Abstract:
Methods for predetermining a contour of a resected bone surface and assessing a fit of a prosthesis on the resected bone surface, for designing prostheses to fit discrete patient populations, and for designing customized prostheses.

Description:
CROSS REFERENCE TO RELATED APPLICATIONS 
       [0001]    The present application is a divisional application of U.S. patent application Ser. No. 11/685,906, docket ZIM0317-01, titled METHODS OF PREDETERMINING THE CONTOUR OF A RESECTED BONE SURFACE AND ASSESSING THE FIT OF A PROSTHESIS ON THE BONE, which claims the benefit under Title 35, U.S.C. §119(e) of U.S. Provisional Patent Application Ser. No. 60/783,630, entitled METHODS OF PREDETERMINING THE CONTOUR OF A RESECTED BONE AND THE FIT OF AN IMPLANT ON THE BONE, filed Mar. 17, 2006, the disclosures of which are hereby expressly incorporated herein by reference. 
     
    
     BACKGROUND 
       [0002]    The present disclosure relates to methods for determining an optimal fit of a prosthesis on a resected bone surface. 
         [0003]    Orthopaedic procedures for the replacement of all, or a portion of, a patient&#39;s joint typically require resecting and reshaping of the bones of the joint to receive prosthetic components. For example, a typical total knee prosthesis has three main components: a femoral component for replacing at least a portion of the distal end of the femur, a tibial component for replacing at least a portion of the proximal end of the tibia, and a bearing insert for replacing at least a portion of the articulating tissue between the femur and the tibia. Procedures for implanting a total knee prosthesis typically involve preparing and reshaping; both the distal end of the femur and the proximal end of the tibia prior to implanting the prosthetic components. The amount of bone removed may be partially determined by the size and type of prosthetic components to be implanted. The size of prosthetic components may be initially determined by measurements taken of the knee prior to and during surgery, and the final determination of size may be made after taking measurements and trialing a provisional prosthesis during the procedure. 
       SUMMARY 
       [0004]    The present disclosure provides methods for predetermining a contour of a. resected bone surface and assessing a fit of a prosthesis on the resected hone surface. The present disclosure also provides methods for designing prostheses to tit discrete patient populations as well as methods for designing customized prostheses. 
         [0005]    In one form thereof, the present disclosure provides a method of virtually assessing the fit of a prosthesis for placement on a resected bone surface, the method including the steps of creating a two-dimensional outline of the resected bone surface; creating a two-dimensional outline of a first prosthesis; and comparing the two-dimensional outline of the resected bone surface with the two-dimensional outline of the first prosthesis. 
         [0006]    In another form thereof, the present disclosure provides an apparatus for virtually assessing the fit of a prosthesis for placement on a resected bone surface, the apparatus including a first computer adapted to create a two-dimensional outline of the resected bone surface; a second computer for creating a two-dimensional outline of a first prosthesis; and a third computer for comparing the two-dimensional outline of the resected bone surface with the two-dimensional outline of the first prosthesis. 
         [0007]    In yet another firm thereof, the present disclosure pro-ides a method of designing a prosthesis to substantially fit a resected bone surface based on a population of bones, the method including the steps of creating a plurality of two-dimensional outlines corresponding to each resected bone surface for each bone of the population; and determining a contour of a bone engaging surface of a prosthesis using the plurality of two-dimensional outlines, wherein the contour substantially matches the plurality of two-dimensional outlines of the resected bone surfaces. 
         [0008]    In still another form thereof, the present disclosure provides an apparatus for designing a prosthesis to substantially fit a resected bone surface based on a population of bones, the apparatus including a first computer for creating a plurality of two-dimensional outlines corresponding to each resected bone surface for each bone of the population; and a second computer for determining a contour of a bone engaging surface of a prosthesis which substantially matches the plurality of two-dimensional outlines of the resected bone surfaces. 
         [0009]    In one form thereof, the present disclosure provides a method of creating a prosthesis for placement on a resected bone surface, the method including the steps of creating a two-dimensional outline of the resected bone surface; and determining a contour of a bone engaging surface of a prosthesis using the two-dimensional outline of the resected bone surface. 
         [0010]    In another form thereof, the present disclosure provides an apparatus for creating a prosthesis for placement on a resected bone surface, the apparatus including a first computer for creating a two-dimensional outline of the resected bone surface; and a second computer for determining a contour of a. bone engaging surface of a prosthesis using the two-dimensional outline of the resected bone surface. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0011]    The above mentioned and other features of the disclosure, and the manner of attaining them, will become more apparent and will be better understood by reference to the following description of embodiments of the disclosure taken in conjunction with the accompanying drawings, wherein: 
           [0012]      FIG. 1  is a perspective view of a digital model of the distal end of a femur including a virtual resection according to an exemplary method of the present disclosure; 
           [0013]      FIG. 2  is a perspective view of the digital model of  FIG. 1 , further illustrating the vertices of the virtual resection; 
           [0014]      FIG. 3  is a top view of the two-dimensional outline of the femoral resection of  FIG. 1 ; 
           [0015]      FIG. 4  a perspective view of an exemplary distal femoral prosthesis which may be used in an exemplary method of the present disclosure; 
           [0016]      FIG. 5  is a perspective view of the prosthesis of  FIG. 4 , further illustrating the step of virtually unfolding the prosthesis; 
           [0017]      FIG. 6  is a top view of the two-dimensional outline of the prosthesis of  FIG. 4  after the unfolding step of  FIG. 5 ; 
           [0018]      FIG. 7  is an illustration of another step of the method of the present disclosure wherein outlines of several exemplary prostheses are compared with outlines of several virtually resected exemplary lemurs; and 
           [0019]      FIG. 8  is another illustration of the step shown in  FIG. 7 . 
       
    
    
       [0020]    Corresponding reference characters indicate corresponding parts throughout the several views. Although the drawings represent embodiments of the present disclosure, the drawings are not necessarily to scale and certain features may be exaggerated in order to better illustrate and explain the present disclosure. Although the exemplifications set out herein illustrate embodiments of the disclosure, the embodiments disclosed below are not intended to be exhaustive or to be construed as limiting the scope of the invention to the precise forms disclosed. 
       DETAILED DESCRIPTION 
       [0021]    The present disclosure may include references to the following terms: anterior (at or near the front of the body, as opposed to the back of the body); posterior back of the body, as opposed to the front of the body); lateral (at or near the side of the body, farther from the midsagittal plane, as opposed to medial); medial (at or near the middle of the body, at or near the midsagittal plane, as opposed to lateral); proximal (toward the beginning, at or near the head of the body as opposed to distal); and distal(further from the beginning, at or near the foot of the body, as opposed to proximal). 
         [0022]    Referring to  FIGS. 1-8 , an exemplary method of the present disclosure may he used to determine how a femoral prosthesis will fit on the distal end of a femur, i.e., to assess whether a prosthesis is of the right size and shape for the distal end of the femur and whether the prosthesis suitably conforms thereto. The method generally includes the steps of obtaining a three-dimensional (3-D) model of a bone based on an acquired image of the bone, virtually resecting the 3-D model of the bone, i.e., creating or simulating a resection of the bone within a computer or other intelligent processing device, preparing a bone profile of the virtual resection, creating a two-dimensional (2-D) outline or footprint of the resection from the bone profile, preparing a prosthesis profile, creating a 2-D outline or footprint from the prosthesis profile, and comparing the 2-D outlines of the bone profile and the prosthesis profile to assess or determine the fit of the prosthesis with the bone. 
         [0023]    More particularly, referring to  FIG. 1 , 3-D digital model  10  of an exemplary femur F is illustrated. Digital model  10  may be obtained by obtaining a computed tomography (“CT”) scan of a femur to produce a 3-D image of the femur and converting the 3-D image to digital model  10 . The conversion of the 3-D CT scan image to 3-D digital model  10  may be performed using any suitable modeling software including, for example, Amira®, available from Mercury Computer Systems, Inc., of Chelmsford, Mass.. Digital model  10  may include femur F having distal end F d . 
         [0024]    Referring still to  FIG. 1  using suitable software such as M.ATLAB®, available from The MathWorks, of Natick, Mass., and Unigraphics®, available from UGS Corp., of Plano, Tex., a virtual resection of distal end F d  of model femur F is performed. Similar to the resection performed in actual knee arthroplasty (procedures, the virtual resection involves defining femoral cut planes  12   a - 12   e  on distal end F d  of model femur F. Femoral cut planes  12   a - 12   e  are calculated using an algorithm of the software. The algorithm calculates femoral cut planes  12   a - 12   e  based on a proposed, exemplary femoral prosthesis and the known surgical technique specified for the proposed. femoral prosthesis More particularly, distal end F d  of model femur F .may be preliminarily measured based on the known surgical technique and using the software described above. The resulting measurements are used to preliminarily select a femoral prosthesis size and type. Resection of distal end F d  of model femur F is determined by the selected femoral prosthesis and involves resecting distal end F d  of femur F to complement and receive the prosthesis, For example, as shown in  FIG. 4 , model femoral prosthesis  20  may be preliminarily selected, Femoral prosthesis  20  is a cruciate-retaining femoral prosthetic component having bone engaging surface  22 , Bone engaging surface  22  includes a plurality of intersecting planar surfaces, including anterior surface  22   a,  distal surface  22   b,  posterior surface  22   c,  anterior chamfer surface  22   d,  and posterior chamfer surface  22   e.  Accordingly, as shown in  FIG. 1 , the virtual resection of distal end F d  of model femur F includes defining a plurality of intersecting cut planes  12   a - 12   e  including anterior cut plane  12   a , distal cut plane  12   b , posterior cut plane  12   c , anterior chamfer cut plane  12   d , and posterior chamfer cut plane  12   e , which correspond to the plurality of intersecting planar surfaces  22   a - 22   e  of model prosthesis  20  ( FIG. 4 ). As illustrated in  FIGS. 2 and 3 , cut planes  12   a - 12   e  intersect one another at femoral cut plane vertices  14   a - 14   d . More particularly, anterior cut plane  12   a  intersects anterior chamfer cut plane  12   d  at vertex  14   a . Anterior chamfer cut plane  12   d  intersects distal cut plane  12   b  at vertex  14   b . Distal cut plane  12   b  intersects posterior chamfer cut plane  12   e  at vertex  14   c . Posterior chamfer cut plane  12   e  intersects posterior cut plane  12   c  at vertex  14   d.    
         [0025]    Referring still to  FIGS. 1 and 2 , femoral profile  16 , shown as a dotted line, of the virtually resected model femur F is prepared by outlining cut planes  12   a - 12   e  extending between cut plane vertices  14   a - 14   d . Two-dimensional outline or footprint  18  of the resected surface of model femur F is then obtained, as shown in  FIG. 3 , by unfolding or bending profile  16  at cut plane vertices  14   a - 14   d  until cut planes  12   a - 12   e  are aligned in a single plane. The suitable software mentioned above may be used to manipulate profile  16  to create two-dimensional outline  18 . 
         [0026]    Referring now to  FIGS. 4-6 , two-dimensional outline or footprint  26  of proposed prosthesis  20  may be made using a process similar to that described above for outline or footprint  18  of femoral profile  16 . More particularly, 3-D digital model  20  of a femoral prosthesis may be obtained using any known method and any suitable software, including those described above. As discussed above, model prosthesis  20  includes bone engaging surface  22 ., which includes anterior planar surface  22   a , distal planar surface  22   b , posterior planar surface  22   c , anterior chamfer planar surface  22   d , and posterior chamfer planar surface  22   e . Planar surfaces  22   a - 22   e  intersect one another at prosthesis vertices  24   a - 24   d . More particularly, anterior planar surface  22   a  intersects anterior chamfer surface  22   d  at vertex  24   a . Anterior chamfer surface  22   d  intersects distal planar surface  22   b  at vertex  24   b . Distal planar surface  22   b  intersects posterior chamfer surface  22   e  at vertex  24   c , and posterior chamfer surface  22   e  intersects postc.&#39;xior surface  22   c  at vertex  24   d . Anterior planar surface  22   a  of prosthesis  20  corresponds to anterior cut plane  12   a  of femur F; anterior chamfer surface  22   d  of prosthesis  20  corresponds to anterior chamfer cut plane  12   d  of femur F; distal planar surface  22   b  of prosthesis  20  corresponds to distal cut plane  12   b  of femur F; posterior chamfer surface  22   e  of prosthesis  20  corresponds to posterior chamfer cut plane  12   e  of femur F; posterior surface  22   c  of prosthesis  20  corresponds to posterior cut plane  12   c  of femur F; vertex  24   a  of prosthesis  20  corresponds to vertex  14   a  of femur F; vertex  24   b  of prosthesis  20  corresponds to vertex  14   b  of femur IF; vertex  24   c  of prosthesis  20  corresponds to vertex  14   c  of femur F; and vertex  24   d  of prosthesis  20  corresponds to vertex  14   d  of femur F. 
         [0027]    Referring to  FIG. 4 , prosthesis profile  25  of model prosthesis  20  is prepared by outlining the perimeter of intersecting planar surfaces  22   a - 22   e  between prosthesis vertices  24   a - 24   d.  Prosthesis profile  25  is represented by the heavy dashed line extending about the perimeter of model prosthesis  20 . Turning to FIGS,  5  and  6 , two-dimensional outline or footprint  26  of prosthesis profile  25  is created by using the suitable software to unfold or bend profile  25  at vertices  24   a - 24   d  until planar surfaces  22   a - 22   e  are aligned within a single plane. 
         [0028]    Prosthesis outline  26  may be visually compared with femur outline  18  to determine and assess whether model prosthesis  20  is a suitable fit for model femur  10 . Thus, a surgeon may compare outline  26  with outline  18  and determine whether prosthesis  20  corresponding to outline  26  is an acceptable prosthesis to use for femur F. Prosthesis outline  26  may be compared with femur outline  18  by superimposing one atop the other and observing the overlapping shapes and the differences therebetween. Furthermore, using the suitable software mentioned. above, quantitative analysis may be made of outlines  26  and  18 . For instance, measurements of outlines  26  and  18  may be taken and the suitable software can calculate deviations between the measurements. For example, width measurements of outlines  26  and  18  at the intersections of each planar surface may be taken and/or at midpoints of each planar surface between such intersections with other planar surfaces. Any deviations between outlines  26  and  18  may then be used to calculate proposed changes in prosthesis  20  to thereby reshape prosthesis  20  to minimize the deviations. Alternatively, any deviations between outlines  26  and  18  may prompt a user to select a different prosthesis  20  and perform the same analysis to assess the tit of the second prosthesis  20  on model femur  10 , i.e., if a surgeon decides that outline  26  of a first prosthesis  20  is unacceptable for femur F, then the surgeon then compares the outline  26  of another prosthesis  20  until an acceptable prosthesis is identified. 
         [0029]    The method described above has several useful, practical applications. For example, the method described above may be used to develop new and improved existing prosthesis designs. It is contemplated that this method may be used to survey a large population of subjects to develop statistics and identify trends in bone shapes, and to adapt prosthesis sizes and shapes accordingly. More specifically, two-dimensional footprints of virtually resected bones of a large population of patients may be obtained and compared to two-dimensional footprints of numerous available prostheses. 
         [0030]      FIGS. 7 and 8  illustrate an exemplary application of the methods of the present disclosure.  FIG. 7  illustrates femur footprints or outlines  18   a - 18 d, shown as dotted lines, taken from a virtually resected model of a lemur of four different subjects compared with footprints or outlines  26   a - 26   c , shown in solid lines, taken from three different models of available prostheses.  FIG. 8  illustrates the same footprints  18   a - 18 d,  26   a - 26   c . The comparison shown in FIGS,  7  and  8  demonstrates that the prosthesis yielding footprint  26   a  is larger in width W ( FIG. 6 ) than the virtually resected bones yielding footprints  18   b - 18   d.  In an exemplary embodiment, outlines  18   a - 18   d  may be used to design or create a prosthesis which substantially matches at least some of outlines  18   a - 18   d.  For example, a prosthesis may be created or designed which is a best fit approximation to a plurality of outlines  18  which may be based on a specific patient population, such as the female population. 
         [0031]    In an exemplary embodiment, a method of the present disclosure may be performed on the femurs of a large population of women to obtain medial/lateral and. anterior/posterior dimensions of the femurs and calculate ratios between the medial/lateral and anterior/posterior dimensions. These dimensions and calculations may be used in designing femoral components for use on female anatomy. In another exemplary embodiment, a method of the present disclosure may also be used to obtain medial/lateral and anterior/posterior dimensions of existing femoral components and calculate ratios between the medial/lateral and anterior/posterior dimensions of the femoral components. The dimensions and calculated ratios may then be used to compare existing femoral components to the dimensions and calculated ratios of the femurs of women to identify potential areas of the femoral component where fit can be optimized. Such a comparison is fully described in US. patent application Ser. No. 11/611,021, entitled DISTAL FEMORAL KNIFE PROSTHESES, assigned to the assignee of the present application the disclosure of which is hereby expressly incorporated herein by reference. The same type of process may be performed tor other populations, such as a population of males, various ethnic populations, populations based on age, stature-based populations, and/or populations based on disease progression or disease status. 
         [0032]    In addition, the method described above may be used in guiding the design and manufacture of custom prostheses. For instance, a patient&#39;s femur may be modeled, virtually resected and footprinted as described above. The footprint could then be used as the footprint for forming a prosthesis. 
         [0033]    Although the method described above is exemplified with reference to the distal end of the femur and femoral prostheses, the methods of the present invention may be applied to any bone and any prosthesis. 
         [0034]    While this invention has been described as having exemplary designs, the present disclosure may be further modified within the spirit and scope of this disclosure. This application is therefore intended to cover any variations, uses, or adaptations of the disclosure using its general principles. Further, this application is intended to cover such departures from the present disclosure as come within known or customary practice in the art to which this disclosure pertains.