Abstract:
A sizing guide for facilitating the sizing of prosthetic devices that are implanted. The sizing guide is designed for consistent mounting at a fixed anatomical reference point to facilitate accurate and repeatable sizing estimations for a variety of bone types and sizes.

Description:
FIELD OF THE INVENTION  
         [0001]    The present invention relates generally to instruments for use in orthopedic procedures, and particularly to a sizing guide to facilitate sizing of certain prosthetic implants.  
         BACKGROUND OF THE INVENTION  
         [0002]    In a variety of orthopedic procedures, a section of bone tissue is removed and an orthopedic prosthesis is attached. For example, it has become relatively common to replace one or more components of a knee joint. One such prosthetic device is mounted to the distal end of a femur. Typically, several cuts are made through the bone tissue at the distal end of the femur to properly shape the bone for receipt and mounting of the prosthetic device.  
           [0003]    However, the prosthetic device must be properly sized relative to the size of the femur to which it is attached or the strength and/or comfort of the implant can be compromised. For example, one of the sizing cuts typically is made along the anterior cortex of the femur. If the size of the implant is too small relative to the area of the cut, a “notch” is left along the bone tissue at the peripheral edge of the implanted prosthetic device. This notch can weaken the bone, making it susceptible to fracture. If, on the other hand, the implant is too large relative to the area of the cut, the anterior tip of the prosthetic device can overhang the face of the bone. This creates a gap between the implant and the bone tissue potentially resulting in reduced strength at the point of attachment between the implant and the bone tissue. Also, if the implant overhangs the bone tissue, the patient may incur soft tissue irritation.  
           [0004]    Examples of current techniques for sizing such prosthetic devices include visually estimating the size of the cuts once a femoral cutting block or guide is fixed into position; comparing the implant to femoral drill guides; and using presurgical X-rays in conjunction with implant X-ray templates to estimate proper implant size. However, these methods involve substantial estimation by the practitioner and create difficulty in producing consistent, accurate sizing of the implant.  
         SUMMARY OF THE INVENTION  
         [0005]    The following passage is intended only to provide a brief summary of limited aspects of the present invention and should not be construed as encompassing all necessary elements or steps of the inventions.  
           [0006]    The present invention relates generally to a technique for sizing a prosthetic device. The technique utilizes a sizing guide able to assess the fit of a range of implants, such as femoral implants. The instrument is designed for consistent positioning at a fixed anatomical reference, such as the intramedullary femoral canal. Because there are established distances or measurements from the intramedullary femoral canal to various perimeter points of the implanted prosthetic device, the desired size of the implant can be readily determined when the sizing guide is consistently positioned at a fixed location with respect to an anatomical reference, e.g. the intramedullary canal. 
       
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0007]    The invention will hereafter be described with reference to the accompanying drawings, wherein like reference numerals denote like elements, and:  
         [0008]    [0008]FIG. 1 is a side view of the distal end of an exemplary femur with a prosthetic device attached;  
         [0009]    [0009]FIG. 2 is a side view of the femur illustrated in FIG. 1 with a sizing guide mounted thereto prior to shaping the femur for receipt of the prosthetic device;  
         [0010]    [0010]FIG. 3 is a top view of the sizing guide illustrated in FIG. 2 positioned adjacent the distal fend of a femur;  
         [0011]    [0011]FIG. 4 is an exploded view of the sizing guide illustrated in FIGS. 2 and 3;  
         [0012]    [0012]FIG. 5 is a front view of a sizing block used in the sizing guide illustrated in FIG. 4;  
         [0013]    [0013]FIG. 6 is a cross-sectional view taken generally along the line  6 - 6  of FIG. 5;  
         [0014]    [0014]FIG. 7 is a front view of a bushing received by the sizing block;  
         [0015]    [0015]FIG. 8 is a cross-sectional view taken generally along line  8 - 8  of FIG. 7;  
         [0016]    [0016]FIG. 9 is a partial cross-sectional view of the sizing guide, illustrating the bushing of FIG. 7 positioned within the sizing block of FIG. 5;  
         [0017]    [0017]FIG. 10 is a top view of an exemplary stylus that may be rotatably mounted to the sizing block of FIG. 5;  
         [0018]    [0018]FIG. 11 is a cross-sectional view taken generally along line  11 - 11  of FIG. 10;  
         [0019]    [0019]FIG. 12 is a side view of the sizing guide positioned against a femur to determine the proper implant size;  
         [0020]    [0020]FIG. 13 is a view similar to FIG. 12 with the sizing guide indicating an implant size that would be too large; and  
         [0021]    [0021]FIG. 14 is a view similar to FIG. 12 in which the sizing guide is indicating an implant size that would be too small. 
     
    
     DETAILED DESCRIPTION OF EXEMPLARY EMBODIMENTS  
       [0022]    It will be appreciated that the present invention can take many forms and embodiments. Some embodiments of the invention are described so as to give an understanding of the invention. It is not intended, however, that the embodiments of the present invention that are described in this specification should limit the invention. The following describes exemplary embodiments and uses of a sizing guide that facilitates proper sizing of prosthetic devices, e.g. implants. The example described is for sizing a prosthetic device to be affixed to a distal end of a femur.  
         [0023]    Referring generally to FIG. 1, an exemplary femur  20  is illustrated as having a distal end  22  to which a prosthetic device  24  is attached. In preparing distal end  22  for attachment of prosthetic device  24 , a distal preparatory cut  26  is made to establish a generally flat plane (see also FIG. 2). Additionally, an anterior cut  28  is made generally along the anterior cortex region of femur  20  to intersect preparatory cut  26 . Additionally, a posterior cut  30  is generally made at an angle that intersects a posterior side of femur  20  and preparatory cut  26 , as illustrated.  
         [0024]    If cuts  26 ,  28  and  30  are properly made, distal end  22  readily receives a corresponding contour  32  of prosthetic device  24 . Anterior cut  28  and posterior cut  30  typically are made with the aid of a cutting block to ensure planar cuts at the appropriate angles, as known to those of ordinary skill in the art. However, the fit of prosthetic device  24  also depends on the size of femur  20  and the selected size of prosthetic device  24 .  
         [0025]    Because prosthetic device  24  is mounted on a stem extending into an intramedullary femoral canal  34 , the distance to or position of various points about the implant are fixed relative to this generally centralized stem location. For example, the exemplary prosthetic device  24  comprises a flange  36  that is a fixed distance from a centerline  37  of the femoral canal  34  when the implant is attached to the femur. Thus, an anterior distance  38  measured from femoral canal  34 , e.g. centerline  38 , to an anterior surface or region  40  of femur  20  provides a strong indication of the desired size of prosthetic device  24 .  
         [0026]    Similarly, the distance from the femoral canal  34  to a posterior region  42  or a medial-lateral region  44  is known for an implant of a given size. Accordingly, making such sizing measurements relative to a fixed reference, such as femoral canal  34  can provide a practitioner with an indication of implant size that will provide the best fit. This, of course, facilitates selection of a properly sized cutting block and formation of proper anterior cut  28  and posterior cut  30 .  
         [0027]    Additionally, in the exemplary prosthetic device  24 , flange  36  has a known flange height  46 . This flange height distance also can be measured from known reference points, such as preparatory cut  26  and femoral canal  34  to ensure that a properly sized prosthetic device is selected. If, for example, the selected prosthetic device  24  is too small, fitment problems result. For example, if anterior cut  28  extends beyond flange  36  a “notch” is left in the bone tissue along the periphery of flange  36 . This notch can weaken the bone, rendering it susceptible to future fracture. Even if the practitioner avoids notching by blending the cut, there may still exist the problem of insufficient cortical bone to support the implant. If, on the other hand, the selected implant is too large, the anterior region of prosthetic device may overhang the face of preparatory cut  26  resulting in a gap between anterior cut  28  and the implant. This can reduce the strength of the attachment and also cause soft tissue irritation to the patient. A prosthetic device  24  that is too large relative to femur  20  also can overhang the bone tissue at medial-lateral regions  44 , potentially leading to problems such as tissue irritation. Furthermore, an oversized implant can result in location of posterior region  42  at a distant too great from the centerline  37  of the femoral canal.  
         [0028]    As illustrated in FIGS. 2 and 3, an exemplary sizing guide  48  is positioned adjacent distal end  22  at a fixed location relative to a fixed anatomical region  50 , in this case, femoral canal  34 . Thus, a variety of sizing measurements may be made that are based on or “driven from” the femoral canal  34 .  
         [0029]    In the example illustrated, sizing guide  48  is mounted over a rod  52  extending from femoral canal  34 . One exemplary rod  52  comprises a trial stem  54  positioned within femoral canal  34  and attached to a trial stem adapter  56  that extends outwardly from femoral canal  34 . Sizing guide  48  is slid along rod  52  into contact with preparatory cut  26  of femur  20 . Once disposed at this fixed position with respect to intramedullary femoral canal  34 , a variety of sizing determinations may be made.  
         [0030]    It should be noted that in at least some implant designs, the implant stem is disposed at a slight angle with respect to certain mounting surfaces of the implant. Consequently, preparatory cut  26  is formed at a similar angle with respect to femoral canal  34 . Accordingly, sizing guide  48  may be designed to receive rod  52  therethrough at such an angle (labeled angle  58 ) with respect to a longitudinal axis or centerline  60  of sizing guide  48 , as illustrated in FIG. 3. An exemplary angle is approximately 6 degrees, however, other angles may be appropriate depending on implant design and bone characteristics.  
         [0031]    With additional reference to FIG. 4, sizing guide  48  generally comprises a sizing block  62  and a stylus member  64  rotatably mounted to sizing block  62 . When sizing block  62  is positioned against distal end  22  of femur  20 , stylus member  64  may be rotated to one or more of a plurality of positions to generally determine anterior distance  38  and thus the optimal size of prosthetic device  24 .  
         [0032]    Although sizing block  62  may have a variety of sizes and configurations, the exemplary sizing block comprises an A-shaped frame  66 , as best illustrated in FIG. 5. An opening  68  extends longitudinally through frame  66  along longitudinal axis  60  of sizing block  62 . Opening  68  is sized to receive a bushing  72 , as described in greater detail below.  
         [0033]    Frame  66  also comprises a pair of legs or extensions  74  that extend generally in a direction towards the posterior region of femur  20  when sizing guide  48  is positioned as illustrated in FIG. 2. Each extension  74  includes medial-lateral sizing features  76  and posterior sizing features  78 . Additionally, frame  66  comprises a generally flat engagement surface  80 , as best illustrated in FIG. 6. Engagement surface  80  is designed to abut against preparatory cut  26 , as illustrated in FIGS. 2 and 3.  
         [0034]    Sizing features  76  and  78  are arranged as outwardly descending steps. The sides of each step provide a medial-lateral measurement indicative of an implant size, and the bottom of each step provides a posterior measurement indicative of the posterior extent of an implant having a given size. The size indications are provided on a visible surface  79  of frame  66 .  
         [0035]    Frame  66  further comprises a threaded opening  84 . Opening  84  is designed to threadably receive a mounting pin  85  (see FIG. 4) by which stylus member  64  is mounted to sizing block  62 .  
         [0036]    Frame  66  also receives rod  52 , e.g. trial stem adapter  56 , therethrough when engagement surface  80  is moved into position adjacent distal end  22  of femur  20 . In the particular embodiment illustrated, rod  52  is slidably received through a bushing openings  86  of bushing  72 . Bushing  72  is best illustrated in FIGS. 7 and 8. Bushing opening  86  passes longitudinally through bushing  72  and may be disposed at angle  58 , e.g. a 6 degree angle, with respect to longitudinal axis  60 . In the embodiment illustrated, longitudinal axis  60  is the longitudinal axis of bushing  72  as well as the overall sizing block  62  and is generally perpendicular to engagement surface  80 . Thus, as sizing block  62  is slid onto rod  52 , sizing block  62  and specifically engagement surface  80  are positioned at the angle  58  with respect to femoral canal  34 .  
         [0037]    In one exemplary embodiment, bushing  72  is designed for rotatable movement such that bushing opening  86  may be moved between at least two angular positions. For example, if bushing  72  is rotated 180(, angle  58  is formed on a directly opposed side of longitudinal axis  60 . This allows easy adaptation of sizing guide  48  for use on either the left femur or the right femur of a patient undergoing the orthopedic procedure. This adaptability can be achieved by permitting the rotation of bushing  72  between two or more locked positions.  
         [0038]    The exemplary bushing  72  comprises a barrel portion  88  having a generally smooth outer surface  90  that is rotatably received in a narrowed portion  92  of opening  68  (see also FIG. 9). Bushing  72  further comprises an annular flange  94  that extends radially outwardly from outer surface  90  to prevent movement past narrowed portion  92 . Between narrowed portion  92  and engagement surface  80 , opening  68  comprises an expanded region  96  sized to receive both barrel portion  88  and a surrounding spring member  96 , as best illustrated in FIG. 9. An exemplary spring member  96  is a coil spring that fits around barrel portion  88  within expanded region  96  while being axially trapped between narrowed portion  92  of opening  68  and annular flange  94  of bushing  72 .  
         [0039]    Bushing  72  is held within opening  68  by a cap member  98  positioned on an opposite side of narrowed portion  92  from flange  94  and affixed to barrel portion  88 . Cap member  98  may be affixed to barrel portion  88  in a variety of ways, including threaded engagement, press fitting, adhesives, weldments, or one or more fasteners. For example, a pin  100  may be radially inserted through a corresponding opening in cap member  98  and threaded or pressed into a radial opening  102  formed through outer surface  90  of bushing  72 . Preferably, pin  100  extends radially outward from cap member  98  for receipt in corresponding recesses  104 , as best illustrated in FIGS. 5, 6 and  9 . In the embodiment illustrated, frame  66  comprises two recesses  104  disposed on generally opposite sides of opening  68 . This placement of recesses  104  permits the 180( rotation of bushing opening  86  to accommodate orthopedic procedures on either leg of the patient.  
         [0040]    To adjust the angle of bushing opening  86 , cap member  98  is gripped and bushing  72  is pulled against the spring bias of spring member  96  until pin  100  clears the corresponding recess  104 . Cap member  98  is then used to rotate bushing  72  and bushing opening  86  to the next predetermined angle or position where pin  100  is aligned with a different, e.g. opposite, recess  104 . The operator then releases cap member  98  and allows spring member  96  to move pin  100  towards and into the desired recess  104 . In the example illustrated, cap member  98  comprises an annular flange  106  that prevents spring member  96  from moving bushing  72  beyond a flush position with respect to engagement surface  80 .  
         [0041]    Referring generally to FIGS. 10 and 11, one exemplary embodiment of stylus member  64  is illustrated. Stylus member  64  comprises a generally central hub  110  having an opening or passage  112  therethrough. Passage  112  extends generally along an axis  114  disposed at the radial center of hub  110 .  
         [0042]    Passage  112  has a narrow portion  116  sized to slidably receive mounting pin  85  when mounting pin  85  is engaged with threaded opening  84  of frame  66  via a mounting pin threaded portion  118  (see FIG. 4). However, narrow portion  116  is not large enough to permit the passage of a mounting pin head  120 , thus preventing inadvertent separation of stylus member  64  from sizing block  62 .  
         [0043]    Passage  112  is further defined by a broad portion  122  separated from narrow portion  116  by an annular shoulder  124 . Broad portion  122  is large enough to receive mounting pin head  120  which is slidably disposed within broad portion  122  at a distance from shoulder  124  when mounting pin  85  is secured to sizing block  62 . This permits stylus member  64  to be lifted or otherwise moved away from sizing block  62  without permitting complete separation of stylus member  64  from sizing block  62 .  
         [0044]    Extending outwardly from hub  110  are a plurality of arms  126 ,  128  and  130 . Each arm is of a different length and corresponds to a prosthetic device of a given size. Specifically, each arm length generally corresponds to flange height  46  indicates the periphery or height of flange  36  when sizing guide  48  is mounted against femur  20 . Although three arms are illustrated, the number of arms may be changed to represent a lesser or greater number of implant size selections. As illustrated best in FIG. 3, a given arm, e.g. arm  126 , can be rotated back and forth across femur  20  to provide the practitioner with a general idea as to the reach or height of flange  36  along anterior region  40  when prosthetic device  24  is attached to femur  20 .  
         [0045]    One of the functions of stylus member  64  is to help determine anterior distance  38  and to correlate this distance to a specifically sized prosthetic device  24 . To accomplish this aspect of sizing the prosthetic device, a plurality of pins  132 ,  134  and  136  are mounted to the outlying ends of arms  126 ,  128  and  130 , respectively. Each pin is oriented to extend from its corresponding arm towards anterior region  40  of femur  20  when positioned adjacent anterior region  40 . The length of each pin is different and corresponds to a prosthetic device having a given size, as explained in greater detail below.  
         [0046]    It should be noted that the use of arms  126 ,  128 ,  130  and pins  132 ,  134 ,  136  can be replaced or supplemented with other types of measuring tools by which a practitioner is able to determine anterior distance  38  and its correlation to a prosthetic device of a given size. For example, hub  110  can be mounted on or adjacent a scale, and a single pin can be moved along the anterior region  40  of femur  20 . As the pin is moved along the bone tissue, hub portion  110  rises or falls along the scale which is appropriately marked for correlation to prosthetic devices of specific sizes. The measurement, and resultant implant size determination, also can be made with a variety of sensors able to determine the desired anterior distance. Accordingly, stylus member  64  is amenable to a variety of alterations depending on the application and the technology available.  
         [0047]    Operation of sizing guide  48  is explained with reference to FIGS. 12, 13 and  14 . In FIG. 12, sizing guide  48  has been slid over rod  52 , e.g. trial stem adapter  56 , via insertion of rod  52  through bushing opening  86 . Sizing block  62  is moved along rod  52  until engagement surface  80  abuts femur  20  at preparatory cut  26 . To ensure proper engagement between surface  80  of sizing block  62  and surface  26  of femur  20 , bushing  72  is rotated to the appropriate position to compensate for the orientation of preparatory cut surface  26  with respect to femoral canal  34 .  
         [0048]    Once sizing block  62  is appropriately positioned, stylus member  64  is rotated until one of the pins  132 ,  134  and  136  is adjacent anterior region  40  of femur  20  without substantially lifting hub  110  away from sizing block  62 , as illustrated in FIG. 12 by indicator arrows  138 . In this example, sizing guide  48  indicates the optimal size for prosthetic device  24  is the size that corresponds to pin  136 , e.g. a size  3 , as opposed to a size  2  (corresponding to pin  134 ) or a size  1  (corresponding to pin  132 ).  
         [0049]    The other sizing features can be used as supplemental or alternative techniques for determining and/or verifying the prosthetic device size selection. Specifically, the length of arm  130  and the longitudinal positioning of pin  136  along anterior region  40  provides the practitioner with an indication of flange height for the prosthetic device of a given size, i.e. size  3 . The arc traversed along anterior region  40  as arm  130  rotates pin  136  across this region provides an indication of where the perimeter of flange  36  would lie when prosthetic device  34  is attached to femur  20 .  
         [0050]    Additionally, medial-lateral sizing feature  76  provides the practitioner with an indication of the medial and lateral extent of a specifically sized prosthetic device. This can be used, for example, to confirm whether the implant size indicated by pin  136  is also appropriate given the medial-lateral extent of distal end  22  of femur  20 .  
         [0051]    Similarly, posterior sizing feature  78  can be used to provide the practitioner with an indication of the posterior extent of prosthetic device  24  so he or she can assess whether the size selected, e.g. size  3 , is appropriate for femur  20 . Each of the sizing features, pins  132 ,  134 ,  136 ; arms  126 ,  128 ,  130 ; medial-lateral sizing feature  76 ; and posterior sizing features  78  are based on or “driven” from the fixed anatomical reference  50  of femur  20 , e.g. the medullary femoral canal  34 .  
         [0052]    When pin  136  is proximate anterior region  40  and minimal or no gap exists between hub  110  and sizing block  62 , as illustrated in FIG. 12, the practitioner has a primary indication of a properly sized prosthetic device  24 . However, if a substantial gap  140  exists between a given pin, e.g. pin  136 , and anterior region  40  of femur  20 , (see FIG. 13) the practitioner is provided with an indication that the implant corresponding to pin  136  is too large for femur  20 . In other words, an implant having a smaller size would be more desirable.  
         [0053]    Alternatively, if the pin, e.g. pin  132  contacts anterior region  40  and lifts hub  110  away from sizing block  62  to create a gap  142 , (see FIG. 14) the practitioner is provided with an indication that the prosthetic device corresponding to pin  132  is too small for the given femur  20 . In other words, a larger prosthetic device  24  would be more appropriate for attachment to the particular femur. Thus, sizing guide  48  provides the practitioner with a consistent, repeatable technique for optimizing the sizing of prosthetic devices.  
         [0054]    It will be understood that the foregoing description is of exemplary embodiments of this invention, and that the invention is not limited to the specific form shown. For example, the sizing guide may be made of a variety of surgical grade materials in various sizes. The configuration of the stylus member as well as various other sizing features may be changed; the orientation of the opening through the sizing block may be changed, fixed or adjustable; and the sizing block may be designed for receipt of other types of rods or fixtures that allow for consistent mounting of the sizing guide relative to a fixed anatomical region. These and other modifications may be made in the design and arrangement of the elements without departing from the scope of the invention as expressed in the dependent claims.