piece 4 is intended to be blocked by screwing into the acetabulum of the iliac bone in the absence of cement.

 This part 4, in the titanium example, has a frustoconical shape with a half-angle at the apex P of 15, both for its external surface and for its internal surface receiving the cup. On the bottom of this part 4, two holes are provided such as 14, which allow the part to be screwed into the bone with an appropriate tool; the studs 10 of the cup then come into these holes.

 On its outer surface, the part 4 carries a thread 13 whose threads are of axis Z perpendicular to the frustoconical surface, that is to say inclined by an angle T of 750 (to within 5 R) relative to the axis T of said frustoconical surface.

 This thread has a pitch -p- of about 5 mm (to within 10%), the cross section of the threads having an angle at the apex -s- of 600 (to within 10 pj). The height -h- of the thread is approximately 3 mm (to within 10%).

 In addition, the thread 15 is notched over its entire height by several notches, in the example five in number distributed around the periphery, so as to form self-tapping fins 13e having an angle of attack -q- d ' about 300 (a 10 Gó near).

 The screwing of the part 4 described above in the acetabulum of the iliac bone ensures an immediate blocking thereof, in the absence of cement. A preliminary tapping can, if necessary, be carried out in the acetabulum with appropriate cutters.

 The geometry of this part 4 and its thread 13 best preserves the bone tissue (low bone sacrifice at the level of the acetabulum due to the taper; minimal bone sacrifice around the acetabular circumference due to the taper angle) and provides excellent hold (progressive tightening in the bone).

 In addition, the geometrical arrangement of the fins allows good penetration with a reduced effort compared to that required in the case of conventional prostheses.

 It should be noted that the geometry of the thread (angle, pitch) gives it good resistance and provides a wide base space between the threads which is filled by a wide, strong and well vascularized bone bridge. This eliminates the risk of necrosis at this level, which guarantees satisfactory behavior over time.

 Furthermore, Figures 7, 8 and 9 show a variant of the acetabulum, intended to cooperate with a shank and a ball joint as previously described.

 This acetabulum is composed of two removable parts: an internal cup 15 in polyethylene and an acetabular cup part 16 to be cemented in titanium.

 As before, the cup 15 has a spherical internal bearing of shape conjugate with that of the ball 2. In addition, it has, on its circular edge, lugs 17 in the example three in number, in order to ensure its locking in the cotylo # piece.

 In the example, the cup 15 is inserted into the acetabular part 16 by surfaces with cut sides, ensuring good immobilization thereof.

 The cotylordian part has on its external hemispherical surface, anchoring pins such as 18, which are distributed over this surface. In the example, twelve pins are divided into three rows as illustrated in Figure 8. Each pin has the shape of a small cylindrical pad 3 mm high. These pins embedded in the cement ensure excellent anchoring of the part and guarantee a cement thickness at least equal to 3 mm.

 In addition, the acetabular part 16 is provided with a peripheral flange 19 so as to carry out the sealing under pressure. This flange is notched with three notches such as 19a in which the lugs 17 of the cup 15 are inserted in order to ensure the locking thereof.

 In addition, the flange 19 is notched with three narrower notches such as 19b which serve as exhaust pipes for the cement when the part is put in place.

Claims (17)

 1 / - Hip prosthesis comprising, on the one hand, a prosthesis tail (1) composed of a femoral stem (5) and an upper pivot (7), on the other hand, a hollowed-out ball joint (2) of a female form (2a) adapted to said upper pivot, finally, an acetabulum (3, 4; 15, 16) adapted to receive the patella and intended to be fixed in the iliac bone, said prosthesis being characterized in that its femoral stem (5) has a lower diaphyseal part (5e) of approximately circular cross-section and a high metaphyseal part (5b) of approximately oval cross-section which is continuously connected to the lower circular part and widens towards the upper pivot.  2 / - Hip prosthesis according to claim 1, characterized in that the circular lower part (5a) and the upper oval part (5b) each extend very approximately over half the height of the femoral stem (5).  3 / - Hip prosthesis according to one of claims 1 or 2, characterized in that, in its upper oval part (5b) the femoral stem (5) has  - according to the frontal plane, a large diameter Dg increasing from the diameter D of the circular lower part (5a) to an upper limit approximately between 2 D and 2.6 D,  - according to the anteroposterior plane, a small diameter Dp increasing from the diameter D of the lower circular part (5a) to an upper limit approximately between 1.1 D and 1.5 D.  4 / - Hip prosthesis according to one of claims 1, 2 or 3, characterized in that the circular lower part (5a) of the femoral stem has a diameter D such that said part is capable of substantially filling the medullary cavity of the femur.  5 / - Hip prosthesis according to one of claims 1, 2, 3 or 4, in which the femoral stem (5) comprises longitudinal ridges (9) for escaping the cement during implantation.  6 / - Hip prosthesis according to one of claims 1, 2, 3, 4 or 5, characterized in that the lower circular part (Sa) ends in a lower end (5c) oblique towards the inside (following the diaphyseal axis from top to bottom).  7 / - Hip prosthesis according to one of claims 1, 2, 3, 4, 5 or 6, characterized in that the upper pivot (7) is attached to the tail by a flange (6) having on its upper face a blind hole with a vertical axis (8) adapted to allow the femoral stem to be oriented or extracted.  8 / - Hip prosthesis according to claim 7, characterized in that the blind hole (8) is formed by a central threaded cylindrical core (8a) for the screwing of a threaded extraction tool, and recesses lateral (8b) for the introduction of a lug orientation tool.  9 / - Hip prosthesis according to one of claims 1, 2, 3, 4, 5, b, 7 or 8, characterized in that the upper pivot (7) is arranged so that its axis (Y) forms at 2 t close, an angle (ol) of 1400 with respect to the diaphyseal axis (X) of the stem.  10 / - Hip prosthesis according to one of the preceding claims, in which the acetabulum is composed of an internal cup (3) having a spherical bearing (3a) and of a self-tapping external acetabular piece (4), characterized in that said cup (3) and said external acetabular piece (4) have frustoconical surfaces, with a half-angle at the top (p) equal to 150 (to within 5 t).  11 / - Hip prosthesis according to claim 10, characterized in that the external acetabular part (4) carries a thread (13), having threads of axis (Z) perpendicular to the frustoconical surface, that is to say say inclined by an angle (T) of 750 (to within 5 o) relative to the axis (T) of said frustoconical surface.  12 / - Hip prosthesis according to claim 11, in which the thread (13) of the external acetabular part is notched, over its entire height by several notches so as to form self-tapping fins (13e), characterized in that said fins have an angle of attack (q) of 300 (to within 10 S).  13 / - Hip prosthesis according to claim 12, characterized in that the thread (13) and the fins (13a) formed by the notches thereof have a height of 3 mm (to within 10 S).  14 / - Hip prosthesis according to one of claims 10, 11, 12 or 13, characterized in that the pitch (p) of the thread (13) is 5 mm (to within 10 S), the cross section of the threads with an angle at the top (s) of 600 (to within 10 t).  15 / - Hip prosthesis according to one of claims 10, 11, 12, 13 or 14, in which the tail (1) of prosthesis and the external cotylo # piece (4) are made of titanium or titanium alloy, the cup (3) being made of polyethylene and the ball joint (2) made of stainless steel or alumina.  16 / - Hip prosthesis according to one of claims 1 to 9, in which the acetabulum is composed of an internal cup (15) having a spherical bearing and of an external acetabular piece to be cemented (16), provided externally with 'a plurality of inking pins (18).  17 / - Hip prosthesis according to claim 16, characterized in that the external acetabular part (16) is provided with a peripheral flange (19) notched, on the one hand, cement exhaust notches (19b) , on the other hand, notches (19a) for locking the cup which is provided with lugs (17) capable of being inserted in said locking notches.