Lower extremity prosthesis

A lower extremity prosthesis solves the problem of relieving the end of the leg stump (S) from a portion of the vertical forces acting on it by pressure from the socket (1) surrounding and supporting the stump, by providing a mechanism capable of transferring forces to the tibia (T). The mechanism includes a first member (13) connected to the tibia and second members (5, 8) detachably connected to the first member and carried by the socket.

BACKGROUND OF THE INVENTION 
The present invention relates to a lower extremity prosthetic fitting which 
is improved in respect of the transfer of loading forces between the 
fitting and the skeleton parts inside the shortened leg, the stump. 
Irrespective of whether the reason for the shortening was morbid changes 
of the lower portion of the leg which made it necessary to remove that 
portion by amputation, for instance due to diabetes, or the result of an 
accident, e.g. a traffic accident, the residual part of the lower 
extremity will assume the shape of a stump in which the skeleton parts are 
both laterally and at their lower ends surrounded by soft tissues. When, 
following healing after the shortening, a lower extremity prosthesis is to 
be fitted, this fitting is provided with a socket open at its top end and 
tapering in the downward direction. It is intended to surround the stump 
and at its lower, closed end connected to an artificial foot. The function 
of the socket is to maintain the fitting connected to the stump. In order 
for the fitting to fulfill that function the internal shape of the socket 
must in a special way match the shape of the stump so that the loading 
forces acting on the foot can be transferred to the stump and from there 
further on to the knee-joint, the thigh-bone and the hip-joint. 
The first step in that force transfer, from the socket to the knee-joint, 
takes place via the residual part of the tibia. However, this transfer 
will be indirect in the sense that as mentioned above, this skeleton 
member is surrounded by soft tissues also at the bottom of the stump. 
These soft portions are deformed, especially during the transfer of the 
substantially vertically oriented forces which occur when the patient is 
standing, when he walks and, generally, when the distance between the 
artificial foot and the thigh is varying. In such situations there occurs 
a relative movement between the stump and the socket. This movement, in 
the vertical direction often referred to as "pumping" and in the 
transversal direction as "staggering", is undesired for two reasons. One 
reason is that the play results in instability and the second reason that 
it may, in combination with the pressure against the soft portions, 
develop infections and wounds, in the most serious cases to such an extent 
that the prosthesis must not be used during a longer or shorter period of 
time. 
There does accordingly exist a need for a possibility, to the extent 
desired, to replace this indirect transfer of forces to the tibia, that is 
via the remaining soft portions of the leg below the knee-joint, by a 
rigid, mechanical transfer direct to the tibia, which would result in a 
corresponding elimination of the possibility of pumping and staggering 
including their detrimental consequences. 
It is prior art in thigh-leg prostheses to attain such a stress-relief by a 
rigid force transfer from the prosthesis directly to the skeleton member 
in the thigh. In such prior art designs the connection to the thigh-bone 
has been created by means of a pin extending vertically upwards from the 
bottom of the socket and entering an axial bore from the bottom end of the 
thigh-bone. It is, however, for several reasons very difficult to use such 
a force transfer arrangement in low extremity prosthetic fittings. Those 
reasons are due to the anatomical differences, in terms of different 
thicknesses and different shapes, which exist between a thigh-bone and a 
tibia. In both those cases the bone tissue proper surrounds an axial 
extending space containing bone-marrow. While the thigh-bone, femur, 
exhibits a great transversal dimension--it is the thickest bone of the 
skeleton--and is almost circular in cross-section, the tibia is 
considerably more narrow and substantially triangular in cross-section. 
Further, its wall thickness is considerably less. Also in a grown up 
person it normally amounts to just 3-5 mms and, quite naturally, in 
children the tibia wall surrounding the central bone-marrow space is still 
thinner. In this context it must also be noted that the force-absorbing 
ability of a pin inserted axially in the bone-marrow space is limited as 
far as lateral forces are concerned. While it is true that such lateral 
forces are of a negligible magnitude when the amputee is standing still in 
an upright position, they increase to considerable magnitudes when he is 
walking in which situation also dynamic force components are added. Should 
the amputee for instance kick or slip, these forces become very high. For 
those reasons orthopaedists and prosthesis-technicians have had to 
establish that so far they have not succeeded to solve the pumping 
problem, its related inconveniences and the risks for the patients, by 
using in lower extremity prosthetic fittings the same type of mechanisms 
for transferring forces between the fitting and the skeleton members of 
the extremity as can be used in thigh-bone prostheses. 
SUMMARY OF THE INVENTION 
However, this problem has now been solved thanks to the present invention 
which makes possible a direct transfer of a greater or smaller share of 
the forces from the prosthesis to the tibia in another way than by the use 
of an axial pin, namely so that some of the loading forces are transferred 
to the tibia in a lateral direction. The remaining part of the forces is 
in the normal manner transferred via the socket. Actually, the share of 
the forces transferred via a mechanism according to the present invention 
needs to amount to a smaller portion of the total force only in order to 
result in a reduction of the stump problems. The realization that this is 
a possible solution of those problems constitutes the new and most 
important part of the inventive concept. It should however be underlined 
that this new principle does not only involve a displacement from one 
position to another of the point of application of the loading forces. 
Just the other way round the situation is that, when the just-mentioned 
inventive concept is to be applied in practice, there will arise several 
detail issues, both of a medical and of a mechanical nature. However, the 
invention includes replies also to such questions, to which reference will 
be made below. A great advantage of the invention is that the surgical 
intervention is small. 
Before proceeding to the special portion of the description I will, 
however, first further illuminate the interaction between the forces in a 
lower extremity prosthesis. 
When an amputee equipped with a prosthesis is standing or sitting still, 
naturally only static pressure forces are in play and--when the leg is in 
a vertical position--these forces become vertically oriented. When the 
foot is suspended freely, there will exist a pulling force between the 
fitting and the stump which force, when the foot rests against the floor, 
will change to a pushing stress. In the latter case the weight of the leg 
or that of all of the body, respectively, will generate a reaction force 
directed upwards which tends to compress the soft tissues between the 
bottom of the socket and the stump. In the first-mentioned case, when the 
foot is lifted above the floor surface, the pulling force directed 
downwards corresponds to the sum of the weight of the fitting proper and 
the shoe of the amputee. As is understood, in that position the just 
mentioned soft tissues are deloaded, which results in an increased degree 
of pumping due to the play between the stump and the socket then arising. 
When the person is walking there are also generated dynamic forces which 
are added to the static forces by an amount typically around 20% and 
sometimes up to approximately 50%. One of those additional forces is the 
longitudinal force (as seen in the longitudinal-axis direction of the leg) 
which is generated each time the artificial foot comes into contact with 
the ground surface and the other foot is more or less deloaded. Another 
such dynamic force is the torsional force generated when the artificial 
foot is exposed to a torque, generally oriented in a horizontal plane and 
counteracted by the friction against the underlying surface. Thirdly, when 
he is walking, there are generated dynamic forces tending to incline the 
fitting relatively the tibia. Those forces can, from a stress-loading 
point of view, be divided into two components, one of them oriented in the 
walking direction and the other perpendicularly thereto. If the 
longitudinal force is assumed to be about 50% greater than the weight of 
the body, it will amount to about 1000N. The two tip forces may generate 
torques of the order of magnitude of 30 Nm. It has empirically been found 
that the resultant of all those forces may grow to such a value that the 
soft tissue layer in the bottom portion of the stump yields pains or 
inconveniences, sometimes injuries. 
A force transmission according to the present invention has the decisive 
advantage that the soft portions of the leg stump are deloaded thanks to 
the fact that they are no longer exposed to all of the vertical pressure 
component of the transfer forces which may instead--to a greater or lesser 
degree--be transferred directly to the skeleton from the lateral wall of 
the socket. As mentioned above, this has not previously been considered 
possible in this type of prostheses. This invention has made it possible 
by the use of a force transfer member extending transversally, i.e. 
"diametrically", through the tibia. Also such transversal forces which 
tend to create staggering of the stump in the socket may to a greater or 
less extent be transmitted via such means.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
The parts of the fitting visible in FIG. 1 are a socket 1 which in a manner 
known per se is shaped to match the leg stump and which suitably consists 
of a plastic material. For cosmetic reasons a sock 2 covers part of the 
socket. A resilient plate 5, is by means of upper fixation members 3 and 
lower fixation members 4, secured substantially above the sock. The 
fixation means may be constituted by for instance screws which engage 
socket 1. The resilient plate consists of a high-quality elastic material 
such as urethan rubber. In the front wall of the socket there is an 
opening 7 which is covered by plate 5. A ball pin 8 extends through the 
plate and through the opening. Its detailed configuration will be 
described below with reference to FIG. 2. In FIG. 1 there is visible only 
the head of the pin which extends through a control arm 9. The head is 
rotatable so that a bayonet catch of the ball pin can be locked and 
unlocked. Numeral 10 designates a cotter pin. The control arm supports two 
wire-shaped arms 12. These can be vertically turned and displaced in a 
sleeve 11, secured to the control arm. Their purpose is to distribute the 
force from the ball pin and the control arm to all of the width of the 
resilient plate. The possibility to displace the two arms longitudinally 
is relied upon when the amputee desires, in a sitting position, to pull 
out the stump somewhat from the socket. The ball pin will then move in the 
elongate hole in plate 5. The control arm has a transversal yoke 9a, which 
by being clamped between the resilient plate and the sleeve limits the 
possibility of the control arm to turn around its vertical longitudinal 
axis. 
The cross-section in FIG. 2 illustrates the parts of the mechanism not 
visible from the outside. As mentioned above, FIG. 2 is a vertical section 
on an enlarged scale through a tibia with a force transfer mechanism and 
an implanted skeleton screw according to the invention. It is important to 
note that the resilient plate 5 is located at the front side of the 
fitting but angularly displaced inwards approximately 45.degree.. In FIG. 
2 reference letter T refers to the tibia. Letters F and B refer to the 
front and back walls of the tibia which along the major portion of its 
circumference is surrounded by muscular tissue M and substantially 
triangular in cross-section. However, on the front side there is 
practically no muscular tissue, just a thin fat coating under the skin H. 
The invention takes advantage of that fact which will be explained below. 
In the area of the window opening 7, suitably in its center, the tibia is 
traversed by a skeleton screw 13 having a thicker cylindrical section 14 
passing through the front wall F of the tibia, a thinner section 15, 
traversing the back wall B of the tibia, and an intermediate conical 
section 16. As illustrated in the drawing, the two cylindrical sections of 
the skeleton screw have threads in engagement with the adjacent portions 
of the tibia in which holes have been drilled as will be explained below. 
In the thicker section 14 of skeleton screw 13 there is a cavity housing a 
member which constitutes a ball retainer with a bayonet lock. It is 
referenced 17 and does likewise comprise an inner, thinner section and an 
outer, thicker section. The latter has external threads engaging 
corresponding internal threads in the smaller section of the cavity. The 
thicker section of the ball retainer has an outwardly open chamber 
receiving the spherical ball 8a and it is shaped so as to form a seating 
for the ball. The contour line of the opening corresponds the shape of the 
ball which has two flat-ground segments so that a bayonet lock is formed. 
The ball pin does, in addition to the head 8b, also comprise a cylindrical 
portion 8c, likewise located outside the window 7 and via an inwardly 
tapering section 8d integrally connected with the ball. 
The mode of operation of the mechanism for transferring forces between the 
skeleton and the socket is as follows. The lateral position of ball 8a is 
substantially central in the bone wall. Accordingly, forces in the 
longitudinal direction of the leg and in the one transversal direction are 
relatively smoothly transferred to the bone tissue from the thicker 
section of the skeleton screw. Forces in the second transversal direction, 
i.e. in the axial direction of the skeleton screw, are also transferred in 
a favourable way via the threads at both ends of the screw. When the 
tibia, in response to various movements in the soft tissues of the stump, 
moves the ball pin, resilient plate 5 will counteract those movements. 
Thanks to the fact that the plate can be biased and given different 
dimensions, the magnitude of the transmitted forces can be selected to 
match the conditions in each individual. The combined functional 
contributions from the skeleton screw, the ball pin, the control arm and 
the resilient plate are characteristic for the invention. The bias has 
been attained by means of fixation screws 3 and 4. Naturally, it is 
necessary that the patient can himself remove the fitting, for example 
when going to bed, which he does by pulling the external parts of the 
mechanism outwards so that ball 8a leaves its seating. 
FIGS. 5-10 exemplify how to mount those parts of the mechanism which are 
permanently secured to the tibia, i.e. osseo-integrated. The drawing shows 
horizontal cross-sections through the tibia T. Adjacent muscular tissues M 
and the skin H have also been shown. The first step of the operation is to 
cut out a substantially semi-circular skin flap having a height of about 
40 mms and a width of about 20 mms, whereupon the flap is folded upwards. 
The next step is to drill a hole about 25 mms above the bottom end of the 
bone. The diameter of the drill should not exceed the core diameter of the 
thread on the narrow end section 15 of the skeleton screw. As shown in the 
drawing, the drilling is performed to such a depth that the rear as well 
as the front wall of the tibia are penetrated. The drill is then replaced 
by a thicker one, shown in FIG. 6, the diameter of which is somewhat less 
than the external diameter of the thread tap 19 and its extension 20, 
shown in FIG. 7. The drill in FIG. 6 does only pass through the front 
hole. The next step is that thread tap 19 and its extension 20, the 
diameter of which corresponds the diameter of the thicker drill in FIG. 6, 
are inserted through the hole in the front wall of the tibia and further 
in so that tap 19 creates internal threads in the rear tibia wall and 
slightly less deep threads in the front wall. This has been shown in FIG. 
7. Retaining the thread tap in its position the next step, shown in FIG. 
8, is to expand the front hole so that its diameter equals the core 
diameter of the thicker section 14 of the skeleton screw. The drill is 
hollow and its cavity receives extension 20. Thanks to the fact that tap 
19 and said extension remain in position the thicker drill will be 
accurately centred relatively the tap. Stated in other words this 
obviously means that the two holes in the front and the rear wall of the 
tibia become coaxial. The last-mentioned drill is removed from the 
extension and replaced by an, equally tubular, thread tap having internal 
threads in engagement with the threads on the tap extension 20, and 
external threads, creating the internal threads in the front wall of the 
tibia which are to engage the thicker section 14 of the skeleton screw. 
Finally, the thread taps are removed whereupon screw 13 can be mounted as 
shown in FIG. 10. Finally, the skin flap is folded back into position and 
secured by sewing. The skeleton screw is now implanted in the tibia and 
the corresponding healing process requires a couple of months. During that 
time period the bone tissue is integrated with the surface layer of the 
skeleton screw. The patient may during that time be temporarily equipped 
with a conventional prosthesis having a orifice around the screw position 
so that he can walk without disturbing the surgery spot. 
The selection of material in the skeleton screw is critical. It is per se 
previously known that pure titanium is superior, not only to other metals 
like stainless steel, but also to titanium alloys. In practical 
applications "pure" means a purity of 99,75%. From other types of 
prostheses, especially within dental surgery, it is since three decades 
known that components of pure titanium quickly oxidize on their surface so 
that there is formed a layer of titaniumperoxide. This layer yields a 
barrier function against chronical tissue inflammation and becomes 
incorporated in the cells of the surrounding tissues thereby counteracting 
repelling phenomena. This phase of the healing period, during which the 
biocompatible material titanium is integrated with the bone tissue to a 
stable compound and the skin flap will become secured on the tibia, is 
typically 6-8 weeks. Following the end of this period a hole is punched 
through the skin flap. Fat tissue under the skin adjacent the screw is 
removed so that the skin gets a possibility to attach itself to the 
underlying bone tissue. Next the skin is left to heal, which again 
requires a few days, whereupon the ball screw 17 is screwed into the 
skeleton screw. This concludes the mounting of all mechanism members to be 
permanently implanted. What now remains is to mount the detachable 
external members, i.e. the ball pin 8 and the other means already 
described above in connection to FIGS. 1 and 2. 
The importance of selecting pure titanium for the implanted members has 
been emphasized above. It should also be underlined that the dimensioning 
of the different members is critical irrespective of the fact that 
variations between individual patients must be allowed to match the 
equipment to the weight and the size of the body. Nevertheless the 
following dimensioning data may serve as a guidance. The diameter of the 
threaded hole in the front wall of the tibia should be about 12 mms and in 
the rear tibia wall about 6 mms. A suitable pitch is about 1 mm. As has 
been mentioned above, it is also to be noted that the force transfer to 
the tibia taking place in a device according to the invention does not 
handle the total force system. Instead, the situation is that the 
implanted mechanism and the socket cooperate. For that reason it is 
important that the socket is, in an anatomical sense, optimally matched to 
the shape of the stump since some of the forces are still transferred from 
the socket to the stump. There does consequently occur a direct force 
transfer from the stump to the socket, to the mechanism and to the tibia 
with a partial "short-circuiting" of the soft tissues in the stump. On the 
other hand, the force transmission between the socket and the stump passes 
those soft tissue regions. 
The cooperation from a force-transfer point of view between the mechanism 
and the socket is especially important after the healing period. By 
exchanging parts of the mechanism, or by making it variable without the 
need of such an exchange, one can during that period of time successively 
increase the share of the total forces which are transmitted via the 
mechanism up to the point of time when it is absolutely certain that the 
skeleton screw is implanted in the tibia in a complete and stable way. 
When the skeleton screw is exposed to loads a successive growth of bone 
material will take place resulting in an increased strength. 
An arrangement according to the invention may be modified in several 
respects as far as the detailed structure of the mechanism is concerned. 
In some cases it may be advantageous to have two screws sharing the load 
between them. The only essential requirement is that the mechanism is 
designed so that the soft portions of the stump can be partially relieved 
of the static and dynamic forces. In this context it is a fundamental 
feature, instead of a, theoretically imaginable, connection comprising an 
axially upwardly directed pin inserted in the tibia, to use a member 
extending laterally into the tibia and anchored into mutually opposite 
parts of its bone wall. However, the switching from axial to lateral 
connection also entails other advantages than those resulting from the 
corresponding displacement of the position at which the forces are 
introduced into the skeleton. These advantages are related to the fact 
that the front of the tibia is not covered by muscular tissues but, 
essentially, only by a thin skin layer. This eliminates the difficult 
sealing problems which in upper extremity prostheses comprising axial pins 
often generate wounds and risk of infection.