A FIXATION DEVICE FOR AN ELONGATE SPINAL SUPPORT ELEMENT

This invention relates to a fixation device for connecting an elongate support element to a patient's vertebra, and to an implant kit which includes a fixation device.

EP-A-470660 discloses apparatus for correcting the shape of a spinal column. The apparatus includes a rod which is formed from a nickel-titanium alloy which has been treated so that it exhibits shape memory properties, in bending or in torsion or both. Articles formed from shape memory alloys can exhibit shape memory properties associated with transformations between martensite and austenite phases of the alloys. These properties include thermally induced changes in configuration in which an article is first deformed from a heat-stable configuration to a heat-unstable configuration while the alloy is in its martensite phase. Subsequent exposure to increased temperature results in a change in configuration from the heat-unstable configuration towards the original heat- stable configuration as the alloy reverts from its martensite phase to its austenite phase. The transformation from austenite to martensite on cooling begins at a temperature known as the M _s temperature, and is completed at a temperature known as the M ₁ - temperature. The transformation of martensite to austenite upon heating begins at a temperature known as the A _s temperature and is complete at a temperature known as the A _f temperature.

The rod of the apparatus disclosed in EP-A-470660 is fastened to a patient's vertebrae while in the configuration from which it has to recover. The temperature of the rod is then increased so that it is greater than the A _s temperature of the alloy. The rod then recovers towards its heat-stable configuration, applying a corrective force to the spinal column.

Rods or other support elements which are used to support a spinal column can be fastened to the patient's vertebrae by means of fastening devices such as screws or hooks. Such devices can include a channel in which the support element can be fitted, and retained by means of a lock screw. The channel can have a through hole in its base, through which the shank of a fixation screw can extend. The channel can have a hook which extends beyond the base of the channel, to engage a projection provided by the bone tissue of the vertebra.

The forces which are imposed on a patient's vertebrae by a rod (or other support element) can be significant. It is important to ensure that the rod is fastened securely to the vertebrae, and that the tissue to which the rod is fastened is capable of carrying the imposed loads. Problems that can arise due to inappropriate fixation include pull-out of a fixation screw, fracture of a fixation screw, and fracture of the vertebra into which the screw is fixed.

It can be preferred to spread the load that is applied by a rod between two fastening devices. This can be done using a bridge connector which includes a channel in which the support element can be fitted, and a pair of transverse arms. Each of the transverse arms can be fastened to a vertebra by means of two bone screws or other fastening devices. This arrangement has been found to be satisfactory to secure a support rod to vertebrae in the sacral, lumbar, and lower thoracic regions of the spinal column. However, the use of this fastening arrangement in the upper thoracic and cervical region can be difficult because of the small size of the vertebrae.

The present invention provides an implant kit for correcting spinal deformities, which includes a housing for an elongate spinal support element, especially a rod, formed by a channel and a closure cap, and having a connection portion such as a screw or a hook which protrudes beyond the base of the channel to engage a vertebra so that the channel is connected directly to the vertebra, and a transverse arm extending from a side wall of the channel which can be connected to a vertebra using a secondary connection portion.

Accordingly, in one aspect, the invention provides a fixation device for connecting an elongate support element to a patient's vertebra in order to correct a spinal deformity, which comprises: a. a primary housing for the support element formed by a channel and a closure cap, in which the element can be received in the channel, and be retained in the channel by the cap, b. a primary connection portion which protrudes beyond the base of the channel to engage a vertebra so that the channel is connected directly to the vertebra, and

c. a transverse arm extending from a side wall of the channel.

In another aspect, the invention provides a fixation device for connecting an elongate support element to a patient's vertebra in order to correct a spinal deformity, which comprises: a. a primary housing for the support element formed by a channel and a closure cap, in which the element can be received in the channel, and be retained in the channel by the cap, b. a first bone screw which protrudes beyond the base of the channel to penetrate a vertebra so that the channel is connected directly to the vertebra, c. a transverse arm extending from a side wall of the channel, d. a secondary connector for engaging the transverse arm at a position that is spaced apart along the transverse arm from the primary housing, which includes a second bone screw which can penetrate a vertebra so that the transverse arm is connected to the vertebra.

The fixation of the invention has the advantage that it enables the forces which are applied to the spinal column by a support element to be applied at two spaced points which are spaced apart across the axis of the spinal column, usually on a single vertebra, but possible on adjacent vertebrae. Furthermore, this is possible in locations in which there is not sufficient space to use a bridge connector of the known kind which is described above. Accordingly, loads can be spread across more than one connection portion which can reduce the risk of damage to a patient's vertebra as a result of a load being applied at a single point. The ability to use multiple points of fixation for a spinal rod or other support element, where there is limited space, represents a considerable advance.

While a bone screw protruding from the base of a channel in which a spinal support rod can be received has been used previously to fasten a rod to a vertebra, it has been found that advantages arise in terms of more secure connection of a spinal support rod to a vertebra when the loads that are placed on the vertebra are spread from the primary housing in which the rod is received, with its bone screw, to a second bone screw which is spaced apart transversely from the housing. In this arrangement, loads arising from a single rod

placed on one side of the spinal column can be spread across the selected vertebrae to which the rod is connected to the opposite side of the column. In addition to providing a more secure connection, this can also help to reduce the risk of damage to bone tissue which might arise due to concentration of applied loads.

In another aspect, the invention provides an implant kit which includes a fixation device as discussed above, and a secondary connection portion for connecting the transverse arm to a vertebra at a position that is spaced apart transversely from the primary connection portion.

In a further aspect, the invention provides an implant kit which includes a fixation device as discussed above, and an elongate support element which can be fastened to a patient's vertebrae using the fixation device in a deformed configuration, from which it attempts to recover after implantation and so to apply a corrective force to the patient's spine.

The primary connection portion or the secondary connection portion or both can comprise a bone screw. The screw should be designed with an appropriate thread having regard to the nature of the tissue into which it is to extend. The factors affecting the suitability of a thread to engage bone tissue of a vertebra are well known, including thread pitch, shank diameter, thread diameter and so on.

The primary connection portion of the secondary connection portion or both can comprise a hook. The design of hooks which are suitable for fixing an elongate spinal support element to a vertebra is known.

When one or each connection portion comprises a bone screw, it can be used with a channel in which the element and/or the transverse arm can be received, which has a through hole in its base. The bone screw can have a head and a shank which are sized so that the shank can extend through the through hole, and the head is retained in the channel. The element or the transverse arm (as the case might be) can be retained in its respective channel by means of a closure cap having threads which can engage corresponding threads on the walls of the channel. The threads can be provided on the internal walls of the channel or on the external walls of the channel. The channel can be provided with threads

on both its internal walls and its external walls to provide different options for engagement of a lock screw.

It can be preferred that the opening in the through hole to the inside of the channel is rounded when viewed from one side, and that the face of the head of the bone screw which faces towards the base of the channel has a corresponding rounded shape. This can enable the bone screw to be screwed into bone tissue with the angle between the axis of the screw and the axis of the through hole being greater than 0°, for example at least about 5°, or at least about 10°, or possibly at least about 15°.

The transverse arm can be permanently connected to the wall of the channel. For example it can be formed as one piece with the channel by a technique such as casting or machining, or it can be fastened permanently to the channel, for example by a technique such as welding. The channel and the transverse arm can be provided as separate parts. For example, the side wall of the channel can provide a socket in to which an end of the transverse arm can be received. The end of the transverse arm can be fastened into the socket by means of cooperating threads. This can provide a suitably secure connection, particularly when the transverse arm is secured against twisting about its axis as a result of the connection at its other end to the secondary connection portion.

The transverse arm can have a through hole towards the end which is remote from the primary housing, through which a bone screw of the secondary connection portion can extend.

Preferably, the bone screw of the secondary connection portion includes a secondary housing for the transverse arm, in which the transverse arm can be clamped. This has the advantage that it allows the position at which the transverse arm is clamped to be selected along the length of the arm, according to the requirements of a particular patient. The features of the housing of the secondary connection portion can include the features of the housing of the primary connection portion.

It can be preferred for the transverse arm to have at least one longitudinally extending spline which extends along at least part of its length. This can help to secure the arm within a housing against twisting about its axis. It can be appropriate for the housing to be provided with splines on at least a portion of at least one wall which are positioned to engage with the splines on the transverse arm to restrict rotation of the arm about its axis. When the housing does not have splines on its internal surfaces, the splines on the transverse arm can still provide the desired restriction of rotation of the arm due to engagement with other housing features, such as the edges of the housing channel in which the arm is received.

It can be preferred for the transverse arm to have a cross-section which is non-circular to enable it to fit securely in a channel in such a way that it can transmit torque to the channel. For example, the arm can have at least one flat face. Polygonal (regular or irregular) shapes can be useful, for example with at least four faces, including square or rectangular or trapezoidal (when the element has four faces when viewed in cross-section), or with six or eight or more faces. An arm which has a generally rounded cross-section might have a flat face.

It can be preferred that the transverse arm is cranked so that the angle between the axes defined by the arms at its opposite ends (when viewed from one side of the primary housing generally along the superior-inferior axis) is less than 180°, for example not more than about 170°, and possibly not more than about 160°. This can help to align the transverse arm with the connection portion of the implant kit which is used to fasten the arm to the vertebra.

The housing can be provided with more than one transverse arm. hi particular, it can be preferred that the housing is provided with a transverse arm extending from each of two opposite side walls. The angle between two such transverse arms at the points where they are connected to the housing, when viewed from above (along the axis of the channel) will generally be 180°. However, the angle might be less than 180°, for example not more than about 170°, and possibly not more than about 160° . This can help to align the transverse

arm with the connection portion of the implant kit which is used to fasten the arm to the vertebra.

Each of the connection portions, the housing and the transverse arm might be made from a metallic material as is generally known for components of spinal implant kits. Suitable metals include certain stainless steels, and titanium and its alloys. It can be preferred that the transverse arm at least is formed from a material which enables the arm to be cut to length to suit a particular application.

The cross-sectional area of the transverse arm will often be approximately constant over at least most of its length, with the possibility that the cross-section might vary in at least one end region to facilitate connection directly or indirectly to a vertebra or to the primary housing. For example, the cross-sectional area of the support element might be at least about 10 mm ² , preferably at least about 20 mm ² , more preferably at least about 30 mm ² , for example about 40 mm ² .

The length of the transverse arm can be selected according to the requirements of a particular application. It is expected that it will seldom be required to be longer than about 50 mm. The length of the transverse arm will usually be at least about 15 mm.

Preferably, the material of the transverse arm is selected so that the arm can be cut to length to suit the requirements of a particular implantation.

Preferably, the support element is capable of recoverable deformation towards its original undeformed configuration (from which it had been deformed) such that the angle between its ends changes through at least about 20°, more preferably at least about 25°, especially at least about 30°. Recoverable deformation is deformation that can be recovered substantially completely back to the undeformed configuration when applied stress is removed, or otherwise when allowed to recover (for example as a result of heating to allow a transformation to austenite phase).

The support element will preferably be a rod, especially with a solid cross-section. A rod support element can be hollow along at least part of its length. One or more of the support elements can be a plate.

The cross-sectional area of the support element will often be approximately constant over at least most of its length, with the possibility that the cross-section might vary in at least one end region to facilitate connection directly or indirectly to a vertebra at the end or to an adjacent support element. For example, the cross-sectional area of the support element might be at least about 10 mm ² , preferably at least about 20 mm ² , more preferably at least about 30 mm ² , for example about 40 mm ² .

The support element might have a rounded cross-sectional shape, especially circular. It can be preferred for the element sometimes to be non-circular to enable it to fit securely in a channel of a connection housing in such a way that it can transmit torque to the housing. For example, the element can have at least one flat face. Polygonal (regular or irregular) shapes can be useful, for example with at least four faces, including square or rectangular or trapezoidal (when the element has four faces when viewed in cross-section), or with six or eight or more faces. An element which has a generally rounded cross-section might have a flat face.

The support element can be formed from a shape memory alloy. The alloy can be treated so that it is implanted while in the martensite phase. The treatment of the alloy can be such that its A _s and A _f temperatures are between ambient temperature and body temperature

(37 ⁰ C), so that the alloy is fully austenite phase at body temperature (for example by virtue of the A _f temperature being about 32°C). This allows the surgeon to make use of the thermally initiated shape recovery properties of the alloy, in which the support element is implanted in the body in the martensite phase, which is stable at ambient temperature. On implantation, the element is exposed to body temperature which leads to the phase of the alloy transforming from martensite to austenite. The element will then tend towards a configuration from which it was transformed while in the martensite phase, applying corrective forces to a patient's vertebrae.

A support element which is formed from a shape memory alloy can apply corrective forces by virtue of the enhanced elastic properties that are available from such materials. Shape memory alloys can exhibit enhanced elastic properties compared with materials which do not exhibit martensite-austenite transformations and it is these properties that the present invention is concerned with in particular. The nature of superelastic transformations of shape memory alloys is discussed in "Engineering Aspects of Shape Memory Alloys", T W Duerig et al, on page 370, Butterworth-Heinemann (1990). Subject matter disclosed in that document is incorporated in this specification by this reference to the document. A principal transformation of shape memory alloys involves an initial increase in strain, approximately linearly with stress. This behaviour is reversible, and corresponds to conventional elastic deformation. Subsequent increases in strain are accompanied by little or no increase in stress, over a limited range of strain to the end of the "loading plateau". The loading plateau stress is defined by the inflection point on the stress/strain graph. Subsequent increases in strain are accompanied by increases in stress. On unloading, there is a decline in stress with reducing strain to the start of the "unloading plateau" evidenced by the existence of an inflection point along which stress changes little with reducing strain. At the end of the unloading plateau, stress reduces with reducing strain. The unloading plateau stress is also defined by the inflection point on the stress/strain graph. Any residual strain after unloading to zero stress is the permanent set of the sample. Characteristics of this deformation, the loading plateau, the unloading plateau, the elastic modulus, the plateau length and the permanent set (defined with respect to a specific total deformation) are established, and are defined in, for example, "Engineering Aspects of Shape Memory Alloys", on page 376.

A preferred way in which non-linear superelastic properties can be introduced in a shape memory alloy involves cold working the alloy by one of several deformation methods, for example, swaging, drawing, pressing, stretching or bending. The cold working step is followed by an annealing step at a temperature less than the recrystallization temperature of the alloy, for a time sufficient to cause dislocations to rearrange, combine and align themselves (so-called "recovery" processes). The resulting recovered dislocation structure should ideally be dense enough to make plastic deformation difficult, but not so dense as to

prevent the martensite phase from transforming upon the application of a load, and growing in a relatively unimpeded manner.

Since many preferred superelastic alloys are thermally unstable in the temperature range in which these recovery processes occur, a second unavoidable result of this recovery heat treatment step is to age the material, that is to cause Ni-rich particles to precipitate, having the effect of enriching the matrix phase in titanium, and thus increasing the transformation temperatures (including the A _f temperature). Optimum superelastic properties are only realized when using shape memory alloys above the A _f temperature, though it should be noted that some indications of superelasticity are observed above the A _s temperature (typically 2 to 2O ⁰ C below A _f ). Thus a second requirement for this recovery heat treatment is that A _f not be increased above the temperature at which the alloy is to be used. Practically speaking this places upper limits on the time and temperature which can be used in the recovery heat treatment.

When it is desired only to rely on the superelastic properties of an alloy without any cont- ribution from any thermally initiated shape memory effect, the alloy should by processed so that its A _f temperature is below temperatures to which the alloy is likely to be subjected during implantation, that is preferably below about ambient temperature. For example, the A _f temperature might be not more than about 20 ⁰ C.

It is often the case that a device is to be used in a shape other than that which can be practically produced by cold working processes. For example, a straight wire can be conveniently produced by cold drawing, but a wire loop or other formed shape cannot be. In this case, it is customary to form the drawn, cold worked wire into the desired "heat stable" shape, to constrain the wire in that shape, and then to perform the above described recovery heat treatment to "shape set" the component. In this case the final annealing operation has two purposes: to adjust the superelastic properties of the alloy, and to properly set the shape of the article. The time and temperature of this heat treatment step are critical. If held too long at temperature, the material over-ages, causing the A _f temperature to rise above the application temperature. If the annealing temperature is too short, or the temperature too low, the shape will be insufficiently formed, and too much of

-l ithe original dislocation structure will remain to allow free martensite movement. This "forming" treatment may introduce still further cold work into the part, but that cold work is usually small compared to that introduced into the wire by drawing. Moreover, forming operations are often not uniform, and thus forming itself is not generally a convenient way to introduce cold work.

Articles of complicated shape require extensive forming and are very difficult to produce according to the above process. If the forming process causes strains which are too severe, the article will fracture as it is heated to the shape setting and recovery temperature (one is able to restrain the formed article, but cannot maintain its shape during the heating process without causing fracture). It is possible to overcome this problem by performing a series of smaller, intermediate shape setting operations which accumulate to provide the desired final shape, but unfortunately each of these shape setting operations requires sufficient annealing time to allow the material to soften, in preparation for the next. When accumulated, these heat treatments cause a cumulative ageing effect that can cause the A _f temperature to rise beyond the expected service temperature (37 ⁰ C for most medical applications, for example).

It is also known that one can introduce superelasticity by solution treating and ageing, abandoning all attempts to retain cold work. Although this approach resolves the above problems, it leads to inferior superelastic properties, producing articles that are susceptible to fatigue and storage problems.

Examples of shape memory alloys which might be used in the first and possibly other support elements in the kit of the invention include nickel-titanium based alloys, especially the binary alloy which contains 50.8 at-% nickel. Suitable alloys include those which satisfy ASTM F2063-00. It will often be particularly preferred for both the first and second support elements to be formed from shape memory alloys, especially for each support element to be formed from shape memory alloys. Other metals which might be used to form support elements which do not exhibit shape memory properties include titanium and alloys thereof, for example Ti6A14V alloys such as satisfy ASTM F136-02a or ASTM F1472-02a or both.

Materials which exhibit shape memory properties, other than alloys, can be used. For example, polymeric materials can be used. Shape memory properties can be imparted to polymeric materials by forming them in a desired ultimate shape (for example by moulding), crosslinking the material, heating the material to a temperature at which it softens, deforming the material while soft and restraining the material in the deformed configuration while it cools. The material will tend to revert towards the initial "as formed" configuration when reheated. Examples of suitable polymeric materials which can be used in this way include oligomers, homopolymers, copolymers and polymer blends which include, as monomers, 1-, d- or d/1-lactide (lactic acid), glycolide (glycolic acid), ethers, ethylene, propylene and other olefins, styrene, norbornene, butadiene, poly- functional monomers such as acrylates, methacrylates, methyl acrylates, and esters such as caprolactone. The use of such polymeric materials in related applications is disclosed in WO-02/34310.

Embodiments of the invention will now be described by way of example with reference to the accompanying drawings, in which:

Figure 1 is a side view of a primary housing for a support element for use in the kit of the invention.

Figure 2 is an isometric view of an implant construct which comprises an elongate spinal support rod and two fixation devices which each include primary housings, connection portions and transverse arms.

Figure 3 is an isometric view of another embodiment of fixation device in which the primary housing and transverse arm are provided as separate parts.

Figure 4 is an isometric view of a portion of an implant kit with another embodiment of fixation device in which a fixation hole is provided in the transverse arm.

Figure 5 is an isometric view of a fixation device in which the primary connection portion comprises a hook to engage the tissue of a patient's vertebra.

Referring to the drawings, Figure 1 shows a fixation device 2 which can be used to provide two-point fixation of an elongate spinal support rod to a patient's vertebra. The device comprises a primary housing provided by a channel part 4 having a base 6 and first and second opposite side walls 8, 10. The gap between the side walls is dimensioned so that the support rod is a tight fit in the channel.

The side walls of the channel part have threads 12 on their internal and external surfaces to engage a locking nut, which can be an internal nut or an external nut, for retaining the rod in the channel. It is also envisaged that both internal and external nuts might be used to provide secure engagement.

A through-hole 16 is provided in the base 6 of the channel, through which the shaft of a fixation screw 18 can extend. The fixation screw 18 has a head whose transverse dimension is too big for to fit through the through-hole. The engaging surfaces of the through-hole and of the screw head are rounded when viewed from one side so that the screw head engages the through-hole surface securely over a range of angles between the axes of the screw and the through-hole.

The fixation device includes a transverse arm 20 which extends from and is formed integrally with one of the side walls 10 of the channel. The transverse arm has a circular cross-section, and has longitudinally extending splines on its lower surface 22.

Figure 2 shows an implant kit which includes two fixation devices 2 of the type described above with reference to Figure 1. An elongate spinal support rod 30 formed from a NiTi shape memory alloy is received in the channels of the fixation device primary housings and is retained there by means of internal nuts 32 which engage ^' the threads 12 on the internal surfaces of the channel parts 4.

The implant kit includes two secondary connectors 34, each having a channel 36 and a fixation screw 38 extending through a through-hole in the base of the channel. Each of the transverse arms 20 fits into the channel on a respective one of the connectors, and is retained in the channel by means of an internal nut 38. Features of the connectors 34 and

of the engagement of the transverse arms in the channels are similar to the corresponding features of the primary housing described above. Note however that the cross-section of the spinal support rod, and therefore of the base of the channel in the primary housing, are square, whereas the cross-section of the transverse arm, and therefore of the base of the channel in the secondary connectors, is rounded.

Figure 3 shows a fixation device 60 in which the side wall 62 of a channel on a housing has a socket 64 formed on it. A transverse arm 66 has a reduced diameter spigot portion 68 at one end which is a tight fit in the socket. Alternatively, the socket and the spigot portion might engage one another by means of mating threads.

Figure 4 shows a part of an implant construct which includes a spinal support rod 70 with a square cross-section, which is received in the channel of a primary housing 72 as discussed above.

The transverse arm 74 which extends from a side wall 76 of the housing has a through-hole at the end which is remote from the housing for receiving a bone screw 78.

Figure 5 shows a fixation device 80 which includes a primary housing for a support element formed by a channel 82. As discussed above with reference to Figure 1, a support rod can be retained in the channel by means of one or more nuts.

A hook 84 extends from the base of the channel and is arranged so that it can engage the patient's vertebra to restrain the support element against movement relative to the vertebra.

A transverse arm 86 with a round cross-section extends from one of the side walls of the channel. Secondary connection portions which can be used to anchor the transverse arm to the patient's vertebra include a bone screw or hook (for engaging the vertebra), having a channel in which the arm can be received and locked. Such screws and hooks are known, as used for example to fasten spinal support rods directly to a patient's vertebrae.