WO2008026135A1

WO2008026135A1 - Lower arm arthroplasty

Info

Publication number: WO2008026135A1
Application number: PCT/IB2007/053371
Authority: WO
Inventors: Malan De Villiers
Original assignee: Malan De Villiers
Priority date: 2006-08-28
Filing date: 2007-08-23
Publication date: 2008-03-06

Abstract

The invention concerns a lower arm arthroplasty, possibly a proximal radial head replacement or a distal radio-ulnar arthroplasty. The arthroplasty includes an intermedullary stem (12, 52) having a first end for anchorage in a hole in a forearm bone and a second end (14, 60). The arthroplasty also has a head (18, 55) with an external, curved surface (44,46, 66) for articulation relative to another bone (53). Another component of the arthroplasty is an intermediate member (16, 54) which locates between the head and the second end of the stem and is enagable with those components. The intermediate member is engagable with at least the stem in an articulating manner which allows the head to pivot relative to the stem through a limited range of angles. In the preferred embodiment, the intermediate member engages both the head and the stem in articulating manner.

Description

"LOWER ARM ARTHROPLASTY"

BACKGROUND TO THE INVENTION

THIS invention relates to a lower arm arthroplasty. In particular the invention relates to a proximal radial head or distal radio-ulnar arthroplasty.

The radial head, which is located at the upper or proximal end of the radius of the forearm, has a concave surface which articulates relative to the capitulum of the humerus. This articulating joint can be fractured in instances where, for instance, a person attempts to break a fall with an outstretched arm. Where severe fracture takes place, the proximal end of the radius may be modified as appropriate in order for the radial head to be replaced by an arthroplasty.

One class of known radial head arthroplasty is of rigid construction and has an intermedullary stem which is cemented in a hole drilled for the purpose in the proximal end of the radius and a head at the proximal end of the stem which is formed with the required concave surface to articulate with the capitulum. An example of a known arthoplasty in this class is the Swanson™ implant in which the stem is aligned relative to the concave surface. Misalignment of the arthroplasty as a result of inaccurate implantation is however a problem. Even in situations where the stem is inclined relative to the concave articulating surface of the head, as in the known Liverpool Radial Head Replacement, misalignment can still be a problem.

In order to address the misalignment problem, another class of arthroplasty has been proposed in which the radial head can float. In one example, known as the Judet floating radial head prosthesis, the stem carries a spherical end which seats in a complementary socket in the radial head and allows the latter to articulate over a wide range of angles relative to the stem. It is however believed that the relatively free articulation of the radial head on the stem is an insecure and inappropriate solution. Other known proposals are the rHead™ Radial Implants. The rHead™ radial implant is a two-component system consisting of an angled intermedullary stem and a head which mates via a morse taper with an abutment on the stem. While the angled stem is designed to enhance the intramedually fit and facilitate fixation, there is no facility for any pivotal movement of the head on the stem, so misalignment can again be problem. The rHead™ Recon™ radial implant is again a two-component system which is similar in principle to the Judet system mentioned above insofar as it has an intermedullary stem with a spherical end that seats in a spherical socket in the head. Misalignment problems are to some degree addressed by the geometry of the stem and head which allows for limited pivotal movement of the head on the stem.

It is an objective of the present invention to provide an alternative arthroplasty for proximal radial head and distal radio-ulnar implantation applications.

SUMMARY OF THE INVENTION

According to the invention there is provided a lower arm arthroplasty comprising:

an intermedullary stem having a first end for anchorage in a hole in a forearm bone and a second end;

a head having an external, curved surface for articulation relative to another bone; and

an intermediate member which is beatable in use between the head and the second end of the stem and which is engagable with the head and with the second end of the stem; wherein the intermediate member is engagable with at least the stem in an articulating manner which allows the head to pivot relative to the stem through a limited range of angles.

In a preferred embodiment of the invention, the intermediate member is engagable with the head in an articulating manner. In this embodiment, the intermediate member has opposite ends with spherically curved, convex surfaces, the ends of the intermediate member being receivable in articulating manner in complemental, spherically curved sockets in the head and in the second end of the stem respectively. Most preferably, the intermediate member has generally a dumb-bell shape with spherically curved, convex surfaces at its opposite ends and a relatively narrow waist between such ends.

According to a preferred feature, the intermediate member and the head are shaped such that the head can translate laterally relative to the stem.

In a practical construction, the socket in the head has a spherically curved base and a radially enlarged space extending radially outwardly of the base to accommodate lateral displacement of an end of the intermediate member which is received in the socket. The socket in the second end of the stem may be similarly configured, with a spherically curved base and a radially enlarged space extending radially outwardly of the base to accommodate lateral displacement of an end of the intermediate member which is received in the socket. Conveniently the ends of the intermediate member and the socket are identical to one another.

The arrangement may be such that each of the sockets has a relatively narrow mouth leading to the radially enlaraged space of that socket. The mouths of the sockets may be narrower than the ends of the intermediate member. Also, the head and the second end of the stem may have annular surfaces, located radially outwardly of their respective sockets, which abut one another at limit positions of pivotal movement of the head relative to the stem. These surfaces may be rerspectively concavely and convexly shaped.

In another embodiment of the invention, the second end of the stem has a convex, spherical curvature and is receivable in a complementally, spherically curved socket in the intermediate member, whereby pivotal movement can take place between the intermediate member and the second end of the stem, and the head is engagable with the intermediate member in a non-pivoting manner. In this embodiment, cooperating morse tapers may be provided on the intermediate member and in a socket in the head which receives the intermediate member, the morse tapers being engagable with one another to secure the head on the intermediate member. Also, the second end of the stem in this embodiment may include a radial recess adjacent the convex curvature thereof and the intermediate member may include a radially inwardly extending formation locatable loosely in the recess in a manner allowing limited pivotal movement of the intermediate member on the second end of the stem.

The arthroplasty of the invention may be a proximal radial head replacement in which case the second end of the stem is a proximal end thereof. In this application, the proximal end surface of the head is concavely curved for cooperation with a capitulum of a humerus.

In an alternative application, the arthroplasty is a distal radio-ulnar arthroplasty and the head has a convexly curved side surface shaped for articulation relative to a radius.

In either case, it is preferred that the arrangement of the components be such that the head can pivot relative to the stem through an angle in the range 6° to 12°, preferably about 8°, from a central orientation in which the head is aligned axially with the stem.

Other features of the invention are described below and set forth in the appended claims. BRIEF DESCRIPTION OF THE DRAWINGS

The invention will now be described in more detail, by way of example only, with reference to the accompanying drawings in which:

Figure 1 shows a diagrammatic side elevation of a proximal radial head arthroplasty according to this invention;

Figure 2 shows a diagrammatic cross-sectional view of the arthroplasty seen in Figure 1 ;

Figure 3 diagrammatically illustrates the arthroplasty of Figures 1 and 2, in use;

Figure 4 shows a diagrammatic side elevation of a distal radio-ulnar arthroplasty according to the invention;

Figure 5 shows a cross-sectional side view of a proximal radial end arthroplasty according to another embodiment of the invention;

Figure 6 shows a perspective view of the arthroplasty seen in Figure 5;

Figure 7 shows a cross-section at the line 7-7 in Figure 6.

DESCRIPTION OF THE ILLUSTRATED EMBODIMENTS

The lower arm arthroplasty 10 seen in Figures 1 and 2 is designed for use as a proximal radial head arthroplasty in the manner shown in Figure 3. The arthroplasty 10 has three components, namely a tapered, elongate intermedullary stem 12 having an integral, enlarged proximal end 14, an intermediate articulating member 16 mounted on the proximal end 14, and a head 18 mounted on the intermediate member.

The stem 12 and proximal end 14 are formed in one piece of titanium or a cobalt chromium molybdenum (CCM) alloy surface-sprayed with titanium. The external surface of the stem may be roughened. As illustrated, the end 14 has a distal portion 20, a proximal head portion 22 with a convexly and spherically curved terminal surface 24 and a radial recess 26 between the portions 20 and 22. In this embodiment the recess 26 is annular and provides a continuous undercut adjacent the portion 22 of the end 14.

The intermediate articulating member 16 is made of titanium or CCM alloy. It has a socket 28 the base of which is defined by a spherically curved surface 30 and a radially inwardly directed, annular rib 32. In the assembled arthroplasty, the head portion 22 of the proximal end 14 is received in the socket with the spherically curved surfaces 24 and 30 making an articulating slide fit with one another and with the rib 32 received in the annular recess 26.

The inner diameter defined by the rib 32 is smaller than the maximum diameter of the portion 22. During assembly the member 16 may be heated up to cause it to expand sufficiently to enable the rib to fit over the portion 22 for location in the recess 26.

In the assembly the rib 32 has a loose fit in the recess. This allows the member 16 to pivot on the head 14 through a limited angle 34 in the range 6° to 12°, typically about 8°, from the aligned condition, as shown in the detail in Figure 2 which illustrates one limit position. The radially outer surface 42 of the member 16 is formed with a male morse taper.

The generally cup-shaped head 18 is made of a cobalt chromium molybdenum (CCM) alloy. It has a concave proximal surface 44 which may have a polished titanium nitride finish for smoothness and hardness. The curvature of this surface is selected for smooth articulation relative to the convex surface of the capitulum of the humerus bone. The external lateral surface 46 of the head 18 is cylindrical. The internal surface 48 of the head is formed with a female morse taper matching that of the surface 42 of the member 16.

The arthroplasty 10 serves as an artificial replacement for a natural proximal radial head which has been damaged or fractured. The damaged radial head is removed and the stem 12 is anchored in a hole 50 (Figure 3) formed for the purpose in the remaining proximal end of the radius 52. This will normally be achieved using a conventional cement. The proximal end 14, carrying the intermediate articulating member 16, is exposed. The head 18 is then tapped home on the member 16 to engage the morse tapers and secure the head to the intermediate member in an accurately aligned relationship.

It will be understood that once the head 18 has been mounted to the member 16 by engagement of the morse tapers it, like the member, is capable of undergoing limited pivotal movement relative to the head 14 and stem 12.

It is believed that the limited pivotal movement which is permitted will allow the external head 18 to be aligned properly with the capitulum 53 even in the event of slight inaccuracy in the positioning and alignment of the stem 12 in the radius. At the same time, the limited range of pivotal movement which can take place will contribute to ensuring that the resultant elbow joint has an appropriate degree of rigidity.

Figure 4 shows another embodiment of arthroplasty designed as a radioulnar replacement. In this application, the arthroplasty 10 is employed to replace a damaged or fractured distal end of the ulna and to provide for lateral articulation relative to the distal end of the radius. It will be understood that in this case the intermedullary stem is anchored in the ulna. The main difference between this embodiment and that of Figures 1 to 3 is the fact that the lateral surface 46 of the head 18 is convexly curved for articulation relative to the distal end of the radius, and the distal end is formed as a flat surface 60 rather than the concavely curved proximal surface 44 of the first embodiment.

Figures 5 to 7 illustrate another, preferred embodiment of the invention. The arthroplasty 50 seen in these Figures again has three components, namely an intermedullary stem 52, an articulating, intermediate member 54 and a head 55. These components may be made of the same materials as the corresponding components 14, 16 and 18 of the first embodiment illustrated in Figures 1 to 3.

In this embodiment, the intermedullary stem has an elongate stem portion 56 formed with external flutes 58 which will promote osseointeg ration of the stem portion in the radius and which will act to prevent rotation of the stem relative to the radius about its axis. The proximal end 60 of the stem is enlarged and is formed with a socket 62 having a relatively narrow mouth 62.1 , a concavely and spherically curved base 62.2 and a radially enlarged region 62.3 between the mouth and the base.

As in the first embodiment, the head 55 has a concavely curved proximal end surface 64 and a cylindrical side wall 66. At its distal end the head 55 is formed with a socket 68, shaped similarly to the socket 62, having a relatively narrow mouth 68.1 , a concavely and spherically curved base 68.2 and a radially enlarged region 68.3 between the mouth and the base.

The intermediate member 54 has generally a dumb-bell shape with enlarged ends 54.1 and 54.2 separated from one another by a relatively narrow, cylindrical waist 54.3. The ends 54.1 and 54.2 have convexly and spherically curved surfaces 54.4 and 54.5 respectively. The curvature of these surfaces matches the curvature of the base surfaces 62.2 and 68.2.

The ends 54.1 and 54.2 of the intermediate member 54 are received in the sockets 62 and 68 respectively, as shown in Figure 5. Although the internal radii of the mouths 62.1 and 68.1 are slightly smaller than the maximum external radii of the ends 54.1 , 54.2, the ends can be inserted into their respective sockets by tilting the components relative to one another and inserting the ends into the sockets at an angle. However, once the ends are located in the sockets they cannot be removed by simple axial separation of the components.

It will be understood that the ability to engage the intermediate member with the stem, and the head with the intermediate member, by a tilting movement, as just described, is advantageous in that it allows the surgeon to implant the stem prior to fitment of the intermediate member and head. In similar manner it is advantageous in the first embodiment of Figures 1 to 3 that the head 18 can be fitted to the intermediate member after implantation of the stem.

The interaction between the spherical surface of the base 62.2 and the surface 54.4 and the corresponding interaction between the spherical surface of the base 68.2 and the surface 54.5 allow the head 55 to pivot relative to the stem 52. The range of pivotal movement is, as in the earlier embodiments, limited. In this case the limits of pivotal movement are reached when a terminal, proximal surface 70 of the stem 52 abuts a terminal, distal surface 72 of the head 55. In practice the range of permitted pivotal movement may be as in the first embodiment, i.e. 6° to 12°, preferably about 8°. This gives rise to the advantages mentioned above.

It will be noted in Figure 5 that the surface 70 is slightly convex and the surface 72 is slightly, correspondingly concave, thereby providing for cooperating abutment between these surfaces at the limits of pivotal movement. The allowed range of such pivotal movement may be the same as that described above for the first embodiment.

As an additional advantageous feature, the embodiment of Figures 5 to 7 allows for some lateral translational movement of the head relative to the stem. This is permitted by the fact that the enlarged ends of the intermediate member can move laterally into the radially enlarged regions 62.3 and 68.3 of the stem and head respectively. Thus both the head and stem can translate laterally relative to the intermediate member, allowing for overall lateral translation of the head relative to the stem, if necessary to take account of lateral misalignment during the implantation of the stem in the radius.

It will be understood that the embodiment of Figures 5 to 7 may, with suitable modifications, also be used in a distal radio-ulnar application.

Claims

1.

A lower arm arthroplasty comprising:

an intermediate member which is beatable in use between the head and the second end of the stem and which is engagable with the head and with the second end of the stem;

wherein the intermediate member is engagable with at least the stem in an articulating manner which allows the head to pivot relative to the stem through a limited range of angles.

2.

A lower arm arthroplasty according to claim 1 wherein the intermediate member is engagable with the head in an articulating manner.

3.

A lower arm arthroplasty according to claim 2 wherein the intermediate member has opposite ends with spherically curved, convex surfaces, the ends of the intermediate member being receivable in articulating manner in complemental, spherically curved sockets in the head and in the second end of the stem respectively.

4.

A lower arm arthroplasty according to claim 3 wherein the intermediate member has generally a dumb-bell shape with spherically curved, convex surfaces at its opposite ends and a relatively narrow waist between such ends.

5.

A lower arm arthroplasty according to either one of claims 3 or 4 wherein the intermediate member and the head are shaped such that the head can translate laterally relative to the stem.

6.

A lower arm arthroplasty according to claim 5 wherein the socket in the head has a spherically curved base and a radially enlarged space extending radially outwardly of the base to accommodate lateral displacement of an end of the intermediate member which is received in the socket.

7.

A lower arm arthroplasty according to claim 5 or claim 6 wherein the socket in the second end of the stem also has a spherically curved base and a radially enlarged space extending radially outwardly of the base to accommodate lateral displacement of an end of the intermediate member which is received in the socket.

8.

A lower arm arthroplasty according to claim 7 wherein the opposite ends of the intermediate member are identical to one another.

9.

A lower arm arthroplasty according to claim 8 wherein the sockets in the head and in the second end of the stem are identical to one another.

10.

A lower arm arthroplasty according to claim 9 wherein each of the sockets has a relatively narrow mouth leading to the radially enlarged space of that socket.

11.

A lower arm arthroplasty according any one of claims 7 to 10 wherein the mouths of the sockets are narrower than the ends of the intermediate member.

12.

A lower arm arthroplasty according to any one of claims 3 to 11 wherein the head and the second end of the stem have annular surfaces, located radially outwardly of their respective sockets, which abut one another at limit positions of pivotal movement of the head relative to the stem.

13.

A lower arm arthroplasty according to claim 12 wherein the annular surface of the head has a concave shape and the annular surface of the second end of the stem has a convex shape.

14.

A lower arm arthroplasty according to any one of claims 2 to 13 wherein an external surface of the stem is fluted.

15.

A lower arm arthroplasty according to claim 1 wherein the second end of the stem has a convex, spherical curvature and is receivable in a complementally, spherically curved socket in the intermediate member, whereby pivotal movement can take place between the intermediate member and the second end of the stem, and the head is engagable with the intermediate member in a non-pivoting manner.

16.

A lower arm arthroplasty according to claim 15 wherein cooperating morse tapers are provided on the intermediate member and in a socket in the head which receives the intermediate member, the morse tapers being engagable with one another to secure the head on the intermediate member.

17.

A lower arm arthroplasty according to either one of claims 15 or 16 wherein the second end of the stem includes a radial recess adjacent the convex curvature thereof and the intermediate member includes a radially inwardly extending formation beatable loosely in the recess in a manner allowing limited pivotal movement of the intermediate member on the second end of the stem.

18.

A lower arm arthroplasty according to any one of the preceding claims wherein the arthroplasty is a proximal radial head replacement.

19.

A lower arm arthroplasty according to claim 18 wherein a proximal end surface of the head is concavely curved for cooperation with a capitulum of a humerus.

20.

A lower arm arthroplasty according to any one of claims 1 to 17 wherein the arthroplasty is a distal radio-ulnar arthroplasty and the head has a convexly curved side surface shaped for articulation relative to a radius.

21.

A lower arm arthoplasty according to any one of the preceding claims wherein the arrangement is such that the head can pivot relative to the stem through an angle in the range 6° to 12° from a central orientation in which the head is aligned axially with the stem.

22.

A lower arm arthroplasty according to claim 21 wherein the arrangement is such that the head can pivot relative to the stem through an angle of 8° from the central orientation.