Abstract:
A method for locating the mechanical axis of a long bone involves generating an image of the long bone and determining the angle between the mechanical axis of the bone and a reference axis, which contains at least two reference points towards one end of the bone. A reference arm is positioned relative to the at least two predetermined reference points, and the mechanical axis is located relative to the reference arm with reference to the previously determined angle.

Description:
BACKGROUND TO THE INVENTION  
       [0001]     The present invention relates to a guide assembly, and in particular a guide assembly for locating the mechanical axis of a bone.  
         [0002]     During surgery it is often necessary to locate the mechanical axis of a bone. For example, during knee surgery it is necessary to find the mechanical axis of the femur so that the femur can be appropriately resected in order to properly fit a femoral prosthesis. It is known to determine the angle between the anatomical and mechanical axes using pre-operative x-ray or other images. During the procedure, the anatomical axis can be determined using an intramedullary rod, which is located in the bone&#39;s intramedullary canal. The mechanical axis can then be determined with reference to the pre-operative image data and the intramedullary rod. A disadvantage of this technique is the requirement to locate an instrument in the intramedullary canal.  
       SUMMARY OF THE INVENTION  
       [0003]     The present invention provides a technique for locating the mechanical axis of a bone with reference to predetermined reference points towards the end of the bone, and with reference to pre-operative image data in which the mechanical axis is located relative to those reference points.  
         [0004]     According to a first aspect of the invention, there is provided a guide assembly for locating the mechanical axis of a long bone comprising: a reference arm for location in direct or indirect contact with predetermined reference points on the long bone towards one end thereof, a locator for the reference arm for retaining it in place relative to the reference points on the long bone; an axis indicator which can rotate relative to the reference arm, and which can extend from the reference arm to indicate the orientation of the mechanical axis of the long bone relative to the predetermined reference points; and a scale for indicating the angle between the reference arm and the axis indicator.  
         [0005]     It is possible to determine, pre-operatively, the angle between the reference arm at its location when in direct or indirect contact with the predetermined reference points, and the mechanical axis of long the bone. For example, the angle can be determined using an image of the long bone. The guide assembly of the invention can then be used during surgery in order to determine the mechanical axis. The reference arm can be located in direct or indirect contact with the predetermined reference points and retained in place using the locator. The axis indicator can be rotated so that the angle between the axis indicator and the reference arm is the same as the angle between the reference arm and the mechanical axis of long the bone that was determined pre-operatively, at which point the axis indicator will indicate the orientation of the mechanical axis.  
         [0006]     Accordingly, the mechanical axis of a bone can be determined accurately during surgery using the guide assembly of the present invention without the use of an intramedullary rod.  
         [0007]     The reference points are points on the bone, which enable a reference axis to be located such that the angle between the reference axis and the mechanical axis can be determined. Preferably, the reference points lie on the reference axis. Preferably, the reference points are located so that they can be viewed when the bone is viewed in the anterior-posterior direction. Preferably, the reference points are the distal most points on the long bone. For instance, when the long bone is a femur, preferably the reference points are the distal most points on the condyles of the femur. Preferably, the axis indicator rotates about a point, which also lies in the reference axis.  
         [0008]     Preferably, the reference arm is configured to extend from the anterior side of the bone towards the posterior side to contact reference points on the distal face of the bone. Such a reference arm can be easier to locate in contact with the predetermined reference points in contrast with reference arms configured to contact reference points not on the distal face of the bone. Preferably, the axis indicator rotates relative to the reference arm in the coronal plane.  
         [0009]     Preferably, the reference arm has at least one contact formation for direct contact with the reference points. Preferably, the at least one contact formation is a planar face. When there at least two contact formations, preferably the at least two contact formations are co-planar faces.  
         [0010]     Providing a planar face, or a plurality of co-planar faces, is advantageous. This is because the predetermined reference points can be the distal most points of the long bone. Accordingly, the reference arm can easily be located in contact with the predetermined reference points by simply urging the planar face against the condyles. This technique is particularly advantageous because it can still be used when the condyles are damaged or corroded.  
         [0011]     Preferably, the axis indicator comprises an axis indicator arm that extends proximally relative to the long bone. Preferably, the axis indicator arm has at least one guide formation for guiding a bone tool into the long bone. This is advantageous because, once the mechanical axis has been found, the long bone can be operated on immediately without the need to remove the guide assembly, and/or to locate an additional guide component. The guide formation can be an opening in the axis indicator arm. The opening can be a bore for guiding a tool, which can cut a bore in the bone. The opening can be a slot that defines a plane on which bone tissue can be cut by a bone saw or burr tool. The opening can be an engagement formation for receiving a corresponding engagement formation on a tool. Accordingly, the tool can be fastened to the axis indicator arm via the engagement formations so that the tool is fixed relative to the axis indicator and hence also to the bone. The tool could be a guide tool, which has formations for guiding a bone tool, which can cut, or drill into a bone. Optionally the tool can be a bone tool, which can cut or drill into a bone.  
         [0012]     Preferably, the axis indicator arm is provided as a separate piece to the reference arm and can be detachably fastened to the reference arm. This is advantageous because it allows the use of types of different axis indicator arms with the reference arm.  
         [0013]     The axis indicator can be configured to rotate relative to the reference arm in predetermined discrete steps. Preferably, the axis indicator can rotate relative to the reference arm so that the axis indicator can be positioned at any angle relative to the reference arm.  
         [0014]     The scale can be provided by way of an indexing mechanism so that the angle between the axis indicator and the reference arm can be determined by counting the number of indexed steps during rotation of the axis indicator. Preferably, the scale is a visual scale. Preferably, the scale comprises a plurality of markings, which can be used to determine the angle between the axis indicator and the reference arm.  
         [0015]     Preferably, the guide assembly further comprises an angle indicator arm, which is fixed relative to the reference arm and can be used in conjunction with the scale to indicate the angle between the reference arm and the axis indicator. The scale can be provided on one of the axis indicator and the angle indicator arm and can indicate the angle between the axis indicator and the angle indicator arm. As the angle indicator arm is fixed relative to the reference arm, the angle between the axis indicator arm and the reference arm can be determined from the angle between the axis indicator and the angle indicator arm. Preferably, the scale is provided on the axis indicator arm.  
         [0016]     The scale can be provided on both of the axis indicator and the angle indicator arm. This can be advantageous because it can provide a more accurate indication of the angle between the axis indicator and the angle indicator arm. For instance, when the scale is provided on both the axis indicator and the angle indicator arm, the scale can be a Vernier scale.  
         [0017]     Preferably, the angle indicator arm is provided as a separate piece to the reference arm and can be detachably fastened to the reference arm. Preferably, the angle indicator arm and the axis indicator arm are provided as a single component, which can be detachably fastened to the reference arm.  
         [0018]     Preferably, the guide assembly further comprises a locking mechanism for locking the axis indicator relative to the reference arm. This is advantageous because, once the mechanical axis has been found by rotating the axis indicator relative to the reference arm, the axis indicator can be locked into position so that the axis indicator does not need to be held by the surgeon in order for it to continue to indicate the axis. This is also particularly advantageous when the axis indicator has guide formations because the locking mechanism prevents the axis indicator from moving due forces caused by a tool being guided by the guide formations acting against the axis indicator.  
         [0019]     Preferably, the locator comprises a locator arm connected to the reference arm. Preferably, the locator arm is configured to contact a second bone adjacent to the long bone, so that the guide assembly can be located between the long bone and the second bone, and so that the locator arm can cause the reference arm to be urged against the end of the long bone. Preferably, the locator further comprises an adjustment mechanism for controlling the force by which the reference arm is urged against the long bone.  
         [0020]     Preferably, the adjustment mechanism comprises a first handle connected to the reference arm and a second handle connected to the locator arm. Preferably, the first and second handles can be manipulated so as to increase the distance between the reference arm and the locator arm.  
         [0021]     Preferably, the adjustment mechanism comprises a lock mechanism, which enables the distance between the reference arm and the locator arm to be maintained against forces acting against the reference arm and the locator arm.  
         [0022]     The guide assembly will generally be made from metallic based materials, which are conventionally used in the manufacture of surgical instruments. Certain stainless steels can be particularly preferred. However, it will be understood that at least one part of the assembly, for instance the reference arm, can be made from polymeric materials. Using polymeric materials can reduce the cost of manufacture of the assembly, especially because an assembly made from polymeric materials can easily be manufactured using a moulding process. Suitable polymeric materials include certain polycarbonates, polyester, polyamides, poly-ether-ether ketones (PEEKs), and polyaryl-ether ketones (PAEKs). Polymeric materials can be reinforced with particulate material, especially fibrous materials, to provide appropriate wear and reinforcement characteristics.  
         [0023]     According to a second aspect of the invention there is provided a method for locating the mechanical axis of a long bone comprising: 
        (a) generating an image of the long bone;     (b) determining the angle between the mechanical axis of the bone and a reference axis, which contains at least two reference points towards one end of the bone;     (c) positioning a reference arm relative to the at least two predetermined reference points; and     (d) locating the mechanical axis relative to the reference arm with reference to the angle that is determined in step (b).        
 
         [0028]     The image can be generated using any suitable method. For instance, the step of generating the image can comprise taking an X-ray of the long bone. Optionally, the image can be generated using any of a CT scan, ultra sound, or a MRI scan.  
         [0029]     Preferably, the reference arm has an axis indicator fastened to it so that it can pivot relative to the reference arm. Preferably, step (d) comprises pivoting an axis indicator relative to the reference arm until the angle between the axis indicator and the reference arm is the same as the angle calculated in step (b). 
     
    
     INTRODUCTION TO THE DRAWINGS  
       [0030]      FIG. 1  is a side elevation view of a femur;  
         [0031]      FIG. 2  shows a perspective view of a part assembled guide assembly according to the present invention, which can be used to indicate the mechanical axis of the femur shown in  FIG. 1 ;  
         [0032]      FIG. 3  shows a perspective view of the guide assembly shown in  FIG. 2  fully assembled; and  
         [0033]      FIG. 4  shows a perspective view of the first and second handles of the guide assembly. 
     
    
     DETAILED DESCRIPTION OF THE INVENTION  
       [0034]     In the described embodiment the guide assembly of the invention is used to determine the mechanical axis of a femur. Nevertheless, it will be understood that the guide assembly can be used to determine the mechanical axis of other types of bone, such as the humerus.  
         [0035]     Referring to the drawings,  FIG. 1  shows a femur  2  having a mechanical axis X. The mechanical axis X of the femur  2  is the line, which extends through the centre point  4  of the head of the femur and the centre point  6  of the intercondylar notch.  
         [0036]     The angle A between the mechanical axis X and a reference axis  12  containing first  8  and second  10  reference points can be determined pre-operatively. In the described embodiment, the first  8  and second  10  reference points are the distal most points of the femurs first  14  and second  16  condyles. Nevertheless, as will be understood, this need not necessarily be the case and the reference points can be any other predetermined reference points on the femur  2 . The angle A can be determined by obtaining an image of the femur  2  and then measuring on the image the angle between the mechanical axis X and the reference axis  12  containing the first  8  and second  10  reference points. The image of the femur  2  can be obtained by taking an X-ray of the femur  2 .  
         [0037]     Referring to  FIGS. 2 and 3 , there is shown a guide assembly  20  according to the present invention located at the distal end of the femur  2 . The guide assembly  20  is located between the femur  2  and the tibia, which is not shown for the sake of simplicity. The guide assembly  20  comprises a first plate  22  for abutment with the femur&#39;s condyles  14 ,  16 , a second plate  24  for abutment with the proximal end of the tibia (not shown), an axis indicator arm  26 , an angle indicator arm  28 , and first  30  and second  32  handles connected to first  20  and second  22  plates respectively.  
         [0038]     The first plate  20  has first  34  and second  36  planar faces separated by a bridge  38 . The bridge provides strength to the first plate  20  as well as provides a housing for the opening  54  (discusses in more detail below). The first  34  and second  36  planar faces and the bridge  38  are shaped and sized so that the first  34  and second  36  planar faces can be brought into contact with the first  14  and second  16  condyles and so that the bridge  38  can sit in the intercondylar notch. The first  34  and second  36  planar faces are co-planar. The second plate  22  has a first face  40  for contact with the tibia.  
         [0039]     The face of the first  20  plates which faces the face of the second  22  plates is configured so that the first  20  and second  22  plates can be brought together and fitted flush against each other.  
         [0040]     The first handle  30  has first  40  and second  42  straight portions connected by a curved portion  44  toward the middle of the handle, such that the angle between straight lines extending along the length of the first  40  and second  42  straight portions is approximately 120E. The second handle  32  is substantially straight along its entire length.  
         [0041]     The curved portion  44  of the first handle  30  is pivotally connected to the second handle  32  so that the first handle  30  can pivot relative to the second handle  32  about an axis Y which extends perpendicularly to the lengths of the first  30  and second  32  handles.  
         [0042]     The first handle  30  is connected to the bridge  38  of the first plate  20  so that the first plate  20  can rotate relative to the first handle  30  about an axis Z that extends along the length of the bridge  38  and is contained within the planes of the first  34  and second  38  planar faces.  
         [0043]     The second handle  32  is connected to second plate  22  so that they cannot move relative to each other. In the embodiment described, the second plate  22  and the second handle  32  are provided as a single moulded piece.  
         [0044]     As shown in  FIG. 4 , a locking arm  70  is attached to the end of the second handle  32  that is distal to the second plate  22  by a pivot  76 . The locking arm  70  has a plurality of teeth  74  arranged on a first face. The locking arm  70  can rotate relative to the second handle  32  about the pivot, so that the first face of the locking arm  70  can be brought into contact with the end of the first handle  30  that is distal to the first plate  20 . Furthermore, the locking arm  70  is biased towards contact with the first handle  30 . The end of the first handle  30  distal to the first plate  20  has a hook  72  which can engage the teeth  74 . The teeth  74  and hook  72  are configured so that the hook  72  can easily slide over the teeth  74  when moved in a direction toward the second handle  32 , but so that the hook  72  is prevented from sliding over the teeth in the opposite direction. A leaf spring  78  is attached to the first handle  30  towards its end distal to the first plate  20 . The leaf spring  78  is biased the against the second handle  32  so that the ends of the first  30  and second  32  handles that are distal to the first  20  and second  22  plates are biased away from each other. Accordingly, the locking arm  70 , hook  72  and leaf spring  78  arrangement provide a ratchet mechanism which enables distance between the first  20  and second  22  plates to easily be selected and locked as described in more detail below.  
         [0045]     The angle indicator arm  28  is provided as a separate piece to the first plate  20 . The angle indicator arm  28  has first  46  and second  48  pegs extending perpendicularly to its length at its first end. The second end of the angle indicator arm  28  narrows to a tip  50 . The first peg  46  can be received in a first  52  bores in the first plate  20 . The second peg  48  can extend through a first opening  54  in the end of the first straight portion  40  of the first handle  30 , and be received in a second bore (not shown) in the first plate  20 . When the first  46  and second  48  pegs are received in the first  52  and second bores of the first plate, the angle indicator arm  28  extends perpendicularly to the planes of the first  34  and second  36  planar faces.  
         [0046]     A first end of the axis indicator arm  26  is connected to the first end of the angle indicator arm  28  so that when the guide assembly  2  is assembled, the axis indicator arm  26  can rotate relative to the angle indicator arm  28  and to the first plate about the axis Z. A scale  58  is provided on a head  56  at a second end of the axis indicator arm  26 . The lengths of the axis indicator arm  26  and the angle indicator arm  28  are such that the tip  50  of the angle indicator arm is in the region of the scale  58  on the head  56 . The axis indicator arm  26  also has first  60  and second  62  limbs extending substantially perpendicularly to the length of the axis indicator arm  26 . The first limb  60  has a first opening  64  formed in it and the second limb  62  has a second opening  66  formed in it. The first  64  and second  66  openings define an axis along which a bone drill bit can be extended. The axes run substantially parallel to the axis Z. A straight line connecting the centre points of the first  64  and second  66  openings runs substantially perpendicular to the length of the axis indicator arm  26 .  
         [0047]     A wheel  68  is provided on the angle indicator arm  28 . The wheel  68  has a threaded spigot (not shown), which extends through and engages a threaded hole (not shown) in the angle indicator arm  28 . Rotating the wheel  68  clockwise causes the spigot to travel through the angle indicator arm  28  towards the axis indicator arm  26 . The length of the spigot is such that the wheel  68  can be rotated so that the end of the spigot can be driven into and engages the axis indicator arm  26 . Once engaged, the spigot prevents the axis indicator arm  26  rotating relative to the angle indicator arm  28 . Rotating the wheel  68  anti-clockwise causes the spigot to travel out of the angle indicator arm  28  away from the axis indicator arm  26 , thereby disengaging the spigot from the axis indicator arm  28 .  
         [0048]     In use, the angle between the reference axis  12  containing the first  8  and second  12  reference points and the mechanical axis X is calculated from an X-ray image of the femur  2 . The first  20  and second  22  plates are then located between the distal end of the femur  2  and the proximal end of the tibia (not shown) so that the first  34  and second  36  planar faces of the first  20  plate face toward the first  14  and second  16  condyles.  
         [0049]     The ends of the first  30  and second  32  handles that are distal to the first  20  and second  22  plates are then squeezed together. This causes the handles  30 ,  32  to rotate relative to each other about axis Y, so that the first  20  and second plates  22  are separated from each other. This causes the first  34  and second  36  planar faces of the first plate  20  to be urged against the first  14  and second  16  condyles of the femur  2 , and the first face  40  of the second plate  22  to be urged against the tibia. This helps to prevent the first plate  20  from moving relative to the femur  2 . As the first  34  and second  36  planar faces are co-planar, urging the first plate  20  against the first  14  and second  16  condyles will help ensure that the first  34  and second  36  planar faces contact the first  8  and second  10  reference points (which are the most distal points of the femur&#39;s condyles). Accordingly, the plane containing the first  34  and second  36  planar faces will also contain the first  8  and second  10  reference points.  
         [0050]     The hook  72  and teeth  74  arrangement will ensure that the ends of the handles  30 ,  32  distal to the first  20  and second  22  plates do not separate under the force of the femur  2  and tibia on the first  20  and second  22  plates. If it is necessary to reduce the distance between the first  20  and second  22  plates, such as when the surgical procedure is complete, then the locking arm  70  can be rotated away from the first handle  30  so as to release the hook  72  from the teeth  74 . The end of the first handle  30  that is distal to the first plate  20  will then be forced away from the second handle  32  due to the leaf spring  78 , thereby causing the distance between the first  20  and second  22  plates to decrease.  
         [0051]     The axis indicator arm  26  and the angle indicator arm  28  are then connected to the first plate  20  by way of receiving the first  46  and second  48  pegs within the first  52  and second bores in the first plate  20 . When initially connected to the first plate  20 , the axis indicator arm  26  and the angle indicator arm  28  extend perpendicularly to the plane of the first  34  and second  36  planar faces.  
         [0052]     The axis indicator arm  26  is then rotated relative to the angle indicator arm  28  until the tip  50  of the angle indicator arm  28  points towards the angle which is equal to 90° minus the angle between the reference axis  12  containing the first  8  and second  12  reference points and the mechanical axis X. Once this position has been reached, the length of the axis indicator arm  26  extends along the length of the mechanical axis X. Accordingly, the mechanical axis X has been found and the first  64  and second  66  openings can be used to guide a drill bit into the femur  2  to prepare bores in femur  2  for receiving corresponding pins of a cutting guide or a cutting tool (not shown). Optionally, the first  64  and second  66  openings can be used to mount a cutting guide directly on the axis indicator arm  26 . The cutting guide can then be used to guide a cutting tool into the femur  2 .