Abstract:
A stylus assembly ( 10 ) adapted to align a surgical tool with a bone, the stylus assembly ( 10 ) comprising: a mounting element ( 12,18 ); a tool guide ( 50 ) adjustably mounted relative to the mounting element ( 12,18 ); a stylus ( 16 ) adjustably mounted relative to the mounting element ( 12,18 ) and having a curved portion for engagement with a predetermined part of the bone; and a lockable retaining member ( 14 ) for simultaneously locking the surgical tool guide ( 50 ) and the stylus ( 16 ) to the mounting element ( 12,18 ).

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
     This application is a 371 U.S. National Stage of International Application No. PCT/GB2010/051275, filed Aug. 3, 2010, and claims priority to British patent application No. 0914116.9 filed Aug. 12, 2009, the disclosures of which are herein incorporated by reference in their entirety. 
     The present invention relates to an instrument for sizing bone and particularly but not exclusively to a stylus assembly for use in positioning a surgical tool guide used in the resection of a proximal human tibia. 
     BACKGROUND 
     It is known to provide a surgical tool guide having a resection guide, which assists a surgeon in making accurate and repeatable cuts during surgery. The accuracy of a cut made using a resection guide is mainly determined by the successful positioning and fastening of the guide in the operative area. Typically, a stylus is attached to the guide, which references from a reference point or marker on the body, for example, a bone. The stylus is either free to translate and/or rotate relative to the guide, so that it may be adjusted, or is rigidly fixed to the guide. An attachment mechanism mounts the stylus to the guide, and a locking mechanism locks the stylus in a desired position relative to the attachment mechanism and hence the guide. Once in the operating position, the guide can be attached to, for example, a bone using pins and/or bone screws. 
     It is also known to use a stylus to position a guide in joint replacement surgery. For example, during a total or partial knee replacement operation, it is necessary to resect the proximal tibia to a specific level. Existing tibial styli reference from the tibial plateau. This area of the joint is subject to considerable wear and, in a knee requiring joint replacement is likely to be damaged. Therefore, the tibial plateau does not provide a reliable reference position. 
     Once a stylus has been used to position a guide, it is usually removed, in order to increase the working space available to the surgeon. A problem of existing arrangements is that the removal of the stylus can cause unintentional and unnoticed movement of the cutting guide. This leads to inaccurate positioning of the cutting guide, with the result that the surgeons&#39; incisions are misdirected. 
     GB 2426198 discloses a stylus assembly which has a stylus that references from the posterior femoral condyle which provides a relatively unworn reference surface. The stylus assembly is easily removed from a guide in order to prevent unwanted movement of the guide. 
     Whilst the stylus used in the assembly of GB 2426198 provides a reference point for performing a repeatable resection of the tibial plateau, it does not provide any feedback regarding the condition of the joint surfaces and that of the ligaments in the knee. The present invention seeks to address this issue. 
     STATEMENTS OF INVENTION 
     According to the present invention there is provided a stylus assembly adapted to align a surgical tool with a bone, the stylus assembly comprising:
         a mounting element;   a tool guide adjustably mounted relative to the mounting element;   a stylus adjustably mounted relative to the mounting element and having a curved portion for engagement with a predetermined part of the bone; and   a lockable retaining member for locking the surgical tool guide and the stylus to the mounting element.       

     The stylus may have a curvature which corresponds to the articular geometry of a surgical implant. 
     The stylus may further comprise a handle portion attached to the curved portion. 
     The curved portion of the stylus may extend past an end of the handle portion to form a protrusion or anterior extension of the stylus. The anterior extension of the stylus may detail the position of cartilage if it had not been eroded and provide a surgeon with an indication of the natural aspect of a healthy bone. 
     The protrusion may comprise an indicator which indicates the correct position for implanting a fixation peg of a surgical implant. 
     The mounting element may further comprises a body, the body having first and second arms with a space therebetween for receiving a portion of the surgical tool guide, wherein actuation of the lockable retaining member from an unlocked to a locked position moves the retaining member axially and causes the body to bear against the surgical tool guide and lock it relative to the body. 
     The lockable retaining member may extends from and be fixed to the first arm of the body. 
     The lockable retaining member may have an enlarged head at its free end, and the stylus may have a slotted portion. The slotted portion of the stylus may be mounted about the retaining member, enabling translational and rotational movement of the stylus relative to the body in the unlocked position of the lockable retaining member. 
     The mounting element may further comprise a bifurcated intermediate element having first and second arms through which the lockable retaining member passes. 
     A portion of the first arm of the intermediate element may be positioned between the body and the stylus and the lockable retaining member may pass through an aperture in the first arm of the intermediate element. 
     The second arm of the intermediate element may extend into the space between the first and second arms of the body. 
     The first arm of the body may extend into in a space between the first and second bifurcated arms of the intermediate element. 
     A first clearance may be provided between the second arm of the intermediate element and the said first arm of the body, and a second clearance may be provided between the retaining head of the retaining member and the stylus, the first clearance being greater than the second clearance. 
     The lockable retaining member may be moved between the locked and the unlocked positions by means of a cam which is pivotally mounted to the body. 
     A surface of the stylus which abuts the mounting element may be substantially parallel to a tool guide surface formed on the tool guide. 
     The curved portion may have a substantially spherical bone engaging surface. The curved portion may be adapted to engage a particular region of bone. For example, it may be shaped such that when correctly aligned the free end of the curved portion contacts the posterior femur. In another embodiment, the posterior aspect of the stylus substantially replicates the posterior aspect of the femur. 
     According to another aspect of the present invention there is provided a stylus assembly kit comprising a mounting element for aligning and fixing a surgical tool guide relative to a bone and a plurality of styli which are selectively connectable to the mounting element, wherein the plurality of styli are of different dimensions. For example, at least the curved portions of the styli may be of different thicknesses. 
     The kit may also comprise a plurality of mounting elements of different dimensions to adapt the stylus assembly for use with a particular patient. For example, the kit may comprise a plurality of mounting elements of different heights, so that by changing one mounting element for another of the mounting elements in the kit, the selected stylus is spaced a different distance from the tool guide. 
     According to another aspect of the present invention there is provided a method of aligning and fixing a surgical tool guide relative to a first bone which articulates with a second bone at a joint, the method comprising the steps of: 
     inserting the curved portion of a stylus, having a handle portion and a curved portion, between said first and second bones, such that a free end of the curved portion contacts a desired region of the first bone; 
     locking a tool guide to the handle portion of the stylus by means of a lockable mounting element, such that the tool guide is correctly aligned with the first bone; fixing the tool guide to the first bone; 
     disconnecting the mounting element and stylus from the tool guide: and 
     resecting the first bone by reference to the tool guide. 
     The step of inserting the curved portion of the stylus between said first and second bones may be repeated with styli having different thickness and/or offset between the handle portion and the curved portion and/or differently shaped or dimensioned portions or spacers which have the effect of changing the resultant laxity in the joint, the method further comprising the step of selecting the stylus which provides the desired laxity, before the step of locking the tool guide to the handle portion. 
     The step of locking a tool guide to the handle portion of the stylus by means of a lockable mounting element, may comprise the step of selecting an appropriate size of lockable mounting element to correspond to a desired thickness of bearing element to fit a particular patient. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       For a better understanding of the present invention, and to show more clearly how it may be carried into effect, reference will now be made, by way of example, to the accompanying drawings, in which:— 
         FIG. 1  is a side view of a stylus assembly in accordance with the invention, wherein the stylus assembly is in use and secured to a surgical guide; 
         FIG. 2  is a second perspective view of the stylus assembly of  FIG. 1 ; and 
         FIG. 3  shows the tool guide of  FIG. 2  following removal of the stylus assembly and resection of the bone. 
     
    
    
     DETAILED DESCRIPTION 
     Referring firstly to  FIG. 1 , a clamp assembly is indicated generally at  10 . The clamp assembly  10  can be made from stainless steel, metal alloy, any suitable material such as plastics or any combination of these and/or other materials. 
     The clamp assembly  10  comprises a substantially C-shaped body  12 , a retaining member  14  attached to the body  12 , a stylus  16 , mounted about and retained by the retaining member  14 , an intermediate element  18 , and a locking member  20 , for securing the clamp assembly  10  to a surgical tool guide  50 . 
     The stylus  16  has an elongate “handle” portion  17  and a curved “bowl” portion  19 . Both the top and bottom surfaces of the curved portion  19  are substantially spherical, but any suitable continuous or discontinuous curved surfaces are contemplated dependent on the joint to which the invention is applied. The curvature of the bowl portion  19  corresponds to the articular geometry of a bearing element of a prosthesis (not shown). The elongate portion  17  is flat and is used to mount the stylus  16  to the retaining member  14 . 
     The stylus  16  has a curved protrusion  64  which comprises the proximal end of the bowl portion  19 . The curved protrusion  64  acts as an indicator which identifies to the surgeon the correct position for the primary peg of the femoral implant and the flexion angle achieved. 
     The C-shaped body  12  has a first arm  22 , which is the upper arm as viewed in the drawings, a second lower arm  24  and a central portion  26  connecting the first and second arms  22 ,  24  together. The inner surfaces  28 ,  30  of the arms  22 ,  24 , i.e. the surfaces facing one another, define a space  23  therebetween for receiving part of the surgical tool guide  50 . 
     The retaining member  14  may be a pin or screw or other suitable fixing, which extends at right angles through the first arm  22  of the body  12  and has an enlarged head  34 . 
     The elongate portion  17  of the stylus  16  has a substantially planar lower surface  13  and a substantially planar upper surface  15  such that the elongate portion  17  has the form of a flat plate. The elongate portion  17  has a longitudinal slot  36  running therethrough, which has an enlarged portion  37  at one end sized to accommodate the head  34  of the retaining member  14 . The retaining member  14  is located within the slot  36  as a clearance fit, thus enabling translation and rotation of the stylus  16  about the pin  14 . The stylus  16  is retained on the retaining member  14  by the enlarged head  34  which is introduced through the enlarged portion  37  (as shown in  FIG. 2 ) of the longitudinal slot  36  and then translated to retain the stylus  16 . 
     The intermediate element  18  is bifurcated and has first and second arms  38 ,  40 , which are disposed one either side of the first arm  22  of the body  12 . An aperture is provided through the first arm  38  of the intermediate element  18 , through which the retaining member  14  passes. The stylus  16  is disposed above the first arm  38  of the intermediate element (as viewed in the drawings) between the intermediate element  18  and enlarged head  34 . The upper surface of the first arm  38  provides a planar reference surface over which the lower surface  13  of the stylus  16  is able to translate and rotate. 
     The second arm  40  of the intermediate element  18  extends into the space  23  between the first and second arms  22 ,  24  of the body  12 . In other words, the first arm  22  of the body  12  is disposed between the first and second arms  38 ,  40  of the intermediate element  18 . 
     The locking member  20  is a cam, which is pivotally mounted to the second arm  24  of the body  12 . A cam lever  44  is integrally formed with the cam, the cam protruding into the space  23  between the first and second arms  22 ,  24  of the body  12 , when actuated to a locked position. 
     The surgical tool guide  50  comprises a support rod  55  having a guide block  53  at one end and an attachment plate (not shown) at the opposite end. The attachment plate is slideably mounted on the support rod and may be fixed at a desired position to be clamped to a limb of a patient. In the illustrated embodiment the attachment plate is clamped to a shin of a patient, although in alternative embodiments, another limb or another part of the leg may be clamped. 
     The guide block  53  is fixed to the support rod  55  and comprises one or more drill guide holes, saw guide slots and/or fixing holes  60 . Chamfered edges  62  (as shown in  FIG. 1 ) are provided to facilitate the insertion of fixings such as pins or screws through the holes  60  in the guide block  53 . 
     As shown in  FIG. 2 , a slot or channel  46  is provided in the rear edge of the body  12 , which receives the cam lever  44  when the lever is in a locked position. An end portion  48  of the cam lever  44  extends out of the channel  46  so the cam leaver  44  can be rotated easily using a thumb and index finger away from the body  12  into an unlocked position. 
     The central portion  26  of the body  12  has an inner surface, between the arms  22 ,  24 , which acts as a stop, against which the guide block  53  of the surgical tool guide  50  can rest. 
     As shown in  FIGS. 1 and 3 , an upper surface of the guide block  53  comprises a planar saw guide surface  49  which is parallel to the elongate portion  17  of the stylus  16  and may also be parallel to the fixing holes  60 , although these fixing holes may be offset where this provides more secure attachment of bone screws or other fixing into the tibia. 
     The intermediate element  18  is able to move a short distance vertically, as indicated by arrow A in  FIG. 1 , relative to the body  12 . A first clearance d 1  is provided between the upper surface of the second arm  40  of the intermediate element and the lower surface of the first arm  22  of the body  12 . A second clearance d 2  is provided between the underside of the head  34  and the upper surface of the elongate portion  17  of the stylus  16 . The first clearance d 1  is greater than the second clearance d 2 , thereby causing movement of the intermediate element  18 , under the action of the cam lever  44 , towards the head  34  until the stylus  16  is trapped between the underside of the head  34  and the first arm  38  of the intermediate element  18 . 
     In use, the clamp assembly  10  is used to position the guide  50  relative to the tibia. In  FIG. 2 , the guide  50  is shown attached to a patient&#39;s tibia  52 , by means of, for example, pins or bone screws, for performing a resection of the proximal tibia (i.e. the tibial plateau) in a knee operation. It will be appreciated that the invention can be applied to other joints in a human or animal body, and that the guide  50  could be fixed to another limb or to another part of the leg by any suitable means. 
     In order to fix the guide  50  to the tibia  52 , it is necessary first to select an appropriate stylus to achieve as closely as possible the natural ligament tension. To achieve this, styli of different thickness are inserted in turn into the gap between the femur and tibia with the joint at approximately 90° of flexion. Insertion of the bowl portion  19  of the stylus  16  into this gap distends the joint, spacing the femur  58  and tibia  52  away from one another. If there is too much laxity in the joint, the stylus  16  is removed and replaced with a thicker stylus  16 . To facilitate this trialling process, the surgeon is provided with a set of styli  16  which have bowl portions  19  of different thicknesses. For example, styli may be provided having bowl portions of 1 mm, 2 mm and 3 mm thickness respectively. For each patient, the thickness of the bowl portion  19  is selected so that resection takes up the desired amount of the laxity in the ligaments of the knee. The thickness of the bowl portion  19  which is selected provides a measure of the articular cartilage erosion from the distal femur and tibial plateau. 
     It will be appreciated that placing a thicker stylus between the femur and the tibia causes the tibia  52  to be translated down relative to the guide block  53  and thus results in less of the tibia  52  being resected. This provides a tighter fit for the prosthesis and reduces the laxity in the joint. 
     Once the correct stylus has been selected, it is attached to the retaining member by inserting the head  34  of the retaining member  14  in the slot  36 . The stylus  16  is then adjusted about the retaining member  14  until the free end of the bowl portion  19  is located against the surface of the posterior femoral condyle  56 . This surface of the posterior femoral condyle provides a reference surface of relatively unworn bone. The degree of movement between the elements of the clamp assembly  10  in the unlocked condition facilitates the positioning of the stylus  16  between the tibia  52  and femur  58  through the operative wound. Alternatively, the stylus  16  may be inserted into the operative wound prior to mounting of the stylus  16  on the retaining member  14 . 
     The chamfered edges  62  of the guide block  53  allow the clamp assembly  10  to be rotated relative to the guide block  53 . Therefore the guide block  53  and stylus  16  may be aligned properly, whilst the clamp assembly  10  is angled to allow access to the holes  60 . Alternatively the body  12  may be dimensioned in such a manner to allow rotation of the clamp assembly  10  about the guide block  53  so that the chamfered edges  62  are not necessary. 
     Once the guide block  53  is positioned relative to the tibia, the cam lever  44  is rotated to the locked position, which causes the cam  20  to extend into the space  23 . This clamps the guide block  53  between the cam  20  and the second arm  40  of the intermediate element  18 . In so doing, the intermediate element is forced upwards, as viewed in the drawings, towards the head  34  of the retaining member  14 , and the first arm  38  of the intermediate element  18  clamps the stylus  16  against the enlarged head  34 . 
     The guide block  53  is then fixed to the tibia by means of bone screws which are screwed into the tibia through the fixing holes in the guide block  53 . 
     With the guide block correctly aligned and fixed to the tibia the clamp assembly  10  can be removed from the guide block  53  by rotating the cam lever  44  downwards to its unlocked position, which causes the cam  20  to retract into the channel  46  of the body  12  thereby releasing the clamp assembly from the guide block  53 . The clamp assembly  10  and guide block  53  are dimensioned so that the guide block  53  has only a small amount of play in the space  23  of the body  12  when in the unlocked condition. This is so that the range of axial movement of the cam  20  in the space  23  can be minimised, and so that the range of movement of the stylus  16  is also minimised. For example, the range of axial movement of the cam may be 0.25 to 2 mm, or more preferably about 1 mm. If the guide block  53  is too small to substantially fill the space  23 , the intermediate element  18  may not be displaced sufficiently to lock the stylus  16 . This problem can be addressed by increasing the effective thickness of the guide block  53 , by means of one or more shims (not shown). 
     With the clamp assembly removed the surgeon can access the guide block  53  with ease and make the necessary resection of the tibia by aligning a surgical saw with the planar saw guide surface  49 . 
       FIG. 3  shows the tibia  52  following resection, with the guide block  53  attached. The level of the resection can be adjusted by selecting a stylus  16  of a different thickness, as mentioned above and/or by the method described in more detail below. 
     The thickness of the intermediate element  18  corresponds to the thickness of the bearing which will be used in the prosthesis. Several clamp assemblies  10  may be provided which correspond to the different bearings and the appropriate clamp assembly  10  and bearing should be chosen for a particular patient. For example, a heavier patient may require a thicker bearing and thus a clamp assembly having a thicker intermediate element will be used. This causes the guide block  53  to be positioned lower on the tibia  52 , so that more of the bone is resected. 
     The stylus  16  is curved to replicate the articular geometry of a bearing element of a femoral implant. As a result of this curvature, even if the femur is not at exactly 90 degrees to the tibia, the stylus  16  will fit the femur accurately and will reference accurately off the posterior aspect of the distal femur. 
     It will be appreciated that, as the proximal end of the bowl portion  19  of the stylus  16  is curved, disruption of cartilage is minimized when the bowl portion  19  is inserted into the joint. By contrast, if a flat stylus is used in place of the above mentioned stylus with a bowl portion  19 , it would only contact the distal femur and tibial plateau at their highest points. A flat stylus therefore does not enable a surgeon to take into account erosion of the articular cartilage when assessing the laxity in the joint. 
     In the above described embodiment a plurality of styli are provided having different thickness. However the required laxity in the joint may be achieved by providing styli having differently shaped or dimensioned portions or spacers.