ENDOPROSTHESIS FOR A JOINT,
ESPECIALLY A FINGER, TOE OR WRIST
TECHNICAL FIELD OF THE INVENTION
The present invention concerns an endoprosthesis for a joint, particularly a finger, toe or hand joint, with a proximal and a distal joint part, of which one joint part in the flexion direction has an essentially convex contact surface around a body axis and the other has a correspondingly essentially concave contact surface, and which joint parts are joined by means of a flexible connection piece that takes up tensile forces, which piece has a thread-type or tape-type element.
The natural metacarpophalangeal joint (MCP) gives the finger a lateral freedom of movement, which differs each time depending on the flexion of the joint. In flexion, the degree of freedom is laterally zero to only a few degrees, but in extension it is approximately 30 degrees. In addition, a limited passive rotation is possible around the axis of the finger. The interphalangeal joint leaves free almost no movement play space laterally, independent of the flexion angle, and the joint parts can rotate only very slightly relative to one another. Therefore, each joint has its own particular degree of freedom with respect to extension and flexion, relative to rotation, but also relative to lateroflexion.
In order to permit natural movements of flexion, extension, and rotation around the joint, while, however, controlling the movement path of the finger opposite the metacarpus, in French patent application FR-A1-2,736,536, an endoprosthesis is proposed for a finger joint, which has a proximal joint part, a corresponding concave distal joint part and a connection and "programmable" part with a rod made of a pliant material. The rod sits in both joint parts in a guide channel, which is arranged axially in the pin of the joint part to be inserted in the bone marrow channel and begins with approximately cylindrical shape and spreads out into trumpet shape with decreasing distance to the condyle surface. The movement play space is defined by the interaction of guide channel and connection rod. Shape, dimensions and possibly the pre-programming of an alloy with "shape memory" of the connection rod determine the course of movement of the joint.
Such a joint has the disadvantage that the pliant connection rod must fulfill a multiple number of tasks simultaneously. Thus it must be pliable differently in different directions, but it must be resistant to abrasion, and in any case, it must have a shape memory. It must have a certain stiffness, yet it must be flexible. In addition, the material should be compatible with the body and long-lasting, i.e., it should not be fatigued.
An orthopedic prosthesis implant is proposed in U.S. Pat. No. 5,534,033, in which the joint parts each have a cup that can be inserted into the bone marrow channel, and in the hollow cavity of this cup is attached a condyle of ceramics or carbon by means of an elastomeric adhesive, whereby the condyles contact one another in a sliding manner and both joint parts are joined by connection means. The connection means comprise either an envelope enclosing the condyles or a thread, which is guided through a hole in the condyles. Also, two threads at a distance relative to one another are proposed. These threads can be guided through holes in the condyles or also guided around the condyle. The function of the threads consists of guiding the joint parts in such a way that they are not dislocated. It cannot be derived from the
document how the connection means react to a movement of the joint. It is assumed that the connection means must be either elastic (knitted DACRON fiber (trademark of E.I. duPont de Nemours and Company, Wilmington, Del.)) or are
5 fastened to a spring. In any case, it can be derived from the drawings and the description that the distance between the attachment points of the connection means changes in length upon movement of the joint. However, this solution brings its own disadvantages. The threads, which are guided
10 through the holes, can be sheared off by the edges of the holes at the condyle surface, or they can end up between the condyles and can be abraded therein. If the threads are guided around the condyles, they rub against the condyles and do not assure sufficient safety from dislocation when the
15 joint is bent. The envelopes are stretched at any flexion on the upper side of the joint, or in the case of the extended joint, folded material of the envelope is present on the upper side.
20 OBJECT OF THE INVENTION
It is thus the object of the invention to create an endoprosthesis for small joints, in which the mentioned disadvantages will be avoided, and a flexible connection of both joint parts that guarantees a play space will be assured. The connection will be loose enough and offers such little resistance, that the two parts can move freely, like the natural joint. The connection piece will not become fatigued. The movement play space of the joint will be definable with respect to rotation, extension, flexion and lateroflexion, and the latter
will be dependent on extension, with a simultaneous loose holding together of the joint parts. An abrasion that is as small as possible will be assured.
DESCRIPTION OF THE INVENTION
According to the invention, this is achieved by the fact that the connection piece assures a defined play space between the contact surfaces, that a groove longitudinally extended in the flexion direction is formed in the convex
40 joint part, that the connection piece is attached in this groove or at the base of this groove, whereby a lateral play space exists between groove and connection piece. In this way, there is a large freedom of movement in the flexion direction and simultaneously there is a definable, usually smaller
45 freedom for a displacement or lateral deflection as well as a rotation. However, the joint is guided and protected against dislocation, since the connection of the two joint parts with a connection piece prevents a luxation.
When fibers of threads and fabrics or of flat tapes and thin
50 membranes are flexed, the material is very slightly stretched and compressed in the direction of the flexion movement, due to the small dimension of the cross section. Fibers, tapes and membranes are consequently more suitable for flexible, pliant connection, the thinner the material cross-section.
55 This property of fibers and tapes also holds true when they are spun and/or woven into large dressings or bandages, in order to be able to take up multiple tensile loads. Such bandages may be formed by spinning, weaving or knotting and have the shape of cords and ropes, tubings, tapes strips
60 or flat woven fabrics. Essentially, the very small dimensions of the material in at least one dimension are suitable for flexion in one direction, and in two dimensions for flexion in several directions. The load capacity of such thread-type, tape-type or membrane-type parts makes them very useful
65 for tensile designs. In addition, their resistance to fatigued fractures is assessed. High resistance to tearing with simultaneous resistance to fracture is realized and utilized, for