Joint prosthesis with contoured pin

A joint prosthesis comprises two relatively inextensible primary components that are spaced apart from one another. Disposed between and spaced from each of the primary components is a pivot member that is also fabricated of relatively inextensible material. When viewed in section taken normal to its central longitudinal axis, the pivot member has two discrete arcuate surfaces that are spaced apart from one another about the circumference of the pivot member. One of the primary components of the prosthesis is resiliently secured to one of the arcuate surfaces of the pivot member, while the other primary component of the prosthesis is resiliently secured to the other arcuate surface of the pivot member. The resilient connections between the pivot member and the primary components permit relative rotation between the pivot member and each of the primary components. As a result, the two primary components can rotate toward and away from each other about an axis that is disposed at least adjacent to and at least approximately parallel to the longitudinal axis of the pivot member. The resilient connections between the arcuate surfaces of the pivot member and the primary components of the prosthesis are preferably accomplished through the use of bodies of elastomeric material. Adjacent the spaced apart arcuate surfaces of the pivot member, the bodies of elastomer have greater circumferential dimensions that the arcuate surfaces so as to reduce stresses occurring at the elastomer-to-pivot member interfaces.

RELATED APPLICATIONS 
The invention described, illustrated, and claimed in the present 
application is similar in structure and function to the joint prostheses 
described, illustrated, and claimed in commonly owned, concurrently filed 
application Ser. No. 852,183 of Leonard J. Schwemmer, entitled, "Joint 
Prosthesis," in commonly owned, concurrently filed application Ser. No. 
852,111 of James B. Koeneman, entitled, "Knee Joint Prosthesis," and in 
commonly owned, concurrently filed joint application Ser. No. 852,182 of 
Leonard J. Schwemmer and Howard T. Wilson, entitled, "Ankle Joint 
Prosthesis." 
BACKGROUND OF THE INVENTION 
Resilient materials, such as elastomers, have long been used in external 
prosthetic devices for the human body to cushion impact or shock loads. 
Because impact loads are necessarily and regularly encountered in walking, 
two common prosthetic devices that have often incorporated resilient 
materials are artificial feet and ankle joint prostheses for use with 
artificial feet. In early designs, an ankle joint prosthesis was typically 
a metallic pivot that included a plain (e.g., sleeve) bearing or a rolling 
element (e.g., ball) bearing. Resilient or elastomeric material was 
disposed both about the pivot to help limit its motion and in various 
portions of an associated artificial foot to cushion or absorb impact 
loads. Typical combinations of a cushioned artificial foot and an ankle 
joint prosthesis that incorporates a metal-on-metal pivot are described 
and illustrated in Ehle U.S. Pat. Nos. 487,697, Rowley 1,090,881, and 
Kaiser 2,183,076. 
Later in the development of ankle joint prostheses for external use, 
resilient or elastomeric material came to be utilized in such prostheses 
for properties other than its ability to absorb or cushion impact loads. 
In Desoutter U.S. Pat. No. 1,911,440, for example, a tubular rubber 
bushing is secured between a pin and a metal sleeve that circumscribes the 
pin to form a pivot for an ankle joint prosthesis. The outer sleeve is 
connected to an artificial foot, while the pin is connected to an 
artificial lower leg. Articulation is permitted by torsional deflection of 
the bushing. Because of the resilience of the bushing material, the ankle 
joint prosthesis automatically returns to a preselected position after it 
is deflected. The prosthesis also does not require lubrication because the 
bushing separates the adjacent metal surfaces of the pin and the sleeve. 
Similar ankle joint prostheses that employ a tubular bushing or body of 
elastomer between an outer rigid sleeve and an inner pin or sleeve are 
described and illustrated in Burger et al U.S. Pat. No. 2,605,475 and 
Prahl U.S. Pat. No. 3,480,972. 
A pivot or pivotable assembly that incorporates a relatively thin, tubular 
body of elastomer secured between a pin and a larger diameter sleeve is 
only capable of extensive rotational movement about a single axis. In a 
typical ankle joint prosthesis, such as the Desoutter and Prahl ankle 
joint prostheses, such an elastomeric pivot is oriented generally 
perpendicular to the longitudinal axis of the wearer's leg and transverse 
to the longitudinal axis of the wearer's artificial foot. In the 
orientation that has been described, the elastomeric pivot permits 
extensive flexion in the dorsal and planter directions. An elastomeric 
pivot so oriented, however, can only provide a limited degree of inversion 
and eversion of a foot about its longitudinal axis or a parallel axis and 
only a limited degree of internal and external rotation of the foot about 
the longitudinal axis of the lower leg. The motions other than flexion are 
all accommodated primarily through compression of the elastomeric bushing, 
which is relatively thin and cannot afford any significant degree of 
deflection. To overcome some of the motion limitations inherent in the 
ankle joint prostheses of the Desoutter and Prahl patents, the ankle joint 
prosthesis of the previously mentioned Burger et al patent incorporates 
two elastomeric pivots disposed at right angles to each other. The Burger 
et al ankle joint prosthesis thus can resiliently permit both extensive 
dorsal and plantar flexion and extensive inversion and eversion. Other 
external ankle joint prostheses attempt to provide the three types of 
movement afforded by a natural ankle joint through the use of relatively 
massive blocks of elastomer, rather than the tubular bushings discussed 
above. The blocks of elastomer may be specially shaped or contoured in 
order to provide appropriate stiffnesses or motion capabilities in the 
three critical rotational directions. Examples of external ankle joint 
prostheses that incorporate large blocks of elastomer are described and 
illustrated in Bennington et al U.S. Pat. No. 2,692,392 and Asbelle et al 
U.S. Pat. No. 3,982,280. 
Although resilient materials, and particularly elastomeric materials, have 
for many years been suggested for use in external joint prostheses, the 
use of resilient or elastomeric materials in internal joint prostheses has 
only recently been proposed. The apparent delay in the appearance of 
proposals for the use of resilient or elastomeric materials internally of 
the human body is probably attributable in part to the lack of a 
physiologically inert elastomeric material that could safely be used in 
the body. Nonetheless, with the development of suitable elastomeric 
materials, such as Dow Corning Corporation's Silastic.RTM. silicone 
elastomer, a number of surgically implantable, elastomeric joint 
prostheses have been proposed, particularly for finger joints. The finger 
joint prostheses, in particular, tend to be entirely formed of elastomer 
or nearly so. Unfortunately, such designs require the elastomer to be bent 
of flexed extensively at some point to provide a pivot. The result is 
alternating tension and compression loading of the elastomer, which is 
detrimental to its long-term fatigue life. The use of notches in the 
elastomer to locate the pivot point further adds to the stresses in the 
elastomer. Examples of finger joint prostheses that are entirely formed of 
elastomer or nearly so are described and illustrated in Swanson U.S. Pat. 
Nos. 3,462,765, Niebauer et al 3,593,342, Lynch 3,681,786, and Swanson 
3,875,594. Other than the finger joint prostheses mentioned above, 
relatively few implantable prostheses that employ resilient or elastomeric 
material have been identified. Nonetheless, the use of elastomeric 
material in an implantable hip joint prosthesis is suggested in Buechel et 
al U.S. Pat. No. 3,916,451, particularly FIG. 1, and in Bokros et al U.S. 
Pat. No. 3,707,006, particularly FIG. 5. 
The ankle joint prostheses described in the previously mentioned patents to 
Desoutter, Burger et al and Prahl appear to represent the best presently 
known designs for use of the desirable properties of elastomeric material 
in a prosthesis that accommodates pivotal or rotational motion. 
Nonetheless, the elastomeric pivots that are incorporated in the ankle 
joint prostheses of these three patents do not make optimal use of 
elastomeric material within the space provided. In particular, the 
relatively thin, tubular bodies of elastomer in the ankle joint prostheses 
of Desoutter, Burger et al, and Prahl are subjected to relatively high, 
torsionally-induced strains which, over periods of extended use, will lead 
to failure of the elastomeric bodies. While the strains experienced by the 
elastomeric bodies of the patented ankle joint prostheses may not be 
detrimental in terms of a few hundred or even a few thousand articulations 
of the prostheses, the strains are critical when one considers several 
million articulations or deflections of the prostheses. Such numbers of 
articulations may easily be experienced during a year or two of normal 
use. In an ankle joint prosthesis that is used externally of the human 
body, replacement of the elastomeric elements of the prosthesis may merely 
represent additional expense and some inconvenience to the user. If such a 
joint prosthesis were implanted in the body of the user, on the other 
hand, failure of the elastomeric elements within one or two years would 
seriously limit the desirability of using such a prosthesis. 
SUMMARY OF THE INVENTION 
The present invention relates to a joint prosthesis for either internal or 
external use which incorporates resilient material to facilitate 
rotational movement and which utilizes the material in a configuration 
that will promote a long service life for the prosthesis. The invention is 
more particularly directed to a pin or pivot member for use in such a 
joint prosthesis. The pivot member has a specially configured outer 
circumference to reduce stress concentrations in selected areas of the 
prosthesis so as to increase the expected service life of the prosthesis. 
A joint prosthesis with a contoured pin according to the present invention 
comprises a pair of relatively inextensible primary components that are 
spaced apart from one another. Disposed between and spaced from each of 
the primary components is a relatively inextensible pin or pivot member. 
The pivot member has a central axis and, when viewed in section taken 
normal to the axis, has two discrete arcuate surfaces that are spaced 
apart from one another about the circumference of the pivot member. The 
pivot member is also resiliently secured to each of the two primary 
components of the prosthesis. The resilient securing structure includes a 
first portion that secures one of the primary components of the prosthesis 
to one of the arcuate surfaces of the pivot member and a second portion 
that secures the other primary component of the prosthesis to the other 
arcuate surface of the pivot member. The two portions of the securing 
structure resiliently permit and accommodate relative rotation or pivotal 
motion between the pivot member and each of the primary components of the 
prosthesis. As a result, the two primary components can rotate toward and 
away from each other about an axis that is disposed at least adjacent to 
and at least approximately parallel to the central axis of the pivot 
member. The two discrete arcuate surfaces of the pivot member facilitate 
attachment of the securing structure to the pivot member with a minimum of 
stress at the interfaces between the pivot member and the securing 
structure. 
In a preferred embodiment of the invention, the two portions of the 
securing structure that are attached to the two discrete arcuate surfaces 
of the pivot member are at least partially formed of elastomer. Adjacent 
its interface with an arcuate surface of the pivot member, each of the two 
portions of the securing structure has a width measured circumferentially 
of the pivot member which is greater than a corresponding width of the 
adjacent arcuate surface of the pivot member. The two resilient portions 
of the securing structure may thus contact and be secured to portions of 
the circumference of the pivot member other than the two spaced apart 
arcuate surfaces. The benefits to be obtained from attaching the securing 
structure to portions of the circumference of the pivot member other than 
its spaced apart arcuate surfaces are greatest if the discrete arcuate 
surfaces of the pivot member are disposed at larger radial distances from 
the central axis of the pivot member than any other portion of the 
circumference of the pivot member, throughout at least a majority of its 
length. The two portions of the securing structure can then extend 
radially inwardly of the pivot member beyond its discrete arcuate 
surfaces, rather than terminating abruptly at the arcuate surfaces. 
Continuing the resilient securing material beyond the arcuate surfaces of 
the pivot member reduces or alleviates the high stresses that would 
otherwise be developed at the bond interface between the resilient 
material and each arcuate surface of the pivot member. Such stresses are 
particularly high when the resilient mateial is being deflected to 
accommodate relative rotation between the pivot member and the primary 
components of the prosthesis.

DESCRIPTION OF EMBODIMENT 
FIG. 1 of the drawing illustrates, in plan view, a finger joint prosthesis 
10 according to the present invention. The finger joint prosthesis 10 
includes a proximal component 12 and a distal component 14 which are 
spaced apart from each other. Both the proximal component 12 and the 
distal component 14 are formed of a relatively inextensible and 
biocompatible material. Suitable materials include high density 
polyethylene, polyester, nylon, rigid silicone resins, stainless steel, 
cobalt-chromium alloys, titanium, and titanium alloys. The relative 
inextensibility of the materials is to be judged in comparison to the 
resilient material that is incorporated into the prosthesis 10. The 
proximal component 12 of the finger joint prosthesis 10 includes a head 
portion 16 and a stem or shank portion 18, as best shown in FIG. 2. The 
distal component 14 of the prosthesis 10 likewise includes a head portion 
20 and a stem portion 22. The stem portions 18 and 22 of the two 
components 12 and 14 are tapered for axial insertion into adjacent 
phalanges or digital bones. Slots 24 and 26 are formed in the surfaces of 
the stem portions 18 and 22, respectively, in order to enhance the 
mechanical engagement between the two stem portions and the bone cement 
that is normally used to secure prostheses in the bones of the human body. 
The head portions 16 and 20 of the proximal and distal components 12 and 
14, respectively, are fixed to the broad or wide ends of the stem portions 
18 and 22 and may each be fabricated, as in the prosthesis 10, in one 
piece with a corresponding stem portion. The head portions 16 and 20 are 
also considerably wider than the adjacent ends of the stem portions 18 and 
22 and each head portion presents an arcuate surface 28 or 30 to the 
other. As best seen in FIG. 2, the surfaces 28 and 30 of the head portions 
16 and 20, respectively, are concavely arcuate when viewed in section 
taken along the longitudinal axes of the two components 12 and 14. 
Disposed between the proximal and distal components 12 and 14 of the 
prosthesis 10 is a pin or pivot member 34. The pin 34 is formed of a 
relatively inextensible material, such as the materials from which the two 
primary components 12 and 14 of the prosthesis 10 may be formed. The 
positioning of the pin 34 with respect to the primary components 12 and 14 
of the prosthesis 10 is such that the center of the pin is displaced from 
the intersection of the longitudinal axes of the primary components. In 
other words, as viewed in FIG. 2, the pin 34 lies below the point at which 
the extended longitudinal axis of the component 12 would intersect the 
extended longitudinal axis of the component 14. Such a positioning of the 
pin 34 locates the center of rotation or pivotal motion for the prosthesis 
10 so as to simulate more closely the functioning of a natural finger 
joint. As best shown in FIG. 4, the pin 34 has, when viewed in section 
taken perpendicular to its central longitudinal axis 36, a pair of 
discrete and convexly arcuate circumferential surfaces 38 and 40. The two 
surfaces 38 and 40 are spaced apart about the circumference of the pin 34 
and extend along the length of the pin. The surfaces 38 and 40 are also 
disposed at greater radial distances from the central axis 36 of the pin 
34 than any of the remainder of the circumference of the pin, except 
perhaps at the ends of the pin. At opposite ends of the pin 34, the 
circumferential surfaces of the pin, including the surfaces 38 and 40, are 
all beveled to avoid sharp, right-angle corners that might produce 
stresses and weaknesses in bonds between the surfaces 38 and 40, for 
example, and adjacent bodies of resilient material. Each of the ends of 
the pin 34 also has formed in it a blind bore 42 that extends part way 
along the central longitudinal axis 36 of the pin. The bores 42 are 
intended to receive pins or lugs to hold the pin 34 in place in a mold as 
resilient material is introduced into the mold and cured about the pin to 
form the prosthesis 10. 
In the prosthesis 10, the convexly arcuate surface 38 of the pin 34 is 
presented to the concavely arcuate surface 28 of the head portion 16 of 
the proximal component 12. The convexly arcuate surface 40 of the pin 34 
is similarly presented to the concavely arcuate surface 30 of the head 
portion 20 of the distal component 14. The pin 34 is secured to the 
proximal and distal components 12 and 14 by a mass of resilient material, 
such as an elastomer, that is essentially separated into two distinct 
portions or bodies 44 and 46. The material used in the resilient bodies 44 
and 46 must be biocompatible, as are certain grades of Dow Corning 
Corporation's Silastic.RTM. silicone elastomer. The body of resilient 
material 44 extends between and is bonded to the concavely arcuate surface 
28 of the head portion 16 of the proximal component 12 and the convexly 
arcuate surface 38 of the pin 34. The resilient body 46 extends between 
and is bonded to the concavely arcuate surface 30 of the head portion 20 
of the distal component 14 and the convexly arcuate surface 40 of the pin 
34. Each of the resilient bodies 44 and 46 has a pair of exposed surfaces 
48 or 50 that extend lengthwise of the pin 34 and outwardly from adjacent 
the circumference of the pin in a generally radial direction. The exposed 
surfaces 48 of the resilient body 44 are spaced throughout their lengths, 
as measured generally radially of the axis 36, from the adjacent surfaces 
50 of the resilient body 46. The surfaces 48 actually diverge from the 
surfaces 50 with increasing radial distance from the axis 36. As a result, 
deflection of the resilient body 44, for example, to permit relative 
rotation between the proximal component 12 and the pin 34 will not 
interfere, at least initially, with similar rotational movement between 
the distal component 14 and the pin. 
Adjacent each of the arcuate surfaces 28 and 38, the resilient body 44 has 
a circumferential width, or a width measured generally along the curve of 
the surface and about the axis 36, that is greater than the corresponding 
width of the adjacent surface 28 or 38. The material in the resilient body 
44 passes around the edges of the surface 28, for example, and along the 
sides of the head portion 16 of the component 12 so as totally to 
encapsulate the head portion 16. Similarly, the material in the resilient 
body 44 passes radially inwardly beyond the arcuate surface 38 of the pin 
34 and over other portions of the circumference of the pin. Adjacent each 
of the arcuate surfaces 30 and 40, the resilient body 46 has a 
circumferential width that is greater than the corresponding width of the 
surface 30 or 40. Like the resilient body 44, the resilient body 46 
totally encapsulates the head portion 20 of the adjacent primary component 
14 of the prosthesis 10. The material of the resilient body 46 also passes 
radially inwardly beyond the arcuate surface 40 of the pin 34 to other 
portions of the circumferential surface of the pin. Together, the bodies 
of resilient material 44 and 46 extend over and are bonded to all of the 
circumference of the pin 34 so as totally to encapsulate the pin, except 
for its ends. By continuing the resilient material of the bodies 44 and 46 
beyond the arcuate surfaces 28, 38, 30, and 40 to other surfaces of the 
inextensible elements 12, 14, and 34 of the prosthesis 10, high stresses 
adjacent the bonded interfaces between the resilient bodies and the 
arcuate surfaces are diminished. Such stresses are particularly large when 
the resilient bodies 44 and 46 are deflected to permit rotation to occur 
between the components 12 and 14 of the prosthesis 10 and the pin 34 of 
the prosthesis. Stress concentrations are also alleviated by the provision 
of rounded corners at the junctures of the various surfaces on the head 
portions 16 and 20 of the proximal and distal components 12 and 14 of the 
prosthesis 10. 
In operation, when implanted in a finger, for example, the prosthesis 10 
permits considerable flexion and extension between proximal and distal 
phalanges. In other words, the prosthesis 10 permits extensive pivoting 
motion between two digital bones, and between the two components 12 and 14 
of the prosthesis which are implanted in the bones, about an axis that is 
at least adjacent to and at least approximately parallel to the 
longitudinal axis 36 of the pin 34, if not coincident with the axis 36. 
Relative pivotal motion or rotation between the proximal component 12 and 
the pin 34 will be accommodated by resilient shearing deflection of the 
body 44. When deflected, the resilient body 44 will also provide a 
resilient restoring force to return the component 12 and the pin 34 to 
their initial relative positions. Relative pivotal motion or rotation 
between the distal component 14 and the pin 34 will be accommodated by 
similar resilient shearing deflection of the body 46, which will also 
offer a resilient restoring action when deflected. The flexural motions 
will be actuated by the muscles and tendons of the finger, which will not 
be displaced or destroyed during implantation of the prosthesis 10. As is 
apparent in FIG. 2, the stem portions 18 and 22 of the two components 12 
and 14 of the prosthesis 10 are normally disposed at an angle other than 
180.degree. with respect to each other about the axis 36. This 
predeflection of the two components 12 and 14 facilitates implantation of 
the prosthesis 10 into a finger, simulates the natural, slightly curved 
position assumed by the digits of the human hand in its relaxed position, 
and avoids having to deflect either of the resilient elements 44 and 46 
entirely in one direction to achieve the maximum desired range of flexural 
movement of the digits. A similar predeflection of the major components of 
a finger joint is described and illustrated in Fixel et al U.S. Pat. No. 
3,990,116. 
The resilient bodies 44 and 46 that interconnect the pin 34 and the primary 
components 12 and 14 of the prosthesis 10 may be deflected to afford 
limited pivotal or rotational movements between the components 12 and 14 
about axes generally perpendicular to the axis 36. In proximal and distal 
interphalangeal joints, such additional rotational movements may not be 
desirable. In metacarpal-phalangeal joints, on the other hand, the ability 
of the prosthesis to accommodate pivotal motions about other axes is 
desirable. The degree of motion that may be afforded about axes other than 
the axis 36 may be changed by adjusting the length of the pin 34, changing 
the thickness of the resilient bodies 44 and 46 as measured generally 
radially of the pin 34, or by shaping the pin to have arcuate surfaces 
along its length. Shortening the pin 34 will increase the motion permitted 
about the other axes, while lengthening the pin will decrease the motion 
permitted. Curving the pin 34 along its length will increase the degree of 
motion afforded about axes other than the axis 36. In a pin 34 that is 
curved along its length, the surfaces 38 and 40 are arcuate not only in 
planes perpendicular to the longitudinal axis 36 of the pin 34, but also 
in planes parallel to and passing through the axis 36. If necessary, the 
longitudinal curvature could be such that the pin 34 would approximate a 
spherical shape. 
The resilient bodies 44 and 46 of the prosthesis 10 are illustrated as 
being entirely formed of elastomer, for example. Nonetheless, it would be 
possible to insert into each of the bodies of resilient material 44 and 46 
one or more shims or plates fabricated of a relatively inextensible 
material and configured to conform to the opposed surfaces 28 and 38 and 
30 and 40 to which the resilient bodies are bonded. Such shims or plates 
would increase the compressive load carrying capability of the resilient 
elements 44 and 46 by restricting the ability of the resilient material to 
bulge or deflect along its free surfaces, such as surfaces 48 and 50. 
It will be understood that the embodiment described above is merely 
exemplary and that persons skilled in the art may make many variations and 
modifications without departing from the spirit and scope of the 
invention. Thus, for example, although the embodiment of the invention 
described above is an internal finger joint prosthesis, the invention 
could equally well be embodied in prostheses for internal use to replace 
other joints of the body and in prostheses intended for use outside the 
body. All such modifications and variations are intended to be within the 
scope of the invention as defined in the appended claims.