Note: Descriptions are shown in the official language in which they were submitted.
WO 95/24875 PCT/GB95/00518
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HAND PROSTHESIS
The present invention relates to hand prostheses and in
particular to such prostheses with movable gripping fingers.
The design of such hand prostheses presents formidable
problems in achieving gripping light enough to handle
fragile objects and strong enough to provide a secure grip
to objects where it is desired to apply substantial forces.
These problems are further compounded by the limited spaced
available, especially within the fingers themselves, and
where it is required to provide for independent movement of
different fingers to a greater or lesser extent, and/or
varying the rate of movement of individual fingers.
A particular problem arises with the fitting of hand
prostheses in patients with relatively long hand stumps.
Conventional electrical hand prostheses use an electric
motor mounted in the body structure of the hand itself.
Some versions have the motor mounted axially parallel to the
long axis of the arm and others have the motor at 90 degrees
to this axis. A variety of transmission systems are used to
link motor and fingers e.g. lead screw and nut, or bevel and
spur gears. Without exception these types of hand
prosthesis require precise alignment between motor and
transmission system. This is usually achieved by locating
all the parts within a hand body consisting of an investment
casting or other moulded structure. Such an arrangement is
relatively cumbersome and the motor cannot be readily
accommodated within the limited space available in the main
body of the prosthesis attached to the patient's hand stump.
Another disadvantage is that patients with some residual
digits cannot have any functional restoration at all because
of the space constraints. The operational characteristics
of such devices also tend to be restricted e.g. allowing
only a single gripping pattern to be employed.
It is an object of the present invention to avoid or
minimise one or more of the above disadvantages.
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The present invention provides a prosthesis for providing at
least one mechanically operable finger member, said
prosthesis having at least one said finger member extending
generally tangentially with respect to a fixed worm gear
wheel means on a support bod~~;o:f said prosthesis and mounted
for rotation about the worm.g~ar wheel spindle, said finger
member having a drive motor'with a worm extending generally
longitudinally of the finger member and in engagement with
the gear teeth of said worm gear wheel so that when said
drive motor is operated, in use of the prosthesis, said
finger member moves around said worm gear wheel towards or
away from another finger member and/or a natural finger for
closing and opening of a hand grip.
Thus the hand prosthesis of the present invention uses a
particularly compact form of finger drive which also can
allow improved operational flexibility.
As used herein the expression "finger member" includes a
"thumb member". Whilst it is possible to provide a useful
hand grip defined by just one movable finger member and one
fixed member, it is generally preferred to have a hand
prosthesis with a plurality of movable finger members, most
preferably with control means formed and arranged so as to
permit more or less, independence of movement of the finger
members or groups of finger members, e.g. to allow higher
speed lower torque movement of a "thumb" finger member, and
slower speed higher torque movement of the other finger(s),
and/or to permit the provision of different gripping
patterns with different combinations of individual/group
finger movement characteristics.
Conveniently a root portion of said at least one said finger
member provided with a drive motor is connected by a linkage
means to the root portion of at least one other (motor-less)
finger member mounted for pivotal movement on said support
body, said linkage means being formed and arranged for
transmitting drive to said other finger member so as to
pivotally move said other finger member towards or away from
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said at least one finger member provided with a drive motor
during direct movement thereof by the drive motor for
closing and opening of a hand grip.
Whilst this kind of arrangement may have less power and
operational flexibility than dther forms of the invention in
which separate motors are used for different fingers, it
does have the advantage of requiring fewer motors and less
power consumption thereby providing a more economical
solution.
Various suitable motors having a relatively high power-to-
weight ratio are known in the art including permanent magnet
DC motors which have a substantially linear relation between
torque and drive current over a reasonably wide range which
facilitates control of the driving of the finger member.
Particularly suitable motors are available from Minimotor SA
of Switzerland, especially their motors which have a
diameter of around 8 to 17 mm. A further advantage of this
type of motor is the availability of a modular gearbox
system coupled to the output shaft of the motor which allows
different torque-output drive speed ratios to be selected
simply by choosing from a range of gearboxes with different
ratios. This has the advantage of facilitating the
provision, in a prosthesis, of different gripping patterns
by simply using different gearboxes in different finger
members.
In general it is preferred to have a faster moving lower
torque thumb finger member with slower moving higher torque
other finger members as this simplifies the provision of a
cosmetically acceptable sheathing or cladding of the
mechanical components of the prosthesis due to the reduced
a range of movement of the other finger members.
The compact form of drive motor and mounting thereof
substantially within the finger member being driven,
particularly when said drive motor is inside a thumb finger
member, allows for a reduction in finger size and hence a
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lighter construction, which is particularly desirable in
application of the prosthesis to children. Moreover the
compactness allows fitting to longer stumps which include a
mobile wrist without any loss of wrist mobility and without
making the hand appear too long.
Further preferred features and advantages of the invention
will appear from the followingy~etailed description given by
way of example of some preferred embodiments illustrated
_, .
with reference to the accompanying drawings in which:
Fig. 1 is an end view of the principal components of the
operating mechanism of a hand prosthesis of the invention;
Fig. 2 is a partly cut-away view of one finger member of
hand prosthesis using the mechanism of Fig. 1;
Figs. 3 (a) and (b) are perspective views of a second
embodiment of hand prosthesis of the invention; and
Fig. 4 is a rear view of the hand prosthesis shown in Figs.
3 (a) and (b) .
Fig. 1 shows the principal components of the operating
mechanism 1 of a hand prosthesis having a main body 2 which
is securely fixed in use to a patient's hand stump (not
shown) in generally known manner, and a plurality of finger
members 3. The main body 2 has mounted thereon a plurality
of fixed worm wheels 4 for engagement, each with a worm 5
mounted in the base 6 of a respective finger member 3 to
extend generally tangentially of the fixed worm wheel 4.
The worm 5 is mounted on the drive shaft 7 of a gearbox and
drive motor combination 8 extending longitudinally of the
finger member 3. The finger member 3 is pivotally connected
9 to the main body 2 for pivoting about the axis 10 of the
fixed worm wheel 4 so that when the motor 8 is actuated to
drive the worm 5, the worm travels around the circumference
11 of the fixed worm wheel 4. At the same time the distal
end 12 of the finger member 3 describes an arc indicated by
the arrows 13 for opening and closing of a hand grip.
The motor 8 is powered by small rechargeable Nickel Cadmium
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batteries 14 (shown schematically) which may be remotely
mounted e.g. in the 4th and 5th fingers (not shown) of the
prosthesis or in any suitable space between the stump (not
shown) and the prosthesis.
The motor 8 is controlled by means of switches 15 actuated
by either residual digit movement or by wrist motion. Two
switches may be used to control both directions of rotation
or alternatively one switch may, by being interfaced through
suitable electronic circuitry, be used to control both
directions in a sequential or toggled manner. In addition
to switches, pressure signals derived from force sensitive
resistor material or signals derived from the
electromyographic activity of residual muscle actions, may
be used as control signals.
The main body 2 of the prosthesis itself may conveniently be
in the form of a simple bar 16 with finger and thumb spigots
17, 18 extending therefrom. The pivotal mounting of the
finger members 3 is effected via lugs 19 which depend from a
generally tubular housing 20 in which are mounted the motor
8 and worm 5.
The components 2 of the prosthesis operating mechanism 1 can
be clad in an overlay 21 of silicone rubber or the like to
provide a more aesthetically acceptable appearance as
similar as practicable to a normal hand appearance, in
generally known manner.
Figs. 3 (a) & (b) and Fig. 4 shows a second embodiment of
hand prosthesis and will be described with reference to the
embodiment shown in Figs. 1 and 2 and indicated by like
reference indicia with suffix 'a' added. The operating
mechanism la of a hand prosthesis has a main body 2_a (only
partially shown) which is securely fixed in use to a
patient's hand stump in generally known manner. The main
body 2a has mounted thereon a single fixed worm wheel 4_a for
engagement with a worm 5_a mounted in the base 6a of a thumb
finger member 3a to extend generally tangentially of the
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ffixed worm wheel 4a. The worm is mounted on the drive shaft
of a gearbox and drive motor combination 8a.
The thumb finger member 3a is pivotally connected 9a to the
main body 2_a for pivoting about the axis of the fixed worm
wheel 4a so that when the motor 8a is:.'.actuated to drive the
T '
worm 5a, the worm travels around th~'r~circumference lla of
the fixed worm wheel 4a. The mai~.::,body 2a_ has pivotally
mounted 22 thereon also a pair o~','finger members 23. The
finger members 23 have an inwardly depending lever 24
pivotally connected by a generally arcuate or cranked
linkage member 25 to a lug 19a which extends from the
housing 20a in which is mounted the motor 8a and the worm
5a. As may be seen from Fig. 4 the pivotal plane of the
thumb finger member 3a is inclined (generally by an inclined
angle a of around 55 to 60°) relative to that of the finger
members 23 to provide a more natural arrangement.
It will be seen from Figs. 3 (a) and (b) that when the motor
8a is actuated to drive the worm 5a, the worm travels around
the circumference lla of the fixed worm wheel whilst driving
the linkage member 25 against the inwardly depending lever
24 on the finger members 23 so as to transmit drive thereto
so as to pivotally move the finger members 23 towards or
away from the thumb finger member 3a for closing (Fig. 3
(b)) and opening (Fig. 3 (a)) of a hand grip.
As with the first embodiment described hereinbefore the
components of the second embodiment of prosthesis can be
clad in an overlay of silicone rubber or the like to provide
a more aesthetically acceptable appearance, in generally
known manner.
The compact form of finger drive and mounting thereof
substantially within the finger being driven thereby, as
well as the essential neutrality i.e. non-handedness of its
configuration, allow an essentially modular approach to the
construction of electrically operated hand prostheses. This
in turn leads to a number of advantages including one or
WO 95124875 ~ PCT/GB95/00518
more of reduced stockholding requirements as various
different prostheses, including left and right handed ones
and ones with different numbers of mechanical fingers may be
readily assembled from a small number of common components.
In addition different grip patterns can be readily achieved
by use of suitable controls with the independent finger
drives, with the 'further advantage that there is no need to
compromise between grip force and speed of operation or
movement.
The modularity and compactness of the units also allows
individual finger prostheses to be used e.g. in the case of
congenital deformities of the type where one or more
fingers, especially the middle fingers which are important
to effective prehension, are missing.
