Note: Descriptions are shown in the official language in which they were submitted.
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MODULAR PROSTHESIS
Field of the Invention
This invention relates to the field of medical prostheses and particularly to
prostheses for use as replacements for diseased or damaged joints.
Bac ground of the Invention
Prostheses for replacement of joints commonly involve two parts having
mutually
articulating surfaces, and structure for mounting the parts to bone. To
duplicate closely
the structure and function of natural joints, the prostheses parts must be
carefully shaped
and sized, and must be properly oriented by the surgeon with respect to each
other and
with respect to the anatomy of the patient.
To achieve good surgical results, a surgeon should have as much freedom as
possible during the surgical implantation procedure to vary the shape, size
and
orientation of prosthesis parts. Mainly for this reason, efforts have been
made to provide
prostheses that are modular in form so that various elements of a prosthesis
can be
individually selected and the prosthesis can be assembled and oriented
according to the
anatomical needs of the patient.
Modular prostheses for the hip joint are shown, for example, in Boleski et
al.,
U.S. patent 5,080,685, Gianezio et al., U.S. patent 4,520,511, Demane et al.,
U.S. patent
4,995,883, Luman, U.S. patent 5,002,578 and Rhenter et al., U.S. patent
4,693,724. Such
prostheses for the most part involve a substantial number of parts that are
held together in
one configuration or another by means of mounting screws which operate to draw
together tapered connections of the parts. Although some freedom of selection
is
provided by previous modular prostheses, the use of threaded mounting screws
and
tapered connections can lead to loosening of the parts and to other problems.
Physical
and chemical corrosion can become substantial problems due to weakening of the
prosthesis and to biologic responses to corrosion debris and byproducts. See
Jacobs, J. J.
et al., Biological Activity of Particulate Chromium-Pho~phatP Corrosion
Products,
Collected Papers of the 21st Annual Meeting of the Society for Biomaterials,
March 18 -
22, 1995, p. 398, and Urban, Robert M., et al., Corrosion Products From
Modular-Head
Femoral Stems ofDifferent Designs a_nd Material Coup, Collected Papers ofthe
21st
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Annual Meeting of the Society for Biomaterials, March 18 - 22, 1995, p. 326.
Fretting
corrosion caused by relative motion between adjoining surfaces leads to the
production
of debris which in turn may lead to accelerated wear between normally
articulating joint
parts of a prosthesis and to osteolysis. When gaps occur between adjacent
surfaces of
prosthesis parts, oxidation of the surfaces may lead to formation of an acidic
environment and hence to chemical attack of the surfaces (commonly referred to
as
crevice corrosion).
It =would be desirable to provide a modular prosthesis kit having elements
that can
be freely cizosen and oriented by the surgeon in the operating arena and that
can be
strongly and firmly fastened to one another without the need for screw
fasteners or
tapered connections that are drawn together.
Summary of the Invention
The present invention makes use of a clamp capable of firmly clamping to a
prosthesis member and that may be used to firmly clamp together selected parts
of a
modular prosthesis. The clamp has a "rest" configuration having a dimension in
one
direction that can be reduced by applying to it physical stress, with
concurrent expansion
of the clamp in a second direction normal to the first direction, so that the
clamp may be
received in a cavity of a prosthesis member. Upon release of the applied
physical stress,
the clamp seeks to return toward its "rest" configuration, the clamp dimension
in the one
direction increasing so that the clamp presses upon the cavity walls to
strongly clamp to
the prosthesis member.
Thus, in one embodiment the invention relates to a modular prosthesis kit
comprising a first member having walls defining a cavity, and a clamp
releasably
clampable within said cavity. The clamp has a rest configuration having a
predetermined dimension in a first direction and being responsive to applied
physical
stress to assume a second configuration having a lesser dimension in said
first direction
with concurrent increase of a dimension in a second direction normal to the
first direction
to permit the clamp to be at least partially received in the cavity. The
predetermined
dimension is so chosen that upon release of the applied physical stress, the
clamp returns
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toward its rest configuration with consequent increase in its dimension in the
first
direction sufficient to strongly clamp to said member.
In another embodiment, the invention comprises a modular prosthesis kit that
includes instrumentation for assembly, comprising a first prosthesis member
having
walls defining a cavity and a clamp releasably clampable within said cavity.
The clamp
has a first, rest configuration having a predetermined dimension in a first
direction. An
instrument is provided for applying a stretching force to said clamp in a
second direction
normal to said first direction to reduce said dimension in the first direction
enough to
permit said clamp to be received in said cavity. The predetermined dimension
is such
that upon removal of the stretching force, the clamp returns toward its rest
configuration
with consequent increase in its dimension in the first direction sufficient to
strongly
clamp to said first prosthesis member. In a preferred embodiment, the
prosthesis kit
includes a second member configured to snugly receive at least a portion of
the first
member in any of several orientations. The cavity walls of the first member
are
configured to expand into clamping contact with the second member as the clamp
returns
toward its rest configuration to fixedly support the second member in a
predetermined
orientation with respect to the first member.
Preferably, the clamp in its rest position is elongated in the second
direction and
the cavity has inner walls similarly shaped to receive the clamp, the clamp
having a distal
end dimensioned to be partially received in the cavity. Force may be exerted
on the
distal end of the clamp directed inwardly of the cavity. As the walls of the
cavity engage
the clamp at any point along its length to resist insertion of the clamp,
continued force on
the distal end of the clamp places in tension that portion of the clamp
between its distal
end and the point of engagement with the cavity, causing that portion to
elongate with a
concurrent reduction in the width of the clamp at the point of engagement by
the cavity
walls. This width reduction, in turn, enables the clamp to move further
inwardly of the
cavity. The force that is exerted on the distal end of the clamp is resisted
by an
essentially equal force in the opposite direction applied to the cavity, the
latter force
being transmitted to the clamp along its length where it is engaged by the
cavity. In this
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embodiment, instrumentation preferably is provided to engage the distal end of
the clamp
and the first prosthesis member.
In yet a further embodiment, the invention relates to a method for assembling
members of a modular prosthesis. A first prosthesis member is provided with
walls
defining a cavity, and a clamp is provided having a first, rest configuration
having a
predetermined dimension in a first direction. The clamp is subjected to
physical
tensioning to expand a clamp dimension in a second direction normal to the
first
direction, with concurrent reduction of the clamp dimension in the first
direction to
enable the clamp to be received in the cavity. This is desirably accomplished
by
applying force directed inwardly of the cavity to a distal end of the clamp
received in the
cavity, as described above. The tensioning force is then withdrawn to allow
the clamp to
return toward its first, rest configuration with consequent increase in its
dimension in the
first direction sufficient to strongly clamp to said first prosthesis member.
In a preferred embodiment, the clamp and the cavity of the first prosthesis
member have confronting clamping surfaces that, when clamped, are
substantially
congruent so as to provide surface-to-surface contact between the clamp and
first
member, and the prosthesis is substantially free of gaps between confronting
surfaces.
Similarly, if a second prosthesis member receives and becomes clamped to the
first
member, preferably the clamping surfaces of these members are substantially
congruent
so as to provide surface-to-surface contact between the clamping surfaces of
the first and
second members, and the prosthesis is substantially free of gaps between
confronting
surfaces. Such surface-to-surface contact promotes uniform loading along the
clamping
surfaces.
Brief Description of the Drawing
Figure 1 is a side view, in partial cross-section, of a portion of a hip joint
prosthesis in accordance with the invention;
Figure 2 is a cross-sectional, broken away view taken across line 2-2 of
Figure 1;
Figure 3 is a schematic front view of the tibial portion of a knee joint in
accordance with the invention;
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Figure 4 is a side view, in partial cross-section, of a portion of another hip
joint
prosthesis similar to that of Figure 1;
Figure 5 is an exploded assembly view of parts of instrumentation for use in
the
assembly of the hip joint prosthesis of Figure 4;
Figure 6 is a view of the parts of Figure 5 as assembled;
Figure 7 is a view of the assembly of Figure 6 together with a manually
operated
force generating device;
Figure 8 is a cross-sectional, broken away view showing another embodiment of
the invention;
Figures 9A, 9B and 9C are side views of the stem, body, and clamping member
of a preferred hip joint prosthesis of the invention;
Figure 10 is a side view, in partial cross-section, of a portion of a hip
joint
prosthesis of which elements are shown in Figures 9A, B, and C;
Figure 11 is a side view of a modified stem of a hip joint;
Figure 12 is a schematic top view of the prosthesis of Figure 10, showing
positional adjustment of the body element with respect to the stem;
Figures 13A, 13B and 13C are cross-sectional views showing different stages in
the assembly of a clamping member and instrumentation;
Figure 14 is a view in partial cross-section and partially broken away of a
prosthesis of the invention during a step in its assembly;
Figure 15 is a perspective view of instrumentation useful in the assembly of a
prosthesis of the invention;
Figure 16 is a schematic view of other instrumentation that can be used in the
assembly of a prosthesis of the invention; and
Figure 17 is a schematic view of a modified instrument that can be used in the
assembly of a prosthesis of the invention.
With reference first to Figure 1, a modular hip prosthesis is designated 10,
and
comprises an elongated stem 12 sized to be received in a surgically prepared
intramedullary canal of the femur. Axial bore 14 is formed in the stem 12. A
body
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member 16 is provided with a bore 18 sized to closely receive the stem 12, the
body
having a generally triangular shape when viewed from the side and configured
to fit the
surgically sculpted proximal end of the intramedullary canal of the femur.
Proximally of
the body 16 is positioned a neck member 20 having a bore 21 sized to closely
receive the
upper end of the stem 12, the neck including an angled extension 22
terminating in a ball
24 sized to articulate with an appropriately sized and shaped socket
prosthesis (not
shown) to be mounted in the acetabular recess of the pelvis.
A clamp 30 is shown in Figure 1 as an elongated metal rod having an axial bore
32 that extends from its proximal end portion 34 to a floor 36 short of the
distal end
portion 38 of the clamp. Near its upper end, the axial bore 32 has a distally
facing body
fashioned to receive a placement instrument, as will be described below.
The clamp 30 is shaped and sized such that at body temperature, its diameter,
when not constrained in the stem 12, will be slightly larger than the diameter
of the bore
14 of the stem. The diameter 21 of the neck bore and the diameter 18 of the
body bore,
on the other hand, are essentially the same as the outer diameter of the stem
12; that is,
the stem is snugly but slidably received in the bores 18, 21 so that the body
and the neck
can be moved by hand upon the stem without difficulty.
The clamp 30, before installation in the bore 14 of the stem, first must be
altered
so that its diameter is slightly less than the bore diameter of the stem. This
is
accomplished by physically stretching the clamp in its long or axial direction
to cause the
diameter of the clamp to shrink sufficiently to enable the clamp to be
inserted in the bore
14. Although the clamp may be made from various metals as described below, a
preferred metal is a shape memory alloy such as nitinol, in its superelastic
state in which
applied stress results in a reversible martensitic phase transition. When a
nitinol clamp
30 is stretched as described above, and providing that its temperature is
maintained
substantially above its austenite finish temperature (the temperature at which
the alloy is
completely in its austenitic form), a transition from the austenite phase to
the martensite
phase occurs. This is known as stress induced martensite formation and is the
basis for
the phenomenon known as pseudoelasticity or superelasticity. The shape memory
alloy
will remain at least partially in the martensite phase as long as the external
stress is
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maintained. Upon release of the stress, however, the clamp 30 will return to
the austenite
phase and toward its original shape and size. Because the clamp is constrained
within
the dimensions of the stem bore 14, however, it will not be able to completely
resume its
original shape and size. As a result, the clamp 30 will exert a continuous
force against
the bore 14 of the stem 12. That is, when externally applied stress is
released, the clamp
tends to return toward a configuration which may be referred to as a "rest"
configuration.
The rest configuration of the clamp has a transverse dimension (the diameter
in the case
of a rod having a circular cross-section) that is slightly larger than the
transverse
dimension of the stem bore, and as a result the clamp pushes outwardly
strongly upon the
stem bore and becomes firmly clamped in the stem bore.
As shown in Figure 1, the walls 42 of the clamp have outer surfaces 44 that
engage and push outwardly upon the bore 14. When appropriately in place, the
outer
wall 44 of the clamp pushes outwardly upon the surface of the stem bore 14,
and the
walls of the stem, in turn, are forced outwardly into contact with the inner
surface 26 of
the body 16 and also with the inner surface 28 of the neck 20.
Preferably, the outer surface of the clamp 30 is generally cylindrical and
makes
substantial surface-to-surface contact with the surface of the bore 14.
Moreover, the stem
wall is sufficiently flexible as to enable the outer wall of the stem to
expand into contact
with the bores of both the body and the shoulder, even when these bores are
slightly
different in diameter. A feature of a preferred embodiment of the invention is
that the
clamped surfaces - that is, the confronting surfaces of the clamp and first
member, and
the confronting surfaces of the first and second members - mate in surface-to-
surface
contact to fairly uniformly distribute the compressive forces over the clamped
surfaces
and preferably to avoid gaps between confronting surfaces. As used herein, a
"gap" is
the thin void space formed between slightly spaced confronting surfaces of a
prosthesis
when assembled, as, for example, the space formed between an elongated, smooth-
walled rod having threads at one end and the bore receiving the rod. If the
clamp is a
cylinder having a circular cross-section and the cavity is a circular bore,
the compressive
clamping force exerted by the clamp against the walls of the bore would be
primarily
radial and substantially uniform along the length of the clamp. One may vary
as desired
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the concentration of compressive forces between the clamp (and between
prosthesis
members) by varying the shapes of the clamping surfaces. For example, if the
cross-
sections of the clamp and recess were oval rather than circular, one would
expect the
compressive clamping force to be somewhat greater in the longer transverse
dimension
than in the shorter transverse dimension.
The invention in another embodiment is shown in Figure 2, in which the ball 24
is firmly mounted to the angled neck extension 22. The ball 24 and the neck
member 20
(from which extends the angled extension 22) generally will be assembled as a
subunit,
and the subunit will then be assembled with the body and stem as mentioned
above.
As shown in Figure 2, the angled extension 22 has an internal bore 50 that is
open
at one end and is closed at its other end 52. The bore 50 extends downwardly
and
laterally as shown in Figures 1 and 2, and opens into the bore 28. The distal
end of the
angled neck has a tapered head 54 that is received within a tapered bore 60
formed in the
ball 24. In this embodiment, the angled neck 22 functions not only as a part
of the
prosthesis but also as the clamp. To positively and firmly connect the ball 24
to the
angled neck, one first elongates the angled neck in the manner described above
in
connection with the clamp 30. Upon elongation of the angled neck 22 sufficient
to
enable the head 54 to be snugly received in the ball, the stretching force
imparted by the
instrument is withdrawn, and the neck 22 returns toward its original, "rest"
configuration,
the outer wall of the head 54 bearing outwardly against the confining walls of
the bore b0
to firmly clamp the ball to the angled neck. Referring to Figure 1, it will be
noted that
the bore 50 is fully accessible through its open end prior to mounting of the
neck 20 upon
the stem 12. It may also be noted that the clamp and the cavity, although
circular in
cross-section and making mutual surface-to-surface contact, are tapered rather
than
cylindrical, illustrating how the shape of the clamp and cavity may be varied.
With reference to Figure 3, a tibial tray component is shown generally as 70
and
comprises a stem 72 adapted to be received in the surgically prepared
intramedullary
canal of the tibia in a known fashion. The stem terminates upwardly in a metal
tray 74
which in turn supports a bearing insert 76 of high molecular weight
polyethylene or the
like. The latter is adapted to articulate with the condyles at the distal end
of the femur, or
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with the condyles of a prosthetic femoral implant, all in a known fashion.
Near the upper
end of the stem is positioned a shoulder 78 which fits in the surgically
prepared upper
end of the tibial intramedullary canal, and serves to support the upper end of
the stem.
A clamp such as that described above is shown at 80 in Figure 3. It is
desirably
cylindrical in cross section, having a diameter at body temperature that is
slightly greater
than the diameter of a bore 82 formed axial within the stem 72. The clamp 80
may be
inserted by the same method described in connection with the clamp 30 of
Figure 1.
When the stretching force is withdrawn, the clamp returns toward its "rest"
configuration
and its walls press outwardly against the walls of the stem 72, causing the
latter in turn to
clamp strongly to the walls of the bore 84 of the shoulder member 78.
A slightly modified hip joint prosthesis is depicted in Fig. 4 as 100, the
prosthesis
having a stem 112 adapted for insertion in the intramedullary canal of the
femur. An
axial bore 114 is formed in the stem, and the walls of the stem near its
proximal end may
have longitudinal slots 116 formed therein, the slots ending in round holes I
18 to avoid
I S stress concentration areas. The slots 116 enable the wall of the stem to
expand more
easily, and are spaced evenly about the circumference of the stem. Four slots
may be
employed. A body 120 is provided with an internal bore 122 sized to snugly
receive the
stem, the body bearing a ball 124 similar to ball 24 of Figure 1. The upper or
proximal
end of the body 120 extends slightly beyond the proximal end 126 of the stem.
Within the stem is received a hollow, tubular clamp 130 similar to the clamp
30
shown in Figure I. Clamp 130 has a proximal, externally threaded end portion
132 that
extends beyond the proximal end 126 of the stem but is yet preferably retained
in the
proximal end portion of the body bore 122, all as shown in Figure 4.
Figures 5 - 7 depict instrumentation for applying tensile stress to the clamp
typified as 130 in Figure 4 of the drawing. Shown at 140 is a tubular gripping
tool
having an open distal end portion 142 that is internally threaded to receive
the external
threads of the proximal end portion 132 of the clamp. Square threads
preferably are
used. An aperture 144 is formed in the gripping tool 140 proximal of its
distal end
portion 142. An elongated pushing rod 150 is received in the hollow clamp, and
has a
distal end 152 shaped to engage the confronting distal end wall 134 of the
clamp in
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surface-to-surface contact. The proximal end 154 of the pushing rod is
accessible
through the aperture 144, as shown best in Figure 6, and has a recessed end
surface 156.
Note that the proximal end wall 146 of the tubular gripping tool similarly has
a recessed
surface 148 facing the recessed end surface 156 of the rod.
5 Figure 7 depicts the assembly of Figure 6 in association with a manually
operated
plier-like force-generating device 170, the device having handles 172,
oppositely facing
nose portions 174 receivable in the aperture in the gripping tool, and a pivot
176
positioned to provide substantial mechanical advantage to the nose portions.
Nose
portions 174 bear against the respective recessed surfaces of the push rod and
gripping
10 tool as shown in Figure 7; squeezing of the handles together results in the
application of
substantial force to the rod I50, causing the clamp 130 to elongate slightly
but
sufficiently to enable the clamp to be inserted in the bore of the stem. A
tooth and pawl
mechanism 178 of known design and commonly used with surgical instruments is
provided at the ends of the handles to hold them together and thus maintain
the stem in
its stressed, elongated configuration. Various other devices capable of
delivering
substantial force to stretch the clamp may be employed using any of a number
of
mechanical, pneumatic, and hydraulic means.
In use, referring again to Figures 4 through 7, a push rod or shaft 150 is
inserted
in an appropriate clamp 130, and the proximal end of the clamp is screwed onto
the end
of the gripping tool 140 to form the assembly shown in Figure 6. The nose
portions 174
of the force-generating device 170 are inserted through the aperture 144 into
contact with
the respective recessed surfaces of the push rod and gripping tool, and the
handles are
squeezed toward each other and locked by the mechanism 178, thus holding the
clamp in
its elongated configuration. Body 120 is received over the stem, and is
positioned where
desired along the stem by the surgeon during the implantation procedure. Once
the stem
and body have been properly oriented with respect to each other, and the body
has been
suitably impacted by the surgeon into the intramedullary canal, the clamp is
inserted into
the stem bore. Mechanism 178 is then released, resulting in the release of
pressure of the
nose elements against the push rod and gripping tool. As the clamp 130 expands
toward
its rest configuration, it bears with substantial force against the walls of
the stem, forcing
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these walls into tight contact with the walls of the bore formed in the body.
The gripping
tool, of course, is then removed, and the open proximal end of the clamp is
capped
appropriately if desired.
It may be particularly valuable to utilize the stem of the prosthesis of
Figure 1
S itself as the clamp, eliminating the clamp 30. Here, the proximal end of the
stem may be
internally threaded to receive the distal threaded end of an externally
threaded gripping
tool similar to that shown at 140 in Figure 6. The gripping tool and push rod
may be
longer than that shown in the drawing to allow placement of the neck and body
over the
gripping tool prior to threading the gripping tool onto the threaded end of
the stem.
By appropriately configuring the gripping tool 140, one may loosely position
the
neck 20 and body 16 on the gripping tool prior to use of the device to
elongate the
proximal portion of the stem. The push rod 150 is placed in the bore of the
stem, and the
gripping tool is threaded onto the stem. Once the stem 12 has been elongated
by
operation of the force generating device and appropriately positioned in the
femoral
cavity, the neck and body may be brought down over the nose portions and
around the
stem and positioned as desired within the intramedullary canal.
Desirably, the various parts of the prostheses of the invention that are
clamped
together are made of metal such as stainless steel, cobalt chrome alloys,
titanium alloys
or the like as are commonly employed for prostheses manufacture. The clamp,
similarly,
may be made of a shape memory alloy or of any metal that exhibits an initial
proportional relationship between stress and strain (in the range of validity
of Hooke's
law). Various metals and metal alloys satisfy this requirement, including
stainless steel.
The ratio of the lateral or transverse strain to the longitudinal or axial
strain, commonly
referred to as Poisson's ratio, can range from 0.2 to 0.5, depending on the
material and its
condition. Poisson's ratio for stainless steel, for example, is about 0.28.
The clamps according to the invention preferably are made of a shape memory
alloy such as nitinol. Nitinol exhibits a Poisson's ratio of about 0.3, but
this ratio
significantly increases up to approximately 0.5 or more when the shape memory
alloy is
stretched beyond its initial elastic limit; that is, when the formation of
stress-induced
martensite begins to occur. Nitinol is a pseudoelastic material, that is, a
material that
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exhibits superelasticity at room temperature. A number of shape memory alloys
are
known to exhibit the superelastic/pseudoelastic recovery characteristic, and
these are
generally characterized by their ability, at room or body temperature, to be
deformed
from an austenitic crystal structure to a stressed-induced martensitic
structure, returning
to the austenitic state when the stress is removed. The alternate crystal
structures give
the alloy superelastic or pseudoelastic properties.
Nitinol clamps of the type referred to above in connection with Figures I and
3
can readily be elongated up to 8% or more through the use of instruments such
as that
shown in Figure 4. Using nitinol with an assumed Poisson's ratio of 0.3, if a
clamp such
as that shown in Figure 6 is elongated 8%, it would be expected to shrink
about 2.4% in
diameter. If the initial diameter of a clamp were in the neighborhood of 1/2
inch, the
decrease in diameter would be on the order of 0.012 inches. Since tooling
tolerances for
the internal bores of stems and other prosthesis parts can easily be held
within t 0.002
inches, a change of 0.012 inches in the clamp diameter allows substantial room
for
design variations in size. It is generally preferred that the diameter of the
stem bore,
however, be only very slightly greater than the outer diameter of the clamp
when the
clamp is longitudinally stretched to an elongation of, for example, 8%.
A surgeon may select the desired sizes of the stem, body and head, and can
assemble the same during a surgical procedure. With reference to the femoral
implant
shown in Figure 4, an articulating ball 124 of the appropriate size is
selected and is
mounted as described above to the neck 120. The femoral prosthesis without the
clamp
130 is then assembled. Assembly may take place away from the patient if the
desired
dimensions and respective angles of the prosthesis parts are known with
accuracy ahead
of time, as by measurement or by use of trial prosthesis parts. The prosthesis
itself can
be assembled in the intramedullary canal of the patient, with the correct
orientations of
the parts noted. Referring to Figures 4 through 7, once the parts have been
arranged and
oriented as desired in the intramedullary canal, a clamp 130 is tensioned to
reduce its
diameter through use of the gripping tool 140, the pushrod 150 and the force
generating
device 170, and is then gently placed in the bore of the stem. When tension on
the clamp
is withdrawn, the clamp expands immediately toward its larger diameter "rest"
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configuration, thereby clamping itself to the stem and clamping the stem 112
to the body
120. It will be noted that the resulting prosthesis desirably has no threaded
fastenings to
come loose. While tension is maintained on the clamp, the body 120 may be
positioned
independently in axial and rotational directions on the stem as the surgeon
may deem
S appropriate for the particular patient. In the same manner in which assembly
was carried
out, disassembly can be afforded by reversing the steps.
Similarly in connection with the prosthesis of Figure 3, once the shoulder 78
and
stem 72 have been mounted in the distal end of the tibia as desired and
oriented with
respect to one another, the clamp 80 may be inserted in the bore 82 and
permitted to
expand toward its "rest" configuration. This, in turn, forces the walls of the
stem
outwardly and to contact with the bore 84 of the shoulder 78 to lock the stem
and
shoulder together.
Although the clamp of the invention has been described in terms of a hollow
rod
with one open end and one closed end, it should be understood that a variety
of clamp
1 S configurations may be employed. Also, while it is desired that the outer
surface of the
clamp and the inner surfaces of the bore or bores within which the clamp is
received be
smooth and regular so as to make good surface-to-surface contact, the outer
surface of
the clamp may, in fact, be ridged or roughened or longitudinally fluted or
otherwise
configured, as desired.
Moreover, as noted above, the clamps of the invention need not be round in
cross
section nor must they have a uniform dimension transverse to the longitudinal
axis. If
desired, the outer surface of the clamp may have a greater transverse
dimension in some
areas than in others. For example, with reference to Figure 1, the transverse
dimension
of the clamp may be greater near the top of the clamp where the stem portion
that is
2S clamped bears also against the bore of the body or vice versa.
The clamp desirably is hollow or tubular in design. Referring to Figure 8,
head
S4 of the neck extension 22 may be formed with a thimble-shaped clamp i80
having an
outwardly flared skirt 182 at its open end. When the head S4 with clamp 180
attached is
forced into the bore 60 of the ball (leaving a gap 62 between the end of the
clamp I 80
and the floor of the bore 60), the rim of the opening 60 encounters the skirt
182 and
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14
forces the walls of the clamp to elongate. Upon release of the pressure
forcing the head
54 into the opening 60, the walls of the clamp increase slightly in thickness,
wedging the
ball onto the head 54 and sealing the opening 60. The interface 61 between the
head 54
and the clamp 180 is also sealed.
In a preferred embodiment, the confronting walls of the clamp and cavity may
be
so configured that any slippage between the clamp and the cavity results in
the clamp
being urged more deeply into the cavity. For example, the confronting walls of
the
clamp or cavity or both may be configured to have circumferential shoulders or
tapered
surfaces or other shapes, that coact to preferentially urge the clamp to move
or "walk" in
one direction rather than the opposite direction upon repeated slippage
between the
confronting surfaces. With reference to Figure 4, for example, the diameters
of the
clamp 130 and the bore in the stem 112 may be slightly greater near the distal
end of the
stem 112 than near the proximal end so that any movement or "walking" of the
clamp
due to repeated slippage of the clamp and the stem bore urges the clamp
distally within
1 S the stem, drawing the widened threaded shoulder at the proximal end of the
clamp into
contact with the proximal end 126 of the stem.
Figures 9A and 9B depict a modified form of a hip prosthesis of the
invention..
Stem 200 has an axially extending bore 202 through a portion of its length and
open at its
proximal end, the walls of the bore including a pair of diametrically opposed
slots 204.
Each slot is formed between two small circular holes 206 which serve to
release stress at
the ends of the slots. A body 210 is provided with a circular bore 212 which
snugly
receives the proximal portion of the stem, and the body has a neck portion of
the type
shown in Figure 1. The proximal end of the stem has a reduced diameter,
externally
threaded portion 208. When the stem is received in the body, as shown in
Figure 10, the
threaded proximal end of the stem is of a diameter less than that of the bore
212 of the
body, enabling the internally threaded end of a tubular instrument (shown at
216 in
Figure 14) to be threaded onto the end of the stem. The tubular instrument is
designed
with an outer diameter enabling it to pass within the bore 212 of the body.
Referring to Figures 9C, 10 and 14, an elongated, hollow clamp 220, preferably
of nitinol or other similar superelastic alloy, is received within the stem
bore 202. The
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clamp may be of the type shown at 42 in Figure 1, that is, with a closed
distal end, or the
clamp may be open throughout its length as shown in Figure 9C. In order to
elongate the
clamp and thereby reduce its transverse dimension sufficiently to enable it to
be received
in the stem bore 202, the distal end of the clamp (shown at 222 in Figure 9C,
and 38 in
S Figure 1) must be subjected to force tending to stretch or elongate the
clamp. Figures 6
and 7 in general show how the proximal end of the clamp may be threaded and
gripped
by a stretching instrument while a push rod 150 engages the closed end of the
clamp to
push against it; that is, tension is applied to the proximal and distal ends
of the clamp.
With respect to Figure 8, on the other hand, the tapered head 54 is moved
axially
10 inwardly of the bore formed in the articulating ball, drawing with it the
thimble-shaped
clamp 180 and causing the clamp to elongate as the interaction between the
clamp and
the bore restrains axial movement of the clamp. This feature also is employed
in the
embodiments of Figures 9-17.
Turning now to Figure 14, a portion of a suitable instrument for forcing the
clamp
15 into the stem bore is shown at 216, this portion being tubular and having
an internally
threaded distal end 218 adapted to thread securely onto the threaded upper end
208 of the
stem 200. Sliding axially within the tubular member 216 is a shaft or push rod
224
which, at its distal end, securely engages the distal end 222 of the clamp.
From Figure
14, it will be understood that as the shaft 224 is forced distally, that is,
downwardly in
this figure, with respect to the stem 200, that portion of the clamp 220
within the stem
will be subject to tensile stress. The distal end 222 of the clamp 220 tapers
inwardly
slightly at its end so that this end can readily be received within the rim
214 of the bore
formed in the stem 200. In this manner, the distal end 222 of the clamp can be
received
partially within the proximal end of the bore without deforming the clamp.
Thereafter,
the force of the shaft 224 pushing distally on the distal end 222 of the clamp
forces this
end of the clamp inwardly of the bore 202. The outer walls of the clamp come
into
contact with the inner walls of the bore as insertion of the clamp proceeds,
and this
contact retards insertion of the clamp. However, application of continued
force upon the
distal end of the clamp causes the clamp to elongate at the points where its
walls come
into contact with walls of the bore, that is, where the clamp becomes "stuck"
in the bore,
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16
with the result that the clamp continues to proceed inwardly of the bore. When
nitinol is
used as the clamp, and cobalt chrome steel or titanium as the material from
which the
stem is made, insertion of the clamp within the bore proceeds smoothly and
without
substantial jerking due to incremental stops.
Referring again to Figures 9-12 and 14, the prosthesis there shown includes
the
stem 200 as a first prosthesis member and the body 210 as a second prosthesis
member.
In this embodiment, the clamp 220 is received within the cavity formed by the
bore 202
in the first member, and the first member in turn is received within the bore
212 of the
second member. Here, it is important that the walls of the first member - the
stem 200 -
be capable of expanding outwardly into contact with the bore 212 as the clamp
220,
returning toward its rest state, forcefully pushes radially outwardly upon the
inner wall
202 of the stem. Axially elongated slots 204, generally two or four in number,
may be
spaced about the circumference of the stem, the slots terminating in circular
bores 206 to
avoid stress concentration at the slot ends. A similar concept is shown in
Figure 11, in
1 S which a pair of diametrically opposed slots (of which one is shown as 205)
extend
proximally from stress relief holes 207 and then curve medially as shown at
209 where
they meet, the slots thus defining a "tongue" shaped portion of the stem wall
which can
more easily be displaced outwardly.
A preferred clamp 220 is shown Figures 13A, B and C. As earlier mentioned, the
distal end 222 of the clamp desirably is tapered inwardly slightly as shown,
to enable it
to enter the outer rim 214 at the proximal end of the stem bore 202. The push
rod or
shaft 224 terminates distally in a reduced diameter portion having a tapered
end section
227 (Figure 13C), the walls of which converge distally. At the end of the
section 227 is a
button 228 of increased diameter. An expandable, annular collar 230 is formed
with a
substantially cylindrical outer surface 232 and a tapered inner bore 234
(Figure 13A)
preferably matching the taper at the end section 227 of the shaft 224. The
walls of the
collar 230 may be provided with axially extending slots, as needed, such that
when the
tapered end portion 227 of the shaft 224 is forced axially in the distal
direction with
respect to the collar, the collar expands into clamping contact with the
interior walls of
the clamp at its distal end 222.
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Refernng to Figure 13A, when the collar 230 is positioned against the terminal
button 228, the collar, as yet unexpanded, can slide through the inner
diameter of the
hollow clamp 220. To wedge the collar within the distal end of the clamp, one
may
utilize a block 236 of a hard material such as steel, the block having a bore
238 formed
through its thickness. The bore 238 is of a diameter just large enough to
admit the button
228, but not large enough to admit the collar 230. As shown in Figure 13B, the
clamp is
oriented with its distal end against the block 238 and with the button 228
extending into
the bore. As the shaft 224 is forced distally (in the direction of the arrow F
in Figure
13B), interaction between the tapered portions of the shaft and collar cause
the collar to
expand into rigid engagement with the inner walls of the clamp 230 near its
distal end,
this configuration being shown in Figure 13C. After insertion of the clamp in
the stem,
the shaft 224 may be left in its position, or may be removed by withdrawing
the staff
proximally. As the shaft is withdrawn from the position shown in Figure 13C to
the
position shown in Figure 13B, the walls of the collar are allowed to collapse
inwardly,
reducing the diameter of the collar and enabling the shaft 224 and collar 230
to be
removed proximally as a unit.
Figure 12 illustrates how the body 210 of a prosthesis of the invention may be
oriented with respect to the stem. Proper angular and axial orientation
between the stem
and body may be initially set by the surgeon, and these elements may be locked
in place
through a clamping element as described above. If it becomes necessary to
reorient the
stem and body, the clamp can again be put under tension to enable the body to
be moved
with respect to the stem.
Figure 15 shows a mechanical device that can be employed to appropriately
elongate a clamp such as that shown at 30 in Figure 1 within the cavity of a
prosthesis
member. The instrument itself, designated 240, includes a tubular member 2I6
having
an internally threaded distal end that is threaded to the proximal end of the
clamp. The
device 240 includes a rigid frame 244 having side walls 246 and end walls 248,
the side
and end walls being rigidly connected to each other. Near its distal end, the
device has a
pivot bar 250 that is guided by the side walls 246 and that is enabled a small
amount of
movement axially of the clamp 30. A pair of elongated compression bars 252 are
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18
positioned within the framework 244, the compression bars having outer ends
that can
pivot against the proximal end wall 248 and the distal pivot bar 250,
respectively. At
their adjacent ends, the compression bars are pivotally attached to a yoke
254. The yoke
has a threaded end which receives the threaded end of a screw drive handle
256, the end
of the screw bearing on the adjacent side wall 246. As shown in Figure 15, the
compression bars 252 extend at a slight angle to one another such that as the
yoke 254 is
forced to the left in this drawing by the turning of the handle 256, the
compression bars
will become more nearly in alignment with one another, forcing the pivot bar
distally.
The ends of the compression bars that pivot on the end walls and pivot bar are
suitably
shaped so that the compression bars cannot be moved "over center" but will,
instead, be
limited to a configuration in which they are in substantial alignment with one
another.
The pivot bar 250 contacts the proximal end of the push rod I50 such that when
the device of Figure 15 is operated, the push rod is moved distally with
respect to the
clamp 30, all in the manner described above.
1 S Another force generating device is shown in Figure 16. Here, the stem 200
is
again shown with an externally threaded proximal end to which the internally
threaded
distal end of a tubular member 216 is attached. In this embodiment, a piston
260 having
a piston rod 264 is positioned within a cylinder 262, the distal end of the
piston rod 264
coming into contact with the proximal end of the shaft 224. In this
embodiment, the
clamp 266 has a closed distal end which is engaged by the distal end of the
shaft 224.
On the other side of the piston 260, the cylinder defines a high pressure
chamber
268. The hydraulic fluid used in this chamber preferably is a sterile saline
solution or
other biologically compatible liquid. Sterile saline is supplied through valve
port 270,
through one-way valve 272 and into a second high pressure chamber 274, the
latter
having a diameter much smaller than that of the diameter of the chamber 268. A
piston
rod 276, itself acting as a piston, is received within the chamber 274.
Movement of the
piston rod 276 distally forces sterile saline from the high pressure chamber
274 through a
one-way valve 278 and into the high pressure chamber 268. Piston rod 276 is
driven, in
turn, by a piston 280 which axially moves in a low pressure chamber 282. The
chamber
282, and the downstream side 284 of that chamber, are connected to sources of
a gas
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such as air or nitrogen under pressure through the use of an appropriate spool
valve
shown schematically at 286 in Figure 16.
During operation of the device of Figure 16, air or other gas under pressure
is
admitted to chamber 282 and withdrawn from chamber 284, causing piston 280 to
descend distally and causing the much smaller diameter piston rod 276 to
compress
sterile saline in the second high pressure chamber 274. The high pressure
sterile saline in
turn enters chamber 268, forcing piston 260 and piston rod 264 downwardly into
contact
with the shaft 224. When the piston 280 has bottomed out within the chamber
284, the
flow of air or other gas is reversed to draw the piston rod 276 proximally and
to admit
sterile saline to the high pressure chamber 274. The one-way valve 278
maintains
pressure in chamber 268. The process is then repeated, with the piston 260
thus traveling
distally in increments as the piston 280 moves back and forth. By
appropriately sizing
the cross-sectional areas of the pistons 280, 276 and 260, low pressure air
can be
employed to generate very high compressive forces in the piston rod 264. The
seals far
piston 260 and piston rod 276 are high pressure seals that can withstand the
substantial
pressures that are generated. The fit between the piston rod 264 and the tube
214 need
not be particularly tight, permitting air in the distal chamber 267 to readily
escape. If
desired, an escape port may be placed in the walls of this chamber.
A similar actuating device is shown in Figure 17 in which the stem 200 is
attached at its upper end via threads to the tube 216. In a fashion similar to
that depicted
in Figure 16 (similar numbering being used to designate similar parts), a
piston rod 264
has its distal end in contact with a staff (not shown) that is in turn
received within a
clamp of the type described above. The piston rod 264 extends downwardly
through the
tube 216 from a piston 260 within a high pressure cylinder 268, the latter
containing,
preferably, a biologically acceptable fluid such as sterile saline. An upper
chamber 282
is provided with a one-way valve 290 for attachment to a high pressure source
of gas
such as nitrogen. A piston 292 operates in the cylinder 282, the piston 292
operating as a
diaphragm to separate the gas side of the cylinder 282 from the sterile saline
side as
shown at 294. Upon actuation of this embodiment by supplying nitrogen gas, for
example, under high pressure to the chamber 282, sterile saline under
substantially the
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same pressure in compartment 294 is forced through flow control valve 296 into
cylinder
268, acting there to force piston 260 and its piston rod 264 distally.
A variety of force-generating devices can be employed in the present
invention.
The clamp element, as noted above, is generally placed under substantial
tensile stress,
5 and the stress-producing elements desirably are so configured and arranged
as to avoid
harm to a patient or medical personnel in the event of a material failure of
any of the
elements. The devices of Figures 16 and 17 offer the advantage of using an
incompressible liquid in the high pressure chambers adjacent the stem of the
prosthesis.
Any material failure that causes even a slight increase in the volume afforded
the liquid,
10 as, for example, a slight increase in the dimensions of the high pressure
chamber, can
substantially completely release the pressure in that chamber. Moreover, by
completely
enclosing the clamp and activating shaft during assembly, as shown in the
embodiments
of Figures 15, 16 and 17, danger from catastrophic failure of the shaft or
clamp is largely
avoided.
15 While a preferred embodiment of the present invention has been described,
it
should be understood that various changes, adaptations and modifications may
be made
without departing from the spirit of the invention and the scope of the
appended claims.
