Asymmetric tibial components for a knee prosthesis

Abstract

An orthopedic tibial prosthesis includes a tibial baseplate with features designed for use with small-stature knee-replacement patients. The tibial prosthesis may include a shortened tibial keel, tibial keel fins which define a large angle with respect to a longitudinal axis of the keel, and/or tibial keel fins which extend along less than the entire longitudinal extent of the keel.

Claims

1. A small-stature tibial baseplate comprising one of a smallest two nominal sizes of baseplate in a family of baseplates of similar design, the baseplate comprising: a tibial plateau comprising: a distal surface sized and shaped to substantially cover a proximal resected surface of a tibia; a proximal surface opposite the distal surface, the proximal surface having a lateral compartment and a medial compartment opposite the lateral compartment, wherein the lateral compartment is asymmetric with respect to the medial compartment about a component anteroposterior axis to define a component asymmetry; and a peripheral wall extending between the distal surface and the proximal surface, wherein a total surface area bounded by the peripheral wall of the tibial plateau is between about 1390 mm.sup.2 and about 1580 mm.sup.2; a tibial keel extending distally from a junction with the distal surface to an opposing distal tip, the tibial keel having a longitudinal extent defined between the junction and the distal tip; and a medial fin and a lateral fin extending from the tibial keel and together having a medial/lateral fin extent at the junction of about 40 mm; wherein the tibial plateau of the small-stature tibial baseplate has a periphery less than a periphery of any other nominal size of baseplate in the family, and a ratio of the medial/lateral fin extent to the longitudinal extent is greater than the same ratio for any other nominal size of baseplate in the family.

2. The small-stature tibial baseplate of claim 1, wherein the longitudinal extent of the tibial keel is about 27 mm.

3. The small-stature tibial baseplate of claim 1, wherein a first diameter of the tibial keel at the junction is greater than a second diameter of the tibial keel at the distal tip, and the tibial keel has a tapered outer profile extending between the first diameter and the second diameter.

4. The small-stature tibial baseplate of claim 3, wherein the tapered outer profile of the tibial keel defines a taper angle of about 9 degrees.

5. The small-stature tibial baseplate of claim 1, wherein at least one of the medial fin and the lateral fin extends along less than an entire longitudinal extent of the tibial keel.

6. The small-stature tibial baseplate of claim 5, wherein at least one of the medial fin and the lateral fin extends along about 74% of the entire longitudinal extent of the tibial keel.

7. The small-stature tibial baseplate of claim 1, wherein the tibial keel has a minimum diameter along the longitudinal extent of at least 13 mm.

8. The small-stature tibial baseplate of claim 1, wherein the tibial keel defines a tapered bore extending proximally into the tibial keel from the distal tip of the tibial keel, the tapered bore sized to receive a correspondingly tapered proximal end of a tibial stem extension, such that the tapered proximal end of the tibial stem extension forms a locking taper connection with the tapered bore.

9. The small-stature tibial baseplate of claim 8, wherein: the tibial keel comprises a set screw aperture extending from an outer surface of the tibial keel to an inner surface defined by the tapered bore; and the tapered proximal end of the tibial stem extension comprises an annular groove positioned to align with the set screw aperture when the locking taper connection is formed between the tibial stem extension and the tapered bore.

10. The small-stature tibial baseplate of claim 9, in combination with a set screw receivable within the set screw aperture, the set screw extending into the annular groove to form a secondary locking mechanism preventing relative axial movement between the tibial stem extension and the tibial keel when the locking taper connection is formed between the tibial stem extension and the tapered bore.

11. The small-stature tibial baseplate of claim 1, wherein the tibial plateau defines an overall medial/lateral extent of between about 57 mm and about 61 mm.

12. The small-stature tibial baseplate of claim 1, wherein the medial fin and the lateral fin each span only a portion of the junction between the tibial keel and the tibial plateau, and wherein each of the medial fin and the lateral fin have a fin edge comprising a planar portion defining an angle of about 45 degrees with respect to a longitudinal tibial keel axis.

13. The small-stature tibial baseplate of claim 12, wherein the medial/lateral fin extent comprises about 40 mm.

14. A small-stature tibial baseplate comprising one of a smallest two nominal sizes of baseplate in a family of baseplates of similar design, the baseplate comprising: a tibial plateau comprising: a distal surface sized and shaped to substantially cover a proximal resected surface of a tibia; a proximal surface opposite the distal surface, the proximal surface having a lateral compartment and a medial compartment opposite the lateral compartment, the lateral compartment is asymmetric with respect to the medial compartment about a component anteroposterior axis to define a component asymmetry; and a peripheral wall extending between the distal surface and the proximal surface, wherein a total surface area bounded by the peripheral wall of the tibial plateau is between about 1390 mm.sup.2 and about 1580 mm.sup.2; a tibial keel extending distally from a junction with the distal surface to an opposing distal tip, the tibial keel having a longitudinal extent defined between the junction and the distal tip, the tibial keel comprising a first diameter at said junction between said distal surface and said tibial keel and a second diameter at said distal tip of said tibial keel, each of the first diameter and the second diameter equal to or greater than 13 mm, the first diameter greater than the second diameter; and a medial fin and a lateral fin extending from the tibial keel and adjoining the distal surface of the tibial plateau, together the medial fin and the lateral fin have a medial/lateral fin extent at the distal surface, wherein the tibial plateau of the small-stature tibial baseplate has a periphery less than a periphery of any other nominal size of baseplate in the family, and a ratio of the medial/lateral fin extent to the longitudinal extent is greater than the same ratio for any other nominal size of baseplate in the family.

15. The small-stature tibial baseplate of claim 14, wherein each of the medial fin and the lateral fin has a fin edge comprising a planar portion defining an angle of about 45 degrees with respect to a longitudinal tibial keel axis.

16. The small-stature tibial baseplate of claim 14, wherein a first diameter of the tibial keel at the junction between the distal surface and the keel is greater than a second diameter of the keel at the distal tip, and the tibial keel has a tapered outer profile extending between the first diameter and the second diameter, and wherein the medial and lateral fins extend along less than an entire longitudinal extent of the tibial keel.

17. The small-stature tibial baseplate of claim 14, wherein the medial fin mates with the distal surface at the medial compartment, the lateral fin mates with the distal surface at the lateral compartment, and wherein the medial/lateral fin extent is about 40 mm.

18. The small-stature tibial baseplate of claim 14, wherein the tibial plateau has an overall medial/lateral extent of between about 57 mm and about 61 mm.

Description

BRIEF DESCRIPTION OF THE DRAWINGS

(1) The above-mentioned and other features and advantages of this invention, 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:

(2) FIG. 1A is an exploded, perspective view of a tibial baseplate and tibial bearing component in accordance with the present disclosure;

(3) FIG. 1B is an assembled, perspective view of the tibial baseplate and tibial bearing component shown in 1A;

(4) FIG. 2A is a top plan view of the peripheries of a set of nine tibial baseplates made in accordance with the present disclosure, in which the peripheries are shown to scale according to the illustrated scales in millimeters in the bottom and right-hand margins of the page;

(5) FIG. 2B is a top plan view of the periphery of a tibial baseplate made in accordance with the present disclosure;

(6) FIG. 2C is a graph illustrating the asymmetric growth of the posterior-medial compartment for the tibial baseplates shown in FIG. 2A;

(7) FIG. 2D is a graph illustrating the asymmetric growth of the posterior-lateral compartment for the tibial baseplates shown in FIG. 2A;

(8) FIG. 3A is top plan view of a periphery of a tibial baseplate made in accordance with the present disclosure, illustrating various arcs defined by the periphery;

(9) FIG. 3B is a partial, top plan view of the periphery shown in FIG. 3A, illustrating an alternative lateral corner periphery;

(10) FIG. 3C is a partial, top plan view of the periphery shown in FIG. 3A, illustrating an alternative medial corner periphery;

(11) FIG. 3D is a top plan view of the periphery of a tibial baseplate made in accordance with the present disclosure, illustrating medial and lateral surface area calculations without a PCL cutout;

(12) FIG. 4A is a top plan view of a tibial baseplate made in accordance with the present disclosure;

(13) FIG. 4B is a side elevation view of the tibial baseplate shown in FIG. 4A;

(14) FIG. 5 is a top plan view of a resected proximal tibial surface with a prosthetic tibial baseplate component and tibial bearing component made in accordance with the present disclosure mounted thereon;

(15) FIG. 6 is a top plan view of a resected proximal tibial surface with a properly sized tibial trial component thereon;

(16) FIG. 7 is a side, elevation view of the tibia and trial component shown in FIG. 6;

(17) FIG. 8 is a side, elevation view of the tibial components shown in FIG. 1A, in conjunction with a femoral component;

(18) FIG. 9 is a bottom, perspective view of a small stature tibial baseplate made in accordance with the present disclosure;

(19) FIG. 10 is a front coronal, elevation view of the small stature tibial baseplate shown in FIG. 9, together with a tibial stem extension; and

(20) FIG. 11 is a rear coronal, perspective view of another small stature tibial baseplate, shown with the tibial stem extension of FIG. 10.

(21) 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.

DETAILED DESCRIPTION

(22) The present disclosure provides an asymmetric knee joint prosthesis which facilitates proper rotational and spatial orientation of a tibial baseplate and tibial bearing component upon a resected proximal tibia, while also offering large-area contact with the resected proximal tibia. The prosthesis permits a wide range of flexion motion, protects natural soft tissue proximate the knee joint prosthesis, and optimizes long term fixation characteristics of the prosthesis.

(23) In order to prepare the tibia and femur for receipt of a knee joint prosthesis of the present disclosure, any suitable methods or apparatuses may be used. 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 the patient.

(24) As used herein, the “periphery” of a tibial prosthesis refers to any periphery as viewed in a top plan view, e.g., in a generally transverse anatomical plane. Alternatively, the periphery of a tibial prosthesis may be any periphery as viewed in bottom plan view, e.g., in a generally transverse plane and looking at the distal surface adapted to contact a resected proximal surface of a tibial bone.

(25) As used herein, the term “centroid” or “geometric center” refers to the intersection of all straight lines that divide a given area into two parts of equal moment about each respective line. Stated another way, a geometric center may be said to be the “average” (i.e., arithmetic mean) of all points of the given area. Stated yet another way, the geometric center is a point in a two dimensional figure from which the sum of the displacement vectors of all points on the figure equals zero.

(26) As used herein, a “disparity” or “difference” between two numerical values (e.g., one value “larger” or “smaller” than another), typically expressed as a percentage, is the difference between the two values divided by the smaller of the two values. For example, a smaller quantity having value 75 and a larger quantity having value 150 would have a percentage disparity of (150−75)/75, or 100%.

(27) Referring to FIG. 5, tibia T includes tibial tubercle B having mediolateral width W, with tubercle midpoint P.sub.T located on tubercle B approximately halfway across width W. While tubercle B is shown as having midpoint P.sub.T at the “peak” or point of maximum anterior eminence, it is recognized that midpoint P.sub.T of tibia T may be spaced from such a peak. Tibia T also includes attachment point C.sub.P representing the geometric center of the attachment area between the anatomic posterior cruciate ligament (PCL) and tibia T. Recognizing that the PCL typically attaches to a tibia in two ligament “bundles,” one of which is relatively anterior, lateral and proximal and the other of which relatively posterior, medial and distal, attachment point C.sub.P is contemplated as representing the anterior/lateral attachment area in an exemplary embodiment. However, it is contemplated that the posterior/medial attachment area, or the entire attachment area, could be used.

(28) As used herein, “anterior” refers to a direction generally toward the front of a patient. “Posterior” refers to the opposite direction of anterior, toward the back of the patient.

(29) In the context of patient anatomy, “home axis” A.sub.H refers to a generally anteroposterior axis extending from posterior point C.sub.P to an anterior point C.sub.A, in which anterior point C.sub.A is disposed on tubercle B and medially spaced from tubercle midpoint P.sub.T by an amount equal to W/6. Stated another way, anterior point C.sub.A is laterally spaced by an amount equal to W/3 from the medial end of mediolateral width W, such that point C.sub.A lies on the “medial third” of the anterior tibial tubercle.

(30) In the context of a prosthesis, such as tibial baseplate 12 described below, “home axis” A.sub.H refers to an axis oriented with respect to baseplate 12 such that the baseplate home axis A.sub.H of baseplate 12 is aligned with home axis A.sub.H of tibia T after implantation of baseplate 12 in a proper rotational and spatial orientation (as shown in FIG. 5). In the illustrative embodiments shown in FIG. 3 and described in detail below, home axis A.sub.H bisects PCL cutout 28 at the posterior edge of periphery 200 of tibial plateau 18 (FIG. 5), and bisects anterior edge 202 at the anterior edge of periphery 200 of tibial plateau 18. It is contemplated that home axis A.sub.H may be oriented to other baseplate features, it being understood home axis A.sub.H of baseplate 12 is positioned such that that proper alignment and orientation of baseplate 12 upon tibia T positions the home axis A.sub.H of baseplate 12 coincident with home axis A.sub.H of tibia T.

(31) Home axis A.sub.H of tibial baseplate 12 may be said to be an anteroposterior axis, as home axis A.sub.H extends generally anteriorly and posteriorly when baseplate 12 is implanted upon tibia T. Tibial baseplate also defines mediolateral axis A.sub.ML, which lies along the longest line segment contained within periphery 200 that is also perpendicular to home axis A.sub.H of baseplate 12. As described below, home axis A.sub.H and mediolateral axis A.sub.ML cooperate to define a coordinate system useful for quantifying certain baseplate features in accordance with the present disclosure.

(32) The embodiments shown and described with regard to FIGS. 1A, 1B, 3A, 4A, 4B, 5 and 6 illustrate a left knee and associated features of a right-knee prosthesis, while the embodiments shown and described in FIGS. 2A, 2B and 3D illustrate the periphery of a right knee prosthesis. Right and left knee configurations are mirror images of one another about a sagittal plane. Thus, it will be appreciated that all aspects of the prosthesis described herein are equally applicable to a left- or right-knee configuration.

(33) 1. Asymmetry of the Tibial Prosthesis.

(34) Referring now to FIGS. 1A and 1B, tibial prosthesis 10 includes tibial baseplate 12 and tibial bearing component 14. Tibial baseplate 12 may include a stem or keel 16 (FIG. 4B) extending distally from proximal tibial plateau 18, or may utilize other fixation structures for securing baseplate 12 to tibia T, such as distally extending pegs. Portions of the outer periphery defined by tibial plateau 18 closely correspond in size and shape with a resected proximal surface of tibia T, as described in detail below.

(35) Tibial bearing component 14 and tibial baseplate 12 have a particular asymmetry, with respect to home axis A.sub.H (shown in FIG. 2A and described above), that is designed to maximize tibial coverage for a large proportion of knee-replacement candidates. This high level of coverage allows a surgeon to cover the largest possible area on the proximal resected surface of the tibia, which in turn offers maximum coverage of cortical bone. Advantageously, the maximized coverage of cortical bone facilitates superior support of tibial baseplate 12. A firm, enduring fixation of tibial baseplate 12 to tibia T is facilitated by large-area contact between the cortical and cancellous bone of tibia T and distal surface 35 of tibial plateau 18 (FIG. 4B), which may be coated with porous ingrowth material and/or bone cement.

(36) In an analysis of a several human specimens, variations in size and geometry for a variety of anatomic tibial features were observed and characterized. Geometrical commonalities between anatomic features, or lack thereof, were noted. Mean tibial peripheral geometries were calculated based on statistical analysis and extrapolation of the collected anatomical data, in view of the observed geometrical commonalities organized around anatomic home axis A.sub.H. These calculated mean geometries were categorized by tibial size.

(37) A comparison between the asymmetric peripheries for the present family of prostheses and the calculated mean tibial geometries was conducted. Based on the results of this comparison, it has been found that substantial tibial coverage can be achieved for a large proportion of patients using tibial components having asymmetric peripheries in accordance with the present disclosure. Moreover, this coverage can be achieved with a relatively small number of sizes, even where particular portions of the prosthesis periphery is intentionally “pulled back” from the tibial periphery in order to confer other orthopaedic benefits. Further, the particular asymmetry of tibial baseplate 12 can be expected to offer such coverage without overhanging any portion of the resected surface.

(38) Thus, periphery 200 including the particular asymmetric profile as described below confers the benefits of maximum coverage, facilitation of proper rotation (discussed below), and long-term fixation as described herein. Such asymmetry may be demonstrated in various ways, including: by a comparison of adjacent radii in the medial and lateral compartments of the asymmetric periphery; by a comparison of the edge length in anterior-medial and anterior lateral corners of the periphery, for a comparable lateral and medial angular sweep; and by a comparison of the location of radius centers for the anterior-medial and anterior-lateral corners with respect to a mediolateral axis. Various comparisons and quantifications are presented in detail below. Specific data and other geometric details of the peripheries for the various prosthesis sizes, from which the below-identified comparisons and quantifications are derived, may be obtained from the draw-to-scale peripheries shown in FIG. 2A.

(39) Advantageously, the asymmetry of tibial component 12 encourages proper rotational orientation of baseplate 12 upon implantation thereof onto tibia T. As described in detail below, the asymmetry of periphery 200 (FIG. 2A) of tibial plateau 18 is designed to provide a close match in selected areas of the lateral and medial compartments as compared to the anatomic bone. As such, a surgeon can select the largest possible component from among a family of different component sizes, such that the component substantially covers the resected tibia T with minimal gaps between the tibial periphery and component periphery 200, as well as little or no overhang over any portions of the tibial periphery. Because the high congruence between prosthesis periphery 200 and the tibial periphery produces only a minimal gap between the peripheries (as shown in FIG. 5), tibial baseplate 12 cannot be rotated significantly without causing tibial plateau 18 to overhang beyond the periphery of the resected tibial surface. Thus, proper rotation of baseplate 12 can be ascertained by the visual acuity between prosthesis periphery 200 and the resected tibial surface.

(40) The following examples and data are presented with respect to tibial baseplate 12. However, as described in more detail below, tibial bearing component 14 defines perimeter wall 54 which follows peripheral wall 25 of baseplate 12 except where noted. Thus, it is appreciated that the conclusions, trends and design features gleaned from data relating to the asymmetric periphery of tibial baseplate 12 also applies to the asymmetric periphery of tibial bearing component 14, except where stated otherwise.

(41) Lateral compartment 20 and medial compartment 22 of tibial plateau 18 are dissimilar in size and shape, giving rise to the asymmetry thereof. This asymmetry is designed so that peripheral wall 25 traces the perimeter of the resected proximal surface of tibia T, such that tibial plateau 18 covers a large proportion of the resected proximal tibial surface as shown in FIG. 5. To achieve this large tibial coverage, tibial plateau 18 closely matches the periphery of tibia T in most areas as noted above. Nevertheless, as shown in FIG. 5, for example, a small gap between periphery 200 of tibial plateau 18 and tibia T is formed to allow some freedom of positioning and rotational orientation. The gap is designed to have a substantially continuous width in most areas, including the anterior edge, anterior-medial corner, medial edge, lateral edge and lateral-posterior corner (all described in detail below).

(42) However, certain aspects of the asymmetric shape are designed to intentionally deviate from the calculated anatomical shape to confer particular features and advantages in the context of a complete, implanted knee prosthesis. Referring to FIG. 5, for example, tibial baseplate 12 and tibial bearing component 14 have anterior-lateral “corners” (described in detail below) which are “pulled back” to create gap 56 between tibia T and prosthesis 10 in the anterior-lateral area of the resected surface of tibia T. Advantageously, gap 56 creates extra space for “soft-tissue friendly” edges of prosthesis 10, thereby minimizing impingement of the iliotibial band. In an exemplary embodiment, gap 56 may range from 0.5 mm for a small-size prosthesis (such as size 1/A described below), to 1 mm for a medium-sized prosthesis (such as size 5/E described below), to 2 mm for a large-sized prosthesis (such as size 9/J described below).

(43) Similarly, the posterior edge of the medial compartment may be “pulled back” from the adjacent edge of tibia T to define gap 58. Gap 58 allows extra space for adjacent soft tissues, particularly in deep flexion as described below. Gap 58 also allows prosthesis 10 to be rotated about a lateral pivot by a small amount, thereby offering a surgeon the freedom to displace medial compartment 22 posteriorly as required or desired for a particular patient. In an exemplary embodiment, gap 58 is about 4 mm.

(44) As described in detail below, the asymmetrical periphery also provides a large overall area for proximal surface 34 of baseplate 12, which creates sufficient space for large contact areas between tibial bearing component 14 and femoral component 60 (FIG. 8).

(45) a. Medial/Lateral Peripheral Curvatures

(46) The particular asymmetric shape of tibial plateau 18 (and of tibial bearing component 14, which defines a similar periphery as described below) gives rise to a generally “boxy” or angular periphery in lateral compartment 20, and a “rounded” or soft periphery in medial compartment 22.

(47) Turning to FIG. 3A, the periphery 200 of tibial plateau 18 surrounds lateral compartment 20 and medial compartment 22, each of which define a plurality of lateral and medial arcs extending between anterior edge 202 and lateral and medial posterior edges 204, 206 respectively. In the illustrative embodiment of FIG. 3A, anterior edge 202, lateral posterior edge 204 and medial posterior edge 206 are substantially planar and parallel for ease of reference. However, it is contemplated that edges 202, 204, 206 may take on other shapes and configurations within the scope of the present disclosure, such as angled or arcuate.

(48) In the exemplary embodiment of FIG. 3A, lateral compartment 20 includes five separate arcs including lateral anterior edge arc 208, anterior-lateral corner arc 210, lateral edge arc 212, posterior-lateral corner arc 214, and lateral posterior edge arc 216. Each of lateral arcs 208, 210, 212, 214 and 216 defines angular sweep 1L, 2L, 3L, 4L and 5L, respectively, having radii R1L, R2L, R3L, R4L and R5L respectively. A radius of a particular angular sweep extends from the respective radius center (i.e., one of centers C1L, C2L, C3L, C4L and C5L) to periphery 200. Radii R1L, R2L, R3L, R4L, and R5L each remain unchanged throughout the extent of angular sweeps 1L, 2L, 3L, 4L and 5L, respectively.

(49) Similarly, medial compartment 22 includes three separate arcs including anterior-medial corner arc 220, medial edge arc 222 and posterior-lateral corner arc 224, defining angular sweeps 1R, 2R and 3R, respectively having radii R1R, R2R and R3R respectively.

(50) In FIG. 2A, peripheries 200.sub.X are shown for each of nine progressively larger component sizes, with 200.sub.1 being the periphery of the smallest size (size “1” or “A”) and 200.sub.9 being the periphery of the largest size (size “9” or “J”). For purposes of the present disclosure, several quantities and features of tibial baseplate 12 may be described with the subscript “X” appearing after the reference numeral corresponding to a component size as set for in the Tables, Figures and description below. The subscript “X” indicates that the reference numeral applies to all nine differently-sized embodiments described and shown herein.

(51) In exemplary embodiments, medial and lateral radii may be any value within the following ranges: for medial radius R1R.sub.X, between about 27 mm and about 47 mm; for medial radius R2R.sub.X, between about 21 mm and about 49 mm; for medial radius R3R.sub.X, between about 14 mm and about 31 mm; for lateral radius R1L.sub.X, between about 46 mm and about 59 mm; for lateral radius R2L.sub.X, between about 13 mm and about 27 mm; for lateral radius R3L.sub.X between about 27 mm and about 46 mm; for lateral radius R4L.sub.X between about 6 mm and about 4 mm; and for lateral radius R5L.sub.X between about 22 mm and about 35 mm.

(52) In exemplary embodiments, medial and lateral angular extents or sweeps may be any value within the following ranges: for medial angle 1R.sub.X, between about 13 degrees and about 71 degrees; for medial angle 2R.sub.X, between about 23 degrees and about 67 degrees; for medial angle 3R.sub.X, between about 23 degrees and about 90 degrees; for lateral angle 1L.sub.X, between about 11 degrees and about 32 degrees; for lateral angle 2L.sub.X, between about 42 degrees and about 63 degrees; for lateral angle 3L.sub.X, between about 23 degrees and about 47 degrees; for lateral angle 4L.sub.X, between about 36 degrees and about 46 degrees; and for lateral angle 5L.sub.X, between about 28 degrees and about 67 degrees;

(53) The unique asymmetry of periphery 200 defined by tibial plateau 18 can be quantified in multiple ways with respect to the curvatures of lateral and medial compartments 20 and 22 as defined by the arrangement and geometry of lateral arcs 208, 210, 212, 214, 216 and medial arcs 220, 222, 224.

(54) One measure of the asymmetry of periphery 200 is found in a simple comparison of radii R2L and R1R, which are the anterior “corner” radii of lateral and medial compartments 20 and 22 respectively. Generally speaking, a corner of a baseplate periphery may be said to be that portion of the periphery where a transition from an anterior or posterior edge to a lateral or medial edge occurs. For example, in the illustrative embodiment of FIG. 3A, the anterior-lateral corner is principally occupied by anterior-lateral corner arc 210, which defines a substantially medial-lateral tangent at the anterior end of arc 210 and a substantially anteroposterior tangent at the lateral end of arc 210. Similarly, the medial corner of periphery 200 is principally occupied by anterior-medial corner arc 220, which defines a substantially medial-lateral tangent at the anterior end of arc 220 and a more anteroposterior tangent at the lateral end of arc 220. For some purposes, the anterior-medial corner of periphery 200 may be said to include a portion of medial edge arc 222, as described below.

(55) A periphery corner may also be defined by a particular angular sweep with respect to an anteroposterior reference axis. Such reference axis may extend posteriorly from an anterior-most point of a tibial prosthesis (e.g., from the center of anterior edge 202 of periphery 200) to divide the prosthesis into medial and lateral halves. In a symmetrical prosthesis, the anteroposterior reference axis is the axis of symmetry.

(56) In the illustrative embodiment of FIG. 3A, the anteroposterior reference axis may be home axis A.sub.H, such that the anterior-medial corner of periphery 200 occupies some or all of the 90-degree clockwise angular sweep between home axis A.sub.H (at zero degrees, i.e., the beginning of the clockwise sweep) and mediolateral axis A.sub.ML (at 90 degrees, i.e., the end of the sweep). Similarly, the anterior-lateral corner of periphery 200 occupies some or all of the 90-degree counter-clockwise angular sweep between home axis A.sub.H and mediolateral axis A.sub.ML.

(57) For example, the anterior-medial and anterior-lateral corners may each occupy the central 45 degree angular sweep of their respective 90-degree angular sweeps as described above. Thus, the anterior-lateral corner of periphery 200 would begin at a position rotated 22.5 degrees counter-clockwise from home axis A.sub.H as described above, and would end at 67.5 degrees counter-clockwise from home axis A.sub.H. Similarly, the anterior-medial corner would begin at a 22.5-degree clockwise rotation and end at a 67.5 degree clockwise rotation.

(58) It is contemplated that the anterior-lateral and anterior-medial corners may occupy any angular sweep as required or desired for a particular design. For purposes of comparison between two corners in a given prosthesis periphery, however, a comparable angular sweep for the lateral and medial sides is envisioned, i.e., the extent and location of the compared angles may be “mirror images” of one another about an anteroposterior axis. For example, in a comparison of anterior-lateral and anterior-medial radii R2L, R1R, it is contemplated that such comparison is calculated across lateral and medial angular sweeps which each begin and end at similar angular end points with respect to the chosen reference axis (e.g., home axis A.sub.H).

(59) As best seen in FIGS. 3A and 5, one aspect of the asymmetric periphery of baseplate 12 arises from R1R.sub.X being substantially larger than R2L.sub.X. Table 1, below, also includes a comparison of radii R1R.sub.X and R2L.sub.X across nine exemplary component sizes, demonstrating that difference Δ-12RL between radius R1R.sub.X and radius R2L.sub.X may be as little as 48%, 76% or 78%, and may be as much as 102%, 103% or 149%. It is contemplated that radius R1R.sub.X may be larger than radius R2L.sub.X by any percentage value within any range defined by the listed values.

(60) TABLE-US-00001 TABLE 1 Comparisons of Values of Respective Medial and Lateral Anterior Corner Radii Δ-12RL SIZE R1R vs. R2L 1/A 103.0% 2/B 149.2% 3/C 82.4% 4/D 74.6% 5/E 90.9% 6/F 78.6% 7/G 102.2% 8/H 86.5% 9/J 48.1% AVG 90.6% All Δ values are expressed as the difference between a given pair of radii, expressed as a percentage of the smaller of the two radii

(61) Stated another way, the smaller R2L.sub.X makes a sharper turn, thereby imparting a relatively more “boxy” appearance to the anterior corner of lateral compartment 20, while the relatively larger radius R1R.sub.X makes a more gradual turn that imparts a more “rounded” appearance to the anterior corner of medial compartment 22. In the exemplary nine sizes illustrated in FIG. 2A and shown in Table 1, an average disparity between the lateral and medial anterior corner radii R2L.sub.X and R1R.sub.X is greater than 90%. In some sizes of periphery 200.sub.X, the anterior-medial “corner” making the more gradual turn may also includes medial edge arc 222.

(62) As described in detail below, this “rounded-medial/boxy-lateral” asymmetry of the anterior corners of tibial plateau facilitates and encourages proper rotational orientation and positioning of baseplate 12 upon tibia T upon implantation by allowing periphery 200 to closely match the periphery of a typical resected tibia T (FIG. 5), while also maximizing the surface area of proximal surface 34 of tibial plateau to allow for use of a tibial bearing component 14 with a concomitantly large proximal surface area.

(63) As noted above, the small-radius “corner” defined by angle 2L may be considered to have a similar angular sweep as a large-radius “corner” defined by angles 1R, 2R (or a combination of portions thereof) for purposes of comparing the two radii. Given this comparable angular sweep, another measure of the asymmetry defined by the medial and lateral anterior corners is the arc length of the corners. More particularly, because medial radii R1R.sub.X and R2R.sub.X are larger than lateral radius R2L.sub.X (as described above), it follows that the medial corner has a larger arc length as compared to the lateral corner arc length for a given angular sweep.

(64) Moreover, while the peripheries of laterl and medial compartments 20, 22 are shown as being generally rounded and therefore defining respective radii, it is contemplated that an asymmetric periphery in accordance with the present disclosure need not define a radius per se, but rather could include one or more straight line segments which, on the whole, define asymmetric corner edge lengths in the medial and lateral compartments. Referring to FIG. 3B, for example, it is contemplated that an alternative anterior lateral corner 210′ could be comprised of three line segments 210A, 210B, 210C which cooperate to span angular extent 2L. Similarly, an alternative anterior medial corner 220′ could be comprised of three line segments 220A, 220B, 220C which cooperate to span angular extent 1R. Any of the other arcs which define periphery 200 could be similarly configured as one or more line segments. In the variant illustrated by FIGS. 3B and 3C, the difference between corner radii would not be an appropriate measure of asymmetry because the straight line segments would not define radii. Asymmetry of the medial and lateral anterior corners would instead be quantified by comparison of the respective lengths of the medial and lateral corner edges across comparable medial and lateral angular extents.

(65) Yet another way to quantify the asymmetry of the anterior corner arcs (i.e., anterior-lateral corner arc 210 and anterior-medial corner arc 220) is to compare the distance of the lateral and medial radius centers C2L and C1R respectively, from anterior edge 202 and/or mediolateral axis A.sub.ML (FIG. 3A). In the boxy anterior-lateral corner, center C2L.sub.X of radius R2L.sub.X is anterior of mediolateral axis A.sub.ML and relatively close to anterior edge 202. For the rounded, anterior-medial corner, centers C1R.sub.X and C2R.sub.X of radii R1R.sub.X and R2R.sub.X, respectively, are posterior of mediolateral axis A.sub.ML and relatively far from anterior edge 202.

(66) Another metric for quantifying the “boxy vs. rounded” asymmetry of periphery 200 is a comparison between ratios of adjacent radii. In the more boxy lateral compartment 20, pairs of adjacent radii define large ratios because the large edge radii (i.e., of lateral anterior edge arc 208, lateral edge arc 212 and lateral posterior edge arc 216) are much larger than the adjacent corner radii (i.e., of anterior-lateral corner arc 210 and posterior-lateral corner arc 214). On the other hand, in the more rounded medial compartment 22, pairs of adjacent radii define small ratios (i.e., nearly 1:1) because the radii of the medial arcs (i.e., anterior-medial corner arc 220, medial edge arc 222 and posterior-medial corner arc 224) are of similar magnitude.

(67) In the illustrated embodiment of FIG. 3A, lateral edge arc 212 is considered an “edge” because arc 212 defines tangent 212A which is substantially perpendicular to anterior edge 202. Just as a “corner” may be considered to be the portion of periphery 200 which makes a transition from anterior or posterior to medial or lateral, an edge is that portion of periphery 200 which encompasses the anterior, posterior, medial or lateral terminus of periphery 200.

(68) Similarly, medial edge arc 222 defines tangent 222A which is also substantially perpendicular to anterior edge 202. The medial “edge” of periphery 200 may be part of the same arc that extends around the anterior-medial corner and/or the anterior-lateral corner, as the medial arcs are similar. Indeed, as noted herein, medial compartment 22 may have a single arc which extends from anterior edge 202 to medial posterior edge 206.

(69) Table 2 shows a comparison between adjacent-radii ratios for lateral and medial compartments 20 and 22. For each adjacent pair of radii, the difference between the radii magnitudes are expressed as a percentage of the smaller radius of the pair, as noted above.

(70) TABLE-US-00002 TABLE 2 Comparisons of Values of Respective Pairs of Baseplate Peripheral Radii Δ-12R Δ-23R Δ-12L Δ-23L Δ-34L Δ-45L R1R vs. R2R vs. R1L vs. R2L vs. R3L vs. R4L vs. SIZE R2R R3R R2L R3L R4L R5L 1/A 18.3% 58.6% 337.3% 141.8% 323.5% 194.1% 2/B 49.0% 62.0% 254.1% 96.7% 361.5% 315.4% 3/C 24.0% 48.8% 247.1% 58.8% 203.4% 214.6% 4/D 44.2% 34.4% 207.0% 59.2% 213.9% 244.4% 5/E 23.3% 57.9% 151.5% 80.6% 250.0% 250.0% 6/F 46.5% 37.6% 122.6% 42.9% 222.6% 260.2% 7/G 25.3% 38.9% 110.8% 64.5% 264.3% 176.2% 8/H 73.6% 21.3% 109.0% 80.9% 198.1% 142.6% 9/J 21.9% 61.2% 70.4% 68.5% 264.0% 172.0% AVG 36.2% 46.7% 178.9% 77.1% 255.7% 218.8% All Δ values are expressed as the difference between a given pair of radii, expressed as a percentage of the smaller of the two radii

(71) As illustrated in Table 2, the “boxy” periphery of lateral compartment 20 gives rise to disparity values Δ-12L, Δ-23L, Δ-34L and Δ-45L that are at least 42%, 48% or 59%, and as great as 323%, 337% or 362%. It is contemplated that the disparity between a pair of adjacent radii in the boxy periphery of lateral compartment 20 may be any percentage value within any range defined by any of the listed values. It is also contemplated that the lateral disparity values may be substantially higher, as required or desired for a particular application.

(72) Meanwhile, the “rounded” periphery of medial compartment gives rise to disparity values Δ-12R and Δ-23R that are as small as 21%, 23% or 25%, and no greater than 61%, 62% or 74%. It is contemplated that the disparity between a pair of adjacent radii in the rounded periphery of medial compartment 22 may be any value within any range defined by any of the listed values. It is also contemplated that the medial disparity values may be less than 21%, and as little as zero %, as required or desired for a particular application.

(73) Moreover, the boxy shape of lateral compartment 20 and rounded shape of medial compartment 22 is also demonstrated by the number of arcs used to define the portion of periphery 200 in lateral and medial compartments 20, 22. In lateral compartment 20, five arcs (i.e., arcs 208, 210, 212, 204, 216) are used to define the lateral periphery, which is indicative of anterior, lateral and posterior “sides” of a box joined by the relatively sharp transitions of corner arcs 210, 214. On the other hand, medial compartment 22 uses only three radii (i.e., 220, 222, 224), leaving no clear definition of any box “sides” or other transitions. Indeed, it is contemplated that medial compartment 22 could join anterior edge 202 to medial posterior edge 206 by a single radius within the scope of the present disclosure.

(74) b. Surface Area of Medial and Lateral Baseplate Compartments

(75) Referring still to FIG. 3A, yet another characterization of the asymmetry of periphery 200 arises from disparities in surface area for lateral and medial compartments 20, 22. For purposes of the present disclosure, surface area of lateral compartment SAL is that area contained within periphery 200, and on the lateral side of home axis A.sub.H. Similarly, the surface area of medial compartment 22 is that area contained within periphery 200, and on the m side of home axis A.sub.H.

(76) In an exemplary embodiment, lateral surface area SAL.sub.X may be as little as 844 mm.sup.2 or may be as much as 1892 mm.sup.2, or may be any area within the range defined by the foregoing values. In an exemplary embodiment, medial surface area SAM.sub.X may be as little as 899 mm.sup.2 or may be as much as 2140 mm.sup.2, or may be any area within the range defined by the foregoing values.

(77) Surfaces areas SAL and SAM do not include any of the area occupied by PCL cutout 28, as any such area is not within periphery 200. However, the asymmetry of surface areas SAL and SAM arises primarily from the differences in the geometry and placement of arcs 208, 210, 212, 214, 216, 220, 222, 224 rather than from any asymmetry of PCL cutout 28. In the illustrative embodiments of FIG. 2A, for example, PCL cutout 28.sub.X is symmetrical with respect to home axis A.sub.H, but extends further posteriorly in medial compartment 22.

(78) Thus, it is contemplated that the asymmetry of surfaces areas SAL, SAM are little changed by exclusion of the PCL cutout 28 from the area calculation. As illustrated in FIG. 3D, PCL cutout 28 is effectively excluded from calculation by extrapolating the line formed by lateral posterior edge 204 and medial posterior edge 206 inwardly to intersect with home axis A.sub.H. In lateral compartment 20, such extrapolation cooperates with the lateral side of PCL cutout 28 to define lateral fill area 80. In medial compartment 22, such extrapolation cooperates with the medial side of PCL cutout 28 to define medial fill area 82.

(79) In the illustrative embodiment of FIG. 3D, lateral surface area SAL.sub.X′ may be as little as 892 mm.sup.2 or may be as much as 2066 mm.sup.2, or may be any area within the range defined by the foregoing values. In an exemplary embodiment, medial surface area SAM.sub.X′ may be as little as 986 mm.sup.2 or may be as much as 2404 mm.sup.2, or may be any area within the range defined by the foregoing values.

(80) Tables 3 and 4 below illustrate that medial surface area SAM.sub.X occupies a greater percentage of the total surface area contained within periphery 200.sub.X, regardless of whether PCL cutout 28 is included in the calculation. That is to say, medial fill area 82 is larger than lateral fill area 80 by approximately the same proportion as medial and lateral surfaces areas SAM.sub.X, SAL.sub.X. In the exemplary embodiments of FIG. 3A, medial surface area SAM.sub.X occupies between 52% and 53% of the total surface area regardless, while lateral surface area SAM.sub.X occupies the remainder. if the PCL cutout is excluded from the calculation as shown in FIG. 3D, medial surface area SAM.sub.X′ occupies between 52% and 54% of the total surface area, while lateral surface area SAM.sub.X′ occupies the remainder. With or without the PCL cutout included in the calculation, it is contemplated that medial surface areas SAM.sub.X, SAM.sub.X′ may occupy as little as 51% of the total surface area, and as much as 60% of the total surface area.

(81) TABLE-US-00003 TABLE 3 Medial vs. Lateral Tibial Baseplate Surface Areas for Baseplates with a PCL Cutout (FIGS. 2A and 3A) With PCL Notch Medial Surface Area SAM.sub.x Site as % of Total Surface Area 1/A 52% 2/B 52% 3/C 52% 4/D 52% 5/E 52% 6/F 52% 7/G 53% 8/H 53% 9/J 53%

(82) TABLE-US-00004 TABLE 4 Medial vs. Lateral Tibial Baseplate Surface Areas for Baseplates without a PCL Cutout (FIG. 3D) Without PCL Notch Medial Surface Area SAM.sub.x′ Size as % of Total Surface Area 1/A 53% 2/B 52% 3/C 53% 4/D 53% 5/E 53% 6/F 53% 7/G 53% 8/H 54% 9/J 54%

(83) c. Anteroposterior Extent of Medial and Lateral Compartments

(84) Still another way to characterize and quantify the asymmetry of tibial periphery 200 is to compare the overall anteroposterior extent of lateral and medial compartments 20, 22.

(85) Turning to FIG. 2A (which is drawn to scale, according to scales 230 and 232) and FIG. 2B, lateral compartment 20 of tibial plateau 18 defines overall lateral anteroposterior extent DAPL.sub.X, while medial compartment 22 of tibial plateau 18 defines overall medial anteroposterior extent DAPM.sub.X, where X is an integer between 1 and 9 corresponding to a particular component size as shown in FIG. 2A, as noted above. As illustrated in Table 5 below, lateral anteroposterior extent DAPL.sub.X is less than medial anteroposterior extent DAPL.sub.X, for all component sizes.

(86) This disparity in anteroposterior extent can be said to result from medial compartment 22 extending posteriorly further than lateral compartment 20. In the illustrative embodiment of FIG. 2B, lateral anteroposterior extent DAPL.sub.X extends from anterior edge 202 to lateral posterior edge 204, while medial anteroposterior extent DAPM.sub.X extends from anterior edge 202 to medial posterior edge 206. Thus, if one takes anterior edge 202 to be the anteroposterior “zero point,” the additional anteroposterior extent defined by medial compartment 22 is due entirely to the further posterior position of medial posterior edge 206.

(87) As set forth in the right-hand column of Table 5, exemplary embodiments of tibial baseplate 12 may define medial anteroposterior extent DAPM.sub.X that is larger than lateral anteroposterior extent DAPL.sub.X by as little as 12.1%, 12.2% or 12.4%, and as much as 13.7%, 14.2% or 14.5%. It is contemplated that such disparity between medial and lateral anteroposterior extents DAPM.sub.X, DAPL.sub.X may be any percentage within any range defined by the listed values of Table 5. Advantageously, the particular asymmetric arrangement of tibial baseplate 12 with respect to anteroposterior extent of lateral and medial compartments 20, 22 facilitates substantially complete coverage of tibia T, without overhanging the edge of tibia T, in a wide variety of patients.

(88) TABLE-US-00005 TABLE 5 Overall A/P and M/L Dimensions for Tibial Baseplates (FIGS. 2A and 2B) Growth in A/P Medial Growth in A/P Lateral Additional A/P Dimension (DAPM), Dimension (DAPL), Extent of DAPM Size from next-smaller from next-smaller vs. DAPL, % of (X) size, mm size, mm DAPL 1/A — — 14.5% 2/B 2.3 2.13 14.2% 3/C 2.4 2.25 13.7% 4/D 2.3 2.27 13.1% 5/E 3 2.8 12.7% 6/F 3.1 2.85 12.4% 7/G 3.2 2.81 12.5% 8/H 3.3 3.11 12.2% 9/J 3.73 3.34 12.1%

(89) For example, in an exemplary family of prosthesis sizes, at least 60% and as much as 90% coverage of the resected proximal surface is provided by tibial plateau 18 of tibial baseplate 12 when rotation is limited to ±5 degrees from home axis A.sub.H. In a majority of all patients, such coverage is between 75-85%. Coverage of up to 100% may be achieved within the scope of the present disclosure, such as by fully extending the posterior-medial and anterior-lateral coverage of tibial plateau (which intentionally leave gaps between tibial plateau 18 and the periphery of tibia T as noted herein).

(90) The additional posteromedial material of tibial plateau 18 includes chamfer 32, described in detail below with respect to the assembly of tibial baseplate 12 to tibial bearing component 14. Chamfer 32 is formed in peripheral wall 25, such that chamfer 32 forms angle α (FIG. 8) with the distal or bone-contacting surface 35 of tibial plateau 18. In the illustrated embodiment, chamfer 32 defines a substantially linear sagittal cross-sectional profile, with angle α between about 35 degrees and about 55 degrees. In addition, it is contemplated that chamfer 32 may have an arcuate profile in a sagittal, coronal and/or transverse plane, and may include convex or concave curvature as required or desired for a particular application.

(91) 2. Progressive Peripheral Growth Between Implant Sizes

(92) In addition to the asymmetry of each individual size/embodiment of tibial baseplate 12, described in detail above, the present disclosure also provides asymmetry in the way periphery 200 grows from one size to the next. Advantageously, this asymmetric peripheral growth accommodates observed growth trends in tibias T of differently-sized patients, while also preserving the optimal fit and coverage provided by baseplate 12, and offering the other advantages of designs in accordance with the present disclosure as described herein.

(93) In symmetrical peripheral growth, a larger size of baseplate is a scaled-up version of a smaller size and vice-versa. In the present asymmetrical peripheral growth, by contrast, certain parameters of tibial baseplate 12 grow faster than others as the overall size of the baseplate gets larger (i.e., from smallest size 1/A through largest size 9/J). Thus, differently-sized components made in accordance with the present disclosure are not proportional to one another in all respects, in that a larger tibial prosthesis is not proportionally larger than a smaller tibial prosthesis in all aspects.

(94) Referring now to FIG. 29, periphery 200.sub.X defines centroid C.sub.X, which is medially biased with respect to home axis A.sub.H owing to medial surface area SAM being larger than lateral surface area SAL (as described in detail above). Posterior-medial distance DMP.sub.X extends from centroid C.sub.X toward the posterior-medial “corner” of periphery 200.sub.X (i.e., toward posterior-medial corner arc 224, shown in FIG. 3A and described above) at an angle of 130 counter-clockwise degrees from home axis A.sub.H. Similarly, posterior-lateral distance DLP.sub.X extends from centroid C.sub.X toward the posterior-lateral “corner” of periphery 200.sub.X (i.e., toward posterior-lateral corner arc 214, shown in FIG. 3A and described above) at an angle of 120 clockwise degrees from home axis A.sub.H. The posterior-lateral and posterior-medial corners are defined in a similar fashion as the anterior-lateral and anterior-medial corners, described in detail above. Moreover, while the asymmetric posterior-medial and posterior lateral growth among consecutive sizes is described below with respect to distances DPL.sub.X, DMP.sub.X, such growth occurs in the entire area occupied by the posterior-medial and posterior-lateral corners.

(95) As illustrated in FIG. 2A and shown in Table 6 below, lateral- and medial-posterior distances DLP.sub.X, DMP.sub.X do not grow linearly as smallest size 1/A progresses among consecutive sizes to eventually reach largest size 9/J. Rather, lateral- and medial-posterior distances DLP.sub.X, DMP.sub.X exhibit an increase in the magnitude of growth as the sizes progress consecutively from size 1/A to size 9/J. This non-linear, asymmetric growth is illustrated in the graphs of FIGS. 2C and 2D and in Table 6 below.

(96) TABLE-US-00006 TABLE 6 Growth of the Posterior-Medial and Posterior-Lateral Corners of Baseplate Periphery (FIGS. 2A and 2B) Growth in medial-posterior Growth in lateral-posterior distance DMP.sub.x from centroid distance (DLP.sub.x) from centroid Size (C.sub.x), compared to next-smaller (C.sub.x), compared to next-smaller (X) size, mm size, mm 1 — — 2 2.42 2.48 3 2.56 2.8 4 2.76 2.55 5 2.86 3.26 6 3.71 2.64 7 3.28 2.83 8 3.52 2.28 9 3.76 3.29

(97) In FIG. 2C, the amount of growth in DMP.sub.X is plotted against size no. X. As illustrated, the family of tibial baseplates 12 illustrated in FIG. 2A exhibit a steadily increasing growth in DMP.sub.X, with nearly 20% average increase in growth from one size to the next consecutive size (as represented by the slope of the linear trend line having equation y=0.1975x+2.0225).

(98) In FIG. 2D, the amount of growth in DLP.sub.X is plotted against size no. X, and illustrates a smaller, but still positive growth increase across baseplate sizes. More specifically, the family of tibial baseplates 12 illustrated in FIG. 2A exhibit a nearly 4% average increase in growth from one size to the next consecutive size (as represented by the slope of the linear trend line having equation y=0.0392x±2.5508).

(99) As used herein a “family” of prostheses refers to a set or kit of prostheses sharing common geometrical and/or performance characteristics. For example, the family of nine tibial baseplates whose peripheries 200.sub.X are shown in FIG. 2A share a common asymmetry as described herein, such that each tibial baseplate is adapted to provide substantial tibial coverage, facilitate proper implant rotation and avoid impingement with various soft tissues of the knee. Typically, a family of prostheses includes a plurality of differently-sized components, with consecutively larger/smaller components sized to accommodate a variety of differently-sized bones. In the exemplary embodiments of the present disclosure, a size “1” or “A” prosthesis is the smallest prosthesis of the family, a size “9” or “J” prosthesis is the largest prosthesis of the family, and each of the intermediate sizes “2” or “B” through “8” or “H” are consecutively larger sizes.

(100) Advantageously, in the family or kit of prosthesis peripheries shown in FIG. 2A, each tibial baseplate 12 (FIG. 1A) having periphery 200.sub.X provides a close match to a particular subset of patient tibias T having a unique size and shape. Particular features of periphery 200.sub.X have been designed with non-linear growth which is calculated to provide the closest possible fit for the largest number of particular natural geometries found in anatomic tibias T, as described in detail herein. This close fit allows for maximum coverage of the resected proximal tibial periphery 200.sub.X, by accommodating the non-linear changes which may occur across anatomic tibial periphery sizes. Lateral- and medial-posterior distances DLP.sub.X, DMP.sub.X are exemplary non-linear growth parameters found in a family of tibial baseplates 12, and are reflective of non-linear growth in mediolateral extent DML.sub.X and anteroposterior extents DAPM.sub.X and DAPL.sub.X across the various sizes.

(101) 3. Tibial Baseplates for Small-Stature Patients

(102) As noted above, tibial baseplate 12 may be provided in a variety of sizes each defining a unique periphery 200.sub.X. Periphery 200.sub.X is described for an exemplary family of baseplate sizes in U.S. Patent Application Publication No. 2012/0022659, filed Jul. 22, 2011 and entitled ASYMMETRIC TIBIAL COMPONENTS FOR A KNEE PROSTHESIS, U.S. Patent Application Publication No. 2012/0022660, filed Jul. 22, 2011 and entitled ASYMMETRIC TIBIAL COMPONENTS FOR A KNEE PROSTHESIS and U.S. Patent Application Publication No. 2012/0022658, filed Jul. 22, 2011 and entitled ASYMMETRIC TIBIAL COMPONENTS FOR A KNEE PROSTHESIS, each of which claims the benefit under Title 35, U.S.C. §119(e) of U.S. Provisional Patent Application Ser. No. 61/381,800, filed on Sep. 10, 2010 and entitled TIBIAL PROSTHESIS FACILITATING ROTATIONAL ALIGNMENT, U.S. Provisional Patent Application Ser. No. 61/367,375, filed Jul. 24, 2010 and entitled TIBIAL PROSTHESIS. The entire disclosures of the aforementioned applications are hereby expressly incorporated by reference herein.

(103) As described in detail below, the smallest two sizes of tibial baseplate 12 include other unique features to accommodate the special needs of smaller stature patients. More particularly, these small sizes of tibial baseplate 12 are not scaled down versions of the larger sizes, but instead include unique geometries suited to the smaller bones for which they are designed. Further, because the small stature tibial baseplates 12 have less material overall, special geometries are employed to selectively strengthen tibial baseplate 12 in areas where such strengthening would not be required for larger baseplate sizes.

(104) In an exemplary embodiment, tibial baseplate 12 is considered “small stature” for nominal sizes 1 and 2. For example, nominal size 1 of tibial baseplate 12 may define a medial/lateral extent DML.sub.1 of about 57 mm, a maximum anterior/posterior extent DAPM.sub.1 of about 40 mm, and a surface area of about 1390 mm.sup.3 within periphery 200.sub.1. Nominal size 2 of tibial baseplate 12 may define a medial/lateral extent DML.sub.2 of about 61 mm, a maximum anterior/posterior extent DAPM.sub.2 of about 43 mm, and a surface area of about 1580 mm.sup.3 within periphery 200.sub.2.

(105) One special feature of the small-stature sizes of tibial baseplate 12 is the shape of the outer surface of keel 16A extending distally from proximal tibial plateau 18. In larger size tibial baseplate 12, such as baseplate 12 shown in FIG. 4B, keel 16 defines a substantially cylindrical outer profile. By contrast, FIG. 10 illustrates that keel 16A of the small-stature size of tibial baseplate 12 has a generally conical, tapered outer profile defining taper angle θ. In an exemplary embodiment, angle θ may be about 9°. This 9° taper may be formed, for example, by tapering keel 16A from a circular outer diameter of about 17.1 mm at the proximal terminus of keel 16A (i.e., at the junction between keel 16A and distal surface 35 of tibial plateau 18) to a circular diameter of approximately 13.4 mm at the distal terminus of keel 16A. Keel 16, on the other hand, maintains a diameter between about 14 mm and about 16 mm that remains constant across the longitudinal extent. Moreover, 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 14 and 28, respectively, of the “Zimmer® Tibial Baseplate, Pocket Guide United States Version,” the entire disclosure of which is hereby expressly incorporated herein by reference, a copy of which is submitted on even date herewith in an information Disclosure Statement.

(106) In an exemplary embodiment, keels 16, 16A are monolithically or integrally formed with tibial plateau 18, though it is contemplated that keels 16, 16A may be separately attachable to tibial plateau 18. Further, in an exemplary embodiment keels 16, 16A themselves are monolithically formed as a single piece, rather than being assembled from multiple partial pieces to form a complete keel.

(107) Referring to FIGS. 9 and 10, another unique feature of small-stature sizes of tibial baseplate 12 is the geometry and arrangement of keel fins 17A as compared to keel fins 17 (FIG. 4B) of larger-stature sizes of baseplate 12. More particularly, fins 17A extend along less than the entire longitudinal extent of keel 16A, as best shown in FIG. 10, such that fins 17A terminate into the conical outer surface of keel 16A at a distance D.sub.F above the distal tip of keel 16A. In an exemplary embodiment, distance D.sub.F is about 7 mm, or about 26% of overall longitudinal extent PD.sub.KA of keel 16A, such that fins 17A extend along the remaining 74% of longitudinal extent PD.sub.KA.

(108) Keel fins 17A of small-stature sizes of tibial baseplate 12 also define keel fin angle γ.sub.A (FIG. 10) with respect to the longitudinal axis of keel 16A, which is larger than keel fin angle γ (FIG. 4B) defined by fins 17 of larger size tibial baseplate 12. In an exemplary embodiment, keel fin angle γ.sub.A is equal to about 45°, as compared to keel fin angle of about 22-27° defined by larger sizes of baseplate 12 and by prior art devices including the Zimmer NexGen MIS Stemmed baseplates shown at pages 4-5 of the “Zimmer® Tibial Baseplate, Pocket Guide United States Version,” the entire disclosure of which is hereby expressly incorporated herein by reference, a copy of which is submitted on even date herewith in an Information Disclosure Statement. The increased magnitude of keel fin angle γ.sub.A concomitantly increases the overall medial/lateral extent in ML.sub.KA of keel fins 17A at the junction with tibial plateau 18 at distal surface 35 for a given proximal/distal extent of keel fins 17A. As illustrated in FIGS. 9 and 10, medial/lateral extent in ML.sub.KA is the maximum medial/lateral distance defined by the medial and lateral fins 17A at the junction thereof with tibial plateau 18. In the illustrated embodiment, medial and lateral fins 17A are the only fins provided as part of small-stature tibial baseplate 12.

(109) Provided that fins 17A extend along a substantial portion of the longitudinal extent PD.sub.KA of keel 16A (e.g., across 74% of longitudinal extent PD.sub.KA, as noted above), medial/lateral keel extent ML.sub.KA may be equal to about 40 mm, which is commensurate with the corresponding medial/lateral keel extent ML.sub.K (FIG. 4B) of larger sizes of tibial baseplate 12. Advantageously, the increased medial/lateral extent ML.sub.KA defined by fins 17A of keel 16A present high resistance to rotation of tibial baseplate 12 in vivo, and enhance the overall strength of baseplate 12.

(110) Yet another unique feature of keel 16A in small stature sizes of tibial baseplate 12 is its overall longitudinal extent PD.sub.KA, which extends in a generally proximal/distal direction as shown in FIG. 10. Longitudinal extent PD.sub.KA of the small-stature sizes of tibial baseplate 12 is substantially reduced with respect to longitudinal extent PD.sub.K (FIG. 4B) of keel 16 in the larger sizes of tibial baseplate 12, and with respect to small baseplate sizes in other, alternative tibial baseplate designs. In an exemplary embodiment, longitudinal extend PD.sub.KA of small stature tibial keel 16A may be about 27 mm, while longitudinal extent PD.sub.K of larger tibial keel 16 may range from about 39 mm to about 48 mm.

(111) Advantageously, the above-described special geometries and features of small stature tibial keel 16A prevent impingement of the conical outer surface of the body of keel 16A and/or fins 17A upon cortical bone when implanted upon the tibia of a small stature patient for which the small stature sizes of tibial baseplate 12 are intended. More particularly, Applicant has found that cortical bone impingement, is most likely to occur (if at all) at or near the distal tip of a tibial keel in small stature patients. To minimize the probability of such impingement small stature tibial keel 16A of tibial baseplate 12 includes the above-described unique features while also retaining a large fixation area for attachment to the surrounding tissues, and maintaining a high minimum material thickness to ensure appropriate strength throughout the material of tibial baseplate 12. For example, the high value of keel fin angle γ.sub.A (described in detail above) increases the surface area for fixation of tibial baseplate 12 to the surrounding bone, while the tapered outer surface of keel 16A ensures that a nominal minimum wall thickness of 1.5 mm is maintained throughout the material of tibial baseplate 12 while presenting a relatively small radius at the distal tip of keel 16A.

(112) The probability of cortical bone impingement by keel 16A is also minimized by medially biasing the position of keel 16A with respect to the tibial baseplate periphery (i.e., peripheries 200.sub.1 and 200.sub.2). More particularly, small-stature sizes of tibial baseplate 12 have keel 16A offset approximately 1 mm from a centered position on distal surface 35 of tibial plateau 18, thereby enhancing the probability of proper alignment with the anatomic intramedullary canal and concomitantly minimizing the probably of cortical bone impingement. Medialization of keel 16A (and of keel 16 for larger sizes of baseplate 12) is described in detail in U.S. Provisional Patent Application Ser. No. 61/562,133, filed Nov. 21, 2011 and entitled TIBIAL BASEPLATE WITH ASYMMETRIC PLACEMENT OF FIXATION STRUCTURES, and in U.S. Provisional Patent Application Ser. No. 61/592,571, entitled TIBIAL BASEPLATE WITH ASYMMETRIC PLACEMENT OF FIXATION STRUCTURES and filed Jan. 30, 2012, and in U.S. Provisional Patent Application Ser. No. 61/594,030, entitled TIBIAL BASEPLATE WITH ASYMMETRIC PLACEMENT OF FIXATION STRUCTURES and filed Feb. 2, 2012, and in U.S. Patent Application Ser. No. 61/621,369, entitled TIBIAL BASEPLATE WITH ASYMMETRIC PLACEMENT OF FIXATION STRUCTURES and filed on even date herewith, the entire disclosures of which are hereby expressly incorporated herein by reference.

(113) Small stature tibial keel 16A also includes some features common to tibial keel 16 of larger sizes of tibial baseplate 12. For example, small stature tibial keel 16A includes a tapered bore 19 (FIG. 9) extending proximally into keel 16A from the distal tip thereof, which is designed to mate with a corresponding locking-taper surface 21 of tibial stein extension 23. The locking taper formed between the inner surface of bore 19 and surface 21 may define an angle of approximately 5° with respect to the shared longitudinal axis of keel 16A and stem extension 23 upon assembly. Further, a secondary locking mechanism may be provided in the form of set screw aperture 27A (FIG. 11) formed in a posterior portion of the outer wall of keel 16A. Set screw aperture 27A is positioned to align with annular groove 27B formed in stem extension 23 when tapered surface 21 is fully, lockingly seated with the correspondingly tapered inner surface bore 19. A set screw may then be threaded into aperture 27A to engage annular groove 27B, thereby offering secondary prevention of relative axial movement between stem extension 23 and tibial baseplate 12.

(114) 4. PCL Cutout Aligned with Home Axis and Associated Technique

(115) In the illustrated embodiment, tibial plateau 18 includes PCL cutout 28 disposed between compartments 20, 22, as described above. PCL cutout leaves PCL attachment point C.sub.P accessible, thereby allowing the PCL to pass therethrough during and after implantation of tibial baseplate 12. Tibial bearing component 14 (FIG. 5) may similarly include cutout 30.

(116) Thus, the illustrated embodiment of tibial prosthesis 10 is adapted for a cruciate retaining (CR) surgical procedure, in which the posterior cruciate ligament is not resected during implantation of tibial prosthesis 10. Further, as noted above, home axis A.sub.H includes reference to PCL attachment point C.sub.P when tibial baseplate 12 is mounted upon tibia T. In order to facilitate alignment of home axis A.sub.H with respect to tibial baseplate 12 and tibia T, alignment indicia 70A, 70P (FIGS. 4A and 4B) may be marked on proximal surface 34 and/or peripheral wall 25. When tibial baseplate 12 is implanted (as described below), anterior alignment indicia 70A (FIGS. 4A and 4B) is aligned with anterior point C.sub.A at the “medial third” of the anterior tibial tubercle T, and posterior alignment indicia 70P is aligned with the natural PCL attachment point C.sub.P of tibia T.

(117) However, it is contemplated that a prosthesis in accordance with the present disclosure may be made for a design in which the posterior cruciate ligament is resected during surgery, such as “posterior stabilized” (PS) or “ultra congruent” (UC) designs. The PS and UC designs may exclude PCL cutout 30 in bearing component 14, thereby obviating the need for PCL cutout 28 in tibial baseplate 12. Continuous material may instead occupy cutout 28 (as schematically shown in FIG. 3D). Moreover, it is contemplated that PCL cutouts 28, 30 may have any shape and/or size within the scope of the present disclosure. For example, PCL cutouts 28, 30 may be asymmetrical with respect to an anteroposterior axis. For purposes of the present disclosure “bisecting” an asymmetric PCL cutout with an anteroposterior axis refers to dividing such cutout into two equal areas for a given anteroposterior section of the anteroposterior axis

(118) 5. Tibial Bearing Component and Deep Flexion Enablement

(119) Turning again to FIG. 1A, tibial bearing component 14 includes lateral portion 39, medial portion 41, inferior surface 36 adapted to couple to tibial baseplate 12, and superior surface 38 adapted to articulate with condyles of a femoral component (such as femoral component 60 shown in FIG. 8 and described in detail below). Superior surface 38 includes lateral articular surface 40 in lateral portion 39 and medial articular surface 42 in medial portion 41, with eminence 44 (FIG. 5) disposed between articular surfaces 40. 42. Referring to FIG. 5, eminence 44 generally corresponds in shape and size with a natural tibial eminence of tibia T prior to resection.

(120) Referring now to FIG. 1A, tibial plateau 18 of tibial baseplate 12 further includes a distal or bone contacting surface 35 and an opposing proximal or superior surface 34, with superior surface 34 having raised perimeter 24 and locking mechanism 26 formed between lateral and medial compartments 20, 22. Raised perimeter 24 and locking mechanism 26 cooperate to retain tibial bearing component 14 upon tibial baseplate 12, as described in detail below.

(121) Inferior surface 36 of tibial bearing component 14 includes recess 46 at the periphery thereof and a tibial bearing locking mechanism (not shown) disposed between lateral and medial articular surfaces 40, 42. Recess 46 is sized and positioned to correspond with raised perimeter 24 of tibial plateau 18, and the tibial bearing locking mechanism cooperates with locking mechanism 26 of tibial plateau 18 to fix tibial bearing component 14 to tibial baseplate 12 in a desired position and orientation as described in detail below. However, contemplated that tibial bearing component 14 may be affixed to baseplate 12 by any suitable mechanism or method within the scope of the present disclosure, such as by adhesive, dovetail tongue/groove arrangements, snap-action mechanisms, and the like.

(122) Exemplary baseplate and tibial bearing locking mechanisms are described in U.S. Patent Application Publication No. 2012/0035737, filed Jul. 22, 2011 and entitled TIBIAL PROSTHESIS, and in U.S. Patent Application Publication No. 2012/0035735, filed Jul. 22, 2011 and entitled TIBIAL PROSTHESIS, the entire disclosures of which are hereby expressly incorporated by reference herein.

(123) As best seen in FIGS. 1B, 5 and 8, the outer periphery of tibial bearing component 14 generally corresponds with the outer periphery of tibial plateau 18, except for the posteromedial extent of plateau 18 as compared with tibial bearing component 14. The anterolateral “corner” of tibial bearing component 14 defines radius R.sub.3 (FIG. 5) having a generally common center with radius R2L of baseplate 12 in a transverse plane, i.e., radii R2L and R.sub.3 are substantially coincident in a plan view. Similarly, the anteromedial “corner” of tibial bearing component 14 defines radius R.sub.4 having a generally common center with radius R1R of baseplate 12 in a transverse plane, i.e., radii R1R and R.sub.4 are substantially coincident in a plan view.

(124) R.sub.3 defines a slightly smaller radial length as compared to R2L, and R.sub.4 defines a slightly smaller radial length as compared to R1R, such that the anterior portion of perimeter wall 54 of tibial bearing component 14 is set back from the anterior portion of peripheral wall 25 (i.e., from anterior edge 202 and adjacent arcs, as described above) of tibial baseplate 12. As with the above-described comparison between radii R2L and R1R, anteromedial radius R.sub.4 is substantially larger than anterolateral radius R.sub.3.

(125) Given that medial portion 41 of tibial bearing component 14 has a lesser anteroposterior extent compared to medial compartment 22 of tibial plateau 18, medial portion 41 must be biased anteriorly in order for the anterior-medial “corners” of tibial bearing component 14 and tibial plateau 18 to coincide as shown in FIG. 5. In view of this anterior bias, it may be said that tibial bearing component 14 is asymmetrically oriented upon tibial plateau 18. More particularly, although lateral articular surface 40 is generally centered with respect to lateral compartment 20 of tibial plateau 18, medial articular surface 42 is anteriorly biased with respect to medial compartment 22 of tibial plateau 18 in order to leave chamfer 32 exposed at the posterior-lateral corner. This asymmetric mounting of tibial bearing component 14 upon tibial plateau 18 ensures a desired articular interaction between tibial prosthesis 10 and femoral component 60, as described in detail below.

(126) Tibial plateau 18 of tibial baseplate 12 deviates from the periphery of tibial bearing component 14 in the posteromedial portion of each component, leaving medial portion 41 incongruent with medial compartment 22 of tibial baseplate 12. More particularly, tibial plateau 18 extends posteromedially to substantially cover the proximal resected surface of tibia T, as shown in FIG. 5 and described in above, while tibial bearing component 14 does not extend posteromedially beyond the superior terminus of chamfer 32 (i.e., tibial bearing component 14 does not “overhang” chamfer 32). In addition, tibial bearing component 14 includes chamfer 50 formed in peripheral wall 54, with chamfer 50 having a profile and geometrical arrangement corresponding with chamfer 32 of tibial plateau 18. More particularly, when tibial bearing component 14 is assembled to tibial baseplate 12 as shown in FIGS. 1B and 8, the anterior orientation or “bias” of the medial portion of tibial bearing component 14 (as described above) aligns chamfers 32, 50, which in turn cooperate to create a substantially continuous chamfer extending from tibia T to medial articular surface 42. Referring to FIG. 8, chamfers 32, 50 further cooperate to define void 52 formed between femur F and tibial plateau 18 when tibial prosthesis 10 is in a deep flexion orientation. In the illustrated embodiment of FIG. 8, the deep flexion orientation is defined by angle β between anatomic tibia axis A.sub.T and anatomic femoral axis A.sub.F of up to about 25 degrees to about 40 degrees, for example (i.e., about 140 degrees to 155 degrees of flexion or more).

(127) Advantageously, void 52 cooperates with the “pulled back” or incongruent posterior medial edge 206 and posterior medial corner 224, as compared to atypical tibial periphery (described above), to allow the deep flexion orientation to be achieved without impingement of femoral component 60 and/or femur F upon tibial plateau 18 and/or tibial bearing component 14. Soft tissues in the region of void 52 are therefore also accommodated with little or no impingement on the surrounding components.

(128) In addition, the relatively large size of tibial plateau 18 (covering a large proportion of the resected proximal surface of tibia T) also allows tibial bearing component 14 to be relatively large, so that tibial bearing component 14 provides sufficient non-articular surface area at chamfers 32, 50 and around the periphery of lateral and medial articular surfaces 40, 42 to allow relatively large-radius, rounded transitions between articular surfaces 40, 42 and peripheral wall 54 of tibial bearing component 14. These gradual, large-radius transitions prevent undue friction between tibial prosthesis 10 and any surrounding soft tissues which may remain in place after implantation of the prosthesis, such as the iliotibial (IT) band.

(129) In certain ranges of prosthesis articulation, for example, the human iliotibial (IT) band may touch the anterolateral “corner”, i.e., the portion of tibial bearing component 14 having radius R.sub.3. Because the anterolateral extent of tibial bearing component 14 follows the anterolateral extent of tibial plateau 18 (as described above), the transition between lateral articular surface 40 and peripheral wall 54 at the point of contact between an IT band and tibial bearing component 14 can have a relatively large convex portion while still leaving sufficient concave space for articular surface 40. This large convex portion results in a large contact area if the IT band does contact tibial bearing component 14, which in turn results in relatively low pressures on the IT band. Further, the anterolateral “pull back” or incongruence between the anterior-lateral corner arc 210 of periphery 200 and a typical tibial periphery, described in detail above, allows the corresponding anterior-lateral corner of bearing component 14 to maintain separation from the IT band through a wide range of flexion, and low contact pressures where contact does occur.

(130) However, to any such contact between the IT band and tibial bearing component 14 may be avoided or minimized by designing periphery 200 such that anterior-lateral corner arc 210 and/or lateral edge arc 212 is brought away from the expected periphery of a typical tibia T (as calculated from anatomical data, described above). This extra spacing designed into periphery 200 provides extra clearance for the iliotibial band. In addition, this extra clearance assures that the substantial proportion of prospective patients lacking Gerdy's tubercle, which is an eminence located at the anterior-lateral portion of tibia T, will not experience any “overhang” of tibial plateau 18 beyond the anatomic periphery of resected tibia T.

(131) Thus, generally speaking, tibial prosthesis 10 can be considered “soft tissue friendly” because the edges of tibial bearing component 14 and tibial plateau 18, including chamfers 32, 50, are smooth and rounded, so that any soft tissue coming into contact with these edges will be less likely to chafe or abrade.

(132) Advantageously, the relatively large inferior/distal surface area of tibial plateau 18 facilitates a large amount of bone ingrowth where bone ingrowth material is provided in tibial baseplate 12. For example, baseplate 12 may also be constructed of, or may be coated with, 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 55%, 65%, or 75% or as high as 80%, 85%, or 90%. An example of such a material is produced using Trabecular Metal™ Technology generally available from Zimmer, Inc., of Warsaw, Ind. 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 manner disclosed in detail in U.S. Pat. No. 5,282,861 to Kaplan, the entire disclosure of which is hereby expressly incorporated herein by reference. 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.

(133) 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 75%, 85%, 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 implant [#] to the patient's bone.

(134) 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 the above-incorporated U.S. Pat. No. 5,282,861, 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.

(135) 6. Trial Tibial Components

(136) Tibial prosthesis 10 may be provided in a variety of sizes and configurations to accommodate different bone sizes and geometries. The choice of one particular size may be planned preoperatively such as through preoperative imaging and other planning procedures. Alternatively, an implant size may be chosen, or a previous size choice modified, intraoperatively. To facilitate proper intraoperative selection of a particular size for tibial prosthesis 10 from among the family of sizes shown in FIG. 2A, and to promote proper orientation of the chosen prosthesis 10, tibial prosthesis 10 may be part of a kit including one or more template or “sizing” components.

(137) Referring now to FIGS. 6 and 7, trial prosthesis 100 may be temporarily coupled to tibia T for intraoperative sizing evaluation of tibial prosthesis 10 and initial steps in the implantation of tibial prosthesis 10. Trial prosthesis 100 is one of a set of trial prostheses provided as a kit, with each trial prosthesis having a different size and geometrical configuration. Each trial prosthesis in the set of trial prostheses corresponds to a permanent prosthesis 10, such as sizes 1/A-9/J of tibial baseplate 12 as described above.

(138) For example, as shown in FIG. 6, trial prosthesis 100 defines superior surface 112 generally corresponding in size and shape to proximal surface 34 of tibial plateau 18, and including lateral portion 102 and medial portion 104. Superior surface 112 is asymmetrical about home axis A.sub.H, with lateral portion 102 having a generally shorter overall anteroposterior extent as compared to medial portion 104 (which includes void indicator 106, discussed below). In addition, the anterolateral “corner” of lateral portion 102 defines radius R2L, which is identical to radius R2L of tibial plateau 18, while the anteromedial “corner” of medial portion 104 defines radius R1R, which is identical to radius R1R of tibial plateau 18 and greater than radius R2L.

(139) Moreover, perimeter wall 114 of trial prosthesis 100 is substantially identical to peripheral wall 25 of tibial plateau 18, and therefore defines periphery 200 with the same features and shapes of perimeter 200 described above with respect to tibial plateau 18. Thus, trial prosthesis 100 is asymmetrical about home axis A.sub.H in a similar manner to tibial plateau 18 of tibial baseplate 12, with the nature of this asymmetry changing across the various other sizes of tibial prosthesis provided in the kit including trial prosthesis 100.

(140) In an alternative embodiment, a trial prosthesis may be provided which extends completely to the posterior-medial edge of the natural tibial resection periphery. Thus, such a would substantially completely cover the resected tibial surface, thereby aiding in determination of a proper rotational orientation of the trial (and, therefore, of the final tibial baseplate 12). In this alternative embodiment, the trial prosthesis lacks the posterior-medial “pull back” of tibial plateau 18, described above.

(141) Trial prosthesis 100 includes void indicator 106 disposed at the posterior portion of medial portion 104, consuming a given posteromedial area of superior surface 34 and peripheral wall 25. Void indicator 106 indicates where void 52 (discussed above) will be located with respect to tibia T after implantation of tibial prosthesis 10. Void indicator 106 facilitates proper rotational and spatial orientation of trial prosthesis 100 on the resected proximal surface of tibia T by allowing a surgeon to visually match tibial bearing component 14 with trial prosthesis 100, as described in detail below. In the illustrated embodiment, void indicator 106 is an area of visual and/or tactile contrast with the remainder of tibial plateau 18. This contrast may include, for example, a contrasting color, texture, surface finish, or the like, or may be formed by a geometric discrepancy such as a step or lip, for example.

(142) Referring specifically to FIG. 6, trial prosthesis 100 further includes a plurality of peg hole locators 108 corresponding to the proper location for peg holes in tibia T to receive pegs (not shown) extending inferiorly from tibial plateau 18 of tibial baseplate 12. Advantageously, peg hole locators 108 allow a surgeon to demarcate the proper center for peg holes in tibia T once the proper size and orientation for trial prosthesis 100 has been found, as discussed in detail below. Alternatively, peg hole locators 108 may be used as drill guides to drill appropriately positioned peg holes while trial prosthesis is still positioned on tibia T.

(143) 7. Tibial Prosthesis Implantation

(144) In use, a surgeon first performs a resection of tibia T using conventional procedures and tools, as are well-known in the art. In an exemplary embodiment, a surgeon will resect the proximal tibia to leave a planar surface prepared for receipt of a tibial baseplate. This planar surface may define a tibial slope, which is chosen by the surgeon. For example, the surgeon may wish to perform a resection resulting in positive tibial slope in which the resected tibial surface slopes proximally from posterior to anterior (i.e., the resected surface runs “uphill” from posterior to anterior). Alternatively, the surgeon may instead opt for negative tibial slope in which the resected tibial surface slopes distally from posterior to anterior (i.e., the resected surface runs “downhill” from posterior to anterior). Varus or valgus slopes may also be employed, in which the resected surface slopes proximally or distally from medial to lateral. The choice of a tibial and/or varus/valgus slope, and the amount or angle of such slopes, may depend upon a variety of factors including correction of deformities, mimicry of the native/preoperative tibial slope, and the like.

(145) In an exemplary embodiment, keel 16 (FIG. 4B) defines a 5-degree, anteriorly-extending angle with respect to bone-contacting surface 35 of tibial plateau 18. Tibial baseplate 12 is appropriate for use with a positive tibial slope of as little as zero degrees and as much as 9 degrees, and with a varus or valgus slope of up to 3 degrees. However, it is contemplated that a tibial baseplate made in accordance with the present disclosure may be used with any combination of tibial and/or varus/valgus slopes, such as by changing the angular configuration of the keel with respect to the bone-contacting surface.

(146) With a properly resected proximal tibial surface, the surgeon selects trial prosthesis 100 from a kit of trial prostheses, with each prosthesis in the kit having a different size and geometrical configuration (as discussed above). Trial prosthesis 100 is overlaid on the resected surface of tibia T. If trial prosthesis 100 is appropriately sized, a small buffer zone 110 of exposed bone of resected tibia T be visible around the periphery of trial prosthesis 100. Buffer 110 is large enough to allow a surgeon to rotate and/or reposition trial prosthesis 100 within a small range, thereby offering the surgeon some flexibility in the final positioning and kinematic profile of tibial prosthesis 10. However, buffer 110 is small enough to prevent trial prosthesis 100 from being rotated or moved to an improper location or orientation, or from being implanted in such as way as to produce excessive overhang of the edge of trial prosthesis 100 past the periphery of the resected tibial surface. In one exemplary embodiment, for example, trial prosthesis may be rotated from a centered orientation by up to +/−5 degrees (i.e., in either direction), though it is contemplated that such rotation may be as much as +/−10 degrees or +/−15 degrees.

(147) To aid in rotational orientation, trial prosthesis may include anterior and posterior alignment indicia 70A, 70P, which are the same marks in the same location as indicia 70A, 70P provided on tibial plateau 18 as described above. The surgeon can align indicia 70A with anterior point C.sub.A and indicia 70P with PCL attachment point C.sub.P, in similar fashion as described above, to ensure the anatomical and component home axes A.sub.H are properly aligned. Alternatively, a surgeon may use indicia 70A, 70P to indicate a desired deviance from alignment with home axis A.sub.H. As noted above, deviation of up to 5 degrees is envisioned with the exemplary embodiments described herein. A surgeon may choose to orient indicia 70A, 70P to another tibial landmark, such as the middle of the patella or the medial end of tibial tubercle B.

(148) Thus, the large coverage of trial prosthesis 100 (and, concomitantly, of tibial plateau 18) ensures that tibial baseplate 12 will be properly positioned and oriented on tibia T upon implantation, thereby ensuring proper kinematic interaction between tibial prosthesis 10 and femoral component 60. If buffer zone 110 is either nonexistent or too large, another trial prosthesis 100 is selected from the kit and compared in a similar fashion. This process is repeated iteratively until the surgeon has a proper fit, such as the fit illustrated in FIGS. 6 and 7 between trial prosthesis 100 and tibia T.

(149) With the proper size for trial prosthesis 100 selected and its orientation on tibia T settled, trial prosthesis 100 is secured to tibia T, such as by pins, screws, temporary adhesive, or any other conventional attachment methods. Once trial prosthesis is an secured, other trial components, such as trial femoral components and trial tibial bearing components (not shown) may be positioned and used to articulate the leg through a range of motion to ensure a desired kinematic profile. During such articulation, void indicator 106 indicates to the surgeon that any impingement of femoral component 60 and/or femur F upon trial prosthesis 100 at void indicator 106 will not occur when tibial prosthesis 10 is implanted. Once the surgeon is satisfied with the location, orientation and kinematic profile of trial prosthesis 100, peg hole locators 108 may be used to demarcate the appropriate location of peg holes in tibia T for tibial baseplate 12. Such peg holes may be drilled in tibia T with trial prosthesis 100 attached, or trial prosthesis 100 may be removed prior to drilling the holes.

(150) With tibia T prepared for receipt of tibial prosthesis 10, tibial baseplate 12 may be provided by the surgeon (such as from a kit or surgical inventory), and is implanted on tibia T, with pegs fitting into holes previously identified and demarcated using peg hole locators 108 of trial prosthesis 100. Tibial baseplate 12 is selected from the family of tibial baseplates illustrated in FIG. 2A to correspond with the trial component 100 chosen, which ensures that tibial plateau 18 will cover a large proportion of the resected proximal surface of tibia T, as trial prosthesis 100 did prior to removal. Tibial baseplate is affixed to tibia T by any suitable method, such as by keel 16 (FIG. 4B), adhesive, bone-ingrowth material, and the like.

(151) With tibial baseplate 12 installed, tibial bearing component 14 may be coupled with tibial baseplate 12 to complete tibial prosthesis 10. However, once attached, tibial bearing component 14 does not fully cover tibial plateau 18 of tibial baseplate 12. Rather, tibial bearing component 14 leaves a posteromedial portion of tibial baseplate 12 uncovered to create void 52 (as shown in FIG. 8 and discussed above). Thus, a surgeon may wish to verify that this anterior-biased, “asymmetrical” orientation of medial articular surface 42 is proper prior to permanent affixation of tibial bearing component 14 to tibial baseplate 12.

(152) To accomplish such verification, tibial bearing component 14 is placed side-by-side with trial prosthesis 100, with inferior surface 36 of tibial bearing component 14 in contact with superior surface 112 of trial prosthesis 100. Tibial bearing component 14 will substantially cover superior surface 112, but will not cover void indicator 106. Put another way, peripheral wall 54 of tibial bearing component 14 will trace perimeter wall 114 of tibial trial prosthesis 100, excluding the posteromedial area defined by void indicator 106. If inferior surface 36 of tibial bearing component 14 is a match with superior surface 112 of trial prosthesis 100 except for void indicator 106 (which is left uncovered by tibial bearing component 14), then tibial bearing component 14 is the proper size component and may be confidently installed upon tibial plateau 18 of tibial baseplate 12.

(153) Tibial baseplate 12 may then be implanted upon the proximal surface of tibia T accordance with accepted surgical procedures. Exemplary surgical procedures and associated surgical instruments are disclosed in “Zimmer LPS-Flex Fixed Bearing Knee, Surgical Technique,” “NEXGEN COMPLETE KNEE SOLUTION, Surgical Technique for the CR-Flex Fixed Bearing Knee” and “Zimmer NexGen Complete Knee Solution Extramedullary/Intramedullary Tibial Resector, Surgical Technique” (collectively, the “Zimmer Surgical Techniques”), copies of which are submitted on even date herewith in an information disclosure statement, the entire disclosures of which are hereby expressly incorporated by reference herein.

(154) When the surgeon is satisfied that tibial bearing component 14 is properly matched and fitted to the installed tibial baseplate 12, bearing component 14 is secured using locking mechanism 26 and the corresponding tibial bearing locking mechanism an appropriate instrumentation (not shown). Proper location and rotational orientation of tibial bearing component 14 upon tibial plateau 18 is ensured by raised perimeter 24 cooperating with recess 46, and locking mechanism 26 cooperating with the corresponding tibial bearing locking mechanism (not shown). Such proper orientation results in medial articular surface 42 being generally anteriorly disposed with respect to medial compartment 22 of tibial plateau 18.

(155) Femoral component 60 may be affixed to a distal end of femur F, if appropriate, using any conventional methods and/or components. Exemplary surgical procedures and instruments for such affixation are disclosed in the Zimmer Surgical Techniques, incorporated by reference above. Femur F and tibia T may then be articulated with respect to one another to ensure that neither femur F nor femoral component 60 impinges upon tibial baseplate 12 and/or tibial bearing component 14 in deep flexion, such as at a flexion angle β of 155° as shown in FIG. 8. When the surgeon is satisfied with the location, orientation and kinematic profile of tibial prosthesis 10, the knee replacement surgery is completed in accordance with conventional procedures.

(156) While this invention has been described as having an exemplary design, the present invention can be further modified within the spirit and scope of this disclosure. This application is therefore intended to cover any variations, uses, or adaptations of the invention using its general principles. Further, this application is intended to cover such departures from the present disclosure as come within known or customary practice in the art to which this invention pertains and which fall within the limits of the appended claims.