Patent Description:
Orthopaedic prostheses are commonly utilized to repair and/or replace damaged bone and tissue in the human body. For a damaged knee, a knee prosthesis may be implanted using a proximal tibial baseplate component, a tibial bearing component, and a distal femoral component. The tibial baseplate component is affixed to a proximal end of the patient's tibia, which is typically resected to accept the baseplate component. The femoral component is implanted on a distal end of the patient's femur, which is also typically resected to accept the femoral component. The tibial bearing component is placed between the tibial baseplate component and the femoral component, and may be fixed or slidably coupled to the tibial baseplate component.

The tibial baseplate component provides support for the tibial bearing component. Forces generated by use of the knee prosthesis are transferred through the tibial bearing component to the tibial baseplate component, and ultimately to the tibia. In order to ensure long term performance of the knee prosthesis, stable and firm securement of the tibial baseplate component to the proximal end of the patient's tibia is desired. <CIT> and <CIT> describe tibial baseplates for implantation on a proximal tibia.

This application is related to <CIT> , to <CIT> , and to <CIT>.

The present invention is as defined in the independent claims. The present disclosure provides an orthopaedic knee prosthesis including a tibial baseplate component having a distal, bone-contacting surface with one or more fixation structures extending distally therefrom, the fixation structures being asymmetrically arranged within the outer periphery of the baseplate.

For designs utilizing a plurality of fixation pegs that extend distally from the bone-contacting surface of the tibial baseplate, fixation pegs are asymmetrically arranged in opposite anterior/lateral and posterior/medial regions of the tibial baseplate, thereby maximizing distance between the fixation pegs, avoiding overlap with the intramedullary canal, avoiding areas of low bone density, and avoiding cortical impingement by positioning the fixation pegs in regions of cancellous bone.

For designs utilizing a single keel - not encompassed by claims - that extends distally from the bone-contacting surface of the tibial baseplate, the keel is medialized with respect to the outer periphery of the tibial baseplate, where the degree of medialization increases as prosthesis sizes grow progressively.

According to an embodiment, the present invention provides a tibial baseplate configured for implantation upon a patient's proximal tibia, the tibial baseplate comprising: a medial compartment; a lateral compartment opposite the medial compartment; a proximal surface; a distal surface opposite the proximal surface, the distal surface sized and shaped to substantially cover the patient's proximal tibia; an outer periphery cooperatively defined by an anterior face, a medial face, a lateral face, and at least one posterior face; at most one medial fixation peg associated with the medial compartment, the medial fixation peg extending distally from the distal surface and positioned for implantation into the patient's proximal tibia; and at most one lateral fixation peg associated with the lateral compartment, the lateral fixation peg extending distally from the distal surface and positioned for implantation into the patient's proximal tibia, the lateral fixation peg being located closer to the anterior face than the medial fixation peg, wherein the anterior face has a flat portion disposed between the medial and lateral compartments and the tibial baseplate further comprises an anterior-posterior axis that bisects the flat portion of the anterior face and bisects a posterior cutout formed in the at least one posterior face of the tibial baseplate.

According to another embodiment thereof, the present invention provides a tibial baseplate configured for implantation upon a patient's proximal tibia, the tibial baseplate comprising: a medial compartment; a lateral compartment opposite the medial compartment; a proximal surface; a distal surface opposite the proximal surface, the distal surface sized and shaped to substantially cover the patient's proximal tibia; an outer periphery cooperatively defined by an anterior face, a medial face, a lateral face, and at least one posterior face; a first, anterior-posterior axis located between the medial face and the lateral face and intersecting the anterior face, the first axis extending centrally between the medial and lateral compartments throughout its length; a first fixation peg extending distally from the distal surface, the first fixation peg being inset from the outer periphery for implantation into the patient's proximal tibia, the first fixation peg being medially spaced from the first axis by a first distance; and a second fixation peg extending distally from the distal surface, the second fixation peg being inset from the outer periphery for implantation into the patient's proximal tibia, the second fixation peg being laterally spaced from the first axis by a second distance, the second distance less than the first distance.

The above-mentioned and other features and advantages of this disclosure, and the manner of attaining them, will become more apparent and the invention itself will be better understood by reference to the following description of embodiments of the invention taken in conjunction with the accompanying drawings, wherein:.

Corresponding reference characters indicate corresponding parts throughout the several views. The exemplifications set out herein illustrate exemplary embodiments of the invention and such exemplifications are not to be construed as limiting the scope of the invention in any manner.

The present disclosure provides a tibial baseplate component for a knee prosthesis including asymmetrically arranged distal fixation structures which promote secure and stable long term fixation of the tibial baseplate to a patient's proximal tibia.

In order to prepare the tibia and femur for receipt of a knee j oint prosthesis of the present disclosure, any suitable methods or apparatuses for preparation of the knee joint may be used. The surgical procedure may involve, for example, forming an incision in the patient's skin near the knee joint, resecting the distal end of the patient's femur (not shown), and resecting the proximal end of the patient's tibia T (<FIG>). Resecting the proximal end of the patient's tibia T (<FIG>), in particular, may involve guiding a saw blade through an appropriate cutting guide slot to form a substantially planar resected surface S of tibia T, as shown in <FIG>.

Exemplary surgical procedures and associated surgical instruments are disclosed in Zimmer's "LPS-Flex Fixed Bearing Knee, Surgical Technique" bearing copyright dates of <NUM>, <NUM> and <NUM>, "NexGen"® Complete Knee Solution, Surgical Technique for the CR-Flex Fixed Bearing Knee" bearing a copyright date of <NUM>, "NexGen"® Complete Knee Solution Extramedullary/Intramedullary Tibial Resector, Surgical Technique" bearing copyright dates of <NUM>, <NUM> and <NUM>, "NexGen"® Trabecular Metal™ Monoblock Tibial Components, Surgical Technique Addendum," bearing copyright dates of <NUM> and <NUM>, "NexGen"® Trabecular Metal™ Tibial Tray, Surgical Technique," bearing copyright dates of <NUM> and <NUM>, and "Trabecular Metal™ Monoblock Tibial Components," bearing a copyright date of <NUM> (collectively, the "Zimmer Surgical Techniques").

As used herein, "proximal" refers to a direction generally toward the torso of a patient, and "distal" refers to the opposite direction of proximal (i.e., away from the torso of a patient). "Anterior" refers to a direction generally toward the front of a patient or knee, and "posterior" refers to the opposite direction of anterior (i.e., toward the back of the patient or knee). "Lateral" refers to a direction generally away from the middle of the patient and the sagittal plane, and "medial" refers to the opposite direction of lateral (i.e., toward the middle of the patient and the sagittal plane). When referring to one of the patient's knees, "lateral" refers to the direction generally away from the other knee, and "medial" refers to the direction generally toward the other knee.

These anatomical regions are labeled in certain drawings for clarity. In <FIG>, for example, the anterior region of tibia T is labeled "A," the posterior region of tibia T is labeled "P," the lateral region of tibia T is labeled "L," and the medial region of tibia T is labeled "M. " Therebetween and moving in a clock-wise direction, the anterior/lateral region of tibia T is labeled "AL," the posterior/lateral region of tibia T is labeled "PL," the posterior/medial region of tibia T is labeled "PM," and the anterior/medial region of tibia T is labeled "AM. " The AL, PL, PM, and AM regions can be described as dividing tibia T into four corners or quadrants. These labels are referenced throughout the following paragraphs.

The embodiments shown and described herein illustrate components for a right knee prosthesis. Right and left knee prosthesis configurations are generally mirror images of one another about a sagittal plane. Thus, it will be appreciated that the aspects of the prosthesis described herein for a right knee configuration are equally applicable to a left knee configuration.

Referring now to <FIG>, tibial baseplate <NUM> is shown disposed atop a proximal resected surface S of a patient's tibia T. The upper or proximal surface <NUM> of baseplate <NUM> is shown in <FIG>. This proximal surface <NUM> of baseplate <NUM> is configured to receive a tibial bearing component <NUM> (<FIG>) in a fixed or a sliding relationship, for example. To arrange baseplate <NUM> and the tibial bearing component <NUM> in a fixed relationship, the tibial bearing component <NUM> may be adhered to, mechanically fastened to, molded directly onto (as discussed further below), or otherwise fixedly coupled to baseplate <NUM>. The illustrative baseplate <NUM> includes a raised rim <NUM> around proximal surface <NUM> to receive, surround, and hold the tibial bearing component <NUM> therein, but it is contemplated that other structures may be provided on baseplate <NUM> to receive and hold the tibial bearing component <NUM> on baseplate <NUM>. In turn, tibial bearing component <NUM> is configured to interact with the patient's distal femur or a prosthetic femoral component, such as femoral component <NUM> shown in <FIG> and described below.

Baseplate <NUM> may be partially or entirely constructed of a highly porous biomaterial. A highly porous biomaterial is useful as a bone substitute and as cell and tissue receptive material. A highly porous biomaterial may have a porosity as low as <NUM>%, <NUM>%, or <NUM>% or as high as <NUM>%, <NUM>%, or <NUM>%. An example of such a material is produced using Trabecular Metal™ Technology generally available from Zimmer, Inc. , of Warsaw, Indiana. Trabecular Metal™ is a trademark of Zimmer, Inc. Such a material may be formed from a reticulated vitreous carbon foam substrate which is infiltrated and coated with a biocompatible metal, such as tantalum, by a chemical vapor deposition ("CVD") process in the manner disclosed in detail in <CIT> to Kaplan. In addition to tantalum, other metals such as niobium, or alloys of tantalum and niobium with one another or with other metals may also be used.

Generally, the porous tantalum structure includes a large plurality of ligaments defining open spaces therebetween, with each ligament generally including a carbon core covered by a thin film of metal such as tantalum, for example. The open spaces between the ligaments form a matrix of continuous channels having no dead ends, such that growth of cancellous bone through the porous tantalum structure is uninhibited. The porous tantalum may include up to <NUM>%, <NUM>%, or more void space therein. Thus, porous tantalum is a lightweight, strong porous structure which is substantially uniform and consistent in composition, and closely resembles the structure of natural cancellous bone, thereby providing a matrix into which cancellous bone may grow to provide fixation of baseplate <NUM> to the patient's bone.

The porous tantalum structure may be made in a variety of densities in order to selectively tailor the structure for particular applications. In particular, as discussed in <CIT>, the porous tantalum may be fabricated to virtually any desired porosity and pore size, and can thus be matched with the surrounding natural bone in order to provide an improved matrix for bone ingrowth and mineralization.

Bearing component <NUM> may be molded directly onto baseplate <NUM>, specifically proximal surface <NUM> of baseplate <NUM>. If baseplate <NUM> is constructed of a highly porous biomaterial, as discussed above, the material that is used to construct bearing component <NUM> (e.g., polyethylene) may interdigitate into the pores of baseplate <NUM> during the molding process. The pores may be located at and beneath proximal surface <NUM> of baseplate <NUM>, so the resulting molded bearing component <NUM> may also be located at and beneath proximal surface <NUM> of baseplate <NUM>. The resulting structure may be a monoblock component having a strong, wear-resistant connection between baseplate <NUM> and bearing component <NUM>, especially along proximal surface <NUM> of baseplate <NUM>.

Baseplate <NUM> includes outer periphery <NUM>, which may be visible in a top plan view (<FIG>) or a bottom plan view (<FIG>) with baseplate <NUM> positioned in a generally transverse anatomical plane. As shown in <FIG>, outer periphery <NUM> is cooperatively defined by anterior face <NUM>, posterior/lateral face <NUM>, posterior/medial face <NUM>, PCL cutout area <NUM>, lateral face <NUM>, and medial face <NUM>. Each of these surfaces is described further below.

Baseplate <NUM> also includes lateral compartment <NUM>, medial compartment <NUM>, and interior compartment <NUM> therebetween. Lateral compartment <NUM> and medial compartment <NUM> are separated by an anterior-posterior home axis AH, which is discussed further below. Because <FIG> is a proximal view of the patient's right tibia T, lateral compartment <NUM> of baseplate <NUM> is located on the right side of <FIG> and medial compartment <NUM> of baseplate <NUM> is located on the left side of <FIG>.

With bearing component <NUM> in place against baseplate <NUM> (<FIG>) to articulate with adjacent femoral component <NUM>, for example, lateral compartment <NUM> of baseplate <NUM> will be positioned generally beneath lateral condyle <NUM> of femoral component <NUM> to support and articulate with lateral condyle <NUM>, and medial compartment <NUM> of baseplate <NUM> will be positioned generally beneath medial condyle <NUM> of femoral component <NUM> to support medial condyle <NUM>. Tibial bearing component <NUM> (<FIG>) may be disposed between medial and lateral condyles <NUM>, <NUM> of femoral component <NUM> and medial and lateral compartments <NUM>, <NUM> to provide a low-friction articular interface, as described below. In the illustrative embodiment, femoral component <NUM> includes cam <NUM> adapted to articulate with a spine of a tibial bearing component, e.g., spine <NUM> of tibial bearing component <NUM> (<FIG>). However, it is contemplated that femoral component <NUM> may omit spine <NUM> to provide an uninterrupted space between medial and lateral condyles <NUM>, <NUM> in some prosthesis designs.

Anterior face <NUM> of the illustrative baseplate <NUM> is disposed anteriorly on periphery <NUM> of baseplate <NUM> (i.e., in the A region of tibia T). Anterior face <NUM> is generally centrally located between lateral and medial compartments <NUM>, <NUM>. More specifically, as shown in <FIG>, anterior face <NUM> includes a linear or flat portion 18a that is generally centrally located between lateral and medial compartments <NUM>, <NUM>. In this illustrated embodiment, flat portion 18a of anterior face <NUM> defines the anterior-most extent of baseplate <NUM>.

Posterior/lateral face <NUM> of the illustrative baseplate <NUM> is disposed generally opposite anterior face <NUM> in the posterior region of lateral compartment <NUM> (i.e., near the PL region of tibia T). Posterior/medial face <NUM> of the illustrative baseplate <NUM> is disposed generally opposite anterior face <NUM> in the posterior region of medial compartment <NUM> (i.e., near the PM region of tibia T). The PCL cutout area <NUM> is disposed between posterior/lateral face <NUM> and posterior/medial face <NUM> (i.e., near the P region of tibia T). From both posterior/lateral face <NUM> and posterior/medial face <NUM>, the PCL cutout area <NUM> extends generally anteriorly until reaching apex 24a.

Lateral face <NUM> of the illustrative baseplate <NUM> is disposed laterally of lateral compartment <NUM> on periphery <NUM> of baseplate <NUM> (i.e., near the L region of tibia T). Medial face <NUM> of the illustrative baseplate <NUM> is located medially of medial compartment <NUM> on periphery <NUM> of baseplate <NUM> (i.e., near the M region of tibia T).

In the context of patient anatomy, such as tibia T described herein, "home axis" AH of tibia T extends anteriorly from a posterior point PP on tibia T to an anterior point PA on tibia T. The posterior point PP and the anterior point PA of tibia T are discussed further below.

The posterior point PP is generally disposed in the area where the patient's posterior cruciate ligament (PCL) attaches to tibia T. More specifically, the posterior point PP is generally disposed at the geometric center of the attachment between the patient's PCL and tibia T. The patient's PCL typically attaches to tibia T in two ligament "bundles," the first bundle having a more anterolateral attachment location and the second bundle having a more posteromedial attachment location. In <FIG>, the posterior point PP is shown at the geometric center of the first bundle. It is also within the scope of the present disclosure that the posterior point PP may be located at the geometric center of the second bundle or at the geometric center of the first and second bundles, together.

The anterior point PA is disposed on the patient's anterior tibial tubercle B. In <FIG>, the anterior point PA is medially spaced from the tubercle midpoint BM (at marking <NUM>/<NUM>) by an amount equal to <NUM>/<NUM> of the overall medial/lateral tubercle width BW (which spans between markings <NUM> and <NUM>). Stated another way, the anterior point PA is laterally spaced from the tubercle medial end BME (at marking <NUM>) by an amount equal to <NUM> of the overall medial/lateral tubercle width BW (which spans between markings <NUM> and <NUM>), such that the anterior point PA lies on the "medial third" of the anterior tibial tubercle B (at marking <NUM>/<NUM>).

In the context of a prosthesis, such as tibial baseplate <NUM> described herein, "home axis" AH of baseplate <NUM> refers to an anterior-posterior extending axis of baseplate <NUM> that aligns with home axis AH of tibia T upon implantation of baseplate <NUM> onto resected surface S of tibia T in a proper rotational and spatial orientation (as shown in <FIG>). According to an exemplary embodiment of the present disclosure, and as shown in <FIG>, home axis AH of baseplate <NUM> is centrally located between the inner-most portion of lateral compartment <NUM> and the inner-most portion of medial compartment <NUM> of baseplate <NUM> throughout its length. In other words, home axis AH of baseplate <NUM> is equidistant from the inner-most portion of lateral compartment <NUM> and the inner-most portion of medial compartment <NUM> of baseplate <NUM> to divide the interior compartment <NUM> therebetween into substantially equal halves.

In the illustrative embodiment of <FIG>, home axis AH of baseplate <NUM> bisects anterior face <NUM> of baseplate <NUM> (which is located anteriorly on periphery <NUM> of baseplate <NUM>) and is generally perpendicular to flat portion 18a of anterior surface <NUM>. Also, home axis AH of baseplate <NUM> bisects PCL cutout area <NUM> of baseplate <NUM> (which is located posteriorly on periphery <NUM> of baseplate <NUM>) and is generally perpendicular to apex 24a of PCL cutout area <NUM>. It is contemplated that home axis AH of baseplate <NUM> may be oriented to other features of baseplate <NUM>, it being understood that proper alignment and orientation of baseplate <NUM> upon resected surface S of tibia T will position home axis AH of baseplate <NUM> coincident with home axis AH of tibia T.

The home axes AH of tibia T and baseplate <NUM> are further described in <CIT>, entitled "ASYMMETRIC TIBIAL COMPONENTS FOR A KNEE PROSTHESIS,".

A pair of reference axes <NUM>, <NUM> is presented in <FIG>. A first reference axis <NUM> extends diagonally across baseplate <NUM> from the back-left PM region of tibia T to the front-right AL region of tibia T, intersecting home axis AH to define a first angle α with home axis AH, as shown in <FIG>. A second reference axis <NUM> extends diagonally across baseplate <NUM> and perpendicularly to the first axis <NUM> from the back-right PL region of tibia T to the front-left AM region of tibia T, intersecting home axis AH to define a second angle β with home axis AH, as shown in <FIG>. The first and second angles α and β are each approximately <NUM> degrees such that, when combined, the first and second angles α and β together total approximately <NUM> degrees.

The first and second reference axes <NUM>, <NUM> illustratively intersect one another and home axis AH at a common point X within periphery <NUM> of baseplate <NUM>. According to an exemplary embodiment of the present disclosure, point X is generally centered within periphery <NUM> of baseplate <NUM> to maximize the aggregated extent of each reference axis <NUM>, <NUM> that is located within periphery <NUM> of baseplate <NUM> while maintaining the desired first and second angles α and β, as discussed above. Point X is illustratively positioned along home axis AH between flat portion 18a of anterior face <NUM> and apex 24a of PCL cutout area <NUM>.

Illustratively, a medial-lateral axis <NUM> also extends through point X in a direction perpendicular to home axis AH. Together, the medial-lateral axis <NUM> (e.g., the x-axis) and the anterior-posterior home axis AH (e.g., the y-axis) cooperate to define a component coordinate system (e.g., an x-y coordinate system) useful for quantifying and identifying certain features of baseplate <NUM>.

According to an exemplary embodiment of the present disclosure, and as shown in <FIG>, baseplate <NUM> has an asymmetric outer periphery <NUM>. The asymmetric outer periphery <NUM> may be designed to closely match the corresponding periphery of resected surface S of tibia T. In the illustrated embodiment of <FIG>, for example, medial compartment <NUM> is larger than lateral compartment <NUM>. Medial compartment <NUM> is wider than lateral compartment <NUM>, so medial face <NUM> is spaced further apart from the anterior-posterior home axis AH than lateral face <NUM>. Medial compartment <NUM> is also deeper than lateral compartment <NUM>, so posterior/medial face <NUM> is spaced further apart posteriorly from the medial-lateral axis <NUM> than posterior/lateral face <NUM>. For at least these reasons, the outer periphery <NUM> of baseplate <NUM> is asymmetric.

The asymmetric shape of baseplate <NUM> is further described in <CIT>, filed July <NUM>, <NUM><NUM>, entitled "ASYMMETRIC TIBIAL COMPONENTS FOR A KNEE PROSTHESIS,".

It is also within the scope of the present disclosure that baseplate <NUM> may have a symmetric outer periphery <NUM>, as shown in phantom in <FIG>. In this embodiment, lateral compartment <NUM> and medial compartment <NUM> are the same shape and size. Lateral compartment <NUM> and medial compartment <NUM> are the same width, so lateral face <NUM> and the modified medial face <NUM> (shown in phantom) are equidistant from the anterior-posterior home axis AH, In this manner, an anterior-posterior axis of symmetry through outer periphery <NUM> of symmetric baseplate <NUM> may overlay "home axis" AH and may serve as a reference for lateral compartment <NUM>, medial compartment <NUM>, lateral face <NUM>, medial face <NUM>, lateral and fixation pegs <NUM>, <NUM> (described below) and other components of baseplate <NUM>. Thus, in addition to being centered within interior compartment <NUM> between lateral compartment <NUM> and medial compartment <NUM> of the symmetric embodiment of baseplate <NUM>, the anterior-posterior home axis AH would also be centered between lateral face <NUM> and the modified medial face <NUM> (shown in phantom). Lateral compartment <NUM> and medial compartment <NUM> also define a common anterior/posterior depth, so posterior/lateral face <NUM> and the modified posterior/medial face <NUM> (shown in phantom) are equidistant from the medial-lateral axis <NUM>. Generally, a symmetric outer periphery <NUM> allows the same baseplate <NUM> to be implanted onto either a patient's right tibia or left tibia.

Referring next to <FIG> and <FIG>, the underside or distal surface <NUM> of baseplate <NUM> is shown. Distal surface <NUM> is the surface which contacts resected surface S of tibia T (<FIG>) after implantation of baseplate <NUM>. As shown in <FIG>, distal surface <NUM> is located opposite proximal surface <NUM>. Baseplate <NUM> includes a plurality of fixation structures, illustratively lateral fixation peg <NUM> and medial fixation peg <NUM>, that extend distally from distal surface <NUM> and into tibia T (<FIG>).

Each fixation peg <NUM>, <NUM> is inset from outer periphery <NUM> of baseplate <NUM>. Each fixation peg <NUM>, <NUM> may have a minimum inset distance <NUM> (<FIG>) that exceeds <NUM>, such as <NUM>, <NUM>, <NUM>, or more, for example. For purposes of the present disclosure, and as shown in <FIG>, the minimum inset distance <NUM> is the smallest distance measured between outer periphery <NUM> of baseplate <NUM> and the outer perimeter of each fixation peg <NUM>, <NUM>.

According to an exemplary embodiment of the present disclosure, fixation pegs <NUM>, <NUM> of baseplate <NUM> are constructed of a highly porous biomaterial, such as the above-described porous tantalum material. Distal surface <NUM> of baseplate <NUM> may also be constructed of a highly porous biomaterial. With distal surface <NUM> of baseplate <NUM> resting against resected surface S of tibia T and fixation pegs <NUM>, <NUM> of baseplate <NUM> extending distally into tibia T, the highly porous biomaterial may provide a matrix into which cancellous bone may grow to provide fixation of baseplate <NUM> to tibia T.

As shown in <FIG>, the illustrative fixation pegs <NUM>, <NUM> are hexagonal in cross-section near distal surface <NUM> of baseplate <NUM>. As fixation pegs <NUM>, <NUM> continue extending distally away from distal surface <NUM> of baseplate <NUM>, fixation pegs <NUM>, <NUM> transition to a circular cross-section. The hexagonal to circular transition of fixation pegs <NUM>, <NUM> is also evident in <FIG>. In <FIG>, by contrast, each fixation peg <NUM>, <NUM> is represented by a phantom circle to schematically show the general location of each fixation peg <NUM>, <NUM>, not necessarily the size or shape of each fixation peg <NUM>, <NUM>. Exemplary fixation pegs <NUM>, <NUM> are shown at pages <NUM>-<NUM> of the "Zimmer® Tibial Baseplate, Pocket Guide United States Version,".

According to an exemplary embodiment of the present disclosure, and as discussed further below, lateral and medial fixation pegs <NUM>, <NUM> are asymmetrically arranged on distal surface <NUM> of baseplate <NUM>. In one exemplary embodiment, fixation pegs <NUM>, <NUM> are asymmetrically arranged about the anterior-posterior home axis AH, such that the anterior-posterior home axis AH is not an axis of symmetry of fixation pegs <NUM>, <NUM>. In another embodiment, fixation pegs <NUM>, <NUM> are asymmetrically arranged about the medial-lateral axis <NUM>, such that the medial-lateral axis <NUM> is not an axis of symmetry of fixation pegs <NUM>, <NUM>. In yet another embodiment, fixation pegs <NUM>, <NUM> are asymmetrically arranged about both the anterior-posterior home axis AH and the medial-lateral axis <NUM>, such that neither the anterior-posterior home axis AH nor the medial-lateral axis <NUM> is an axis of symmetry of fixation pegs <NUM>, <NUM>.

Returning now to <FIG>, lateral fixation peg <NUM> in lateral compartment <NUM> of baseplate <NUM> is positioned anteriorly relative to the medial-lateral axis <NUM> and anteriorly of medial fixation peg <NUM>. Thus, lateral fixation peg <NUM> is more generally positioned in the AL region of tibia T while being substantially distanced from the PL region of tibia T. The AL bias of lateral fixation peg <NUM> is evident in <FIG>, because from the center point X, the first axis <NUM> extends toward the AL region and approaches or even intersects lateral fixation peg <NUM>, while the second axis <NUM> extends toward the PL region extends further away from lateral fixation peg <NUM>.

In the medial compartment <NUM> of baseplate <NUM>, medial fixation peg <NUM> is positioned posteriorly relative to the medial-lateral axis <NUM> and posteriorly of lateral fixation peg <NUM>. Thus, medial fixation peg <NUM> is more generally positioned in the PM region of tibia T while being substantially distanced from the AM region of tibia T. The PM bias of medial fixation peg <NUM> is evident in <FIG>, because from the center point X, the first axis <NUM> extends toward the PM region and approaches or even intersects medial fixation peg <NUM>, while the second axis <NUM> extends toward the AM region and travels away from medial fixation peg <NUM>. In this exemplary embodiment, both fixation pegs <NUM>, <NUM> are generally positioned along the same first reference axis <NUM> which spans the PM and AL regions.

An alternative baseplate <NUM>' is shown in <FIG> for contrast. Outer periphery <NUM>' of the alternative baseplate <NUM>' of <FIG> is generally the same as outer periphery <NUM> of baseplate <NUM> (shown in solid lines in <FIG>) - both are asymmetric in shape. However, unlike fixation pegs <NUM>, <NUM> of <FIG>, which are located on opposite sides of the medial-lateral axis <NUM>, fixation pegs <NUM>', <NUM>' of <FIG> are aligned along and intersect with medial-lateral axis <NUM>'. With respect to baseplate <NUM>', both the anterior-posterior home axis AH' and the medial-lateral axis <NUM>' are axes of symmetry for fixation pegs <NUM>', <NUM>', such that fixation pegs <NUM>', <NUM>' may be said to be symmetrically oriented with respect to the component coordinate system.

Another alternative baseplate <NUM>" is shown in <FIG> for contrast. Outer periphery <NUM>" of the alternative baseplate <NUM>" of <FIG> is generally the same as outer periphery <NUM> of baseplate <NUM> (shown in phantom in <FIG>) - both are symmetric in shape. Lateral compartment <NUM>" of the alternative baseplate <NUM>" is generally the same size and shape as medial compartment <NUM>" of the alternative baseplate <NUM>". Therefore, the anterior-posterior home axis AH" is an axis of symmetry for outer periphery <NUM>" of baseplate <NUM>". Like fixation pegs <NUM>', <NUM>' of <FIG>, fixation pegs <NUM>", <NUM>" of <FIG> are aligned along and intersect with medial-lateral axis <NUM>". With respect to baseplate <NUM>", both the anterior-posterior home axis AH" and the medial-lateral axis <NUM>" are axes of symmetry for fixation pegs <NUM>", <NUM>", such that that fixation pegs <NUM>", <NUM>" may be said to be symmetrically oriented with respect to the component coordinate system.

Returning again to <FIG>, the asymmetric positioning of lateral and medial fixation pegs <NUM>, <NUM> near opposite AL and PM corners or quadrants, respectively, allows fixation pegs <NUM>, <NUM> to be widely spaced apart across distal surface <NUM> of baseplate <NUM>. Advantageously, this wide spacing facilitates avoidance of the anatomic intramedullary canal of tibia T upon implantation (which may be located near the intersection point X), particularly where baseplate <NUM> is used for a small-stature patient. By avoiding placement of fixation pegs <NUM>, <NUM> within the intramedullary canal of tibia T, the associated areas of low bone density are avoided and, instead, fixation pegs <NUM>, <NUM> may be implanted into areas of higher bone density, thereby promoting firm and stable long-term fixation of tibial baseplate <NUM> to tibia T. If fixation pegs <NUM>, <NUM> are constructed of a highly porous biomaterial, as discussed above, this firm and stable long-term fixation may be achieved by cancellous bone growth into the porous fixation pegs <NUM>, <NUM>. Also advantageously, the wide spacing between fixation pegs <NUM>, <NUM> encourages bone ingrowth therebetween. By contrast, if fixation pegs <NUM>, <NUM> are too close together, there may not be enough space for bone to grow therebetween.

Also, the asymmetric arrangement of lateral and medial fixation pegs <NUM>, <NUM> on opposite sides of the medial-lateral axis <NUM> may enhance the torsional stability of baseplate <NUM> when implanted upon tibia T (<FIG>). During normal use, a significant portion of the forces generated on baseplate <NUM> are directed anteriorly or posteriorly. Activities which primarily generate such anteriorly-directed or posteriorly-directed forces include wallcing, running, squatting, and climbing stairs, for example. As shown in <FIG>, such anteriorly-directed and posteriorly-directed forces give rise to anterior torsional moments MA and posterior torsional moments MP, respectively, which urge rotation of baseplate <NUM> anteriorly and posteriorly about the medial-lateral axis <NUM>. Having lateral and medial fixation pegs <NUM>, <NUM> positioned on opposite sides of the medial-lateral axis <NUM> (i.e., the axis of rotation), as illustrated in <FIG> and discussed in detail above, presents greater resistance to such rotation.

Furthermore, positioning lateral and medial fixation pegs <NUM>, <NUM> in the AL and PM regions of tibia T, rather than the PL and AM regions of tibia T, may avoid impingement of pegs <NUM>, <NUM> on adjacent cortical bone upon implantation of baseplate <NUM>. Advantageously, the AL and PM regions of tibia T (where fixation pegs <NUM>, <NUM> are located) are typically populated with substantial areas of cancellous bone, thereby promoting firm and stable long-term fixation of tibial baseplate <NUM> to tibia T and promoting bone ingrowth. By contrast, the PL and AM regions of tibia T (where fixation pegs <NUM>, <NUM> are not located) are typically populated with substantial areas of cortical bone. By avoiding the PL and AM regions of tibia T, the potential for impingement of fixation pegs <NUM>, <NUM> upon cortical bone is minimized.

Because lateral fixation peg <NUM> extends from lateral compartment <NUM> and medial fixation peg <NUM> extends from medial compartment <NUM>, as discussed above, lateral fixation peg <NUM> can be said to be positioned "more laterally" on distal surface <NUM> of baseplate <NUM> than medial fixation peg <NUM>. Similarly, medial fixation peg <NUM> is positioned "more medially" on distal surface <NUM> of baseplate <NUM> than lateral fixation peg <NUM>. Thus, as shown in <FIG>, fixation pegs <NUM>, <NUM> are spaced apart by a medial-lateral separation distance <NUM>. For purposes of the present disclosure, the medial-lateral separation distance <NUM> is measured on center between fixation pegs <NUM>, <NUM> along a direction perpendicular to home axis AH and parallel to medial-lateral axis <NUM> (<FIG>). In an exemplary embodiment, the medial-lateral separation distance <NUM> is between <NUM> and <NUM>, with smaller separation distances <NUM> corresponding to smaller nominal prosthesis sizes, and larger separation distances <NUM> corresponding to larger nominal prosthesis sizes.

According to an exemplary embodiment of the present disclosure, lateral fixation peg <NUM> and/or medial fixation peg <NUM> are medially biased in their respective compartments <NUM>, <NUM>. In lateral compartment <NUM>, the illustrative lateral fixation peg <NUM> is medially biased toward home axis AH. In medial compartment <NUM>, the illustrative medial fixation peg <NUM> is medially biased away from home axis AH. The medial bias of fixation pegs <NUM>, <NUM>, is evident in <FIG>, for example, where central peg axis <NUM> (which is centered along the medial-lateral separation distance <NUM> between fixation pegs <NUM>, <NUM>) is medially biased toward medial compartment <NUM> and away from home axis AH. Because central peg axis <NUM> is centered along medial-lateral separation distance <NUM>, central peg axis <NUM> divides medial-lateral separation distance <NUM> into equal halves - one half being located between lateral fixation peg <NUM> and central peg axis <NUM> and the other half being located between medial fixation peg <NUM> and central peg axis <NUM>.

If fixation pegs <NUM>, <NUM> were equally spaced apart from home axis AH, central peg axis <NUM> would coincide with home axis AH. However, in <FIG>, pegs <NUM>, <NUM> are not equally spaced apart from home axis AH, Instead, lateral fixation peg <NUM> is located closer to home axis AH than medial fixation peg <NUM>. As a result, central peg axis <NUM> between fixation pegs <NUM>, <NUM> is medially spaced or offset toward medial compartment <NUM> and away from home axis AH by offset distance <NUM>. Therefore, fixation pegs <NUM>, <NUM> may be said to be asymmetrically, medially biased relative to home axis AH. In an exemplary embodiment, offset distance <NUM> is between <NUM> and <NUM>. Smaller prosthesis sizes may have smaller values for offset distance <NUM>, while larger prosthesis sizes may have larger values for offset distance <NUM>.

As discussed above, lateral fixation peg <NUM> is positioned relatively more anteriorly on distal surface <NUM> of baseplate <NUM> than medial fixation peg <NUM>. Stated differently, medial fixation peg <NUM> is positioned relatively more posteriorly on distal surface <NUM> of baseplate <NUM> than lateral fixation peg <NUM>. Thus, as shown in <FIG>, pegs <NUM>, <NUM> are spaced apart by an anterior-posterior separation distance <NUM>. For purposes of the present disclosure, the anterior-posterior separation distance <NUM> is measured on center between fixation pegs <NUM>, <NUM> along a direction parallel to home axis AH, In an exemplary embodiment, the anterior-posterior separation distance <NUM> is between <NUM> and <NUM>, with smaller separation distances <NUM> corresponding to smaller prosthesis sizes, and larger separation distances <NUM> corresponding to larger prosthesis sizes.

The alternative baseplates <NUM>', <NUM>" of <FIG> are provided for contrast. Because lateral and medial fixation pegs <NUM>', <NUM>' of the alternative baseplate <NUM>' of <FIG>, for example, are aligned in an anterior-posterior direction, lateral and medial fixation pegs <NUM>', <NUM>' lack an anterior-posterior separation distance analogous to the anterior-posterior separation distance <NUM> of <FIG>. Or stated differently, lateral and medial fixation pegs <NUM>', <NUM>' have an anterior-posterior separation distance equal to <NUM>. Similarly, lateral and medial fixation pegs <NUM>", <NUM>" of the alternative baseplate <NUM>" of <FIG> are aligned in an anterior-posterior direction and, therefore, have an anterior-posterior separation distance equal to <NUM>.

Turning now to <FIG>, another way of quantifying the anterior/posterior asymmetry of fixation pegs <NUM>, <NUM> is by contrasting their different positions relative to a common reference marker. In <FIG>, for example, the common reference marker is flat portion 18a of anterior face <NUM> of baseplate <NUM>, with measurements being taken posteriorly therefrom in a direction parallel to home axis AH. Lateral fixation peg <NUM> is spaced posteriorly from anterior face <NUM> by a relatively smaller lateral peg distance <NUM>, while medial fixation peg <NUM> is spaced posteriorly from anterior face <NUM> by a relatively larger medial peg distance <NUM>. The lateral anterior/posterior depth <NUM> of lateral compartment <NUM> of baseplate <NUM> is also shown being measured from anterior face <NUM> to posterior/lateral face <NUM> of baseplate <NUM>, and this lateral anterior/posterior depth <NUM> exceeds both peg distances <NUM>, <NUM>. Similarly, medial anterior/posterior depth <NUM> of medial compartment <NUM> of baseplate <NUM> is also shown being measured from anterior face <NUM> to posterior/medial face <NUM> of baseplate <NUM>, and medial anterior/posterior depth <NUM> exceeds both peg distances <NUM>, <NUM>, as well as lateral anterior/posterior depth <NUM>. If baseplate <NUM> had a symmetric outer periphery <NUM> (shown in phantom in <FIG>) instead of the asymmetric outer periphery <NUM> of <FIG>, lateral depth <NUM> and medial depth <NUM> would be the same.

The alternative baseplates <NUM>', <NUM>" of <FIG> are provided for contrast. Because lateral and medial fixation pegs <NUM>', <NUM>' of the alternative baseplate <NUM>' of <FIG>, for example, are aligned in an anterior-posterior direction, the lateral peg distance <NUM>' from anterior face <NUM>' to lateral fixation peg <NUM>' is the same as the medial peg distance <NUM>' from anterior face <NUM>' to medial fixation peg <NUM>'. The same is also true for lateral peg distance <NUM>" and medial peg distance <NUM>" of the alternative baseplate <NUM>" of <FIG>. Because the alternative baseplate <NUM>' of <FIG> has an asymmetric outer periphery <NUM>', medial depth <NUM>' differs from lateral depth <NUM>'. Because the alternative baseplate <NUM>" of <FIG> has a symmetric outer periphery <NUM>", on the other hand, medial depth <NUM>" is the same as lateral depth <NUM>".

Baseplate <NUM> may be provided in a kit or set of different prosthesis sizes. In one embodiment, nine baseplates <NUM> are provided in the set, with baseplates <NUM> growing progressively in lateral anterior/posterior depth <NUM> and/or other dimensions, for example. The progressive growth of periphery <NUM> of baseplates <NUM> across the set or family of baseplate sizes is described in detail in <CIT> filed July <NUM>, <NUM><NUM> and entitled ASYMMETRIC TIBIAL COMPONENTS FOR A KNEE PROSTHESIS.

Referring next to <FIG>, exemplary peg distances <NUM>, <NUM> are graphically presented for a set of prostheses of different sizes. More specifically, exemplary peg distances <NUM>, <NUM> are graphically presented for a set of prostheses having different lateral depths <NUM>. The vertical axis of <FIG> shows peg distances <NUM>, <NUM> (in millimeters), while the horizontal axis of <FIG> shows various lateral depths <NUM> (also in millimeters) and the corresponding nominal size indicator (<NUM>-<NUM>). The data points located farther to the left represent smaller lateral depths <NUM> (and therefore smaller nominal prosthesis sizes), and data points located farther to the right represent larger lateral depths <NUM> (and therefore larger nominal prosthesis sizes). In accordance with <FIG>, peg distances <NUM>, <NUM> and lateral depth <NUM> are measured posteriorly from flat portion 18a of anterior face <NUM>.

For each given prosthesis size (i.e., each discrete value of lateral depth <NUM>), a pair of points are presented for lateral and medial peg distances <NUM>, <NUM>, respectively, with a space between the pair of points. This space indicates that peg distances <NUM>, <NUM> are different for each of the nine given prosthesis sizes. Medial peg distances <NUM> consistently exceed the corresponding lateral peg distances <NUM> for each of the nine given prosthesis sizes. For example, each medial peg distance <NUM> may exceed the corresponding lateral peg distance <NUM> by <NUM> to <NUM>. In this manner, each of the given prostheses has anterior/posterior asymmetry of fixation pegs <NUM>, <NUM> with respect to anterior face <NUM>.

<FIG> also demonstrates that, as the prosthesis size increases, medial peg distances <NUM> may increase at a faster rate than lateral peg distances <NUM>. In the illustrated embodiment of <FIG>, medial peg distances <NUM> increase at a rate (i.e., slope) of approximately <NUM>, while lateral peg distances <NUM> increase at a rate of approximately <NUM>. As a result, the difference between medial peg distance <NUM> and its corresponding lateral peg distance <NUM> increases as the prosthesis size increases, causing fixation pegs <NUM>, <NUM> to become more and more spaced apart as the prosthesis size increases.

With respect to the alternative baseplate <NUM>' of <FIG>, by contrast, where the lateral peg distance <NUM>' is the same as the medial peg distance <NUM>', the peg distances <NUM>', <NUM>' would overlap graphically in <FIG>. Thus, for any given prosthesis size, a single point corresponding to both lateral peg distance <NUM>' and medial peg distance <NUM>' would be presented in <FIG>, without a space therebetween. Also, because fixation pegs <NUM>', <NUM>' of baseplate <NUM>' are aligned along medial-lateral axis <NUM>', and not forward of or behind medial-lateral axis <NUM>' like fixation pegs <NUM>, <NUM> of baseplate <NUM>, the overlapping peg distances <NUM>', <NUM>' of the alternative baseplate <NUM>' would fall somewhere between the spaced-apart peg distances <NUM>, <NUM> of <FIG>. The same result would occur with the overlapping peg distances <NUM>", <NUM>" of the alternative baseplate <NUM>" of <FIG>.

According to an exemplary embodiment of the present disclosure, the above-described distances, including inset distance <NUM>, medial-lateral separation distance <NUM>, offset distance <NUM>, anterior-posterior separation distance <NUM>, lateral peg distance <NUM>, and medial peg distance <NUM>, are measured along distal surface <NUM> of baseplate <NUM>. As a result, the distances are measured near the intersection of each peg <NUM>, <NUM> with distal surface <NUM> (e.g., near the proximal end of each peg <NUM>, <NUM>). In embodiments where pegs <NUM>, <NUM> are perpendicular to distal surface <NUM>, the distances could also be measured away from distal surface <NUM> (e.g., near the distal end of each peg <NUM>, <NUM>) without impacting the measurements. In embodiments where pegs <NUM>, <NUM> are canted relative to distal surface <NUM>, however, the measurements could vary if taken away from distal surface <NUM> (e.g., near the distal end of each canted peg <NUM>, <NUM>). Therefore, for consistency, the measurements are taken along distal surface <NUM> of baseplate <NUM>.

A first prosthesis was manufactured, as shown in <FIG>, by mounting bearing component <NUM> onto baseplate <NUM>, with baseplate <NUM> having an asymmetric outer periphery <NUM> and asymmetrically arranged lateral and medial fixation pegs <NUM>, <NUM> (<FIG>). A second prosthesis (not shown) was manufactured by mounting a similar bearing component <NUM> onto an alternative baseplate <NUM>', with the alternative baseplate <NUM>' having an asymmetric outer periphery <NUM>' but aligned lateral and medial fixation pegs <NUM>', <NUM>' (<FIG>). A third prosthesis (not shown) was manufactured by mounting a similar bearing component <NUM> onto another alternative baseplate <NUM>", with the other alternative baseplate <NUM>" having a symmetric outer periphery <NUM>" and aligned lateral and medial fixation pegs <NUM>", <NUM>" (<FIG>).

The illustrative bearing component <NUM> has lateral articular surface <NUM>, medial articular surface <NUM>, and spine <NUM> located therebetween. When bearing component <NUM> is assembled onto baseplate <NUM>, as shown in <FIG>, lateral articular surface <NUM> of bearing component <NUM> aligns with lateral compartment <NUM> of baseplate <NUM>, medial articular surface <NUM> of bearing component <NUM> aligns with medial compartment <NUM> of baseplate <NUM>, and spine <NUM> aligns with interior compartment <NUM> (<FIG>) of baseplate <NUM>. For the first and second prostheses, bearing component <NUM> had a thickness T of <NUM>. For the third prosthesis, bearing component <NUM> had a thickness T of <NUM>. Bearing component <NUM> and its associated articular surfaces <NUM>, <NUM> are described in detail in <CIT>, filed November <NUM>, <NUM><NUM>, and are further described in <CIT> , and are further described in <CIT> , and are further described in <CIT> , and are further described in <CIT>, and are further described in <CIT> , and are further described in <CIT> , and are further described in <CIT> , and are further described in <CIT> , and are further described in <CIT> , and are further described in <CIT> , all entitled "TIBIAL BEARING COMPONENT FOR A KNEE PROSTHESIS WITH IMPROVED ARTICULAR CHARACTERISTICS,".

As shown in <FIG>, a lateral compressive force FCL was applied onto lateral articular surface <NUM> of each bearing component <NUM>, and a medial compressive force FCM was applied onto medial articular surface <NUM> of each bearing component <NUM>. The compressive forces FCL, FCM measured <NUM> N.

Simultaneously with application of the compressive forces FCL, FCM, an anterior-facing force FAP was applied to the distal/posterior base of spine <NUM>, as shown in <FIG>. The anterior-facing force FAP measured <NUM> N for the first and second prostheses and was scaled up to <NUM> N for the third prosthesis to account for the thinner bearing component <NUM>.

Forces FCL, FCM, and FAP were designed in magnitude and area of application to replicate forces exerted on tibial bearing component <NUM> by a prosthetic femoral component, e.g., femoral component <NUM>, during a kneeling motion. An exemplary femoral component which articulates with tibial bearing component <NUM> is described in <CIT> , and is further described in <CIT>, and is further described in <CIT>, and is further described in <CIT> , and is further described in and in <CIT> , and are further described in <CIT> , and are further described in <CIT> , and are further described in <CIT> , and are further described in <CIT> , and are further described in <CIT> , all entitled "FEMORAL COMPONENT FOR A KNEE PROSTHESIS WITH IMPROVED ARTICULAR CHARACTERISTICS,".

Finite element analysis was performed on the first, second, and third prostheses to evaluate and compare stresses experienced at the interface of baseplates <NUM>, <NUM>', <NUM>" and a simulated tibial bone that was well fixed to each respective baseplate. Peak stresses experienced in the above-described loading scenario were substantially reduced for the first baseplate <NUM> having asymmetrically arranged fixation pegs <NUM>, <NUM> as compared to the second baseplate <NUM>' having aligned fixation pegs <NUM>', <NUM>' and the third baseplate <NUM>" having aligned fixation pegs <NUM>", <NUM>". More particularly, a <NUM>% reduction in peak stress was observed in the first baseplate <NUM> as compared to the second baseplate <NUM>', and a <NUM>% reduction in peak stress was observed in the first baseplate <NUM> as compared to the third baseplate <NUM>".

In addition to lateral fixation peg <NUM> described above, lateral compartment <NUM> of tibial baseplate <NUM> may further include at least one additional lateral fixation peg <NUM>. As shown in <FIG>, the additional lateral fixation peg <NUM> is substantially centered within the PL quadrant. The illustrative lateral fixation peg <NUM> is positioned anteriorly/posteriorly between lateral fixation peg <NUM> and medial fixation peg <NUM>, such that lateral fixation peg <NUM> is the anterior-most fixation peg on tibial baseplate <NUM> and medial fixation peg <NUM> is the posterior-most fixation peg on tibial baseplate <NUM>. As a result of lateral fixation peg <NUM> being medially biased toward home axis AH, as described above, the illustrative lateral fixation peg <NUM> is located laterally outward of lateral fixation peg <NUM> and is the lateral-most fixation peg on tibial baseplate <NUM>.

In addition to medial fixation peg <NUM> described above, medial compartment <NUM> of tibial baseplate <NUM> may further include at least one additional medial fixation peg <NUM>. As shown in <FIG>, the additional medial fixation peg <NUM> is substantially centered within the AM quadrant. The illustrative medial fixation peg <NUM> is positioned anteriorly/posteriorly between lateral fixation peg <NUM> and medial fixation peg <NUM>, such that lateral fixation peg <NUM> is the anterior-most fixation peg on tibial baseplate <NUM> and medial fixation peg <NUM> is the posterior-most fixation peg on tibial baseplate <NUM>. As a result of medial fixation peg <NUM> being medially biased away from home axis AH, as described above, the illustrative medial fixation peg <NUM> is located laterally inward of medial fixation peg <NUM>.

Turning to <FIG> and <FIG>, tibial baseplate <NUM> is provided which is not encompassed by the claims but is similar to baseplate <NUM> of <FIG>, except that baseplate <NUM> includes a single fixation structure, illustratively keel <NUM>, that extends distally from distal surface <NUM> and into tibia T (<FIG>). Keel <NUM> may be monolithically or integrally formed as part of tibial baseplate <NUM>,
or keel <NUM> may be separately attachable to distal surface <NUM> of tibial baseplate <NUM>. Structures of baseplate <NUM> that correspond to structures of baseplate <NUM> have corresponding reference numerals, with the number <NUM> being added to the reference numerals of baseplate <NUM> to arrive at the corresponding reference numerals of baseplate <NUM>, except as otherwise noted.

The illustrative keel <NUM> of <FIG> has a cylindrical core <NUM> defining longitudinal axis AK (i.e., the axis of the cylinder defined by cylindrical core <NUM>) and having two or more fins <NUM> extending radially outwardly therefrom, the fins being arranged symmetrically relative to the cylindrical core <NUM>. More particularly, fins <NUM> extend along substantially all of the longitudinal extent PDK (<FIG>) of keel <NUM>, as best shown in <FIG> and <FIG>, such that fins <NUM> terminate at or near the distal tip of keel <NUM>. In an exemplary embodiment, longitudinal extent PDK of tibial keel cylindrical core <NUM> may range from <NUM> to <NUM>, with smaller nominal sizes of baseplate <NUM> having relatively lesser extents PDK and larger nominal sizes of baseplate <NUM> having relatively greater extents PDK.

Keel fins <NUM> also define keel fin angle γ with respect to longitudinal axis AK of cylindrical core <NUM> of keel <NUM>. In an exemplary embodiment, keel angle γ is equal to between <NUM> degrees and <NUM> degrees. Keel fin angle γ and longitudinal extent longitudinal extent PDK of cylindrical core <NUM> cooperate to define a medial/lateral keel extent MLK (<FIG>) of between <NUM> and <NUM>, with smaller nominal sizes of baseplate <NUM> having relatively lesser extents MLK and larger nominal sizes of baseplate <NUM> having relatively greater extents MLK. Advantageously, this medial/lateral extent MLK defined by fins <NUM> of keel <NUM> present high resistance to rotation of tibial baseplate <NUM> in vivo, and enhance the overall strength of baseplate <NUM>.

Keel <NUM> defines a substantially cylindrical outer profile as illustrated in <FIG>. Where such cylindrical outer profile is employed, an exemplary embodiment of core <NUM> of keel <NUM> may maintain an outer diameter between <NUM> and <NUM>, with such diameter remaining constant across the longitudinal extent. However, it is contemplated that core <NUM> of keel <NUM> may have a conical, tapered outer profile, particularly for small-stature baseplate sizes. The taper angle may be formed, for example, by tapering core <NUM> of keel <NUM> from a circular outer diameter of <NUM> at the proximal terminus of keel <NUM> (i.e., at the junction between keel <NUM> and distal surface <NUM> of tibial baseplate <NUM>) to a circular diameter of <NUM> at the distal terminus of keel <NUM>. An exemplary conical keel used in conjunction with a small-stature baseplate size is disclosed in <CIT> and in <CIT> (Attorney Docket No. ZIM0919-<NUM>), both entitled ASYMMETRIC TIBIAL COMPONENTS FOR A KNEE PROSTHESIS.

Prior art tibial baseplates include constant-diameter keels in this diameter range, such as the Zimmer NexGen Stemmed Tibial Plates and Natural Knee II Modular Cemented Tibial Plates. The NexGen Stemmed Tibial Plates and Natural Knee II Modular Cemented Tibial Plates are shown at pages <NUM> and <NUM>, respectively, of the "Zimmer® Tibial Baseplate, Pocket Guide United States Version,".

In <FIG> (not falling within the scope of the claims), keel <NUM> is represented by a phantom oval to show the general location of keel <NUM>, not necessarily the size or shape of keel <NUM>. Rather than being cylindrical in shape, it is also within the scope of the present disclosure that core <NUM> of keel <NUM> may be conical in shape, with an outer diameter that tapers distally.

As discussed above, fixation pegs <NUM>, <NUM> of baseplate <NUM> (<FIG>) may be designed to interact with cancellous bone surrounding the intramedullary canal of the patient's tibia T. To enhance this interaction with the cancellous bone, fixation pegs <NUM>, <NUM> may be constructed of a highly porous biomaterial that accepts bone ingrowth. Keel <NUM> of baseplate <NUM> (<FIG> and <FIG>), by contrast, may be designed to fit into the intramedullary canal of the patient's tibia T. Like fixation pegs <NUM>, <NUM>, keel <NUM> may also be constructed of a highly porous biomaterial that accepts bone ingrowth. Alternatively, rather than achieving fixation via bone ingrowth, keel <NUM> may be constructed of a solid metal that achieves fixation via a tight interference fit with the patient's surrounding bone.

Although keel <NUM> may be the only fixation structure on baseplate <NUM>, it is also within the scope of the present disclosure to combine keel <NUM> with additional fixation structures. In one embodiment, keel <NUM> may be combined with the above-described fixation pegs <NUM>, <NUM> (<FIG>). On another embodiment, keel <NUM> may be combined with sharp spikes (not shown). Such spikes may be located in the same general areas discussed above with respect to fixation pegs <NUM>, <NUM>. However, unlike the blunt-tipped and porous fixation pegs <NUM>, <NUM>, the spikes may be sharp-tipped to pierce the patient's bone and may be solid in construction. The spikes may also have external ribs or barbs to enhance fixation with the patient's bone.

Keel <NUM> may also include a tapered bore (not shown) extending proximally into the distal tip of keel <NUM>, designed to mate with a corresponding locking-taper surface of a tibial stem extension.

As shown in <FIG>, keel <NUM> is asymmetrically disposed on distal surface134 of baseplate <NUM> with respect to home axis AH. More particularly, the longitudinal keel axis AK of keel <NUM> is biased medially with respect to the vertical plane that contains home axis AH, i.e., keel axis AK is offset toward medial compartment <NUM> and away from lateral compartment <NUM> by offset distance <NUM>. Throughout the following paragraphs, home axis AH and the vertical plane that contains home axis AH are used interchangeably.

Offset distance <NUM> is measured along distal surface <NUM> of baseplate <NUM>. As a result, offset distance <NUM> is measured medially from the intersection of home axis AH and distal surface <NUM> to the intersection of keel axis AK and distal surface <NUM> (e.g., near the proximal end of keel <NUM>). In embodiments where keel axis AK is perpendicular to distal surface <NUM>, offset distance <NUM> could also be measured away from distal surface <NUM> (e.g., near the distal end of keel <NUM>) without impacting the measurement. In embodiments where keel axis AK is canted relative to distal surface <NUM>, however, the measurement could vary if taken away from distal surface <NUM> (e.g., near the distal end of the canted keel <NUM>). Therefore, for consistency, the measurement is taken along distal surface <NUM> of baseplate <NUM>.

Where baseplate <NUM> has a symmetric outer periphery <NUM>, an anterior-posterior axis of symmetry through outer periphery <NUM> may be used as a "home axis" AH for referencing medial face <NUM>, lateral face <NUM>, keel <NUM>, and other components of baseplate <NUM>. This home axis AH would be substantially centered between medial face <NUM> and lateral face <NUM>. With keel axis AK being medially offset from the central home axis AH, keel axis AK would be positioned closer to medial face <NUM> than lateral face <NUM>. Thus, medial distance <NUM> between keel axis AK and the medial-most portion of medial face <NUM> would be less than lateral distance <NUM> between keel axis AK and the lateral-most portion of lateral face <NUM>.

Where baseplate <NUM> has an asymmetric outer periphery <NUM>, as shown in <FIG> and <FIG>, home axis AH would not constitute an axis of symmetry and would be positioned closer to lateral face <NUM> than medial face <NUM>. Depending on the degree to which keel axis AK is medially offset from home axis AH, keel axis AK may still be positioned closer to medial face <NUM> than lateral face <NUM>. Thus, medial distance <NUM> between keel axis AK and the medial-most portion of medial face <NUM> may be less than lateral distance <NUM> between keel axis AK and the lateral-most portion of lateral face <NUM>.

The degree of medialization of keel <NUM> may be expressed as a ratio or a percentage and may be calculated by dividing the offset distance <NUM> between keel axis AK and home axis AH by the total medial/lateral width of distal surface <NUM> (i.e., medial distance <NUM> plus lateral distance <NUM>). For baseplate <NUM> having the dimensions set forth in Table <NUM> below, for example, the degree of medialization would be approximately <NUM>% (calculated as <NUM> / <NUM> x <NUM>%).

Advantageously, the medial bias of keel <NUM> (i.e., the relatively short medial distance <NUM> and the relatively long lateral distance <NUM>) more closely aligns keel <NUM> with the intramedullary canal of the patient's tibia T (<FIG>). Thus, upon implantation of baseplate <NUM> onto the patient's tibia T, keel <NUM> may be centered or nearly centered within the intramedullary canal. In this manner, keel <NUM> may avoid impinging onto hard, cortical bone around the intramedullary canal, thereby promoting firm and stable long-term fixation of tibial baseplate <NUM> to tibia T. The medial bias of keel <NUM> may also be important if it becomes necessary to attach a distal stem extension (not shown) to keel <NUM>, such as during a revision surgical procedure. In this manner, tibial baseplate <NUM> may achieve an optimum metaphyseal fit on tibia T in the region of keel <NUM> and diaphyseal fit on tibia T in the region of the distal stem extension.

Baseplate <NUM> may be provided in a kit or set of different prosthesis sizes. In one embodiment, nine nominal sizes of baseplate <NUM> are provided in the set, with baseplates <NUM> growing progressively in size.

According to an exemplary embodiment of the present disclosure, the degree of medialization of keel <NUM> increases as the prostheses in the set grow in size. Thus, rather than maintaining a fixed relationship between medial distance <NUM> and lateral distance <NUM> as the prostheses grow in size, medial distance <NUM> makes up a smaller and smaller portion of the total width as the prostheses grow in size, and lateral distance <NUM> makes up a larger and larger portion of the total width as the prostheses grow in size. Stated differently, the rate at which keel <NUM> moves toward medial face <NUM> exceeds that rate at which the prostheses grow in size.

The dimensions of another sample baseplate <NUM> are provided in Table <NUM> below. Baseplate <NUM> of Table <NUM>, which has a total width of <NUM>, is smaller than baseplate <NUM> of Table <NUM> above, which has a total width of <NUM>.

As baseplates <NUM> of the present set grow in size from Table <NUM> to Table <NUM> (i.e., from a small nominal size having a <NUM> total width to a large nominal size having an <NUM> total width), the degree of medialization of keel <NUM> increases relative to home axis AH (from <NUM>% to <NUM>%). Also, as keel <NUM> moves medially from the small size of Table <NUM> to the large size of Table <NUM>, medial distance <NUM> makes up a smaller portion of the total width (from <NUM>% to <NUM>%), and lateral distance <NUM> makes up a larger portion of the total width (from <NUM>% to <NUM>%).

Advantageously, increasing the degree of medialization of keel <NUM> as baseplate <NUM> grows in size may better track the position of the intramedullary canal as the patient's tibia T (<FIG>) grows in size. Therefore, keel <NUM> may be positioned inside the intramedullary canal rather than in hard, cortical bone around the intramedullary canal.

The increasing medialization of keel <NUM> is presented graphically in <FIG>, where exemplary offset distances <NUM> between keel axis AK and home axis AH are shown for a set of prostheses of different sizes. More specifically, exemplary offset distances <NUM> between keel axis AK and home axis AH are shown for a set of prostheses having different medial/lateral widths (i.e., medial distance <NUM> plus lateral distance <NUM>). The data points located farther to the left represent smaller medial/lateral widths (and therefore smaller prosthesis sizes), and data points located farther to the right represent larger medial/lateral widths (and therefore larger prosthesis sizes). Although adjacent nominal prosthesis sizes may share the same offset distance <NUM> between keel axis AK and home axis AH (compare, for example, the corresponding offset distances <NUM> of the size <NUM> and size <NUM> implants, shown as the fifth- and sixth- from left data points respectively), the overall trend in <FIG> is that offset distance <NUM> increases as total medial/lateral width increases.

In a smaller bone, the metaphyseal region of tibia T is more closely aligned with the diaphyseal region of tibia T. Therefore, keel <NUM> may achieve an optimum metaphyseal and diaphyseal fit with a relatively small offset distance <NUM> (e.g., <NUM>, <NUM>). In a larger bone, by contrast, the metaphyseal region of tibia T is more offset from the diaphyseal region of tibia T. Therefore, keel <NUM> may require a relatively large offset distance <NUM> (e.g., <NUM>, <NUM>) to achieve an optimum metaphyseal and diaphyseal fit. <FIG> presents exemplary offset distances <NUM>, but for any given size, offset distance <NUM> may vary by +/- <NUM>, +/- <NUM>, +/- <NUM>, or +/- <NUM>, for example.

As discussed above, the degree of medialization of keel <NUM> may be expressed as a percentage by dividing the offset distance <NUM> between keel axis AK and home axis AH by the total medial/lateral width. In <FIG>, the offset distances <NUM> from <FIG> are shown as percentages of the total medial/lateral width. The overall trend in <FIG> is that the degree of medialization of keel <NUM> increases as medial/lateral width increases. With respect to a relatively small nominal size <NUM> implant, for example, the medial offset of keel <NUM> from home axis AH is <NUM>% of the total medial/lateral implant width. With respect to a relatively large nominal size <NUM> implant, the medial offset of keel <NUM> from home axis AH is <NUM>% of the total medial/lateral implant width.

As shown in <FIG> (not falling within the scope of the claims), the anterior/posterior keel distance <NUM> may be measured posteriorly from flat portion 118a of anterior face <NUM> to keel axis AK, for example. The lateral depth <NUM> of lateral compartment <NUM> is also shown being measured posteriorly from flat portion 118a of anterior face <NUM> to posterior/lateral face <NUM> of baseplate <NUM> in <FIG>, and this lateral depth <NUM> exceeds keel distance <NUM>.

According to an exemplary embodiment of the present disclosure, keel distance <NUM> is measured along distal surface <NUM> of baseplate <NUM>. As a result, keel distance <NUM> is measured posteriorly from the intersection of flat portion 118a of anterior face <NUM> and distal surface <NUM> to the intersection of keel axis AK and distal surface <NUM> (e.g., near the proximal end of keel <NUM>). In embodiments where keel axis AK is perpendicular to distal surface <NUM>, keel distance <NUM> could also be measured away from distal surface <NUM> (e.g., near the distal end of keel <NUM>) without impacting the measurement. In embodiments where keel axis AK is canted relative to distal surface <NUM>, however, the measurement could vary if taken away from distal surface <NUM> (e.g., near the distal end of the canted keel <NUM>). Therefore, for consistency, the measurement is taken along distal surface <NUM> of baseplate <NUM>.

Across a set of different tibial baseplates <NUM> having varying nominal sizes, the anterior/posterior positioning of keel <NUM> may vary. In <FIG>, for example, exemplary anterior/posterior keel distances <NUM> are shown for a set of prostheses of different sizes. The overall trend in <FIG> is that keel distance <NUM> increases as lateral depth <NUM> increases. Moving keel <NUM> further and further from anterior face <NUM> as baseplate <NUM> increases in size may avoid anterior cortical bone impingement by keel <NUM>, especially if keel <NUM> also increases in size (e.g., diameter, length) along with baseplate <NUM>. <FIG> depicts exemplary keel distances <NUM>, but for any given size, keel distance <NUM> may vary by +/- <NUM>, +/- <NUM>, +/- <NUM>, or +/- <NUM>, for example.

As shown in <FIG>, the illustrative keel <NUM> includes a blind proximal bore <NUM> therein that is sized to receive a fixation structure, such as a set screw (not shown), from proximal surface <NUM> of baseplate <NUM>. The fixation structure may be used to attach a tibial bearing component onto proximal surface <NUM> of baseplate <NUM>, for example.

The illustrative bore <NUM> of <FIG> is centered along home axis AH. However, because keel axis AK is offset from home axis AH, bore <NUM> becomes offset in keel <NUM>. To ensure that the walls of keel <NUM> surrounding bore <NUM> are adequately thick along the axial extent of bore <NUM> (e.g., <NUM>), keel <NUM> may expand radially outwardly around bore <NUM> to form bulge <NUM>.

As keel <NUM> becomes more and more offset from home axis AH and bore <NUM>, bulge <NUM> may become larger and larger in size. For example, for medium nominal prosthesis sizes (e.g., sizes <NUM> and <NUM>) having medium offset distances <NUM> between keel axis AK and home axis AH (e.g., <NUM>), bulge <NUM> may increase the diameter of keel <NUM> by <NUM>. For large nominal prosthesis sizes (e.g., sizes <NUM>-<NUM>) having large offset distances <NUM> between keel axis AK and home axis AH (e.g., <NUM>, <NUM>), bulge <NUM> may increase the diameter of keel <NUM> by <NUM>. For small nominal prosthesis sizes (e.g., sizes <NUM>-<NUM>) having small offset distances <NUM> between keel axis AK and home axis AH (e.g., <NUM>, <NUM>), bulge <NUM> may be excluded.

Claim 1:
A tibial baseplate (<NUM>) configured for implantation upon a patient's proximal tibia (T), the tibial baseplate comprising:
a medial compartment (<NUM>);
a lateral compartment (<NUM>) opposite the medial compartment;
a proximal surface (<NUM>);
a distal surface (<NUM>) opposite the proximal surface, the distal surface sized and shaped to substantially cover the patient's proximal tibia;
an outer periphery (<NUM>) cooperatively defined by an anterior face (<NUM>), a medial face (<NUM>), a lateral face (<NUM>), and at least one posterior face (<NUM>, <NUM>);
at most one medial fixation peg (<NUM>) associated with the medial compartment (<NUM>), the medial fixation peg extending distally from the distal surface (<NUM>) and positioned for implantation into the patient's proximal tibia; and
at most one lateral fixation peg (<NUM>) associated with the lateral compartment (<NUM>), the lateral fixation peg extending distally from the distal surface (<NUM>) and positioned for implantation into the patient's proximal tibia, the lateral fixation peg being located closer to the anterior face (<NUM>) than the medial fixation peg
wherein the anterior face (<NUM>) has a flat portion (18A) disposed between the medial and lateral compartments (<NUM>, <NUM>) and in that
the tibial baseplate further comprises an anterior-posterior axis (AH) that bisects a posterior cutout (<NUM>) formed in the at least one posterior face (<NUM>, <NUM>) of the tibial baseplate,
characterised in that the anterior-posterior axis bisects the flat portion (18A) of the anterior face.