Patent Publication Number: US-2022233203-A1

Title: Customized patient-specific contact segments for orthopaedic surgical instrument using bone silhouette curves

Description:
CROSS-REFERENCE TO RELATED APPLICATION 
     The present application is a divisional application of U.S. application Ser. No. 16/729,351, entitled “CUSTOMIZED PATIENT-SPECIFIC CONTACT SEGMENTS FOR ORTHOPAEDIC SURGICAL INSTRUMENT USING BONE SILHOUETTE CURVES,” which was filed on Dec. 28, 2019 and which issued on Apr. 19, 2022 as U.S. Pat. No. 11,304,710, which is incorporated herein by reference in its entirety. 
    
    
     TECHNICAL FIELD 
     The present disclosure relates generally to orthopaedic surgical instruments and, more particularly, to customized patient-specific orthopaedic surgical instruments. 
     BACKGROUND 
     Joint arthroplasty is a well-known surgical procedure by which a diseased and/or damaged natural joint is replaced by a prosthetic joint. For example, in a total knee arthroplasty surgical procedure, a patient&#39;s natural knee joint is partially or totally replaced by a prosthetic knee joint or knee prosthesis. A typical knee prosthesis includes a tibial tray, a femoral component, and a polymer insert or bearing positioned between the tibial tray and the femoral component. In a hip replacement surgical procedure, a patient&#39;s natural acetabulum is replaced by a prosthetic cup and a patient&#39;s natural femoral head is partially or totally replaced by a prosthetic stem and femoral ball. 
     To facilitate the replacement of the natural joint with a prosthesis, orthopaedic surgeons use a variety of orthopaedic surgical instruments such as, for example, cutting blocks, drill guides, milling guides, and other surgical instruments. Typically, the orthopaedic surgical instruments are reusable and generic with respect to the patient such that the same orthopaedic surgical instrument may be used on a number of different patients during similar orthopaedic surgical procedures. 
     The orthopaedic surgical instruments may also be customized to a specific patient. Such “customized patient-specific orthopaedic surgical instruments” are single-use surgical tools for use by a surgeon in performing an orthopaedic surgical procedure that is intended, and configured, for use on a particular patient. It should be appreciated that these instruments are distinct from standard, non-patient-specific orthopaedic surgical instruments that are intended for use on a variety of different patients. These customized patient-specific orthopaedic surgical instruments are distinct from orthopaedic prostheses, whether patient-specific or generic, which are surgically implanted in the body of the patient. Rather, customized patient-specific orthopaedic surgical instruments are used by an orthopaedic surgeon to assist in the implantation of orthopaedic prostheses. 
     SUMMARY 
     According to one aspect of the disclosure, a customized patient-specific surgical instrument includes a polymeric body including a bone-facing surface and an outer surface positioned opposite the bone-facing surface, a first bone-contacting segment coupled to the bone-facing surface and having a bone-contacting surface that is raised relative to the bone-facing surface, and a surgical guide defined by inner walls that extend from the outer surface to the bone-facing surface or to the bone-contacting surface of the bone-contacting segment. The bone-contacting surface defines a customized patient-specific first negative contour shaped to match and receive a corresponding first positive contour of the patient&#39;s bone. The first positive contour corresponds to a silhouette curve of a three-dimensional model of the patient&#39;s bone. The silhouette curve corresponds to a contour of the patient&#39;s bone captured in a first two-dimensional image that is used to generate the three-dimensional model. In an embodiment, the bone-facing surface is devoid of any negative contour shaped to match and receive a corresponding positive contour of the patient&#39;s bone. 
     In an embodiment, the surgical instrument further includes a second bone-contacting segment coupled to the bone-facing surface and having a bone-contacting surface that is raised relative to the bone-facing surface, wherein the bone-contacting surface of the second bone-contacting segment defines a customized patient-specific second negative contour shaped to match and receive a corresponding second positive contour of the patient&#39;s bone, wherein the second positive contour corresponds to a silhouette curve of the three-dimensional model that corresponds to a contour of the patient&#39;s bone captured in a second two-dimensional image that is used to generate the three-dimensional model. In an embodiment, the second bone-contacting segment is arranged generally perpendicular to the first bone-contacting segment. In an embodiment, the first two-dimensional image and the second two-dimensional image are captured in orthogonal imaging planes. 
     In an embodiment, the polymeric body includes a base and an elongated first arm coupled to the base, and the first bone-contacting segment extends from the base to a posterior end of the first arm. In an embodiment, the surgical instrument further includes a second bone-contacting segment coupled to the bone-facing surface and having a bone-contacting surface that is raised relative to the bone-facing surface, wherein the bone-contacting surface of the second bone-contacting segment defines a customized patient-specific second negative contour shaped to match and receive a corresponding second positive contour of the patient&#39;s bone, wherein the second positive contour corresponds to a silhouette curve of the three-dimensional model that corresponds to a contour of the patient&#39;s bone captured in a second two-dimensional image that is used to generate the three-dimensional model. The second bone-contacting segment extends generally perpendicular to the first bone-contacting segment from a lateral side of the first arm to a medial side of the first arm. 
     In an embodiment, the surgical instrument further includes a second bone-contacting segment coupled to the bone-facing surface, spaced apart from the first bone-contacting segment, and having a bone-contacting surface that is raised relative to the bone-facing surface, wherein the bone-contacting surface of the second bone-contacting segment defines a customized patient-specific second negative contour shaped to match and receive a corresponding second positive contour of the patient&#39;s bone and wherein the second positive contour corresponds to a silhouette curve of the three-dimensional model that corresponds to a contour of the patient&#39;s bone captured in the first two-dimensional image. The polymeric body further includes an elongated second arm coupled to the base, and the second bone-contacting segment extends from the base to a posterior end the second arm. The first bone-contacting segment is positioned on a medial side of the polymeric body and the second bone-contacting segment is positioned on a lateral side of the polymeric body. In an embodiment, the bone-facing surface is positioned between the first bone-contacting segment and the second bone-contacting segment, and the bone-facing surface is devoid of any negative contour shaped to match and receive a corresponding positive contour of the patient&#39;s bone. 
     In an embodiment, the surgical instrument further includes a third bone-contacting segment coupled to the bone-facing surface, spaced apart from the first bone-contacting segment and the second bone-contacting segment, and having a bone-contacting surface that is raised relative to the bone-facing surface, wherein the bone-contacting surface of the third bone-contacting segment defines a customized patient-specific third negative contour shaped to match and receive a corresponding third positive contour of the patient&#39;s bone, wherein the third positive contour corresponds to a silhouette curve of the three-dimensional model that corresponds to a contour of the patient&#39;s bone captured in the first two-dimensional image. The third bone-contacting segment is positioned between the first bone-contacting segment and the second bone-contacting segment. 
     In an embodiment, the surgical instrument further includes a fourth bone-contacting segment coupled to the bone-facing surface and having a bone-contacting surface that is raised relative to the bone-facing surface, wherein the bone-contacting surface of the fourth bone-contacting segment defines a customized patient-specific fourth negative contour shaped to match and receive a corresponding fourth positive contour of the patient&#39;s bone, wherein the fourth positive contour corresponds to a silhouette curve of the three-dimensional model that corresponds to a contour of the patient&#39;s bone captured in a second two-dimensional image, and a fifth bone-contacting segment coupled to the bone-facing surface and having a bone-contacting surface that is raised relative to the bone-facing surface, wherein the bone-contacting surface of the fifth bone-contacting segment defines a customized patient-specific fifth negative contour shaped to match and receive a corresponding fifth positive contour of the patient&#39;s bone, wherein the fifth positive contour corresponds to a silhouette curve of the three-dimensional model that corresponds to a contour of the patient&#39;s bone captured in the second two-dimensional image. The fourth bone-contacting segment extends generally perpendicular to the first bone-contacting segment from a lateral side of the first arm to a medial side of the first arm. The fifth bone-contacting segment extends generally perpendicular to the second bone-contacting segment from a lateral side of the second arm to a medial side of the second arm. 
     In an embodiment, the surgical guide includes a cutting slot defined by the inner walls. In an embodiment, the surgical guide includes a cylindrical bone-pin guide slot defined by the inner walls. 
     According to another aspect, a method for creating a patient-specific resection guide includes generating a three-dimensional model of a patient&#39;s bone based on a plurality of images, wherein each image views the patient&#39;s bone in a different imaging plane of a plurality of imaging planes, mapping the three-dimensional model to a plurality of silhouette curves, wherein each silhouette curve corresponds to a contour of the patient&#39;s bone captured in an image of the plurality of images, and determining a plurality of customized patient-specific negative contours based on the plurality of silhouette curves, wherein each customized patient-specific negative contour is shaped to match and receive a corresponding positive contour of the patient&#39;s bone, and wherein each positive contour corresponds to a silhouette curve of the plurality of silhouette curves. In an embodiment, the method further includes imaging the patient&#39;s bone in the plurality of imaging planes to generate the plurality of images. 
     In an embodiment, the method further includes generating a guide having a polymeric body including a bone-facing surface and an outer surface positioned opposite the bone-facing surface and a plurality of bone-contacting surfaces coupled to the bone-facing surface and raised relative to the bone-facing surface, wherein each bone-contacting surface defines a customized patient-specific negative contour of the plurality of patient-specific negative contours. In an embodiment, the bone-facing surface is devoid of any negative contour shaped to match and receive a corresponding positive contour of the patient&#39;s bone. In an embodiment, generating the guide includes molding the polymeric body. In an embodiment, generating the guide includes additively manufacturing the polymeric body. 
     In an embodiment, the polymeric body further includes a cutting guide defined by inner walls that extend from the outer surface to the bone-facing surface and to a bone-contacting surface of the bone-contacting segment. 
     In an embodiment, the method further includes attaching the plurality of bone-contacting surfaces of the guide to the patient&#39;s bone, wherein the negative contour of each bone-contacting surface contacts the patient&#39;s bone at a position of the corresponding positive contour of the patient&#39;s bone; and performing a surgical operation using the cutting guide in response to attaching the plurality of bone-contacting surfaces of the guide to the patient&#39;s bone. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       The detailed description particularly refers to the following figures, in which: 
         FIG. 1  is a posterior perspective view of a customized patient-specific surgical instrument; 
         FIG. 2  is an anterior perspective view of the customized patient-specific surgical instrument shown in  FIG. 1 ; 
         FIGS. 3A and 3B  are illustrations of medical images of a distal end of a patient&#39;s femur; 
         FIG. 4  is an illustration of a three-dimensional model of the patient&#39;s femur shown in  FIGS. 3A and 3B ; 
         FIG. 5  is a perspective view of the customized patient-specific surgical instrument shown in  FIGS. 1-2  positioned on the distal end of the patient&#39;s femur of  FIGS. 3A, 3B, and 4 ; and 
         FIG. 6  is a perspective view of another customized patient-specific surgical instrument positioned on the distal end of the patient&#39;s femur of  FIGS. 3A, 3B, and 4 . 
     
    
    
     DETAILED DESCRIPTION OF THE DRAWINGS 
     While the concepts of the present disclosure are susceptible to various modifications and alternative forms, specific exemplary embodiments thereof have been shown by way of example in the drawings and will herein be described in detail. It should be understood, however, that there is no intent to limit the concepts of the present disclosure to the particular forms disclosed, but on the contrary, the intention is to cover all modifications, equivalents, and alternatives falling within the spirit and scope of the invention as defined by the appended claims. 
     Terms representing anatomical references, such as anterior, posterior, medial, lateral, superior, inferior, etcetera, may be used throughout the specification in reference to the orthopaedic implants and surgical instruments described herein as well as in reference to the patient&#39;s natural anatomy. Such terms have well-understood meanings in both the study of anatomy and the field of orthopaedics. Use of such anatomical reference terms in the written description and claims is intended to be consistent with their well-understood meanings unless noted otherwise. 
     Referring now to  FIGS. 1 and 2 , an orthopaedic surgical instrument  10  is illustratively embodied as a customized patient-specific orthopaedic surgical instrument. What is meant herein by the term “customized patient-specific orthopaedic surgical instrument” is a surgical tool for use by a surgeon in performing an orthopaedic surgical procedure that is intended, and configured, for use on a particular patient. As such, it should be appreciated that, as used herein, the term “customized patient-specific orthopaedic surgical instrument” is distinct from standard, non-patient-specific orthopaedic surgical instruments (i.e., “patient-universal instruments” such as patient-universal cutting blocks) that are intended for use on a variety of different patients and were not fabricated or customized to any particular patient. Additionally, it should be appreciated that, as used herein, the term “customized patient-specific orthopaedic surgical instrument” is distinct from orthopaedic prostheses or implants, whether patient-specific or generic, which are surgically implanted in the body of the patient. Rather, an orthopaedic surgeon uses customized patient-specific orthopaedic surgical instruments to assist in the implantation of orthopaedic prostheses. Examples of “customized patient-specific orthopaedic surgical instruments” include customized patient-specific drill/pin guides, customized patient-specific tibial cutting blocks, customized patient-specific femoral cutting blocks, and customized patient-specific alignment guides. 
     The customized patient-specific orthopaedic surgical instrument  10  is a femoral cutting guide block in the illustrative embodiment. The anterior contact surfaces and the distal contact surfaces of the cutting block  10  facilitate securing the cutting block  10  on the patient&#39;s femur. As described in greater detail below, the cutting block  10  is configured to be coupled to the patient&#39;s femur in a unique pre-determined location and orientation on the patient&#39;s condyles and an anterior portion of the femur extending proximally from the condyles. The cutting block  10  includes a grid of raised contact segments that are configured to contact the patient&#39;s femur along predetermined contours that correspond to silhouette curves determined via three-dimensional modeling of the patient&#39;s femur. As discussed in more detail below, the silhouette curves correspond to parts of the three-dimensional model that have higher accuracy relative to the patient&#39;s anatomy. Thus, the femoral cutting block  10  may contact the patient&#39;s bone with higher accuracy as compared to typical techniques for generating surgical cutting guides. This improved accuracy may provide for more secure and accurate fixation of the cutting block  10  to the patient&#39;s bone, which in turn may allow the surgeon to produce more accurate resection angles. Further, although illustrated in the present disclosure as a femoral cutting guide block  10 , it should be understood that the concepts of this disclosure may also be applied to other customized patient-specific orthopaedic surgical instruments, including tibial cutting blocks, drill/pin guides, milling guides, or other surgical guides. 
     The femoral cutting block  10  includes a curved body  16  that includes a number of arms or lobes that extend outwardly from a center  18  of the body  16 . In the illustrative embodiment, the femoral cutting block  10  is a single monolithic component formed from a polymeric material, such as polyphenylsulfone (PPSU), polyethylene, or another plastic material. In that way, the body  16  and its arms form a single monolithic polymeric body. It should be appreciated that in some embodiments, the femoral cutting block  10  may be fabricated using one or more forms of additive manufacturing technology such as, for example, resin printing, optical fabrication, photo-solidification, or Direct Metal Laser Sintering (DMLS). Although illustratively formed from polymeric material, it should be understood that in some embodiments, the femoral cutting block  10  may be formed from metallic material such as, for example, stainless steel. 
     The body  16  includes a pair of condyle arms  22 ,  24  that are configured to engage the distal end  14  of the condyles of the patient&#39;s femur  12  (see  FIG. 5 ). The arms  22 ,  24  are spaced apart from each other such that a notch  26  is defined between the inner edges of the arms  22 ,  24 . The notch  26  is sized and shaped to correspond to the natural intercondylar notch  28  of the patient&#39;s femur  12 , which is defined between the natural condyles  30 ,  32  of the patient&#39;s femur  12  (see  FIG. 5 ). The body  16  also includes a proximally extending lobe  40  that is configured to engage the anterior side of the distal end  14  of the patient&#39;s femur  12 . Together, the arms  22 ,  24  and the lobe  40  form a concave body that faces the condyles  30 ,  32  and an anterior portion of the femur  12  extending proximally from the condyles  30 ,  32 . 
     The cutting block  10  further includes a bone-facing surface  34  and an outer surface  38  that is positioned opposite the corresponding bone-facing surface  34 . In the illustrative embodiment, each surface  34 ,  38  is substantially smooth. As used herein, the term “substantially” should be understood to refer to the normal tolerances created by manufacturing variation and other design criteria. As such, a “substantially smooth surface” is one that is smooth within the normal tolerances created or permitted by manufacturing variation and other design criteria. 
     The cutting block  10  further includes a grid of elongated plateaus, rails, or other contact segments  36  formed in the body  16  and raised relative to the bone-facing surface  34 . As illustrated, the arm  22  includes an elongated segment  44  that extends from the center  18  of the body  16  to a posterior end  46  of the arm  22 , and an elongated segment  48  that extends from an outer side  50  of the arm  22  to the notch  26  such that the segments  44 ,  48  run generally perpendicular to each other. Similarly, the arm  24  includes an elongated segment  52  that extends from the center  18  of the body  16  to a posterior end  54  of the arm  24 , and an elongated segment  56  that extends from an outer side  58  of the arm  24  to the notch  26  such that the segments  52 ,  56  run generally perpendicular to each other. The lobe  40  includes elongated segments  60 ,  62 ,  64  that each extend from the center  18  of the body  16  to the proximal end  66  of the lobe  40 . Each of the segments  60 ,  62 ,  64  defines a separate bone-contacting surface  42 . As shown, the segments  60 ,  62 ,  64  are separated by sections of the bone-facing surface  34 . 
     The segments  36  of the body  16  each include a bone-contacting surface  42  that is configured to engage part of the patient&#39;s femur  12  (as illustrated in  FIG. 5 ). Each of the bone-contacting surfaces  42  are raised relative to the bone-facing surface  34  such that only the bone-contacting surfaces  42  contact the patient&#39;s femur  12  when the cutting block  10  is positioned on the patient&#39;s femur  12 . Each bone-contacting surface  42  defines one or more negative contours that are configured to contact the patient&#39;s femur  12  along a corresponding predetermined positive contour of the femur  12 . Those positive contours include or otherwise correspond to silhouette curves that are determined based on a three-dimensional model of the patient&#39;s femur  12 , as described further below in connection with  FIGS. 3A, 3B, 4-5 . Thus, the bone-contacting surface  42  is configured to engage the patient&#39;s femur  12  at a unique predetermined location and orientation. Note that the bone-facing surface  34 , which does not contact the patient&#39;s femur  12 , does not include a negative contour corresponding to a positive contour of the patient&#39;s bone. Thus, the bone-facing surface  34  may have a non-patient-specific shape. 
     As shown, the bone-contacting surface  42  of each of the segments  44 ,  48 ,  52 ,  56 ,  70 ,  72 ,  74  defines a negative contour that corresponds to a respective positive contour of the patient&#39;s femur  12 . It should be understood that in other embodiments the cutting block  10  may include a different number and/or arrangement of elongated segments  36 , based on the number and/or arrangement of the corresponding positive contours of the patient&#39;s femur  12 . Additionally, although the segments  44 ,  48 ,  52 ,  56 ,  70 ,  72 ,  74  are illustrated as extending from end-to-end and side-to-side of the cutting block  10 , it should be understood that in some embodiments one or more of the segments may not extend entirely from end-to-end or side-to-side. For example, in some embodiments, the bone-contacting surfaces  42  may be defined by one or more islands raised relative to and surrounded by the bone-facing surface  34 . 
     As shown, the cutting block  10  includes a number of surgical tool guides  20  that are each defined by inner walls that extend from the outer surface  38  to the bone-facing surface  34  and/or to the bone-contacting surface  42 . As described further below, the surgical tool guides  20  may include cutting guides as well as drilling/fixation pin guides. It should be understood that in other embodiments, the surgical tool guides  20  may additionally or alternatively include milling guides and/or other surgical guides. 
     As shown in  FIG. 2 , the body  16  includes a flange  68  that extends anteriorly from the center  18  of the body  16  to a free end  70  that is spaced apart from the body  16 . The flange  68  includes an elongated opening  72  that is defined in the free end  70  and a number of inner walls  74  that extend inwardly from the opening  72 . Returning to  FIG. 1 , the inner walls  74  extend to another opening  76  that is defined in the bone-facing surface  34  and the bone-contacting surfaces  42 . The opening  76  cooperates with the inner walls  74  and the elongated opening  72  to define a cutting guide slot  78 , which is sized and shaped to guide a surgical tool such as, for example, a surgical saw or other cutting blade, into engagement with the patient&#39;s bone. The cutting guide slot  78  is positioned to guide a customized, patient-specific resection of the distal end  14  of the patient&#39;s femur  12 . 
     As shown in  FIG. 2 , a pair of openings  80 ,  82  are defined in the outer surface  38  of the proximal lobe  40 . An inner wall  84 ,  86  extends inwardly from each respective opening  80 ,  82 . As shown in  FIG. 1 , the inner wall  84  extends to another opening  88  in the bone-contacting surface  42  of the segment  64  to define a guide slot  90  extending through the cutting block  10 . Similarly, the inner wall  86  extends to another opening  92  in the bone-facing surface  34  to define a guide slot  94  extending through the cutting block  10 . In the illustrative embodiment, each guide slot  90 ,  94  is a drill guide and fixation pin guide hole, which is sized and shaped to guide a surgical drill or self-drilling fixation pin to prepare the patient&#39;s bone to receive a fixation pin to couple the cutting block  10  to the bone. 
     As shown in  FIG. 1 , an opening  96  is defined in the bone-contacting surface  42  of the segments  44 ,  48  of the arm  22 . An inner wall  98  extends outwardly from the opening  96  to another opening in the outer surface  38  of the arm  22  to define a guide slot  100  through the cutting block  10 . Similarly, an opening  102  is defined in the bone-contacting surface  42  of the segment  52  of the arm  24 . An inner wall  104  extends outwardly from the opening  102  to another opening in the outer surface  38  of the arm  24  to define a guide slot  106  through the cutting block  10 . In the illustrative embodiment, each guide slot  100 ,  106  is also a drill guide and fixation pin guide hole, which is sized and shaped to guide a surgical drill or self-drilling fixation pin to prepare the patient&#39;s bone to receive a fixation pin to couple the cutting block  10  to the bone. 
     Prior to surgery, a three-dimensional model of the patient&#39;s femur  12  is developed based on imaging of the patient&#39;s femur  12 . To generate the three dimensional model, a number of medical images of the relevant bony anatomy or joint of the patient are generated. To do so, the orthopaedic surgeon or other healthcare provider may operate an imaging system to generate the medical images. The medical images may be embodied as any number and type of medical images capable of being used to generate a three-dimensional rendered model of the patient&#39;s bony anatomy or relevant joint. For example, the medical images may be embodied as a number of X-ray images or other two-dimensional images from which a three-dimensional rendered model of the patient&#39;s relevant bony anatomy may be generated. Additionally, in some embodiments, the medical image may be enhanced with a contrast agent designed to highlight the cartilage surface of the patient&#39;s joint. Additionally or alternatively, the medical images may be embodied as any number of computed tomography (CT) images, magnetic resonance imaging (MRI) images, or other three-dimensional medical images. 
     Referring now to  FIGS. 3A and 3B , illustrative medical images  300 ,  302  are shown. The images  300 ,  302  are x-ray images of the patient&#39;s knee joint, including the femur  12 . The image  300  is an anteroposterior view of the knee joint, and the image  302  is a lateral view of the knee joint. Thus, the images  300 ,  302  represent views of the femur  12  in different imaging planes that are substantially orthogonal to each other. Additionally, although illustrated as two x-ray images  300 ,  302 , it should be understood that in some embodiments, the medical images may include a different number and/or type of x-ray images, magnetic resonance images, or other scans of the patient&#39;s femur  12 . 
     After generating or otherwise receiving the medical images, a three-dimensional model of the patient&#39;s femur  12  is generated based on the medical images. In particular, a computing device or other modeling system may perform an x-ray segmentation process to model the patient&#39;s bone based on the input x-ray images. The computing device receives a set of x-ray images (e.g., the images  300 ,  302 , a set of three x-ray images, or other images). The computing device accesses a bone library that includes models or other measurements of many sample bones. The computing device generates a three-dimensional model based on the bone library and then morphs (interpolates) that model to match the patient&#39;s specific geometry represented in the medical images. 
     Referring now to  FIG. 4 , a visual representation of an illustrative three-dimensional model  400  of the patient&#39;s femur  12  is shown. As shown, the model  400  represents the femur  12  that was imaged in the medical images  300 ,  302 . The model  400  may represent a best match to the patient&#39;s specific geometry determined using an interpolation process as described above. 
     After generating the three-dimensional model (e.g., the model  400  of  FIG. 4 ), the computing device maps the three-dimensional model onto a number of silhouette curves. To map the silhouette curves, the computing device may map the three-dimensional model onto a number of two-dimensional projections or other curves, with each curve corresponding to a contour of the patient&#39;s bone captured in a particular input medical image (and thus in a particular imaging plane). The computing device may use any appropriate algorithm for mapping the three-dimensional model to the silhouette curves. For example, the computing device may map the silhouette curves using a ray tracing algorithm (e.g., tracing rays from the viewpoint of an x-ray source, identifying polygons in the model that intersect a ray, identifying intersecting polygons that are adjacent to each other and are arranged in opposing orientations, and determining common edges between the intersecting, adjacent, opposing polygons). 
     As described above, each silhouette curve corresponds to an outline of the patient&#39;s bone represented in the three-dimensional model that corresponds to a particular input medical image. Thus, the silhouette curves may match the patient&#39;s bone geometry with higher accuracy as compared to other parts of the three-dimensional model. For example, because the silhouette curves map directly to bone features shown in the plane of the input medical images, the silhouette curves may be more accurate than other parts of the three-dimensional model that are determined through interpolation. 
     For example, and still referring to  FIG. 4 , a number of silhouette curves  402  are shown mapped in their corresponding positions on the model  400 . As described above, each silhouette curve  402  corresponds to an outline of the patient&#39;s bone as represented in the three-dimensional model that corresponds to a particular input image  300 ,  302 . For example, a silhouette curve  404  follows the outline of the condyle  32  as viewed in the imaging plane of the image  302 . Continuing that example, a silhouette curve  406  follows the outline of the condyle  30  as viewed in the imaging plane of the image  302 , and a silhouette curve  408  follows the outline of the trochlear groove between the condyles  30 ,  32  as viewed in the imaging plane of the image  302 . A silhouette curve  410  follows the outline of the condyles  30 ,  32  as viewed in the imaging plane of the image  300 . Note that the silhouette curves  404 ,  406 ,  408  intersect with and are roughly perpendicular to the silhouette curve  410 . 
     After generating the three-dimensional model and mapping the silhouette curves, a cutting block  10  is manufactured to include bone-contacting surfaces  42  that define negative contours that match the positive contours of the three-dimensional model at the silhouette curves (e.g., the silhouette curves  402  of the illustrative model  400 ). For example, in the illustrative embodiment the negative contours defined by the bone-contacting surfaces  42  of the segments  44 ,  60  match the silhouette curve  404 , the negative contours defined by the bone-contacting surfaces  42  of the segments  52 ,  64  match the silhouette curve  406 , the negative contour defined by the bone-contacting surface  42  of the segment  62  matches the silhouette curve  408 , and the negative contours defined by the bone-contacting surfaces of the segments  48 ,  56  match the silhouette curve  410 . As described above, the bone-facing surface  34  does not include any negative contours that match positive contours of the femur  12 . Accordingly, because the bone-facing surface  34  need not be patient-specific, and thus manufacturing of the cutting block  10  may be simplified as compared to manufacturing the entire bone-facing surface as patient-specific. 
     Referring now to  FIG. 5 , during use, the orthopaedic surgeon prepares the patient&#39;s femur  12  by positioning the cutting block  10  on the distal end  14  of the patient&#39;s femur  12 . The negative contours defined by the bone-contacting surfaces  42  engage the matching positive contours of the patient&#39;s femur  12  that correspond to the silhouette curves  402  as described above. For example, as is visible in  FIG. 5 , the segment  56  contacts the patient&#39;s femur  12  along the silhouette curve  410 , the segment  64  contacts the patient&#39;s femur  12  along the silhouette curve  406 , and the segment  62  contacts the patient&#39;s femur  12  along the silhouette curve  408 . Of course, the other segments  44 ,  48 ,  52 ,  60 , of the cutting block  10  also contact the patient&#39;s femur  12  along a corresponding silhouette curve  402  as described above in connection with  FIG. 4 . As shown in  FIG. 5 , when the bone-contacting surfaces  42  engage the patient&#39;s femur  12 , the bone-facing surface  34  is positioned apart from and does not contact the patient&#39;s femur  12 . 
     After positioning the cutting block  10  on the femur  12 , the surgeon can then position a fixation pin in each of the guide slots  90 ,  94 ,  100 ,  106  to secure the cutting block  10  to the patient&#39;s femur  12 . A distal resection is then performed on the distal end  14  of the patient&#39;s femur  12  by advancing a surgical saw through the guide slot  78 . In some embodiments, the fixation pins inserted through the guide slots  100 ,  106  may be removed before the distal resection of the distal end  14  of the patient&#39;s femur  12  so that the fixation pins do not interfere with the surgical saw. 
     Referring now to  FIG. 6 , another embodiment of an orthopaedic surgical instrument  600  is shown. The instrument  600  is illustratively a customized patient-specific orthopaedic surgical instrument. The customized patient-specific orthopaedic surgical instrument is a femoral cutting guide block  600  in the illustrative embodiment, similar to the cutting block  10  of  FIGS. 1-2, 5 . 
     The femoral cutting block  600  includes a body  616  that has a number of arms that extend outwardly from a center  618  of the body  616 . In the illustrative embodiment, the femoral cutting block  600  is a single monolithic component formed from a polymeric material, such as polyphenylsulfone (PPSU), polyethylene, or another plastic material. In that way, the body  616  and the arms form a single monolithic polymeric body. It should be appreciated that in some embodiments, the femoral cutting block  600  may be fabricated using one or more forms of additive manufacturing technology such as, for example, resin printing, optical fabrication, photo-solidification, or Direct Metal Laser Sintering (DMLS). Although illustratively formed from polymeric material, it should be understood that in some embodiments, the femoral cutting block  600  may be formed from metallic material such as, for example, stainless steel. 
     The body  616  includes a pair of condyle arms  622 ,  624  that are configured to engage the distal end  14  of the condyles of the patient&#39;s femur  12 . The arms  622 ,  624  are spaced apart from each other such that a notch  626  is defined between the inner edges of the arms  622 ,  624 . The notch  626  is sized and shaped to correspond to the natural intercondylar notch  28  of the patient&#39;s femur  12 , which is defined between the natural condyles  30 ,  32  of the patient&#39;s femur  12 . The body  616  also includes a proximally extending arms  640 ,  641  that are configured to engage the anterior side of the distal end  14  of the patient&#39;s femur  12 . Together, the arms  622 ,  624 ,  640 ,  641  form a concave body that faces the condyles  30 ,  32  and an anterior portion of the femur  12  extending proximally from the condyles  30 ,  32 . 
     The cutting block  600  further includes a bone-contacting surface  642  an outer surface  638  that is positioned opposite the corresponding bone-contacting surface  642 . In the illustrative embodiment, the surface  638  is substantially smooth. As used herein, the term “substantially” should be understood to refer to the normal tolerances created by manufacturing variation and other design criteria. As such, a “substantially smooth surface” is one that is smooth within the normal tolerances created or permitted by manufacturing variation and other design criteria. 
     The bone-contacting surface  642  defines one or more negative contours that are configured to engage parts of the patient&#39;s femur  12  as shown in  FIG. 6 . Similar to the bone-contacting surfaces  42  of  FIGS. 1-5 , the bone-contacting surface  642  is configured to contact the patient&#39;s femur  12  along one or more predetermined positive contours of the femur  12 . Those positive contours include or otherwise correspond to silhouette curves that are determined based on a three-dimensional model of the patient&#39;s femur  12  as described above in connection with  FIGS. 3A, 3B, 4 . Thus, the bone-contacting surface  642  is configured to engage the patient&#39;s femur  12  at a unique predetermined location and orientation. 
     For example, as shown in  FIG. 6 , the bone-contacting surface  642  defined by the arm  624  contacts the condyle  30  at positive contours that corresponds to the silhouette curves  406 ,  410 , and the bone-contacting surface  642  defined by the arm  622  contacts the condyle  32  at positive contours that corresponds to the silhouette curves  404 ,  410 . The bone-contacting surface  642  defined by the arm  640  contacts the trochlear groove between the condyles  30 ,  32  at a positive contour that corresponds to the silhouette curve  408 . The bone-contacting surface  642  defined by the arm  641  contacts the condyle  30  at a positive contour that corresponds to the silhouette curve  406 . 
     As shown, the arms  622 ,  624  and  640 ,  641  are spaced apart such that the bone-contacting surface  642  does not contact the patient&#39;s femur at locations that do not include a silhouette curve  402 . For example, a gap  644  is defined between the arms  640 ,  641  such that the bone-contacting surface  642  does not contact the patient&#39;s femur  12  between the silhouette curves  406 ,  408 . Thus, by contacting the bone only at locations that include a silhouette curve  402 , the cutting block  600  may bridge over areas of the bone with higher potential for the formation of osteophytes or other areas of the bone that are difficult to map accurately in the three-dimensional model. Additionally, by including the gap  644  and otherwise contacting the bone only at locations that include a silhouette curve  402 , the cutting block  600  may allow the surgeon to fully visualize the bone contact, for example to determine if the cutting block  600  is being lifted out of position by an osteophyte. If so, the surgeon may cut away or grind off the osteophyte for more accurate placement of the cutting block  600 . Thus, the cutting block  600  may provide more stability and/or improved contact to the bone as compared to conventional cutting guides. 
     In some embodiments, the bone-contacting surface  642  may be raised relative to a bone-facing surface  34 . In those embodiments, the bone-contacting surfaces  642  may be separated from each other by the bone-facing surface  34 , similar to the cutting block  10  of  FIGS. 1-2 . Thus, in those embodiments, the bone-facing surface  34  does not contact the patient&#39;s bone and does not include a negative contour corresponding to a positive contour of the patient&#39;s bone. 
     As shown, the cutting block  600  includes a number of surgical tool guides  620  that are each defined by inner walls that extend from the outer surface  638  toward the bone-contacting surface  642 . As described further below, the surgical tool guides  620  may include cutting guides as well as drilling/fixation pin guides. It should be understood that in other embodiments, the surgical tool guides  620  may additionally or alternatively include milling guides and/or other surgical guides. 
     The body  616  includes a flange  668  that extends anteriorly from the body  616  to a free end  670  that is spaced apart from the body  616 . The flange  668  includes an elongated opening  672  that is defined in the free end  670  and a number of inner walls  674  that extend inwardly from the opening  672 . The inner walls  674  extend to another opening defined in the bone-contacting surface  642  and/or the bone-facing surface  34 . That opening cooperates with the inner walls  674  and the elongated opening  672  to define a cutting guide slot  678 , which is sized and shaped to guide a surgical tool such as, for example, a surgical saw or other cutting blade, into engagement with the patient&#39;s bone. The cutting guide slot  678  is positioned to guide a customized, patient-specific resection of the distal end  14  of the patient&#39;s femur  12 . 
     Each arm  622 ,  624  includes a guide boss  680  that is attached to, and extends distally from, the outer surface  638  of the arms  622 ,  624 , respectively. Each guide boss  680  includes a guide slot  682  that is sized and shaped to guide a surgical drill and a fixation pin into engagement with the patient&#39;s bone to couple the cutting block  600  to the bone. Each guide boss  680  includes a post  684  that extends from a base  686  attached to the outer surface  38  of one of the arms  622 ,  624  to a free end  688  that is spaced apart from the outer surface  638 . 
     An opening is defined in the free end  688  of each boss  680 . An inner wall  690  extends inwardly from the opening to another opening that is defined in a bone-contacting surface  642  of the respective arm  622 ,  624 . Those openings and the inner wall  690  cooperate to define the guide slot  682 . As described above, each guide slot  682  is a drill guide and fixation pin guide hole, which is sized and shaped to guide a surgical drill or self-drilling fixation pin to prepare the patient&#39;s bone to receive a fixation pin to couple the cutting block  10  to the bone. 
     As shown, the arm  641  includes an anterior guide boss  692  that is attached to, and extends anteriorly from the outer surface  638  of the arm  641 . A bracket  694  is coupled to the guide boss  692  and extends laterally to another anterior guide boss  696 . The bracket  694  is also coupled to the outer surface  638  of the arm  640 . The bracket  694  does not include a bone-contacting surface  642 . 
     Similar to the guide bosses  680 , each of the guide bosses  692 ,  696  includes a guide slot  682  that is sized and shaped to guide a surgical drill and a fixation pin into engagement with the patient&#39;s bone to couple the cutting block  600  to the bone. Each guide boss  692 ,  696  includes a post  684  that extends from a base  686  to a free end  688  that is spaced apart from the outer surface  638 . The base  686  of the boss  692  is attached to the outer surface  638  of the arm  641 , and the base  686  of the boss  696  is attached to the bracket  694 . 
     As with the bosses  680 , an opening is defined in the free end  688  of each boss  692 ,  696 . An inner wall  690  extends inwardly from the opening to another opening that is defined in the base  686  of the respective boss  692 ,  696 . Those openings and the inner wall  690  cooperate to define the guide slots  682 . As described above, each guide slot  682  is a drill guide and fixation pin guide hole, which is sized and shaped to guide a surgical drill or self-drilling fixation pin to prepare the patient&#39;s bone to receive a fixation pin to couple to the cutting block  10  to the bone. 
     While the disclosure has been illustrated and described in detail in the drawings and foregoing description, such an illustration and description is to be considered as exemplary and not restrictive in character, it being understood that only illustrative embodiments have been shown and described and that all changes and modifications that come within the spirit of the disclosure are desired to be protected. 
     There are a plurality of advantages of the present disclosure arising from the various features of the devices and assemblies described herein. It will be noted that alternative embodiments of the devices and assemblies of the present disclosure may not include all of the features described yet still benefit from at least some of the advantages of such features. Those of ordinary skill in the art may readily devise their own implementations of the devices and assemblies that incorporate one or more of the features of the present invention and fall within the spirit and scope of the present disclosure as defined by the appended claims.