Patent Publication Number: US-10758360-B2

Title: Tibial implant having an anatomic stem

Description:
CROSS-REFERENCE TO RELATED APPLICATION 
     This application is a divisional of pending U.S. patent application Ser. No. 14/386,152, filed Sep. 18, 2014, entitled “Tibial Implant Having an Anatomic Stem”, which is a United States National Stage filing under 35 U.S.C. § 371 of International Application No. PCT/US2013/032115, filed on Mar. 15, 2013, which claims the benefit of U.S. Provisional Patent Application No. 61/613,733, filed Mar. 21, 2012, each of which is hereby incorporated by reference herein in its entirety. International Application No. PCT/US2013/032115 was published under PCT Article 21(2) in English. 
    
    
     BACKGROUND 
     Knee implants are often used to replace knees that become disabled or cause a patient pain due to normal wear of the bone or due to degenerative disease. A total knee replacement involves replacement of the native bone with at least two components: a femoral component to replace the distal end of the femur, and a tibial component to replace the proximal end of the tibia. The femoral and tibial components are positioned and designed to mimic the native bone and provide an articulation interface that allows normal anatomic movement of the knee joint following implantation surgery. 
     A standard tibial implant includes two main portions—a tray and a stem. The proximal portion of the tibial implant is a tray that forms the articulation interface with a femoral component. Often, the tray holds a liner made of a compliant material, such as polyethylene, that provides a smooth surface for articulation with the femoral component. The distal portion of the tibial implant forms a stem that is designed to extend down into a tibia into which the component is implanted. In order to provide normal anatomic movement of the knee implant, the tibial implant must be held firmly in place to prevent the implant from moving down further into the bone and also from rotating in place within the bone. 
     In order to resist movement of the tibial implant relative to the bone, the implant must form a strong interface with the bone into which it is being implanted. During implantation, the interface between the implant and the bone is first formed by an initial fixation, for example with bone screws or cement, when the tibial component is first placed into the bone. The initial fixation is sometimes supplemented by later ingrowth of the surrounding bone into the surfaces of the implant if the implant includes textured ingrowth surfaces. This ingrowth can provide some resistance to rotation and subsidence of the tibial component down into the bone. However, the cancellous bone that grows into the implant surface is soft, spongy bone that is able to resist only small forces. A substantial force applied to the implant can break the interface of the bone and the ingrowth surface, leading to subsidence and rotation of the tibial tray. 
     In some current approaches, the stem of a tibial implant is placed into the cancellous bone of a patient&#39;s tibia, and the areas between the implanted component and the hard cortical bone are filled with cement. While this cement provides a connection between the implant and the stronger cortical bone, the cement is often not strong enough to provide the initial and long term fixation and resistance against subsidence and rotation that is needed to resist high forces that can occur during normal use of a total knee arthroplasty. As with cancellous bone ingrowth, the cement interface can be broken by these forces, and the implant function can be compromised by the effects of stress shielding and subsidence. 
     SUMMARY 
     Disclosed herein are systems, devices and methods for providing a tibial implant that includes an anatomic stem that forms a geometrically defined interface between the implant and the cortical bone of a patient&#39;s tibia. The systems, devices and methods also provide tibial implants that contact and transmit forces incident on the implant to surrounding bone. The interface provides physical presence in areas and regions of the tibia bone that support improved fixation on the implant while still transferring adequate stress to both the cancellous and cortical bone to ensure that the bone remains strong. Thus, the interface between the implant and the cortical bone can help reduce the occurrence of stress shielding and subsidence into the tibial bone while also resisting rotation of the tibial implant in the bone. 
     According to some embodiments, a tibial implant includes a tray configured to abut a patient&#39;s bone and a stem extending from a surface of the tray. The stem has a proximal portion at which a first cross-section of the stem has a first shape and a first center positioned at a first location relative to the tray, and a distal portion at which a second cross-section of the stem has a second shape and a second center positioned at a second location relative to the tray. 
     In certain implementations, at least one of the first and second cross-sections includes a corner configured to engage the patient&#39;s bone, for example an interior surface of the patient&#39;s cortical bone. In certain implementations, the first cross-section includes a first corner that extends at a first angle relative to the longitudinal axis of the stem, and the second cross-section includes a second corner that extends at a second angle relative to the longitudinal axis of the stem. The first and second angles correspond to an anatomic landmark at each of the first and second cross-sections. 
     In certain implementations, the center of the second cross-section is located anterior relative to the center of the first cross-section. In certain implementations, the center of the second cross-section is located medial relative to the center of the first cross-section. In certain implementations, the center of the second cross-section is located posterior relative to the center of the first cross-section. In certain implementations, the second cross-section has a smaller area than the first cross-section. 
     In certain implementations, the implant includes a fin extending outward from the stem. The fin extends from an inferior surface of the tray, and an inferior perimeter of the fin includes a tapping mechanism configured to cut into the patient&#39;s bone. The implant also includes a plurality of engagement portions extending from the fin. The fin extends from the stem at a first angle relative to a longitudinal axis of the stem, and each of the plurality of engagement portions extends from the fin at a second angle perpendicular to the first angle. The engagement portions include at least one of circular projections, triangular projections, square projections, and sawtooth projections. An outer portion is shaped to engage the patient&#39;s bone, and the outer portion of the fin may include a cloverleaf shape or a hook shape. 
     In certain implementations, the implant includes first and second cross-sections that include corners that are offset relative each other based on second cross-sections of a cortical bone. A bounding box may be defined by four line segments along the peripheries of the first and second cross-sections, and the line segments may be connected by four or more radial segments. The first and second cross-sections may have an aspect ratio between approximately 1.0 and approximately 2.0. The long axis of the second cross-section may be rotated relative to a long axis of the first cross section. The second cross section may be rotated less than 70.degree. relative to the first cross section. In certain implementations, the first and second cross-sections of the implant are optimized to target both maximum and minimum bone sizes while maintaining a defined offset to cortical bone. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       The foregoing and other objects and advantages will be appreciated more fully from the following further description thereof. With reference to the accompanying drawings, these depicted embodiments are to be understood as illustrative and not as limiting in any way. 
         FIG. 1  shows a front view of an illustrative tibial implant having an anatomic stem; 
         FIG. 2  shows an anterior view of an illustrative tibia bone; 
         FIG. 3  shows an illustrative longitudinal cross-section of a proximal portion of the tibia bone shown in  FIG. 2 ; 
         FIGS. 4A and 4B  show illustrative transverse cross-sections of the tibia bone shown in  FIG. 2 ; 
         FIG. 4C  shows an overlay of the cross-sections shown in  FIGS. 4A and 4B ; 
         FIG. 5  shows the tibial implant of  FIG. 1  implanted into the tibial bone of  FIG. 2 ; 
         FIGS. 6A and 6B  show illustrative transverse cross-sections of the implant and bone shown in  FIG. 5 ; 
         FIG. 6C  shows an overlay of the cross-sections shown in  FIGS. 6A and 6B ; 
         FIG. 7  shows an illustrative tibial implant having an anatomic stem and fin extensions; 
         FIGS. 8A and 8B  show bottom views of illustrative tibial implants having an anatomic stem and fin extensions; 
         FIG. 9  shows aspect ratio data for tibial intramedullary cavities; and 
         FIG. 10  shows a wireframe model of cross-sections of a tibial intramedullary cavity. 
     
    
    
     DETAILED DESCRIPTION 
     To provide an overall understanding of the systems, devices and methods described herein, certain illustrative embodiments will now be described. For the purpose of clarity and illustration these systems and methods will be described with respect to orthopedic tibial implants. It will be understood by one of ordinary skill in the art that the systems, devices and methods described herein may be adapted and modified as is appropriate, and that these systems, devices and methods may be employed in other suitable applications, such as for other types of joints and orthopedic implants, and that other such additions and modifications will not depart from the scope hereof. 
       FIG. 1  shows a front view of a tibial implant  100  having a base plate  102  and an anatomic stem  104 . In the depicted embodiment, the tibial implant  100  having the base plate  102  and the anatomic stem  104  is monolithic. The implant  100  is configured to be implanted into a patient&#39;s tibia with the base plate  102  disposed at the proximal end of the tibia. In this position, the top surface  103  of the base plate  102  forms an articulation interface with a patient&#39;s femoral bone or an implant placed into the native femoral bone. A liner made of a compliant material, such as polyethylene, is coupled with the top surface  103  to provide a smooth articulation surface. The bottom surface  105  of the base plate  102  forms an interface with the tibial bone into which the implant  100  is implanted. The bottom surface  105  contacts a proximal surface of the tibial bone and is affixed to the bone by cement, bone screws, growth of the tibial bone into a textured surface, or a combination thereof. 
     The anatomic stem  104  of the implant  100  extends from the bottom surface  105  and is designed to form a geometrically defined interface with the tibial bone into which it is implanted. In particular, the stem is designed to substantially extend longitudinally through the center of the tibial intramedullary cavity, transmit longitudinal and radial forces into surrounding bone, preferably in a way that disperses the forces equally, and maintain a close interface with the cortical bone. The combination of these features helps strengthen the bone to reduce stress shielding effects and subsidence of the base plate  102  further down into the tibia, and helps resist rotation of the implant  100  when torsional forces are applied to the implant  100 . 
     A standard tibia bone exhibits a changing anatomy as it extends distally from the proximal end of the bone. In particular, the size, center, and shape of the inner intramedullary cavity, formed of spongy cancellous bone, vary throughout the tibia. In order to maintain a close contact with the bone, the anatomic stem  104  is varied from the bottom surface  105  of the base plate to the tip  112  of the stem to accommodate the changing anatomy. For example, an upper portion  108  of the stem  104  is tapered from proximal level  115  to distal level  117 , and a lateral side  107  of the upper portion tapers at an angle θ 1  that is greater than the angle θ 2  at which a medial side  109  of the upper portion tapers. This taper shifts the center of the stem  104  longitudinally along the stem to maintain the alignment of the stem  104  with the changing position of the center of the intramedullary cavity. In particular, at the proximal level  115 , the center is center point  111 , and at the distal level  117 , the center is at center point  113 , which is shifted medially from center point  111 . The stem  104  also has a rounded corner  106  that shifts laterally relative to the longitudinal axis of the stem from the upper portion  108  to the lower portion  110  of the stem  104  in order to accommodate the anatomy of the anterior portion of the tibia to maintain a close contact with the hard outer cortical bone of the tibia. While the stem  104  in  FIG. 1  shows a lateral side having a sharper taper angle θ 1  than the angle θ 2  on the medial side, the taper angles of the stem may vary to accommodate varying patient tibial anatomies. In other implementations, the stem  104  may taper at approximately the same angle on both medial and lateral sides. As a result, the center points  111  and  113  may be co-located and not shifted relative to each other, or may shift only in an anterior or a posterior direction. Alternatively, the stem  104  may taper on the medial side at an angle θ 2  that is sharper than angle θ 1  on the lateral side. As a result, the center of the stem will shift in a lateral direction moving distally down the stem. 
       FIG. 2  shows a front view of a tibial bone  120  and its anatomical variations that the stem  104  is designed to accommodate. The bone  120  has a proximal end  122  and a distal end  124 . The anatomy of the bone  120  and, in particular, the shape and location of the center of the intramedullary cavity of the bone  120  may vary from the proximal end  122  to the distal end  124 . The bone  120  narrows from the proximal end  122  as the lateral side  126  and medial side  128  of the bone taper toward the center of the bone  120 . As can be seen in  FIG. 2 , the lateral side of the bone  126  tapers at an angle θ 3  toward the center of the bone  120 , which is sharper than the angle θ 4  at which the medial side  128  tapers. As a result, the center of the bone and the intramedullary cavity inside the bone shift in a medial direction. The movement of the intramedullary cavity is shown by the center axes  130  and  132 , taken at different levels of the bone  120  shown by lines AA and BB. The center axis  130  passes through proximal center point  121  at proximal line AA, while the axis  132  passes through the distal center point  123  of the bone  120  at distal line BB. As shown in  FIG. 2 , the center axis moves slightly towards the medial side of the bone as a result of the sharper tapering angle of the lateral side of the bone  120 . While the center of the cavity shifts medially in bone  120 , patient tibial anatomies may vary naturally or due to certain medical conditions and thus the shift of the cavity may vary across a population. In other bones, the tibia may taper at approximately the same angle on medial and lateral sides, or the medial side may taper at a sharper angle than the lateral side. Additionally, the intramedullary cavity may exhibit a different change than the outer cortical bone. Regardless of the tapering of the medial and lateral sides of the tibial cortical bone, the center of the intramedullary cavity may shift either medially or laterally, and either posteriorly or anteriorly, moving distally down the bone. 
     The changes in the external anatomy of the tibia shown in  FIG. 2  cause corresponding changes in the size, location, and shape of the inner cancellous bone of the tibia.  FIG. 3  shows a front view of a longitudinal cross-section taken of the bone shown in  FIG. 2 , exposing the interior cancellous bone  142 . The outer shell of cortical bone  140  surrounds the spongy cancellous bone  142 . The changes in the intramedullary cavity caused by the changes in anatomy moving away from the proximal end  122  of the bone  120  are highlighted by the nonsymmetrical shape of the cancellous bone  142 . The tapering of the lateral and medial sides of the cortical bone  140  causes the cancellous bone  142  to become narrower in the distal tibia. The uneven tapering of the lateral and medial sides also shifts the center of the cancellous bone  142  medially in the distal tibia. In other bones, the tapering of medial and lateral sides of the cortical bone may be about equal, and the center of the cancellous bone  142  may shift either medially or laterally, or may not shift in a medial or lateral direction. 
     An anatomical stem designed to contour to the inner shapes of the bone  120  can be designed by creating two main portions—one portion corresponding to a wider proximal region of the bone and one portion corresponding to a narrower distal portion of the bone. A critical point on the tibial anatomy, such as a point corresponding to a shape change or other physical transition, can be identified and used to determine the size and design of the two portions of the stem. In some implementations, one portion is designed to accommodate the proximal anatomy above the critical point and one portion designed to accommodate the distal anatomy below the critical point. An example of such a critical point is shown in  FIG. 3  by the transverse line  134 . The transverse line  134  separates a top portion and a bottom portion of the bone  120  at an inflection point on the medial and lateral sides of the bone  120 . In the region of the bone  120  above the line  134 , the medial and lateral sides of the bone  120  exhibit a concave shape as shown on the lateral side by the bone portion  136   a  and medial side by bone portion  136   b . Below the line  134 , the medial and lateral sides of the bone exhibit a concave shape, as shown by lateral bone portion  138   a  and medial bone portion  138   b . In certain implementations, an anatomic stem is designed using the inflection point indicated by line  134  as a critical point and structuring the stem to have an upper portion such as the upper portion  108  of implant  100 , which is designed to accommodate the shape and size of the intramedullary cavity above the inflection point and a second portion of the stem, such as the lower portion  110  of the implant  100  designed to accommodate the shape and size of the intramedullary cavity below the inflection point. The line  134  corresponds to the shape or other physical transition that in turn corresponds to the change or transition in the bone&#39;s structure. Though the exact location of the inflection point and the line  134  varies from bone to bone and patient to patient, the shape of a tibial bone does not vary significantly and thus the line  134  does not vary significantly from patient to patient. Thus, structuring a stem based on such a point can provide a better fit for a larger number of patients. 
     In addition to the size and the changing center of the intramedullary cavity, the cross-sectional shape of the bone  120  changes along its structure from the proximal end  122  to the distal portion  125 . This change in shape can be shown by viewing transverse cross-sections at different levels of the bone  120 . For example, one cross-section can be viewed above a critical point of a tibia, and a second cross-section can be viewed below the critical point. By using one cross-section above the line  134  and cross-section below the line  134 , for example cross-sections taken at lines AA and BB in  FIG. 2 , one can highlight the differing shapes of the bones for which the different portions of an anatomic stem is designed. Although the size, shape, and center of the intramedullary cavity may change, other characteristics such as the aspect ratio of the cavity, and of an anatomic implant designed to accommodate the cavity geometry, may remain generally constant at various levels of the cavity. For example, the aspect ratio of intramedullary cavities across a population of bone geometries may fall within the range of about 1.0 to about 2.0 even as shape and size of the intramedullary cavity varies at different levels of the bones. The aspect ratio of the cavity and anatomic implants is discussed below with respect to  FIG. 9 . 
       FIG. 4A  shows a cross-section  150  of the bone  120  taken at the level AA indicated in  FIG. 2 , which is above the inflection point discussed above with respect to  FIG. 3 . The cross-section  150  shows the outer cortical bone  140   a  surrounding the intramedullary cavity formed by the cancellous bone  142   a  along with a major axis  154  and minor axis  152  that cross at the center point  156  of the intramedullary cavity. Due to the shape of the cortical bone  140   a , the center point  156  is not equidistant from the anterior and posterior sides of the bone, nor is the center point  156  equidistant from the medial and lateral sides of the bone. Thus, a stem which is merely symmetrical and designed to hit the center point  156  of the inner cavity is not able to form a close interface in all directions with the cortical bone  140   a.    
     The center point  156  is not equidistant from the sides of the cortical bone  140   a  because of the nonsymmetrical shape of the bone at cross-section  150 . In the anterior-posterior direction, the cortical bone  140   a  extends in the anterior direction on the anterior side of the bone to form a tibial tuberosity  158  that is standard for the tibial anatomy, and there is an indent formed on the posterior side  159  side of the bone. The positioning of the notch and the tuberosity cause the center point  156  of the two axes to be closer to the posterior side of the bone than the anterior side. In addition, the nonsymmetrical shape of the cortical bone  140   a  creates a wider area of cancellous bone between the center point  156  and both the lateral side  157  of the bone and medial side  155  of the bone than between either of the center point  156  and the anterior or posterior sides  158  and  159 . A stem designed to form a close contact with the cortical bone  140   a  and occupy a significant portion of the cavity within the cortical bone  140   a  preferable accommodates these nuances of the anatomy. 
     Viewing transverse cross-sections at various levels along the longitudinal axis of the bone  120  highlights the change in the size and shape of the inner cavity of the bone as well as the different positions of the center point of the bone.  FIG. 4B  shows a cross-section  160  taken at the level indicated by line BB in  FIG. 2 , which is below the inflection point depicted in  FIG. 3 . The cross-section  160  shows the cortical bone  140   b  and cancellous bone  142   b  of the tibial bone  120 , and the shape of cross-section  160  is different than the cross-section  150 . A major axis  164  and a minor axis  162  are shown in the cross-section  160 , and the axes intersect at the center point  166  of the inner cavity of the bone. 
     The size and shape of the cortical bone  140   b  in cross-section  160  is significantly different from the cortical bone  140   a  shown in cross-section  150 . The ridge  168  on the anterior surface of the bone is shifted in a lateral direction relative to the center  166  of the cavity, shown by arrow  151 , relative to the location of the tibial tuberosity  158  on the anterior portion of the cross-section  150  relative to the center  156 . The lateral shift of the ridge  168  can be measured in terms of an angle of rotation about the center of the cavity at that level of the bone. This rotation may be up to 70.degree. from epiphysis to diaphysis, depending on an individual patient&#39;s tibial anatomy. While  FIG. 4B  shows a ridge  168  that is shifted in a lateral direction  151 , the ridge  168  may shift in a medial direction, particularly in more distal parts of the tibia. An anatomic implant stem can be designed to conform to the shape and rotation of the anterior ridge to improve fixation and rotation resistance. For example, the implant stem may include an anterior ridge that also rotates from a proximal portion of the stem to the distal portion to account for the rotation of the ridge  168 . Additionally, the posterior portion  169  of the bone is now rounded compared to the posterior side  159  of the cross-section  150 , with no indent in the posterior portion  169 . Furthermore, the medial side  165  and lateral side  167  are more rounded compared to the medial side  155  and lateral side  157  of cross-section  150 . The changes in the shape of the bone create new landmarks for an anatomic stem to accommodate, and changes in the size of the cancellous bone portion provides a smaller cavity into which the stem is implanted. 
     In addition to the changes in the size and shape of the inner bone cavity between the cross-sections  150  and  160 , the center point  166  of the cross-section  160  is shifted relative to the center point  156  of the cross-section  150 . The movement of the center point of the inner bone cavity is highlighted in  FIG. 4C , which shows an overlay of the cross-sections  150  and  160 . The center point  166  in the distal cross-section is shifted in an anteromedial direction relative to the center point  156  in the proximal cross-section. The shift may be caused by the sharper tapering angle of the lateral side of the bone compared to the medial side of the bone discussed above with respect to  FIG. 2 . In other tibia bones, the center point  166  may shift in other directions due to variations in patient tibial anatomy. For example, the center point may shift in an anterior direction, a medial direction, a lateral direction, or an anteromedial direction. Alternatively, the center point may shift in a posterior direction, although generally tibial anatomies will likely shift in an anterior direction, if any shift in the anterior-posterior direction is present. In other bones, for example in patients having a different tibial anatomy, the center points  156  and  166  may be co-located and not shifted relative to each other. 
     The combination of changes in the size, position, and shape of the inner cavity of the bone from proximal to distal cross-sections create a need for changes in the design of a tibial stem to maintain an adequate interface with the bone anatomy at different levels of the tibia. In order to maintain the alignment of the stem with the center of the intramedullary cavity, the stem is designed so that the center point of the stem shifts from the proximal to the distal end of the stem in order to accommodate the shift from the center point  156  to the center point  166 . In addition, the overall size of the stem can be varied, as the cavity in the proximal cross-section  150  is much larger than the inner cavity of the distal cross-section  160 . In order to maintain a close interface with the cortical bone  140  and transmit forces to the bone, the stem shape preferably also changes to accommodate the changing contours and landmarks of the proximal and distal cross-sectional shapes. Although the size, shape, and center of an anatomic stem may change between a proximal cross-section and a distal cross-section of the stem to accommodate corresponding changes in bone anatomy, the aspect ratio may remain generally constant. The generally constant aspect ratio, or any other characteristic of the bone that remains generally constant, can provide an improved approximation of bone anatomy and allow the implant to fit a larger population of patient bone sizes. 
     The close interface with the varying anatomy of the tibial bone provided by an anatomic stem is shown in  FIG. 5 , which depicts the tibial implant  100  of  FIG. 1  implanted into the tibial bone  120  of  FIG. 2 . The varying size and shape of the stem  104  from the proximal end to the distal end of the stem accommodates variations in the cross-sectional anatomy of the bone  120  at the different levels along the longitudinal axis of the bone. For example, the upper portion  108  of the stem  104  is wider than the lower portion  110  of the stem to accommodate the larger size of the inner cavity of the cancellous bone  142  in the proximal region of the bone  120 . As the upper portion  108  tapers from line CC to line DD, the lateral side  107  of the upper portion  108  tapers at a sharper angle θ 1  than the tapering angle θ 2  of the medial side  109 , thus shifting the center point  111  of the stem  104  at proximal line CC medially to center point  113  at distal line DD to accommodate the corresponding shifting center of the bone  120 . 
     The close interface between the stem  104  and the bone  120  is shown by cross-sectional views at different levels of the bone  120 , for example at the levels indicated by lines CC and DD in  FIG. 5 . The lines CC and DD correspond to the lines AA and BB discussed above with respect to  FIG. 2 , with the line CC above the critical point of inflection of bone  120  and the line DD below the point of inflection. 
       FIG. 6A  shows a cross-section  170  of the implant  100  in bone  120  taken at the line CC in  FIG. 5 . In cross-section  170 , the stem  104   a  fills a significant portion of the inner cavity formed by the cancellous bone  142   a . The stem  104   a  also includes rounded corners  103   a ,  105   a , and  106   a  that provide stabilization and an interface between the stem  104   a  and the cortical bone  140   a . For example, the rounded corner  106   a  is positioned near the cortical bone  140   a  of the tibial tuberosity  158  adjacent inner surface  141  of the cortical bone  140   a . In addition, the corners  105   a  and  103   a  extend toward the lateral and medial sides of the bone while the posterior side  107   a  between the two corners accommodates the notch on the posterior side  159  of the bone. The shape of the stem  104   a  defines an aspect ratio between the width of the implant from corner  103   a  to corner  105   a  and the height of the implant from corner  106   a  to posterior side  107   a . In certain embodiments, the aspect ratio may be between about 0.5 and about 4.0, between about 1.0 and about 2.0, or may be any other suitable value to accommodate varying patient anatomies. Modeling bone anatomy across a patient population may establish an aspect ratio range used in the design of anatomic implants. For example, data shown in  FIG. 9  and discussed below shows aspect ratios of various intramedullary cavities that have aspect ratios generally falling in the range 1.5+/−0.4. 
     The position and shape of the side  107   a  allows a surgeon to place the stem  104   a  into the bone without hitting the posterior side  159  of the bone. The position of the three corners also provides anti-rotation contact between the stem  104   a  and cortical bone  140   a . While a torsional force applied to the stem  104   a  may move the stem a small amount, interfering contact is created at least between one of the corners  103   a  and  105   a  and the inner surfaces  143  and  145  of the posterior side  159  of the cortical bone, and also between the rounded corner  106   a  and the inner surface  141  of the tibial tuberosity  158 . 
     Changes in the stem  104  that accommodate corresponding changes in the bone anatomy are shown in a second transverse cross-section viewed at a distal region of the bone.  FIG. 6B  shows a cross-section  180  taken at the level of line DD shown in  FIG. 5 . In the cross-section  180 , the size and shape of the stem  104   b  is changed to accommodate the varied anatomy. The rounded corner  106   b  on the stem  104   b  is shifted in a lateral direction, shown by arrow  153 , as the anterior ridge  168  of the bone has moved laterally in the direction of arrow  153  relative to the tibial tuberosity  158  on the anterior side of the bone in the proximal region. The tibial implant stem is designed such that it generally mimics the shape of the tibial intramedullary cavity and takes into account the rotation of this ridge. The lateral shift of the ridge  168  can be measured in terms of an angle of rotation about the center of the bone, and the rotation may be up to 70.degree. from a proximal portion of the stem to the distal portion of the stem.  FIG. 6C  illustrates the relative change in the stem from proximal to distal portions to accommodate the tibial intramedullary cavity and the rotation of the ridge  168 . The rounded corner  106   a  of the implant stem  104   a  in cross-section  170  is shifted laterally in the distal cross section  180  of the implant stem  104   b  to enhance fixation against cortical bone throughout the bone cavity. The rounded corners  103   b  and  105   b  are closer together, and the side  107   b  is shorter, to accommodate the decreased width of the inner cancellous bone  142   b.    
     The cross-section  180  shows the interaction between the stem  104   b  and the cortical bone  140   b  that provides resistance to movement cause by torsional forces applied to the stem  104   b . As in the cross-section  170 , rotation of the stem  104   b  in the cross-section  180  causes interfering contact between rounded corner  106   b  and the anterior ridge  168  of the cortical bone  140   b  as well as between one of rounded corners  103   b  and  105   b  and the posterior side  169  of the bone. Because the interfering contact is maintained from the proximal portion to the distal region of the stem, the rotation of the stem is resisted and the torsional forces applied to the stem are distributed along the longitudinal access of the stem into the surrounding cortical bone in both the proximal and distal regions. 
     In addition to the differential size and shape of the stem between the cross-section  170  and the cross-section  180 , the distal center point  113  of the stem is shifted relative to the proximal center point  111  to maintain alignment with the changing center point of the bone. The shift from center point  111  to center point  113  is shown in  FIG. 6C , which shows an overlay of cross-sections  170  and  180 . The overlay shows the distal stem  104   b  shifted anteromedially relative to the distal stem  104   a . The shift, caused by the uneven tapering of the stem, mimics the uneven tapering of the tibia and accommodates the shifting center point of the intramedullary cavity. In other implementations, for example for implants designed based on different tibial anatomies, the center points  111  and  113  may be co-located, and the distal stem  104   b  may not be shifted relative to the distal stem  104   a , or the center point  113  may shift laterally relative to the center point  111 . 
     In certain implementations, an anatomic stem of a tibial implant may match the shape of a specific distal cross-section of the intramedullary canal and leverage this shape to provide improved fixation over traditional cylindrical shapes. This anatomic stem may use a single shape, but change in size from the proximal to distal end. Additionally, the stem may have a center aligned with the distal cross-section to guide insertion. For example, a distal cross-sectional shape, such as the tear-drop shape shown in the cross-section  180 , or an oval shape, may be used for the implant stem shape. In this case, the tear-drop shape of the stem  104   b  is used over the length of the stem. The stem may also be offset or angled from the center of the tray to accommodate for the changing center of the tibial intramedullary cavity. In other implementations, the shape of the anterior wall or ridge may be leveraged to provide improved fixation. A stem may be designed to conform primarily to the anterior portion of the tibial intramedullary canal to provide improved rotation resistance and fixation against the anterior ridge. 
     Additional methods for taking advantage of the tibial anatomy may include using fixation elements, such as bone screws, to change an implant&#39;s effective shape. A tibial implant may be cannulated so that a bone screw passes through the stem and provides additional fixation with the bone. For example, an implant may have an opening that runs longitudinally down the stem so that a bone screw may engage the intramedullary canal near or through the anterior ridge to provide additional fixation. Furthermore, a screw opening may be provided in other areas of the stem or tray to provide additional support. For example, a screw hole may be provided as a cutout on a side of the tray to wedge the implant against bone. At the level of the stem at which the screw exits the stem and enters surrounding bone, the effective shape of the stem is changed in the direction the screw exits, as the cross-section taken at that level includes both the stem and the additional screw material. 
     The close interface between an implant and bone provided by an anatomic stem can be supplemented with one or more additional fixation features. In areas where there is more cancellous bone between the implant and cortical bone than other areas, for example in the proximal region of the tibia that exhibits a wider intramedullary cavity, such fixation features can be disposed to supplement the fixation of the stem and reduce the negative effects of stress shielding and rotation. A proximal portion of the stem that is placed into the wider region of the intramedullary cavity can incorporate fixation extensions to maintain a close bone interface.  FIG. 7  shows an example of a tibial implant  200  that includes fixation extensions, with fin extensions  208  and  210  extending outward from the stem  204  and the bottom surface  203  of the tray  202 . When the implant  200  is placed into a tibia, the fin extensions  208  and  210  extend into the spongy cancellous bone of the intramedullary cavity and supplement the stress transmission and rotation resistance of the anatomic features of the stem  204 , such as the rounded corner  206 . Though the cancellous bone is not as strong as cortical bone, the contact between the fins  208  and  210  and the cancellous bone provides added stability to the implant  200 , as rotation of the implant would require additional force to both overcome the contact between the stem  204  and the cortical bone and crush the cancellous bone contacting the fins  208  and  210 . In order to facilitate insertion of the implant  200  into the bone without crushing the cancellous bone, the inferior perimeters  207  and  209  of the fins  208  and  210  can include a tapping mechanism, such as a knife edge, that creates a seam in the cancellous bone rather than crushing it down. 
     The fin extensions may also facilitate insertion of an anatomic implant into an intramedullary cavity at an offset angle so that the stem is inserted without impinging on cortical bone. The non-symmetrical shape of the anatomic stem and the changing bone anatomy may cause interference if the implant is inserted straight into the bone. The straight insertion into the bone, however, is easiest for a surgeon to perform and requires force to be applied to the implant in only one direction down into the bone. This straight insertion is difficult to accommodate if an anatomic stem has a corner or other feature that rotates to match distal anatomy but interferes with proximal cortical bone if the stem is inserted straight into the intramedullary cavity. In order to accommodate the anatomy and the shape of the stem, the anatomic implant is rotated as it is inserted into the bone to avoid interference from the cortical bone. Fin extensions, such as the fins  208  and  210  in implant  200 , can be designed to guide the twisting rotation of the implant during insertion such that the surgeon can insert the implant with a single force down into the bone, with the fins guiding the twisting rotation of the implant during insertion. Such fins are designed with a curved or corkscrew-shaped profile extending distally down the stem from the implant tray. The fins engage the cancellous bone, and the twisting shape of the fins translates the axial insertion force into a rotational force that rotates the implant through a desired angle as it is inserted. The fin shape and resulting insertion rotation provide for insertion of the stem with only an axial force, simplifying the implantation for the surgeon, while still rotating the implant to reduce potentially harmful contact between the implant and cortical bone. 
     As the anatomic implant is inserted further into the intramedullary cavity, a rounded corner of the stem may begin to engage the anterior ridge of the intramedullary cavity. The anatomic stem may follow the rotation of the anterior ridge while moving distally until the tibial tray is seated against the proximal bone cut. To guide twisting during placement, curved or corkscrew-shaped fins may extend from the implant. These curved fins may engage cortical or cancellous bone to act as a guide for twisting the implant in addition to providing additional fixation to bone. The end of the curved fin may act as a reference point for twisting. For example, the end of a curved fin may be lined up with a major or minor axis of the proximal tibial cut prior to entering the bone. As the curved fins are twisted into the bone, the distal movement of the stem may be synchronized with the twisting to accurately guide placement of the implant. 
     The outward extension of fins on the tibial implant extends the interface between the bone and implant into areas of the bone where the stem does not extend in close proximity to the cortical bone.  FIG. 8A  shows an implant  220  having fin extensions  226  and  228  extending outward from stem  224  and bottom surface  222 . The fins  226  and  228  extend into areas of the bottom surface  222  in which there is a larger area of the surface between the stem  224  and the perimeter of the surface. By extending into these areas, the fins  226  and  228  provide a closer interface with surrounding bone when the component  220  is implanted, and the bottom surface  222  abuts a patient&#39;s tibia. 
     In order to further supplement the fixation of an anatomic stem, fin extensions can include fixation elements that increase the surface area on which the fins contact surrounding bone. The fixation elements may include the shape of the fins or further extensions off of the fins, as shown in  FIG. 8B . An implant  240  includes two fin extensions  246  and  248  that extend outward from stem  244  and bottom surface  242  of the implant. In contrast to the fin extensions  226  and  228 , each of fin extensions  246  and  248  include an outer portion that is shaped to supplement bone engagement and individual engagement portions extending outward perpendicular to the angle at which the fin extends from the stem  244 . 
     The outer portions of the fin extensions  246  and  248  are shaped as a rounded tip  243  and a cloverleaf  241 , respectively, but any suitable shape may be used. The shaped outer portion increases the surface area over which the fins  246  and  248  contact cancellous bone when the component  240  is implanted, and may also supplement contact between the stem  244  and outer cortical bone. When the component  240  is implanted and the bottom surface  242  abuts a tibial bone, the engaging outer portions  243  and  241  of the fins can further limit the angle over which the implant  240  can be rotated before interfering contact is made with cortical bone. While an implant having an anatomic stem without fins can rotate until a corner of the stem contacts cortical bone, the outer portions  241  and  243  may contact cortical bone before a corner of the stem  244  contacts the bone, and thus may limit the possible rotation angle more than if the implant  240  included only the stem  244 . 
     The engagement portions  245 ,  247 , and  249  that extend outward from fins  246  and  248  supplement the fixation provided by the fins. While the engaging extensions are shown as square projections  245 , triangular projections  247 , and sawtooth projections  249 , the extensions may have any other suitable shape. The engagement portions further increase the surface area of contact between the fins and the cancellous bone and also provide resistance to movement in directions other than rotation of the implant. For example, the square projections  245  and triangular projections resist movement in a direction parallel to the direction in which the fin  246  extends from the stem  244 , and the sawtooth projections  249  resist movement in a direction parallel to the direction in which the fin  248  extends from the stem  244 . 
     Other suitable shapes for the fin extensions, outer portions, and engagement portions may provide various advantages in improving fixation, resistance to rotation, and guidance during insertion. Shapes that may be too costly or otherwise difficult to form using standard machining or casting techniques may be formed using rapid manufacturing techniques. For example, standard machining techniques may not be able to form corkscrew shapes or clover-shaped edges due to limitations of the machining tools, but rapid manufacturing machinery can create these shapes with precision. Fin extensions, including the outer portions and engagement portions, can also be formed integrally with the implant with rapid manufacturing techniques. Additionally, rapid manufacturing facilitates the creation of fin extensions, and nuanced stem designs, for a specific patient. Patient-specific implants designed with fin extensions, including engagement portions, may provide a better fit with the surrounding bone or may take into account a patient-specific deformity or medical condition. For example, it may be advantageous to design a specific fin shape, outer portion shape, engagement portion shape, or arrangement of the three to accommodate for diseased or missing bone for a specific patient to provide fixation not offered by a standard implant designed for a larger population of patients. 
     The features and variations of the tibial anatomy used to design an anatomic tibial implant are obtained by imaging and modeling the intramedullary cavities of a population of patients. To model the tibia, imaging data is acquired using medical imaging techniques. The model is then viewed at various cross-sectional views at different depths of the bone to study the size, shape, and changes of the intramedullary anatomy. Non-limiting examples of medical imaging techniques used to obtain the bone models include x-ray, computed axial tomography, magnetic resonance imaging (MM), and ultrasound. The features extracted from the model at various levels of the bone are then used to design the tapering and changes in shape and size of the implant stem so that the anatomical implant provides a better fit against cortical bone in the tibial intramedullary cavity. 
     In some implementations, the imaged tibial anatomy of multiple patients is combined into a composite model to be used in designing a robust anatomic implant that provides an improved fit, and improved fixation, over a varied patient population compared to a standard implant. The composite model provides a comparison of the different anatomies across the studied population, including the variations in shapes and sizes that are accommodated by the anatomic implant. The range of variations in the modeled bones are summarized by defining two conditions indicating the largest and smallest cavity anatomies across the population at each level of the bone. The maximum and minimum limits are defined by a “maximum-material condition” (MMC) and a least-material condition (LMC). The MMC is defined by superimposing the models of all of the bones studied and creating an envelope along the outer-most boundary of the superimposed model throughout the intramedullary cavity. The LMC is then defined by creating an envelope along the inner-most boundary of the superimposed model throughout the cavity. The MMC and LMC act as limits, and the anatomy of the intramedullary cavities of each bone studied falls either on or between these two limits at each level of the bone. Thus, the MMC and LMC define the full range of anatomies that the anatomic implant accommodates, and these two conditions are used to design the anatomic stem. Because the MMC represents the largest cavities, an implant designed to match the MMC may break into cortical bone at some locations of the intramedullary cavity when implanted into bones that are smaller than the MMC in some areas. Thus, it is preferable to design the anatomic stem to match the LMC model, as that model is at least as small as the intramedullary cavity of each bone in the population at all levels of the bone. For a bone having the intramedullary cavity of the MMC condition, the LMC-designed stem may not contact the cortical bone at all levels of the bone, but may still provide an improved fit over a standard implant by accommodating anatomical changes and variations that are extracted from the LMC model. In other embodiments, an anatomic implant can be designed to match the MMC model, or a combination of the MMC and LMC. 
     In some implementations, a tibial anatomy model of a single patient&#39;s bone is used to design a “patient-matched” implant for the specific patient&#39;s anatomy. The patient-specific implant accommodates the variations and features of the patient&#39;s tibial anatomy and provides close fit with the patient&#39;s cortical bone. The precision in the design provides improved fixation and fit compared to a standard implant designed for implantation into a wide variety of tibial bone anatomies. 
     Modeling tibial bones for a single patient or across a population of patients provides useful data on the changes and trends in the different areas of the bone that dictate implant design. In some instances, rather than exhibiting large changes from one area of the bone to another, features extracted from the model may exhibit a generally consistent trend that can be leveraged in designing an implant. For example, although the size and shape of the intramedullary cavity changes throughout the tibial bone, the aspect ratio of the cavity measured at different levels of patient bones exhibits a consistent trend.  FIG. 9  shows a graph  300  that plots the aspect ratio measured from cross-sections taken along tibial intramedullary cavities from a reference location to a distal location across a patient population of tibia bones. The reference location may be at the knee joint, at a proximal bone cut such as the lines CC or DD in  FIG. 5 , or another suitable reference point. The location of each cross-section plotted is measured as a location, in inches in  FIG. 9 , in inches from the reference point. The groups of tibia bones plotted in graph  300  include smaller bones (“size 3”), average bones (“size 6”), and larger bones (“size 8”). For each bone size, a sample of bones was imaged to model the population of bones of that particular size, and a computer model was generated using the techniques discussed above. A composite model of each size of bones was then used to define a LMC and a MMC for each group. Aspect ratios were calculated for the LMC and MMC models for each bone size at cross-sections taken at multiple levels of the bone. 
     The bones modeled in  FIG. 9  exhibit aspect ratios generally falling in the range 1.5+/−0.2. For example, line  302  shows the aspect ratio trend of the LMC of the “size 8” bones. The aspect ratio of the LMC for those bones is about 1.55 at a proximal bone cut of the tibial intramedullary cavity and varies by about 0.15, to a value of about 1.70 at a distal cross-section taken two inches from the bone cut. As a comparison, line  304  shows the aspect ratio of the MMC “size 6” bones, a different bone size and material condition than line  302 . At a proximal bone cut, the aspect ratio of this model is about 1.45 and varies by about 0.4 through the cross-sections, to a distal cross section where the aspect ratio is about 1.47. The other models plotted in  FIG. 9  also show different trends, but all data points fall within the fairly narrow range of 1.5+/−0.2 for all sizes, conditions, and levels of the models. This narrow range is leveraged in design of an anatomic stem by maintaining the aspect ratio of the stem within this range at various levels of the stem while size and shape are varied to match corresponding changes in the modeled anatomy. 
     In addition to extracting trends in aspect ratio of the modeled bones to design an anatomical implant, the shift and rotation of the tibial anterior ridge can be measured in the models and used to design a corresponding change in an anatomic stem.  FIG. 10  shows a wireframe model  320  of cross-sections  322   a - e  of a tibial intramedullary cavity where the anterior ridge or prominence of the bone shifts by an angle about the center of the bone. The wireframe model  320  may be created using data from a single patient, or may be a model of the LMC or MMC of bones imaged across a patient population. The shift in the ridge and the anterior portion of the intramedullary cavity is the result of the changing cross-sectional shape of the tibia. The shift in the anterior portion of the intramedullary cavity caused by this shape change is shown in wireframe model  320  by the major and minor axes of each of the cross-sections  322   a - e , for example minor axis  324  and major axis  326  shown for cross-section  322   a . The major and minor axes of the cross-sections rotate about the longitudinal axis of the tibia as cross sections are taken moving distally down the tibia from cross-section  322   a . For example, the short axis  328  of cross-section  322   c  is rotated by an angle θ in the plane of the cross-sections from the short axis  324  of cross-section  322   a , represented by a dotted line  330  in the cross-section  322   c . This angle θ shows the angle of rotation of the anterior ridge at the level of cross-section  322   c  and generally increases moving distally down the tibia. 
     The rotation of the anterior ridge is accounted for in the design of the anatomic implant stem by including a corresponding ridge feature on the stem. For example, the angle of rotation of the modeled ridge is used to enhance the fit of the anatomic stem by defining the extent to which a rounded corner, such as rounded corners  106   a  and  106   b  of implant  100 , shift or rotate along the implant stem to conform to the tibial intramedullary cavity. The enhanced fit provides rotation resistance and resists loosening of the implant postoperatively by seating the implant against structural cortical bone, as opposed to spongy cancellous bone, over the length of the stem. Depending on the anatomy of the bone or bones modeled to create a model, the rotation of the short axis of the modeled cross-sections may be between 0.degree. and 70.degree. over the length of the stem from a proximal end to a distal end. 
     It is to be understood that the foregoing description is merely illustrative and is not to be limited to the details given herein. While several embodiments have been provided in the present disclosure, it should be understood that the disclosed systems, devices, and methods, and their components, may be embodied in many other specific forms without departing from the scope of the disclosure. 
     Variations and modifications will occur to those of skill in the art after reviewing this disclosure. The disclosed features may be implemented, in any combination and subcombinations (including multiple dependent combinations and subcombinations), with one or more other features described herein. The various features described or illustrated above, including any components thereof, may be combined or integrated in other systems. Moreover, certain features may be omitted or not implemented. 
     Examples of changes, substitutions, and alterations are ascertainable by one skilled in the art and could be made without departing from the scope of the information disclosed herein. All references cited herein are incorporated by reference in their entirety and made part of this application.