Multiaxis joint, especially artificial knee joint

Prosthesis (i.e. – artificial body members) – parts thereof – or ai – Leg – Knee

Reexamination Certificate

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Details

C623S044000

Reexamination Certificate

active

06749640

ABSTRACT:

BACKGROUND OF THE INVENTION
The invention relates to a multiaxis joint, especially an artificial knee joint, whose axes of rotation are perpendicular to one plane.
DESCRIPTION OF THE RELATED ART
In prosthetics, artificial knee joints are known which swivel about a single axis, their respective swiveling movement being unsatisfactory, and which in particular permit only an unnatural gait pattern. Multiaxis knee joints have therefore been disclosed whose axes are perpendicular to one plane, for example in accordance with U.S. Pat. No. 5,314,498.
In multiaxis knee joints of this type, a polar curve describes the relative movement of the instantaneous center of rotation during swiveling. In this way, a harmonious gait pattern is obtained, because walking on uneven surfaces and stepping over small obstacles and the like is made easier by a shortening of the knee joint. To do this, the leg has to be consciously lifted and, in the process, the instantaneous center of rotation generally shifts forward before the joint comes into the actual flexion angle. In this forwardly directed movement, the points of articulation move away from one another, which results in a lengthening of the knee joint. One reason for this, among others, is that the polar curve and thus the instantaneous center of rotation generally lies outside the actual knee joint. In this respect, an unnatural pattern of movement is also produced here, because the leg which is actually to be shortened is first lengthened and must therefore be lifted higher than an obstacle or the normal pattern of movement actually requires.
SUMMARY OF THE INVENTION
Against this technical background, the invention has the object of making available a multiaxis joint, especially an artificial knee joint, which permits a harmonious gait modeled after the natural function of a knee.
To achieve this object in a multiaxis joint, especially an artificial knee joint, whose axes of rotation are perpendicular to one plane, a relative swiveling of an upper platform toward a lower platform from a rest position to a position of maximum excursion is permitted, upon which swiveling the distance between the platforms is immediately reduced with an excursion from the rest position. The polar curve which describes the relative movement of the instantaneous center of rotation will as a result also immediately describe a convergence of the two platforms which are attached to a thigh and lower leg, respectively. Thus, upon a walking movement and a lifting of the prosthesis, the distance between a ground surface and a foot of the prosthesis is immediately increased to such an extent that irregularities and small obstacles can be surmounted in an almost natural way.
In particular, it is also provided for here that, during the swiveling of the platforms relative to one another, the instantaneous center of rotation at all times lies inside the actual joint.
Here, the platforms are each understood to be an attachment element which serves to bind the joint according to the invention to a femoral stump and to a below-knee prosthesis, respectively. In this respect, the spatial configuration of the platforms is open.
In terms of construction, provision can be made for the joint to be designed to be self-retaining or self-locking in the rest position and/or the position of maximum excursion, or for these positions to be marked by dead-point positions of the joint. This affords the advantage, in particular in the rest position, that the joint is as it were locked in the respective position even when loaded. This results, on the one hand, in a stable and unstressed stance and, on the other hand, buckling of the joint upon heel contact in the walking cycle is avoided. In this respect, a particular aim is to inhibit the joint in a range of excursion from one of the positions of between 0° and 10°. Nevertheless, initiation of the flexion phase starting from the rest position, in particular also starting at 0°, is possible without any problem.
Alternatively, or in addition, provision can be made for the rest position and/or the position of maximum excursion to be fixed by one or more limit stops. Such limit stops guarantee that the swiveling can take place only within a predetermined angle range. By means of a limit stop, it is also possible for a considerably greater load to be taken up by the joint in said positions. It has proven expedient to provide for a cushioned run-in into the rest position and/or the position of maximum excursion. In this connection, it is conceivable on the one hand to provide special dampers, for example also on limit stops, or in some other way to reduce the angular velocity of the axes of rotation during the run-in into the positions. A cushioned, gentle run-in into one of the positions can also be achieved by the predetermined kinematics of the joint itself, for example by the run-in into self-retaining, self-locking areas or dead-point positions.
In terms of construction, provision is made for the platforms to be connected by two levers of different length articulated respectively at their ends on the platforms.
This measure ensures that the upper platform is displaced variably not only in height relative to the lower, but that the platforms are actually swiveled about an angle relative to one another. As a further construction measure, provision can be made that in one position, in particular in the rest position, the opening angle of the lines connecting the axes of the levers is less than 95°, and in particular these can also be approximately perpendicular to one another. In this respect, it is preferable that, in this position in particular, an axis of the second lever is arranged lying substantially on the connecting line of the axes of a first lever. A setting of the joint is thus obtained such that in normal standing, when the joint assumes the rest position, a stable and unstressed stance is guaranteed. This up to and including a certain excursion, if loading takes place in this excursion.
In this respect, it is also preferably provided that, during the swiveling, the levers cross each other, or at least in continuation the connecting lines of the first and second axes. These measures ensure, inter alia, that the instantaneous center of rotation during swiveling lies inside the joint according to the invention and that, as a consequence, an approximately natural pattern of movement is obtained.
In a further embodiment, it is provided that in one position, in particular in the rest position, the first axes of the levers articulated on the lower platform are arranged substantially at the same height or, alternatively, that a first axis of a lever articulated on the lower platform is arranged lying higher than the first axis of the other lever. Opposite the plane spanned by these two first axes in one position, in particular in the rest position, an angle of between 45° and 75°, in particular of between 50° and 60°, is preferably spanned between the first lever and this plane. This is the angle between the line connecting the axes of the first lever and the line connecting the first axes in the lower platform.
These construction measures are geared toward the natural attachment of the cruciate ligaments to the lower leg, and the natural articulation of the anterior cruciate ligament on the lower leg in the standing position. Corresponding to the distance between the attachment of anterior and posterior cruciate ligament to the lower leg, it is also preferably provided that the distance between the first axes is between 36 mm and 62 mm. Likewise, the distance of the first axis from the second axis of the first lever articulated on the upper platform can be between 45 mm and 80 mm, once again geared toward the length of an anterior cruciate ligament between thigh and lower leg of a person of average height. Corresponding to the distance between the attachment of the cruciate ligaments to the thigh, a distance between the second axes of between 16 mm and 46 mm is provided. The simulation of the human knee in respect of the cruciate ligaments

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