Abstract:
A measuring instrument ( 2 ) for use during an orthopaedic surgical procedure to measure first and second distances from a reference point on a bone to first and second measurement points respectively. The measuring instrument has a first scale ( 24 ) for displaying the first distance and a second scale ( 28 ) for displaying the second distance. It includes a mask ( 26 ) which can be positioned against the second scale after the first distance has been measured in a position that is selected relative to the second scale dependent on the measured first distance, to restrict the length of the second scale that is visible to the user.

Description:
CROSS REFERENCE TO RELATED APPLICATION 
       [0001]    This application is a continuation of U.S. patent application Ser. No. 14/407,208, filed Dec. 11, 2014, which is a National Stage 35 U.S.C. 371 of International Patent Application PCT/GB2013/051367 filed May 24, 2013, which claims priority to United Kingdom Application No. 1211886.5, filed Jul. 4, 2012 (now abandoned) and which claims priority to United Kingdom Application No. 1301741.3 filed Jan. 31, 2013, all of which are incorporated by reference in their entireties. 
     
    
     BACKGROUND OF THE INVENTION 
       [0002]    This invention relates to a measuring instrument for use in orthopaedic surgery. 
         [0003]    It is important for the success of a surgical procedure to replace an orthopaedic joint to determine as accurately as possible the configuration of the patient&#39;s natural joint. This information facilitates the correct selection of the components of a joint prosthesis for use during the procedure, and the correct positioning of the prosthesis components during the procedure. Correct selection and positioning of the prosthesis components can help to ensure that the joint provides the desired biomechanical performance after implantation. 
         [0004]    Accurate determination of the configuration of a patient&#39;s joint is particularly important in relation to the implantation of a knee joint prosthesis. For example, it can be important to determine (a) the size of the femur measured along the anterior-posterior axis, (b) the proximal extent of the femoral sulcus, and (c) the distance between the femur and the proximal tibia, and the rotational orientation of the tibia relative to the femur, when the joint soft tissue is appropriately tensioned. These factors can determine the selection of components which are to be implanted in the knee replacement procedure, including the size of the femoral component and the size of the spacer which is to be provided between femoral component and the tibial component. The factors can be inter-related in that, for example, a change in the size of the femoral component that is selected will affect the distance between that component and the proximal face of the tibial component. This will have an effect on the tension in the joint soft tissue for a particular size of bearing or spacer component. 
         [0005]    It is known to select the size of the femoral component of a knee joint prosthesis by measuring the distance from the posterior condylar surface to the anterior condylar surface. This measurement can be performed before the start of the joint replacement procedure using images of the femur. However, it is generally desirable to measure this distance during the procedure. This can be done through the incision which is made to access the joint by placing the knee in flexion. 
         [0006]    The size of the bearing spacer component which is used between the femoral component and the tibial component can also be selected before the start of the joint replacement procedure using images of the joint. Again, it is generally desirable to obtain additional distance data during the procedure. Measurements can be made with the knee in flexion. Additional measurements can be made with the knee in extension. The measurements of the distance between the femur and the tibia requires that the soft tissue extending between the femur and the tibia is placed in tension. 
         [0007]    A preferred approach to measuring the femur in a knee replacement procedure makes use of a sizing guide which has a pair of feet which contact the posterior condyles, and a stylus which contacts the anterior cortex, extending beyond the femoral sulcus. The position of the stylus can be adjusted to adjust the point along the length of the anterior cortex at which the anterior cortex is contacted by the stylus. Such a sizing guide is disclosed in WO-A-2011/141722. 
         [0008]    US-A-2007/162036 discloses a device which can be used to determine the size of a femoral component and the size of a bearing component for use in knee replacement procedure. The device is located on a rod which is inserted into the intramedullary canal of the patient&#39;s femur. The size of the femoral component is measured in a first step by placing a pair of feet in contact with the femoral condyles while the knee is in flexion, and placing a stylus anteriorly in contact with the anterior cortex. In a subsequent step, the plate which carries the pair of feet is moved away from the posterior condyles towards the tibia to place the ligaments spanning the joint in tension. This can then be used to measure the space between the femur and the tibia. This information can be used to ensure that an appropriately sized bearing component is used in the implanted prosthesis, between the femoral and tibial components. 
       BRIEF SUMMARY OF THE INVENTION 
       [0009]    The present invention provides a measuring instrument for use during an orthopaedic surgical procedure to measure first and second distances from a reference point on a bone to first and second measurement points respectively, the measuring instrument having a first scale for displaying the first distance and a second scale for displaying the second distance, the instrument including a mask which can be positioned against the second scale after the first distance has been measured in a position that is selected relative to the second scale dependent on the measured first distance, to restrict the length of the second scale that is visible to the user. 
         [0010]    The invention also provides a measuring instrument for use during an orthopaedic surgical procedure to measure first and second distances from a reference point on a bone to first and second measurement points respectively, which comprises:
       a. a reference structure which can be located relative to the reference point on the bone,   b. a first measurement structure for measuring the first distance between the reference point and the first measurement point, the first measurement structure and the reference structure being capable of movement relative to one another, and the instrument including a first scale provided by a first scale portion on the reference structure and a second scale portion on the first measurement structure, the first scale indicating the position of the first measurement structure relative to the reference structure and so indicating the first distance,   c. a second measurement structure for measuring the second distance between the reference point and a second measurement point, the second measurement structure and the reference structure being capable of movement relative to one another, the instrument including a second scale which is provided by a third scale portion on the second measurement structure and the first scale portion, the second scale indicating the position of the second measurement structure relative to the reference structure and so indicating the second distance,   d. a mask which can be located against the first scale portion in a position that is selected according to the size of the first measurement, the mask restricting the length of at least one of the first and third scale portions that is visible.       
 
         [0015]    The instrument of the invention has the advantage that the interpretation of measurement data when measuring the distance from the reference point to the second measurement point is made simpler because of the restriction on the length of the second scale that is visible to the user. This can help to reduce the incidence of errors when using the instrument. 
         [0016]    The instrument can include a reference structure which can be located relative to reference point, and at least one measurement structure which can be moved relative to the reference structure to a position which correlates to one of the first and second measurement positions. When the instrument is for use in a knee replacement procedure to measure a patient&#39;s femur, the reference point might be the femoral anterior cortex or the posterior femoral condyles. 
         [0017]    The reference structure can include a formation which can be positioned against the reference point to enable the reference structure to be located relative to the reference point. By way of example, when the reference point is the femoral anterior cortex, the reference structure might be a stylus which contacts the anterior cortex, extending beyond the femoral sulcus. The position of the stylus can be adjusted to adjust the point along the length of the anterior cortex at which the anterior cortex is contacted by the stylus. By way of another example, when the reference point is a posterior femoral condyle, the reference surface might be a plate or other formation which can be positioned against the posterior condyle. 
         [0018]    The measurement structure can include a plate member having a first face which is directed towards the reference structure and a second face which is directed away from the reference structure, the first face forming part of the measurement structure. The first face can be used to position the measurement structure relative to the first measurement point and the second face can be used to position the measurement structure relative to the second measurement point. The instrument can include a driver for moving the plate member between the first and second measurement points. A plate member which is used in the instrument of the invention can be a solid plate without openings extending through it. This will be preferred for many applications. However, the plate might have one or more openings or hollows formed in it so that, in some constructions, it can be seen as for example a frame. 
         [0019]    The instrument can include (a) a first measurement structure for measuring the first distance between the reference point and the first measurement point, the first measurement structure and the reference structure being capable of movement relative to one another, and (b) a second measurement structure for measuring the second distance between the reference point and a second measurement point, the second measurement structure and the reference structure being capable of movement relative to one another. 
         [0020]    The first and second measurement structures can have components in common. Each of the measurement structures can include components such as a housing for mounting the measurement structure, a driver which can be operated to adjust the measurement structure, one or more scale portions. The first and second measurement structures can have any of these components (and other components) in common. 
         [0021]    The measurement instrument can include a plate member which has a first face which is directed towards the reference structure and a second face which is directed away from the reference structure, the first face forming part of one of the first and second measurement structures and the second face forming part of the other of the first and second measurement structures. The plate member can be fixed as part of the measurement instrument (for example as a result of being formed with one or more other parts of the instrument such as by a moulding or casting technique, or by a joining technique such as welding or brazing or soldering, or by use of one or more fasteners such as screws or rivets), or it might be capable of be detached. This can facilitate use of the instrument. 
         [0022]    The measurement instrument can include (a) a first scale provided by a first scale portion on the reference structure and a second scale portion on the measurement structure, the first scale indicating the position of the measurement structure relative to the reference structure and so indicating the first distance, and (b) a second scale which is provided by a third scale portion on the measurement structure and the first scale portion, the second scale indicating the position of the measurement structure relative to the reference structure and so indicating the second distance. The mask can be used to restrict the length of at least one of the first and third scale portions that is visible to the user. It will be apparent that the first and second scales share the first scale portion on the reference structure in this construction. 
         [0023]    The measurement structure can include a component for fixing the measurement structure against movement relative to the bone in at least one degree of freedom. In particular, it can be preferred that the measurement structure is prevented from translating in the plane which includes the femoral anterior-posterior axis and the femoral medial-lateral axis. It can be preferred that the measurement structure can rotate at least through a limited angle about the femoral superior-inferior axis. This can enable the instrument to be aligned with both medial and lateral posterior condyles. 
         [0024]    This can allow a measurement structure to be used to measure the first distance, and to be adjusted to measure the second distance. The measurement structure can include at least one part which is used to measure the first distance and not the second distance, and at least one part which is used to measure the second distance and not the first distance. For example, the measurement structure can include a plate member having a first face which is directed towards the reference structure and which is used to measure the first distance and a second face which is directed away from the reference structure and which is used to measure the second distance. The measurement structure can include the second scale portion which is used to measure the first distance and the third scale portion which is used to measure the second distance. 
         [0025]    The measuring instrument can include an intramedullary rod which can be fitted in the intramedullary canal of the patient&#39;s bone. The intramedullary rod can be fixed to the measurement structure. The measurement structure can include a component by which it can be fixed to the intramedullary rod. Preferably, the rod is fitted in the intramedullary canal so that it will not move within the canal when subjected during use of the instrument to forces which might otherwise cause it to move in translation and/or rotation. 
         [0026]    The measurement structure can include (a) a formation for locating the measurement structure relative to the first measurement point or the second measurement point, and (b) a driver for moving the formation relative to the connector component. The formation can be provided by a plate member. A plate member which is used in the instrument of the invention can be a solid plate without openings extending through it. This will be preferred for many applications. However, the plate might have one or more openings or hollows formed in it so that, in some constructions, it can be seen as for example a frame. 
         [0027]    The formation for locating the measurement structure relative to an intramedullary rod can comprise a socket in which the end of the rod can be received. Preferably, the rod and the measurement structure can be connected to one another such that relative movement between them is prevented, at least in some degrees of freedom, for example by means of a clamp. The intramedullary rod might have a plate attached to it at its end, and the formation on the measurement structure can then enable the measurement structure to be fastened on to the plate. When the rod has a plate attached to it at its end, it will frequently be unnecessary for the measurement structure to include a socket in which the end of the rod can be received. 
         [0028]    It can be preferred that the reference structure can be positioned on the measurement structure relative to the reference point while the measurement structure is connected against movement relative to the bone. This can be appropriate when the instrument includes a component by which the measurement structure can be fixed against movement relative to the bone in at least one degree of freedom. Accordingly, use of the instrument can involve fixing the measurement structure to the patient&#39;s bone in a position to measure one of the first and second distances, and then positioning the reference structure on the measurement structure so that it is appropriately located relative to the reference point on the bone. For example, when the reference point is on the anterior cortex on a patient&#39;s femur, the reference structure can be positioned on the measurement structure such that a stylus on the reference structure contact the anterior cortex. Preferably, the instrument includes a lock mechanism by which the reference structure can be locked against movement relative to the measurement structure. 
         [0029]    The measurement structure can include a housing having tower portion. The reference structure can have an opening formed in it in which the tower portion of the measurement structure is received. A lock mechanism can comprise a collar on the reference structure which can be used to compress a collet against the surface of the measurement structure housing. 
         [0030]    A driver can be used to adjust the measurement structure so that the position of a formation by which the measurement structure is located relative to a measurement point is adjusted relative to a connector for the measurement structure. A convenient driver can be based on a threaded connection between a first threaded member which is connected to the connector for the measurement structure, and a second threaded member which can mate with the first threaded member, the second threaded member being connected to the formation by which the measurement structure is located relative to a measurement point. For example, the driver can be based on a threaded connection between a threaded shaft, and a threaded bore in a rotatable knob. 
         [0031]    The measurement structure can comprise an elongate housing. The housing can be hollow along at least part of its length. A connector for fixing the structure relative to a bone can be provided within the housing, for example in the form of a socket in which the end of an intramedullary rod can be received, or in the form of features by which the housing can be attached to a plate or other structural component on the end of a intramedullary rod. The structure can include a driver knob which is mounted on the housing along the axis of the housing so that it can rotate. The axis of rotation is parallel to or coincident with the axis of the housing. The knob has a threaded bore within it. The bore can be open at the end of the knob. This can facilitate cleaning and can mean that the length of the bore does not restrict the range of movement of the shaft within the bore. The connector can have a shaft extending from it which is threaded at one end so that the shaft can be received in the bore in the knob. When the connector is fixed to the bone, rotating the knob relative to the housing causes the shaft to translate relative to the knob. This results in translation of the measurement structure housing relative to the connector. 
         [0032]    Preferably, at least one of the first and second scales comprises a scale portion on one component which has a single indicium and a scale portion on another component which has a plurality of spaced apart indicia. The components (the reference structure and the measurement structure) are capable of movement relative to one another. The indicium on the other component that is adjacent to the single indicium on the one component indicates the measured distance. Preferably, the single indicium that is used for the second scale in the second measurement is one of the plurality of spaced apart indicia of the first scale. For example, the first scale can be provided by (a) a first scale portion on the reference structure and (b) a second scale portion on the measurement structure that is positioned relative to the first measurement point, the first scale indicating the position of the measurement structure relative to the reference structure and so indicating the first distance between the reference point and the first measurement point. The first scale portion on the reference structure can present a plurality of spaced apart indicia and the second scale portion on the measurement structure can provide a single indicium. The first distance is indicated by the indicium on the first scale portion that is adjacent to the single indicium on the second scale portion. 
         [0033]    The second scale can be provided by (a) a third scale portion on the measurement structure that is positioned relative to the second measurement point and (b) the first scale portion, the second scale indicating the position of the measurement structure relative to the reference structure and so indicating the second distance between the reference point and the second measurement point. The third scale portion on the measurement structure can present a plurality of spaced apart indicia and the first scale portion on the reference structure can provide a single indicium, that being the indicium on the first scale portion that was indicated in the first measurement step. 
         [0034]    The mask can be positioned against the second scale in a position that is selected relative to the second scale dependent on the measured first distance, to restrict the length of the second scale (the first scale portion or the third scale portion or each of them) that is visible to the user. This helps the user to interpret the measurement data that is provided by the instrument. This can be particularly helpful when a common component (especially the housing of a measurement structure) provides a scale portion which is used to perform the first measurement and a different scale portion which is used to perform the second measurement. The mask can then be used to restrict the visibility of a scale portion (or a portion thereof) which is not used to perform the second measurement. 
         [0035]    It can also be helpful when one of several indicia on a scale which is used to perform the first measurement is used as a selected indicium on a scale which is used to perform the second measurement. The mask can then be used to identify the selected indicium, restricting the visibility of other indicia on the scale which is used in the first measurement and which are not selected for use in the second measurement. 
         [0036]    The mask can be fastened against the second scale in a plurality of different positions, each position corresponding to a different size of the first measured distance. For example, the reference structure and the mask can fit together slidingly by means of cooperating rib and groove features. For example, one of the reference structure and the mask can have one or ribs formed on it, and the other can have one or more grooves formed on it. The position of the mask on the reference structure is determined by the choice of rib and groove. 
         [0037]    The mask can be fastened against the second scale so that it can slide relative to the second scale to allow it to be placed in a selected position corresponding to the size of the first measured distance. 
         [0038]    The mask can be used to provide an indication to the user of the location of posterior references on the femur. For example, it can be used to provide an indication to the user of the location of the posterior cuts (through the posterior condyles) for the chosen size of femoral component of a knee joint prosthesis. Accordingly, the invention provides a kit for use in a surgical procedure which comprises: 
         [0000]    (a) a measuring instrument according to the invention, in which the mask includes at least one posterior cut reference surface, and
 
(b) a femoral component of a knee joint prosthesis which includes an anteriorly facing bone contacting surface on a posterior condyle, in which the said bone contacting surface defines a posterior cut plane,
 
in which the posterior cut reference surface when the mask is positioned on the second scale of the instrument appropriately having regard to the measured first distance, lies in the posterior cut plane which is defined by the femoral component when the femoral component is implanted.
 
         [0039]    The invention also provides a kit for use in a surgical procedure to implant a knee joint prosthesis, which comprises: 
         [0000]    (a) a measuring instrument according to the invention, in which the mask includes at least one posterior cut reference surface, and
 
(b) a femoral component trial which includes an anteriorly facing bone contacting surface on a posterior condyle, in which the said bone contacting surface defines a posterior cut plane,
 
in which the posterior cut reference surface when the mask is positioned on the second scale of the instrument appropriately having regard to the measured first distance, lies in the posterior cut plane which is defined by the femoral component trial when the femoral component trial is positioned on the resected femur.
 
         [0040]    Components of the instrument of the invention can be made from materials which are conventionally used in the manufacture of surgical instruments. Example of such materials include metals (for example stainless steels) and polymers (for example polyolefins such as polyethylenes and polypropylenes, polycarbonates, acetals and so on). It can be preferred that the measurement structure is made from a metal. It will frequently be preferred that the reference structure is made from a metal. The mask can be made from a polymer. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0041]    Embodiments of the invention are described below by way of example with reference to the accompanying drawings, in which: 
           [0042]      FIG. 1  is a lateral view of a measuring instrument (without its mask), shown schematically in relation to a femur and a tibia as they might be positioned in flexion during a surgical procedure to replace a knee joint. 
           [0043]      FIG. 2  is an anterior view of the measuring instrument shown in  FIG. 1  (with its mask). 
           [0044]      FIG. 3  is an exploded view showing separated components of the instrument shown in  FIGS. 1 and 2 . 
           [0045]      FIG. 4  is a front view of the measuring instrument shown in  FIG. 1  (without its mask). 
           [0046]      FIG. 5  is a view from behind of the mask which can be fitted to the measuring instrument shown in  FIG. 4 . 
           [0047]      FIG. 6  is an isometric view of the instrument when the plate member in contact with the femoral posterior condyles. 
           [0048]      FIG. 7  is an enlarged view of the first scale on the instrument as shown in  FIG. 6 . 
           [0049]      FIG. 8  is an isometric view of the instrument when the plate member in contact with the tibia. 
           [0050]      FIG. 9  is an enlarged view of the first scale on the instrument as shown in  FIG. 7 . 
       
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
       [0051]    The drawings show a measuring instrument which can be used in a surgical procedure to implant a knee joint prosthesis. Such a prosthesis comprises a femoral component which is fitted to the femur and a tibial component which is fitted to the tibia. The femoral component has a convex bearing surface which provides medial and lateral bearing surfaces corresponding to those provided by the anterior, distal and posterior surfaces of the medial and lateral condyles of the natural knee. The tibial component generally presents a flat proximal surface. A bearing component is provided between the femoral and tibial components. The bearing component has medial and lateral concave recesses on its proximal surface in which the medial and lateral condylar bearing surfaces of the femoral component can be received and can articulate. The distal surface of the bearing component is planar so that the bearing component can slide on the tibial component, in rotation or in translation or in both rotation and translation. Knee prostheses of these general types are well known. 
         [0052]    It is common in knee replacement surgery to resect the tibia before cutting the femur. The distal cut of the femur is frequently performed after the tibial resection. The instrument that is provided by the invention can then be used to ensure that the size of the femoral component that is used is appropriate having regard to the size of the femur. It can also be used to ensure that the bearing component that is used as the appropriate thickness having regard to (a) the size of the femoral component and (b) the soft tissue which extends between the femur and the tibia. 
         [0053]    Referring to the drawings,  FIGS. 1 and 2  show a measuring instrument  2  that is in use in a surgical procedure to implant a knee joint prosthesis, illustrating the way in which the instrument is used in such a procedure. The drawings show the tibia  4  which has been resected proximally to define a flat surface  5  on to which the tibial component (not shown) of the prosthesis can be implanted, and the femur  6  which has been resected distally. 
         [0054]    The instrument has a plate member  8  and a stylus  10 . The instrument is mounted on an intramedullary rod  12  which is located in the intramedullary cavity in the femur. The instrument allows the position of the plate member  8  to be moved relative to the intramedullary rod  12  until the upper face  16  of the plate member is in contact with the femoral posterior condyles  18 . The instrument is shown in this configuration in  FIG. 1 . As discussed below, the location of the tip of the stylus can be adjusted relative to the reference portion (on which it is mounted) along the superior-inferior axis, and the location of the reference portion (with the stylus) can be adjusted relative to other component of the instrument along the anterior-posterior axis. These adjustments allow the position of the stylus  10  to be adjusted until the tip of the stylus is in contact with the anterior cortex  20 , beyond the femoral sulcus  22 . 
         [0055]    The instrument provides a first scale  24  (see  FIGS. 4 and 6 ) from which information relating to the distance between the posterior condyles and the anterior cortex, measured in a direction parallel to the anterior-posterior axis, can be obtained. As discussed below, this is provided by locating a single indicium  104  on the measurement structure against a set  108  of indicia on the reference structure, where the set of indicia provides a first scale portion and the single indicium provides a second scale portion. 
         [0056]    The position of the plate member  8  can be moved relative to the intramedullary rod  12  until the lower face  27  of the plate member is in contact with the flat face  5  of the resected tibia. The movement of the plate member can result in the soft tissue which extends between the tibia and the femur to be placed under tension. The instrument is shown in this configuration in  FIG. 2 . 
         [0057]      FIG. 2  shows the instrument shown in  FIG. 1  with its mask  26  attached. The mask provides a window  132  through which a limited length of a second scale  28  (see  FIG. 7 ) on the instrument is visible, which provides information relating to the distance between the anterior cortex and the flat face of the resected tibia, measured in a direction parallel to the anterior-posterior axis, can be obtained. This information can be used to select a bearing component having an appropriate thickness having regard to the size of the femoral component, and the soft tissue which extends between the femur and the tibia. 
         [0058]      FIG. 3  shows separate components of the instrument. A measurement structure  50  comprises a hollow elongate housing  52  which is hollow and has a circular cross-section when viewed along its length. The housing has upper and lower ends  53 ,  54 . A pair of slots are formed in the housing wall. The front slot  55  is visible in  FIG. 3 . A turret portion  57  extends from the upper end  53  of the housing  52 . 
         [0059]    The hollow housing contains a connector block  56  which has a transverse bore  58  extending through it. A shaft  60  extends through the housing  52  and through the turret  57  along the housing axis from the connector block towards the upper end  53  of the housing. The shaft is threaded at the end  61  which is remote from the connector block. 
         [0060]    The measurement structure includes a driver assembly  62  which is mounted on the upper end  53  of the housing  52 . The driver assembly includes a driver knob  75  and a driver sleeve  76 . The driver sleeve fits over the turret on the housing so that the lower end of the driver sleeve sits on the upper end of the housing. The turret is a close fit within the driver sleeve so that the driver sleeve and the driver knob can rotate on the turret relative to the housing. The driver knob has a threaded bore  63  within it, the thread in the bore of the knob  62  cooperating with the thread on the end  61  shaft  60  so that, when the driver knob and the driver sleeve are rotated relative to the housing and turret, the shaft is cause to move within the housing and the turret along the housing axis. The bore can be open at the top end of the knob. This can facilitate cleaning of the bore. 
         [0061]    The measurement structure cooperates with an intramedullary rod  64  which is a sliding fit in the transverse bore  58  in the connector block  56 . The intramedullary rod extends through one or both of the slots in the wall of the housing when it is received in the transverse bore in the connector block. The measurement structure can rotate around the axis which is defined by the intramedullary rod. 
         [0062]    The measurement structure includes a plate member  66  which is fixed to the elongate housing  52  at its lower end  54  when the instrument is in use. The plate member has two lobes  68 ,  70  whose upper faces are shaped so as to replicate approximately the shape of the proximal face of the tibia, and which are intended to cooperate with the posterior faces of the medial and lateral femoral condyles respectively. It has upper and lower faces  72 ,  74  which are for contacting the posterior femoral condyles and the proximal tibia respectively. 
         [0063]    The instrument includes a reference structure  80  which comprises a bridge portion  82  and medial and lateral limbs  84 ,  86 . The bridge portion has a circular hole  88  extending through it which is slightly bigger than the housing  52  of the measurement structure  50  so that the reference structure can be slid on to the housing of the measurement structure. 
         [0064]    The reference structure includes a locking collet  90  which comprises a plurality of flexible fingers  92  and a collar  94  which is threaded on to the bridge portion of the reference structure. The fingers can be deformed inwardly so as to grip the housing of the measurement structure by tightening the collar  94  down on to the bridge portion of the reference structure. 
         [0065]    The reference structure includes a stylus  96  having a body portion  98  which has a slot  100  formed in it. The stylus is mounted on a spigot on the bridge portion of the reference structure so that it can slide relative to the bridge portion, with the spigot sliding in the slot. In this way, the effective length of the stylus can be adjusted so that it extends just beyond the sulcus to the anterior cortex. This allows the tip of the stylus to be moved along the superior-inferior axis. The stylus can include indicia  102  on the edges of the slot  100  to indicate the lengths of the different sizes of femoral component that correspond to the different positions of the stylus. 
         [0066]    The housing  52  of the measurement structure  50  bears a single indicium  104 , which is marked “SZ” in the drawings (for example see  FIGS. 6 and 7 ) which forms part of a first scale, and a set of indicia  106  which form part of a second scale (which is discussed in more detail below). The set of indicia of the second scale is marked “mm” in the drawings (for example see  FIG. 3 ). The reference structure  80  has a set of indicia  108  (a first scale portion) in two groups on its medial and lateral limbs respectively which cooperate with the single indicium  104  (a second scale portion) on the measurement structure to provide the first scale. The two groups of indicia on the reference structure are marked “SZ” on each of the medal and lateral limbs. The first scale is used to obtain information concerning the distance between the posterior condyles and the anterior cortex, measured in a direction parallel to the anterior-posterior axis, for use in selecting the appropriate size of a femoral component of the knee joint prosthesis. In the example shown in the drawings, the first scale indicates that the appropriate size of femoral component having regard to the measured anterior-posterior dimension is size 5. 
         [0067]    As can be seen in particular in  FIG. 4 , the outside edges of the medial and lateral limbs  84 ,  86  of the reference structure are shaped with a plurality of parallel grooves  87  extending in a direction which is parallel to the intramedullary rod  64 . The spacing between the grooves can (but need not) correspond to the spacing between the indicia  108  on the reference structure. 
         [0068]      FIGS. 3 and 5  show the mask component  120  of the instrument. The mask has a front wall  122  and a pair of side walls  124 ,  126 . The inside surface of each of the side walls carries a series of inwardly extending ribs  127 . The spacing between the ribs corresponds to the spacing between the grooves  87  in the outside edges of the limbs of the reference structure. The ribs are a sliding fit in selected grooves of the medial and lateral limbs of the reference structure so that the side walls of the mask can be slid on to the edges of the reference structure. 
         [0069]    The front wall  122  of the mask  120  is shaped to fit over the reference structure  80  and the housing  52  of the measurement structure  50  when the reference structure is fitted on to the measurement structure. Accordingly, the front wall has a central portion  128  which is arcuate. It also has a slot  130  formed in it which is aligned with the front slot  55  in the housing of the measurement structure when the mask is fitted to the reference and measurement structures, to accommodate the end portion of the intramedullary rod  64 . The mask has a pair of windows  132  formed in it. The windows are generally T-shaped (with the “T” turned through  90  so that the limb of the “T” that is generally referred to as the upright limb is horizontal and the limb of the “T” that is might be referred as the cross limb is vertical). When the mask is fitted on to the assembled reference and measurement structures with the ribs on the inside surfaces of the side walls of the mask are a sliding fit in selected grooves in the outside edges of the medial and lateral limbs of the reference structure, the horizontal limb of each of the windows exposes a selected indicium from the set of indicia  108  (the first scale portion) on the reference structure, and exposes one of the indicia from the set of indicia  106  (a third scale portion) on the measurement structure  50 . This can be seen in  FIGS. 2 and 9 , which show that the appropriate size of bearing component as 5 mm. 
         [0070]    The instrument that is shown in the drawings has a first scale  24  which provides indicia corresponding to ten different sizes of femoral component. The distance between the anterior cortex and the posterior condyles differs between consecutive sizes of component differs by 3 mm. The distance between adjacent grooves  87  on the outside edges of the limbs of the reference structure is 3 mm. 
         [0071]    The instrument that is shown in the drawings has a second scale which provides for bearing components to be used which have thicknesses between 5 and 22 mm. The second scale  28  indicates bearing component thickness increments of 1 mm for bearing component thicknesses from 5 to 8 mm, and bearing component thickness increments of 2 mm for bearing component thicknesses from 8 to 22 mm. 
         [0072]    The instrument that is shown in the drawings can be used in a surgical method to replace a knee, which includes the following steps:
   1. Perform the proximal tibial cut to form the planar proximal surface  5  on the tibia on which a tibial component of a knee joint prosthesis can be seated.   2. Perform the distal cut on the femur.   3. Insert the intramedullary rod  64  in the intramedullary cavity of the femur  6 .   4. Locate the measurement structure  50  on the intramedullary rod  64  by positioning the end of the rod through the slots  55  in the wall of the housing  52  and through the hole  58  in the connector block  56 , so that the measurement structure housing is pressed against the distal face of the resected femur.   5. Turn the knob  62  on the measurement structure (in the direction indicated by the “SZ arrow”—see  FIG. 2 ) to draw the plate member  66  upwardly towards the femoral posterior condyles until the upper face  72  of the plate member is in contact with the posterior condyles.   6. Slide the reference structure  5480  on to the measurement structure  50  so that the measurement structure housing  52  extends through the hole  88  in the bridge portion  82  and through the collar  94 , effectively moving the tip of the stylus along the anterior-posterior axis until the tip of the stylus  96  touches the femoral anterior cortex, adjusting the length of the stylus as necessary by sliding it along the anterior-posterior axis relative to the bridge portion.   7. Note the indicated size of the femoral component using the first scale provided by the single indicium  104  on the measurement structure  50  and the set  108  of indicia on the reference structure  80 .   8. Fit the mask  120  to the measurement and reference structures  50 ,  80  by sliding the ribs on the mask into the grooves  87  on the reference structure, the grooves being selected so that the femoral component size as indicated on the scale portion on the reference structure appears towards the outward end of the horizontal limb of one of the windows  130  in the mask.   9. Turn the knob  62  on the measurement structure  50  (in the direction indicated by the “mm arrow”—see  FIG. 2 ) to move the measurement structure including the plate member  66  downwardly towards the resected tibia until the lower face  74  of the plate member is in contacted with the tibia, and until the soft tissue extending between the tibia and the femur is placed under appropriate tension.   10. Ensure that the tip of the stylus  98  touches the femoral anterior cortex.   11. Note the size of the bearing component as indicated towards the inward end of the horizontal limb of the mask window which shows the femoral component size in towards its outward end.   12. Assess the location of the posterior cuts (through the posterior condyles) for the chosen size of femoral component of a knee joint prosthesis using the posterior edges of the mask.   13. Tighten the collar  94  on the reference structure to prevent relative sliding movement between the reference structure and the measurement structure housing.   14. Insert pins through the vertical limbs of the mask window and into the holes in medial and lateral limbs ( 84 ,  86 ) of reference structure to provide location for cutting block.