Patent Publication Number: US-2023157831-A1

Title: Tibial component of endoprosthetic knee implant, kits and instruments therefore, and methods of use

Description:
BACKGROUND OF THE INVENTION 
     1. Reference to Related Application 
     This application claims the benefit of priority to U.S. Provisional Application No. 63/264,332 filed on Nov. 19, 2021. The disclosure of this related application is hereby incorporated into this disclosure in its entirety. 
    
    
     2. Technical Field 
     The present disclosure relates generally to the field of knee implants, and more particularly to tibial components for endoprosthetic knee implants adapted for kinematic alignment, anatomic alignment, and methods of implantation thereof. 
     3. Related Art 
     Knee arthroplasties are procedures in which an orthopedic surgeon replaces portions of severely diseased knee joints with an artificial endoprosthetic implant that is intended to restore joint function and alleviate pain. The procedure itself generally consists of the surgeon making a vertical midline anterior incision on the bent knee (i.e., a knee in flexion). The surgeon then continues to incise tissue to access the joint capsule. Once pierced, the patella is retracted and the distal condyles of the femur, the cartilaginous meniscus, and the proximal tibial plateau are exposed. 
     The surgeon then removes the cartilaginous meniscus and may use instrumentation to measure and resect the distal femur and the proximal tibia to accommodate the endoprosthetic knee implant. The endoprosthetic knee implant generally comprises three primary components: a femoral component, a tibial component, and a meniscal insert component that is disposed between the installed femoral and tibial components. 
     The resections themselves often remove areas of diseased bone. These resections also necessarily modify the profiles of the remaining distal femur and proximal tibia to better accommodate complementary shapes of the associated implant component. That is, the resected distal femur will eventually fit into a complementary femoral component. Likewise, the resected proximal tibia will eventually support a complementary tibial component. 
     The tibial component of the endoprosthetic implant typically includes a unitary keel descending downwardly and perpendicularly from a lower surface of a tibial base plate. When installed on the resected proximal tibia, the keel extends into a bore in the intramedullary canal of the tibia. The keel of an installed tibial component can be thought of as being disposed in a coronal plane and is oriented perpendicularly to the lower surface of the tibial base plate. 
     In the field of knee arthroplasties, there are several schools of thought concerning the angles at which resection of the distal femoral condyles and the proximal tibia should be made. The angles of resection largely determine how the implant components will sit in the joint and can influence how the artificial joint performs over time. 
     One school of thought is the anatomic alignment method. In the anatomic alignment method, the surgeon resects the tibia at 3 degrees of varus because this is believed to be the average angle of the native joint line. Femoral resections and ligament releases are then performed to keep a straight hip-knee-ankle axis of the limb. 
     Another school of thought is the mechanical alignment method. The priority in mechanical alignment is to resect the tibia perpendicular to the length or axis of the tibial shaft (i.e., parallel to the transverse body plane). A mechanically resected tibial plateau would have a 0 degree varus tilt. The distal femur is then adjusted to account for the 0 degree varus tilt of the resected tibia and any necessary ligament releases are performed to maintain a straight hip-knee-ankle axis. 
     By contrast, with the kinematic alignment method, the surgeon seeks to restore the patient&#39;s specific natural pre-diseased joint line based on data made available to the surgeon both pre-operatively and intra-operatively. Most kinematic alignment techniques start with referencing the distal femur and generally adjusting the slope of the tibial resections to be parallel to the distal femoral resection when the knee is in extension, and parallel to the posterior resections of the femoral condyles when the knee is in flexion. 
     The utility of traditional tibial components can be severely limited when the proximal tibia is resected at an angle. On average, a unitary tibial component may have a keel having a length in the range of about 35 millimeters (“mm”) to about 65 mm. When the tibial base plate is disposed at an angle on the resected tibia, the keel is likewise disposed at an angle relative to the longitudinal axis of the operative tibia. If the angle of the tibial resection is too great, or if the keel is too long, the keel will abut the medial or lateral inner cortical wall of the tibia. In extreme cases, repeated pressure of the keel on the inner cortical wall could weaken or penetrate the cortex (i.e., the compact, generally non-spongy outer wall of the bone). 
     Shortening the keel excessively is usually not possible. A shortened keel risks ineffective force transfer from the knee into the tibia during normal use. This thereby increases the risk that the tibial component may become unseated during normal activity. 
     The problem of having the keel abut the inner cortical wall is particularly pronounced in revision or trauma cases. In such cases, the surgeon typically resects more of the proximal tibia to expose heathy bone. For context, in revision cases, the prior-installed implant is usually cemented to the tibia. The surgeon removes the prior-installed implant by resecting the underlying bone. As a result, there is often less healthy bone available for the surgeon to work with after the prior-installed implant has been removed. In trauma cases, complex fractures may encourage resecting the tibial below the complex fracture area to simplify reconstruction. However, the tibia tapers distally. This causes a narrowing of the intramedullary canal (particularly in the metaphysis and diaphysis) and of the abutting inner cortical walls. Stated simply, with available tibial components, the more that the surgeon resects the proximal tibia, the less room the surgeon has when setting the resection angle of the tibia. If the angle it too steep, the keel of the tibial component will abut or penetrate the tibial cortex. 
     To avoid this problem, surgeons may choose to resect the tibia at 0 degrees varus and proceed with a mechanical alignment procedure. The mechanical alignment technique can provide good stability when the patient&#39;s leg is in extension (e.g., when the patient is standing), but the implants that are commonly used with this technique often require the release of the anterior cruciate ligament (“ACL”). In some circumstances, the posterior cruciate ligament (“PCL”) may also be released. The ACL normally prevents the tibia from sliding too far anteriorly and from rotating too far relative to the femur. The absence of either of these ligaments can lead to feelings of weakness when the leg is in flexion. Furthermore, changing the location of the patient&#39;s natural joint line can lead to feelings of discomfort. Patients who alter their gait to accommodate the new joint line may chronically stress the remaining muscles, which can further exacerbate the feelings of discomfort and contribute to additional musculoskeletal problems in the future. 
     SUMMARY OF THE INVENTION 
     The problem of the limited range of motion for tibial components of endoprosthetic knee implants in situations in which the proximal tibia is resected at an angle relative to a transverse plane is mitigated by a tibial component of an endoprosthetic knee implant comprising: a tibial baseplate; and a keel extending from a lower surface of the tibial baseplate, wherein a keel axis extends axially through the keel, wherein the tibial baseplate is disposed at a keel posterior angle relative to the keel axis, and wherein the tibial baseplate is disposed at a keel varus angle relative to the keel axis. 
     In these and other situations, it would be advantageous to have a tibial component of an endoprosthetic knee implant that is configured to provide a built-in varus tilt (i.e., keel varus angle) and a built-in posterior slope (i.e., keel posterior angle). 
     It would therefore be unique and advantageous to have tibial component of an endoprosthetic knee implant that is adapted to kinematic and/or anatomic knee arthroplasty techniques having the characteristics and features described herein. 
     It is contemplated that certain exemplary embodiments in accordance with the present disclosure may provide tibial components of endoprosthetic knee implants that are particularly adapted for use in kinematic or anatomic knee arthroplasty procedures. 
     It is further contemplated that certain exemplary embodiments in accordance with the present disclosure can include exemplary instruments for surgically installing exemplary tibial components of an endoprosthetic knee implants that are particularly adapted for use in kinematic or anatomic knee arthroplasty procedures. 
     It is still further contemplated that certain exemplary embodiments in accordance with the present disclosure can include kits comprising the exemplary tibial components, exemplary instruments therefore, or a combination thereof. 
     It is still further contemplated that the exemplary tibial components, instruments therefore, and kits thereof may be useful in revision procedures and primary procedures and of the primary procedures, particularly in stemmed primary procedures. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       The foregoing will be apparent from the following more particular description of exemplary embodiments of the disclosure, as illustrated in the accompanying drawings. The drawings are not necessarily to scale, with emphasis instead being placed upon illustrating the disclosed embodiments. 
         FIG.  1 A  is a side view of an exemplary embodiment of a tibial component of an endoprosthetic knee implant, featuring a right knee unitary tibial component comprising a base having a varus tilt and a posterior slope relative to the keel. 
         FIG.  1 B  is a top view of the exemplary tibial component of  FIG.  1 A . 
         FIG.  1 C  is a front view of the exemplary tibial component of  FIG.  1 A . 
         FIG.  2 A  is a top view of an exemplary embodiment of a left sided tibial component of an endoprosthetic knee implant, featuring a left knee unitary tibial component comprising a base having a varus tilt and a posterior slope relative to the keel. 
         FIG.  2 B  is a front view of the exemplary tibial component of  FIG.  2 A  shown implanted on and in a resected tibia. The resected tibia is shown in cross-section. 
         FIG.  2 C  is a side view of the exemplary tibial component of  FIG.  2 A  shown implanted on and in a resected tibia. The resected tibia is shown in cross-section. 
         FIG.  3 A  is a side view of an exemplary embodiment of a tibial component of an endoprosthetic knee implant, featuring a right knee modular tibial component comprising a baseplate having a varus tilt and a posterior slope relative to a transverse plane. 
         FIG.  3 B  is a top view of the exemplary tibial component of  FIG.  3 A . 
         FIG.  3 C  is a front view of the exemplary tibial component of  FIG.  3 A . 
         FIG.  4 A  is a top view of an exemplary embodiment of a tibial component of an endoprosthetic knee implant, featuring a left knee modular tibial component comprising a baseplate having a varus tilt and a posterior slope relative to a transverse plane. 
         FIG.  4 B  is a front view of the exemplary tibial component of  FIG.  4 A  shown implanted on and in a resected tibia. The resected tibia is shown in cross-section. 
         FIG.  4 C  is a side view of the exemplary tibial component of  FIG.  4 A  shown implanted on a resected tibia. The resected tibia is shown in cross-section. 
         FIG.  5    is a perspective view of a person used to illustrate a transverse plane, coronal plane, and sagittal plane. 
         FIG.  6    shows an example endoprosthetic knee implant installed on a resected distal femur and a resected proximal tibia. 
         FIG.  7 A  is a front view of a modular keel for a right knee in an unassembled configuration. 
         FIG.  7 B  is a top down view of the modular right-sided keel of  FIG.  7 A . 
         FIG.  7 C  is a front view of a modular keel for a left knee in an unassembled configuration. 
         FIG.  7 D  is a top down view of the modular left-sided keel of  FIG.  7 C . 
         FIG.  7 E  is a side view of a modular keel in an unassembled configuration. 
         FIG.  7 F  is another side view of a modular keel in an unassembled configuration. 
         FIG.  8 A  is an expanded front view of an exemplary modular keel and modular baseplate. 
         FIG.  8 B  is an expanded side view of an exemplary modular keel and modular baseplate. 
         FIG.  9    is an expanded top down perspective view of an exemplary keel punch and punch guide disposed above a trial baseplate (i.e., in a non-assembled configuration). 
         FIG.  10 A  is a top down view of the exemplary assembly of  FIG.  10 D . 
         FIG.  10 B  is a cross sectional view of the assembly of  FIG.  10 A  taken along line B-B. 
         FIG.  10 C  is a cross sectional view of the assembly of  FIG.  10 A  taken along line A-A. 
         FIG.  10 D  is an anterior view of an exemplary assembly (i.e., the components of  FIG.  9    in an assembled configuration) comprising a keel punch and a punch guide engaging a trial baseplate. 
         FIG.  11 A  is a top down view of an exemplary tibial component comprising a symmetric tibial baseplate. 
         FIG.  11 B  is an anterior view of an exemplary tibial component wherein the keel longitudinal axis is offset from an anterior-posterior plane that bisects the tibial baseplate of  FIG.  11 A . 
         FIG.  11 C  is a medial side view of the exemplary tibial component of  FIG.  11 B . 
     
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     The following detailed description of the preferred embodiments is presented only for illustrative and descriptive purposes and is not intended to be exhaustive or to limit the scope and spirit of the invention. The embodiments were selected and described to best explain the principles of the invention and its practical application. One of ordinary skill in the art will recognize that many variations can be made to the invention disclosed in this specification without departing from the scope and spirit of the invention. 
     Similar or the same reference characters indicate corresponding parts throughout the several views unless otherwise stated. Although the drawings represent embodiments of various features and components according to the present disclosure, the drawings are not necessarily to scale and certain features may be exaggerated to better illustrate embodiments of the present disclosure, and such exemplifications are not to be construed as limiting the scope of the present disclosure. 
     Except as otherwise expressly stated herein, the following rules of interpretation apply to this specification: (a) all words used herein shall be construed to be of such gender or number (singular or plural) as such circumstances require; (b) the singular terms “a,” “an,” and “the,” as used in the specification and the appended claims include plural references unless the context clearly dictates otherwise; (c) the antecedent term “about” applied to a recited range or value denotes an approximation with the deviation in the range or values known or expected in the art from the measurements; (d) the words, “herein,” “hereby,” “hereto,” “hereinbefore,” and “hereinafter,” and words of similar import, refer to this specification in its entirety and not to any particular paragraph, claim, or other subdivision, unless otherwise specified; (e) descriptive headings are for convenience only and shall not control or affect the meaning of construction of part of the specification; and (f) “or” and “any” are not exclusive and “include” and “including” are not limiting. Further, the terms, “comprising,” “having,” “including,” and “containing” are to be construed as open-ended terms (i.e., meaning “including but not limited to”). 
     References in the specification to “one embodiment,” “an embodiment,” “an exemplary embodiment,” etc., indicate that the embodiment described may include a particular feature, structure, or characteristic, but every embodiment may not necessarily include the particular feature, structure, or characteristic. Moreover, such phrases are not necessarily referring to the same embodiment. Further, when a particular feature, structure, or characteristic is described in connection with an embodiment, it is submitted that it is within the knowledge of one skilled in the art to affect such feature, structure, or characteristic in connection with other embodiments, whether explicitly described. 
     To the extent necessary to provide descriptive support, the subject matter and/or text of the appended claims are incorporated herein by reference in their entirety. 
     Recitation of ranges of values herein are merely intended to serve as a shorthand method of referring individually to each separate value falling within the range of any sub-ranges there between, unless otherwise clearly indicated herein. Each separate value within a recited range is incorporated into the specification or claims as if each separate value were individually recited herein. Where a specific range of values is provided, it is understood that each intervening value, to the tenth or less of the unit of the lower limit between the upper and lower limit of that range and any other stated or intervening value in that stated range of sub range thereof, is included herein unless the context clearly dictates otherwise. All subranges are also included. The upper and lower limits of these smaller ranges are also included therein, subject to any specifically and expressly excluded limit in the stated range. 
     It should be noted that some of the terms used herein are relative terms. For example, the terms, “upper” and, “lower” are relative to each other in location, i.e., an upper component is located at a higher elevation than a lower component in each orientation, but these terms can change if the orientation is flipped. 
     The terms, “horizontal” and “vertical” are used to indicate direction relative to an absolute reference, i.e., ground level. However, these terms should not be construed to require structure to be absolutely parallel or absolutely perpendicular to each other. For example, a first vertical structure and a second vertical structure are not necessarily parallel to each other. 
     Throughout this disclosure, various positional terms, such as “distal,” “proximal,” “medial,” “lateral,” “anterior,” and “posterior,” will be used in the customary manner when referring to the human anatomy. More specifically, “distal” refers to the area away from the point of attachment to the body, while “proximal” refers to the area near the point of attachment to the body. For example, the distal femur refers to the portion of the femur near the tibia, whereas the proximal femur refers to the portion of the femur near the hip. The terms, “medial” and “lateral” are also essentially opposites. “Medial” refers to something that is disposed closer to the middle of the body. “Lateral” means that something is disposed closer to the right side or the left side of the body than to the middle of the body. Regarding “anterior” and “posterior,” “anterior” refers to something disposed closer to the front of the body, whereas “posterior” refers to something disposed closer to the rear of the body.” 
     However, when referring to any component of the exemplary endoprosthetic implant or instruments described herein (e.g., an exemplary tibial component), particularly when such components are in an uninstalled or unassembled configuration, the various positional terms such as “distal,” “proximal,” “medial,” “lateral,” “anterior,” and “posterior,” absolute terms that refer to the associated position depicted in the accompanying figures and that do not change relative to the component&#39;s orientation is space. For example, a “proximal end” of a modular keel refers to the end indicated in the accompanying figures regardless of the orientation such “proximal end” relative to a reference (e.g., a person or the surface of the Earth). 
     “Varus” and “valgus” are broad terms and include without limitation, rotational movement in a medial and/or lateral direction relative to the knee joint. 
     The phrase, “mechanical axis of the femur” refers to an imaginary line drawn from the center of the femoral head to the center of the distal femur at the knee. 
     The phrase, “mechanical axis of the tibia” refers to an imaginary line drawn from the center of the proximal tibia to the center of the distal tibia, which is just above the ankle. 
     The term, “anatomic axis” refers to an imaginary line drawn lengthwise down the middle of femoral shaft or tibial shaft, depending upon use. The mechanical axis of the tibia and the anatomic axis of the tibia are generally considered to be co-linear. 
     Referring to  FIG.  6   , an example endoprosthetic knee implant  1  is depicted installed on a resected distal end  12  of a femur  10  and on the resected proximal end  13  of the tibia  20 . Unless otherwise stated, “endoprosthetic knee implant” can refer to either a trial implant, such as the one depicted in  FIG.  6   , or an actual endoprosthetic knee implant that is designed to be surgically implanted into the patient and used as an artificial joint for an extended period. The endoprosthetic knee implant  1  generally comprises: a femoral component  30  configured to be engaged to the resected distal end  12  of the femur  10 , a tibial component  40  configured to be engaged to the resected proximal end  13  of the tibia  20 , and a meniscal insert  50 , which is disposed between the femoral component  30  and the tibial component  40  when installed in the patient. 
     To initiate a typical revision knee arthroplasty, the surgeon makes a vertical midline incision on the anterior side of the operative knee. The incision is generally made with the knee in flexion at or below the tibial tuberosity and may extend several inches above the patella. 
     In a primary knee arthroplasty, the surgeon then continues to incise the fatty tissue to expose the anterior aspect of the joint capsule. The surgeon may then perform a medial parapatellar arthrotomy to pierce the joint capsule and resect the medial patellar retinaculum. A retractor is then commonly used to move the patella generally laterally to expose the distal condyles of the femur and the cartilaginous meniscus resting on the proximal tibial plateau. The surgeon then removes the meniscus and uses instrumentation to measure and resect the distal femur and proximal tibia to accommodate trial implants. Trial implants are test endoprostheses that generally have the same functional dimensions of the actual endoprostheses, but trial implants are designed to be temporarily installed and removed for the purposes of evaluating the fit of the actual endoprostheses and for the purposes of evaluating the knee joint&#39;s kinematics. The surgeon removes the trial implants and installs the actual implants once the surgeon is satisfied with the trial implant&#39;s sizing and the knee joint&#39;s kinematics. 
     Measurement and installation methods differ. Surgeons generally execute a mechanical alignment, anatomic alignment, or kinematic alignment technique according to preference, patient anatomy, the state of the operative joint, and available instruments. 
     To highlight the kinematic alignment technique and by way of example, a surgeon may proceed as described in U.S. Pat. No. 11,246,603 to Steensen et. al. The principle of kinematic alignment is that the surgeon uses instrumentation and implants to ascertain and restore the patient&#39;s natural pre-diseased joint line. The instruments described in U.S. Pat. No. 11,246,603 solve several issues encountered in kinematic techniques, such as allowing: the angle of the current or natural joint surface to be measured on both the femur and tibia; a wear factor to be used so that the measured amount of bone resection restores the joint surface to its pre-diseased level on both the femur and tibia; the surgeon to resect a specific amount of bone from the medial and lateral aspects of the joint surface on both the femur and tibia; the surgeon to visualize the angle of resection; the angle of resection to float infinitely, rather than in specific increments (within an acceptable range on both the femur and tibia); the surgeon to selectively lock the angle if desired; and the surgeon to measure the resection of the medial and lateral femoral condyles or of the medial and lateral tibial hemi-plateaus. 
     To summarize the distal resection step, the surgeon may ascertain the amount of cartilage wear on the distal condyles of the femur, attach a movable resection guide instrument to the exposed distal femur, and adjust the resection guide instrument to account for the measured loss of articular cartilage and the size of the implant. For example, if the implant is 10 mm in size and if the surgeon measures 2 mm of missing cartilage on the medial distal femoral condyle and 1 mm of missing cartilage on the lateral femoral condyle, the surgeon can adjust the resection guide instrument to position a resection slot to resect 8 mm of bone on the medial condyle and 9 mm of bone of the lateral condyle. The surgeon then inserts a saw or other cutting instrument through the resection slot to create the distal resection surface  5  at the desired angle and location. 
     By resecting the distal femur at this angle, the surgeon creates the first mating surface for the femoral component at an angle that is consistent with the angle of the patient&#39;s natural pre-diseased joint line when the knee is in extension (i.e., as depicted in  FIG.  6   ). The 8 mm of resection on the medial side plus the 2 mm of measured cartilage loss thereby is sized to accommodate a 10 mm endoprosthetic implant  1  on the medial side M. Likewise, the 9 mm resection of the lateral side plus the 1 mm of measured cartilage loss is thereby sized to accommodate the 10 mm endoprosthetic implant  1  on the lateral side L. 
     The surgeon may then place the knee in flexion (i.e., bend the knee) and repeat the measurement and resection process to create the posterior resection surface  3 . 
     After creating the distal resection surface  5  and possibly the posterior resection surface  3 , the surgeon may place a four-in-one cutting block (or separate resection guides) on the distal resection surface  5  to create the chamfer resection surfaces  8   a ,  8   b , the anterior resection surface  2 , and the posterior resection surface  3  if not made previously. The femoral component  30  has complementary faces that are disposed against the respective resection surfaces  5 ,  8   a ,  8   b ,  2 , and  3  when the femoral component  30  is disposed in an installed configuration as shown in  FIG.  6   . 
     It will be appreciated that the presence of the respective resection surfaces  5 ,  8   a ,  8   b ,  2 , and  3  and the complementary mating faces of the femoral component  30  are the primary means by which the femoral component  30  is “configured to be engaged” to the resected distal end  12  of the femur  10 . It will be further appreciated that the engagement side of the femoral component  30  may further comprise one or more projections (e.g., spikes) designed to be inserted into any one of the resection surfaces  5 ,  8   a ,  8   b ,  2 , and  3  to further facilitate the engagement of the femoral component  30  to the resected distal end  12  of the femur  10 . By way of further example, press-fit femoral components typically have a porous roughened surface on the engagement side. The porous surface permits regrowth of the bone into these pours over time. 
     Surgeons can also apply biocompatible “bone cement” to help secure the femoral component  30  to the resected distal end  12  of the femur  10 . It will be appreciated that “bone cement” is a term of art used by people in the orthopedic industry even though bone cements themselves generally do not have adhesive properties. Bone cements generally rely on a close mechanical interlock between the irregular surface of the bone and the surface of the connective side of the endoprosthesis. Bone cements may or may not be laden with antibiotics depending upon the intended use. Common bone cements include polymethyl methacrylate (“PMMA”), calcium phosphate cements (“CPCs”), and glass polyalkenoate isomer cements (“GPICs”) . It will be appreciated that when present, the use of projections, porous roughened surfaces, and/or “bone cement” to further facilitate engaging the femoral component  30  to the distal end  12  of the femur  10  and can therefore also fall within the scope of the “femoral component  30  configured to be engaged to the resected distal end  12  of the femur  10 ” language. Bone cement is difficult to remove once cured. Bone cement&#39;s presence is a significant factor that contributes to the need to resect the supportive bone in revision procedures. 
     Creating the resected proximal end  13  of the tibia  20  can be completed before or after the femur  10  is resected. A cutting guide is typically placed on the anterior surface of the tibia  20 , and the surgeon can adjust the varus and valgus angle of resection and optionally the posterior slope of resection depending upon the elected knee alignment method (e.g., anatomic, mechanical, or kinematic). Once the resection angle is set, the surgeon inserts a saw or other cutting instrument through a resection slot in the tibia resection guide to create the resected tibial surface  23 . For example, in a kinematic operation, the tibial longitudinal axis LA (i.e., a projected stem axis) is determined and the tibia  20  is resected relative to the tibial longitudinal axis LA. The tibia  20  is resected at a varus tilt  90  -δ sloping down from the lateral L to the medial M side, commonly at about three degrees relative to the keel longitudinal axis KLA. The tibia is further resected at a posterior slope  90 −θ that slopes down from the anterior side A to the posterior side P of the patient&#39;s tibia  20 , commonly at about three degrees relative to the keel longitudinal axis KLA. The surgeon may then use the exemplary punch guide  83  described further below to insert a reamer and/or punch (see  73 ,  FIG.  9   ) into the exposed epiphysis or metaphysis of the exposed intramedullary canal of the resected tibial surface  23  to create a cavity. This cavity can be desirably sized to accommodate the keel  43  ( FIG.  1 A ) of the tibial component  40 . 
     The keel  43  is then inserted into the cavity (see  FIG.  2 B ) and the surgeon may hammer the tibial component  40  into place such that the lower surface  41  of the baseplate  45  rests upon the resected proximal end  13  of the tibia  20  (i.e., on the resected tibial surface  23 ). In this manner, the tibial component  40  can be said to be “configured to be engaged” to the resected proximal end  13  of the tibia  20 . As with the femoral component, the lower surface  41  of the baseplate  45  may comprise projections (e.g., spikes) in certain embodiments. By way of further example, press-fit tibial components typically have a porous roughened surface on the engagement side. The porous surface permits regrowth of the bone into these pours over time. In certain applications, the surgeon may elect to place bone cement between the resected tibial surface  23  and the lower surface  41  of the baseplate  45 . When present, the projections, porous roughened surface, and/or the use of bone cement as described can fall within the scope of the “tibial component  40  configured to be engaged to the resected proximal end  13  of the tibia  20 ” language. 
     In a revision procedure (i.e., a subsequent knee arthroplasty in which the surgeon removes and replaces a prior installed endoprosthetic knee implant), the surgeon may release scar tissue around the patellar tendon after the surgeon has performed the initial incision. The surgeon can then move (i.e., preform a subluxation of) the patella or patellar implant generally laterally to expose the prior-installed implant, which usually has a femoral component installed on the distal femur, a tibial component installed on the proximal tibia, and a meniscal insert disposed between the femoral component and the tibial component. The surgeon then removes the prior-installed implant. 
     It will be appreciated that the type of prior-installed implant can vary from case to case. Prior-installed implants can include static spacers that have been inserted into aligned intramedullary bores in the distal femur and proximal tibia to immobilize the knee joint, or complex implants that have been used to reconstruct portions of the knee joint that had undergone trauma. However, common prior-installed implants include implants installed during a primary total or partial knee arthroplasty, or prior revision implants. 
     Removal of the prior-installed implants generally involves cutting away the bone underlying the bone cement—or in the case of press-fit implants, the bone underlying the press-fit implant. This resection exposes fresh bone capable of receiving a revision press-fit or bone cement bondable implant. Removing bone to remove the prior-installed implant would move the joint line if the revision implant was not sized to replace the newly resected bone. 
       FIG.  2 B  is an anterior view of an exemplary tibial component  40  disposed in a resected tibia  20 . The resected tibia  20  is shown in cross section. The exemplary tibial component  40  generally comprises a baseplate  45  having an upper surface  42  that is distally disposed from a lower surface  41  and an anterior side A that is distally disposed to a posterior side P (see  FIG.  2 A ). The baseplate  45  has a medial side M distally disposed from a lateral side L. A medial-lateral line M-L can be imagined to extend from the medial side M directly across the baseplate  45  (and horizontally when viewed in the orientation depicted in  FIG.  2 A ) to the lateral side L of the baseplate  45 . Likewise, an anterior-posterior line A-P can be imagined to extend from the anterior side A directly across the baseplate  45  (and vertically when viewed in the orientation depicted in  FIG.  2 A ) to the posterior side P of the baseplate  45 . The anterior-posterior line A-P is perpendicular to the medial lateral line M-L. 
     In the depicted embodiment, the resected tibial surface  23  of a given tibia  20  is not perfectly symmetrical. Moreover, the left tibia is chiral to the right tibia. In the depicted embodiment, a given baseplate  45  is typically designed for either the left tibia or the right tibia. The anterior side A, posterior side P, medial side M, and the lateral side L of a given baseplate  45  are desirably sized and shaped to closely approximate the profile (i.e., perimeter) of the resected tibial surface  23  (see  FIG.  2 A ). In other exemplary embodiments, the baseplate  45  can be symmetrical. Such exemplary symmetrical embodiments can be used with the modular keel  43  that is further described with reference to  FIGS.  7 A- 8 B  below. It is contemplated that in such exemplary embodiments, a symmetrical baseplate  45  can become a subcomponent of a left or right tibial component  40  upon being placed in an assembled configuration together with a left or right modular keel  43 . In other exemplary embodiments, the symmetrical tibial baseplate and the keel  43  can be unitary. 
     In the exemplary embodiments shown in  FIGS.  1 A- 4 C , a keel  43  extends downwardly from the lower surface  41  of the baseplate  45 . The keel  43  can comprise one or more fins  46  extending outwardly from the keel  43 . 
     In general, the femoral component  30 , tibial component  40 , and meniscal insert  50  can be made from any biocompatible material designed to withstand the stress of repeated and normal use of the knee. In practice, the femoral component  30  and tibial component  40  are frequently made from cobalt-chromium alloys, titanium, titanium alloys, zirconium, zirconium alloys, nickel, or nickel alloys. In certain embodiments, these components, particularly the femoral component  30 , may have a ceramic coating (or may comprise an outer ceramic layer) on the articular surface. In other embodiments, the entire femoral component  30  can be made from ceramic. In still other embodiments, the entire tibial component  40  can be made from ceramic. The meniscal insert  50  is typically made from ultra-high molecular weight polyethylene (“UHMWPE”) or from a ceramic. 
     Reference is made to  FIG.  5   , which illustrates three standard anatomical body planes commonly used in reference to human anatomy. The coronal plane CP or frontal plane is any imaginary vertical plane that divides the body  50  into a ventral section V and a dorsal section D. Although  FIG.  5    depicts the coronal plane CP intersecting the midline ML of the body  50 , it will be understood that the coronal plane CP can be imagined at any vertical location through the body  50  provided that the coronal plane CP divides the body  50  into a ventral section V and a dorsal section D. The sagittal plane SP, or longitudinal plane, is an imaginary vertical plane that divides the body  50  into left and right parts. The sagittal plane SP is depicted as intersecting the body  50  at the midline ML, but it will be understood that a sagittal plane SP can be imagined away from the midline to designate unequal right and left parts. The transverse plane TRP, or axial plane is an imaginary plane that divides the body  50  into superior and inferior parts (i.e., upper, and lower parts). The transverse plane TRP is perpendicular to both the coronal plane CP and the sagittal plane SP. The transverse plane TRP can be imagined to extend generally horizontally from the vertical midline ML of the body  50  at any location along the midline ML provided that the transverse plane TRP divides the body  50  into superior and inferior sections. Reference to these anatomical planes will be used to describe the relationship of certain parts of exemplary embodiments of the present disclosure. It will be appreciated that the exemplary implants described herein do not need to be physically implanted into a patient in order to refer to the anatomical planes. 
     As shown in  FIGS.  1 A- 4 C , exemplary embodiments in accordance with this disclosure can comprise, generally, a tibial component  40  of an endoprosthetic knee implant  1  having a baseplate  45  and a keel  43  extending downwardly from the baseplate  45 , wherein the baseplate  45  defines a baseplate plane BPP that is coplanar with the length l and width w of the baseplate  45 , wherein the baseplate  45  (and therefore the baseplate plane BPP) is oriented at a compound angle relative to a keel longitudinal axis KLA, the keel longitudinal axis KLA extending lengthwise through the keel  43  (i.e., parallel to the height dimension h of the keel  43 ), wherein the compound angle can be said to comprise a “keel varus angle” δ and a “keel posterior angle” θ. 
     For the purposes of this disclosure, a “keel posterior angle” θ is the acute angle defined by the intersection between the anterior-posterior line A-P of the baseplate plane BPP (represented by BPP in  FIGS.  1 A and  3 A ) and the keel longitudinal axis KLA, wherein the acute angle is disposed closer to the posterior side P of the keel  43  than the anterior side A. In exemplary embodiments, the keel posterior angle θ is less than 90 degrees and preferably greater than or equal to 75 degrees. It is contemplated that within this provided range, a keel posterior angle θ of or about 87 degrees may be desirable in some embodiments because it is believed that the native pre-diseased joint line for the average patient may be replicated in part by orienting the resected tibial surface  23  at a three degree posterior slope (e.g.,  90 −θ). However, in other exemplary embodiments, the posterior slope  90 −θ can be between greater than zero degrees and less than or equal to about fifteen degrees, depending on surgeon preference and natural anatomy of the patient. 
     A “posterior slope” in this context is a term of art that typically refers to the anterior to posterior orientation of the resected tibial surface  23 . A “posterior slope” is typically thought of as the angle of the anterior-posterior line A-P of the resected tibial surface  23  relative to an intersecting transverse plane TRP. Because the lower surface  41  of the baseplate  45  is desirably disposed on the resected tibial surface  23  and oriented parallel to the resected tibial surface  23  in an installed configuration, the “posterior slope” can also refer to the anterior to posterior orientation of the baseplate  45  of the exemplary tibial components  40  described herein regardless of orientation; however, this relationship is especially pronounced when said baseplate  45  is oriented as it would be in the installed configuration.  FIGS.  1 A and  3 A  illustrate this concept by showing the anterior posterior line A-P of the baseplate  45  (represented by the baseplate plane BPP) intersecting a transverse plane TRP at the posterior side P of the baseplate  45 . Because the relationship of posterior slope to the keel posterior angle θ is 90 degrees minus θ (where θ is the value of the keel posterior angle) a compound angle that comprises a keel posterior angle θ can necessarily be said to comprise a posterior slope ( 90 −θ). 
     For the purposes of this disclosure, a “keel varus angle” δ can be described as the acute angle defined by the intersection between the medial lateral line M-L of the baseplate plane BPP (represented by BPP in  FIGS.  1 C and  3 C ) and the keel longitudinal axis KLA, wherein the acute angle is disposed closer to the medial side M of the keel  43  than the lateral side L. In exemplary embodiments, the keel varus angle δ is less than 90 degrees and preferably greater than or equal to 83 degrees. It is contemplated that within this provided range, a keel varus angle δ of or about 87 degrees may be desirable in certain embodiments because it is believed that the native pre-diseased joint line for the average patient may be replicated in part by orienting the resected tibial surface  23  at a three degree varus tilt (e.g.,  90 −δ). However, in other exemplary embodiments, the varus tilt  90 −δ can be between greater than zero degrees and less than or equal to about seven degrees, depending on surgeon preference and natural anatomy of the patient. 
     A “varus tilt” in this context is a term of art that refers to the medial to lateral orientation of the resected tibial surface  23 . A “varus tilt” is typically thought of as the angle of the medial-lateral line M-L of the resected tibial surface  23  relative to an intersecting transverse plane TRP. Because the lower surface  41  of the baseplate  45  is desirably disposed on the resected tibial surface  23  and oriented parallel to the resected tibial surface  23  in an installed configuration, the “varus tilt” can also refer to the medial to lateral orientation of the baseplate  45  of the exemplary tibial components  40  described herein regardless of orientation; however, this relationship is especially pronounced when said baseplate  45  is oriented as it would be in the installed configuration.  FIGS.  1 C and  3 C  illustrate this concept by showing the medial-lateral line M-L of the baseplate  45  (represented by the baseplate plane BPP) intersecting a transverse plane TRP. Because the relationship of varus tilt to the keel varus angle δ is 90 degrees minus δ (where δ is the value of the keel varus angle) a compound angle that comprises a keel varus angle δ can necessarily be said to comprise a varus tilt ( 90 −δ). 
     In this manner, the compound angle for the exemplary tibial components  40  described herein can also be said to comprise a posterior slope ( 90 −θ) and a varus tilt ( 90 −δ). 
     The exemplary tibial component  40  of an endoprosthetic knee implant  1  can be provided in a unitary (i.e., non-modular; unibody; comprising a single piece) embodiment. In other exemplary embodiments, the keel  43  and the baseplate  45  can be modular, as described below. 
       FIG.  1 A  shows a side view of one embodiment of an exemplary tibial component of an endoprosthetic knee implant  1  in an uninstalled configuration. The depicted embodiment is unitary or non-modular right knee tibial component  40 . It will be appreciated that the exemplary tibial components  40  described herein have an installed configuration (see e.g.,  FIG.  2 B ) in which the tibial component  40  is disposed on and in a resected proximal tibia  20 , and an uninstalled configuration (see e.g.,  FIGS.  1 A- 1 C ) in which the tibial component  40  is not disposed on and in a resected proximal tibia  20 . 
     The tibial component  40  includes a baseplate  45  (which is also known as a “tibial tray”) and a unitary keel  43  extending downwardly from a lower surface  41  of the baseplate  45 . The keel  43  can comprise one or more fins  46  extending outwardly from the keel  43 . The fins  46  are typically disposed in the transverse plane TRP when the tibial component  40  is in the installed configuration. In exemplary embodiments, the fins  46  can be disposed at a compound angle relative to the baseplate plane BBP, wherein the compound angle comprises a keel posterior angle and a keel varus angle as described herein. In other exemplary embodiments, the fins  46  can be oriented relative to the keel  43  as the fins  46  would be for a mechanically aligned tibial component. 
     It will be appreciated that the “non-modular” or “unitary” embodiments described herein can refer to tibial components  40  that have been manufactured as a single piece (e.g., by casting, machining, additive manufacturing, etc.) and to tibial components  40  that have been manufactured as separate pieces (e.g., a baseplate portion  45  and keel portion  43 ) but that are fixedly engaged to each other in a non-readily-removable manner when made available to the surgeon for the operative procedure (for modular embodiments, see  FIGS.  7 - 8   ). In both modular and unitary systems, the keel  43  is typically sized and configured for use in stabilizing the tibial component  40  on the proximal tibia  20 . In modular systems, a proximal end  61  of the keel  43  can be configured to selectively attach to the baseplate  45 . In exemplary embodiments of either non-modular or modular embodiments, the distal end  44  of the keel  43  can be configured to selectively attach to distal stem extensions  100  (see  FIG.  3 C ) to further stabilize the tibial component  40  in the tibia  20 . 
     In certain exemplary embodiments, the distal end  44  of the keel  43  is preferably rounded. The rounded distal end  44  reduces the risk of unnecessarily shaving off of proximate cancellous bone and bone marrow during the installation process. In other exemplary embodiments, the distal end  44  of the keel  43  can be substantially straight, wedge shaped, wedge shaped with a rounded end, conical, conical with a rounded end, frustoconical, frustoconical with a rounded end, pyramidal, pyramidal with a rounded end, frustopyramidal, frustopyramidal with a rounded end, or combinations thereof. Shapes that have a generally convex profile are generally preferable to reduce the risk of unnecessarily ablating cancellous bone and bone marrow during the installation process. All shapes having a generally convex profile are considered to be within the scope of this disclosure. An upper surface  42  of the baseplate is configured to selectively secure the meniscal insert  50  to the baseplate  45 . The keel  43  has a keel longitudinal axis KLA extending lengthwise therethrough (i.e., the keel longitudinal axis KLA is disposed parallel to the height h dimension of the keel  43 ). Unlike prior tibial components  40 , the baseplate  45  is not oriented perpendicular to the keel  43  but is instead oriented at a compound angle comprising a keel posterior angle θ and a keel varus angle δ. Without being bound by theory, it is thought that such embodiments can assist in kinematic or anatomic alignment procedures. Moreover, it is contemplated that use of the exemplary tibial components  40  described herein may permit a surgeon to use kinematic alignment or anatomic alignment procedures in revision knee arthroplasties that might not otherwise have been possible using conventional tibial components. 
     As can be seen in the side view of  FIG.  1 A , the baseplate  45  slopes downwardly from the anterior side A to the posterior side P of the tibial component  40  relative to the keel longitudinal axis KLS. A baseplate plane BPP can be imagined to be coplanar with the anterior — posterior line A-P and the medial-lateral line M-L ( FIG.  1 B ) of the baseplate  45  (stated differently, the baseplate plane BPP can be imagined to be coplanar with a width w dimension and a length l dimension of the baseplate  45 ). Because the baseplate plane BPP is co-planar with the anterior-posterior line A-P and the medial-lateral line M-L of the baseplate  45 , the baseplate plane BPP can be described as having an anterior-posterior line A-P and a medial-lateral line M-L that are co-linear with the respective anterior-posterior line A-P and medial-lateral line M-L of the baseplate  45 . When measured with reference to the keel longitudinal axis KLA, the keel posterior angle θ can be described as the acute angle defined by the intersection between the anterior-posterior line A-P of the baseplate plane BPP (represented by BPP in  FIG.  1 A ) and the keel longitudinal axis KLA, wherein the acute angle is disposed closer to the posterior side P of the keel  43  than the anterior side A. 
     Stated differently, the intersection of the anterior-posterior line A-P line of the baseplate plane BPP and a transverse plane TRP defines a posterior slope ( 90 θ). 
     The transverse plane TRP can be imagined to be disposed perpendicular to the keel longitudinal axis KLA of the keel  43  in both the coronal plane CP ( FIG.  5   ) and the sagittal plane SP ( FIG.  5   ). In exemplary embodiments, the posterior slope  90 −θ may be in a range of greater than zero degrees to less than or equal to 15 degrees, preferably between 1 degree and ten degrees. However, in many applications, the posterior slope  90 −θ is set at about three degrees. 
       FIG.  1 B  shows a top down view of an exemplary tibial component  40 . Locking features  47  are shown on the upper surface  42  of the tibial baseplate  45  for reference. In embodiments, the locking features  47  can comprise a partial perimeter lip  52  that can be disposed at the anterior side A of the baseplate  45 , at the posterior side P, and partially at the lateral side L and the medial side M. These locking features can also comprise a keying element  53  that can be inserted into (or receive, depending upon configuration) a complementary keying element  54  ( FIG.  6   ) in the meniscal insert  50 . In this manner, these locking features  47  mate with complementary locking features (see  54 ) on the meniscal insert  50  when the meniscal insert  50  is installed on the upper surface  42  of the tibial baseplate  45 . As such, the upper surface  42  of the baseplate  45  can be said to be “configured to selectively secure” the meniscal insert  50  to the baseplate  45 . 
       FIG.  1 C  is a frontal view (i.e., an A-P view) of the tibial component  40 . As  FIG.  1 C  more clearly illustrates, the tibial baseplate  45  slopes downwardly from the lateral side L to the medial side M of the tibial component  40  relative to the keel longitudinal axis KLA. Because the baseplate plane BPP is co-planar with the anterior-posterior line A-P and the medial-lateral line M-L of the baseplate  45 , the baseplate plane BPP can be described as having an anterior-posterior line A-P and a medial-lateral line M-L that are co-linear with the respective anterior-posterior line A-P and medial-lateral line M-L of the baseplate  45 . When measured with reference to the keel longitudinal axis KLA, the keel varus angle δ can be described as the acute angle defined by the intersection between the medial-lateral line M-L of the baseplate plane BPP (represented by BPP in  FIG.  1 C ) and the keel longitudinal axis KLA, wherein the acute angle is disposed closer to the medial side M of the keel  43  than the lateral side L. 
     Stated differently, the medial-lateral line M-L of the baseplate plane BPP intersects the transverse plane TRP at the medial side M to define a varus tilt  90 −δ. The varus tilte  90 −δ may be greater than zero degrees to less than or equal to seven degrees. However, in many applications, the varus tilt  90 −δ is set at about three degrees. It will be appreciated that tibial components  40  having different combinations of preset keel varus  90 −δ and keel posterior angles θ (and therefore corresponding present varus tilts  90 −δ and posterior slopes  90 −θ) are considered to be within the scope of this disclosure. 
       FIG.  2 A  is a top down view of one embodiment of an exemplary tibial component  40 . The tibial component  40  of  FIG.  2 A  is configured for use on a left knee and is therefore a substantially mirror image of the tibial component shown in  FIGS.  1 A- 1 C . 
       FIG.  2 B  is an anterior view of the exemplary tibial component of  FIG.  2 A  installed in a resected tibia  20  (i.e., the tibial component  40  is in the installed configuration). As shown, the baseplate  45  has a keel varus angle δ and the baseplate  45  also slopes downwardly from the anterior side A to the posterior side P of the knee (i.e., has a keel posterior angle θ, see  FIG.  2 C ). A medial inner cortical wall  25   m  and a lateral inner cortical wall  251  are depicted to represent the bounds of the intramedullary canal  27  of the tibia  20 . The fibula  60  is also shown for reference.  FIG.  2 B  more clearly illustrates that as the eye moves from the proximal end  13  of the tibia  20  to the distal end of the tibia  20 , the medial inner cortical wall  25   m  and the lateral inner cortical wall  251  taper as the metaphysis transitions into the diaphysis. In revision procedures in which a prior-installed tibial component is removed through undercutting the proximal tibia below the prior resected tibial surface, the total usable area of the intramedullary canal  27  decreases, particularly at the proximal end  13  (i.e., note how the metaphysis (i.e., the area of the generally wedge shaped portion) of the intramedullary canal  27  near the proximal end  13  would further decrease if the position of the resected tibial surface  23  were positioned any lower). 
       FIG.  2 C  provides a side view of the tibial component  40  of  FIG.  2 A  implanted on the resected tibia  20 . As can be seen in the side view of  FIG.  2 C , the baseplate  45  has a keel posterior angle θ that slopes downwardly from the anterior side A to the posterior side P of the knee. Furthermore, the keel longitudinal axis KLA of the tibial component  40  is shown essentially aligned with the longitudinal axis LA of the tibia  20  while being perpendicular to sagittal plane SP and the coronal plane CP when the tibial component  40  is in the installed configuration. The longitudinal axis of the tibia is also known as the anatomic axis of the tibia by those skilled in the art. “Anatomic axis of the tibia” is also sometimes used interchangeably with “mechanical axis of the tibia” because the anatomic axis of the tibia and the mechanical axis of the tibia are co-axial for most patients. 
     Without being bound by theory, it is contemplated that by having a sloped baseplate  45  relative to the keel longitudinal axis KLA in the manner described, that extends into the intramedullary canal  27  when the tibial component  40  is in the installed configuration such that the keel  43  is substantially aligned with (i.e., is coaxial with) the longitudinal axis LA of the tibia  20  and such that the keel  43  is disposed perpendicular to a sagittal plane SP (see  FIG.  5   ) and a coronal plane CP (see  FIG.  5   ), the exemplary tibial component  40  permits the keel  43  to extend generally downwardly into the intramedullary canal  27  of the tibia  20  without having the distal end  44  of the keel  43 , or any other part of the keel  43  contact the inner cortical wall  25  of the tibia  20 . In this manner, exemplary tibial components  40  substantially reduce the risk of the distal end  51  of the stem construct breaching the proximal cortical bone while orienting the baseplate  45  at a compound angle that can be used to more accurately reconstruct the patient&#39;s native, pre-diseased joint line. In  FIG.  2 C , a posterior inner cortical wall  25   p  and an anterior inner cortical wall  25   a  are depicted. 
     However, it will be appreciated that the exemplary tibial components  40 , when in the installed configuration, do not necessarily need to have the keel longitudinal axis KLA to be fully aligned with the longitudinal axis LA of the tibia  20 . In such embodiments, the keel  43  is not necessarily disposed perpendicular to both the sagittal plane SP and the coronal plane CP (see  FIG.  5   ). Full alignment is considered desirable because it is thought to maximize available remaining space in the intramedullary canal  27 , but partial alignment may be more appropriate in a given situation depending upon a number of factors, including patient anatomy, position of reactions, and surgeon preference. As such, disposing the keel longitudinal axis KLA at an angle relative to the longitudinal axis LA of the tibia  20  is considered to be within the scope of this disclosure. 
     As a result, and without being bound by theory, it is contemplated that the exemplary embodiments disclosed herein may permit the surgeon to insert longer keels  43  into the tibia  20  than was previously possible, especially in cases where the resected proximal end  13  of the tibia  20  was disposed at a slope (particularly at a compound angle comprising a varus tilt  90 −δ and a posterior slope  90 −θ). The longer keels  43  can more firmly stabilize and secure the tibial component  40  to the tibia  20  in kinematic alignment procedures and in anatomic alignment procedures over what was previously possible. It is still further contemplated that by aligning the keel longitudinal axis KLA with the longitudinal axis LA of the tibia  20 , the force vectors that result from the natural ambulatory movement of the knee can be more stably transferred through the tibial component  40  to the tibia  20  and thereby prolong the useful life of the endoprosthetic knee implant  1 . For example, when a person is standing, it is thought that the force exerted on the tibial component  40  from the mass of the person above the tibial component  40  can be more stably transferred to the feet of the person because the keel  43  of the tibial component  40  is desirably oriented closer to the mechanical axis of the tibia  20  (i.e., the longitudinal axis LA). The tibia has naturally evolved to distribute the force of the mass of a person generally along the mechanical axis toward the feet. By substantially aligning the keel longitudinal axis KLA with the longitudinal axis of the tibia, it is thought that the exemplary embodiments described herein may preserve the natural force distribution and kinematics of a pre-diseased natural knee over what was previously possible. 
       FIGS.  3 A- 4 C  show exemplary embodiments of a tibial component  40  of an endoprosthetic knee implant  1 , wherein the distal end  44  of the keel  43  is configured to selectively attach to a distal stem extension  100 . A surgeon may elect to use a distal stem extension  100  when there is a need to further stabilize the tibial component  40  in the intramedullary canal  27  of the tibia  20 .  FIG.  3 A  shows a side view of a right knee tibial component  40  having a keel varus angle δ (better visualized in  FIG.  3 C ) and a keel posterior angle θ. The depicted tibial component  40  is similar to the exemplary embodiment shown in  FIGS.  1 A- 2 C . However, rather than having a rounded distal end  44 , the depicted keel  43  is configured to selectively connect to a distal stem extension  100 . It will be appreciated that the connecting elements on the distal end  44  of the keel  43  and the proximal end  49  of the stem extension  100  can be complementary mating elements. For example, the distal end  44  may have one or more projections and the proximal end  49  may have one or more complementary receivers designed to selectively and closely receive the one or more projections, or vice versa. In this manner, the keel  43  can be said to be “configured to selectively connect” or “configured to selectively attach” to a distal stem extension  100 . Likewise, the distal stem extension  100  can be said to be “removably engaged” to the keel  43 . In certain exemplary embodiments, the projections and complementary receivers desirably lockingly engage one another in an installed configuration. For example, certain exemplary embodiments can have a frustoconical or a frustopyramidal projection on either the distal end  44  of the keel  43  or the proximal end  49  of the stem extension  100 . To continue the example, the receiver (i.e., the proximal end  49  of the stem extension  100  if the projection comprises the distal end  44  of the keel  43 , and vice versa) can be a complementary frustoconical or frustopyramidal receiver designed to selectively and closely receive the projection of the mating element (e.g., either the distal end  44  of the keel  43  or the proximal end  49  of the stem extension  100 ). In other exemplary embodiments, the projections and complementary receivers can be screw threads. Combinations of the foregoing are considered to be within the scope of this disclosure. 
     In the depicted embodiment, the distal end  44  of the keel  43  comprises a tapered bore, the bore is sized to closely receive a tapered extension on the proximal end  49  of the distal stem extension  100 , in a press-fit secured configuration. As can be seen in  FIG.  3 A , the keel  43  and the distal stem extension  100  together form a stem construct  143  having a substantially continuous keel longitudinal axis KLA. 
     Although  FIGS.  3 A- 4 C  depict exemplary tibial components  40  comprising selectively attachable stem extensions  100 , it will be appreciated that offset adaptors and angle adaptors can be configured to attach to the distal end  44  of the keel  43  in the same manner as the distal stem extension  100 . Such offset adapters or angle adaptors can be used in lieu of or in addition to the distal stem extension  100 . When both an adaptor and a stem extension are present, an offset adaptor, an angle adaptor, or a combination thereof would commonly be disposed between the distal end  44  of the keel  43  and the proximal end  49  of the distal stem extension  100 . However, in exemplary embodiments comprising a modular keel  43  (see  FIGS.  7 A- 8   ), the offset adaptor or angle adaptor, or a combination thereof, could be disposed between the proximal end  61  of the modular keel  43  and the lower end  41  of the baseplate  45 . 
     As can be seen in the side view of  FIG.  3 A , the baseplate  45  is configured to slope downwardly from the anterior side A to the posterior side P of the tibial component  40  relative to the keel longitudinal axis KLA to define a keel posterior angle θ as described above. Stated differently, the intersection of the anterior-posterior line A-P line of the baseplate plane BPP with the transverse plane TRP at the posterior side P of the baseplate  45  defines a posterior slope  90 −θ. The posterior slope  90 −θ may be greater than zero degrees degree to less than or equal to fifteen degrees, preferably between 1 degree and ten degrees. However, in many applications, the posterior slope  90 −θ is set at about three degrees. 
     The distal stem extension  100  may have grooves  98  disposed generally parallel to the keel longitudinal axis KLA when installed on the keel  43 . These grooves  98  can facilitate the installation of the distal stem extension  100  into the intermedullary canal  27  ( FIG.  4   ), especially when the distal stem extension  100  is a press fit stem extension. These grooves  98  can mitigate the displacement of medullary bone and marrow when the distal stem extension  100  is inserted into the tibial diaphysis. Displaced medullary bone and marrow can flow upwardly through these grooves  98  toward the still exposed tibial resected surface  23  to thereby be collected and disposed of. Without these grooves  98 , the distal stem extension  100  could capture air in the intramedullary canal  27  or otherwise build up pressure and risk breaching the proximal cortex of the bone. The grooves  98  can also secure the distal stem extension  100  rotationally within intramedullary canal  27 . When the distal stem extension  100  is a cemented distal stem extension such as the one depicted in  FIG.  3 A , the grooves  98  also provide additional space in which bone cement can be disposed between the distal stem extension  100  and the walls of the intramedullary canal  27 . 
     The distal end  51  of the distal stem extension  100  can be selected from a variety of shapes. In certain exemplary embodiments, the distal end  51  of the distal stem extension  100  can be rounded, substantially straight, wedge shaped, wedge shaped with a rounded end, conical, conical with a rounded end, frustoconical, frustoconical with a rounded end, pyramidal, pyramidal with a rounded end, frustopyramidal, frustopyramidal with a rounded end, or combinations thereof. Shapes that have a generally convex profile are generally preferable to reduce the risk of unnecessarily ablating cancellous bone and bone marrow during the installation process. All such shapes having a generally convex profile and considered to be within the scope of this disclosure. 
     Without being bound by theory, it is contemplated that a removable distal stem extension  100  may permit surgeons to select from a group of available distal stem extensions  100  provided in a kit on the day of the procedure. Common of types of distal stem extensions  100  include press-fit stem extensions and cemented stem extensions. The surgeon or the technician can select from the provided group of available stem extensions  100  to effectively change the overall height of the tibial component  40  without having to rely on selectively testing and removing multiple unitary tibial components  40 . The number of distal stem extensions  100  available to the surgeon can depend upon a number of factors, including the existence of a pre-operative measurements taken of the interior of the tibia  20  through radiography or other imaging methods. 
       FIG.  3 B  shows a top down view of the exemplary tibial component  40  of  FIG.  3 A . As can be seen in the frontal view or A-P view of the exemplary tibial component  40  in  FIG.  3 C , the baseplate  45  is configured to slope downward from the lateral side L to the medial side M of the tibial component  40  relative to the keel longitudinal axis KLA to thereby define the keel varus angle δ as described above. Stated differently, the medial-lateral line M-L of the baseplate plane BPP intersects the transverse plane TRP to define a varus tilt  90 −δ. The varus tilt  90 −δ can be in a range of greater than zero degrees to less than or equal to seven degrees. However, in many applications, the varus tilt  90 −δ is set at about three degrees. It will be appreciated that tibial components  40  having different combinations of preset posterior slopes  90 −θ and varus tilts  90 −δ are considered to be within the scope of this disclosure. 
       FIG.  4 A  is a top-down view of one embodiment of an exemplary tibial component  40  in accordance with this disclosure.  FIG.  4 B  shows the tibial component  40  implanted on a resected proximal tibia  20 . The tibial component  40  of  FIG.  4 A  is configured for use on a left knee and is therefore substantially a mirror image of the tibial component  40  shown in  FIGS.  3 A- 3 C . As can be seen in the anterior view of  FIG.  2 B , the tibial baseplate  45  has a keel varus angle δ that slopes downwardly from the lateral side L to the medial side M of the knee. 
       FIG.  4 C  provides a side view of the exemplary tibial component  40  of  FIG.  4 A  implanted on the resected tibia  20 . As can be seen in the side view of  FIG.  4 C , the tibial baseplate portion  45  has a keel posterior angle θ that slopes downwardly from the anterior side A to the posterior side P. In certain cases (e.g., revision cases), augments can be attached to the lower surface  41  of the baseplate  45  to compensate for poor or missing bone. 
     Without being bound by theory, it is contemplated that by having a sloped baseplate  45  relative to the keel longitudinal axis KLA in the manner described that extends into the intramedullary canal  27  when the tibial component  40  is in the installed configuration such that the stem construct  143  is substantially aligned with the longitudinal axis LA of the tibia  20  and such that the stem construct  143  is disposed perpendicular to a sagittal plane SP (see  FIG.  5   ) and a coronal plane CP (see  FIG.  5   ), the exemplary tibial component  40  permits the stem construct  143  to extend generally downwardly into the intramedullary canal  27  of the tibia  20  without having the distal end  51  of the stem construct  143 , or any other part of the stem construct  143  contact the inner cortical wall  25  of the tibia  20 . In this manner, exemplary tibial component  40  substantially reduces the risk of the distal end  51  of the stem construct breaching the proximal cortical bone while orienting the baseplate  45  at a compound angle that can be used to more accurately reconstruct the patient&#39;s native, pre-diseased joint line. In  FIG.  4 C , a posterior inner cortical wall  25   p  and an anterior inner cortical wall  25   a  are depicted for reference. 
     As a result, it is contemplated that the exemplary embodiments disclosed herein may permit the surgeon to insert longer stem constructs  143  into the tibia  20  than was previously possible in cases where the resected proximal end  13  of the tibia  20  was disposed at a slope (i.e., in a prior kinematic alignment or anatomic alignment procedure. Previously, stemmed revision-style tibial components were only usable with a revision mechanical alignment procedure regardless of whether the patient had undergone a primary kinematic alignment or a primary anatomic alignment procedure. Such procedures were previously not possible because of the amount of bone that had to be removed to extract the pre-existing implant and because the removal of existing bone necessarily reduced the amount of available volume (particularly the length and width dimensions) in the remaining intramedullary canal  27  in which to insert a stabilizing keel  43  or stem construct  143 . 
     It is further contemplated that the exemplary embodiments described herein may permit a surgeon to perform a kinematic alignment or an anatomical alignment procedure in a revision procedure (i.e., a secondary surgery in which the original implant is removed) or in trauma cases that feature severely compromised tibial bone. Without being bound by theory, it is conceivable that a patient who underwent a mechanical alignment knee arthroplasty during a primary or prior revision procedure may now be able to benefit from a kinematic or anatomic alignment procedure. 
     Nothing in this disclosure limits the use of the exemplary tibial components and related instruments therefore to revision procedures, however. It is contemplated that such exemplary tibial components and/or exemplary instruments described herein can be used in primary procedures. It is believed that surgeons may especially benefit from the embodiments of the present disclosure in stemmed primary procedures. 
     Stemmed primary procedures are often used with patients with a high body mass index or that suffer from poor bone quality. It is thought that the use of a distal stem extension  100  in these cases can more reliably and more effectively distribute the stationary and ambulatory forces that a patient and the tibial component  40  with a stem construct  143  experiences. 
     In this manner, it is contemplated that the longer stem constructs  143  can more firmly stabilize and secure the tibial component  40  to the tibia  20  in kinematic alignment procedures and in anatomic alignment procedures over what was previously possible. It is still further contemplated that by aligning the keel longitudinal axis KLA with the longitudinal axis LA of the tibia  20 , the force vectors that result from the natural ambulatory movement of the knee can be more stably transferred through the tibial component  40  to the tibia  20  and thereby prolong the useful life of the endoprosthetic knee implant  1 . 
     As will be appreciated from the foregoing discussion, all embodiments of the tibial component  40  can be configured for use on right or left knees. In other exemplary embodiments, the exemplary tibial component can comprise a symmetric baseplate  45 . 
     Referring to  FIGS.  7 A- 7 D , in certain exemplary embodiments the keel  43  is modular. That is, the keel  43  itself can selectively disengage the lower surface  41  of the baseplate  45  (see  FIGS.  8 A and  8 B ). The modular keel  43  can be produced in a number of different dimensions. It will be appreciated that the connecting elements on the lower surface  41  of the baseplate  45  and the proximal end  61  of the modular keel  43  can be complementary mating elements. For example, the lower surface  41  of the baseplate  45  may have one or more projections and the proximal end  61  of the modular keel  43  may have one or more complementary receivers designed to selectively and closely receive the one or more projections, or vice versa. In certain exemplary embodiments, the projections and complementary receivers desirably lockingly engage one another in an installed configuration. Combinations of the foregoing are considered to be within the scope of this disclosure. In this manner, the proximal end of the modular keel  43  is configured to engage the lower surface  41  of the baseplate  45 . 
     It will be appreciated that the proximal end  61  of the modular keel can define a compound angle comprising a keel varus angle δ and a keel posterior angle θ. As a result, it is contemplated that the modular keel  43  can be chiral. That is, a left-sided modular keel  43  is configured to engage a left-sided baseplate  45  while a right-sided modular keel  43  is configured to engage a right-sided baseplate  45 . 
       FIG.  7 A  depicts a modular keel  43  for a right knee in an unassembled configuration. The depicted modular keel  43  comprises a body  48  and fins  46  extending transversely therefrom. A keel longitudinal axis KLA extends axially along the height h of the body  48  of the modular keel  43 . The modular keel  43  comprises a proximal end  61  that is distally disposed from a distal end  44 , a medial side M that is distally disposed from a lateral side L, and an anterior side A that is distally disposed from a posterior side P. 
     The modular keel  43  defines a compound angle at the proximal end  61 , wherein the compound angle comprises a keel posterior angle θ (see  FIG.  7 E ) and a keel varus angle S.  FIG.  7 A  is an anterior view of the exemplary modular keel  43  in which the modular keel  43  is oriented generally vertically (i.e., the keel longitudinal axis KLA extends generally up and down). The depicted orientation illustrates both the keel varus angle δ and the varus tilt  90 −δ more clearly. Because the depicted embodiment is for a right knee and because the depicted modular keel  43  is oriented vertically in an anterior view, the proximal end  61   l  of the lateral fin  46   l  (depicted on the left side of the image in  FIG.  7 A ) is disposed above the proximal end  61   m  of the medial fin  46   m  (depicted on the right side of the image in  FIG.  7 A ). In this manner, the proximal end  61  of the keel  43  can be imagined to define medial-lateral line M-L. The keel varus angle δ can be visualized as the acute angle that results from the intersection of the medial-lateral line M-L with the keel longitudinal axis KLA, wherein the acute angle is disposed closer to the medial side M than the lateral side L. 
     When viewed in the depicted orientation, the proximal end  61  of the keel  43  can also be imagined to be disposed at a varus tilt  90 −δ relative to a transverse plane TRP. It will be appreciated that when the depicted modular keel  43  is assembled with a modular baseplate  45  (see  FIG.  8   ) (i.e., an assembled configuration), the medial-lateral line M-L of the proximal end  61  of the modular keel  43  is disposed parallel to the medial-lateral line M-L of the baseplate plane BPP. 
     As with the unitary embodiments, the varus tilt  90 −δ of the modular keel  43  embodiments can be about three degrees for most patients, but angles greater than zero and less than or equal to seven degrees are considered to be within the scope of this disclosure. Multiple modular keels  43  may be provided at the time of a surgical procedure. Of the provided modular keels  43 , one or more provided keels may have different compound angles (including but not limited to, for the sake of example, provided keels  43  having different keel varus angles δ, different keel posterior angles θ, or combinations thereof). 
       FIG.  7 B  is a top down view of the modular right-sided keel  43  of  FIG.  7 A . In  FIG.  7 B , the orientation of the keel  43  has been adjusted such that the proximal end  61  of the modular keel  43  is coplanar with the transverse plane TRP. In this orientation, the compound angle can be better visualized. The keel varus angle δ is visualized by the showing the distal end  44  of the modular keel  43  extending more toward the medial side M of the modular keel  43  than toward the lateral side L. The keel posterior angle θ is illustrated by showing the distal end  44  of the modular keel  43  also extending more toward the proximal side P of the modular keel  43  than the anterior side A. As shown more clearly with reference to  FIG.  8   , the proximal end  61  of the modular keel  43  can be selectively fixedly engaged (which can also be known as being “removably engaged”) to a modular baseplate  45  in a number of ways appreciated by those having ordinary skill in the art, including all of the ways that a stem extension  100  can be selectively connected or attached to the distal end  44  of a keel  43 .  FIGS.  7 A- 7 D  depict a receiver  56  that can closely receive a projection  65  ( FIGS.  8 A- 8 B ) extending from the lower surface  41  of a modular tibial baseplate  45  to thereby selectively fixedly engage the modular keel  43  to a modular tibial baseplate  45 . 
       FIG.  7 C  depicts a modular keel  43  for a left knee in an unassembled configuration. The modular keel  43  of  FIG.  7 C  is essentially a mirror image of the modular keel  43  depicted in  FIG.  7 A . Because the depicted embodiment is for a left knee and because the depicted modular keel  43  is oriented vertically in an anterior view, the proximal end  61   l  of the lateral fin  46   l  (depicted on the right side of  FIG.  7 C ) is disposed above the proximal end  61   m  of the medial fin  46   m  (depicted on the left side of  FIG.  7 C ). In this manner, the proximal end  61  of the keel  43  can be imagined to define medial-lateral line M-L. The keel varus angle δ can be visualized as the acute angle that results from the intersection of the medial-lateral line M-L with the keel longitudinal axis KLA, wherein the acute angle is disposed closer to the medial side M than the lateral side L. 
     When viewed in the depicted orientation, the proximal end  61  of the keel  43  can also be imagined to be disposed at a varus tilt  90 −δ relative to a transverse plane TRP. It will be appreciated that when the depicted modular keel  43  is assembled with a modular baseplate  45  ( FIG.  8   ) (i.e., an assembled configuration), the medial-lateral line M-L of the proximal end  61  of the modular keel  43  is disposed parallel to the medial-lateral line M-L of the baseplate plane BPP. 
       FIG.  7 D  is a top down view of the exemplary modular left-sided keel  43  of  FIG.  7 C .  FIG.  7 D  is essentially a mirror image of the modular keel  43  depicted in  FIG.  7 B . In  FIG.  7 D , the orientation of the keel  43  has been adjusted such that the proximal end  61  of the modular keel  43  is coplanar with the transverse plane TRP. In this orientation, the compound angle can be better visualized. The keel varus angle δ is visualized by the showing the distal end  44  of the modular keel  43  extending more toward the medial side M of the modular keel  43  than toward the lateral side L. The keel posterior angle θ is illustrated by showing the distal end  44  of the modular keel  43  also extending more toward the proximal side P of the modular keel  43  than the anterior side A. 
       FIG.  7 E  is a medial side view of an exemplary modular keel  43  that is positioned to highlight the keel posterior angle θ. The keel posterior angle θ can be visualized as the acute angle that results from the intersection of the anterior-posterior line A-P with the keel longitudinal axis KLA, wherein the acute angle is disposed closer to the posterior side P than the anterior side A. When viewed in the depicted orientation, the proximal end  61  of the keel  43  can also be imagined to be disposed at a posterior slope  90 −θ relative to a transverse plane TRP. It will be appreciated that when the depicted modular keel  43  is assembled with a modular baseplate  45  (see  FIGS.  8 A- 8 B ) (i.e., an assembled configuration), the anterior-posterior line A-P of the proximal end  61  of the modular keel  43  is disposed parallel to the anterior-posterior line A-P of the baseplate plane BPP. 
     As with the unitary embodiments, the posterior slope  90 −θ of the modular keel  43  embodiments can be about three degrees for most patients, but angles greater than zero and less than or equal to fifteen degrees are considered to be within the scope of this disclosure. 
       FIG.  7 F  is a lateral side view of an exemplary modular keel  43  wherein the proximal end is disposed at a compound angle relative to the keel longitudinal axis KLA, wherein the compound angle comprises a keel posterior angle θ and a keel varus angle δ.  FIG.  7 F  is essentially a mirror image of the exemplary modular keel  43  depicted in  FIG.  7 E . 
     In certain exemplary embodiments, the modular keel  43  may have a distal end  44  that is not configured to engage a distal stem extension  100 . In such embodiments, the distal end  44  of the modular keel  43  can be rounded, substantially straight, wedge shaped, wedge shaped with a rounded end, conical, conical with a rounded end, frustoconical, frustoconical with a rounded end, pyramidal, pyramidal with a rounded end, frustopyramidal, frustopyramidal with a rounded end, or combinations thereof. Shapes that have a generally convex profile are generally preferable to reduce the risk of unnecessarily ablating cancellous bone and bone marrow during the installation process. Such generally convex shapes can also reduce stress risers in the bone cement. 
     In other exemplary embodiments comprising a modular keel  43 , the distal end  44  can be configured to selectively engage a distal stem extension  100  in substantially the same manner as described above (see generally  FIG.  3 A  and  FIG.  3 C ). 
     Without being bound to theory, it is contemplated that combinations of the modular keel  43  and of the distal stem extensions  100  can allow the surgeon to select a constructed tibial component  40  that more closely matches the patient&#39;s anatomy after the tibia  20  has been resected at a compound angle (comprising a posterior slope  90 −θ and a varus tilt  90 −δ) than what was previously available. The modular keel  43  and distal stem extension  100  may allow the surgeon to insert the stem construct  143  of an optimal height such that the keel longitudinal axis KLA is substantially aligned with the tibial longitudinal axis LA while being disposed generally perpendicular to an intersecting sagittal plane SP and a coronal plane CP. In this manner, surgeons can practice kinematic and anatomic alignment methods without risking using a shorter keel  43  or stem construct  100  on the tibial component  40  and without risking the keel  43  or stem construct  100  contacting an inner cortical wall  25   a ,  25   l ,  25   m ,  25   p  of the tibia  20 . 
       FIG.  8 A  is a front view of an exemplary tibial component  40  comprising a modular keel  43  and a modular tibial baseplate  45  shown in an expanded view. It will be appreciated that when the modular keel  43  and the modular baseplate  45  do not engage one another, the modular keel  43  and/or the modular baseplate  45  can be said to be in an uninstalled and unassembled configuration. When the baseplate  45  fixedly engages the modular keel  43  (e.g., when the projection  65  descending from the lower surface  41  of the baseplate  45  is disposed in the receiver  56  at the proximal end  61  of the keel  43 ), the keel  43  and/or baseplate  45  can be said to in an assembled configuration. In this manner, the assembled keel  43  and baseplate  45  can be said to comprise an exemplary tibial component  40 . When the assembled keel  43  and baseplate  45  are surgically implanted in a patient, the assembled keel  43  and baseplate  45  can be said to be in an installed configuration. 
     In the depicted embodiment, the projection  65  is a tapered projection and the receiver  56  is a tapered receiver that closely receives the tapered projection and, in this manner, “fixedly engages” the modular baseplate  45  to the modular keel  43 . However, all methods of mechanically and securely engaging the modular keel  43  to the modular baseplate  45  are considered to be within the scope of this disclosure. Examples of such other mechanical engagement mechanisms include screws, pins, and any other locking projection and receiver mechanism known by those having ordinary skill in the art.  FIG.  8 A  highlights the keel varus angle δ of the compound angle of the keel proximal end  61  and depicts the varus tilt  90 −δ as described further above. 
       FIG.  8 B  is a side view of the exemplary tibial component  40  depicted in  FIG.  8 A .  FIG.  8 B  highlights the keel posterior angle θ of the compound angle and depicts the posterior slope  90 −θ as described further above. 
       FIG.  11 A  is a top down view of an exemplary tibial component  40  wherein the baseplate  45  is symmetric around the A-P bisecting line. As a subcomponent in an unassembled configuration, the depicted baseplate  45  can be used in either the right or left knee. 
       FIG.  11 B  is a front view of an exemplary tibial component  40  wherein the keel  43  is offset from a central anterior-posterior plane APP (see also  FIG.  11 C ) that extends along the A-P line depicted in  FIG.  11 A . In the depicted embodiment and orientation, the central anterior-posterior plane APP bisects the baseplate  45  vertically along the A-P line to define the anterior-posterior plane APP and the keel longitudinal axis KLA is offset from the anterior-posterior plane APP medially. In other exemplary embodiments, the keel longitudinal axis KLA can be offset from the anterior-posterior plane APP laterally. In still other exemplary embodiments, it is contemplated that the keel longitudinal axis KLA can be offset from a medial-lateral plane anteriorly or posteriorly. Combinations of anterior, posterior, medial, and lateral offsets are considered to be within the scope of this disclosure. It is contemplated that most offsets would be less than 10 mm because the available space in most resected tibias  20  is limited. 
     The embodiment of  FIG.  11 B  illustrates that the general placement of the fins  46  does not necessarily have to change to accommodate the offset keel  43 . The medial fin  46   m  is smaller than the lateral fin  46   l  to accommodate the further reduced space in the medial side of the tibia. The space is further reduced due in part to the presence of the offset keel  43 . As in the before discussed exemplary embodiments, the proximal surface  61  of the keel  43  is disposed at a compound angle relative to the keel longitudinal axis KLA wherein the compound angle comprises a keel posterior angle θ ( FIG.  11 C ) and a keel varus angle δ. 
     The depicted embodiment shows the distal end of the keel  43  engaging a distal stem extension  100  to define a stem construct  143 . However, in other exemplary embodiments, the distal end of the offset keel  43  can be generally rounded as described above. In other exemplary embodiments, the offset keel  43  can be modular and selectively attachable to the tibial baseplate  45 . It is contemplated that modular offset keels  43  and unitary offset tibial components  40  can be provided in left knee and right knee configurations. 
     The tibial components  40  described herein can be provided in the form of a kit. For example, any of the tibial components  40 , modular tibial baseplate  45 , distal stem extensions  100 , and modular keels  43  (including but not limited to non-offset modular keels and offset modular keels) can be provided with a plurality of different sizes. Chiral components can be provided for a left knee and a right knee. The tibial components  40  and/or the modular keels  43  can be provided in a number of different compound angles, wherein of the number of different compound angles, the compound angles differ by at least one of a difference in keel posterior angle θ and/or keel varus angle S. Optionally, exemplary kits may further comprise offset adaptors, angle adaptors, augments, meniscal inserts, femoral components, trial components, or combinations of any thereof. The components of the kit are preferably arranged in a convenient format, such as in a surgical tray or case. However, the kit components do not have to be packaged or delivered together, provided that they are assembled or collected together in the operating room for use at the time of surgery. 
     An exemplary kit can include any suitable embodiment of an exemplary tibial component  40 , variations of the exemplary tibial components  40  described herein, and any other exemplary tibial component  40  according to an embodiment (including sub-components thereof such as a modular keel  43  and a modular baseplate  45 ). While it is contemplated that an exemplary kit may include one or more tibial components  40  and one or more distal stem extensions  100 , it will be appreciated that certain kits may lack some or all of these components. 
     Any suitable embodiment of a tibial baseplate  45 , variations of the tibial baseplates  45  described herein, and any other tibial baseplate  45  according to an embodiment are considered to be within the scope of this disclosure. 
     Likewise, any suitable embodiment of a modular keel  43 , variations of the modular keels  43  described herein, and any other modular keel  43  according to an embodiment are considered to be within the scope of this disclosure. 
     Still further likewise, any suitable embodiment of a distal stem extension  100 , variations of the distal stem extensions  100  described herein, and any other distal stem extension  100  according to an embodiment are considered to be within the scope of this disclosure. 
     Selection of a suitable number or type of tibial components  40 , tibial baseplates  45 , modular keels  43 , and distal stem extensions  100  to include in a kit according to a particular embodiment can be based on various considerations, such as the procedure intended to be performed using the components included in the kit. 
       FIG.  9    is a perspective view of an exemplary instrument assembly  70  depicted in an expanded (e.g., unassembled configuration). The components of the exemplary assembly comprise a keel punch  73  and a punch guide  83 . The depicted punch guide  83  can also serve as a reamer guide. The keel punch  73  has a punch proximal end  71  distally disposed from a punch distal end  74  along a punch body  78 . The punch proximal end  71  is disposed at a keel posterior angle θ relative to a keel longitudinal axis KLA extending along a height dimension kh of the keel punch  73 . The punch proximal end  71  is disposed at a keel varus angle δ relative to the keel longitudinal axis KLA extending along the height dimension kh of the keel punch  73 . The punch distal end  74  comprises a sharp edge. Punch fins  76  extend transversely from the punch body  78 . The distal edges  77  of the punch fins  76  are desirably sharpened. The sharp edge of the punch distal end  74  and the sharp distal edges  77  of the punch fins  76  facilitate the displacement of intramedullary bone and marrow when inserted into the resected surface  23  of the tibia  20 . 
     The punch guide  83  has a guide distal end  84  distally disposed from a guide proximal end  81  along a guide body  88 . As seen more clearly in  FIGS.  10 A- 10 C , an inner wall  89  of the guide body  88  defines a through channel  85  that extends through the guide body  88  from the guide proximal end  81  to the guide distal end  84 . The through channel  85  is complementary to the perimeter profile of the keel punch  73  and slightly larger than the perimeter profile of the keel punch  73 . In this manner, the punch guide  83  can be said to be “configured to closely receive” the keel punch  73 . Protrusions can extend from the guide distal end  84  to secure the punch guide  83  to the resected tibial surface  23  when in use. 
     The center portion of the through channel  85  of the depicted punch guide is further sized and dimensioned to guide a reamer into the tibial intramedullary canal  27 . As seen in  FIG.  10 A , the inner wall  89  of the guide body  88  can comprise an anterior projection  89   a  that is distally disposed from a posterior projection  89   c  and a medial projection  89   b  that is distally disposed from a lateral projection  89   d . The most projected surface of the projections  89   a ,  89   b ,  89   c ,  89   d  can desirably be concavely curved to accommodate the generally cylindrical profile of the reamer. 
     In practice, when the exemplary instruments are arranged in an assembled and installed configuration (discussed further below), the surgeon can first insert a reamer (which generally resembles a large threaded drill bit) through the through channel  85  to create an initial intramedullary cavity. Depending upon the size of the patient&#39;s tibia, multiple reamers of progressively larger sizes may be inserted through the through channel  85  to iteratively enlarge the dimensions of the initial intramedullary cavity. After the initial intramedullary canal has been reamed to the desired dimensions, the surgeon can remove the reamer and then insert an exemplary keel punch  73  through the through channel  85  to define an intramedullary cavity that is generally complementary to the profile of the keel punch  73 . In certain exemplary procedures, multiple keel punches  73  of progressively larger sizes may be inserted through one or more punch guides  83  to define an intramedullary cavity that is generally complementary to the profile of the desired size of an exemplary keel  43  of an exemplary tibial component  40 . It is contemplated that multiple exemplary punch guides  83  may be provided in different sizes at the time of the surgical procedure. 
     Referring back to the structure of the depicted exemplary punch guide  83  with particular reference to  FIG.  10 B , the guide distal end  84  is disposed at a guide posterior angle gθ relative to a guide longitudinal axis GLA ( FIG.  10 D ) extending along a height dimension gh of the punch guide  83 . In like manner, an anterior side  89   a   1  of the inner surface  89  and a posterior side  89   c   1  of the inner surface  89  are desirably disposed at the same guide posterior angle gθ as the distal guide end  84  relative to the guide longitudinal axis GLA such that the through channel  85  of the punch guide  83  slopes anteriorly to posteriorly from the guide proximal end  81  to the guide distal end  84 . However, it will be appreciated that in other exemplary embodiments, the guide posterior angle gθ of the anterior side  89   a   1  and the posterior side  89   c   1  of the inner surface  89  can be different from the guide posterior angle gθ of the guide distal end  84 . The guide posterior supplemental angle  180 -gθ is also shown. 
     Moreover, as better seen in  FIG.  10 C , the guide distal end  84  is disposed at a guide varus angle gδ relative to the guide longitudinal axis GLA extending along the height dimension gh of the punch guide  83 . In like manner, a medial side  89   b   1  of the inner surface  89  and a lateral side  89   d   1  of the inner surface  89  are desirably disposed at the same guide varus angle gδ as the distal guide end  84  relative to the guide longitudinal axis GLA such that the through channel  85  of the punch guide  83  slopes laterally to medially from the guide proximal end  81  to the guide distal end  84 . However, it will be appreciated that in other exemplary embodiments, the guide varus angle gδ of the medial side  89   b   1  and the lateral side  89   d   1  of the inner surface  89  can be different from the guide varus angle gδ of the guide distal end  84 . The guide varus supplemental angle  180 -gδ is also shown. 
       FIG.  10 D  depicts an exemplary instrument assembly  70  in an assembled configuration. In operation, a surgeon may first place a trial tibial baseplate  45  to the resected surface  23  of the tibia  20 . Once positioned, the surgeon can insert the protrusions  75  (e.g., spikes) of the punch guide  83  through receiving holes in the trial baseplate  45 . The surgeon may use a mallet to hammer the protrusions  75  of the punch guide  83  through the receiving holes and into the resected tibia  20  to thereby secure the trial baseplate  45  and the punch guide  83  to the resected surface  23  of the tibia  20 . 
     The keel punch  73  can be affixed to the distal end of a broach handle. The surgeon can insert the keel punch  73  through the through channel  85 , which guides the keel punch  73  into the resected surface  23  of the tibia  20  at the desired keel posterior angle θ and keel varus angle δ. The surgeon may use a mallet to hammer the proximal end of the broach handle to insert the keel punch  73  into the resected surface  23  of the tibia  20 . In this manner, the surgeon can create a keel cavity in the intramedullary canal  27  of the tibia  20  at the desired keel posterior angle θ and keel varus angle δ to accommodate an exemplary keel  43  of an exemplary tibia component  40 . 
     It will be appreciated that the keel punch  73  and the punch guide  83  can be provided in different sizes. A surgeon may insert and remove successively larger sizes of the punch guide  83  and keel punch  73  to iteratively create a successively larger keel cavity in the intramedullary canal  27  until the desired size of the keel  43  of the tibial component  40  is achieved. Upon achieving the desired size and optionally evacuating any residual material from the keel cavity, the surgeon may insert the keel  43  of any of the exemplary tibial components  40  described herein into the keel cavity at the desired angle and position. 
     The exemplary instrument assemblies  70  described herein can be provided in the form of a kit. For example, the exemplary keel punch  73  and exemplary punch guide  83  can be provided with a plurality of different sizes. Chiral components can be provided for a left knee and a right knee. The keel punch  73  and/or punch guide  83  can be provided in a number of different compound angles, wherein of the number of different compound angles, the compound angles differ by at least one of a difference in keel posterior angle θ and/or keel varus angle δ. Optionally, exemplary kits may further comprise mallets, trial tibial baseplates, broach handles, or combinations of any thereof. The components of the kit are preferably arranged in a convenient format, such as in a surgical tray or case. However, the kit components do not have to be packaged or delivered together, provided that they are assembled or collected together in the operating room for use at the time of surgery. 
     An exemplary kit can include any suitable embodiment of an exemplary keel punch  73 , variations of the exemplary keel punches  73  described herein, and any other exemplary keel punch  73  according to an embodiment. While it is contemplated that an exemplary kit may further include one or more trial tibial baseplates  45 , it will be appreciated that certain kits may lack some or all of these components. 
     Any suitable embodiment of a punch guide  83 , variations of the punch guides  83  described herein, and any other punch guide  83  according to an embodiment are considered to be within the scope of this disclosure. 
     Selection of a suitable number or type of keel punches  73  and punch guides  83  to include in a kit according to a particular embodiment can be based on various considerations, such as the procedure intended to be performed using the components included in the kit. 
     An exemplary tibial component of an endoprosthetic knee implant comprises: a tibial baseplate; and a keel extending from a lower surface of the tibial baseplate, wherein a keel longitudinal axis extends axially through the keel, wherein the tibial baseplate is disposed at a keel posterior angle relative to the keel longitudinal axis, and wherein the tibial baseplate is disposed at a keel varus angle relative to the keel longitudinal axis. 
     In an exemplary tibial component, the keel posterior angle can be less than 90 degrees and can be greater than or equal to about 75 degrees. 
     In an exemplary tibial component, the keel varus angle can be less than 90 degrees and can greater than or equal to about 83 degrees. 
     In an exemplary tibial component, a distal stem extension can be removably engaged to the keel. 
     In an exemplary tibial component, the keel can be a modular keel, and wherein the modular keel is removably engaged to the tibial baseplate. 
     In an exemplary tibial component, the keel longitudinal axis can be aligned with the longitudinal axis of the tibia in both the sagittal plane and the coronal plane when the tibial component is disposed in an installed configuration. 
     An exemplary tibial component comprises: a tibial baseplate having an upper surface, a lower surface, an anterior side distally disposed from a posterior side, and a medial side distally disposed from a lateral side, wherein a first line connecting the anterior side and the posterior side, defines an anterior-posterior line, wherein a second line connecting the medial side to the lateral side defines a medial-lateral line, and wherein the anterior-posterior line is disposed perpendicular to the medial-lateral line on a tibial baseplate plane; and a keel descending from the lower surface of the tibial baseplate, wherein the anterior-posterior line of the tibial baseplate is disposed at a posterior slope relative to a transverse plane intersecting the tibial component, and wherein the medial-lateral line of the tibial baseplate is disposed at a varus tilt relative to the transverse plane intersecting the tibial component. 
     In an exemplary tibial component, the posterior slope can be greater than zero degrees and can be less than or equal to about 15 degrees. 
     In an exemplary tibial component, the varus tilt can be greater than zero degrees and can be greater than or equal to about 7 degrees. 
     In an exemplary tibial component, a keel longitudinal axis can extend along a height of the keel, the keel longitudinal axis can be aligned with the longitudinal axis of the tibia when the tibial component is in an installed configuration. 
     In an exemplary tibial component, the tibial baseplate plane can be parallel to the anterior-posterior line and the medial-lateral line. 
     An exemplary modular keel comprises: a keel body; and a keel proximal end, wherein a keel longitudinal axis extends axially through the keel body, wherein the keel proximal end is disposed at a keel posterior angle relative to the keel longitudinal axis, and wherein the keel proximal end is disposed at a keel varus angle relative to the keel longitudinal axis. 
     An exemplary modular keel can further comprise fins extending transversely from the keel body. 
     An exemplary modular keel can further comprise a receiver in the keel proximal end, wherein the receiver is configured to selectively fixedly engage a complementary projection extending from a lower end of a tibial baseplate. 
     An exemplary modular keel can further comprise a projection in the keel proximal end, wherein the projection is configured to selectively engage a complementary receiver defined by a tibial baseplate. 
     An exemplary instrument assembly can comprise: a keel punch, the keel punch having a punch proximal end distally disposed from a punch distal end along a body, wherein the punch proximal end is disposed at a keel posterior angle relative to a keel longitudinal axis extending along a height dimension of the keel punch, and wherein the punch proximal end is disposed at a keel varus angle relative to the keel longitudinal axis extending along the height dimension of the keel punch; and a punch guide configured to closely receive the keel punch, the punch guide having a guide distal end distally disposed from a guide proximal end along a guide body, wherein the guide distal end is disposed at a guide posterior angle relative to a guide longitudinal axis extending along a height dimension of the punch guide, and wherein the guide distal end is disposed at a guide varus angle relative to a guide longitudinal axis extending along the height dimension of the punch guide. 
     An exemplary instrument assembly can further comprise a trial tibial baseplate, wherein the guide distal end further comprises spikes configured to extend through holes in the trial tibial baseplate. 
     An exemplary instrument assembly can further comprise a reamer extending through a through channel defined by an inner wall of the guide body of the punch guide to from the guide proximal end to the guide distal end. 
     Although the present invention has been described in terms of specific embodiments, it is anticipated that alterations and modifications thereof will no doubt become apparent to those skilled in the art. It is therefore intended that the following claims be interpreted as covering all alterations and modifications that fall within the true spirit and scope of the invention.