Bone fixation device

A bone fixation device has a housing on the exposed head of a screw mounted in a bone part from which it projects. A clamp for a fixation rod is mounted on a plate extending from each of opposite lateral sides of the housing in such a manner that allows the plate to rotate about a lateral axis relative to the housing. Each clamp is itself rotatable on its plate about an axis perpendicular to its respective lateral axis, and thereby provides universal movement relative to the housing. Mechanisms are provided for closing the clamps, for locking each plate against rotation about its lateral axis, and blocking each clamp against rotation about its perpendicular axis. In this way rods can be fixed in the clamps and relative to the device.

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 one aspect of the present invention 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 one preferred embodiment of the 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.

In another preferred embodiment of the invention the housing includes a bracket with pairs of spaced part-cylindrical surfaces on opposite lateral sides thereof on which the plates are supported for pivotal movement about the lateral axis. Each surface pair defines either side of a groove extending beneath shoulders under the respective surfaces. Each clamp is mounted on a boss extending through its respective plate to a locking element in the groove extending beneath the shoulders under the part-cylindrical surfaces. Each clamp also has jaws forming a groove for receiving a fixation rod and a clamping element over the rod. The groove depends into the boss such that a rod received in the groove engages the plate at either end of the groove. Activation of the clamping element locks the rod and clamp relative to the plate and the plate relative to the bracket to prevent any movement of the rod relative to the head of the screw. Each locking element can be integral with its respective clamp boss, but this requires the plate to be formed with a slot to enable the device to be assembled. Preferably, the element is separate, and attached to the clamp boss for example, by means of a bayonet coupling. A friction pad can be disposed between the locking element and the shoulders under the respective part-cylindrical surfaces, to assist the locking process.

A bone fixation device according to another aspect of the invention comprises a screw for mounting in a bone part with an exposed head projecting from the bone. A housing on the exposed head of the screw has jaws defining adjacent grooves for receiving fixation rods. Each groove has a central section in which the base has a part-spherical surface and opposed end sections in which the base has a surface divergent from the central section base. In each groove is a split ball element with a diametral bore therethrough; and a clamping element for compressing each ball element, contracting the bore therethrough, and clamping the element against said part-spherical surface.

The diametral bore through at least one split ball element can be sized to receive, contract around and clampingly engage a fixation rod, or receive, contract around and clampingly engage a sleeve for slidingly receiving at least one fixation rod.

Fixation devices of the invention 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.

The present invention also has 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 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. Such a system is described in our co-pending International Patent Application entitled “Bone Fixation System” claiming priority from British Patent Application No: 0820252.5, incorporated by reference. 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 such a fixation system 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.

FIG. 1shows the head2of a bone screw4as it might project from a bone (not shown) after installation during surgery. On the head is mounted a housing6comprising a base8and a drum10. The base8will normally be formed with a frusto conical recess which received the head2of the screw4. 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 a circlip12.

As shown inFIG. 2, the drum10is formed with two threaded recesses (only one is shown) facing in opposite directions. When assembled, a locking ring14holds a plate assembly16in the recess by virtue of an annular projection18from the base of each plate16engaging an annular shoulder20within the respective locking ring. A split ring22(only one is shown) holds the annular projection18and shoulder20in engagement during assembly of the device as a whole, and until the locking ring is tightened.

The locking rings14are axially aligned around two cylindrical projections that also extend through the boss sections26of the plates16and define lateral axes along which the plates16can move. The distal end surface of the wall of each projection14has a concave spherical surface for reasons which are described below.

Each plate16carries a clamp unit28which has a circular base received in a circular recess in the plate. This provides for rotation of each clamp unit28about an axis perpendicular to the common lateral axis of the plates16. The circular base of the clamp unit will be held in the plate recess30by means of a resilient circlip (not shown).

Each clamp unit has an outer jaw32and an inner 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 the inner 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 the respective projection24. The respective spherical surfaces match, such that there is uniform contact between them when they engage.

When the device is assembled, each locking ring14may be rotated in its respective drum recess, the threads causing the ring to drive the respective plate inwards by virtue of engagement between the annular projection18on each plate, and the shoulder20on each locking ring. This movement causes the end face of the projections24to engage the convex face of the movable jaw34serving simultaneously to close the clamp 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 the projection24and the movable jaw34may 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 locking ring14and the base of the respective drum recess, by means of a resilient washer therebetween.

The flexibility of the inner jaw34in each clamp unit28facilitates 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 engaging openings36in each locking ring14to enable such locking to be quickly and effectively accomplished.

FIG. 3shows 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 the drums10of 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. 4illustrates a second embodiment of the invention. Specifically, it shows how two fixation rods40and42can be mounted in alignment in two fixation devices of the kind described above, and interconnected by a sleeve44. The rod40is fixed in the sleeve44by a clamping screw (now shown) in the bell46. The rod42is slidably received in the other end of the sleeve44. 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 the rod42were to exit the sleeve44, its outward movement is restricted by an inwardly directed screw or spigot mounted in bell48that extends into a groove or similar (not shown) in the rod42which terminates before its end within the sleeve44. 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. 4shows rods extending on both sides of drums10mounted on screws4inserted 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 each screw4. 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. 5shows an alternative sleeve and rod assembly. In this embodiment, each sleeve is integral with one fixation rod section52to be clamped in a fixation device, for example of the kind illustrated inFIGS. 1 and 2. The other end of each sleeve50receives a curved section54of the other fixation rod, which merges with a straight section56for clamping in a fixation device installed in an adjacent bone part such as another vertebra. The curved section54is formed with a groove56on its surface aligned with its axis. A screw58driven through the wall of the sleeve50extends into the groove56. 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 the rod section54is curved in the same sense. Thus, as the rod54is 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. 6illustrates a similar arrangement to that ofFIG. 5, but here the two sleeves are coupled together to provide greater stability as the rod sections54are withdrawn from the sleeves50. The coupling unit60preserves 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. 7illustrates 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. 8illustrates 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, a housing66is mounted on a head64of a bone screw62. In the embodiment ofFIG. 8, the housing is secured on the head64by means of a locking pin68. The housing itself comprises a bracket70on which are formed pairs of spaced part-cylindrical surfaces72. These surface pairs define opposite sides of grooves extending beneath shoulders74which extend under the respective surfaces72.

Plates76are formed with part-cylindrical lower surfaces which complement and rest on the surfaces72. 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 the respective plates76are clamps78. Each clamp has a boss80which extends through an opening in its respective plate76and into the groove defined under and between the surfaces72. There it is attached to a locking element82which extends laterally under the shoulders74. Interposed between its locking element82and the respective shoulders74is a friction pad84to assist the locking process.

Each clamp has two jaws86defining a groove for receiving a fixation rod (not shown). A clamping element88in the form of a screw with an Allen Key socket engages a complementary screw thread defined on the internal faces of the jaws86.

As can be seen inFIG. 9, the holes in the plates76through which the bosses80extend have a chamfered perimeter. This enables the groove defined between the jaws86of 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 clamping element88driven downwards to engage it, the rod itself is forced not against the base of the groove, but against the upper surface of a plate76. Thus, when the clamping element is tightened, the rod is locked in the groove; the clamp86is locked in the plate76; and the plate76is locked against the surfaces72of the bracket70as the locking element82is drawn against the friction pad84and the shoulders74. As with the device ofFIG. 1then, a single locking action secures a fixation rod against axial movement in the groove, and pivotal movement in any direction.

FIG. 10illustrates 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 a housing90mounted on the head of a bone screw92, secured by means of a locking pin94. However, in the embodiment ofFIG. 10the clamps are defined side-by-side in a unitary body96. Each clamp has a locking element98similar to element88inFIG. 8, in the form of a screw engaging a complementary screw thread defined on the juxtaposed sides of a groove for receiving a fixation rod100. However, in the embodiment ofFIG. 10the fixation rod passes through a split ball element102having an external spherical surface which rests in an internal spherical surface at a central section of the groove over which the locking element98is 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 clamping element98is disengaged or only loosely engaged with the ball 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 a rod100fitted in the ball element102limited pivotal movement also about the centre of the ball 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. 10is essentially similar, but the bore through the ball element102is larger. This enables it to receive a sleeve104, 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, the fixation rod100is locked in orientation, but can still move axially within the sleeve. The reason for this is explained below.

FIG. 11illustrates how fixation devices according to the invention can be used in spinal surgery. Three devices106are shown, mounted on alternate vertebra with a pair of fixation rods108. Each rod108is secured by a clamp in each of the three devices106. 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. 11is to reinforce the spine and prevent undue curvature.

FIG. 12illustrates a fixation system that serves an additional purpose. In the system ofFIG. 12, three devices are once again installed on alternate vertebra, but device110installed on the central vertebra clamps not fixation rods108, but sleeves112. Rods114are not continuous between the devices106, but are discontinuous. This enables the spine to grow; ie, the vertebra on which the devices106are 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 the devices106and110are attached, and the intervening vertebra therebetween.

It will be appreciated that the fixation devices shown inFIGS. 11 and 12could 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 8can 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, the drum10of the housing6can be grasped either directly by the surgeon or remotely by a gripping device. In the devices ofFIGS. 8 and 10one 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. 10provides a particularly convenient alternative to these techniques. As shown inFIG. 13, a manipulator tool116having two pivotal arms118can lock onto the housing90by means of conical elements120at 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.