Tibial prosthesis having a U-shaped intramedullary stem

A tibial prosthesis which interacts with a femoral prosthesis to provide a knee joint replacement is disclosed. The prosthesis comprises a tibial plateau platform provided with an intramedullary stem having a generally U-shaped cross section allowing for retention of the cruciate ligaments of the knee joint while still achieving adequate anchorage of the device to the proximal end of the tibia.

BACKGROUND OF THE INVENTION 
The present invention relates to a tibial prosthesis which interacts with a 
femoral prosthesis to provide a knee joint replacement. The prosthesis is 
intended to be implanted in the proximal end of the tibia. The prosthesis 
of the present invention utilizes an intramedullary stem having a 
generally U-shaped cross section. The U-shaped cross section stem allows 
for retention of the cruciate ligaments of the knee while also increasing 
stability of the tibial prosthesis component. 
The natural knee joint comprises the bottom (distal) end of the femur and 
the upper (proximal) end of the tibia. Bearing action occurs between two 
condyles on the lower surface of the femur and the complementary upper 
surface plateaus of the tibia, separated by intermediary cartilage pads, 
the meniscii. Connection of the tibia to the femur is provided by means of 
ligaments, including the cruciate ligaments of the knee, which are strong 
thick bundles situated between the condyles. 
Movement of the normal knee is complex, including rolling, gliding and 
axial rotational motions. Not only do the cruciate ligaments provide 
connection between the femur and the tibia, assure joint stability and 
help to absorb stresses applied to the knee but they are also largely 
responsible for providing the proper combination of rolling and gliding 
motions and transitions between such motions which characterize the normal 
knee action. 
Hip joint prosthetic devices are becoming relatively common and have had a 
fairly good record of success. Knee joint implants are of more recent 
design, have generally been less successful than hip implants and their 
long term stability has not yet been proven. The knee joint is subject to 
greater stresses than any other joint in the body. It must support the 
entire weight of the body above the knee and must do so throughout the 
various relative angular relationships of femur and tibia. Ideally, a knee 
joint prosthesis should provide the same action as the natural human 
knee--a complex combination of rotational, rolling and sliding/gliding 
movements. 
Until recently, endoprosthetic knee joint devices comprised separate 
femoral and tibial components linked together with a coupling pin in the 
form of a mechanical hinge, having a single axis of rotation, fixed to the 
femur and the tibia, respectively, each component having a long 
intramedullary stem for bone fixation. These so-called hinge-type devices 
were not always duplicative of knee joint biological hingings since they 
had a single axis of rotation while the human knee joint which these 
devices sought to emulate involves polycentric movements. Thus, many 
variations of hinges, spindles, ball and socket and double hinges were 
used in an attempt to duplicate the complex natural knee motions; however, 
none of the simplified approximations could accurately do so. A common 
feature of these so-called hinged implants was that they provided positive 
mechanical connection between the femoral and tibial components so that 
the natural connective ligaments did not need to be and, in fact, could 
not be retained. Also, because of the bulk of these devices, it was 
necessary to remove a considerable amount of natural bone to allow space 
for the device, such removal reducing bone reserve which might be needed 
for future corrective measures. 
A further disadvantage of the hinge-type devices was their limited axial 
rotation causing direct transmission of end limit forces through the 
structure thereby tending to loosen fixation of the femoral and tibial 
components. For this reason, practically all of these devices used 
all-metal, deep-bone penetrating intramedullary stems for purposes of 
fixation. 
Examples of these hinge-type knee prosthetic devices can be found in 
Lagrange U.S. Pat. No. 3,688,316; Bousquet U.S Pat. No. 3,696,446; 
Goldberg U.S. Pat. No. 3,765,033; Findlay U.S. Pat. No. 3,886,601; 
Lagrange U.S. Pat. No. 3,918,101 and Arkangel U.S. Pat. No. 4,001,896. 
The newer implantable knee devices generally mechanically uncouple the 
femoral and tibial components; instead of connecting the components by a 
pivot pin or other mechanical linkage they are held in mutual bearing 
engagement by the biological structure of the knee, that is, by indirect 
coupling through the muscular, capsular and ligamentous components of the 
natural joint. These devices usually comprise a combination of a femoral 
device constructed of metal with spaced runners to replace the natural 
condyles, and a plastic tibial device to replace the natural plateau, the 
devices having usually convex and concave mating surfaces, respectively. 
SUMMARY OF THE INVENTION 
The present invention relates to these newer type knee devices of the 
non-hinged type and further relates only to the tibial component thereof. 
The device of the present invention can be used in combination with a 
large number of femoral components. Some exemplary femoral devices are 
disclosed in Link U.S. Pat. No. 3,715,763; Averill U.S. Pat. No. 
3,728,742; Helfet U.S. Pat. No. 3,748,662; Marmor U.S. Pat. No. 3,852,830 
and Lee U.S. Pat. No. 3,958,278. 
The tibial prosthesis of the present invention comprises a tibial plateau 
platform provided with an intramedullary stem having a generally U-shaped 
cross section thereby allowing for retention of the cruciate ligaments of 
the knee joint while still achieving adequate anchorage of the device to 
the proximal end of the tibia.

DETAILED DESCRIPTION OF THE INVENTION 
The present invention relates to the tibial component of a non-hinged knee 
prosthesis. A problem encountered in the designing of these devices has 
been the apparent need to compromise between the provision of firm and 
lasting mechanical attachment of the components to the leg bones on the 
one hand and the need for retention of the natural ligaments on the other. 
Of particular concern are the cruciate ligaments which are probably the 
most important ligaments of the knee joints. Unfortunately, these cruciate 
ligaments work through the center of the knee so that retention of these 
important ligaments precludes the usage of the intramedullary stem 
attachment which was provided on the majority of hinged-type knee 
replacements since these stems or spikes would also need to be located at 
the center of the components. In other words, it appeared impossible to 
have both an opening for the cruciate ligaments in the center of the 
implantable components and intramedullary fixation stems in the center of 
the components. 
The problem was met in the case of the femoral component since, because of 
its wrap-around design, it could make good geometric and area contact with 
the end of the femur. In addition, the knobbed end of the femur is quite 
large and thus provides sufficient bone around the cruciate ligaments so 
that several smaller stems can be securely placed within solid bone. 
Further, the femoral component is usually made of metal and hence is not 
particularly subject to wear or breakage. 
The tibial component presented more of a problem. Being quite small and 
being normally made of plastic, it does not have the mechanical stability 
and strength of the metal femoral component. Examination of the prior art 
shows that various protrusions, short pegs and keying designs were 
provided, mainly in an effort to increase the contact surface area so that 
the bone cement used to adhere the component to the tibia would be more 
effective. However, it is well known in the art that bone cement does not 
guarantee firm and lasting adhesion to bone. Further, these devices are 
frequently employed in treatment of arthritic conditions where bone 
quality is generally poor and usually subject to even further 
degeneration. The means for attachment of these prior art tibial devices 
is so shallow as to provide a poor moment arm to resist displacement 
torque yet these devices are used in a joint which is subject to 
substantial tensional loadings. 
Essentially, the surgeon understood that a problem existed. However, if the 
cruciate ligaments were sound, it was felt to be prudent to retain the 
ligaments and use a potentially unstable tibial component with full 
knowledge that a high proportion of operating stresses would have to be 
carried by the ligaments. Then, if the device loosened at a later date, 
the cruciate ligaments could be removed at that time and a more stable 
hinged knee prosthesis having an intramedullary stem substituted. 
The present invention provides a tibial component 10 which substantially 
resolves the problem and eliminates the need for compromise. The tibial 
component of the present invention provides for both cruciate ligament 
retention and intramedullary stem attachment. Referring more particularly 
to the drawings, it will be seen that the tibial component 10 is provided 
with an intramedullary stem or spike 11 which has a generally tapered 
U-shaped cross section so that such spike 11 can be attached to the tibial 
plateau platform 12 around the periphery of a central opening 13 which 
allows for passage of the important cruciate ligaments. The attachment 
area is U-shaped so that the central opening 13 remains fully open 
posterially to allow for passage of and not to impede the action of the 
cruciate ligaments. 
It is preferred that the spike 11 be of constant "radius" as it diminishes 
in width toward a tapered point 14 so that the spike 11 can be easily 
driven or forced into the intramedullary cavity of the resected tibia. 
Similarly, the cross section can diminish in thickness toward the point 14 
because the need for strength and stiffness diminishes with distance from 
the plateau platform 12. Thus, spike 11 will be inclined upwardly and 
outwardly from the point 14 to its area of attachment to tibial plateau 
platform 12. It is preferred that the penetration tip of the spike 11 be 
flattened or slightly rounded rather than sharp and pointed. While it is 
preferred that the intramedullary stem have a generally U-shaped cross 
section, it should be understood that variations such as a crescent shaped 
channel are also contemplated. It is, however, preferred to use a cross 
sectional form which is other than a straight flat shape as such would be 
more subject to bending and hence be less stiff although portions of the 
spike, particularly near the point, could be flat. 
Since plastic does not generally possess the mechanical stability, 
stiffness and strength of metal, it is preferred to make the spike 11 of 
metal, preferably of medical grade cobalt-chromium alloy. Alternatively, 
stainless steel or titanium may be used. For similar reasons, the plateau 
platform 12 or at least the underlying portion of the platform which is 
directly attached to the spike 11 should also be made of similar metal. 
The bearing surface 26 of the tibial device 10 which must react against 
the condylar surfaces of a mating femoral device (not shown) should 
preferably be made of a durable, low-friction, physiologically acceptable 
plastic, as for example, Ultra High Molecular Weight Polyethylene. The 
plastic bearing inserts 25 should be firmly attached to the metal plateau 
platform 12 and be suitably restrained from movement. 
The plateau platform 12 is generally U-shaped as will clearly be seen in 
FIG. 3. As presently preferred, each leg portion 15 is recessed at its 
juncture with the thicker body portion 17 of the U to provide a flat, 
generally D-shaped platform 16 for bearing insert 25. A pair of spaced 
apertures 18 is provided in each platform 16. Although the apertures 18 
are shown as being square, other shapes are contemplated provided that the 
aperture has at least one straight edge for a purpose to be described 
hereafter. Each aperture 18 is formed with a depending ear 19 along one 
edge thereof. Each platform 16 is formed with an upwardly extending 
retainer wall member 20 along the interior of each leg portion 15. The 
portion of the wall member 20 adjacent platform 16 is smoothly upwardly 
arcuate (convex) while the opposite side follows the contour of the spike 
11 and thus is angularly contoured. (See FIG. 3). An arcuately contoured 
upwardly extending retaining ridge 21 is provided along the edge of each 
platform 16 at the juncture of the leg portion 15 with the body portion 17 
of the U. It will be appreciated that numerous other means for removably 
attaching the bearing inserts 25 to plateau platform 12 are possible and 
such alternatives are considered to be within the scope of the present 
invention. 
It is preferred that the surfaces of the spike 11 be texturized in some 
fashion as with grooves, perforations, ridges or the like to increase the 
interaction of same with the bone of the tibia or with intervening bone 
cement. Even more preferred would be the provision of porous surfaces so 
as to permit for bone ingrowth, as is known in the art. It is also 
preferred that portions of the underside of the tibial plateau platform 12 
be provided with similar surface texture or porosity. 
Bearing insert 25 is generally kidney shaped and is sized to fit onto 
platform 16 and within the confines of retainer wall member 20 and 
retaining ridge 21. The outer portion of the upper surface of bearing 
insert 25 is scooped out to form a smoothly arcuately concave bearing 
surface 26 for the mating condylar surface of a femoral prosthesis (not 
shown). The inner edge of bearing insert 25 is provided with an arcuate 
recess 27 shaped complementarily to the smoothly upwardly arcuate contour 
of the wall member 20. The bottom surface of bearing insert 25 is 
substantially flat. In order to allow bearing insert 25 to fit onto 
platform 16, the portion of the bottom surface of said insert extending 
beyond the recess formed in the leg portion 15 of platform 16 is removed 
to a depth correspondng to the depth of said recess. The thus undercut 
portion forms ledge 28 which fits and lies upon body portion 17 of the U 
when bearing insert 25 is positioned onto platform 16. Projecting from the 
bottom surface of insert 25 and positioned in alignment with the spaced 
apertures 18 in platform 16 are a pair of spaced pegs 29. The free end of 
each peg 29 is larger than the peg itself such that a ridge 30 is provided 
along at least one edge of the peg. In the illustrated embodiment, ridge 
30 is provided along two opposite edges although only the interior ridge 
30 is required. As clearly shown in FIG. 2, ridges 30 engage the depending 
ears 19 of the corresponding apertures 18, thus securely attaching insert 
25 to platform 12. 
In order to facilitate insertion of pegs 29 into apertures 18, the leading 
edges of the enlarged ends of peg 29 are chamfered as shown in the 
drawings. 
The tibial prosthesis of the present invention, in combination with a 
femoral prosthesis having a medial slot for passage of the cruciate 
ligaments, provides a prosthetic device affording substantial congruence 
between the femoral condyles and the tibial plateau thus providing the 
rotational, sliding, rolling and gliding action of a functional knee.