Anatomically neutral joint

A joint prosthesis is disclosed with stems affixed to the flexing main body at a natural, angle; with positive flexion and extension stops; and with stress-relieving radii at intersections between the stems and the main body.

BACKGROUND OF THE INVENTION 
This application claims the benefit of U.S. Provisional application Ser. 
No. 60/017,477, filed Apr. 26, 1996. 
FIELD OF THE INVENTION 
This invention relates to surgically implantable prosthetic finger joints 
and a design for such joints which permits a wider range of use and 
provides a more natural appearance. The joints of this invention exhibit 
an anatomically neutral resting position and are fitted with anatomically 
correct flexion limits. 
DESCRIPTION OF THE PRIOR ART 
U.S. Pat. No. 4,871,367, issued Oct. 3, 1989 on the application of 
Christensen et al., discloses a prosthetic knuckle implant wherein the 
implant stems are in a straight line and the flexion limits are 
incomplete. 
U.S. Pat. No. 3,462,765, issued Aug. 26, 1969 on the application of Swanson 
discloses a straight rubber joint prosthesis for implantation into hands. 
U.S. Pat. No. 3,993,342, issued Jul. 20, 1971 on the application of 
Niebauer et al, discloses a straight flexible plastic joint prosthesis 
having a narrowed body to facilitate flexibility and movement of the 
joint. 
Russian Patent No. RU 2,026,653, published Jan. 20, 1995, discloses a 
prosthetic knuckle joint in which the stems of the joint are at an angle 
with one another. 
SUMMARY OF THE INVENTION 
A surgically implantable, anatomically neutral, joint, comprising a 
one-piece main body of flexible, elastomeric, material including first and 
second spaced-apart end sections with first and second outer faces and a 
thinner web section extending between and joining said end sections for 
flexing movement about a particular flexing axis through the web section 
section, whereby the first and second outer faces are diverging toward the 
top of the main body and the main body functions as a hinge with said end 
sections and said web section together defining a v-shaped groove across 
the top of the main body, having first and second v-shaped groove sides 
converging toward each other, and extending into the main body; and a 
key-hole shaped groove across the bottom of the main body, having a 
key-hole hole segment of circular cross-section through the main body 
defining the bottom of the web section, and first and second key-hole side 
segments converging toward each other, extending into the main body, and 
opening into said key-hole hole segment between said end sections; and 
first and second stems of flexible, elastomeric, material connected to and 
extending out from and perpendicular with the first and second end 
sections of said main body in directions normal to said flexing axis and 
away from one another, said first and second stems being implantable 
within cooperating intramedullary canals of adjacent bones, defining said 
joint.

DETAILED DESCRIPTION 
In the art of surgically implantable prostheses, there is a continual 
search for devices which provide more natural appearance and more 
convenient, efficient, and long-lasting use. 
The prosthetic joint of this invention permits a natural appearance by its 
anatomically neutral design. By "anatomically neutral" is meant that the 
joint is made such that a hand or finger utilizing the joint is in a 
normal, slightly flexed, attitude at rest. For example, the proximal 
phalanx relative to the metacarpal is 20 to 30 degrees in flexion. 
Anatomic neutrality is important not only for the sake of appearance but, 
also, because it provides significant functional benefit. The usual, 
straight-through, prosthetic knuckle device with the stems set at 180 
degrees, requires that the phalangeal stem portion of the joint must be 
rotated nearly 30 degrees in flexion and maintained in that position by 
constant use of muscle force just to achieve a natural appearance. The 
present device achieves a natural appearance with muscles relaxed. In 
addition, the present device permits full flexion using considerably 
reduced energy because the present device is at rest within, and near the 
middle of, the desired range of motion rather than at the extreme, near 
full extension. 
The device of this invention finds utility joining metacarpal and proximal 
phalanx bones as a prosthetic metacarpal phalangeal joint and, also, 
joining proximal phalanx and middle phalanx bones as a prosthetic proximal 
interphalangeal joint. 
Referring now to FIG. 1, surgically implantable finger joint 10 of this 
invention comprises a one-piece main body 11 with spaced-apart end 
sections 12 and 13--each having outer faces 14 and 15, respectively. Main 
body 11 has a thin web section 16 between end sections 12 and 13. It is 
web section 16 which provides for flexing or hinge movement of the joint 
and which includes flexing axis 17 extending through web section 16. 
Stems 18 and 19 are connected to end sections 12 and 13, respectively, and 
are adapted for implantation within cooperating intramedullary canals in 
adjacent bones. Stems 18 and 19 have longitudinal axes 18a and 19a, 
respectively, and those axes are perpendicular with outer faces 14 and 15, 
respectively. Stems 18 and 19 can have round, square, or other 
cross-sectional shape and are generally sized to be small at the outer end 
and larger closer to the end sections. 
The inventors herein have discovered that breakage of the stems from the 
main body, which has in the past been a considerable problem, can be 
greatly mitigated or eliminated by providing radii of at least 0.07 
centimeter at the intersections 20 of the stems with the outer faces. It 
has been determined that sharp corners at intersections 20 greatly 
increases the likelihood of rupture and tear propagation between the stem 
and the main body and that a radius of 0.07 to 0.13 centimeter 
dramatically relieves stresses in the material at those points and, 
thereby, reduces the likelihood of failure. A radius of about 0.075 
centimeter is preferred. 
In order to maintain a natural angle for the finger joint, main body 11 is 
constructed to have outer faces 14 and 15 diverging upward toward the top 
of the main body at an angle of 20 to 40, preferably about 30, degrees. 
Having such an angle, at rest, permits a user to both, flex and extend the 
knuckle using muscle forces. Starting from a knuckle position which is 
straightened at 180 degrees, it is desirable that 5 to 40, preferably 10 
to 20, degrees of further extension should be possible; and, with that in 
mind, v-shaped groove 21 across the top of main body 11 is made having 
v-shaped groove sides 22 and 23 converging toward each other into main 
body 11 at an angle of 25 to 80, preferably 45, degrees. As the knuckle is 
extended, groove sides 22 and 23 are moved closer together and the 
extension limit is reached when the groove sides meet. In the neutral or 
resting position of the knuckle, groove sides 22 and 23 can join at the 
end of the groove in a radius 24 which is 0.07 to 0.23, preferably 0.17, 
centimeter. 
Again, starting from a knuckle position which is straightened at 180 
degrees, it is desirable that 85 to 95, preferably 90, degrees of flexion 
should be possible. Key-hole shaped groove 25 across the bottom of main 
body 11 is made having a key-hole hole segment 26 of circular 
cross-section through main body 11. Hole segment 26 defines the bottom, 
and radius 24 of v-shaped groove 21 defines the top of web section 16. 
Key-hole side segments 27 and 28 converge toward hole segment 26 at an 
angle of 50 to 85, preferably 60, degrees. As the knuckle is flexed, side 
segments 27 and 28 are moved closer together and the flexion limit is 
reached when the side segments come together. 
The key-hole construction is used for flexion due to the desire for a clear 
limit to flexion extreme. Key-hole hole segment 26 permits compression of 
web section 16 without interference by material at the bottom of the 
key-hole shaped groove and contact of side segments 27 and 28 provides a 
sure limit to the range of movement. V-shaped groove 21 can, also, be 
constructed to include a key-hole hole segment. However, because the 
primary function for implanted finger joints is to restore grasping, 
flexion is more important than extension and there is less need for a sure 
limit to the range of movement for extension of the knuckle. The key-hole 
construction is, therefore, not so important for v-shaped groove 21. 
The nature of the material used in construction of main body 11 and the 
thickness of web section 16 can be used to control flexing stiffness of 
the joint. Suitable elastomeric materials are resilient, biocompatible and 
resistant to tearing. Such elastomeric materials may include silicone 
rubber, polyurethane rubber, polyurethane urea rubber, rubber reinforced 
polypropylene, polycarbonate based polyurethane, and the like. One 
characteristic by which an elastomer can be evaluated is called Durometer 
hardness. Durometer hardness is defined by ASTM test number D-2240. In 
general, the hardness of an elastomer is reported as a number and a Shore 
designation. The number is on a scale of 10 to 100, the higher the number 
the harder the elastomer. The Shore designation determines the hardness 
test method and equipment, and is designated by letters A through D. 
Generally, as the Shore A hardness of a material increases, the modulus of 
elasticity for that material also increases--leading to a stiffer joint. 
Moreover, the joint can be made from more than one material or it can be 
made from a single material treated to exhibit different characteristics 
in different sections. As an example, a joint can be made using rubber 
reinforced polypropylene with a Durometer hardness of 64 Shore A for web 
section 16, which serves as a flexing hinge, and a Durometer hardness of 
87 Shore A for the remainder of the joint. The joint of this invention is 
preferably made from elastomers having a hardness of 45 to 100. 
In joints having material of different characteristics in different 
sections, the hinge or web section should have a Durometer hardness of 
about 45 to 80 Shore A and the stem sections should have a Durometer 
hardness higher than that for the web section and within the range of 75 
to 100 Shore A. 
EXAMPLE 1 
This example describes flexural fatigue test results for a finger joint 
prosthesis of the current invention and for two commercially available 
finger joint prostheses. The implants of the current invention displayed a 
higher resistance to tear formation during flexural fatigue testing than 
prior art implants. 
Metacarpophalangeal joint prostheses of the current invention (size 40) 
were prepared from a silicone rubber having a Durometer hardness of 55 
Shore A by injection molding according to the general design shown in FIG. 
1, except that the corners of the end sections 12 and 13 were rounded. The 
outer faces 14 and 15 diverged upward toward the top of the main body at 
an angle of 30 degrees with the inner groove sides 22 and 23 joining at 
the end of groove 21 in a radius 24 of 0.066 in (0.168 cm). The 
intersection 20 of the stems with the outer faces had a radius of 0.040 in 
(0.102 cm). The key-hole shaped groove 25 across the bottom of the main 
body included first and second key-hole side segments 27 and 28 converging 
at an angle of 75 degrees and a key-hole hole segment 26 having a radius 
of 0.050 in (0.127 cm). The thickness of the web section 16 was 0.090 in 
(0.229 cm). Four implants of the current invention were gas plasma 
sterilized (4 hr cycle) prior to testing and two unsterilized joints were 
also tested. 
Commercially available sterilized metacarpophalangeal joint prostheses were 
tested for comparison with the implants of the current invention. Three 
size 40 AVANTA MCP joints (commercially offered by Avanta, San Diego, 
Calif.) having a design described in U.S. Pat. No. 4,871,367 with a 
keyhole-shaped groove on the top and a V-shaped groove on the bottom, as 
shown in FIG. 2A, and five size 6 Swanson MCP joints (commercially offered 
by Wright Medical, Arlington, Tenn.) having a design described in U.S. 
Pat. No. 3,462,765 and shown in FIG. 2B were used. The implants were all 
of equivalent size. A flexural fatigue machine was set to cycle the 
implants through a total 90 degree rotation ranging from 5 degrees of 
extension to 85 degrees of flexion. The angles of extension and flexion 
are measured relative to the finger being in a straightened state where 
the axes of the two stems are at an angle of 180 degrees. The collets 
which supported the stem region of the implants during testing were 
machined to the profile of the stem so that there would be adequate 
support of the stem (representing bone support in vivo) while allowing the 
stem to piston in an unconstrained fashion (again, as seen in vivo). The 
implants were immersed in a 37 degrees C. recirculating saline bath 
buffered to a pH of 7.4 to simulate the environment in the body, and 
cycled at 3 Hz for a total of ten million cycles. After approximately 
every half million cycles, the implants were removed from the test 
machine, dried, and visually inspected under a light microscope at 
40.times.. Both the palmar and dorsal sides of the hinge were examined for 
signs of damage. 
The implants of the current invention survived the ten million cycles of 
fatigue with negligible damage. No visible damage was observed on three of 
the four sterilized implants. One implant showed the initiation of a small 
tear (0.025 mm wide and 1.6 mm in length) on the palmar surface of the 
hinge after 2.5 million cycles. The tear after ten million cycles measured 
0.05 mm in width and 3 mm in length. Of the unsterilized implants, one 
implant had no visible damage while the other had a slight tear on the 
palmar surface of the hinge, first observed at 4.1 million cycles and 
progressing slightly over the duration of the fatigue cycling to a final 
width of 0.037 mm and 3 mm in length. No other anomalies were observed on 
the implants. 
The three AVANTA MCP implants showed some minimal damage during testing, 
with all implants displaying a similar damage mode over time. Small voids 
were observed spaced along the entire length of the palmar surface of the 
hinge after 2.4 million cycles, the voids ranging from 0.125 to 0.200 mm 
in width and 0.125 to 1 mm in length. Over time, these voids coalesced to 
form a closely associated series of voids, resulting in a small tear 
spanning the entire width of the palmar surface of the hinge after ten 
million cycles. The failure sites are similar to those reported in 
clinical failures of AVANTA MCP implants. 
The Swanson MCP implants also showed some damage during testing, with each 
of the five implants displaying the same damage mode over time. Tears 
formed in the proximal stem/hinge interface on the palmar surface, with 
the tears first observed after two million cycles and progressing through 
ten million cycles. After two million cycles, the tears measured 1.24-3.75 
mm in length and progressed to a crack opening more and penetrating into 
the depth of the part to a greater degree after ten million cycles. This 
mode of damage is consistent with failure sites reported clinically for 
the Swanson MCP implants. 
EXAMPLE 2 
A finite element analysis was performed on an anatomically neutral 
metacarpophalangeal joint of the current invention and compared to the 
AVANTA MCP joint described in Example 1 using the advanced nonlinear 
ABAQUS finite element code. In this example, the joint of the current 
invention was identical to that described in Example 1 except that the 
stem profiles were rounded and the angle at which segments 27 and 28 
converged was 60 degrees. 
The results of the finite element analysis are shown in FIG. 3 which is a 
plot of the maximum von Mises strain versus the angle of flexion/extension 
for each design. In this example, an angle of zero degrees corresponds to 
the finger being in a straightened state where the axes of the two stems 
are at an angle of 180 degrees. Negative angles are angles of extension 
and positive angles are angles of flexion. The angles which range from 
35-75 degrees represent the functional range of motion for a patient 
performing daily activities and are the regions in which most movement is 
performed. In the functional range, it is clear that the 
metacarpophalangeal joint of the current invention has significantly lower 
strain than the AVANTA MCP joint. This indicates that the AVANTA MCP joint 
is strained much more through the functional range of motion, thus 
stressing the material to a greater degree, potentially resulting in a 
reduced fatigue life of the implant. 
The superior results obtained for the metacarpophalangeal implant of the 
current invention shown in FIG. 3 is believed to result from the 
synergistic combination of two factors: the 30 degree bend and the keyhole 
geometry in the palmar hinge region in the metacarpophalangeal joint of 
the current invention versus the straight geometry and V-shaped groove in 
the palmar hinge of the AVANTA MCP joint.