US20110282391A1 - Bone Fixation System - Google Patents

Abstract

In a fixation system for controlling the alignment of adjacent vertebra after spinal surgery, aligned fixation rods are held in a mount on each of such vertebra, with their juxtaposed ends received in a sleeve between the mounts. At least one of the rods is movable in the sleeve relative to the other to allow controlled movement of adjacent vertebra. In some embodiments, the sleeve can itself be held in a mount installed on one of such vertebra with the aligned fixation rods being capable of relative movement therein. The system allows the spine to grow without the need for further surgery.

Description

• This invention relates to fixation systems for bones after surgery, and has particular application to the setting or controlling of bone parts. It is applicable to the fixation of bone sections after fracture, and also to the relative orientation of bone parts such as vertebra between which some movement must be permitted after spinal surgery.
• Fixation devices for bone parts are known, and in this respect reference is directed to International Patent Publication No: WO 02/03868 in the name of Vagn Erik Dall, incorporated herein by reference. That specification discloses a device in which one or two bone fixation rods are mounted on a bone screw to couple that screw to a similar screw in an adjacent bone part. The fixation rods are gripped relative to a support body mounted on bone screw between jaws which close in response to pressure generated by engagement against the wall of the support body. The present invention is directed at a similar device and related fixation systems, but which offer greater flexibility and versatility.
• According to the invention of our co-pending International Application entitled “Bone Fixation Device” claiming priority from British Patent Application No: 0820251.7, incorporated by reference, a bone fixation device comprises a screw for mounting in a bone part with an exposed head projecting therefrom. A housing on the exposed head of the screw supports a plate extending from each of two opposite sides of the housing upon which is mounted a clamp for a fixation rod. Each plate is rotatable about a lateral axis relative to the housing, and each clamp is rotatable on its plate about an axis perpendicular to its respective lateral axis. Each clamp is therefore capable of universal movement relative to the housing. The housing will normally be rotatable on the exposed head of the screw, but locked against such rotation once its preliminary orientation has been established. Mechanisms are provided for closing the clamps, for locking each plate against rotation about its lateral axis, and for locking each clamp against rotation about its perpendicular axis. Thus, once these mechanisms have been activated, rods held in the clamps are locked in position relative to the screw head.
• In preferred embodiments of the above invention, each clamp has an outer jaw and a movable inner jaw on a support rotatably mounted on the respective plate. The clamp locking mechanism comprises an element within the housing for engaging the inner jaw, to move it towards the outer jaw and thereby secure a fixation rod therebetween. This engagement can also lock one or both jaws against rotation about one or both of its lateral and perpendicular axes. The engaging surfaces may be textured or otherwise treated to achieve this locking effect. They would in any event, normally be matchingly spherical to maximise the surface engagement. The element itself may be fixed within the housing, with the respective plates being movable parallel to their lateral axes. Such movement inwards urges the inner jaw against the element. This inward movement may be effected by a locking ring with an external screw thread received in a complementary internal screw thread in the housing. The locking ring can overlay an inner shoulder on the respective plate against which it engages to urge the plate inwards. Rotation of the locking ring can in this way, by urging the inner jaw of the clamp against the housing element, simultaneously lock a rod in the clamp and the clamp against rotation about both of the lateral and perpendicular axes.
• In one simple form, the inner jaw of each clamp is mounted simply for pivotal movement relative to the outer jaw. Further, in order to minimise the number of moving parts, the inner and outer jaw are preferably integral parts of the same unit, and the pivotal movement is allowed by the deformation of the material used.
• Fixation devices of the kind referred to above are useful in many aspects of surgery, and are very effective when bone parts must be fixed relative to one another. A pair of fixation rods can extend between adjacent devices, to provide a very stable locking mechanism. At the same time, it will be appreciated that prior to the clamps and plates being locked, the loosely assembled structure of screws, fixation rods and bone parts can be manoeuvred into precise position. Once that position has been found, a single movement of each locking ring in the preferred embodiment can set the assembly.
• Fixation devices also have application in surgery where bone parts must be located relative to one another, but in a manner which allows for controlled relative movement. This is particular important after spinal surgery on a young patient who is still growing. For example, in a child treated for scoliosis a surgeon will want to set adjacent vertebra in such a manner that permits them to grow apart, but in a predetermined direction. In other words, the relative movement of the vertebra should be restricted to one which counteracts the deformity. This can be achieved in a fixation system according to the present invention in which fixation rods held in mounts on each of adjacent vertebra are coupled in a sleeve which allows the mounts (and therefore the vertebra) to separate, but only in a direction determined by the fixation rods and the respective sleeve or sleeves. The sleeve can receive juxtaposed rod ends between mounts, or can itself be secured in a mount. Generally, the end of one rod will be fixed in an end of the sleeve with the juxtaposed end of the other rod slidably received in the other end of the sleeve. The extent of movement of the other rod will normally be restricted to retain at least a minimum end length of each rod in the sleeve, and this can be accomplished by the end of the movable rod being formed with a longitudinal groove extending axially on its surface, but terminating short of the rod end. A spigot in the sleeve is received in the groove. The spigot is typically the end of a screw installed from the exterior of the sleeve, and therefore withdrawable to allow for initial insertion of the rod in the sleeve.
• A single rod and sleeve combination extending between mounts in adjacent vertebra provides some control over their relative movement, but it is preferred to use two such combinations side by side. The respective rods can be fixed in separate mounts on the vertebra, or on opposite sides of a housing on the exposed head of a bone screw, using a device of the kind described above. The use of two rods side by side does of course provide a much more stable and controlled link between the adjacent bone parts.
• If the permitted movement of adjacent bone parts using a fixation system according to the invention is to be linear, then the fixation rod and sleeve combination or preferably combinations will be straight. However, and particularly in surgery to combat scoliosis, the permitted movement may need to be along a curved path. This can be accomplished by selecting a rod or rods with appropriate curvature, and matching that curvature with a correspondingly curved sleeve, as necessary. Different circumstances will of course require different kinds and degrees of curvature, and different geometric figurations can be achieved by selecting appropriate rods and sleeves, and the respective orientations of the rods when they are fixed in the mounts in the respective bone parts.
• The fixation rods themselves will normally be of circular cross-section, but there may be situations in which a polygonal cross-section might have benefits. A polygonal cross-section of the rod and the sleeve interior will of course prevent rotation of one relative to the other. This can be additional to the use of the spigot and groove referred to above which does of course serve a similar purpose but can lock if a substantial twisting force is applied to a rod.
• As a further mechanism for stabilising relative movement of adjacent bone parts in a fixation system of the invention, the sleeves in which adjacent rods are received can be coupled together. This provides some additional resistance to adjacent vertebra twisting relative to one another.
• The invention will now be described by way of example and with reference to the accompanying illustrative drawings wherein:
• FIG. 1 is a perspective view of a bone fixation device according to a first embodiment of the invention;
• FIG. 2 is a part exploded view of the device ofFIG. 1;
• FIG. 3 illustrates two devices of the invention with fixation rods extending therebetween;
• FIG. 4 is a perspective view illustrating a second embodiment of the invention;
• FIG. 5 illustrates a variation on the embodiment ofFIG. 4;
• FIG. 6 illustrates a further variation on the embodiment ofFIG. 4; and
• FIG. 7 illustrates schematically how the embodiment ofFIG. 6 can be applied in the treatment of spinal deformations.
• FIG. 8 is a perspective view of a bone fixation device according to a second embodiment of the invention;
• FIG. 9 is an exploded view of the device ofFIG. 8;
• FIG. 10 is a perspective view of a bone fixation device according to a third embodiment of the invention;
• FIG. 11 illustrates schematically how devices of the kind shown inFIG. 10 can be applied in the treatment of spinal deformations;
• FIG. 12 shows a view similar to that ofFIG. 11 in which spinal growth is accommodated; and
• FIG. 13 illustrates how a tool can be coupled to a device of the kind shown inFIG. 10 after installation on a vertebra to manipulate a vertebra during the course of an operation.
• FIG. 1 shows thehead2 of abone screw4 as it might project from a bone (not shown) after installation during surgery. On the head is mounted ahousing6 comprising abase8 and adrum10. Thebase8 will normally be formed with a frusto conical recess which received thehead2 of thescrew4. This initially allows rotation of the base on the head, but once this is set in a desired orientation, it is locked by installation of acirclip12.
• As shown inFIG. 2, thedrum10 is formed with two threaded recesses (only one is shown) facing in opposite directions. When assembled, a lockingring14 holds aplate assembly16 in the recess by virtue of anannular projection18 from the base of eachplate16 engaging anannular shoulder20 within the respective locking ring. A split ring22 (only one is shown) holds theannular projection18 andshoulder20 in engagement during assembly of the device as a whole, and until the locking ring is tightened.
• The locking rings14 are axially aligned around two cylindrical projections that also extend through theboss sections26 of theplates16 and define lateral axes along which theplates16 can move. The distal end surface of the wall of eachprojection14 has a concave spherical surface for reasons which are described below.
• Eachplate16 carries aclamp unit28 which has a circular base received in a circular recess in the plate. This provides for rotation of eachclamp unit28 about an axis perpendicular to the common lateral axis of theplates16. The circular base of the clamp unit will be held in theplate recess30 by means of a resilient circlip (not shown).
• Each clamp unit has anouter jaw32 and aninner jaw34. The clamp unit itself is an integral body, and the movement of the jaws towards and away from each other is as a consequence of the resilient flexure of the material of the clamp and particularly of theinner jaw34. As can be seen, each inner jaw is formed as a section extending from the outer jaw, and its mass and dimensions are generally less than those of the outer jaw to facilitate such relative movement. The outer face of each inner jaw has a convex spherical shape and is in juxtaposition with the distal end of therespective projection24. The respective spherical surfaces match, such that there is uniform contact between them when they engage.
• When the device is assembled, each lockingring14 may be rotated in its respective drum recess, the threads causing the ring to drive the respective plate inwards by virtue of engagement between theannular projection18 on each plate, and theshoulder20 on each locking ring. This movement causes the end face of theprojections24 to engage the convex face of themovable jaw34 serving simultaneously to close theclamp unit28, and lock it against rotation about either the lateral axis of the plate, or the perpendicular axis of the clamp unit. The engaging faces of theprojection24 and themovable jaw34 may be knurled or otherwise roughened or treated to enhance the locking effect. Additional locking may be established by friction between the inner end face of each lockingring14 and the base of the respective drum recess, by means of a resilient washer therebetween.
• The flexibility of theinner jaw34 in eachclamp unit28 facilitates the initial insertion of fixation rods and once a rod is installed the device may be locked as described above, in a relatively swift and often single movement. A key can be provided for engagingopenings36 in each lockingring14 to enable such locking to be quickly and effectively accomplished.
• FIG. 3 shows how two fixation rods can be installed to extend between two devices of the kind illustrated inFIGS. 1 and 2, and it will be noted that thedrums10 of the respective devices are not aligned in a common plane. This illustrates how the respective devices can be manoeuvred to ensure that the various locking mechanisms can be secured.
• FIG. 4 illustrates a second embodiment of the invention. Specifically, it shows how twofixation rods40 and42 can be mounted in alignment in two fixation devices of the kind described above, and interconnected by asleeve44. Therod40 is fixed in thesleeve44 by a clamping screw (now shown) in thebell46. Therod42 is slidably received in the other end of thesleeve44. This construction enables the respective fixation devices to be installed in adjacent vertebra for example, but in a manner which allows the vertebra to grow apart without restriction by the fixation system. However, as damage is likely to be caused if therod42 were to exit thesleeve44, its outward movement is restricted by an inwardly directed screw or spigot mounted inbell48 that extends into a groove or similar (not shown) in therod42 which terminates before its end within thesleeve44. The diameter of the fixation rods used in devices and systems of the invention will normally be around 4 mm. The depth to which the rod ends are inserted and retained in the cylinders will normally be controlled to be at least equal to the rod diameter.
• FIG. 4 shows rods extending on both sides ofdrums10 mounted onscrews4 inserted or to be inserted in adjacent bone parts. It will though, be appreciated that there may well be circumstances in which only a single rod is required to be mounted on eachscrew4. In those circumstances of course, it is not necessary to use a fixation device of the kind illustrated inFIGS. 1 to 3. A single rod mounting device, for example of the kind illustrated in FIG. 1 of International Publication No: WO 02/03868, incorporated by reference, or similar, may be used. Generally though, it is preferred to use aligned rods in parallel, for the reasons set out below.
• FIG. 5 shows an alternative sleeve and rod assembly. In this embodiment, each sleeve is integral with onefixation rod section52 to be clamped in a fixation device, for example of the kind illustrated inFIGS. 1 and 2. The other end of eachsleeve50 receives acurved section54 of the other fixation rod, which merges with astraight section56 for clamping in a fixation device installed in an adjacent bone part such as another vertebra. Thecurved section54 is formed with agroove56 on its surface aligned with its axis. Ascrew58 driven through the wall of thesleeve50 extends into thegroove56. The groove stops short of the end of the rod within the sleeve to prevent it from withdrawing entirely from the sleeve. The engagement of the screw and groove also of course inhibits rotation of the rod relative to the sleeve. The interior of the sleeve which receives therod section54 is curved in the same sense. Thus, as therod54 is withdrawn from the sleeve as the attached bone parts (vertebra) separate, such separation is constrained to be along the line of the curve, as indicated. With two sleeves and respective rod sections, the curves are matched, and as a consequence the aligned rod sections define what is effectively a curved plane in which the adjacent bone parts are constrained.
• FIG. 6 illustrates a similar arrangement to that ofFIG. 5, but here the two sleeves are coupled together to provide greater stability as therod sections54 are withdrawn from thesleeves50. Thecoupling unit60 preserves the spacing between the sleeves, and any relative rotation. Where relatively large amount of movement must be accommodated between coupled bone parts, this additional control can be very valuable.
• FIG. 7 illustrates how fixation systems of the invention can be used after spinal surgery to control the alignment of adjacent vertebra while permitting them to grow apart. The vertebra are illustrated as rectanguloid blocks and as can be seen, each fixation system constrains the respective interconnected vertebra to move out of alignment as they grow apart. This serves to move adjacent vertebra, between which no fixation system is installed, to move out of alignment in the opposite sense, but in practice as a spine grows and develops the result will be that the vertebra grow in substantial alignment. It will be appreciated of course, that the degree of curvature in the rods and sleeves has to be selected with great care in order to achieve the desired result.
• FIG. 8 illustrates a device according to the invention which operates in a manner similar to that shown inFIG. 1, in that a rod is held in each of two clamps on the screw, each clamp being allowed to pivot until the orientation of the rod and clamp are secured. In a manner similar to the device ofFIG. 1, ahousing66 is mounted on ahead64 of abone screw62. In the embodiment ofFIG. 8, the housing is secured on thehead64 by means of a lockingpin68. The housing itself comprises abracket70 on which are formed pairs of spaced part-cylindrical surfaces72. These surface pairs define opposite sides of grooves extending beneathshoulders74 which extend under the respective surfaces72.
• Plates76 are formed with part-cylindrical lower surfaces which complement and rest on thesurfaces72. Each plate can therefore pivot about a lateral axis defined by the common axis of the respective cylindrical surfaces. Of course, it is not essential that both plates pivot about the same axis, but in practice this will almost always be the case.
• Supported on therespective plates76 areclamps78. Each clamp has aboss80 which extends through an opening in itsrespective plate76 and into the groove defined under and between thesurfaces72. There it is attached to a lockingelement82 which extends laterally under theshoulders74. Interposed between its lockingelement82 and therespective shoulders74 is afriction pad84 to assist the locking process.
• Each clamp has twojaws86 defining a groove for receiving a fixation rod (not shown). A clampingelement88 in the form of a screw with an Allen Key socket engages a complementary screw thread defined on the internal faces of thejaws86.
• As can be seen inFIG. 9, the holes in theplates76 through which thebosses80 extend have a chamfered perimeter. This enables the groove defined between thejaws86 of each clamp to extend below the surface of the plate when the device is assembled. As a consequence, when a fixation rod is fitted in a groove, and the clampingelement88 driven downwards to engage it, the rod itself is forced not against the base of the groove, but against the upper surface of aplate76. Thus, when the clamping element is tightened, the rod is locked in the groove; theclamp86 is locked in theplate76; and theplate76 is locked against thesurfaces72 of thebracket70 as the lockingelement82 is drawn against thefriction pad84 and theshoulders74. As with the device ofFIG. 1 then, a single locking action secures a fixation rod against axial movement in the groove, and pivotal movement in any direction.
• FIG. 10 illustrates a bone fixation device according to a third embodiment of the invention. As with the embodiments ofFIGS. 1 and 8, two separately lockable clamps are defined as part of ahousing90 mounted on the head of abone screw92, secured by means of a lockingpin94. However, in the embodiment ofFIG. 10 the clamps are defined side-by-side in aunitary body96. Each clamp has a lockingelement98 similar toelement88 inFIG. 8, in the form of a screw engaging a complementary screw thread defined on the juxtaposed sides of a groove for receiving afixation rod100. However, in the embodiment ofFIG. 10 the fixation rod passes through asplit ball element102 having an external spherical surface which rests in an internal spherical surface at a central section of the groove over which thelocking element98 is disposed. When the locking element is driven into the groove, it compresses the ball element to contract the bore therein through which the fixation rod extends, locking the rod in the ball element and the ball element in the groove.
• While the clampingelement98 is disengaged or only loosely engaged with theball element102, the ball element can pivot in the groove within the central spherical surface. The end sections of the groove diverge from the central section to allow arod100 fitted in theball element102 limited pivotal movement also about the centre of theball element102. The base of each groove end section preferably has a frusto-conical surface extending around more than 180°, for reasons that will become apparent.
• The above description of the installation of a fixation rod is described above with reference to the clamp shown on the righthand side in the device ofFIG. 10. The clamp on the lefthand side of the device shown inFIG. 10 is essentially similar, but the bore through theball element102 is larger. This enables it to receive asleeve104, which has limited pivotal movement while the clamping element is disengaged, but can be locked in a similar way when the element is engaged. However, thefixation rod100 is locked in orientation, but can still move axially within the sleeve. The reason for this is explained below.
• FIG. 11 illustrates how fixation devices according to the invention can be used in spinal surgery. Threedevices106 are shown, mounted on alternate vertebra with a pair offixation rods108. Eachrod108 is secured by a clamp in each of the threedevices106. As shown therefore, the devices and rods secure the alignment of the vertebra to which the devices are attached, and thereby the vertebra in between. The primary purpose of a fixation system of the kind illustrated inFIG. 11 is to reinforce the spine and prevent undue curvature.
• FIG. 12 illustrates a fixation system that serves an additional purpose. In the system ofFIG. 12, three devices are once again installed on alternate vertebra, but device110 installed on the central vertebra clamps notfixation rods108, butsleeves112.Rods114 are not continuous between thedevices106, but are discontinuous. This enables the spine to grow; ie, the vertebra on which thedevices106 are mounted can move away from each other during normal growth, while the rods and sleeves preserve the orientation and alignment of the vertebra to which thedevices106 and110 are attached, and the intervening vertebra therebetween.
• It will be appreciated that the fixation devices shown inFIGS. 11 and 12 could be any of the embodiments described above, as each provides the flexibility needed to accommodate rods and sleeves at the required orientation. It will be appreciated therefore, that while the mounting of a sleeve in a fixation device is only described with reference to the embodiment ofFIG. 10, the devices ofFIGS. 1 and 8 can be readily adapted to perform the same function.
• In the conduct of delicate spinal surgery, it is often necessary to manipulate individual vertebra in the spine before fitting a fixation system of the kind described above. Such manipulation has to be conducted with great care and accuracy. This is not always possible when using a surgeon's hand. Devices of the present invention provide a very convenient means by which an individual vertebra on which a device has been installed, can be manipulated with minimal direct contact between the surgeon's hands and the spine under surgery. In the embodiment ofFIG. 1, thedrum10 of thehousing6 can be grasped either directly by the surgeon or remotely by a gripping device. In the devices ofFIGS. 8 and 10 one of the clamping elements (88,98) can be removed and a manipulator probe having a suitably threaded end installed in one of the clamps. The embodiment ofFIG. 10 provides a particularly convenient alternative to these techniques. As shown inFIG. 13, amanipulator tool116 having twopivotal arms118 can lock onto thehousing90 by means ofconical elements120 at the distal ends of the arms engaging the frusto-conical surfaces of the end sections of one of the grooves. This technique has the advantage that the distal ends of the arms can be swiftly engaged or disengaged, and the length of the tool can enable manipulation to be conducted from a distance with a maximum amount of the vertebra being manipulated and adjacent vertebra, being in view.
• Devices and systems of the invention will normally be formed in titanium or, stainless steel or some other material which can be safely used in surgical applications. The use of resilient synthetic materials is best avoided, and for this reason the components must be manufactured with considerable precision.

Claims (21)

1. A fixation system for controlling the alignment of adjacent vertebra after spinal surgery, which system comprises aligned fixation rods; a mount for installation on each of said vertebra and for holding a fixation rod; and a sleeve for receiving the juxtaposed ends of the rods in alignment between the mounts, at least one of the rods being movable in the sleeve relative to the other to allow controlled movement of adjacent vertebra.

2. A fixation system for controlling the alignment of adjacent vertebra after spinal surgery, which system comprises aligned fixation rods; a mount for installation on each of said vertebra and for holding a fixation rod; and a sleeve for receiving the juxtaposed ends of the rods in alignment, the sleeve being secured in a said mount at least one of the rods being movable in the sleeve relative to the other to allow controlled movement of adjacent vertebra.

3. A fixation system according toclaim 1 wherein the sleeve includes a stop whereby said movement of at least one of the rods is restricted to retain a minimum end length thereof in the sleeve.

4. A fixation system according toclaim 3 wherein an internal spigot in the sleeve is received in a longitudinal groove extending axially on the respective rod surface, the groove terminating short of the rod end.

5. A fixation system according toclaim 1 wherein at least one of the fixation rods is curved.

6. A fixation system according toclaim 5 wherein the rod movable in the sleeve has a curved section extending into the sleeve, and wherein the sleeve is correspondingly curved.

7. A fixation system according toclaim 1 wherein the movable rod and the section of the sleeve in which it is received have complementary non-circular cross-sections preventing rotation of the rod in the sleeve.

8. A fixation system according toclaim 1 including a pair of said mounts for holding fixation rods on each said vertebra; and two said sleeves for receiving juxtaposed ends of the respective rods, the rods and sleeves defining a plane of permitted movement.

9. A fixation system according toclaim 8 wherein the two sleeves are coupled together.

10. A fixation system according toclaim 8 wherein each pair of mounts is part of the same device secured to a said vertebra.

11. A fixation system according toclaim 1 wherein at least one rod is fixed relative to its respective sleeve in an integral unit.

12. A method of controlling the alignment of adjacent vertebra after spinal surgery, which method comprises fitting a mount for fixation rods on each of a plurality of vertebra in the spine under surgery; securing a plurality of aligned fixation rods in the mounts with juxtaposed ends of aligned rods in a sleeve in which at least one of the rods is slideable, movement of said at least one rod in the sleeve allowing the spine to grow without further intervention.

13. A method according toclaim 12 wherein the sleeve is secured in a mount.

14. A method according toclaim 12 wherein at least one of the fixation rods is curved.

15. A method according toclaim 14 wherein the rod movable in the sleeve has a curved section extending into the sleeve, and wherein the sleeve is correspondingly curved.

16. A method according toclaim 12 wherein two pluralities of aligned fixation rods are secured in mounts on vertebra with juxtaposed rod ends in respective sleeves, the rods and sleeves thereby defining a plane of permitted movement.

17. A fixation system according toclaim 2 wherein the sleeve includes a stop whereby said movement of at least one of the rods is restricted to retain a minimum end length thereof in the sleeve.

18. A fixation system according toclaim 2 wherein at least one of the fixation rods is curved.

19. A fixation system according toclaim 2 wherein the movable rod and the section of the sleeve in which it is received have complementary non-circular cross-sections preventing rotation of the rod in the sleeve.

20. A fixation system according toclaim 2 including a pair of said mounts for holding fixation rods on each said vertebra; and two said sleeves for receiving juxtaposed ends of the respective rods, the rods and sleeves defining a plane of permitted movement.

21. A fixation system according toclaim 2 wherein at least one rod is fixed relative to its respective sleeve in an integral unit.

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