Abstract:
Disclosed herein are methods of designing and fabricating prosthetic implants having a sagittal wall in which at least a portion thereof traverses a non-linear path. A method of fabricating such a prosthetic implant may include generating a virtual bone model based on image information obtained from at least one bone, determining a proposed height of the prosthetic implant at a first location on the virtual bone model, determining a proposed resection depth into the at least one bone at the first location based at least in part on the proposed height of the prosthetic implant, and determining a curved resection path across a portion of the virtual bone model. The curved resection path may intersect the first location and the prosthetic implant may have a curved sagittal wall corresponding to the curved resection path.

Description:
BACKGROUND OF THE INVENTION 
     Current knee arthroplasty tibial implants that retain the tibial eminence contain a straight sagittal wall portion which banks against the eminence. Typically, the sagittal wall resection of the eminence corresponding to the straight sagittal wall of an implant is made with a reciprocating saw normal to the transverse tibial resection plane. When placing an eminence-preserving implant, preferences to component placement on a cut plane include avoiding ligaments, optimizing bony coverage, avoiding deep cuts into the eminence, and permitting possible kinematic trialing feedback, for example. 
     However, a straight walled implant may not be the most optimal design to retain constant resection depth on both sides of the eminence and to avoid ligaments because the native eminence wall itself is not a perfectly straight line. A straight walled implant also may not provide sufficient anterior-posterior stability. 
     SUMMARY OF THE INVENTION 
     Tibial implants having a curved sagittal wall portion taking into account both general anatomic features of the proximal tibia as well as providing greater anterior-posterior stability than implants having a straight sagittal wall portion are described herein. Further, by modifying the sagittal wall of the implant itself, issues like cutting too close to the anterior cruciate ligament (“ACL”) and cutting deep into the eminence to optimize placement can be avoided over large populations. 
     An aspect of the invention is bone preservation of the proximal tibia, namely the eminence. A consistent eminence height or equal height to final implant geometry allows for more conservation of bone than a straight cut affords. For a proposed implantation position, a curved geometry is more forgiving of ligaments and high peaks of the eminentia. Therefore, if positional changes of the implant are required to optimize the implant fit or function, there will be more positional freedom before the implant is limited by interfering with these structures. 
     According to one aspect of the invention, a method of designing a prosthetic implant comprises generating a virtual bone model based on image information obtained from at least one bone, determining a proposed height of the prosthetic implant at a first location on the virtual bone model, determining a proposed resection depth into the at least one bone at the first location based at least in part on the proposed height of the prosthetic implant, determining a curved resection path across a portion of the virtual bone model, the curved resection path intersecting the first location, and providing a prosthetic implant having a curved sagittal wall corresponding to the curved resection path. 
     In accordance with one embodiment, the curved resection path follows at least one outer surface contour of the virtual bone model. According to a further embodiment, the image information obtained from the at least one bone includes a database of knee data measurements taken from a plurality of knees. According to still further embodiment, the information from at least one bone consists of a plurality of measurements taken from a single knee. 
     According to one embodiment, the prosthetic implant is a tibial implant. According to a further embodiment, the tibial implant is a unicompartmental tibial implant. According to a still further embodiment, the tibial implant is a bicompartmental tibial implant. 
     According to one embodiment, the curved resection path substantially corresponds to the geometry of the tibial eminence. According to a further embodiment, at least a portion of the sagittal wall is defined by a rotation about a first pivot point. According to a still further embodiment, the curved resection path substantially corresponds to the geometry of the tibial eminence. According to a further embodiment, at least a first portion of the sagittal wall is defined by a rotation about a first pivot point at a first diameter, and a second portion of the sagittal wall is defined by a rotation about the first pivot point at a second diameter. 
     According to a further aspect of the invention, a prosthetic implant comprises a first surface for facing bone, a second surface, opposite the first surface, for facing a joint, and a sagittal wall extending between the first and second surfaces, wherein the sagittal wall traverses a non-linear path across the first and second surfaces, and wherein the non-linear path is based on the dimensions of at least one bone. 
     According to one embodiment, the implant is a tibial implant. According to a further embodiment, the tibial implant is a unicompartmental prosthetic implant. According to a still further implant, the prosthetic tibial implant is a bicompartmental tibial implant. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       A more complete appreciation of the subject matter of the present invention and the various advantages thereof can be realized by reference to the following detailed description in which reference is made to the accompanying drawings in which: 
         FIG. 1  is a top view of a virtual bone model of a proximal tibia with a point field map on a medial condyle thereof. 
         FIG. 2  is a contour plot comparing medial eminence height to resection level. 
         FIG. 3  is a contour plot comparing lateral eminence height to resection level. 
         FIGS. 4A-C  are top, front and side views of a virtual bone model of a proximal tibia having a medial point field map demonstrating measurements from surface points along contours of the bone to a proposed resection plane. 
         FIG. 5  is a top view of a virtual bone model of a proximal tibia having a point field map on the lateral condyle showing a high point on the virtual bone model. 
         FIGS. 6A-C  are top, front and side views of a virtual bone model of a proximal tibia having a lateral point field map demonstrating measurements from surface points along contours of the bone to a proposed resection plane. 
         FIGS. 7A-B  are top plan views and  FIGS. 7C-D  are lateral and medial side views of a virtual bone model of a proximal tibia illustrating proposed bone resections with a straight sagittal wall. 
         FIGS. 8A-B  are top plan views and  FIGS. 8C-D  are lateral and medial side views of a virtual bone model of a proximal tibia illustrating proposed bone resections having a curved sagittal wall. 
         FIGS. 9A-C  are top plan views of a virtual bone model of a proximal tibia illustrating surface contours and proposed bone resections having a curved sagittal wall. 
         FIGS. 10A-D  are perspective views of a virtual bone model of a proximal tibia illustrating bone resections with corresponding medial and lateral tibial components each having a curved sagittal wall. 
         FIG. 11  is a top plan view of a virtual bone model of a proximal tibia illustrating bone resections with corresponding medial and lateral tibial components each having a curved sagittal wall. 
         FIG. 12  is a top view of a bicompartmental tibial implant having medial and lateral portions each having a curved sagittal wall as to allow a rotational or helical insertion path about a pivot point. 
         FIG. 13  is a perspective view of a virtual bone model of a proximal tibia illustrating a bone resection and a plurality of stackable bicompartmental tibial implants each with medial and lateral portions each having a curved sagittal wall. 
         FIG. 14A  is a perspective view of a virtual bone model of a proximal tibia illustrating a bone resection and a bicompartmental tibial implant with medial and lateral components each having a curved sagittal wall. 
         FIG. 14B  is a top plan view of the virtual bone model of the proximal tibia and bicompartmental tibial implant of  FIG. 14C . 
         FIG. 14C  is a cross-sectional view showing the bicompartmental implant engaged to the resected proximal tibia taken along line  14 C- 14 C of  FIG. 14B . 
     
    
    
     DETAILED DESCRIPTION 
     Referring to the drawings, wherein like reference numerals represent like elements, there is shown in the figures, in accordance with embodiments of the present invention, prosthetic implants and virtual bone models.  FIGS. 1-4  illustrate one method of designing a prosthetic implant for fabrication having a curved sagittal wall using a virtual bone model  102 . In this embodiment, the method includes obtaining or generating at least one virtual bone model  102 . Bone model  102  is a bone model of a proximal tibia  103  having lateral  104  and medial  106  portions or sides. The lateral  104  and medial  106  sides are separated by a tibial eminence  108 . 
     A height or high point  110  is identified on bone model  102  representing a location on the bone model that is the greatest linear distance away from a proposed resection depth defined by a plane  114  measured about a longitudinal axis of bone model  102 . In the present embodiment, high point  110  corresponds to the high point of the medial tibial spine of bone model  102  of the proximal tibia shown. The high point  110  serves as a reference for additional points on bone model  102  to create a point field map  112 . Measurement locations or points  112   a ,  112   b , and  112   c  are defined as millimeter offsets from one of the medial and lateral identified spine high points  110 , and as percentages of the total anterior to posterior distance at each offset plane. Measurements are made from each measurement point  112   a ,  112   b , and  112   c , for example, on point field map  112  to a point on the proposed resection plane  114 . The respective measurement point  112   a ,  112   b , and  112   c  and point on the proposed resection plane defining a line representing the height and/or resection depth, the line being parallel to the longitudinal axis of the bone model  102 . 
       FIGS. 2 and 3  show the results of measurements taken on over  540  tibias generated using a database including information on patient morphology such as size, shape, density, and inner and outer cortical boundaries drawn from diverse populations to represent a broad range of patients. Point field maps  112  created on the medial and lateral condyles each contain  104  measurement points to define contour lines on either side of the tibial eminence  108 . Additionally, tibiae were split into multiple sizes based on their primary sizing measure such as anterior-posterior length, for example. Measurements from each size group then had a contour plot created based on its point field height measurements and variability of each measurement. 
     A prosthetic implant can then be designed based on the measurements taken and/or the created contour plots. In one embodiment, the prosthetic implant being designed for later fabrication is a medial tibial implant. The sagittal geometry of the tibial implant can be created to follow or mirror the contours of the virtual bone model at a proposed height, or a determined offset from the proposed height of the tibial implant. The resultant tibial implant has a sagittal wall that allows for a constant eminence height on average along its length. In one embodiment, the tibial implant can be designed to have the same height of the bone cut by both curving the cut and changing the implant height depending on region of the sagittal cut height. 
     If it is preferable to have the implant below the eminence, then acceptable bounding criteria can be developed, and using the variability of the normally distributed data of each height point measured, a periphery can be defined that captures the largest population of patients within the acceptable bounding criteria. 
     In  FIGS. 5-6 , point field maps are created on the lateral portion  204  of the bone model  202 . The bone model  202  is a model of a proximal tibial bone model having lateral  204  and medial  206  sides. The lateral  204  and medial  206  sides are separated by a tibial eminence  208 . 
     One or more heights or high points  210  are identified on the surface of the bone model  202 , representing a surface location that is the greatest distance away from one or more proposed depths defined by resection planes  214 . The high point  210  then serves as a reference for additional measurement points on the bone model to create a point field map  212 . In this example, the high point  210  corresponds to the high point of the lateral tibial spine. Measurement locations or points  212   a ,  212   b , and  212   c  are defined as millimeter offsets from one of the one or more identified high points  210 , and percentages of the total anterior to posterior distance at each offset plane. Measurements were made from each described point on the point field map to the proposed resection plane. 
     The implant sagittal geometry is then created to follow the contour plot at the implants proposed height, or an offset of the implants height for each size. The resultant tibial sagittal cuts then follow a constant eminence height on average along their length, which is mirrored in implant design. 
       FIGS. 7A-D  illustrate different views of a bone model  302 ,  302 ′ showing proposed lateral  316  and medial  316 ′ resections having a straight sagittal wall  318 ,  318 ′. In these illustrations, bone models  302 ,  302 ′ are of a proximal tibia having a lateral side  304 ,  304 ′ and a medial side  306 ,  306 ′, respectively. A resection plane  314 ,  314 ′ defines the depth of resection into the proximal tibia that will be made. The proposed resected portion  316 ,  316 ′ corresponds to an area of bone that will be removed and made available for replacement by a tibial implant. As shown best in  FIGS. 7A and 7B , the sagittal wall  318 ,  318 ′ defining the interface between the bone  302 ,  302 ′ and the proposed resected portion  316 ,  316 ′ is a linear line bisecting a proximal outer surface of bone model  302 ,  302 ′, respectively. 
       FIGS. 8A-D  illustrate different views of a bone model  402 ,  402 ′ showing proposed lateral  416  and medial  416 ′ resections having a curved sagittal wall  418 ,  418 ′, wherein the curvature is based, in part, on the tibial eminence geometry. In the embodiments shown, bone model  402 ,  402 ′ is a proximal tibia having a lateral side  404 ,  404 ′ and a medial side  406 ,  406 ′, respectively. A resection plane  414 ,  414 ′ defines the depth of resection into the proximal tibia that will be made. A proposed resected portion  416 ,  416 ′ corresponds to an area of bone that will be removed and made available for replacement by a tibial implant. 
     Moreover, the proposed resection level  414 ,  414 ′ is preferably a planar resection level having an orientation about a longitudinal axis of the bone that is determined based on desired component placement in order to achieve planned internal-external rotation, varus-valgus angle, and flexion-extension axis of the knee joint once the component is positioned thereon. As shown best in  FIGS. 8A and 8B , the sagittal wall  418 ,  418 ′ defining the interface between the bone  402 ,  402 ′ and the proposed resection portion  416 ,  416 ′ is curved and/or non-linear. The curvature in this example is based at least in part on the outer contour of medial and lateral portions of the tibial eminence. The curved path of the sagittal wall  418 ,  418 ′ substantially follows a surface contour representing a constant bone height as measured from resection plane  414 ,  414 ′. 
       FIGS. 9A-C  illustrate surface contour plots along a bone model  502  having proposed resection areas  516 ,  516 ′. A surface contour plot corresponds to bone model  502  that is a proximal tibia having a lateral side  504  and a medial side  506 . The lateral  504  and medial  506  sides are separated by a tibial eminence  508 . The contour plot identifies a resection portion  516 ,  516 ′ having a sagittal wall  518 ,  518 ′ that is curved. The curved sagittal wall  518 ,  518 ′ substantially follows a constant tibial height as measured from a proposed resection level. 
     In  FIGS. 10A-D  and  11 , a proximal tibia  702  is shown having resected areas on both lateral  704  and medial  706  sides, i.e., on either side of tibial eminence  708 . Tibial implant portions  720   a  and  720   b  have curved sagittal walls  722   a  and  722   b , respectively, corresponding to the height contours of the adjacent bone of the proximal tibia  702  such that there is a substantially smooth transition between native bone of the tibial eminence and an outer or articular surface of tibial implant portions  720   a  and  720   b.    
     In the embodiment shown, the tibial implant is a bicompartmental tibial implant including both lateral  720   a  and medial  720   b  implant portions. In other embodiments, the tibial implant may be a unicompartmental or unicondylar implant in which only one of lateral  720   a  and medial  720   b  implant portions is utilized. The lateral portion  720   a  corresponds to the lateral side  704  of the tibia, and the medial portion  720   b  corresponds to the medial side  706  of the tibia. Each portion  720   a  and  720   b  has a respective bone contacting surface  712   a  and  712   b , an articular surface  714   a  and  714   b , and a sagittal wall  722   a  and  722   b  extending between the bone contacting and articular surfaces. The sagittal wall  722   a  and  722   b  traverses a non-linear path from an anterior portion  716   a ,  716   b  to a posterior portion  718   a ,  718   b  of the implant portions  720   a ,  720   b . Sagittal wall  722   a  and  722   b  have curvature corresponding to the contours of the tibial eminence  708  such that the height of the resected bone interfacing with the sagittal wall of the implant portions  720   a  and  720   b  remains constant with respect to the resection level  714   a  and  714   b , respectively. 
     In a further embodiment, an implant may have an eminence geometry further defined as a rotation about a pivot point as described, for example, in U.S. Pat. Pub. No. 2012/0330429, titled “Prosthetic Implant and Method of Implantation,” the entirety of which is hereby incorporated by reference herein. As illustrated in  FIGS. 12-13 , a prosthetic implant  820  has a lateral portion  820   a  and a medial portion  820   b . The implant  820  further has a connecting portion  824  connecting the lateral  820   a  and medial  820   b  portions, such that the implant  820  has an open central portion  808  for, as an example, circumscribing a tibial eminence. 
     Additionally, the sagittal walls  822   a  and  822   b  of the lateral  820   a  and medial  820   b  portions have a curved geometry such that the walls  822   a  and  822   b  are swept about a single pivot point  826 . In other words, the geometry of the sagittal walls  822   a  and  822   b  substantially follow curvatures  828   a  and  828   b  that circumscribe a common pivot point  826  such that the curvatures  828   a  and  828   b  are portions of a circumference of concentric circles about pivot point  826 . Each curvature  828   a  and  828   b  has a respective radius R 1  and R 2  with pivot point  826 . The implant  820  is therefore inserted into place at the proposed resection level  814  in a rotational motion, which once fully positioned, increases the stability of implant  820  with respect the resected proximal tibia. Further, the implant could also be tapered towards the eminence, which may decrease the chance for eminence avulsion, aid in decreasing component micromotion, and ease the insertion process. An implant design with the features described should be more resistant to physiological loading than current designs with straight walls contacting the eminence. 
       FIG. 14A-C  show a bicompartmental tibial implant  920  engaged to a resected virtual bone model  900  of a proximal tibia. The bicompartmental tibial implant  920  has lateral  920   a  and medial  920   b  component portions each having a curved sagittal wall. The implant  920  further has a connecting portion  924  connecting the lateral  920   a  and medial  920   b  portions, such that the implant  920  has an open central portion for, as an example, circumscribing a tibial eminence  908 . 
     Additionally, the sagittal walls  922   a  and  922   b  of the lateral  920   a  and medial  920   b  portions have a curved geometry such that the walls  922   a  and  922   b  may be swept about a single pivot point. Further, implant  920  is tapered  926 ,  928  towards eminence  908 , as shown for example in  FIG. 14C , which may decrease the chance for eminence avulsion, aid in decreasing component micromotion, and ease the insertion process. Here, an interference fit between a fixation post  930  of tibial implant  920  acts to push eminence  908  down an provide a secure engagement between tapered  926 ,  928  sections of tibial implant  920  and eminence  908 . 
     The methods of designing the implants described herein can be applied to other joints of the body, such as the hip, elbow, shoulder, wrist and ankle, for example. Effective resection depth and natural bony contours of these joints can be taken into account in designing such implants in order to stabilize the implants with respect to resected bone in order to restore joint motion. 
     Although the invention herein has been described with reference to particular embodiments, it is to be understood that these embodiments are merely illustrative of the principles and applications of the present invention. It is therefore to be understood that numerous modifications may be made to the illustrative embodiments and that other arrangements may be devised without departing from the spirit and scope of the present invention as defined by the appended claims.