Hip Joint Prosthesis

Abstract

A ball-shaped portion is to form part of the hip joint and a second portion extends from this ball-shaped portion and is to be inserted into an opening in an human femur. The second portion is provided with a passage through which an elongated expander rod is extended which is also to be inserted into the opening of the femur, and on the expander rod is arranged a series of expansion elements in form of a row which, as the expander rod is moved longitudinally of the row, are all expanded to anchor the prosthesis in the femur.

Description

Background of the Invention

The present invention relates generally to a surgical prosthesis, and more particularly to a hip joint prosthesis.

There are instances where a hip joint prosthesis is required, for instance in the event of destruction of the ball of the hip joint which may be the result of arthrosis. Hip joints for this purpose are already known. The problem with the prior-art constructions is primarily the manner in which they must be anchored in the femur. Most commonly used heretofore has been a method involving the cementing of a portion of the artificial hip joint into the channel of the femur. It is known to use a special cement for this purpose, namely methylmentharcrylate. Unfortunately, when this cement hardens after having been applied, it tends to develop a not inconsiderable amount of heat. This, in turn, can be dangerous in the particular area in which a hip joint prosthesis--which is, after all, the articulation between the femur and the innominate bone--must be installed and which could lead to damage to the bone substance and to adjacent tissue. For this reason, special complicated measures must be taken by the surgeon to cool and thus protect the bone substance and the adjacent tissue during the liberation of heat resulting from hardening of the aforementioned cement.

This is evidently highly undesirable, and a further disadvantage is added to this prior-art approach, namely the fact that the constant stresses to which the artificial hip joint will be exposed after installation, lead to a loosening of the cemented-in portion of the joint and to a constant if slow sliding of the joint deeper into the femur. The prior art has attempted to avoid this, or at least to reduce it, by providing at the junction of the ball-shaped portion of the prosthesis and of the portion which is inserted into the channel of the femur an abutment surface which provides an abutment for the prosthesis against the corticalis of the femur. However, in the long run this also is not sufficient to prevent the undesired shifting of the prosthesis with reference to the femur, so that over a period of time the bone becomes damaged. Of course, when this takes place, the prosthesis becomes unuseable. Aside from the fact that this would at the very least involve a further operation to insert another prosthesis, there exists the danger that the femur may have become damaged to such an extent that it is incapable of supporting another prosthesis.

SUMMARY OF THE INVENTION

It is, accordingly, a general object of the present invention to overcome the disadvantages of the prior art.

More particularly, it is an object of the present invention to provide an improved hip joint prosthesis which avoids the aforementioned disadvantages.

Still more particularly, it is an object of the invention to provide such an improved hip joint prosthesis which can be readily inserted and anchored in the femur without any difficulties.

Another object of the invention is to provide such a hip joint prosthesis which can also be readily removed, for instance in the event of infection.

An additional object of the invention is to provide such a hip joint prosthesis which affords a uniform pressure distribution upon the femur, so as to protect the bone material as much as possible against damage and to prevent shifting of the prosthesis into the bone under simultaneous destruction of the bone substance.

In keeping with the above objects and with others which will become apparent hereafter, one feature of the invention resides in a hip joint prosthesis, which, briefly stated, comprises first means including a ball-shaped portion which is to form part of the hip joint, and a second portion which is to be inserted into the human femur and extends from the first portion. Second means is provided for anchoring the second portion in the femur, and this second means comprises an elongated expander rod insertable through the second portion and into the femur longitudinally thereof, a row of tubular expansion elements each surrounding the rod and being angularly displaceable with reference to the longitudinal axis of the same, and actuating means for effecting axial movement of the rod with reference to the row and for thereby causing expansion of the expansion elements into anchoring contact with the femur.

The axially successive expansion elements of the row of such elements cooperate with one another, so that each of them expands on axially adjacent one, with the leading element of the row becoming expanded by an expander portion which is provided at the leading end of the expander rod and which is drawn into the leading element. The number of elements in the row can of course be selected at will, for instance in dependence upon the size and/or weight of the patient on whom the operation is to be performed, or upon other factors such as the activities to be performed by the patient.

It might be conceivable to use a single expansion element instead of a row of them. However, in such case, the entire outwardly directed force resulting from expansion of the expansion elements and which acts upon the surrounding bone material, would all be concentrated upon a very small area of the bone and would very likely crack the bone. With the arrangement according to the present invention, however, the pressures resulting from expansion are distributed over several areas of the bone which are spaced from one another, so that no particular area is subjected to unduly high pressure and cracking of the bone need not be feared. Moreover, the tubular expansion elements can be permitted to become angularly displaced with reference to the longitudinal axis of the expander rod, so that they can more readily adapt themselves to the contour of the bone channel, that is the channel in the femur which, as is well known, is a hollow bone. The channel is usually somewhat curved, and this also militates against the use of a single and rather long expansion element, so that the insertion of a row of shorter expansion elements is much simpler and in many instances is the only manner in which such elements can be inserted at all. The outer diameter of the expansion elements in the unexpanded condition thereof, may--but need not--be so selected as to correspond to the inner diameter of the channel in the bone. Even if this is not the case and the channel is rather large in its diameter as compared to the tubular expansion elements, a reliable anchoring is still obtainable because the individual elements can then engage the inner wall bounding the channel, in zig-zag fashion.

When the individual expansion elements are expanded, they each form with respect to the axially preceding element a step at which they dig into the wall of the bone. These various steps in conjunction with the frictional engagement of the elements against the bone wall, prevent the undesired lowering of the prosthesis into the bone that is the gradual sliding of the prosthesis axially of the bone with the resultant destruction of the material of the bone. This means that the position of the prosthesis with reference to the femur remains the same, even over a prolonged period of time and under constant stress. If the individual expansion elements are so turned with reference to one another, that is angularly offset relative to one another, that their axially extending slots which are provided to facilitate expansion are offset through 90.degree. from element to element, the formation of the steps and the distribution of the pressure can be further improved.

The novel features which are considered as characteristic for the invention are set forth in particular in the appended claims.

The invention itself, however, both as to its construction and its method of operation, together with additional objects and advantages thereof, will be best understood from the following description of specific embodiments when read in connection with the accompanying drawings.

BRIEF DESCRIPTION OF THE DRAWING

FIG. 1 is an axial section through the upper portion of a human femur, showing a hip joint prosthesis according to the present invention anchored in the bone; and

FIG. 2 is a fragmentary axial sectional detail view, showing the leading one of the expansion elements of FIG. 1 in expanded condition.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

Referring now to the specific embodiment illustrated in FIGS. 1 and 2, it will be seen that reference character F designates the femur, that is the tubular bone in which the novel prosthesis is to be anchored. The prosthesis is designated in toto with reference numeral 1 and has a ball-shaped portion which is to form part of the hip-joint, and a second portion which is to be inserted into the femur F as shown, and which extends from the first portion at an angle thereto. The portion 4 is provided with a bore as illustrated, and it has an end face which faces outwardly away from the femur F, and against which the head of a hollow cap nut 6 abuts, the cap nut 6 constituting the actuating means which meshes with external screw threads on the trailing end of an expander rod 7 to thereby draw the rod outwardly, that is upwardly in FIG. 1, to effect expansion of the expansion elements. The leading end of the rod is provided with an expander portion 8 of the illustrated configuration (compare FIGS. 1 and 2).

The rod is surrounded by a row of tubular expansion elements, including a trailing expansion element 9a, a leading expansion element 9a which is located adjacent the expander portion 8, and intermediate expansion elements 9. When the rod 7 is drawn upwardly in FIG. 1 by requisite turning of the nut 6, the expander portion 8 is drawn into the element 9b, expanding the same. The trailing end of the element 9b is in turn drawn into the leading end of the next adjacent element 9 which is expanded as a result of this, and this continues from element to element until finally the trailing element 9a is also expanded.

Each of the elements 9, 9a and 9b is provided with at least one axially extending slot 10 extending from its respective leading towards its trailing end to facilitate the desired expansion. The number and axial length of these slots 10 depend upon the hardness and resistance of the material to spreading of the elements 9, 9a and 9b. If, for instance, titanium is used, which is a material that is widely favored for such surgical insertions but is difficult to work, the simplicity of configuration of the expansion elements and consequently, the simplicity of the manufacture of them, is advantageous. The extent to which the elements 9, 9a and 9b can be angularly displaced with reference to the longitudinal axis of the rod 7 depends upon the difference of the diameter of their central passage 11 on the one hand, and the outer diameter of the rod 7 on the other hand. A large difference provides for a significant angular displaceability and a small difference for a lesser displaceability. The particular relationship will be selected by the surgeon in dependence upon the diameter and the curvature of the channel in the femur.

The passages 11 through which the rod 7 extends conically diverge in the direction towards the respective leading end, that is downwardly in FIG. 1 and in FIG. 2, so that the trailing end of each axially following expansion element can enter into the passage of each preceding element. The trailing ends, incidentally, are part-spherical in configuration as indicated by reference numeral 12, to facilitate their introduction into the passages 11, and to reduce the force which is applied to be exerted for spreading purposes by the rod 7. Furthermore, this also facilitates the angular displacement of the expansion elements with reference to the longitudinal axis of the rod 7.

When the rod 7 is drawn upwardly in FIG. 1 by turning of the nut 6, the elements 9 can accommodate themselves to the curvature of the passage in the femur F by angularly shifting about the longitudinal axis of the rod 7. This means that if the width of the bone passage is greater than the expansion capability of the elements 9, the latter can assume a zig-zag shaped configuration in that alternated ones of them become inclined to opposite sides of the rod 7. In this manner, the arrangement can still be reliably anchored due to the wedging action resulting from such inclination, in which the opposite axial ends of the respective elements 9 become wedged against the bone at opposite sides of the passage.

If the channel 14 in the femur F is curved to a substantial extent, a further improvement in the accommodation of the elements 9, 9a and 9b to this curvature can be achieved by making the rod 7 itself curved, in accommodation to the curvature of the channel 14. As a general rule, all channels in the various femurs are curved to a greater or lesser extent; it is very rare that a completely straight channel is encountered. Curving the rod 7 has the advantage not only of facilitating the introduction of the rod and of the elements 9, 9a, 9b into the channel 14, but also if the latter is curved significantly, the angular displaceability of the expansion elements about the longitudinal axis of the rod 7 is in some circumstances no longer sufficient to compensate for this curvature.

If the rod 7 is itself curved, then the nut 5 is used so that the rod 7 need not be turned. It will be appreciated that if the rod 7 is straight or curved only to a very slight extent, it will be possible to eliminate the nut 6 and instead to merely provide the outer end portion of the rod 7 itself with a head having a slot or the like, by means of which the rod could be turned due to engagement with an appropriate tool. It would then be this head that would bear against the outer surface of the portion 4. In that case, it will be appreciated that the portion 8 would not be of one piece with the rod 7, but instead the rod 7 would have to be threaded into the portion 8 and, as the rod 7 would be turned, the portion 8 would be drawn into the leading expansion element 9b, in the manner described earlier.

If, however, the nut 6 is used, then the portion 8 can be of one piece with the rod 7. In such an instance, which is illustrated in FIGS. 1 and 2, the axial length of the nut 6 is somewhat greater than the distance through which the nut 6 must be drawn outwardly (upwardly in FIG. 1) to effect complete expansion of all of the expansion elements 9, 9a, and 9b. This assures that the outer end of the rod 7, will be located within the central passage of the nut 6 at all times, and that the possiblity of a development of a painful pressure point resulting from engagement of an outwardly extending end portion of the rod with the surrounding tissue is avoided.

The cross section of the portion 4 corresponds at least approximately to the form of the channel 14 at the upper end of the femur F, which cross sectional configuration is usually somewhat elliptical. Also, the trailing expansion element 9a is configurated to accommodate itself to the conically converging contour of the channel 14. This facilitates the support and anchoring of the element 9a in the channel 14. The trailing end of the element 9a is provided with a recess 15 into which the adjacent end portion of the portion 4 extends, to prevent relative turning of the element 9a with reference to the portion 4.

The portion 4 extends at an angle of approximately 120.degree. with reference to the axis of the base part on which the ball-shaped portion 2 is provided and this angle corresponds approximately to the natural angle of the hip joint. A spring washer or the like can be used intermediate the outerwardly directed surface of the portion 4 and ahead of the rod 7, or, as in the illustrated embodiment, the head of the nut 6, to fix them in place against undesired turning. Such spring washers serve, incidentally, also to compensate for the effect of pulling forces acting upon the rod 7 and provide a certain amount of elastic yieldability when a load is placed upon the hip joint prosthesis.

FIG. 2 shows the leading expansion element 9b in axial section. It will be seen that it has the portion 8 is drawn into the passage 11 of the element 9b, the latter is spread apart, i.e. expanded, which is made possible by the provision of the slot 10. At the same time, the trailing end 12 of the element 9b is drawn into the passage 11 of the axially adjacent element 9, thereby expanding the latter. As this takes place for all of the elements 9, 9a, and 9b, adjacent elements form steps with one another, which steps are identified with reference numeral 16. At these steps the elements bite into the bone material surrounding the channel 14 and this in combination with the frictional engagement of the outer surfaces of the elements 9, 9a and 9b with the surrounding bone material provides for a reliable anchoring and prevents the sliding with reference to the femur, which, as outlined earlier can lead to the destruction of the bone material.

Undesired relative turning between the rod 7 and the element 9 can be prevented by making the cross-section of the rod 7 and of the passages 11 non-circular and mating, without, however, thereby interfering with the desired angular displaceability of the elements 9. In the illustrated embodiment a spring portion 17 is provided on the rod 7 which engages the corresponding groove 18 of the element 9, as shown in FIG. 2. However, a simple use of a non-circular mating cross-section, for instance quadratic or the like, can suffice also.

Finally, it is also possible to roughen the outer surfaces of the elements 9, 9a and 9b for instance by knurling them or by providing fluting or the like to increase the friction betweem them and the bone material of the femur F and to thereby improve the anchoring effect.

It will be understood that each of the elements described above, or two or more together, may also find a useful application of other types of constructions differing from the types described above.

While the invention has been illustrated and described as embodied in a hip joint prosthesis, it is not intended to be limited to the details shown, since various modifications and structural changes may be made without departing in any way from the spirit of the present invention.

Without further analysis, the foregoing will so fully reveal the gist of the present invention that others can, by applying current knowledge, readily adapt it for various applications without omitting features that, from the standpoint of prior art, fairly constitute essential characteristics of the generic or specific aspects of this invention and, therefore, such adaptations should and are intended to be comprehended within the meaning and range of equivalence of the following claims.

What is claimed as new and desired to be protected by Letters Patent is set forth in the appended claims.

Claims

I claim:

1. A hip joint prosthesis, comprising first means, including a ball-shaped portion which is to form part of a hip joint, and a second portion which is to be inserted into a human femur and extends from said first portion; and second means for anchoring said second portion in the femur, comprising an elongated expander rod inserted through said second portion and insertable into the femur longitudinally thereof, a row of tubular expansion elements each surrounding said rod and being angularly displaceable with reference to the longitudinal axis of the same, and actuating means on said expander rod for effecting axial movement of said rod with reference to said row and for thereby causing expansion of said expansion elements into anchoring contact with the femur.

2. A prosthesis as defined in claim 1, said rod having a leading end provided with an expander portion; and said row including a leading expansion member into which said expander portion is drawn in response to said axial movement.

3. A prosthesis as defined in claim 2, wherein each of said elements has a leading end and a trailing end, a center passage through which said rod extends which conically diverges towards said leading end, and a slot in its peripheral wall extending from said leading towards said trailing end.

4. A prosthesis as defined in claim 2, wherein each of said elements has a leading end and a part-spherical trailing end, and a center passage through which said rod extends and which conically diverges towards said leading end, said trailing end of each of said elements extending into said center passage at the leading end of the respective axially adjacent element for expanding the latter, in response to said movement of said rod.

5. A prosthesis as defined in claim 4, wherein said elements have an exterior configuration which coaxially converges toward the respective leading end.

6. A prosthesis as defined in claim 1, wherein said elements have outer surfaces which are roughened.

7. A prosthesis as defined in claim 1, wherein said row includes a leading and a trailing element; and wherein at least said trailing element has an exterior configuration which conically converges in direction towards said leading element.

8. A prosthesis as defined in claim 1, the femur having a curved internal channel; and wherein said rod is curved in longitudinal direction for facilitating its accommodation of said channel.

9. A prosthesis as defined in claim 1, said second portion having an inner end remote from said first portion; and wherein said row includes a leading element, and a trailing element which is adjacent said second portion, said trailing element having an end face provided with a recess into which said inner end extends.

10. A prosthesis as defined in claim 1, said second portion extending at an angle to said first portion and having a bore for said rod, and a surface which faces outwardly from the femur; and said rod having an outer end adapted to bear against said surface.

11. A prosthesis as defined in claim 1, said second portion extending at an angle to said first portion and having a bore for said rod, and a surface which faces outwardly away from the femur; said rod having a trailing portion provided with screw threads; and further a cap nut threadedly engageable with the screw threads of said trailing portion and provided with a head which bears against said surface.

12. A prosthesis as defined in claim 11, wherein said cap nut has an axial length which is at least equal to the distance through which said rod must be moved with reference to said row in order to cause said expansion of said elements.

13. A prosthesis as defined in claim 1, wherein said elements each have a center passage through which said rod extends, said passages and said rod having respective non-circular mating cross-sections for preventing undesired relative angular displacement.