HIP COMPRESSION SCREW ASSEMBLIES AND JOINTS THEREFOR

Abstract: A two-dimensional joint for use in hip compression screw assemblies has first and second elongate elements (2, 4) formed with complementary male and female surfaces (10, 12) defining the plane of adjustment. A setting member (20) extends from a pivot point on the first element (2) into the second element (4) along a set axis (18) inclined to the axis of the second element (4), and adjustably coupled thereto. Such adjustment alters the angle between the elements (2, 4) when the cylindrical surfaces (10, 12) are in sliding engagement.

Description: 1 HIP COMPRESSION SCREW ASSEMBLIES AND JOINTS THEREFOR This invention relates to two-dimensional joints and their use in compression screw assemblies, particularly for use in surgical operations and in the treatment of proximal femur fracture. In the treatment of proximal femur fracture the typical orthopaedic surgical process is to secure the fractured bone using a compression screw consisting of a lag screw and an angle plate secured thereto at a fixed angle. Such screws are available with different angles between the lag screw and plate, typically increasing at 5° intervals from 120° to 150°. The angle plates are coupled to the lag screw by a system which allows axial movement of the plate relative to the screw. Because the location of the lag screw in the treatment of a fracture of this kind is critical, the limitation on the angular relationship between the lag screw and the angle plate means that it is difficult in almost every case to fit the compression screw at the ideal position. The above problem is addressed in my International Patent Application published under No. WO98/01078. That Application discloses an adjustable two-dimensional joint for use in compression screw assemblies, which allows for relative alignment of the angle plate of such an assembly with the lag screw, even when these components are effectively fitted to the respective bone sections. That joint, adjustable in two dimensions, comprises first and second elements formed with complementary male and female cylindrical surfaces defining the plane of adjustment, the first element defining a hinge point at the axis of the male cylindrical surface, and the second element including a fulcrum extending to the axis of the female cylindrical surface. A setting device extends between pivot points on the first and second elements which is adjustable in length between the pivot points to alter the angle 2 between the elements. In a hip compression screw assembly, the first element will constitute or be effectively part of the angle plate and the second element constitute or be part of the lag screw. The present invention seeks to improve on the joint disclosed in my International Application referred to above, and particularly to simplify its assembly. This is primarily achieved by modifying the geometry of the joint such that adjustment of the setting device is substantially linear. This enables the joint to be set by means of a relatively simple linear coupling, such as a screw thread. A joint according to the present invention, as in my previous design, comprises first and second elongate elements formed with complementary male and female cylindrical surfaces defining the plane of adjustment in two dimensions. However, the setting member extends from a pivot point on the first element into the second element along a set axis inclined to the axis of the second element, and it is adjustably coupled to the second element to alter the angle between the elements when the cylindrical surfaces are in sliding engagement. As noted above, the adjustable coupling will typically comprise complementary male and female screw threads, with the male thread normally being formed on the setting member. The coupling between the setting member and the second element can be deliberately designed to provide a degree of freedom accommodating some lateral shift of the pivot point on the first element relative to the set axis during adjustment of the coupling. This can be accomplished by providing for some translational movement of the point on the first element, or permitting some pivotal movement between the setting member and the set axis during adjustment of the coupling. Generally, the latter is preferred as it provides more positive locking of the first element in the joint, and it can be accomplished by establishing no 3 more than a loose fit between screw threads on the setting member and the second element. In all embodiments of the invention some form of mechanism will usually be provided for locking the elements at an angle set by the setting member. A simple such mechanism comprises a pin or screw which engages the setting member to prevent it pivoting away from the locked position. However, a preferred mechanism exploits the optional use of a block mounted in the second element to which the second member is adjustably coupled. Such a block can be adjustably mounted in the second element, preferably for selective axial movement parallel to the set axis. A locking mechanism can thus comprise engagement of the exposed end of such a block with a projection or other part of the first element; before, during or after setting of the angle between the elements. The block can be mounted in the second element by means of a screw thread assembly, and if a similar mechanism is employed for coupling the second member to the block, then by using oppositely sensed threads adjustment of the elements and the locking of such adjustment can be accomplished substantially simultaneously. The invention will now be further described by way of example and with reference to the accompanying drawing wherein: Figure 1 shows in cross-section a two-dimensional joint according to the invention for use in a hip compression screw assembly; and Figure 2 shows a view similar to that of Figure 1, but illustrating the use of an alternative form of setting member. The joint shown in Figure 1 has first and second elements 2, 4 which are pivotal relative to each other around an axis 6. Such pivotal movement is constrained to be in a single two-dimensional plane by complementary male (10) and female (12) cylindrical surfaces 4 respectively on the elements 2 and 4. The pivot axis 6 is located at the axis of the male cylindrical surface 10 on the first element 2, which coincides with the axis of the female surface 12, when the cylindrical surfaces 10, 12 are in sliding engagement with each other. The female cylindrical surface 12 is formed on an enlarged peripheral segment 14 of the second element 4 which, in a hip screw assembly, would constitute the lag screw secured to or to be secured in the caput of a femur in the treatment of a typical proximal femur fracture. Within the second element 4, and mounted against the inner flank of the segment 14, is a block 16 mounted for translational movement along a set axis 18, inclined to the axis itself of the element 4. The block 16 is typically cylindrical, and mounted on the second element 4 by means of a screw thread. A setting member 20 is coupled to the block 16 for axial movement relative thereto substantially along the set axis 18. The setting member 20 comprises a ball 22 and shaft 24, the shaft 24 bearing a male screw thread received in a complementary female thread within the block 16. The ball 22 is mounted in a suitable recess in the first element 2, and the geometry of the joint is such that rotation of the ball 22 and therefore the shaft 24 to cause axial movement of the setting member 20 relative to the block 16 and therefore the second element 4, adjusts the angle between the elements 2 and 4 while maintaining the complementary cylindrical surfaces 10 and 12 in sliding engagement. This geometry requires the coupling between the setting member 20 and the block 16 and/or the second element 4 to accommodate some lateral movement of the ball 22 relative to the set axis 18, and in the illustrated embodiment this is accomplished by the adoption of a slightly loose threaded coupling between the member 20 and the block 16 and/or the block 16 and the body of the second element 4. This permits some pivotal movement of the member 20 5 in the second element 4. Alternatively, the complementary socket 26 in the first element 1 which receives the ball 22 may be enlarged in a direction substantially perpendicular to the set axis but of course in the plane of the angular adjustment of the joint . The arrangement described above sets the angle of the joint described against turning in one sense. However, the joint can still be turned in the other sense by separation of the complementary cylindrical surfaces 10 and 12 and/or withdrawal of the ball 22 from its seat 26. To prevent this, a locking mechanism is employed which effectively prevents such pivotal movement of the shaft 24 relative to the ball seat 26. A locking screw or pin can be used to achieve this end, or a simple wedge. However, a preferred mechanism exploits the use of the block 16 as the means for receiving the shaft 24 of the setting member 20. The block 16 is adjustably mounted in the second element 4 and as shown can be withdrawn from the element to engage a projection 28 at the proximal end of the first element 2. The block 16 is normally mounted in the element 4 by means of a conventional screw thread, which particularly facilitates this locking adjustment. It is further facilitated if the thread coupling the shaft 24 to the block 16 is of the opposite hand relative to that of the thread coupling the block 16 to the second element 4. With this arrangement the exercise of adjusting and tightening the setting member 20 can simultaneously draw the block 16 up and into locking engagement with the projection 28 on the first element 2. In the modification shown in Figure 2, the setting member 20 comprises a main body consisting of two threaded portions 30,32. One threaded portion 32 defines the adjustable coupling to the second element 4 in a manner similar to that described above with reference to Figure 1. The other portion 30 has mounted 6 thereon a ball unit 34 with a complementary female thread, enabling the position of the ball 34 relative to the setting member 20 itself to be adjusted. This adjustment can be effected by a tool 36 adapted to engage appropriate elements such as castellations 38 on the exposed end of the ball unit 34. The preferred ball unit is in the form of a sleeve whereby the end of the setting member 20 is accessible therethrough, enabling the coupling of the setting member 20 to the second element 4 to be adjusted independently of the ball unit 34 relative to the setting member 20. Conveniently, the end face of the setting member is provided with an alien key junction (not shown in the drawing) accessible through the tool 36 for adjusting the ball unit 34, enabling both to be adjusted simultaneously. One or both of the tool and key on the one hand, and the ball and setting member on the other can be locked for this purpose . In the use of a joint according to the invention in a hip compression screw assembly, the second element 4 will be installed in the caput of the femur as a separate exercise. If it is not already fitted, then the block 16 will be coupled to the element 4, and the element 2 laid against the femur with the ball seat 20 generally located on the axis 18 of the block 16. The setting member is then fitted to the block 16 and tightened against its seat with the elements 2 and 4 in place. With the threads on the block 16 being oppositely handled as noted above, rotation of the setting member can serve effectively simultaneously to set the angle between the two elements, and lock it against subsequent straightening. As noted above, the angle of the set axis relative to the axis of the element 4 must be chosen carefully to enable sufficient adjustment to be made within the tolerence of the coupling used, normally a screw threaded coupling. Suitable angles are typically 7 between 8 and 15° with the preferred angle being around 10.5°. With the angle within this range, sufficient movement can be accommodated by the use of loosely engaging threads. Even if the ball seat 26 does not accommodate translational movement of the ball 22, a loose thread can quite safely accommodate the requisite pivotal movement . While the invention has been described above specifically in the context of a hip compression screw assembly, or as a device for accomplishing the internal fixation of limbs, it should be noted that it can also be used in the external fixation of limbs. Respective bone sections can be coupled by appropriate bone screws to components which are themselves coupled by means of a joint according to the invention. The joint will allow individual adjustment to alter the relative orientation of the bone sections to be fixed.

Claims: 8 CLAIMS 1 A joint adjustable in two dimensions comprising first and second elongate elements formed with complementary male and female cylindrical surfaces defining the plane of adjustment, and a setting member extending from a pivot point on the first element into the second element along a set axis inclined to the axis of the second element, and adjustably coupled to the second element to alter the angle between the elements when the cylindrical surfaces are in sliding engagement. 2 A joint according to Claim 1 wherein the adjustable coupling between the setting member and the second element comprises complementary male and female screw threads . 3 A joint according to Claim 2 wherein the male screw thread is formed on the setting member. 4 A joint according to any preceding claim wherein the coupling provides a degree of freedom permitting pivotal movement between the setting member and the set axis during adjustment of the coupling. 5 A joint according to Claim 2 and Claim 4 wherein the complementary screw threads form a loose fit . 6 A joint according to any preceding claim including a mechanism for locking the elements at an angle set by the setting member. 7 A joint according to Claim 6 wherein the locking mechanism comprises a pin engageable with the setting member. 9 8 A joint according to Claim 6 wherein the locking mechanism comprises a screw. 9 A joint according to any preceding claim wherein the setting member is adjustable coupled to a block mounted in the second element . 10 A joint according to Claim 9 wherein the block is adjustably mounted in the second element. 11 A joint according to Claim 10 wherein the block is mounted for selective axial movement parallel to the set axis . 12 A joint according to Claim 11 wherein the block is formed with a male screw thread and received in a complementary female screw thread in the second element . 13 A joint according to Claim 3 and Claim 12 wherein the thread between the setting member and the block is the opposite hand from that between the block and the second element. 14 A joint according to Claim 6 and any of Claims 11 to 13 wherein the first element is formed with a projection for engaging the exposed end of the block when adjusted thereto after setting of the angle between the elements. 15 A joint according to any preceding claim wherein the angle between the set axis and the axis of the second element is in the range 8° and 15°. 16 A joint according to Claim 15 said angle is substantially 10.5°. 10 17 A joint according to any preceding Claim wherein the setting element has at one end a ball unit mounted on a first end length thereof, the ball serving to define the pivot point between the first and second elements, the first end length being contiguous with a second length thereof adjustable coupled to the second element . 18 A joint according to Claim 17 wherein the ball unit is mounted for axial adjustment on the first length of the setting element. 19 A joint according to Claim 18 wherein the axial adjustment is by means of complementary screw threads on the ball unit and setting element end length. 20 A joint according to Claim 2 and Claim 19 wherein the screw threads on the ball unit and setting element end length are of the opposite hand to that of the threads between the setting member and the second element . 21 A joint according to any of Claims 18 to 20 including means on the ball unit and on the end length of the setting element enabling their independent turning relative to each other and to the second element . 22 A joint according to Claim 21 wherein said means comprises castellations on an end face of the ball unit and an alien key junction on the end face of the setting member . 23 A hip compression screw assembly comprising a joint according to any preceding claim wherein the first element is adapted to lay against a section of femur adjacent the hole cut for the lag screw, and the second 11 element comprises the lag screw for securement in the caput . 24 A method of treating a proximal femur fracture using a hip compressing screw assembly according to Claim 23, comprising securing the lag screw in the caput with the first element at the surface of the femur, adjusting the setting device to orient the first element of the joint on the femur surface; and securing the first element to the femur. 25 The use of a joint according to any of Claims 1 to 22 in the external fixation of a limb, wherein the first and second elements are attached to the limb sections to be fixed, and the setting device adjusted to establish the desired orientation thereof.

Inventors: DALL VAGN ERIK, DALL, VAGN, ERIK
Applicants: DALL VAGN ERIK, DALL, VAGN, ERIK
Application year: 1999
Main classification: A61B17
IPC codes: A61B17