Wrist prosthesis

Details Wrist prosthesis Wrist prosthesis

1996-11-11

1998-06-16

Isabella, David

Prosthesis (i.e., artificial body members), parts thereof, or ai

Implantable prosthesis

Bone

A61F 242

Patent

active

057662584

DESCRIPTION:

BRIEF SUMMARY
BACKGROUND OF THE INVENTION

The invention relates to a wrist prosthesis with a radial part having a shaft that can be received in the distal region of the radial medullary canal and fixed therein, a metacarpal part having two shafts that extend distally, one of which can be received and fixed within the proximal region of the third metacarpal bone, whereas the other can be anchored in the carpal region, and a joint connection between the radial part and the metacarpal part.
A wrist prosthesis of this kind is generally known. Of the constructions most commonly used in practice, one is described in U.S. Pat. No. 4,784,661 and others in U.S. Pat. No. 4,106,128, U.S. Pat No. 4,063,314, and U.S. Pat. No. 4,180,871. The three last publications are each concerned with wrist prostheses having a ball-joint connection between the radial and the metacarpal part. Accordingly, these wrist prostheses allow both dorsal-palmar flexion and radial-ulnar deflection plus axial rotation. The disadvantage of this joint connection is its deficient stability, denoted by the term "unconstrained" joint. The ball-joint connection in itself would be ideal, as it most closely resembles the natural wrist joint. However, if during implantation the mechanical midpoint of the ball-joint connection is not found in conjunction with the tendons present in the wrist, a one-sided load is imposed on the joint connection, with the consequence that it becomes incorrectly positioned and is hence subject to abrasion. This effect is particularly evident when polyethylene is used as bearing material. For this reason the useful life of such a wrist prosthesis is very limited.
The wrist prosthesis according to U.S. Pat. No. 4,784,661 has a joint connection between the radial and metacarpal parts that comprises an ellipsoidal joint element at the metacarpal part and a complementary bearing surface at the radial part. This joint prosthesis has come to be preferred over the prosthesis with ball-joint connection. However, a problematic feature of the construction according to U.S. Pat. No. 4,784,661 is that axial rotation is not ruled out, and during such rotation the ellipsoidal bearing element is lifted away from the complementary bearing surface. The same applies to angular movements outside the axes for flexion-extension and radial/ulnar deflection. The result is that the bearing surfaces wear out prematurely. The possibility of luxation is also not excluded.


SUMMARY OF THE PRESENT INVENTION

It is the object of the present invention to create a wrist prosthesis of the kind described at the outset that is characterized by a simple and abrasion-resistant joint connection between the radial and the metacarpal part, whle corresponding closely to the mobility of the anatomical wrist joint.
The wrist prosthesis in accordance with the invention thus permits no axial rotation between the radial and metacarpal parts. Axial rotation instead occurs in the forearm. It has been found that the wrist itself does not need to be capable of axial rotation. On the basis of this finding, the joint connection between radial and metacarpal part can be very simply constructed, and in particular is substantially simpler than in the case of the joint according to U.S. Pat. No. 4,307,473, which likewise incorporates a ball-joint connection. Furthermore, the choice of swivel radii in accordance with the invention achieves a close approximation to the anatomical wrist. In this regard it should be kept in mind that the forces transmitted into the connective-tissue supporting apparatus per angular degree are considerably less for "dorsal/palmar" than for "radial/ulnar". Correspondingly, in the wrist prosthesis in accordance with the invention the radial/ulnar swivel radius is distinctly larger than the dorsal/palmar swivel radius. Preferably the radial/ulnar swivel radius is about 15 to 30 mm, in particular 15 to 20 mm, whereas the dorsal/palmar swivel radius is only about 2 to 5 mm, in particular 2 to 3 mm. Owing to the swivel radii chosen in accordance with the invention, the

REFERENCES:
patent: 4229841 (1980-10-01), Youm et al.
patent: 4307473 (1981-12-01), Weber
patent: 4714476 (1987-12-01), Ranawat et al.

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