Note: Descriptions are shown in the official language in which they were submitted.
210~3~
JOINT PROSTHESIS
The invention relates to a joint prosthesis having
two projections pivotally mounted relative to one another
about an axis and intended for connection to bones, in
which one of the projections includes a fork-like head
and the other projection has a head portion which defines
a bearing eye and is insertable within said fork-like
head, wherein the joint axis extends through aligned
bores in the fork-like head and in the bearing eye.
Prostheses of the kind mentioned above can be found,
for example, in US-PS 4 193 139. A further joint
endoprosthesis, which can be found, for example, in DE-A
2351,912, is, in accordance with its construction,
primarily intended for the elbow joint. Generally, the
greater part of the prostheses known up till now are more
or less particularly adapted for special uses or for the
replacement of a particularly selected joint.
Joint prostheses which, like the joint prosthesis
according to the present invention, are primarily
intended to replace a finger joint, are found
particularly in the above-mentioned US-PS 4 193 139 and
also in EP-B 1 214 773. These two known joints are
limited with respect to the fact that they are intended
for use a8 finger joints having a single-axis hinging of
the bone portions intended to be pivotally connected
together, wherein the apparent intention is to use the
prosthesis primarily in the second joint inward from the
end of the finger. However, the known construction of
this kind of joint prosthesis requires that a relatively
large length of the bone on its front surface, where the
~oint is intended to be inserted, be removed in order to
create enough room to install the joint. This operation
results in the removal of a region of the bone to which
originally the ~oint ligaments were connected, with the
consequence that a new location for the connection of
ligament~ to the bone must be sought. Experience has
shown that an attempt to attach bone ligaments to a
`` ~102~30
location different from the original natural location i5
attended by considerable problems.
Mechanical joints proposed as prostheses, in
addition to possessing substantial mechanical strength,
must also be simple to assemble, in order that the
operation time for insertion can be kept correspondingly
short.
An aim of this invention is to provide a hinge
joint, i.e. a joint prosthesis particularly for use as a
metacarpal bone joint, which because of its construction
i5 such that, during insertion, only small portions of
the adjacent front surfaces of the bones intended to be
pivotally connected together must be removed, and in
p~rticular that the removal of the frontal bone surface
is sufficiently limited as to leave undamaged the
connection locations for ligaments. A further aim is to
provide A joint prosthesis of the kind described above,
which is characterised by a particularly simple
construction and assembly, without expensive instruments.
To realize these aims, the ~oint prosthesis in accordance
with the invention i8 such that the joint axis is
provided by a bolt which tapers toward one end and/or
steps down to a smaller outer diameter, wherein the said
end i8 configured as a circular locating member which
cooperates with corresponding circular locating members
on one of the aligned bores of the fork-like head.
Because the joint axis is defined by a bolt, it is
possible to keep the joint itself relatively small and to
ensure a secure anchoring of the projections in the
respective bones before undertaking the simple assembly
of the joint within a relatively small space.
Specifically, all that is necessary to assemble the joint
is simply to push the bolt into the corresponding bores,
for which a relatively small space is required, this
having the further result that the bones intended to be
connected together by this kind of artificial joint need
be trimmed or cut away over only a short length. In
' ' . : .
addition to this guiding capability, particularly
advantageous in terms of a simple assembly, the end of
the bolt can have a tapering configuration in order to
facilitate centering and entry of the bolt, 80 that the
final assembled condition is achieved by the guiding of
the free end of the bolt into the corresponding locating
members of the fork-like head. Assembly can be further
facilitated to a great degree by providing the axis-
defining bolt, at its end remote from the locating
member, with a blind bore for receiving an implement,
wherein a particularly simple insertion of an implement
for the manipulation of the relatively small-scale axis-
defining bolt is possible if the construction is such
that the blind bore has an internal thread. Generally,
with this construction, relatively small-scale axis-
defining bolts for corresponding small-scale joints can
be securely assembled in the least possible space, due to
the capability of using a correspondingly light and
manipulable implement.
Advantageously, the construction of the ~oint
prosthesis according to the invention is such that the
loc~ting members are constructed a~ circular grooves
andlor circular ribs. In order to make possible the
elastic registration of such circular grooves with
circular ribs, it is advantageous to utilize
physiologicAlly harmless synthetic material, wherein the
~s~e~bly or manufacture of the ~oint i8 considerably
simplified in that the ~ligned bores of the fork-like
head have substantially the same inner diameter, and in
that the locating members are provided in a bearing
insert which is securely installed in one of the two
bores of the fork-like head. Providing the aligned bores
with essentially the same inner diameters permits the
selection of correspondingly simple assembly instruments
for the projections and the fork-like head, whereby to
ensure the necessary small tolerances through the
corresponding, precision-built insert portions,
~ln~30
particularly the bearing insert. Further, it is po~Aible
to select corresponding high-strength material,
particularly metallic material, for the pro~ections and
the construction of the head. By using bearing inserts
with correspondingly small tolerances, and in particular
bearing inserts of physiologically harmless material, the
durability of the joint is significantly improved and
frictional wearing of metal parts against each other is
avoided. Such an undesirable frictional wear between
metallic parts of the joint, using up the self-
lubricating characteristics of various physiologically
harmless synthetic materials, also arises if the
configuration is such that the bearing insert has a ring
flange with an outer diameter greater than the inner
diameter of the bores, the said outer diameter, when the
~oint i8 assembled, being positioned on the inside of the
fork of the fork-like head, by which it is possible to
achieve lateral stabilization of the joint with a
correspondingly smaller tolerance, seen looking in the
direction of the joint axis. This effect can be still
further improved by providing in the bearing eye of the
head portion of the sQcond projection a bearing insert
which includes a ring flange having an outer diameter
greater than the inner diameter of the receiving bore for
the be~ring insert, whereby it can be ensured that, to
either side of the head portion carrying the bearing eye,
wearing of metallic portions of the inner walls of the
fork receiving this head portion can be avoided.
With reference to the use of the inventive joint
prosthesis as a ~oint in the metacarpal or finger region,
all that is needed i8 a hinge articulation of which the
construction, which broadly imitates the actual natural
movement characteristics of such hand joints, is
advantageously so arranged that the head portion carrying
the bearing eye has at least one abutment shoulder which
cooperates with the fork-like head of the other
projection. In this manner, at least one of the limit
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positions, for example the stretched position of the
joint, can be clearly limited by mechanical inter~erence,
without making it necessary to provide another spacQ-
occupying component.
In order to achieve a secure anchoring of the
projections, even with a small-scale joint of thi~ kind,
the configuration is aavantageously such that the
projections be essentially symmetrical about the
elongational axis which crosses the bearing axis, wherein
advantageously the projections have preferably sharp-
edged ribs extending in the direction of the long axis
and distributed over the periphery. Such ribs,
particularly with sharp edges, facilitate the anchoring
of the projections in the end region of the bones, and
offer a number of advantages during the installation of
the projections in the bones. Whereas with known
maximally asymmQtrical projections, stringent
requirements as to the precision of the corresponding
bone recess must be met to allow quick and secure
installation, the largely centrally symmetrical
pro~ections herein have substantially lower requirements
a8 to the maximum permissable tolerances of such
recesses, without prejudicing the sQcurity of the
installation. In order to secure a sufficient anchoring
effect immediately following insertion, the configuration
is advantageously 80 arranged that the sharp-edged ribs
decreasQ in height in the direction toward the free end
of the projection, while for the subsequently necessary
binding to the bone material, the configuration is
advantageously 80 arranged that the projections carry a
coating of biologically degradable ceramic material. The
insertion of the projections into the easily prepared
recesses in the adjacent ends of the bones intended for
pivotal connection is substantially simplified and
attainable with a smaller degree of force, by providing a
conical taper at the free ends of the projections.
-`~ 21 0~
The invention will now be described in greater
detail with reference to an example embodiment
illustrated schematically in the drawings. The latter
illustrate: in Fig. 1 a schematic section, ~or example
through a metacarpal joint, with an installed ~oint
prosthesis in accordance with the invention; in Fig. 2 a
plan view of the one joint portion or projection
supporting the fork-like head, for the inventive joint
prosthesis of Fig. l; in Fig. 3 a side elevation of the
projection or joint portion seen in the direction of
arrow III in Fig. 2; in Fig. 4 an elevation view in the
direction of arrow IV of Figs. 2 and 3; in Fig. 5 a side
elevation of the second joint portion or projection
carrying the bearing eye, of the inventive joint
prosthQsis according to Fig. 1, analogous to the
illustration in Fig. 3; in Fig. 6 a plan view of the
second projection seen in the direction of arrow VI of
Fig. 5; in Fig. 7 an elevation of the joint-axis-defining
bolt of the joint prosthesis according to the invention;
in Fig. 8 a sectional view through a bearing insert
adapted to be inserted in one part of the fork-like head,
the latter carrying a locating member for cooperation
with the bolt illustrated in Fig. 7; and in Fig. 9 a
section through a further bearing insert, which is
insertable in the bearing eye of the head portion of the
second projection of the ~oint prosthesis in accordance
with this invention.
Fig. 1 shows generally at 3 a joint prosthesis
installed as a replacement for the natural joint between
two bones 1 and 2, wherein the joint prosthesis 3
includes two projections 4 and 5 which are intended to be
inserted in bored or hollow regions of the bones, and
which are more fully described below in detail. The two
projections or joint portions 4 and 5 are pivotally
connected together about a pivot axis which is identified
schematically in Fig. 1 by the number 6. The pivot axis
is provided by a bolt insertable in aligned bore~ of the
~10~
projections 4 and 5, the bolt being illustrated in Pig.
7. The actual joint regions of the ~oint prosthesis, i.e
the portions which project out of the bone~ 1 and 2, are
short enough, when using a bolt as the ~oint axis, that
the natural growth locations for the ligaments in the end
regions of the bones do not need to be removed, the said
growth locations for the ligaments being schematically
identified by the numeral 7.
Figs. 2 and 3 illustrate more fully the projection
or joint portion 5 which supports a fork-like head.
Directly adjacent the projection 5, which tapers
conically to its free end 8, two fork-portions are
provided which define two aligned bores 10 which have
essentially the same diameter. Again, the pivot axis is
identified by the numeral 6. As can be clearly seen in
Fig. 4, the pro;ection 5 defines, extending in the
direction of its long axis 11 and distributed around the
periphery, sharp-edged ribs 12. Fig. 4 also clearly
shows the conically tapering configuration of the
pro~ection.
In assembled condition, the fork portions 9 enclose
the bearing eye 13 of the second projection or joint
portion 4, which is illustrated in detail in Figs. 5 and
6. The projection 4 or bearing eye 13 defines an opening
14 aligned with the bores 10 of the second projection 5,
the opening 14 in the assembled condition also receiving
the bolt illustrated in Fig. 7. The projection
illustrated in Fig. 5 and 6 additionally includes a
conically tapering cross-section in the direction of its
free end 15, and is likewise provided with sharp-edged
ribs 12. Generally, the two projections or joint
portions 4 and 5 are such that the portions extending
into the bones are both essentially symmetrical and
identically constructed, so that the joint can be
installed in both positions.
The sharp-edged ribs 12 distributed about the
circumference are arranged such that, when installed in
-` 21 02~13~
the bones, only a single rib is located on the inside,
i.e. on the bending side of the ~oint, wherea~ the two
remaining ribs 12 shown in the drawing are directed
toward the outside, as is schematically indicated ln Fig.
1. Beginning at the respective free ends 8 and 15, the
ribs have a gradually increasing height, thereby to
facilitate the insertion of the respective pro~ections
into the interior of the bones. Generally, the ribs
serve to anchor the pro~ections in the bones, and to
avoid any rotation of the joint portions in the bones.
Further, because of the presence of the ribs 12, the
geometric configuration of the recesses in the bones does
not need to meet excessively stringent requirements,
since the ribs, due to their graduated heights, can
compensate for irregular recesses in the bones.
In one of the fork portions 9 within the
corrQsponding bore 10, there is provided a bearing insert
16 illustrated in Fig. 8, including a ring flange 17,
which is placed to the inside of the corresponding fork
portion. In a similar manner, the bearing insert 18
illustratQd in Fig. 9 is inserted into the bearing eye 13
of the second pro~ection, the item 18 also having a ring
flange 19 which lies to the inside of the bearing eye,
thus being located between the bearing eye 13 and the
ad~acently lying fork portion 9. Naturally, the bearing
insert 16 is installed in the sQcond fork portion 9
provided.
For the assQmbly of the joint prosthesis, the bolt
20 shown in Fig. 7 is insQrted either through the
openinge 10 and 14 of the pro~ections 5 and 4, or through
the openings provided in the bearing inserts 16 and 18,
the bolt 20 defining the joint axis. The bolt 20 is
configured such that it tapers at one end to a smaller
diameter, wherein at the free end 21 of the bolt 20, a
ring-shaped locating member 22, in the form of a circular
rib, is provided. When the bolt 20 is inserted from that
side of the assembled joint prosthesis which is adjacent
"` ~102~.3~
the fork portion 9 containing the bearing insert 16, th~
bolt initially passes through the ~ir~t opening of the
first fork portion 9, thereafter pa~sing through the
opening 23 of the bearing insert 18 in the bsaring eye,
and finally reaching the bearing in~ert 16 in the ~econd
fork portion. As can be seen in Fig. 8, the bearing
insert 16 has a circular projection 24 determined by the
diameter of the circular rib 22 of the bolt 20, 80 that,
when the joint locating bolt is fully inserted, the
circular rib is guided by the circular projection 24 of
the bearing insert 16, and the joint is closed in a
simple manner, i.e., the two projections or ~oint
portions 4, 5 are pivotally linked with each other.
For the insertion of a handling implement, the joint
bolt 20 is provided with a blind bore 25 which has an
internal thread over a schematically indicated region 26,
80 that there can be provided, for example, a threaded
member or the like for the simple simple manipulation of
the bolt or the application of the required force,
whereby the implement, after assembly, can be removed in
a simple manner.
In the use of the present ~oint prosthesis for the
bones of the fingers or the metacarpus, only a single-
axis hinged connection is necessary, a limit position for
the ~oint prosthesis 3 being illustrated in Fig. 1,
wherein a bend angle of roughly 90 is obtainable. In
order to prevent excessive stretching or bending in the
opposite direction, the bearing eye 13 illustrated in
Fig. 5 and 6 has an abutment shoulder 27, which in the
stretched condition makes contact with the inner surface
28 of the pro~ection 5 between the forked portions 9,
thus preventing excessive bending beyond the stretched
position. Because of this contact, the angle attainable
beyond the stretched position is limited to about 10.
As materials for the joint prosthesis special
titanium-aluminum alloy8 are utilized, which are noted
for their low specific weight, high elasticity, high
21Q~.3~
strength and excellent corrosion resistance. The bearing
inserts 16 and 18 illustrated in Fig. 8 and 9 are
provided not only to lock the ~oint portions together,
but also to prevent the rubbing of metallic parts again~t
each other. To this end, the bearing inserts are
constructed of a biologically compatible material, having
good sliding characteristics with the least possible
wear, along with optimal damping characteristics. For a
rapid installation of the prosthesis, in particular for
the projections 4 and 5 in the bones, a suitable
biologically degradable coating of material is applied to
the projections 4 and 5, for example through plasma
spraying, making it possible to shorten the discomfort
time, and permitting an earlier use of the joint.
BecausQ of the complete accommodation of the bones to the
implanted projections 4 and 5, attained as just
mentioned, the artificial joint achieves an early and
satisfactory load-carrying capability.
In order to accommodate different bone sizes, the
ao projQctionB 4 and 5 can be provided with different
external diameters. The conical taper of the projections
4 and 5 is selectQd such that, during the anchoring of
the ribs 12, a self-locking connection is attained which
opposQs the removal of the projQctions from the bones,
wherQin, corresponding to the material used and the
bonQs, a bending angle of 5 to 7 is selected. In the
evQnt that, contrary to the illustrated embodiment, a
smaller number Or ribs is found to be sufficient, the rib
on the bending side can be removed, based on the
kinematics of the joint.
