Patent ID: 12213889

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

DETAILED DESCRIPTION

The present disclosure provides a femoral component for a knee prosthesis which contributes to preservation of healthy bone stock, enhanced articular characteristics, and reduced impact on soft tissues of the knee.

In order to prepare the tibia and femur for receipt of a knee joint prosthesis of the present disclosure, any suitable methods or apparatuses for preparation of the knee joint may be used. 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”), the entire disclosures of which are hereby expressly incorporated herein by reference, copies of which are filed in an information disclosure statement on even date herewith. A surgeon first provides a prosthetic component by procuring an appropriate component (e.g., such as femoral component20) for use in the surgical procedure, such as from a kit or operating-room container or storage receptacle. The surgeon then implants the component using suitable methods and apparatuses, such as the methods and apparatuses described in the Zimmer Surgical Techniques.

As used herein, “proximal” refers to a direction generally toward the torso of a patient, and “distal” refers to the opposite direction of proximal, i.e., away from the torso of a patient. “Anterior” refers to a direction generally toward the front of a patient or knee, and “posterior” refers to the opposite direction of anterior, i.e., toward the back of the patient or knee. In the context of a prosthesis alone, such directions correspond to the orientation of the prosthesis after implantation, such that a proximal portion of the prosthesis is that portion which will ordinarily be closest to the torso of the patient, the anterior portion closest to the front of the patient's knee, etc.

Similarly, knee prostheses in accordance with the present disclosure may be referred to in the context of a coordinate system including transverse, coronal and sagittal planes of the component. Upon implantation of the prosthesis and with a patient in a standing position, a transverse plane of the knee prosthesis is generally parallel to an anatomic transverse plane, i.e., the transverse plane of the knee prosthesis is inclusive of imaginary vectors extending along medial/lateral and anterior/posterior directions. However, it is contemplated that in some instances the bearing component transverse plane will be slightly angled with respect to the anatomic transverse plane, depending, e.g., on the particular surgical implantation technique employed by the surgeon.

Coronal and sagittal planes of the knee prosthesis are also generally parallel to the coronal and sagittal anatomic planes in a similar fashion. Thus, a coronal plane of the prosthesis is inclusive of vectors extending along proximal/distal and medial/lateral directions, and a sagittal plane is inclusive of vectors extending along anterior/posterior and proximal/distal directions. As with the relationship between the anatomic and bearing component transverse planes discussed above, it is appreciated that small angles may be formed between the bearing component sagittal and coronal planes and the corresponding anatomic sagittal and coronal planes depending upon the surgical implantation method.

As with anatomic planes, the sagittal, coronal and transverse planes defined by the knee prosthesis are mutually perpendicular to one another. For purposes of the present disclosure, reference to sagittal, coronal and transverse planes is with respect to the present knee prosthesis unless otherwise specified.

In the context of the femoral component in some knee prostheses, a sagittal plane may be a plane this is equidistant from intercondylar walls bounding the intercondylar gap formed by the component condyles. For example, referring toFIG.5A, femoral component220defines intercondylar notch or gap268formed between lateral and medial intercondylar walls238,239(FIG.5C). In this context of component220, a sagittal plane may the plane which bisects intercondylar gap268and is equidistant from intercondylar walls238,239.

Where the sagittal plane discussed above forms the basis for the component coordinate system, a coronal plane would be defined as a plane perpendicular to the sagittal plane and extending along the same proximal/distal direction as the sagittal plane. A transverse plane is the plane perpendicular to both the sagittal and coronal planes.

In other instances, it may be appropriate to define transverse plane as the plane perpendicular to one or both of distal most points30,32(FIG.1B) defined by lateral and medial condyles24,26. Generally speaking, the “distal-most points” of a femoral component of a knee prosthesis are those points which make the distal-most contact with the corresponding tibial bearing component or natural tibial articular surface when the knee is fully extended. Similarly, the “posterior-most points” of a femoral component of a knee prosthesis are those points which make contact with the corresponding tibial bearing component when the knee is at 90-degrees flexion, i.e., when the anatomic femoral and tibial axes form an angle of 90 degrees.

In the illustrative embodiment ofFIG.1A, lateral and medial condyles24,26each define bearing surfaces that are three-dimensionally convex at distal-most points30,32. Stated another way, the lateral and medial articular bearing surfaces have no planar portions at distal-most points30,32. Recognizing that a three-dimensionally convex surface can define only one tangent plane at a particular point, the transverse plane of femoral component20may be defined as the plane tangent to one or both of distal-most points30,32. For many femoral components, transverse planes tangent to each of distal-most points30,32, are coplanar or nearly coplanar, such that a selection of either of distal-most points30,32is suitable as a reference point for definition of the component transverse plane.

Where the above-described transverse plane is the basis for the component coordinate system, a coronal plane may be defined as being perpendicular to the transverse plane and extending along the same medial/lateral direction as the transverse plane. Alternatively, the coronal plane may be defined as a plane tangent to one or both of posterior-most points34,36in similar fashion to the tangency of the transverse plane to distal-most points30,32as discussed above. In either instance, the sagittal plane can then be defined as a plane perpendicular to the coronal and transverse planes.

Practically speaking, femoral prostheses are sold with a particular surgical procedure envisioned for component implantation. Depending on the particular geometry and accompanying surgical procedure, a person having ordinary skill in the art of orthopaedic prostheses will be able to define “distal-most points” of a femoral prosthesis component, and will be able to identify the sagittal, coronal and transverse component coordinate planes based on their relationship to the corresponding anatomic planes upon implantation.

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

Prosthesis designs in accordance with the present disclosure may include posterior stabilized (PS) prostheses and mid level constraint (MLC) prostheses, each of which includes spine278(FIG.6) on the tibial bearing component and femoral cam276(FIG.5A) on the femoral component. Spine278and cam276are designed to cooperate with one another to stabilize femoral component220with respect to tibial bearing component240in lieu of a resected posterior cruciate ligament (PCL).

Another contemplated design includes “cruciate retaining” (CR) prostheses, such as those using components configured as shown inFIGS.1A,2A(shown by solid lines) and4. CR designs omit spine278from the tibial bearing component and femoral cam276from the femoral component (e.g.,FIG.9A), such that cruciate-retaining femoral component20defines an intercondylar space between lateral and medial condyles24,26that is entirely open and uninterrupted by femoral cam276. CR tibial components are generally used in surgical procedures which retain the PCL.

Yet another design includes “ultra congruent” (UC) prostheses, which may use a femoral component lacking femoral cam276, and may be similar or identical to the femoral component used in a CR prosthesis (i.e., femoral component20shown inFIG.9A). Like CR prostheses, UC prostheses also omit spine278(e.g., the solid-line embodiment ofFIG.2A). However, UC prostheses are designed for use with a patient whose PCL is resected during the knee replacement surgery. “Congruence,” in the context of knee prostheses, refers to the similarity of curvature between the convex femoral condyles and the correspondingly concave tibial articular compartments. UC designs utilize very high congruence between the tibial bearing compartments and femoral condyles to provide prosthesis stability, particularly with respect to anterior/posterior relative motion.

Except as otherwise specified herein, all features described below may be used with any potential prosthesis design. While a particular design may include all the features described herein, it is contemplated that some prostheses may omit some features described herein, as required or desired for a particular application.

1. Articular Features: Bulbous Sagittal Posterior Geometry.

Referring toFIG.1B, femoral component20includes anterior flange22, lateral condyle24and opposing medial condyle26, and fixation pegs28. Lateral and medial condyles24,26define articular surfaces which extend from respective lateral and medial distal-most contact points30,32(FIG.4), through respective lateral and medial posterior-most contact points34,36(FIG.7) and terminate at respective deep flexion contact areas as described in detail below. The articular surfaces are rounded and convex in shape, and sized and shaped to articulate with a tibial articular surface through a full range of motion from full extension of the knee (i.e., zero degrees flexion) through mid-flexion and deep-flexion. In an exemplary embodiment, such tibial articular surfaces are correspondingly concave dished surfaces of a prosthetic tibial component (e.g., tibial bearing component240ofFIG.6). However, it is appreciated that in some instances the tibial articular surface may be the natural articular compartments of a patient's tibia.

Distal-most contact points30,32contact a tibial bearing component of the knee prosthesis (such as tibial bearing component40shown inFIG.2A) when the knee prosthesis is at zero degrees of flexion, i.e., when the knee is fully extended, as noted above. As the knee is flexed from full extension, the lateral and medial contact points between femoral component20and the adjacent tibial articular surface shift posteriorly and proximally into an initial-flexion segment along medial and lateral J-curves27M,27L (FIG.1A), passing through intermediate levels of flexion to eventually reach posterior most contact points34,36at 90 degrees flexion. Further flexion transitions such contact points further proximally, and also anteriorly (i.e., toward anterior flange22) into a deep-flexion segment of J-curves27M,27L.

For convenience, the present discussion refers to “points” or “lines” of contact between tibial bearing component40and femoral component20. However, it is of course appreciated that each potential point or line of contact is not truly a point or line, but rather an area of contact. These areas of contact may be relatively larger or smaller depending on various factors, such as prosthesis materials, the amount of pressure applied at the interface between tibial bearing component40and femoral component20, and the like. In an exemplary embodiment, for example, tibial bearing component40is made of a polymeric material such as polyethylene, while femoral component20is made of a metallic material such as cobalt-chrome-molybdenum (CoCrMo).

Moreover, it is appreciated that some of the factors affecting the size of the contact area may change dynamically during prosthesis use, such as the amount of applied pressure at the femoral/tibial interface during walking, climbing stairs or crouching, for example. For purposes of the present discussion, a “contact point” may be taken as the point at the geometric center of the area of contact. The “geometric center”, in turn, 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. Similarly, a “contact line” is the central line of contact passing through and bisecting an elongate area of contact.

Taken from the sagittal perspective (FIG.1B), anterior flange22and condyles24,26cooperate to define an overall U-shaped profile of femoral component20. The articular surface of femoral component20, along the outer surface of this U-shaped profile, defines medial and lateral J-curves27M,27L respectively (FIG.1A). More specifically, the articular surface of lateral condyle24cooperates with the articular surface of anterior flange22to define lateral J-curve27L, which is inclusive of distal-most contact point30and posterior-most contact point34. Similarly, medial J-curve27M is defined by the articular surfaces of anterior flange22and medial condyle26, taken in a sagittal cross-section and inclusive of distal-most contact point32and posterior-most contact point36.

Where J-curves27L,27M define the sagittal articular profile of femoral component20, coronal curves64L,64M define the corresponding coronal articular profile. Lateral coronal curve64L extends along a generally medial/lateral direction, passing through lateral distal-most contact point30perpendicular to J-curve27L. Similarly, medial coronal curve64M extends along a generally medial/lateral direction, passing through medial distal-most contact point32perpendicular to J-curve27M. The articular surfaces of lateral and medial condyles24,26may be defined or “built” by sweeping coronal curves64L,64M along J-curves27L,27M respectively to produce convex three-dimensional articular surfaces generally corresponding with the shape of the natural femoral condyles. The specific curvatures of coronal curves64L,64M may vary over the extent of J-curves27L,27M, such as by having a generally larger radius at distal-most points30,32as compared to posterior-most points34,36. It is contemplated that coronal curves64L,64M may have a variety of particular geometrical arrangements as required or desired for a particular application.

The portions of J-curves27L,27M which articulate with lateral and medial articular compartments46,48(FIG.6) of tibial bearing component40extend from approximately distal-most points30,32, through posterior-most contact points34,36and into the portion of J-curves27L,27M including bulbous profile42, shown inFIG.1C. Stated another way, the condylar articular portions of J-curves27L,27M are a collection of the contact points between femoral condyles24,26and tibial articular compartments46,48respectively. The J-curve geometry illustrated inFIG.1Cis common to both lateral condyle24and medial condyle26. For clarity, however, such geometry is described herein only with respect to lateral condyle24.

Condyle24A of a predicate design is shown schematically inFIG.1Cas dashed lines, while condyle24of femoral component20is shown in solid lines. As compared with condyle24A, condyle24defines bulbous profile42in the portion of lateral J-curve27L of condyle24corresponding to greater than 90 degrees of prosthesis flexion. Medial J-curve27M of medial condyle26(shown behind lateral condyle24inFIG.1Band extending further proximally, as described in detail below) also defines a similar bulbous geometry in the portion of J-curve27M corresponding to greater than 90 degrees flexion. For simplicity, the bulbous condylar geometry of condyles24,26is described with reference to lateral condyle24only.

As illustrated, bulbous profile42extends further posteriorly and proximally than the corresponding predicate profile42A. This bulbous geometry arises from a reduction in the average magnitude of radius R defined throughout angular sweep α of profile42, such that radius R is less than the corresponding average magnitude of radius RAof profile42A through angular sweep αA. It is contemplated that one or more radii may be defined through angular sweeps α, αA. Comparisons of the average radii, rather than individual radius values, are appropriate where multiple different radii cooperate to form profile42of J-curve27L and/or the corresponding predicate profile42A. For example, in certain exemplary embodiments femoral component20may define an average radius R of 10 mm while the average magnitude of radius RAmay be 10.8 mm over a similar angular sweep. As described in detail below, the resulting bulbous overall arrangement of profile42advantageously influences the articular characteristics of femoral component20in deep flexion while minimizing bone resection.

Prior art devices relevant to deep-flexion bulbous sagittal geometry include the femoral components of the NexGen CR Flex prosthesis system and the femoral components NexGen LPS Flex prosthesis system, all available from Zimmer, Inc. of Warsaw, Ind. The prior art Zimmer NexGen CR Flex prosthesis system is depicted in “NEXGEN COMPLETE KNEE SOLUTION, Surgical Technique for the CR-Flex Fixed Bearing Knee,” incorporated by reference above. The prior art Zimmer NexGen LPS Flex prosthesis system is depicted in “Zimmer LPS-Flex Fixed Bearing Knee, Surgical Technique,” also incorporated by reference above.

As noted above, radii R are swept through angular extents α, αA. Angular extents α, αAbegins in the area of posterior most point34, such as within 10 degrees of posterior-most point34, and ends at or near the proximal-most point of the articular surface of lateral condyle24. Referring toFIG.1C, this proximal-most point of the articular surface is at the intersection between the end of J-curve27L and posterior bone-contacting surface58. It is contemplated that terminal profile44may be disposed between the proximal end of bulbous profile42and posterior bone contacting surface58(As shown inFIG.1C). If included, terminal profile44is a nearly flat or very large-radius nonarticular portion of condyle24which bridges the gap between bulbous profile42and posterior bone contacting surface58. In an exemplary embodiment, however, bulbous profiles42extend all the way to posterior bone-contacting surface58. Further, this exemplary femoral component20has a substantially planar bone-contacting surface58which forms obtuse angle57with distal bone-contacting surface54. Anterior bone-contacting surface50also diverges proximally from posterior bone-contacting surface58in the sagittal perspective, such that femoral component20is implantable onto a resected distal femur along a distal-to-proximal direction.

In the illustrated embodiment, the proximal terminus of angular extent a (i.e., the deepest-flexion portion of bulbous profile42) corresponds with up to 170 degrees of knee flexion. Because femoral component20facilitates this high level flexion of the knee, component20may be referred to as a “high flexion” type component, though it is appreciated that any component which enables flexion of at least 130 degrees would also be considered “high flexion.” In exemplary embodiments, a high-flexion knee prosthesis may enable a flexion range of as little as 130 degrees, 135 degrees, or 140 degrees and as large as 150 degrees, 155 degrees or 170 degrees, or may enable any level of flexion within any range defined by any of the foregoing values.

For example, as illustrated inFIGS.2A and2B, femoral component20is illustrated in a deep flexion orientation, i.e., an orientation in which flexion angle θ between longitudinal tibial axis ATand longitudinal femoral axis AFis between 130 degrees and 170 degrees. As best shown inFIG.2B, bulbous profile42remains in firm contact with lateral articular compartment46of tibial bearing component40at this deep flexion configuration, thereby establishing femoral component20as a component which is deep flexion enabling. As described in detail below, femoral component20accomplishes this high-flexion facilitation with a reduced condyle thickness as compared to prior art high-flexion type components.

Determination of whether the sagittal profiles42,42A are relatively more or less “bulbous” within the meaning of the present disclosure can be accomplished by a comparison of radii R, RAas described above. However, because angular sweeps α, αAmay differ, a suitable comparative quantity may be the amount of arc length per degree of angular sweep referred to herein as the “bulbousness ratio.” A more bulbous geometry, (i.e., one having a smaller average radius) defines a shorter arc length per degree of sweep as compared to a comparable less-bulbous geometry. That is to say, a lower bulbousness ratio value corresponds to a more bulbous sagittal geometry across a given angular sweep. Given the direct correspondence between bulbousness and radius, a relatively smaller average radius (i.e., radius R as compared to radius RA, as shown inFIG.1C) yields a correspondingly larger bulbousness ratio across a comparable angular sweep.

Turning now toFIG.1D, a comparison of bulbousness ratios defined by profiles42,42A are shown across various prosthesis sizes for lateral condyles24and24A. For purposes of the bulbousness comparisons discussed herein, angular sweeps α, αA(FIG.1C) are taken from posterior-most points34,36, (i.e., at 90-degrees flexion) through the end of the corresponding J-curve (i.e., at the intersection between J-curves27L,27M,27A and posterior bone-contacting surface58,58A respectively).

As illustrated inFIG.1D, a dotted-line data set illustrates that the lateral condyles of the femoral components of the prior art Zimmer NexGen CR Flex prosthesis system define a bulbousness ratio of between 0.190 mm/degree (for the smallest nominal size) and 0.254 mm/degree (for the largest nominal size), while the dashed-line data set illustrates an alternative subset of lateral condyles within the prior art Zimmer NexGen CR Flex prosthesis system defining a bulbousness ratio of between 0.231 mm/degree and 0.246 mm/degree across a range of sizes. Femoral components made in accordance with the present disclosure define a bulbousness ratio of between 0.177 mm/degree (for the smallest nominal size) and 0.219 mm/degree (for the largest nominal size), with each comparable size of the present components having a bulbousness ratio below the comparable size of the prior art devices (as shown).

For purposes of the present disclosure, anteroposterior sizing extent340(FIG.13A) can be considered a proxy for nominal sizes of the present femoral component and prior art devices. Anteroposterior sizing extent340may also be referred to the “functional” anterior/posterior extent of femoral component20, because extent340traverses the portion of femoral component20which is most relevant to tibiofemoral articulation (and excludes the articular portions of anterior flange22, which is relevant to patellofemoral articulation). More information regarding specific, enumerated definitions of nominal sizes is provided inFIG.13B, a detailed discussion of which appears below.

Similar to the lateral condylar bulbousness illustrated inFIG.1D,FIG.1Eillustrates a comparison of bulbousness ratios defined by the portions of medial J-curves27M corresponding to greater than 90 degrees of prosthesis flexion, shown across various prosthesis sizes as compared to prior art devices. As illustrated, a dotted-line data set illustrates that the medial condyles of the femoral components of the prior art Zimmer NexGen CR Flex prosthesis system define a bulbousness ratio of between 0.185 mm/degree (for the smallest nominal size) and 0.252 mm/degree (for the largest nominal size), while the dashed-line data set illustrates the above-mentioned alternative subset of medial condyles within the prior art Zimmer NexGen CR Flex prosthesis system defining a bulbousness ratio of between 0.209 mm/degree and 0.259 mm/degree across the same range of sizes depicted inFIG.1D. Femoral components made in accordance with the present disclosure define a bulbousness ratio of between 0.172 mm/degree (for the smallest nominal size) and 0.219 mm/degree (for the largest nominal size), with each comparable size of the present components having a bulbousness ratio below the comparable size of the prior art devices (as shown).

Thus,FIGS.1D and1Equantify the bulbous geometry for profiles42of lateral and medial condyles24,26of cruciate-retaining type femoral component20. Similarly,FIG.1Fquantifies the corresponding bulbous J-curve geometry for lateral and medial condyles224,226of posterior-stabilized type femoral component220(shown, for example, inFIG.2Ainclusive of the dashed lines andFIG.5A) as compared to the femoral components of the prior art Zimmer NexGen LPS Flex prosthesis system, described above. As illustrated, a dotted-line data set illustrates that the medial and lateral condyles of the femoral components of the prior art Zimmer NexGen LPS Flex prosthesis system define a bulbousness ratio of between 0.209 mm/degree (for the smallest and second-smallest nominal sizes) and 0.282 mm/degree (for the second-largest nominal size). Femoral components made in accordance with the present disclosure define a bulbousness ratio of between 0.208 mm/degree (for the smallest nominal size) and 0.240 mm/degree (for the largest nominal size), with each comparable size of the present components having a bulbousness ratio below the comparable size of the prior art devices (as shown).

Advantageously, the above-described bulbous geometry of condyles24,26,224,226facilitates a reduced anterior/posterior condylar thickness TCin such condyles as compared to the larger anterior/posterior condylar thickness TAwhile also enabling high flexion (i.e., flexion of at least 130 degrees, as noted above). For such high-flexion enablement to exist, angular sweep α must be sufficiently large such that an articular portion of J-curves is available at deep-flexion orientations. Stated another way with reference to lateral condyle24shown inFIG.1C, profile42of J-curve27L must “make the turn” completely from 90-degrees flexion at posterior-most point34through a deep flexion orientation at 130 degrees or greater.

The reduction in condylar thickness TCas compared to prior art condylar thickness TAis facilitated by the bulbous geometry of the portion of J-curves27L,27M occupied by profile42, which in turn flows from a reduction in average radius R as compared to prior art radius RAas discussed above. More particularly, these geometrical features of the portions of J-curves27L,27M occupied by profile42allow J-curves27L,27M to “make the turn” required in a smaller allotted anterior/posterior space. In an exemplary embodiment, the relatively greater arc length per degree of angular sweep and smaller radius R defined by bulbous profile42allows the approximately 80-degree angular sweep α from posterior-most contact point34to terminal profile44to be completed in a shorter anterior/posterior span, thereby allowing the overall thickness TCof condyle24to be reduced relative to thickness TAof predicate condyle24A.

Advantageously, this reduced condylar thickness TCshifts posterior bone contacting surface58posteriorly with respect to the predicate posterior bone contacting surface58A, as illustrated inFIG.1C, while preserving high-flexion enablement. Thus, femoral component20satisfies an unmet need by safely allowing very deep flexion (e.g., between 130 and 170 degrees) while also allowing the posterior portions of lateral and medial condyles24,26to be relatively thin, thereby reducing the amount of bone that must be resected as compared to predicate devices. For example, the family of femoral component sizes provided by the prior art Zimmer CR Flex prior art designs define thickness TAof between 8.5 mm and 8.6 for the two smallest prosthesis sizes and in excess of 11 mm for the remaining larger prosthesis sizes. An alternative prior art Zimmer CR Flex prior art design, referred to in the present application as the “CR Flex Minus” prosthesis system, defines thickness TAof between 9.1 mm and 9.6 mm across the range of prosthesis sizes.

In an exemplary cruciate-retaining embodiment (FIGS.1D and1E), bulbous profile42facilitates a condylar thickness TCof 8 mm for the smallest two prosthesis sizes and 9 mm for the remaining prosthesis sizes, as measured by the maximum material thickness between posterior-most points34,36and posterior bone-contacting surface58. This thickness TCis less than thickness TAfor comparable prosthesis sizes in the above-described prior art high-flexion devices.

Thus up to 2.3 mm of bone adjacent posterior bone contacting surface58is preserved through the use of femoral component20as compared to comparably-sized prior art high-flexion femoral prostheses. In an exemplary embodiment, the overall anterior/posterior space APF(FIG.1B) between anterior and posterior bone-contacting surfaces50,58, which corresponds to the anterior/posterior extent of the distal femur after preparation to receive femoral component20, is between 33 mm and 56 mm. The numerical value of anterior/posterior space APFis relatively smaller or larger in direct correspondence to the size of component20within a family of component sizes.

In an exemplary posterior-stabilized embodiment (FIGS.1F and5A), bulbous profile42facilitates a condylar thickness TCof 9 mm for the smallest two prosthesis sizes and 10 mm for the remaining prosthesis sizes, as measured by the maximum material thickness between posterior-most points34,36and posterior bone-contacting surface258. This thickness TCis less than thickness TAfor comparable prosthesis sizes in the prior art high-flexion devices. For example, a family of prior art femoral component sizes in the Zimmer NexGen LPS Flex prosthesis system, which is a posterior-stabilized design which enables high flexion, defines thickness TAof between 10.4 mm and 10.5 for the two smallest prosthesis sizes and between 12.2 mm and 12.4 for the remaining larger prosthesis sizes.

Thus between 1.4 mm and 2.4 mm of bone adjacent posterior bone contacting surface258is preserved through the use of femoral component220as compared to comparably-sized prior art high-flexion femoral prostheses. In an exemplary embodiment, the overall anterior/posterior space APFbetween anterior and posterior bone-contacting surfaces250,258, which corresponds to the anterior/posterior extent of the distal femur after preparation to receive femoral component220, is between 33 mm and 56 mm. The numerical value of anterior/posterior space APFis relatively smaller or larger in direct correspondence to the size of component220within a family of component sizes.

2. Articular Features: “Standard” and “Narrow” Femoral Components for Each Component Size.

Turning toFIG.3A, an anterior elevation view of regular femoral component20is shown juxtaposed against a corresponding narrow component120. Regular component20includes articular geometry in accordance with the present disclosure and adapted for a particular subset of potential knee replacement patients, while narrow component120has articular geometry different from component20and adapted for a different subset of patients. As best seen inFIG.3B, femoral components20,120share a common sagittal geometry such that component120is adapted to selectively mount to a femur which has been prepared to accept femoral component20. Advantageously, this common sagittal geometry allows a surgeon to choose intraoperatively between components20,120.

As shown inFIG.3B, regular femoral component20has five bone contacting surfaces disposed opposite the articular surfaces of anterior flange22and lateral and medial condyles24,26. These five bone contacting surfaces include anterior bone contacting surface50, anterior chamfer surface52, distal bone contacting surface54, posterior chamfer surface56, and posterior bone contacting surface58. Anterior, distal and posterior bone-contacting surfaces50,54,58are adapted to abut a resected surface of a femur upon implantation of femoral component20. In an exemplary embodiment, anterior chamfer and posterior chamfer surfaces52,56are sized and positioned to leave a slight gap between surfaces52,56and the respective adjacent chamfer facet of the resected femur upon implantation, such as about 0.38 mm. However, because this gap is small and may be filled in with fixation material adhering the resected chamfer facets to chamfer surfaces52,56, anterior chamfer and posterior chamfer surfaces52,56are also referred to as “bone-contacting” surfaces herein.

As detailed in the Zimmer Surgical Techniques, a surgical procedure to implant a femoral component such as component20includes resecting the distal end of a femur to create five facets corresponding with bone contacting surfaces50,54,58and chamfers52,56. Relatively tight tolerances between the distal end of the femur and the five bone-contacting surfaces of femoral component20ensure a snug fit.

Femoral component20is provided in a family or kit of differing component sizes, as graphically portrayed inFIGS.3C-3Fand described in detail below. Consideration in choosing an appropriately sized femoral component20from among the set of components include the amount of bone resection necessary to accommodate the component20, and the ability for resected surfaces to make full-area, flush contact with the adjacent bone-contacting surfaces50,52,54,56,58of femoral component20(see, e.g.,FIG.11Bshowing femoral component220implanted upon femur F). To implant femoral component20, the anterior/posterior distance defined by the anterior and posterior facets of the resected femur must match the corresponding anterior/posterior distance APF(FIG.1B) between anterior bone contacting surface50and posterior bone contacting surface58. An appropriately sized femoral component20provides snug abutting contact between all five of the bone-contacting surfaces of femoral component20and the distal resected facets, while also resulting in a desired articular profile in the knee prosthesis.

In the interest of preserving as much natural bone stock as practical, it is desirable to maximize the anterior/posterior distance APFof femoral component20provided the articular profile is acceptable to the surgeon. However, no two patients are exactly alike. In some cases, for example, the overall sagittal geometry of bone contacting surfaces50,54,58and chamfers52,56may represent an ideal match for the femur of a particular patient, but the peripheral characteristics of femoral component20(described in detail below) may not present an adequate match to the other anatomical features of the femur. The present disclosure addresses this eventuality by providing alternative femoral component designs sharing a common sagittal geometry, as illustrated inFIG.3B.

For example, the height HSFand geometry of anterior flange22of regular femoral component20(FIGS.3A,3B and3D) may result in “overhang” thereof past the associated anterior facet of the resected femur. Similarly, the overall medial/lateral width MLSof regular femoral component20(FIGS.3A and3C) may be too large, as indicated by overhang of one or more bone-contacting surfaces50,52,54,56,58past the medial and/or lateral edge of the patient's femur. Yet another possibility is that the overall proximal/distal heights HSM, HSLof medial and lateral condyles26,24, respectively (FIGS.3A,3B,3E, and3F) may be too large, also potentially resulting in overhang of the component beyond the resected posterior facets of the femur. In each of these cases, femoral component20would normally be considered too large, possibly resulting in the use of a smaller component size with its associate reduction in anterior/posterior distance APF(FIGS.1B and3B).

Moreover, Applicants have found that for a substantial subset of knee replacement candidates, “regular” or standard femoral component sizes may have an appropriate anterior/posterior distance APFand spatial arrangement of bone contacting surfaces50,54,58and chamfers52,56, but are too large with respect to one or more of the aforementioned characteristics of the component periphery, and usually all three (i.e., height HSFand geometry of anterior flange22, overall width MLS, and condyle heights HSM, HSL).

To accommodate a wider variety of femoral geometries while facilitating maximum preservation of healthy bone stock during the surgical procedure, a prosthesis system in accordance with the present disclosure provides a set of “narrow” femoral components120which share a common spatial arrangement of bone contacting surface geometry with a corresponding set of femoral components20(i.e., a common anterior/posterior distance APFand associated sagittal profile of resected facets), but includes anterior flange122, lateral condyle124and medial condyle126which are strategically downsized.

In the anterior elevation view ofFIG.3A, the periphery of narrow femoral component120is aligned with the periphery of regular femoral component20such that lateral distal-most contact points30,130and medial distal-most contact points32,132are superimposed over one another. Moreover, the articular profile and geometry of condyles24,26of femoral component20, including medial and lateral J-curves27M,27L described above (FIG.3B), are substantially identical to the corresponding profile of condyles124,126of narrow femoral component120, with the exception of the reduction in various peripheral aspects of femoral component120as compared to component20as described below. Taking account of such reductions, the articular surfaces of femoral component120are subsumed by the articular surfaces of femoral component20when the articular surfaces of components20,120are superimposed, as illustrated inFIGS.3A and3B. Thus, both of femoral components20and120may be used interchangeably with a selected abutting tibial component, such as tibial bearing component240(FIG.6).

However, anterior flange122of narrow femoral component120defines a shorter overall flange height HCF, as illustrated inFIGS.3A,3B and3D. In an exemplary embodiment, height HCFmay be reduced by 1 mm from the corresponding height HSFof anterior flange22of regular femoral component20for any given prosthesis size. As shown inFIG.3D, height HSFof femoral component20ranges from 38 mm to 51 mm, and grows progressively larger across a range of prosthesis sizes (starting from a nominal size 3 and ending at a nominal size 12). By contrast, height HCFof femoral component120ranges from 35 mm to 47 across an overlapping range of prosthesis sizes (starting from a nominal size 1 and ending at a nominal size 11). As illustrated in the lines connecting the data points ofFIG.3D, anterior flange heights HCFof each size of femoral component120are consistently less than the corresponding flange heights HSFfor corresponding sizes of femoral component20. A common nominal size for femoral components20,120denotes a substantially identical spatial arrangement of bone contacting surface geometry, including a common anterior/posterior distance APF, such that either of a particular size of component20,120can be implanted onto the same resected femur.

Medial condyle height HCMof medial condyle126is also shorter than the corresponding medial condyle height HSMof standard medial condyle26. In an exemplary embodiment, height HCMmay be reduced by 1 mm from the corresponding height HSMof medial condyle26of regular femoral component20for any given prosthesis size. As shown inFIG.3F, height HSMof medial condyle26of regular femoral component20ranges from 24 mm to 33 mm, and grows progressively larger across a range of prosthesis sizes (starting from a nominal size 3 and ending at a nominal size 12). By contrast, height HCMof femoral component120ranges from 21 mm to 31 mm across an overlapping range of prosthesis sizes (starting from a nominal size 1 and ending at a nominal size 11). As illustrated in the lines connecting the data points ofFIG.3F, medial condyle heights HCMof femoral component120are consistently less than the corresponding medial condyle heights HSMof femoral component20across a range of corresponding sizes.

Similarly, lateral condyle height HCLof lateral condyle124is less than lateral condyle height HSLof lateral condyle24. In an exemplary embodiment, height HCLmay be reduced by 1 mm from the corresponding height HSLof lateral condyle24of regular femoral component20for any given prosthesis size. As shown inFIG.3E, height HSLof lateral condyle24of regular femoral component20ranges from 22 mm to 31 mm, and grows progressively larger across a range of prosthesis sizes (starting from a nominal size 3 and ending at a nominal size 12). By contrast, height HCLof lateral condyle124of femoral component120ranges from 19 mm to 29 mm across an overlapping range of prosthesis sizes (starting from a nominal size 1 and ending at a nominal size 11). As illustrated in the lines connecting the data points ofFIG.3E, lateral condyle heights HCLof femoral component120are consistently less than the corresponding lateral condyle heights HSLof femoral component20across a range of corresponding sizes.

Referring now toFIG.3A, the overall width MLCof narrow femoral component120is also consistently less than the overall width MLSof femoral component20across a range of prosthesis sizes. In an exemplary embodiment, width MLCmay be reduced by between 1 mm from the corresponding width MLSof regular femoral component20for any given prosthesis size. As shown inFIG.3C, width MLSof regular femoral component20ranges from 62 mm to 78 mm, and grows progressively larger across a range of prosthesis sizes (starting from a nominal size 3 and ending at a nominal size 12). By contrast, width MLCof femoral component120ranges from 55 mm to 70 mm across an overlapping range of prosthesis sizes (starting from a nominal size 1 and ending at a nominal size 11). As illustrated in the lines connecting the data points ofFIG.3C, width MLCof femoral component120is consistently less than the corresponding width MLSof femoral component20across each size in a range of corresponding sizes.

The above-described changes in peripheral characteristics to femoral component120, as compared to femoral component20, advantageously leave the overall sagittal profile of components20,120similar, and with substantially identical anterior/posterior spaces between anterior bone-contacting surfaces50,150and posterior bone-contacting surfaces58,158(including distance APF). However, it is appreciated that the shortening of anterior flange122and posterior condyles124,126do alter the sagittal profile of component120in that such profile is “shortened” overall. However, the sagittal profile of component120is subsumed by the corresponding profile of regular component20(as illustrated inFIG.3B), such that narrow component120will fit the same resected femur as component20. Advantageously, this shortening prevents potential overhang of component120past the resected portions of some femurs, as discussed above.

In addition to the differences in the peripheral characteristics described above, articular features of anterior flange122also vary as compared to anterior flange22of regular femoral component20. Referring toFIG.3A, standard anterior flange22defines flange taper angle βS, which is the taper angle defined by the medial and lateral walls adjoining anterior bone-contacting surface50to the opposed articular surface of flange22. In the illustrative embodiment ofFIG.3A, taper angle βSangle is measured between lines tangent to points along the rounded frontal profile defined by the medial and lateral walls of anterior flange22at the base of anterior bone-contacting surface50(i.e., where anterior bone-contacting surface50meets anterior chamfer surface52). However, it is appreciated that taper angle βSmay be defined at any point along such rounded edges, provided the medial and lateral tangent lines are drawn at common proximal/distal heights for purposes of comparison between femoral components20,120.

In contrast to standard anterior flange22, narrow anterior flange122defines taper angle βCwhich is different from taper angle βSfor any given nominal prosthesis size. This disparity of taper angles facilitates a relatively smaller disparity in overall heights HSF, HCFof anterior flanges22,122as compared to the relatively larger disparity in overall widths MLC, MLSthereof (as shown by comparison ofFIGS.3C and3D, and detailed above). Advantageously, this differing taper defined by taper angles βS, βCin anterior flanges22,122accommodates a wide range of natural patient anatomies for larger- and smaller-stature patients.

Yet another difference between regular femoral component20and narrow femoral component120is the angle defined by patellar grooves60,160(also referred to a patellar sulcus) formed in anterior flanges22,122respectively. As best illustrated inFIG.8, anterior flange22defines patellar groove60, which is a longitudinal concavity or trough extending along the proximal/distal extent of anterior flange22, as shown inFIG.3A. A natural or prosthetic patella articulates with groove60during normal flexion and extension of the knee. Turning back toFIG.3A, the path of the deepest portion of the patellar trough defined by patellar groove60is represented by the illustrated sulcus axis, which is extrapolated proximally and distally for clarity. The sulcus axis of patellar groove60defines angle γSwith a transverse plane tangent to distal most points30,32of lateral and medial condyles24,26. In the illustrated embodiment ofFIG.3A, this transverse plane appears as an imaginary line connecting distal-most points30,32(and also, therefore, connecting distal-most points130,132of the superimposed narrow femoral component120).

As illustrated, standard patellar groove angle γSis greater than the corresponding groove angle γCdefined by patellar groove160of anterior flange122. In an exemplary embodiment, standard patellar groove angle γSis 83 degrees, while the narrow-component patellar groove angle γCis 80 degrees.

It is contemplated that for each regular femoral component size within the range of available sizes (i.e., for a range of unique, differing anterior distances APF), one narrow femoral component including the features described above may be provided. In an exemplary embodiment, up to twelve or more unique femoral component sizes may be provided, with each of the12sizes including both regular and narrow femoral components20,120. Thus, a surgeon may intraoperatively elect to implant narrow femoral component120if it becomes apparent that regular femoral component20is too large in certain respects (as described above).

An exemplary surgical technique and apparatus for intraoperatively choosing between regular femoral component20and narrow femoral component120is described in U.S. patent application Ser. No. 13/161,624, filed Jun. 16, 2011 and entitled FEMORAL PROSTHESIS SYSTEM, the entire disclosure of which is hereby expressly incorporated herein by reference.

However, it is also contemplated that multiple narrow components may be provided corresponding to each standard component size. Each of the plurality of narrow components may feature different widths, heights and/or anterior flange arrangements in accordance with the principles described above.

3. Articular Features: Differential Condyle Height.

Referring again toFIG.1C, medial condyle26is taller (i.e., defines a greater proximal/distal extent) as compared to lateral condyle24to define height differential ΔH. In an exemplary embodiment, height differential ΔH may be between 1.1 and 2.3 mm depending on prosthesis size. As described in detail below, an exemplary family or set of femoral components20may include twelve prosthesis sizes, with the smallest size defining height differential ΔH at 1.1 mm and the largest size defining height differential ΔH at 2.3 mm. Intermediate sizes define intermediate height differentials ΔH within the aforementioned range.

In an exemplary embodiment, each adjacent pair of prosthesis sizes have respective height differentials ΔH that vary by 0.1 mm, with larger sizes having proportionally larger variance in height differentials ΔH. Thus, for example, a prosthesis having a nominal size of 1 may have a height differential ΔH of 1.1 mm, while a prosthesis having nominal size 2 has a height differential ΔH of 1.2 mm.

By contrast, the femoral components of the prior art Zimmer NexGen CR Flex prosthesis system have medial condyles which are taller than the lateral condyles by between 1.3 mm and 2.1 mm. Further, families of femoral components of the prior art Zimmer NexGen CR Flex prosthesis system have variability in the condyle height differential which do not grow proportionally larger as nominal sizes increase, instead having differentials which grow at varying rates across the range of sizes.

Advantageously, providing a relatively shorter lateral condyle24allows such lateral condyle24to roll back and externally rotate when the knee prosthesis is in deep flexion (FIG.2A). This deep-flexion rollback and rotation is permitted by shortened lateral condyle24, while any potential impingement between condyle24and adjacent structures and/or soft tissues is avoided. This facilitation of femoral roll back is particularly effective in combination with the other features of a cruciate-retaining femoral component, such as component20, which lacks a femoral cam as described herein.

4. Soft Tissue Accommodation: Femoral Cam Geometry.

Turning now toFIG.5A, posterior stabilized (PS) femoral component220having femoral cam276is illustrated. Femoral component220is substantially similar to femoral component20described above, with reference numerals of component220corresponding to the reference numerals used in component20, except with200added thereto. Structures of femoral component220correspond to similar structures denoted by corresponding reference numerals of femoral component20, except as otherwise noted.

However, femoral component220is specifically adapted for use in a surgical procedure wherein the posterior cruciate ligament (PCL) is resected. More particularly, femoral component220includes femoral cam276spanning intercondylar notch268formed between lateral and medial condyles224,226. Intercondylar notch268is bounded at its lateral and medial sides by lateral and medial condylar walls238,239(FIG.5C), which face inwardly toward one another and each extend proximally from distal bone-contacting surface254. Condylar walls238,239are engageable with spine278of tibial bearing component240(FIG.6) to provide medial/lateral stability to femoral component220from full extension to at least mid-flexion; therefore, in an exemplary embodiment condylar walls238,239are substantially parallel to one another to define a total medial/lateral width MLTwhich remains constant across the anterior/posterior extent of intercondylar notch268.

Femoral cam276is sized, shaped and positioned to articulate with spine278of tibial bearing component240(FIG.6) along posterior articular surface280thereof (as described in detail below). Spine278extends proximally from the articular surface of tibial bearing component240, and is disposed between lateral and medial articular compartments246,248thereof. Additional details of spine278and its interaction with femoral cam276are described in: U.S. Provisional Patent Application Ser. No. 61/561,657, filed Nov. 18, 2011 and entitled “TIBIAL BEARING COMPONENT FOR A KNEE PROSTHESIS WITH IMPROVED ARTICULAR CHARACTERISTICS”; U.S. Provisional Patent Application Ser. No. 61/577,293, filed Dec. 19, 2011 and entitled “TIBIAL BEARING COMPONENT FOR A KNEE PROSTHESIS WITH IMPROVED ARTICULAR CHARACTERISTICS”; U.S. Provisional Patent Application Ser. No. 61/592,576, filed Jan. 30, 2012 and entitled “TIBIAL BEARING COMPONENT FOR A KNEE PROSTHESIS WITH IMPROVED ARTICULAR CHARACTERISTICS”; U.S. Provisional Patent Application Ser. No. 61/621,361, filed on even date herewith and entitled “TIBIAL BEARING COMPONENT FOR A KNEE PROSTHESIS WITH IMPROVED ARTICULAR CHARACTERISTICS”; and U.S. Provisional Patent Application Ser. No. 61/621,363, filed on even date herewith and entitled “TIBIAL BEARING COMPONENT FOR A KNEE PROSTHESIS WITH IMPROVED ARTICULAR CHARACTERISTICS”. The entire disclosures of each of the above-identified patent applications are hereby expressly incorporated herein by reference.

Femoral cam276includes central articular area282defined by a plurality of cylindrical surfaces tangent to one another, with the longitudinal axes defined by such cylindrical surfaces all substantially parallel to one another and extending in a medial/lateral direction. Central articular area282is flanked by medial and lateral transition areas284M,284L which provide a rounded transition from the cylindrical central articular area to lateral and medial condyles224,226, as shown inFIG.5Aand described in detail below.

More particularly,FIG.5Billustrates four cylindrical surface curves286,288,290,292as viewed in a sagittal cross-section bisecting femoral cam276. As described in detail below, curves286,288,290,292are indicative of surfaces when viewed from a perspective other than the sagittal perspective ofFIG.5B. Proximal curve286extends posteriorly from posterior bone contacting surface258, and defines a relatively large curvature radius R1. In an exemplary embodiment, radius R1may be as little as 10 mm or as large as 11.5 mm, with larger values for radius R1corresponding to larger prosthesis sizes within a family of different prosthesis sizes.

Posterior curve288tangentially adjoins proximal curve286, thereby creating a smooth transition between curves286,288. As viewed from the sagittal perspective ofFIG.5B, posterior curve288extends posteriorly and distally from its junction with proximal curve286. Posterior curve288defines radius R2which is smaller than radius R1. In an exemplary embodiment, radius R2may be as little as 2.5 mm, 6.5 mm or 7 mm and large as 8 mm or 12 mm, or may be any size within any range defined by the foregoing values. Similar to radius R1discussed above, larger values of radius R1may correspond to larger prosthesis sizes within a family of prostheses.

Distal curve290tangentially adjoins posterior curve288to create another smooth transition between curves288,290. As viewed from the sagittal perspective ofFIG.5B, distal curve290extends distally and anteriorly from its junction with posterior curve288. Distal curve290defines radius R3which is smaller than radius R2of posterior curve288. In an exemplary embodiment, radius R3may be between 2 mm and 3 mm across all sizes in the aforementioned family of prostheses.

Anterior curve292tangentially adjoins distal curve290, and extends anteriorly and proximally therefrom, to rejoin posterior bone contacting surface258. Anterior curve292defines a very large radius, or is substantially flat. As noted above, curves286,288,290each define a medially/laterally extending cylindrical face, such that centers C1, C2, C3of radii R1, R2, R3, respectively, lie on respective medially/laterally extending longitudinal cylinder axes. Stated another way, the cylindrical faces and longitudinal axes of curves286,288,290extend into and out of the page ofFIG.5B.

Although the sagittal curve arrangement described above utilizes three articular curves to define central articular area282, it is contemplated that any number of mutually tangent curves may be used. For example, in certain exemplary embodiments posterior curve288may be broken up into two sections, in which a transitional curve portion between radii R1, R2has a relatively smaller radius than either of radii R1, R2, thereby providing a decisive transition from the mid-flexion articular characteristics provided by posterior curve288(as described below) and the deep-flexion articular characteristics of proximal curve286(also described below).

As described above with regard to the exemplary embodiment of femoral component220, the articular surfaces defined by curves286,288,290are shown and described as cylindrical and therefore are depicted as straight lines in the coronal cross-section ofFIG.5C. However, it is contemplated that central articular area282may have a slight medial/lateral curvature, such as a slight convex curvature resulting in a slightly curved coronal profile. Moreover, for purposes of the present disclosure, a geometric shape defined by a component of a knee prosthesis (such as a cylindrical surface) refers to a shape having the nominal characteristics of that geometric shape, it being appreciated that manufacturing tolerances and circumstances of in vivo use may cause such nominal characteristics to deviate slightly.

Turning now toFIG.5C, the cylindrical surfaces including curves286,288,290define varying medial/lateral extents along the respective longitudinal axes defined by the curves. As described in detail below, these varying axial extents cooperate to accommodate the unique demands on central articular area282through the range of prosthesis flexion.

Medial/lateral extent MLPis defined by proximal cylindrical surface286, which corresponds to a deep-flexion portion of central articular area282, i.e., that part of femoral cam276which contacts spine278during deep flexion of femoral component220. In the context of the varying widths defined by central articular area282, medial/lateral extent MLPis relatively small. In an exemplary embodiment, medial/lateral extent MLPmay be as small as 1.5 mm or 3 mm, and may be as large as 3.5 mm or 5 mm, or may be any size within any range defined by the foregoing values. For example, in an exemplary family of femoral components having different component sizes, medial/lateral extent MLPmay grow larger as the component sizes increase. In this exemplary family of components, medial/lateral extent MLPis between 10% and 25% of total intercondylar width ML-r, which in turn ranges from 14 mm to 22 mm.

By contrast, medial/lateral extent MLDis defined by distal cylindrical surface290, which corresponds to an initial-flexion portion of central articular area282. Medial/lateral extent MLDof distal cylindrical surface290is relatively larger than medial/lateral extent MLP, and represents the largest medial/lateral extent of central articular area282. In an exemplary embodiment, medial/lateral extent MLDmay be as small as 12 mm, 14.8 mm or 15 mm, and may be as large as 16.1 mm, 19.5 mm or 20 mm, or may be any size within any range defined by the foregoing values. As best seen inFIG.5A, posterior cylindrical surface288defines a steadily expanding medial/lateral extent which smoothly transitions from the narrower proximal medial/lateral extent MLPto the wider distal medial/lateral extent MLD. For example, in the above-mentioned exemplary family of femoral components having different component sizes, medial/lateral extent MLDmay grow larger as the component sizes increase. In this exemplary family of components, medial/lateral extent MLDis between 85% and 95% of total intercondylar width MLT.

Lateral and medial transition areas284L,284M (FIG.5C) flank central articular area282and extend laterally and medially to join articular area282to the adjacent lateral and medial condyles224,226, respectively. In an exemplary embodiment, medial and lateral transition areas284M,284L are mirror images of one another about a sagittal plane, i.e., the section plane ofFIG.5Bwhich is parallel to and equidistant from lateral and medial condylar walls238,239. However, it is contemplated that differing transition areas may be employed as required or desired for a particular application.

Transition areas284M,284L define transition surfaces corresponding to the respective central articular surfaces to which they are adjoined. For example,FIG.5Cillustrates a representative coronal cross-section of femoral cam276, in which the curvature of transitions areas284M,284L is depicted. Convex lateral and medial transition surfaces defining coronal radius R4flank the lateral and medial terminus of proximal central articular surface286, forming a tangent with surface286and extending medially and laterally toward lateral and medial condyles224,226respectively. In an exemplary embodiment, radius R4may be as small as 6 mm, 6.5 mm or 7 mm, and may be as large as 8 mm or 12 mm, or may be any size within any range defined by the foregoing values. In an exemplary family of prosthesis sizes, larger values for radius R4correspond to larger prosthesis sizes. Across all sizes, however, radius R4represents a significant portion of the total medial/lateral width MLT. For example, radius R4may be equal to as little as 40%, 41% or 44% of total medial/lateral width MLT, or may be as large as 46% or 56% thereof, or may be any percentage within any range defined by the foregoing values.

Referring still toFIG.5C, the widely radiused and convex coronal curvature defined by radius R4gives way to a tighter concave curvature having radius R5as lateral and medial transitional areas284L,284M approach intersection with lateral and medial condyles224,226respectively. This concave curvature is tangent to radius R4and to the adjacent surfaces of condyles224,226, thereby forming a smooth transition therebetween. Similarly, the portion of transition areas284L,284M which join distal and anterior surfaces290,292(FIG.5B) of femoral cam276to condyles224,226are composed only of concave curvature having radius R6, owing to the substantial width of surfaces290,292(as discussed above). In an exemplary embodiment, both radius R5and radius R6are at least 1 mm. As noted above, all other radii defined by the surfaces of femoral cam276are substantially larger than 1 mm. Thus, femoral cam276defines a minimum radius of at least 1 mm at all parts subject to articulation with any adjacent soft tissues or prosthesis structures (i.e., excluding the portion of posterior bone-contacting surface258, which only abuts the corresponding facet of the bone after implantation).

Moreover, the concave transitional radii R5, R6are not generally considered a portion of the “articular” surfaces of femoral cam276, because these concave surfaces will not come into contact with spine278of tibial bearing component240(FIG.6). Rather, central articular area282and lateral and medial transitional areas284L,284M form the potential articular surfaces with regard to spine278, and these areas combine to occupy a large proportion of total medial/lateral width MLT. In an exemplary embodiment, the overall portion of total medial/lateral width MLToccupied by the combination of central articular area282and the convex portions of transition areas284L,284M is as little as 80%, 85% or 88%, and as much as 89% or 91%, or may be any percentage within any range defined by the foregoing values. Thus, only surfaces which are broadly convex and/or cylindrical are presented to surrounding tissues and anatomical structures, thereby maximizing surface area contact (and reducing contact pressure) between femoral cam276and spine278during articulation.

As illustrated inFIGS.5A and5B, femoral cam276is disposed between lateral and medial condyles224,226near the proximal-most portion thereof. In use, the relative positioning of femoral cam276and tibial spine278results in initial contact therebetween in mid-flexion. As femoral component220as articulates with tibial bearing component240through the range of flexion, a portion of distal curve290initially contacts spine278along proximal contact line294(FIG.6). In an exemplary embodiment, this initial contact occurs at a prosthesis flexion angle θ (FIG.2A) of between 75 degrees and 93 degrees. In this mid-flexion configuration, external rotation of femoral component220has not yet begun, and the wide medial/lateral extent MLD) of the cylindrical distal surface290is in articular contact with a comparably wide medial/lateral extent of proximal contact line294to provide a large contact area and associated low contact pressure.

As femoral component220transitions into deeper flexion orientations (i.e., larger flexion angles θ as shown inFIG.2A), contact between femoral cam276and posterior articular surface280of spine278moves distally toward distal contact line296(FIG.6). Simultaneously, the contact area on cam276transitions from distal surface290, through posterior surface288, and ultimately to proximal surface286once in deep flexion (e.g., when angle θ approaches and surpasses 155 degrees, as shown inFIG.2A). In deep flexion, femoral component220also externally rotates, thereby altering the orientation of cylindrical surfaces286,288,290of femoral cam276with respect to posterior articular surface280of spine278. To accommodate this altered orientation, posterior articular surface280angles or “turns” as cam276moves from proximal contact line294toward distal contact line296. Thus, the anterior/posterior thickness defined by spine278along distal contact line296is greater near lateral articular compartment246as compared to the corresponding thickness near medial articular compartment248.

This configuration of posterior articular surface280and attendant change in thickness is described in detail in: U.S. Provisional Patent Application Ser. No. 61/561,657, filed Nov. 18, 2011 and entitled “TIBIAL BEARING COMPONENT FOR A KNEE PROSTHESIS WITH IMPROVED ARTICULAR CHARACTERISTICS”; U.S. Provisional Patent Application Ser. No. 61/577,293, filed Dec. 19, 2011 and entitled “TIBIAL BEARING COMPONENT FOR A KNEE PROSTHESIS WITH IMPROVED ARTICULAR CHARACTERISTICS”; U.S. Provisional Patent Application Ser. No. 61/592,576, filed Jan. 30, 2012 and entitled “TIBIAL BEARING COMPONENT FOR A KNEE PROSTHESIS WITH IMPROVED ARTICULAR CHARACTERISTICS”; U.S. Provisional Patent Application Ser. No. 61/621,361, filed on even date herewith and entitled “TIBIAL BEARING COMPONENT FOR A KNEE PROSTHESIS WITH IMPROVED ARTICULAR CHARACTERISTICS”; and U.S. Provisional Patent Application Ser. No. 61/621,363, filed on even date herewith and entitled “TIBIAL BEARING COMPONENT FOR A KNEE PROSTHESIS WITH IMPROVED ARTICULAR CHARACTERISTICS”. The entire disclosures of each of the above-identified patent applications are hereby expressly incorporated herein by reference.

As external rotation of femoral component220initiates in deep flexion, engagement of posterior articular surface280of spine278shifts from distal surface290to posterior surface288of cam276. As this shift takes place, the convex portions of transition areas284M,284L (described in detail above) move into position near the medial and lateral edges of posterior articular surface280. As flexion (and external rotation) of femoral component220progresses, contact between femoral cam276and posterior articular surface280transitions from posterior surface288and to proximal surface286. Proximal surface286defines a smaller medial/lateral width MLPcompared to width MLDof distal surface290creating the medial/lateral space for the large-radius, broadly convex portions of transition areas284M,284L flanking proximal surface286(FIG.5A). These large portions of transition areas284M,284L facilitate solid contact between the relatively narrower proximal surface286when femoral component220internally or externally rotates in deep flexion, thereby ensuring that a large area of contact and concomitantly low contact pressure between femoral cam276and tibial spine278is maintained.

Stated another way, the potential for internal/external rotation of femoral component220increases with increasingly deep flexion. Such internal/external rotation also causes the longitudinal axis of femoral cam276to rotate with respect to posterior surface280of tibial spine278, thereby potentially misaligning one of cylindrical surfaces286,288with posterior surface280(depending on the level of flexion). This misalignment is accommodated by the progressive narrowing of cylindrical surface288(and resulting narrow width MLPof proximal surface286), which concomitantly increases the medial/lateral extent of transition areas284M,284L. The narrower cylindrical surfaces286,288present a smaller area of contact with posterior surface280of spine278, which in turn allows femoral cam276the requisite rotational freedom to accommodate internal/external rotation while maintaining area contact between the cylindrical surface of proximal surface286of femoral cam and the angled distal contact line296along posterior surface280of spine278.

Advantageously, medial and lateral transition areas284M,284L provide a space or “trough” that is strategically located to accommodate the edges of spine278adjacent posterior articular surface280, as femoral component220rotates externally and/or internally. This accommodation prevents any potential for impingement of cam276upon spine278in deep flexion. At the same time, radii R4are relatively large, thereby providing a widely rounded, convex and “soft tissue friendly” surface to reduce contact pressure in the event of soft tissue impingement upon transition areas284L,284M. Convex radii R5similarly eliminate any sharp edges in the vicinity of femoral cam276, further minimizing potential contact pressures caused by impingements thereupon.

By contrast, predicate femoral components utilize an articular surface that is concave along its medial/lateral extent, and includes transition area radii that are substantially less than 1 mm. One such prior art femoral component forms a part of the NexGen LPS Flex prosthesis system (described above).

5. Soft Tissue Accommodation: Asymmetric Intercondylar Notch.

Referring toFIG.7, for cruciate retaining (CR) femoral component designs, such as femoral component20, intercondylar notch68is laterally and medially bounded by lateral inner sidewall76and medial inner sidewall77, respectively. As described in detail below, inner sidewalls76,77define angular orientations with respect to femoral component20which operate to protect the posterior cruciate ligament (PCL) during prosthesis articulation. As noted above, the PCL is retained in the surgical procedure implanting cruciate retaining femoral component20and associated prosthesis components.

Referring toFIG.7, femoral component20defines bisecting axis80, which divides femoral component20into medial and lateral halves. In the context of component20, bisecting axis80bisects the arcuate anterior terminus82of intercondylar notch68, and is perpendicular to a posterior coronal plane defined by posterior bone contacting surface58. However, it is contemplated that bisecting axis80may be defined in a number of other ways, provided that axis80generally divides a femoral component made in accordance with the present disclosure into medial and lateral halves. In the context of patient anatomy, bisecting axis80corresponds to Whiteside's line when implanted onto a femur. Whiteside's line is defined as the line joining the deepest part of the anatomic patellar groove, anteriorly, and the center of the anatomic intercondylar notch, posteriorly.

Lateral inner sidewall76defines angle σLwith respect to bisecting axis80, while medial sidewall77defines angle σMwith respect to bisecting axis80. Intercondylar notch68may be said to be “asymmetric” because medial sidewall angle σMis greater than lateral sidewall angle σL. Advantageously, this asymmetric angular arrangement of sidewalls76,77of intercondylar notch68facilitates external rotation of femoral component20in deep flexion (described in detail above) by providing additional space for the posterior cruciate ligament on the medial side. This additional medial space avoids potential contact between the PCL and medial inner sidewall77which might otherwise occur when femoral component20externally rotates.

6. Soft Tissue Accommodation: Rounded Anterior Flange.

FIG.8illustrates a cross-section of anterior flange22of femoral component20. As illustrated inFIG.1B, the cross-sectional profile ofFIG.8is taken at the junction of anterior bone-contacting surface50and anterior chamfer surface52(and through the middle of thickness ridge300, as described below). The plane of theFIG.8cross section is taken generally perpendicular to the adjacent surfaces, i.e., such that the minimum material thicknesses are shown. For simplicity, the geometric features of anterior flange22are described with reference to the cross section ofFIG.8, it being understood that such geometric features also propagate through the remainder of anterior flange22.

As shown inFIG.8, anterior flange22includes lateral condylar portion62and medial condylar portion63, with a concave patellar groove60disposed therebetween. As noted above, a natural or prosthetic patella articulates with the concave patellar groove60during prosthesis articulation. During such articulation, lateral and medial condylar portions62,63provide constraint to medial and lateral movement of the patella. The level of medial/lateral constraint depends in part on “jump heights” JHL, JHM, defined by condylar portions62,63. Jump heights JHL, JHM, illustrated inFIG.8, represent the amount of anterior travel, i.e., travel outwardly away from patellar groove60, that a patella would have to traverse in order for subluxation of the patella component from the lateral and medial sides of anterior flange22, respectively to occur. In anterior flange22, jump heights JHL, JHMare arranged to prevent such subluxation under normal operating conditions of the prosthesis. In an exemplary embodiment, medial jump height JHMis between 3.0 mm and 4.6 mm and lateral jump height JHLis between 3.5 mm and 5.7 mm. These jump height value ranges are comparable to the prior art femoral components of the Zimmer NexGen prosthesis series, e.g., the NexGen CR Flex prosthesis system and the NexGen LPS Flex prosthesis system.

Anterior flange22defines large-radius, convex lateral and medial condylar portions62,63respectively. Lateral edge98extends from peak62P of the convex lateral condylar portion62, to the lateral edge of anterior bone contacting surface50. Similarly, medial edge99extends from peak63P of the convex medial condylar portion63to the medial edge of anterior bone contacting surface50. Peaks62P,63P cooperate with patellar groove60to define lateral jump height JHL, JHMrespectively, as illustrated inFIG.8. As compared with alternative anterior flange profiles (schematically illustrated inFIG.8using dashed lines), anterior flange22includes lateral and medial edges98,99which define larger radii of curvature R7, R8, respectively. These large radii of curvature R7, Rxadvantageously present a large, convex surface which minimizes pressure applied to adjacent soft tissues such as the retinaculum and extensor mechanism. In an exemplary embodiment, radius R7is equal to radius R8, with each of radii R7, R8sized as small as 5.0 mm, 5.3 mm or 5.5 mm and as large as 6.5 mm, 6.8 mm or 7.0 mm, or are any size within any range defined by any of the foregoing values.

In some instances, the radii defined by the cross-sectional profile of patellar groove60are larger than radii R7, R8, such that the smallest radii presented across the entire medial/lateral extent MLGof the articular surface of anterior flange22are radii R7, R8. In these instances, no small radii are potentially presented to any adjacent soft tissues.

Moreover, these radii represent a large proportion of the overall medial/lateral width MLA(FIG.8) of anterior flange22at any given medial/lateral cross-section. For example, at the cross-section ofFIG.8, medial/lateral flange width MLCranges from 37 to 53 mm across a family of prosthesis sizes, such that radii R7, Rxeach define between 10% and 16% of overall medial/lateral width MLGof anterior flange22.

By contrast, the corresponding radii defined by the prior art femoral components of the Zimmer NexGen CR Flex prosthesis system define medial and lateral flange radii (analogous to radii R7, R8of the present prosthesis) of between 2.0 mm and 2.6 mm across a range of seven nominal prosthesis sizes. Each of these prior art radii define between 3.5% and 5.9% of the overall medial/lateral width (analogous to width MLGof the present prosthesis) of the respective anterior flanges of the prior art femoral components.

7. Bone Conservation: Uniform Thickness of Anterior Flange.

FIG.9Aillustrates femoral component20having thickness ridge300, which is disposed on the bone-contacting side of anterior flange22and spans across portions of anterior bone contacting surface50and anterior chamfer surface52. As described in detail below, thickness ridge300defines a sagittally-oriented peak302, which advantageously allows minimum thicknesses TT(FIG.8), TS(FIG.10A) in anterior flange22to be maintained while preserving a surgeon's ability to implant femoral component20on a distal femur with planar anterior and anterior chamfer facet cuts.

Turning toFIG.9B, thickness ridge300includes ramped lateral facet304and ramped medial facet306, which gradually ascend toward one another to meet at peak302. By contrast, a non-peaked thickness ridge may include a single flat surface (illustrated schematically as surface300′ inFIG.8), which extends medially/laterally without any peaked structure. Viewed from a sagittal perspective, such as shown inFIG.10A, such non-peaked thickness ridge would follow the inner sagittal profile of anterior bone contacting surface50and anterior chamfer surface52(shown in dashed lines). In contrast, as best seen inFIGS.10A and10B, peak302of thickness ridge300protrudes inwardly from bone contacting surface50and anterior chamfer surface52. In an exemplary embodiment, the amount of such inward protrusion may be up to 1.5 mm to allow for implantation of femoral component20upon a bone with planar resected surfaces, as discussed below.

Bone-contacting surfaces50,52,54,56,58(FIG.9A) each extend from a lateral edge to a medial edge of femoral component20. Posterior surface58and posterior chamfer surface56are each interrupted by intercondylar notch68, such that surfaces56,58each extend from the medial edge of condyle26to medial condylar wall39, and from the lateral edge of lateral condyle24to lateral condylar wall38. Together, bone-contacting surfaces50,52,54,56,58define the inner sagittal profile of femoral component20, which is the profile as it appears when the medial and lateral edges are superimposed over one another (i.e., aligned as illustrated inFIG.1B).

Referring still toFIG.9A, femoral component20includes lateral and medial rails59L,59M which bound recessed pocket31adapted to receive bone cement, porous material, or other fixation material (e.g., fixation material33as shown inFIG.10B) for adhering femoral component20to the distal femur upon implantation. Where rails59L,59M are provided, rails59L,59M are considered to define the inner sagittal periphery of femoral component20rather than the recessed profile of pocket31.

Advantageously, peaked thickness ridge300allows for transverse thickness TT(FIG.8) and sagittal thickness TS(FIG.10A) to be maintained above a desired minimum thickness by providing extra material following the path of patellar groove60(FIG.8). Thicknesses TT, TSare measured as the shortest distance between the trough of patellar groove60(described above) and peak302, and are equal when measured between common points. The extra material provided by peak302, corresponds with the profile of the deepest portion of the trough defined by groove60. In the exemplary embodiment illustrated in the drawings, this deepest portion of groove60is also the portion that defines a series of points closest to the adjacent anterior and anterior-chamfer bone-contacting surfaces50,52(e.g.,FIGS.7and8). Thus, what would normally be the thinnest portion of anterior flange22is made thicker by peak302. The overall minimum thickness of anterior flange22may be as little as 1 mm, 1.1 mm or 1.3 mm and may be as large as 1.8 mm, 1.9 mm or 2 mm, or may be any thickness within any range defined by any of the foregoing values. Generally speaking, larger prosthesis sizes have larger minimum thicknesses. Thicknesses TT, TS, are at least as large as, or greater than, the minimum.

Moreover, as illustrated inFIGS.8and10A, the overall thickness of anterior flange22is also more consistent across the medial/lateral and proximal/distal extent of anterior flange22, as compared with a thickness ridge having surface300′ with a flat medial/lateral profile. This consistent thickness allows for the overall average thickness of anterior flange22to be reduced to a value closer to the desired minimum thickness, rather than providing the minimum thickness only near patellar groove60and excess thickness in the remainder of flange22. This reduction in average flange thickness allows for reduced bone resection in the anterior facet and anterior chamfer, thereby facilitating preservation of healthy bone stock. Further maintaining uniformity of thickness across medial/lateral extent MLGfacilitates manufacture of femoral component20by allowing for more even, consistent dissipation of heat, such as after forming, forging and machining operations.

The uniformity of thickness across the medial/lateral cross-section of anterior flange22may be expressed as the maximum deviation of any given thickness dimension as a percentage of the average thickness. In an exemplary embodiment, this deviation may be as little as 38%, 39% or 44% and as large as 55%, 58% or 65% of the average thickness, or may be any percentage of the average thickness within any range defined by any of the foregoing values. The nominal range of average thicknesses across the range of prosthesis sizes is between 2.2 mm and 3.7 mm. The above-mentioned thicknesses take into account the presence of recessed pocket31, which defines recess depth DRof between 1.1 and 1.2 mm.

By contrast, the prior art Zimmer NexGen CR Flex prosthesis system includes femoral components exhibit a corresponding maximum thickness deviation of between 35% and 46%, with the nominal range of average thicknesses across a range of prosthesis sizes being between 3.4 mm and 4.4 mm.

Peak302defines a relatively sharp edge along its longitudinal extent (FIG.9B). In an exemplary embodiment, this sharp edge is manufactured as an edged surface, such that the edge defines no appreciable radius as viewed in the medial/lateral cross section ofFIG.8. Because peak302protrudes inwardly from bone contacting surface50and anterior chamfer surface52(as viewed from the sagittal perspective ofFIG.10A), this sharp edge operates to compact adjacent bone of the anterior facet and anterior chamfer facet when femoral component20is implanted on a distal femur. Such compaction is shown inFIG.10B, where peak302is shown extending into the anterior and anterior chamfer facets of resected femur F. More particularly, referring toFIG.10C, femur F may be prepared with planar anterior facet AF and planar anterior chamfer facet ACF. Once femoral component20is implanted upon femur F as shown inFIG.10B, indentation I mimicking thickness ridge is formed by local compaction of bone on facet AF and planar anterior chamfer facet ACF, thereby disrupting the planarity of facets AF, ACF in the region of indentation I.

As compared with flat a prior art surface (shown schematically as surface300′, shown inFIG.8and described above), the additional volume of bone displaced by the edge defined by peak302and the associated elevation of lateral and medial facets304,306is minimal. In an exemplary embodiment, the displaced volume may be as little as 0.8 mm3, 1.2 mm3or 1.5 mm3and as large as 13.5 mm3, 13.7 mm3or 13.8 mm3, or may be any volume within any range defined by any of the foregoing values. Moreover, the maximum inward protrusion of the edged peak302is 1.5 mm past the sagittal geometry of anterior bone-contacting surface50and anterior chamfer surface52, as noted above.

Thus, the cancellous or cortical bone of the planar resected anterior and anterior chamfer facets is easily compacted upon implantation of femoral component20to accommodate such additional volume. A surgeon may make facet cuts in the femur which are substantially planar (as shown inFIG.10C), thereby simplifying the surgical procedure. These facet cuts may, for example, include five cuts to create five facets sized to receive anterior, anterior chamfer, distal, posterior chamfer and posterior bone-contacting surfaces50,52,54,56,58. Femoral component20is provided by the surgeon, who then implants femoral component20on the resected femur along a distal-to-proximal direction, until peaked portion302of thickness ridge300compresses the adjacent bone fully (as shown inFIG.10B). When such full compression has occurred, indentation I is formed (FIG.10D) such that the entire periphery of thickness ridge300will be in contact with the adjacent facets of the bone.

Optionally, to further ease bone compaction to accommodate peak302, additional resection of the bone at the intersection of the anterior facet and anterior chamfer facet may be performed. For example, a small osteotomy in the vicinity of peak302may be made prior to implantation, such as with a small saw blade, so that peak302sits within the osteotomy upon implantation. Similarly, a small hole may be made in this area, such as with a drill. However, testing performed by Applicants has revealed that no such osteotomy is necessary, and peak302, lateral facet304and medial facet306all seat firmly and completely on cortical and cancellous bone upon implantation.

An additional advantage conferred by peak thickness ridge300is additional medial/lateral fixation of femoral component20upon implantation. Once peak302has impacted the abutting bone, such facets are no longer planar but instead include a ridge-shaped depression occupied by peak302. Thus, lateral and medial facets304,306act as barriers to medial and lateral translation of femoral component20, and thereby confer additional medial/lateral stability. This additional stability aids in secure component fixation, particularly initially after implantation.

It is contemplated that the overall size and geometry of thickness ridge300may be constant across multiple femoral sizes, or may grow and shrink as femoral sizes grow larger or smaller. In an exemplary embodiment, twelve femoral sizes are provided (as described in detail below), with the ten largest sizes including thickness ridge300having a common size, shape and volume across all ten sizes. For the smallest sizes, a reduced-size thickness ridge300A (FIG.12A) may be used.

Overall medial/lateral extent MLR(FIGS.8and9B) and proximal/distal height HR(FIGS.9B and10A) are calculated to be as small as possible while maintaining a minimum desired thickness across the entirety of anterior flange22(as discussed above). In an exemplary embodiment, proximal/distal height HRmay be as little as 7.4 mm and as large as 14.5 mm, 14.6 or 15.0 mm, or may be any height within any range defined by any of the foregoing values. Medial/lateral extent MLRmay be as little as 12.5 mm and as large as 15.0 mm, 15.1 or 15.5 mm, or may be any volume within any range defined by any of the foregoing values. Within these dimensional bounds, the overall peripheral shape of thickness ridge300is designed to follow the contours of anterior flange22, advantageously providing visual acuity therebetween.

For example, the changes in geometry for narrow anterior flange122of narrow femoral component120result in corresponding changes to the overall shape of the corresponding thickness ridge (not shown), thereby providing visual acuity with the narrow shape of component120. However, the overall coverage area and design principles of thickness ridge300apply to any femoral component made in accordance with the present disclosure.

Advantageously, maintaining medial lateral width MLRand proximal/distal height HRat minimum values serves to maximize the area on anterior bone contacting surface50and anterior chamfer surface52for fixation material, as described in detail below.

8. Bone Conservation: Intercondylar Notch with Sloped Sidewalls.

FIGS.11A and11Billustrate a sagittal cross-sectional view of posterior stabilized femoral component220, both before and after implantation upon resected femur F. The cross section ofFIGS.11A and11Bare taken along the outer (i.e., lateral-facing) surface of lateral wall238of intercondylar notch268. A similar cross-sectional view, taken at the medially-facing side of medial wall239of intercondylar notch268, would be a mirror image ofFIGS.11A and11B. As illustrated, lateral wall238extends proximally from distal bone-contacting surface254to define a height HIWalong the proximal/distal direction (e.g., the direction perpendicular to distal bone contacting surface254).

A posterior portion of wall238defines proximal edges (extending along distance D ofFIGS.11A and11B) which are substantially parallel with distal bone-contacting surface from the sagittal perspective ofFIG.11A, while lateral wall238includes a downwardly sloping (i.e., in a distal direction) anterior portion320. In an exemplary embodiment, the posterior and anterior portions define an overall anterior/posterior extent of between 35 mm and 54 mm. The downward sloping anterior portion320initiates at a distance D spaced anteroposteriorly from posterior bone contacting surface258, which is between 27 mm and 48 mm in the exemplary embodiment. Both distance D and the overall anterior/posterior extent grow as sizes grow within a family of prosthesis sizes; across such a family of prosthesis sizes, distance D represents between 77% and 89% of the overall anterior/posterior extent of wall238.

Distance D is calculated to provide sufficient proximal/distal wall height across the posterior portion of intercondylar notch268, such that impingement of femur F upon spine278of tibial bearing component240(FIG.6) is avoided throughout the prosthesis range of motion.

Similarly, the angle322of sloped portion320, taken with respect to a transverse plane (which, in the illustrated embodiment, is parallel to distal bone contacting surface254), is also calculated to prevent spine278from extending proximally beyond walls238,239throughout the range of prosthesis motion. In extension, spine278sits between the non-sloped portions of walls238,239occupied by distance D (FIG.11A). As flexion progresses, the proximal tip of spine278advances toward sloped portion320as femoral component220rotates with respect to tibial bearing component240. Angle322is calculated to provide space above the proximal tip of spine278in deep flexion, while avoiding unnecessary resection of bone. Depending on the geometry of spine278and the particular articular characteristics of the prosthesis, angle322may be any acute angle greater than zero but less than 90 degrees. In an illustrative embodiment ofFIGS.11A and11Bangle322is 60 degrees. The anterior location and gentle slope of anterior portion320cooperate to position the anterior terminus of sloped portion320at anterior chamfer252. As shown inFIGS.11A and11B, sloped portion320terminates into anterior chamfer252.

Advantageously, positioning the terminus of sloped portion320in a relatively anterior location, i.e., at anterior chamfer252, prevents the junction between walls238,239and the adjacent bone-contacting surfaces (252,254,256,258) from interfering with any portion of intercondylar notch268. By contrast, for example, a very steep or vertical angle322for sloped portion320would cause sloped portion320to terminate into an area occupied by intercondylar notch268, potentially necessitating a change in the geometry and/or location of intercondylar notch268.

Advantageously, sloped portion320preserves bone stock of femur F within area A in the anatomic intercondylar notch, thereby reducing the amount of bone which must be removed upon implantation of femoral component220. By contrast, anterior sagittal profile320′, which excludes anterior sloped portion320and extends anteriorly along the same profile as the top of lateral wall238, would necessitate the removal of the bone within area A. Although femur F is shown inFIGS.11A and11Bas having resection profiles that follow the sagittal profile of intercondylar walls238,239, it is contemplate that in certain exemplary procedures the portion of the bone resection corresponding to sloped portion320may be extrapolated to the posterior facet (thereby yielding a substantially planar distal facet).

9. Bone Conservation: Intercondylar Fixation Lug.

For posterior stabilized femoral prosthesis designs, e.g., those including a femoral cam which articulates with a tibial bearing component spine during articulation, fixation pegs28(FIG.1B) may be omitted in favor of utilizing lateral and medial walls238,239of intercondylar notch268for fixation of femoral component220to the femur.

For example,FIG.12Ashows femoral component220in a relatively smaller component size which omits fixation pegs, instead offering uninterrupted distal bone contacting surfaces254. In order to fix component220to femur F (FIGS.11A and11B), a function normally provided in part by pegs28, walls238,239of intercondylar notch268may double as a fixation device. For example, a close tolerance between the central lug defined by walls238,239and the adjacent resected bone within the anatomic intercondylar notch may result in a friction-fit therebetween, thereby providing axial fixation of component220to femur F. In an exemplary embodiment, femoral component220including such a central lug is implanted onto a femur with a nominal clearance of 0.76 mm, and a range of clearances between 0.43 mm and 1.49 mm. These clearances may be provided through use of an appropriately sized cut guide designed for resection of the anatomic intercondylar fossa.

Advantageously, these exemplary clearances allow walls238,239to be used as an axial fixation structure as described above, while maintaining acceptable stresses on the surrounding bone upon implantation of femoral component220. Further, because the natural intercondylar notch naturally defines an anatomic void, use of walls238,239for fixation allows for only minimal resection of bone around the periphery of the existing void, rather than creation of an entirely new void within the bone stock of the distal femur.

Referring now toFIG.12B, for example, lateral wall238may include recessed cement pocket330formed therein. Medial wall239may include a similar, laterally facing recessed cement pocket (not shown). When femoral component220is implanted upon femur F, bone cement or porous fixation material may be disposed in the lateral and medial cement pockets330for fixation to the adjacent, resected bone within the intercondylar notch of the femur to augment the fixation of femoral component220at bone contacting surfaces250,254,258and chamfers252,256.

For example, pockets330, bone contacting surfaces250,254,258and/or chamfers252,256may be at least partially coated with a highly porous biomaterial to facilitate firm fixation thereof to the abutting resected surfaces of the distal femur. 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%, or may have any porosity within any range defined by any of the foregoing values. An example of such a material is produced using Trabecular Metal™ Technology generally available from Zimmer, Inc., of Warsaw, Indiana. Trabecular Metal™ is a trademark of Zimmer, Inc. Such a material may be formed from a reticulated vitreous carbon foam substrate which is infiltrated and coated with a biocompatible metal, such as tantalum, by a chemical vapor deposition (“CVD”) process in the manner disclosed in detail in 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.

Generally, the porous tantalum structure includes a large plurality of struts (sometimes referred to as ligaments) defining open spaces therebetween, with each strut generally including a carbon core covered by a thin film of metal such as tantalum, for example. The open spaces between the struts 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 implant10to the patient's bone.

The porous tantalum structure may be made in a variety of densities in order to selectively tailor the structure for particular applications. In particular, as discussed in 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.

Alternatively, as shown inFIG.12C, the laterally facing surface of lateral wall238may include surface texture332to aid in initial and long term fixation of femoral component220to bone. Surface texture332may include knurling, striations or scales, or any other suitable texture. Similar to cement pocket330, surface texture332may also be provided on the medially facing surface of medial wall239, such that surface texture332abuts resected bone in the intercondylar notch of femur F when femoral component220is implanted.

Omitting fixation pegs28and utilizing walls238,239of intercondylar notch268is particularly advantageous in the context of small component sized for use with small stature patients. In these instances, a limited amount of distal bone area is available for fixation of femoral component220, which may leave insufficient fixation space between fixation pegs28and walls238,239of intercondylar notch268. By omitting femoral fixation pegs28and instead using walls238,239for fixation as described above, additional natural bone may be preserved to provide enhanced structural integrity and robustness of the distal femur.

For small stature patients, the medial/lateral width or gap between lateral and medial walls238,239of intercondylar notch268may be reduced. This may allow for walls238,239to have increased contact with cortical bone in a relatively narrower anatomic intercondylar notch typical of small stature distal femurs.

Referring now toFIG.12D, an optional auxiliary fixation lug334may be provided to further enhance fixation of femoral component220to the femur. Auxiliary lug334extends laterally from the lateral face of lateral wall238, and spans the angular corner formed by lateral wall238and the adjacent portion of distal bone contacting surface254, thereby forming a fin-like structure protruding outwardly from wall238. A similar auxiliary fin (not shown) may also extend medially from the medial face of medial wall239.

Auxiliary lug334increases the bone-contacting surface area provided by femoral component220, thereby enhancing the strength of fixation of component220to the distal resected femur. The surfaces of auxiliary lug334may be affixed to the bone by porous material, bone cement or surface texture, for example, in a similar fashion to the lateral and medial faces of walls238,239as discussed above.

In use, a slot is resected in the distal resected surface of the femur, with the slot sized and positioned to accommodate auxiliary lug334. Advantageously, the resected slots in the femur are clearly visible to the surgeon as femoral component220is advanced toward the femur upon final implantation. If the anterior and distal facets of the femur (i.e., the resected surfaces created to abut anterior and posterior bone-contacting surfaces250,258respectively) are obscured during implantation, such as by the adjacent tissues of the knee, the surgeon will nevertheless be able to visualize the proper implanted orientation of femoral component220by aligning auxiliary lug324to the visible resected slot in the distal femur, and then verify such alignment by tactile feedback as femoral component220is seated upon the resected bone surface.

In the illustrated embodiment, auxiliary lug334has a generally triangular shape and is substantially perpendicular to lateral wall238. However, it is contemplated that auxiliary lug334may have other shapes and/or spatial arrangements. For example, lug334may have rounded corners, squared corners, and/or leading edges that are pointed, rounded or squared.

10. Bone Conservation: Reduced Incremental Growth Between Sizes.

Referring now toFIG.13A, anteroposterior sizing extent340of femoral component20is illustrated. Extent340is measured beginning from intersection point342between anterior bone contacting surface50and distal bone contacting surface54, with surfaces50,54, extrapolated distally and anteriorly to form intersection point342. The other end of extent340is posterior-most contact points34and/or36(discussed in detail above).

As noted herein, an exemplary knee prosthesis system in accordance with the present disclosure includes twelve separate component sizes, each of which defines a different and unique anteroposterior sizing extent340. As between any adjacent pair of sizes (e.g. sizes 1 and 2, sizes 6 and 7 or sizes 11 and 12), a common difference344is defined between the respective anteroposterior extents340of the pair of sizes, as shown inFIG.13B.FIG.13Billustrates that difference344is 2 mm across a range of prosthesis sizes, while corresponding prior art size ranges have corresponding differences that are larger than 2 mm and not consistent across the range of sizes. In an exemplary embodiment, the associated family of femoral prostheses may be as little as 3 sizes and as large as 12 sizes. The prior art devices shown inFIG.13Binclude cruciate-retaining designs, in particular the femoral components of the prior art Zimmer NexGen CR Flex prosthesis system, discussed above, and femoral components of the prior art Zimmer NexGen CR prosthesis system, shown in the “NexGen Complete Knee Solution, Implant Options, Surgeon-Specific,” submitted on even date herewith in an Information Disclosure Statement, the entire disclosure of which is hereby expressly incorporated herein by reference.FIG.13Balso includes posterior-stabilized prior art designs, in particular the femoral components of the prior art Zimmer NexGen LPS Flex prosthesis system, and the femoral components of the prior art Zimmer NexGen LPS prosthesis system, shown in the “Zimmer® NexGen® LPS-Flex Mobile and LPS-Mobile Bearing Knees” product brochure and “Zimmer® NexGen® LPS Fixed Knee, Surgical Technique”, both submitted on even date herewith in an Information Disclosure Statement, the entire disclosures of which are hereby expressly incorporated herein by reference.

Advantageously, measuring anteroposterior extent340from the virtual intersection point342to posterior most contact point34establishes size increments irrespective of changes to anterior flange22across sizes. For example, as shown inFIG.13A, anterior flange50A of the next incrementally larger-size femoral component20A is longer and wider. Therefore, difference344, designed to be constant among respective adjacent pairs of sizes, would be effected by this changing geometry of flange22A.

However, it is desirable to include only incremental anteroposterior growth/shrinkage of posterior most contact point34A in selecting size increments, so that a change in size has a predictable effect on mid-flexion soft tissue balancing of the knee. Thus, incremental size growth having a common anteroposterior difference344defined between any respective pair of sizes provides a uniform and consistent effect on soft tissue balancing as between any pair of sizes across the size range. This, in turn, promotes shorter operative times and allows for implant designers to optimize anterior flange22without impacting the consistency of growth between sizes. Further, by providing twelve standard sizes with unique anteroposterior extents340, greater patient specificity may be achieved as compared with alternative systems having fewer size options.

In an exemplary embodiment, a surgeon may resect a patient's femur to accept the largest of a range of candidate prosthesis sizes identified by the surgeon (such as, for example, by pre-operative imaging). If the surgeon subsequently decides to “downsize” to the next-smallest size of femoral component20, the posterior and posterior-chamfer facets of the resected bone surface (i.e., the facets corresponding to posterior chamfer surface56and posterior surface58) may be further resected, with 2 mm of bone removed from posterior surface58to correspond to anteroposterior difference344. To effect such further resection, an appropriately configured cutting guide may be used. Alternatively, the surgeon may employ a provisional femoral component utilizing appropriately sized resection slots, such as by using the system and method disclosed in U.S. Patent Application Publication Serial No. 2012/0078263, filed Sep. 9, 2011 and entitled BONE PRESERVING INTRAOPERATIVE DOWNSIZING SYSTEM FOR ORTHOPAEDIC IMPLANTS, the entire disclosure of which is hereby expressly incorporated herein by reference.

11. Bone Conservation: Revisable Bone Contacting Fixation Area.

As shown inFIG.14A, femoral component20includes recessed pocket336formed as part of bone contacting surfaces50,54and58and chamfers52,56. Recessed pocket336is surrounded by peripheral rail337, similar to medial and lateral rails59M,59L shown inFIG.9Aand discussed in detail above. Recessed pocket336is interrupted by fixation pegs28and thickness ridge300. Aside from the small areas occupied by rail337, pegs28and ridge300, the entirety of bone contacting surfaces50,54and58and chamfers52and56are available to receive cement or porous ingrowth material for fixation of femoral component20to the adjacent resected facets on the distal femur. In an exemplary embodiment, rails59M,59L are elevated above the surfaces of recessed pocket336by between 1.1 and 1.2 mm.

Advantageously, recessed pocket336is larger than alternative devices by up to 40%, thereby providing a larger fixation area for more robust fixation to the distal femur. More particularly, in an exemplary embodiment femoral component20may have a total fixation area within recessed pocket336of as little as 2272 mm3for a small-size prosthesis and as much as 5343 mm3for a large-size prosthesis, representing between 79% and 88% of the total aggregated surface area of bone-contacting surfaces50,52,54,56,58across all prosthesis sizes. Advantageously, this range of surface area coverage represents an increase in surface area coverage of at least 14%, as compared to comparable prosthesis sizes in the above-mentioned prior art cruciate-retaining prostheses.

In some instances, it may be necessary to perform a revision surgery in which femoral component20is removed from the distal femur and replaced with a new femoral component. In order to facilitate this process, osteotome350having blade352may access the entirety of recessed pocket336either from the outer periphery along rail337, or via intercondylar notch68and the intercondylar portion of rail337. When blade352is worked around the entirety of rail337in this way, all of the bone cement or porous fixation material may be dislodged from the distal femur by osteotome350. Full dislodging femoral component20from the distal femur prior to removal in a revision surgery protects the integrity of the remaining bone.

Turning now toFIG.14B, posterior stabilized femoral component220includes recessed pocket338surrounded by rail237, which are generally similar to recessed pocket336and rail337described above. In an exemplary embodiment, rail237is elevated above the surfaces of recessed pocket338by between 1.1 and 1.2 mm. However, the proximally extending lateral and medial intercondylar walls238,239of intercondylar notch268(described in detail above) preclude blade352of osteotome350from accessing the bone-contacting space between walls238,239and adjacent fixation pegs28.

To facilitate potential revision surgery, femoral component220includes recessed pocket interruptions in the form of lateral and medial ridges346,348. Lateral ridge346directly abuts the distal resected facet on femur F (FIG.11) when femoral component220is implanted thereon, thereby preventing bone cement or porous ingrowth material from inhabiting the space between lateral wall238and peg28. Similarly, medial ridge348occupies the space between medial wall239and peg28, also preventing bone cement or porous ingrowth material from inhabiting this space upon implantation. In an exemplary embodiment, ridges346,348are elevated above the surrounding surfaces of recessed pocket338by the same amount as rail337, i.e., between 1.1 and 1.2 mm.

Referring still toFIG.14B, lateral and medial ridges346,348define ridge sidewalls disposed entirely anterior or posterior of the periphery of pegs28, (i.e., as viewed “from the side” in a sagittal plane or “from the top” in a transverse plane). Thus, no portion of the sidewalls of ridges346,348is inaccessible to blade352of osteotome350as blade352enters from rail237and sweeps along a medial-to-lateral or lateral-to-medial direction. Accordingly, blade352can reach every other portion of recessed pocket338via rail237surrounding outer periphery of femoral component220in similar fashion as described above. Accordingly, femoral component220may be fully dislodged from femur F prior to removal therefrom during revision surgery.

Similar to recessed pocket336discussed above, recessed pocket338is also larger than alternative devices by up to 40%, thereby providing a larger fixation area for more robust fixation to the distal femur. More particularly, in an exemplary embodiment femoral component220may have a total fixation area within recessed pocket338of as little as 2128 mm3for a small-size prosthesis and as much as 4780 mm3for a large-size prosthesis, representing between 77% and 85% of the total aggregated surface area of bone-contacting surfaces50,52,54,56,58across all prosthesis sizes. Advantageously, this range of surface area coverage represents an increase in surface area coverage of at least 15%, as compared to comparable prosthesis sizes in the above-mentioned prior art posterior-stabilized prostheses.

While the disclosure has been described as having exemplary designs, the present disclosure can be further modified within the spirit and scope of this invention. This application is therefore intended to cover any variations, uses or adaptations of the disclosure 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 disclosure pertains.