Great toe joint implant and method of implantation

A nonconstrained, total great toe joint implant for the metatarsophalangeal joint made of a first component with a convex, partially spherical surface ending in a rear surface from which a longitudinally asymmetric implantation stem projects and having a flange on the dorsal side of the implant which extends the convex surface past the rear surface. The rear surface is inclined 10.degree. relative to a normal plane which intersects this surface. The metatarsal bone is resected accordingly. The phalangeal implant component is made of a base with a stem for placement in a bone cavity, projecting from a rear side thereof and having a low-friction, concavely curved insert affixed to the base which slidably engages and cooperates with the convex surface of the metatarsal implant component. The base has an outline corresponding approximately to the outline of the resected surface on the phalangeal bone to eliminate bone-overhang and bony overgrowth that may result from such an overhang and which can compromise the proper functioning of the implant. The metatarsal implant component has right foot and left foot configurations resulting from the relative positioning and orientation of the asymmetrically shaped stem projecting from the rear surface thereof.

BACKGROUND OF THE INVENTION 
The present invention relates to a nonconstrained prosthetic implant for 
the replacement of metatarsophalangeal joints. 
The replacement of degenerated natural joints with man-made prosthetic 
replacements is well known, including the replacement of the 
metatarsophalangeal joint (MPJ) of the great toe. One such replacement, to 
which the present invention relates, uses a two-piece, nonconstrained 
prosthesis to replace the first MPJ where the metatarsal head is resected 
and replaced with a metal component and the proximal phalanx is resected 
and replaced with a metal-backed polyethylene component. 
The early implants for the great toe were hemiarthroplasty devices, where 
only one side of the joint was replaced-either the metatarsal head or the 
proximal phalanx. As early as the 1950's, one Swanson developed a metal 
replacement for the metatarsal head, fixed with an intramedullary stem. 
The procedure was deemed unsuccessful, and failure was attributed to 
resorption of bone around the stem. In the mid-1960's, Swanson became 
aware of a medical-grade silicone developed by Dow Corning. First, he 
tried replacing the metatarsal head with the material, but because of high 
shear and compressive loads across the first MPJ during gait, the 
prosthesis failed. Because of the failures incurred by using a 
hemiarthroplasty on the metatarsal side, he turned his attention to the 
proximal phalanx. He designed a silicone implant having an intramedullary 
stem and proximal portion that acted as a spacer, replacing the bone which 
is resected in a Keller procedure. Because the implant was only acting as 
a spacer, it lacked full functional motion and caused severe reactions of 
the surrounding bone due to debris thought to be generated by wear between 
the implant and metatarsal head. 
In the 1970's, a double-stem hinged implant similar to one for the 
metacarpophalangeal joint (base of the finger) was developed using a new 
High Performance Silastic Elastomer. Various forms of such constrained, 
silastic implants have been in use ever since, and they have been 
relatively successful. Pain relief is obtained in almost all cases, 
patients have the ability to walk, and functional scores have been shown 
to increase. While the range of motion has increased compared to the other 
available options (fusion, Keller procedure), a fully functional toe has 
not been the result. Often, due to the constraining nature of the devices, 
the implant pistons in and out of the medullary canal, causing debris 
formation and synovitis. In view of the recent silicone breast implant 
scare, there is now much concern about the use of silicone-based implants 
in the hand and foot. Thus, the two-piece metal on poly designs that have 
worked so well in the hip and knee areas are now being considered by many 
for MPJ replacements. 
The somewhat unconstrained implants on the market today have a metatarsal 
component (generally cobalt chromium alloy) with a convex articulating 
surface that mates with a concave medical-grade ultra high molecular 
weight polyethylene (UHMWPE) metal-backed (generally titanium alloy) 
phalangeal component. Both components utilize an intramedullary stem for 
stability and fixation purposes. 
A great toe replacement, developed by one Richard Koenig, DPM, and marketed 
under the trademark Biomet, utilizes a medial surgical approach to address 
the plantar articulation of the sesamoids with the plantar surface of the 
metatarsal head. The metatarsal component wraps around the dorsal to 
plantar aspect of the metatarsal head. This replacement, with its somewhat 
difficult surgical implantation technique, was introduced in the late 
1980's. The dorsal flange of this replacement allows a normal range of 
dorsiflexion, but may cause interfered motion of the sesamoid/plantar 
metatarsal articulation. While there is unconstrained motion from dorsal 
to plantar, the motion of this implant is in fact semi-constrained because 
of the flatter radius of curvature medial to lateral, which inhibits the 
motion. 
In the early 1990's, podiatrist Kerry Zang developed a simpler replacement 
which is available from MicroAire Surgical Instruments, Inc. of Valencia, 
Calif., under the trademark Bio-Action. Neutral cuts (generally 
perpendicular to the long axis of the bone) are made on both the phalanx 
and metatarsal head. The metatarsal head is replaced with less than a 
hemisphere of a somewhat spherical surface, while the phalangeal base is 
covered by a circular metal-backed polyethylene insert. The range of 
motion during walking is limited due to the absence of a dorsal flange; 
however, the sesamoid/metatarsal articulation is relatively undisturbed. 
This implant, due to its generally spherical surface, is unconstrained. 
Its medial to lateral alignment is maintained by the soft tissue and not 
by the implant design. 
In early 1993, another great toe implant became available from Acumed. This 
implant combines some of the positive aspects of the two previously 
discussed implants--the dorsal flange of the Biomet-Koenig implant with 
the noninvasive plantar aspect of the Bio-Action implant. The Acumed toe 
is similar to the Koenig implant because it is also semi-constrained due 
to a flatter radius limiting the motion in the medial to lateral 
direction. Both the Acumed and Biomet toes require a proper, perpendicular 
alignment of the neutral cut with the convex surface of the metatarsal 
implant component. 
SUMMARY OF THE INVENTION 
The great toe implant of the present invention uniquely combines a number 
of positive design features to provide a totally unconstrained implant 
having an optimal range of motion dorsally without interference of the 
plantar metatarsal/sesamoid articulation. The implant procedure employs a 
10.degree. resection relative to the distal surface or neutral plane of 
the metatarsal head. This allows for uninterrupted motion of the 
metatarsal/sesamoid articulation, requires minimal bone resection, and 
rotates the articulating surface of the metatarsal head 10.degree. 
dorsally to provide a more normal range of motion during gait. In 
combination, the somewhat spherical surface dorsally provides for 
unconstrained motion medial to lateral as well as plantar to distal. 
Because of the sphericity of the articulating surface, the initial 
resection is not alignment sensitive (medial to lateral), as are some of 
the known, semi-constrained implants mentioned above. While known implants 
and the implant of the present invention have intramedullary stems to 
provide stability and fixation, the intramedullary stems of the implant of 
the present invention are uniquely adapted to achieve maximum stability 
and optimal fixation. The plantar surface of the metatarsal stem is 
inclined to follow the anatomic inclination of the metatarsal shaft, and 
the stem is alignment insensitive by providing for the anatomic right and 
left angles of the metatarsal shaft. The stem is squared, providing 
additional torsional stability (the dorsal flange also prevents axial 
rotation of the component). Grooves are further provided on both the 
metatarsal and phalangeal stems to provide for bony ingrowth or cement 
interdigitation. The grooves on the phalangeal component prevent axial 
rotation and inhibit pull-out of the stem itself.

DESCRIPTION OF THE PREFERRED EMBODIMENTS 
Referring to FIGS. 1-3, a great toe implant 2 constructed in accordance 
with the present invention forms a nonconstrained joint between the 
metatarsal bone 4 and phalangeal bone 6 of a great or big toe 8 so that 
the toe can be deflected in dorsiflexion (FIG. 2) or plantarflexion (FIG. 
3) as well as side to side (in FIG. 1 but not illustrated). 
Referring to FIGS. 1-8, the implant has a metatarsal component 10 and a 
phalangeal component 12 which slide and move relative to each other along 
their convex and concave bearing surfaces 14, 16. 
The metatarsal component has a head 18 which, on one side, defines the 
convex surface which terminates in a rear face 20, a portion 22 of which 
is angularly inclined relative to a line perpendicular to the rear face by 
an angle .alpha. of about 55.degree. to define a flange 24 which extends 
the convex surface rearwardly past the rear face 20 as best seen in FIGS. 
6 and 7. Flange 24 is located at the dorsal side of implant. 
The convex surface 14 is spherically shaped except for a relatively small 
section 26 at the plantar side which has a slightly lesser radius (as 
indicated in FIG. 16) than the major part of the convex surface. Further, 
the plantar side of the head is truncated to define an end wall 28 which 
is inclined relative to the rear face 20 of the head by an angle .beta. of 
about 80.degree. to maintain the anatomic articulation of the sesamoids 
(shown in FIG. 18 only). FIG. 6 schematically illustrates the portion 26 
of reduced curvature radius in the area where that portion slopes away 
from the circular phantom line of the circle defining the spherical 
curvature of the major portion of the convex surface. The reduced 
curvature portion 26 of the convex surface 14 lessens pressure on the 
underlying sesamoid and thereby enhances the comfort of the implant. 
A stem 30 projects from the rear face 20 of the implant away from the 
convex surface 14, and it determines whether the metatarsal implant 
component 10 is intended for the right foot or the left foot. The stem has 
the shape of a longitudinally asymmetric, truncated pyramid and includes a 
side 32 inclined with respect to a line perpendicular to rear face 20 of 
the implant by about 13.degree. (or inclined about 103.degree. with 
respect to the rear face 20 as shown in FIG. 12). As FIGS. 12 and 15 
illustrate, this side 32 is mirror-imaged on the left and right foot 
implants. By providing left and right foot implants, a relatively larger 
stem can be utilized so that a more secure and long-lasting stem-bone 
connection is obtained. Preferably, the stem includes surface grooves 34 
which become filled with bone cement or, over time, will become filled 
with bone tissue to better anchor the stem. 
Preparation of the metatarsal bone for the metatarsal implant component 
includes a 10.degree. resection of the metatarsal head relative to the 
distal surface of the metatarsal, as is further discussed below. This 
minimizes the bony resection of the metatarsal head and leaves the plantar 
surface of the metatarsal head intact. The component is designed to sit at 
the 10.degree. angle with the dorsal surface of the stem parallel to the 
axis of the metatarsal. By providing left and right foot implants, the 
stem optimally fits the shape of the medullary canal of the metatarsal 
bone. 
Referring to FIGS. 1-11, the phalangeal component 12 includes a generally 
kidney-shaped base 36, a rear side 38 of which is placed against the 
resected phalangeal bone and from which a generally cylindrical stem 40 
projects. The stem includes longitudinal grooves 42 (not shown in FIGS. 4 
and 5) to prevent rotation of the stem, and therewith of the base, when it 
is implanted in the bone canal. Alternatively, stem 40 can be given a 
noncylindrical; e.g. a triangular, square, rectangular or other polygonal, 
cross-section (not shown). The stem may also include circumferential 
grooves 45 to facilitate the cementing of the implant and/or bony ingrowth 
into the grooves to provide a secure connection to the bone. 
A generally cylindrical (in cross-section) insert 44 constructed of a 
relatively low-friction material such as polyethylene is nonmovably 
affixed to the side of the base opposite the stem so that the periphery of 
insert 44 is spaced inwardly from the periphery of the base. The insert 
includes a concavity 46 which is spherically shaped at a radius just 
slightly (e.g. in the order of 0.005 to 0.010 inches) larger than the 
radius of curvature of the main portion of convex surface 14 of the 
metatarsal implant to facilitate rotation and a slight translation of the 
convex metatarsal implant surface. By forming the cooperating convex and 
concave surfaces 14, 46 of the two implant components spherical, a full 
range of motion for the joint is assured. In addition, forces acting 
between the cooperating spherical surfaces are relatively evenly 
distributed over the surfaces to lower bearing pressures and thereby 
prevent undesirably high line pressures generated, for example, by 
implants having cooperating surfaces with differing surface 
configurations, as are encountered on some of the known implants mentioned 
above. 
Base 36 of the phalangeal implant is shaped so that its rear side 38 has 
substantially the same shape and size as resected phalangeal surface 60; 
e.g. it is somewhat kidney-shaped, as best seen in FIG. 11. By making the 
base of the implant component about the same size as the resected surface, 
the heretofore frequently encountered bone-overhang; i.e. the extension of 
the resected surface past the periphery of the implant base, is 
eliminated. Such an overhang, if present, can lead to bony overgrowth, 
which, in time, can interfere with the proper functioning of the implant. 
To avoid such a possibility on phalangeal implant components which have 
the heretofore more common cylindrical (round) base, the overhanging bone 
has sometimes been resected. Such resection is not only time consuming, it 
also involves an undesirable loss of bone structure and a resulting 
weakening of the phalangeal bone. This aspect of the present invention 
eliminates both. 
The metatarsal component of the present invention can be made of any 
suitable material. However, cobalt chromium, a hard material which has 
good wear characteristics, is presently preferred. The phalangeal 
component is preferably made from medical-grade titanium alloy, for 
maximum bone apposition, and the insert is preferably made from ultra high 
molecular weight polyethylene (UHMWPE) to provide a bearing surface that 
is optimal for articulation with the cobalt chromium of the metatarsal 
component. 
Further, it is preferred to provide a number of different sized components; 
e.g. three or four sizes, for both the metatarsal component and the 
phalangeal component to take into account different toe sizes. In 
addition, as discussed above, the metatarsal component is made in left and 
right configurations. 
Turning now to the implantation of the implant of the present invention, a 
dorsal linear incision is initially made over the first metatarsal 
phalangeal joint parallel to the extensor hallucis longus tendon to expose 
and provide access to the metatarsal head and base. After the removal of 
any osteophytes from the dorsal surfaces of the proximal phalanx and 
distal metatarsal, the ends of the bones are resected. 
Referring particularly to FIG. 18 (not drawn to scale), the metatarsal bone 
4 is resected to form a first resected surface 48 which is angularly 
inclined, preferably by about 10.degree., relative to a neutral cut or 
normal plane 50 which is perpendicular to the long axis of the bone and 
which typically intersects both the first resected plane 48 and the 
sesamoid 52 beneath the metatarsal bone. A further resected surface 54 is 
formed on the metatarsal bone which is angularly inclined relative to 
normal plane 50 by about 45.degree. or 35.degree. relative to rear face 
20, and which is sized and shaped so that rear face 20 and portion 22 
thereof abut against resected surfaces 48, 54, respectively, upon the 
implantation of the implant. 
Next the medullary canal (not separately shown in the drawings) is broached 
out to define interior bone surfaces that engage stem 30 of the metatarsal 
implant. To do so, the bone is broached to define an interior surface 53 
on the dorsal side of the canal for engaging a dorsally facing stem 
surface 55 that is substantially perpendicular to normal plane 50 and, 
therefore, angularly inclined relative to rear face 20 by about 
80.degree.. The medullary canal of the metatarsal bone is broached so that 
it additionally defines a surface 56 on the plantar side of the canal 
which has the same angularity as stem surface 57 on the plantar side 
thereof; i.e. an angle of 103.degree. relative to the rear face 22 of the 
implant. 
The phalangeal bone is resected to define a resected surface 60 which is 
generally perpendicular to the floor, and therefore parallel to normal 
plane 50, so that the rear side 38 of the phalangeal implant 12 can abut 
it when implanted. Further, the phalangeal bone is appropriately broached 
to form a cavity 62 therein into which the stem 40 of the phalangeal 
implant fits snugly. 
Following resection, the implant components are implanted by pushing and 
tapping the stem of each component into the bone cavity until rear faces 
20, 22 of the metatarsal implant and rear side 38 of the phalangeal 
implant abut the respective resected surfaces 48, 54 and 60. The implants 
are conventionally secured to the bone in any desirable manner such as 
with cement, frictionally and/or by way of bony ingrowth. 
It will be noted that the 10.degree. inclination of rear face 22 relative 
to normal plane 50 and the rearwardly extending flange 24 of the 
metatarsal implant provide a number of advantages. First, the 10.degree. 
angular inclination of the rear face of the metatarsal implant, coupled 
with the geometry of the stem as described above so that it is effectively 
aligned with the medullary canal of the metatarsal bone, rotate the 
metatarsal implant upwardly (dorsally) and, in conjunction with flange 24 
and the rearwardly extended concave surface 16 formed by it, extend the 
range of dorsiflexion of the implant, where a maximum range of motion is 
needed, by about 45.degree.. This is particularly important because the 
implant is subjected to maximum forces during toe-off. At the same time, 
the stem of the metatarsal implant component is oriented so that it 
follows the inclination of the metatarsal bone. 
Still further, the 10.degree. inclination of the rear face of the 
metatarsal component limits the amount of bone that must be resected and 
rotates its plantarly facing end away from the underlying sesamoidal bone. 
As a result, except during plantar flexion, which occurs relatively less 
frequently and during which the joint is normally not heavily stressed, 
the natural metatarsal bone overlies the sesamoid and need not be resected 
to properly receive the metatarsal implant component, which enhances the 
performance of the implant.