Abstract:
A spinal support rod kit for the treatment of spinal column shape deformations, comprises a first spinal support rod whose cross-section is non-circular at least at one end thereof and a second spinal support rod whose cross-section is circular along at least part of its length. A socket attached to the second support rod at one end is shaped so that the non-circular end of the first support rod can be received in the socket. The second support rod can include a mechanical locking component which can engage the non-circular end of the first support rod when it is inserted into the socket to prevent it from being withdrawn.

Description:
BACKGROUND  
       [0001]     The present invention relates to a spinal support rod kit for the treatment of spinal column shape deformations.  
         [0002]     Support rods which are used to support a spinal column can be fastened to the patient&#39;s vertebrae by means of fastening devices such as for example by bone screws or hooks. The support rods can help to support the spine in a desired alignment, for example during fusion of vertebrae.  
         [0003]     Support rods are deformed prior to fixation to a patient&#39;s vertebrae, and forces are then applied to the vertebrae as the rods attempt to recover elastically toward their original undeformed configuration. Deformation of the rods can involve application of bending stresses and of torsional stresses. When it is desired to apply forces to a patient&#39;s spinal column through torsional deformation of a support rod, it can be preferred that the support rod has a non-circular cross-section, especially a square cross-section, which can be fitted into a channel in a fixation device whose cross-section is such that twisting of the rod in the channel is inhibited.  
         [0004]     Maintaining a torsional deformation in a support rod requires that the rod is received in appropriate fixation devices at spaced apart points along its length. When this is done by fitting the rod into a channel in a fixation device with an appropriate non-circular cross-section, it can be difficult to locate the fixation device appropriately having regard to the variation in the cross-section of the rod as presented to the channel and the available vertebral tissue to which the fixation device might be fitted. This difficulty can be particularly acute in the thoracic region.  
       SUMMARY  
       [0005]     The present invention provides a spinal support rod kit which includes a first spinal support rod, and a second spinal support rod with a socket at one end in which the end of the first rod can be received in an end-to-end arrangement.  
         [0006]     Accordingly, in one aspect, the invention provides a spinal support rod kit for the treatment of spinal column shape deformations, comprising: 
        a first spinal support rod having a first cross-section at least at one end thereof, and     a second spinal support rod which has a socket which is permanently connected to the said second support rod at one end which is shaped so that the said end of the first support rod can be received in the socket, the second support rod including a mechanical locking component which can engage the said end of the first support rod when it is inserted into the socket, to prevent it from being withdrawn.        
 
         [0009]     The spinal support rod kit of the invention has the advantage that the second support rod enables the support rod assembly to be aligned torsionally with a channel in a fixation device which is intended to engage a selected vertebra, accommodating torsional misalignment of the first support rod, especially when the cross-section of the first support rod at its end is different from the cross-section of the second support rod.  
         [0010]     Preferably, the cross-section of the first spinal support rod at the said end thereof is non-circular, especially approximately square. However, the cross-section of the first spinal support rod at the said end thereof might be approximately circular.  
         [0011]     Preferably, the cross-section of the second support rod is circular along at least part of its length. The second support rod can then be received in a channel in a fixation device in any torsional orientation, enabling a desired torsional deformation to be maintained along the first and second support rods to ensure that appropriate torsional forces are applied to the vertebrae to which they are connected to ensure the desired correction of deformities.  
         [0012]     The second support rod can have a non-circular cross-section. For example, it might have at least one flat side. It can be preferred that the second support rod has a plurality of flat sides. The second support rod can then be fitted into a channel in a fixation device whose cross-section is such that twisting of the rod in the channel is inhibited. The second support rod might have a square cross-section. The first support rod might have a square cross-section. When the second support rod and the socket for the first support rod have the same cross-section, it can be preferred that they are offset about the axis of the rod. For example, when the rod and the socket are both square, it can be preferred for one to be offset by about 45°.  
         [0013]     A first support rod which has a non-circular cross-section can have at least one flat face. Preferably, the first spinal support rod has a regular or irregular polygonal shaped cross-section. The cross-section can be a polygonal shape having at least four faces, including square or rectangular or trapezoidal (when the rod has four faces when viewed in cross-section), or with six or eight or more faces. A first support rod which has a generally rounded cross-section might have a flat face. Preferably, the first support rod has a generally square cross-section.  
         [0014]     The second support rod and the socket can be formed as one piece by a technique such as casting or machining, or the socket can be fastened permanently to the second support rod, for example by a technique such as brazing or welding.  
         [0015]     Preferably, at least one of the first and second support rods is capable of recoverable deformation towards an original undeformed configuration (from which the support rods have been deformed) such that the angle between the ends of a support rod 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 austentite phase). Accordingly, it can be preferred for the first support rod or the second support rod or each of the first and second support rods to be formed from a shape memory alloy.  
         [0016]     The first and second support rods will preferably have solid cross-sections. A support rod can be hollow along at least part of its length. One or more of the support rods can be in the form of a plate.  
         [0017]     The cross-sectional area of a support rod 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 rod. For example, the cross-sectional area of the support rod 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 .  
         [0018]     The first and second support rods can differ from one another by features which include one or more of material, physical properties (for example modulus, elastic limit, etc which might for example be introduced through different processing techniques), and dimensions.  
         [0019]     One or more support rods can be formed from a shape memory alloy. Preferably, the first support rod is made 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° 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 rod is implanted in the body in the martensite phase, which is stable at ambient temperature. On implantation, the support rod is exposed to body temperature which leads to the phase of the alloy transforming from martensite to austenite. The support rod will then tend towards a configuration from which it was transformed while in the martensite phase, applying corrective forces to a patient&#39;s vertebrae.  
         [0020]     A support rod 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.  
         [0021]     Examples of shape memory alloys which might be used in the first and possibly other support rods in the kit of the invention include nickel-titanium based alloys, especially the binary alloy which contains 50.8% nickel. Suitable alloys include those which satisfy ASTM F2063-00. It will often be particularly preferred for both the first and second support rods to be formed from shape memory alloys, especially for each support rod to be formed from shape memory alloys. Other metals which might be used to form support rods which do not exhibit shape memory properties include titanium and alloys thereof, for example Ti6Al4V alloys such as satisfy ASTM F136-02a or ASTM F1472-02a or both.  
         [0022]     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, l-, d- or d/l-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.  
         [0023]     Preferably, the surface of the second spinal support rod has at least one spline. The at least one spline should preferably be located so that it is on the surface of the second support rod which is directed into the channel in a fixation device. The engagement of one or more splines on the second rod with corresponding splines in the channel in a fixation device can help to inhibit rotation of the second rod in the channel due to torsional stresses in the first support rod or the second support rod or both.  
         [0024]     It will generally be preferred for each spline on the second support rod to extend approximately along the axis of the rod.  
         [0025]     The spinal support rod kit will generally include a fixation device having a channel in which a support rod can be received, having a bone connection feature by which the device can be fastened to a patient&#39;s vertebra. The fixation device can be a bone screw, in which the channel is provided in or is otherwise fastened to the head of the screw, with a threaded shank extending from the head which can be screwed into a bore in the patient&#39;s bone tissue. The fixation device can be a hook which can be fitted on to a protrusion or recess in the patient&#39;s bone. The bone connection feature preferably extends below the lower surface of the support rods. Fixation devices having bone connection features of this general type (such as screws and hooks) for fastening spinal support rods to a patient&#39;s spinal column are known.  
         [0026]     The fixation device can cooperate with a locking screw to retain a support rod in its channel. For example, the internal walls of the channel can have a circular cross-section when the channel is viewed along the axis of insertion of the support rod, and be threaded to engage a screw having a threaded peripheral surface, which is inserted into the channel. Alternatively or in addition, the external walls of the channel can have a circular cross-section when the channel is viewed along the axis of insertion of the support rod, and be threaded to engage a locking ring having a threaded internal surface, which can be positioned on the device so that the threads on the internal surface of the ring engage the corresponding threads on the external walls of the channel.  
         [0027]     The fixation device 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.  
         [0028]     The second support rod can have a fixation device by which the rod can be fastened to a patient&#39;s vertebra. The fixation device can be connected to the socket of the second spinal support rod, especially in such a way that the fixation device cannot be separate from the socket during or after implantation. For example, the socket can have a through hole extending through it to receive a bone screw. Alternatively, the socket can have a hook extending from its bone facing surface.  
         [0029]     It can be preferred that the opening in the through hole to the inside of the socket is rounded when viewed from one side, and that the face of the head of the bone screw which faces towards the lower surface of the socket can have a correspondingly 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°.  
         [0030]     The socket provided by the second spinal support rod can have a closed cross-section at its open end when viewed along the longitudinal axis of the rod. The end of the first support rod can be inserted through the open end into the socket of the second support rod in a direction along the longitudinal axis of the second spinal support rod. In order to connect the first and second support rods, there must be sufficient space in the direction of the longitudinal axes of the rods between the end of the first support rod and the socket of the second support rod.  
         [0031]     Alternatively, the socket can have an open cross-section at its open end when viewed along the longitudinal axis of the second spinal support rod. The end of the first spinal support rod can be inserted into the socket of the second spinal support rod by moving the end of the first support rod relative to the socket of the second support rod in a direction which is generally transverse to the longitudinal axis of the second support rod. This socket has the advantage that the amount of space required to connect the first and second support rods, when measured in a direction along the longitudinal axis of the rods, is reduced. Prior to the connection of the first and second support rods, the end of the first support rod can overlap with the socket at the end of the second support rod. This reduces the space required by the surgeon to connect the spinal support rods.  
         [0032]     The socket can be shaped and dimensioned to receive the correspondingly shaped end of the first spinal support rod. The end of the first support rod preferably forms a tight sliding fit with the socket.  
         [0033]     The outer surface of the wall of the socket can also include one or more engagement members, such as for example a groove, a recess or a protrusion which can cooperatively engage corresponding engagement members, such as for example a ridge, a protrusion or a recess on surgical instruments, such as for example a rod approximator instrument. This has the advantage that the surgeon can use a surgical instrument to locate the socket to help connect or disconnect the end of the first support rod to or from the socket of the second support rod.  
         [0034]     The locking component of the second support rod preferably comprises a threaded fastener, which can engage a surface of the socket so that it can be driven towards the surface of the first support rod when received in the socket. It will generally be preferred for the internal wall of the socket to be threaded and for the fastener to have a threaded peripheral surface.  
         [0035]     Preferably, the kit of the invention includes a transverse arm which can extend from one of the first and second spinal support rods generally transversely to a patient&#39;s spinal column. Preferably, the transverse arm is provided as a separate component and the kit can then include a connector by which the transverse arm can be connected to one of the first and second support rods.  
         [0036]     A transverse arm which is fastened to the second support rod can help to inhibit rotation of the second rod due to torsional stresses in the first support rod or the second support rod or both. The connection between the second support rod and a transverse arm should preferably be capable of preventing relative movement (for example involving rotation of the second support rod about its axis relative to the transverse arm) due to torsional stresses in the second support rod.  
         [0037]     The transverse arm can be fastened to the patient&#39;s vertebra at its end which is remote frm the first and second support rods. Preferably, the kit includes a third spinal support rod for fixation to a patient&#39;s spinal column approximately parallel to the first and second support rods, and a connector by which the said transverse arm can be connected to the third support rod. The kit can include appropriate fixation devices by which the third support rod can be fixed to the patient&#39;s vertebrae.  
         [0038]     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 spline can cooperate with a cooperating spline on the first or second support rod. This spline helps to secure the arm and can prevent the transverse arm from twisting about its axis. This spline can therefore increase the torsional stability of the system.  
         [0039]     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 first or second spinal support rods. For example, the cross-sectional area of the transverse arm 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 .  
         [0040]     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.  
         [0041]     The support rods, the transverse arm and the fixation devices can be made from one or more metallic materials 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. 
     
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0042]     Embodiments of the invention will now be described by way of example with reference to the accompanying drawings, in which:  
         [0043]      FIG. 1  is a perspective view of the socket of the second support rod of one embodiment of the present invention,  
         [0044]      FIG. 2  is a view from above of the socket of the second support rod of a further embodiment of the present invention connected to an end of the first support rod,  
         [0045]      FIG. 3  is a perspective view of the second support rod of the embodiment of  FIG. 2 ,  
         [0046]      FIG. 4  is a perspective view of the first and second support rods of the embodiment of  FIG. 2 ,  
         [0047]      FIG. 5  is a side view of the first and second support rods of the embodiment of  FIG. 2 ,  
         [0048]      FIG. 6  is a perspective view of the transverse arm and the second support rod of the embodiment of  FIG. 1 , and  
         [0049]      FIG. 7  is a perspective view of the assembled kit of the embodiment of  FIG. 2 . 
     
    
     DETAILED DESCRIPTION  
       [0050]     In general, the present invention provides a spinal support rod kit comprising first and second support rods for the treatment of spinal column shape deformations.  
         [0051]     With reference to  FIG. 1 , the second spinal support rod  2  is composed of stainless steel and has a circular cross-section. The second support rod  2  has a solid cross-section. The cross-sectional area of the second support rod  2  is approximately constant over the entire length of the second support rod  2 .  
         [0052]     A socket  4  is permanently connected to one end  6  of the second support rod  2 . The socket  4  has a closed cross-section at its open end  8  when viewed along the longitudinal axis A-A′ of the second support rod  2 . The open end  8  provides an opening  10  having a square shape which is dimensioned to receive an end of a first spinal support rod (not shown) which has a square cross-section. The opening  10  is in communication with a channel (not shown) which extends in the direction of the longitudinal axis A-A′ of the second support rod  2 .  
         [0053]     The upper surface  16  of the socket  4  provides an aperture  14 . The aperture  14  is in communication with a threaded bore  18  which extends substantially perpendicular to the longitudinal axis A-A′ of the second support rod  2 . The threaded bore  18  also extends substantially perpendicular to the channel (not shown) which is in communication with the opening  10 . The upper surface  16  of the socket  4  also provides an opening  20  which is dimensioned to receive an end of a surgical tool (not shown) for locating the socket  4 .  
         [0054]     In use, the surgeon uses a surgical tool to locate the opening  20  provided by the socket  4  of the second support rod  2 . An end of a first spinal support rod (not shown) having a square cross-section is inserted through the opening  10  in the open end  8  of the socket  4  of the second support rod  2 . The first support rod (not shown) is inserted into the socket  4  in the direction of the longitudinal axis A-A′ of the second support rod  2 .  
         [0055]     A locking component (not shown) of the second support rod  2  is inserted into the aperture  14  provided by the upper surface  16  of the socket  4 . The locking component (not shown) comprises a threaded screw (not shown) which cooperatively threadingly engages the threaded bore  18  provided by the socket  4  of the second support rod  2 . The threaded screw (not shown) engages the end of the first support rod (not shown) and prevents the first support rod (not shown) from being withdrawn from the socket  4  of the second support rod  2 .  
         [0056]     Once the end of the first support rod (not shown) is secured within the socket  4  of the second support rod  2  torsional stability is maintained along the first and second support rods  2  of the invention. The second support rod  2  is not able to rotate relative to the first support rod (not shown).  
         [0057]     With reference to FIGS.  2  to  5  and  7 , the socket  24  of the second support rod  22  has an open configuration provided by a rectangular opening  30  at its open end  28  when viewed along the longitudinal axis A-A′ of the second support rod  22 . The rectangular opening  30  in the open end  28  is in communication with a channel  31  which extends in the direction of the longitudinal axis A-A′ of the second support rod  22 . The opening  30  and the channel  31  of the second support rod  22  are dimensioned to receive an end  42  of the first support rod  44 .  
         [0058]     The end  42  of the first support rod  44  has a square cross-section. The rectangular opening  30  provided at the open end  28  of the second support rod  22  has a width which is slightly greater than the width of the end  42  of the first support rod  44 .  
         [0059]     The socket  24  provides an aperture  34  in the upper surface  36  of the socket. The aperture  34  is in communication with a threaded bore  38  which extends substantially perpendicular to the longitudinal axis A-A′ of the second support rod  22 . The threaded bore  38  also extends substantially perpendicular to the longitudinal axis of channel  31 . Prior to connection of the first  44  and second  22  rods, the end  42  of the first support rod  44  overlaps with the socket  4  of the second support rod  22 .  
         [0060]     The second support rod  22  comprises a locking component  46  which is dimensioned to be inserted through the aperture  34  provided by the socket  24 . The locking component  46  comprises a threaded screw.  
         [0061]     The lower surface  48  of the support rod  22  has a spline  50 . The spline  50  extends along the lower surface  48  of the socket  24  in a direction which is substantially perpendicular to the longitudinal axis A-A′ of the second support rod  22 .  
         [0062]     In use, the square end  42  of the first support rod  44  is inserted through the opening  30  into the channel  31  of the socket  24  by moving the end  42  of the first support rod  44  relative to the socket  24  of the second support rod  22 . The end  42  of the first support rod  44  is moved in a direction which is generally transverse to the longitudinal axis A-A′ of the second support rod  22 .  
         [0063]     A locking component  46  of the second support rod  22  is inserted through the aperture  34  into the channel  31  of the socket  24 . The locking component  46  cooperatively threadingly engages the threaded bore  38  provided by the socket  24  of the second support rod  22 . The threaded screw  46  engages the end  42  of the first support rod  44  and prevents the first support rod  44  from being withdrawn from the socket  24  of the second support rod  22 .  
         [0064]     Once the end  42  of the first support rod  44  is secured within the socket  24  of the second support rod  22  torsional stability is maintained along the first  44  and second support rods  22  of the invention. The second support rod  22  is not able to rotate relative to the first support rod  44 .  
         [0065]     With reference to  FIGS. 6 and 7 , the second support rod  2  and  22  can be connected to a transverse arm  60 . The transverse arm  60  extends from the second support rod  2  and  22  substantially perpendicular to the longitudinal axis A-A′. The transverse arm  60  and the second support rod  2  and  22  are provided in the kit as separate parts. The transverse arm  60  includes a connector  62  which is permanently connected to one end of the arm  60 . The connector  62  has a through hole  64  extending substantially perpendicular to the longitudinal axis B-B′ of the transverse arm  60 . The through hole  64  has a circular cross-section. The through hole  64  is dimensioned to receive the second support rod  2  and  22 . The diameter of the through hole  64  is slightly greater than the diameter of the second support rod  2  and  22 .  
         [0066]     A threaded bore  66  extends from the upper surface  68  of the connector  62 . The threaded bore  66  extends substantially perpendicular to the longitudinal axis B-B′ of the transverse arm  60 . The threaded bore  66  also extends substantially perpendicular to the longitudinal axis of the through hole  64 .  
         [0067]     The end  70  of the transverse arm  60  which is remote from the connector  62  of the transverse arm  60  also provides a through hole  72 . The through hole  72  has a circular cross-section. Through hole  72  extends substantially perpendicular to the longitudinal axis B-B′ of the transverse arm  60 . The longitudinal axis of through hole  72  is substantially parallel to the longitudinal axis of through hole  64 .  
         [0068]     The upper surface  74  of the end  70  of the transverse arm  60  provides an aperture  76 . The aperture  76  is in communication with a threaded bore (not shown). The threaded bore (not shown) in the end  70  extends substantially perpendicular to both the longitudinal axis of the through hole  72  and the longitudinal axis B-B′ of the transverse arm  60 . The threaded bore (not shown) extends in a direction substantially parallel to the threaded bore  66  of the connector  62 .  
         [0069]     In use, the second support rod  2  and  22  is inserted into the through hole  64  of the connector  62  of the transverse arm  60 . The longitudinal axis A-A′ of the second support rod  2  and  22  extends substantially perpendicular to the longitudinal axis B-B′ of the transverse arm  60 . A threaded screw (not shown) is inserted into the threaded bore  66  of the connector  62 . The threaded screw cooperatively engages the threaded bore  66  and the second support rod  2  and  22  is secured within the through hole  64  of the connector  62 .  
         [0070]     A third rod  78  having a circular cross-section is inserted through the through hole  72  at the end  70  of the transverse arm  60 . The diameter of the through hole  72  is slightly greater than the diameter of the third rod  78 . A threaded screw (not shown) is inserted into the threaded bore  76 . The threaded screw (not shown) cooperatively engages the threaded bore  76  and the third rod  78  is secured within the through hole  72  of the transverse arm  60 . The third rod  78  extends substantially parallel to the first  44  and second  2  and  22  rods.  
         [0071]     With reference to  FIG. 7 , the kit includes six fixation devices  80 . Each fixation device  80  comprises a hook  82  and a housing  84 . The housing  84  comprises a channel (not shown) which is adapted to receive a support rod having a circular cross-section. The inner surface of the housing  84  also provides a threaded bore (not shown).  
         [0072]     In use, the second  2  and  22  and third  78  rods are inserted through the channel (not shown) of a fixation device  80 . The hook  82  extends below the plane of the lower surface  48  of the socket  4  and  24  and the rods  2 ,  22  and  78 . A threaded screw  86  is inserted into the threaded bore (not shown) of each fixation device  80  to engage the support rod  2 ,  22  or  78 . The assembled kit includes three fixation devices  80  which are engaged to the second support rod  2  and  22 . Three fixation devices  80  are engaged to the third rod  78 . The fixation devices  80  which are engaged to the second support rod  2  and  22  are spaced apart from the fixation devices  80  which are engaged to the third rod  78  by the transverse arm  60 . The spacing is sufficient so that three fixation devices  80  can be connected to each side of the spine in a claw formation.