Articulation prosthesis, in particular a hip prosthesis, with a self-damping property

The present invention relates to a joint prosthesis formed from an implantable elongated body at one end in the medullary canal of a bone and receiving at its other end a spherical head.

The most common joint prostheses are femoral prostheses of which the ball head can reconstruct the natural shape of the articulation of the femur, at the end of a massive body sealed in the medullary canal of the femur to a support special cement.

However, it is not uncommon that, under the effect of repeated leg movements, jolts or other stresses to which the spherical head and transmitted to the elongate body cause cracking of the cement and thus a loosening of the prosthesis. This results in severe pain of the thigh that we can not remove that recimentant the body of the prosthesis into the femur, which requires practicing a new surgery on the patient.

To overcome this, some seek to transfer the shock of the head to the stem implanted via a joint. But any articulation finished by breaking. A joint prosthesis according to the preamble of claim 1 is disclosed in FR-A-23 05 961.

The present invention proposes to overcome this drawback and, to do this, it relates to a joint prosthesis of the aforementioned type which is characterized in that its body is divided in the direction of its length, in a supporting part the spherical head and an anchoring portion in the bone, joined to one another by a damping elastic connection.

This damping connection is preferably made by a direct connection of the support portions and anchor, at a junction of branch offset towards the outer face of the body of the prosthesis and holding said support portion in alignment with said anchoring portion, in conferring a possibility of slight bending or deformation to the inner face of the body, the outer and inner faces of the body of the prosthesis being here defined as those which are turned respectively towards the outer side and the inner side thigh once the prosthesis implanted in the femur.

The prosthesis according to the invention is thus equipped with a self-damping power which puts the anchor portion immune to jolts or other stresses to which may be subject spherical head.

Under these conditions, the produced cement seal around the anchor portion for securing the body of the prosthesis to the femur bone, will not be achieved by these shocks and will maintain its integrity. unsealing the risk of the prosthesis and the appearance of thigh pain are removed or at least reduced to a very large extent.

According to a first embodiment of the invention, the connecting leg of said support portions and anchoring is formed between the outer face of the body and the bottom of a slot in the latter from its internal face, said slot defining said support portions and anchor.

In a prosthesis, including a femoral prosthesis, whose body is formed of an intramedullary rod surmounted by a neck which receives at its free end the spherical head, the slot is advantageously formed in the epiphyseal portion of the intramedullary rod.

Preferably, this slot is located in a plane substantially perpendicular to the axis of the neck and its bottom expands in a rounded shape, these two latter characteristics resulting to provide the best possible damping effect.

According to another characteristic of the invention, the slot extends to a depth such that the width that it determines for said junction region, is approximately equal to the height that at the support portion in a plane substantially perpendicular to plane of the slot, the law of that background. This is the optimal compromise between the size and the positioning of the slit to eliminate any risk of breakage of the support portion at its junction zone with the anchoring part.

Can additionally fill the slot, at least partially the side of its opening, of a compressible material, avoiding the introduction of cement into the slot.

According to a second embodiment of a prosthesis according to the invention, the body is also formed from an intramedullary shaft topped by a neck which receives at its free end the spherical head, the anchor portion is formed by the entire intramedullary rod and the support part of the spherical head is formed by the neck which, by its foot constituting said junction leg, is connected to the rod of the outer face side of the latter and inclines towards its inner face.

While maintaining its conventional function of supporting the spherical head on the inside of the prosthesis, the neck can flex slightly when the latter is sought, and thus play alone the role of the damping element of the prosthesis.

To avoid breaking the neck at its connection with the rod, it will be preferable to carry out in the form of an element whose constituent material has an inherent elasticity.

Alternatively or in addition, however one can obtain the same result, in a prosthesis according to a third embodiment of the invention, giving the neck shaped such that with the bottom of a recess formed in the face next to the intramedullary rod, it defines a rounded cavity opening on the inner face of the prosthesis body and opening on the front and rear faces thereof.

Preferably, the opening through which the cavity opens on the outer face of the body of the prosthesis is demarcated by two abutments projecting to face each other, respectively on the collar and the rod. These stops have the function, in cases of extreme stress of the spherical head, to limit the amplitude of flexing the neck for the shelter of a break at its junction zone with the rod.

The present invention will now be described in more detail, but without limitation, with reference to the accompanying drawings in which:<ul><li>Figure 1 is a side elevational view of a prosthesis according to the first embodiment of the invention;</li><li>2 shows the upper portion of the prosthesis of Figure 1;</li><li>Figure 3 is an enlarged sectional view along the line III-III of Figure 1;</li><li>Figure 4 is an elevational view of the upper part of a prosthesis according to the second embodiment of the invention;</li><li>Figure 5 is an elevational view of a prosthesis according to the third embodiment of the invention;</li><li>FIG 6 is an enlarged view of the upper portion of the prosthesis of Figure 5, and;</li><li>Figure 7 is a sectional view taken in the plane VII-VII of Figure 4.</li></ul>

In its three modes exemplified embodiment, the prosthesis of the invention, namely a femoral prosthesis comprises an intramedullary rod, designated as a whole by 1, 1a or 1b, which is topped beyond its widened upper face 2, 2a, by a neck 4, 4a or 4b carrying at its free end a head 3 of general spherical shape.

The intramedullary rod has, at its end opposite the neck, a substantially cylindrical portion 5 to equal to the diameter of the medullary canal of the femur F C, by which it can be implanted in the latter and be anchored by means of a sealing cement, as shown partially in K in Figure 1 only. Between this cylindrical portion 5 and the upper face 2 of the rod 1, 1a, 1b, extends widened epiphyseal portion 13 thereof, which is received in the upper flared part of the medullary canal C.

In the first embodiment of the invention shown in Figure 1, a continuous slot 6 has been practiced, for example by a simple kerf in the epiphyseal section 13 of the rod 1, near its upper face 2 . this slit 6 which opens on the broad anterior and posterior faces of the stem 1, opens also on the inner edge or face 7 of the rod 1, that is to say the one which faces the groin once the prosthesis implanted in the patient's thigh, and ends on the other side by a bottom 8 which widens in a rounded form. The slot 6 extends also in a plane substantially perpendicular to the axis X of the cylindrical neck 4, which is positioned, as in conventional femoral prostheses, on the inner side of the prosthesis.

The slot 6 and the prosthesis divided into two parts, namely a long anchoring portion 9 comprising the cylindrical end portion 5 and the lower portion of the epiphyseal portion 13 of the rod 1, and a support portion 10 of the spherical head, consisting of the short upper portion of the epiphyseal portion and the neck 4. the support part 10 is on the inner side of the prosthesis, maintained in line or cantilever above the part anchor 9 by a short branch junction 11 that links the two parts to one another between the bottom 8 of the slot 6 and the outer portion 12 of the intramedullary rod 1.

This branch junction 11, which is integral with the anchor 10 and support portions 9 of the prosthesis usually made of titanium or a titanium based alloy, is an elastic connection which permits a slight drop towards the inside, the first relative to the second when the ball head is subjected to jolts or other stresses.

The prosthesis according to the invention thus has a self-damping power which isolates the K sealing cement, present only around the anchor portion 9, jolts or other forces exerted on the spherical head and preserves of any cracking or other degradation. To avoid penetration of cement in the slot 6, we can fill it with a compressible resin, as indicated by 14 in Figure 2.

Figure 2 further shows that the width <u>l</u> this branch junction 11, that is to say its dimension measured between the outer edge 12 of the rod 1 and the bottom 8 of the slot 6 is substantially equal to the height H has the support portion 10 to bottom right of the slot 8 in a plane perpendicular thereto. This condition determines the positioning and depth<u>p</u> the slot 6 which, in the example shown, corresponds approximately to 2/3 of the width L has the stem 1 in the same plane (see Figure 3). The slot 6 has also a height<u>h</u> about 1 to 2 mm.

These characteristics of dimensioning and positioning of the slit are those which will maximize the breaking strength of the support portion 10 at the junction of leg 11.

In the second embodiment of the invention shown in Figure 4, the intramedullary stem 1a of the prosthesis is a conventional solid rod, while the neck 4a is connected to the rod 1a, not the inner side of the prosthesis, but near its outer edge 12 by an enlarged foot 15 covering approximately the outer half of the section of the rod. It tilts further toward the inside of the prosthesis at about a 45 ° angle to terminate at its free end by a portion 16 bent upward which holds the spherical head in the position it had on the prosthesis of Figure 1. the collar 4a has in fact the general shape of a swan neck.

The neck 4, thanks to this inclination, can flex slightly inward when the head 3 is subjected to stresses and thus plays itself the role of the damping element of the prosthesis, filled in the embodiment of the FIG 1 by the supporting part 10.

To ensure optimum protection against the risk of breaking the neck 4a is preferably made of a composite material having an inherent elasticity, and reported on the rod 1a as shown in dashed lines 17.

Figures 5 and 6 show a third embodiment of the prosthesis of the invention, the collar 4b formed integrally with the metal rod 1b or attached thereto, has the same general shape as that of 4a mode embodiment of Figure 4, with the difference that widened slightly, as indicated at 18, facing the top of the stem 1b. In the latter is also a recess 19 substantially semicircular and located directly opposite the recess 16 of the neck 4b to define without interruption therewith a cavity 20 of generally circular shape.

This cavity 20, which opens at 21 in the inner face of the prosthesis, the neck 4b gives a possibility of slight elastic flexing while increasing its tensile strength at its connection to the rod 1b.

Can still be seen in Figure 6 that the opening 21 of the cavity 20 is a slot with a height of 2 to 3 mm, delimited by stops 22, 23 facing each other, formed in slight relief, one 22 under the end portion 16 of the neck 4b and the other 23 on the cortical edge of the stem 1b. Both stops 22, 23 for utility limit by mutual contact, the amplitude of flexing the neck and consequently prevent a rupture of the latter, in very occasional cases, particularly after overexertion exerted on the spherical head 3.

It is further specified with reference to Figure 7, that in the embodiment shown in Figure 4, the collar 4a of the prosthesis has a rectangular section, with its long sides or plates 24, 25 facing the internal faces 7 and 12 external of the prosthesis and its short sides or edges 26, 27 directed towards the front and rear faces thereof. This form of characteristic, which can also be found in the embodiment of Figures 5 and 6, strengthens the elasticity of the neck of the prosthesis.