Endoprosthesis for a shoulder joint

Prosthesis (i.e. – artificial body members) – parts thereof – or ai – Implantable prosthesis – Bone

Reexamination Certificate

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C623S019110

Reexamination Certificate

active

06749637

ABSTRACT:

FIELD OF THE INVENTION
The invention relates to an endoprosthesis for a shoulder joint as generically defined by the preamble to claim
1
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PRIOR ART
From European Patent Disclosure EP 0 712 617, a shoulder joint prosthesis is known whose shaft part has an indentation with a hemispherical bottom in its metaphysial end, into which indentation a ball is introduced. This ball has a central continuous bore and a slot that communicates with the bore over its entire length and splits the ball ring, making it C-shaped. A stem connected to the cap is inserted into the bore. Grub screws are provided in the shaft part, in order to immobilize the ball in the indentation in a selectable angular position and to deform the ball ring and thereby firmly clamp the stem in it.
A disadvantage of this prosthesis is that the cap must be precisely placed in every direction before it can be fixed in this position. To that end, it must be held in this position by a surgeon while the grub screws are being tightened forcefully. Furthermore, the optimal position of the cap can be only approximated, since the spacing between the spherical cap axis and the shaft axis is constant. An alignment of the cap center in the anterior/posterior direction always necessarily involves a displacement in the cranial or caudal direction. Furthermore, the grub screws must be tightened before the prosthesis is inserted, because afterward they are no longer accessible. This requires multiple test manipulations of the arm being operated on and corrections to the cut face of the bone during the operation and makes a correction of the alignment after implantation has been completed impossible.
French Patent Disclosure FR 2 773 469 discloses a shoulder joint prosthesis with a shaft part and on it a pivotable adapter or directional piece, and the adapter has a ball cap. In this prosthesis, a hemispherical pit with a threaded bore on its bottom is provided on the metaphysial end of the shaft part. The pit and the threaded bore are aligned with a neck axis, whose angle to the shaft axis is selected in advance. Seated in the pit is a pivotable adapter, with a hemispherical surface, concentric with the hemispherical surface of the pit, on its humeral end. On its other end, the adapter has a conical surface, which is eccentric with regard to the center of the sphere common to the two spherical surfaces, on which conical surface the spherical cap is seated with a suitable conical indentation. The axis of this indentation is eccentric to the center point of the cap. The adapter has an axial bore with a hemispherical bottom. A screw with a ball head is introduced, with the shaft leading, into the bore, and the threaded portion of the shaft is punched through a conical bore in the hemispherical bottom of the adapter and screwed into the threaded bore in the shaft part. The centers of the spherical surfaces of the pit in the shaft part, the inner and outer hemispherical surface on the adapter, and the spherical surface of the screw head are located at a common central point, once the screw has been tightened. As a result, the adapter is pivotable relative to the shaft part in all directions and simultaneously rotatable. Pivoting the adapter causes its longitudinal axis to shift parallel to the desired neck axis direction, and rotating the adapter about its longitudinal axis and rotating the cap in a manner adapted to that rotation makes it possible to compensate for the deviation of the neck axis to be attained from the approximated neck axis. Once the optimal location of the adapter is definite, it can be fixed to the shaft part by tightening the screw.
In this prosthesis as well, the direction of the joint neck is adjustable infinitely variably, depending on the given conditions of the original joint. By adjusting the inclination and rotation relative to the shaft axis with the aid of a manipulation head, the neck axis is adjustable parallel to the optimal neck axis. This approximation can then be corrected infinitely variably afterward transversely to the direction of the axis with the cooperating eccentrically disposed conical surfaces on the adapter and the cap. The length of the joint neck is regulated by a set of caps of different thicknesses. In this way, a spherical cap can be made firm at any arbitrary site in any desired position, so that the site of the cap can be oriented optimally to the original conditions of the joint or the intended correction and the original center of rotation of the shoulder joint can be restored.
Since the success of a shoulder joint depends essentially on the restoration of the muscular equilibrium of the shoulder musculature, it is important that the original center of rotation of the shoulder joint be restorable by means of this individual placeability of the spherical cap. The closest possible spacing between the cut face of the bone and the artificial cap is also attainable, so that in every motion, the muscles are offered a continuous support point. It is also advantageous that the inclination and rotation of the adapter relative to the shaft part can be oriented to the cut face of the bone or to the desired orientation of the cap and can be fixed in that position even after the implantation of the shaft part, since the screw that fixes the adapter remains accessible from the side of the cap.
A disadvantage of this prosthesis, however, is that the spherical surfaces must be produced exactly concentrically, which is complicated and expensive. Ball joints furthermore offer a very small support face. In addition, the support face is reduced in size, in the region of the main direction for tightening the screw, by the conical bore in the spherical wall of the adapter. Ball joints in general have the disadvantage of not allowing a press fit in the sense of a conical clamping action. Therefore even after the operation is concluded, the fixation of this ball joint remains insecure.
Furthermore, the adapter must be placed correctly in every respect before it can be allowed to be fixed. In particular, a rotation about the directional axis, namely its own longitudinal axis, in order to rotate the eccentric cone into the desired position, is possible only if the angular position of the adapter is also unstable. Unintentional shifting of the adapter can occur upon removal of the manipulation head, insertion of the screw, tightening of the screw, and adjustment and placement of the spherical cap.
Finding the correct rotary position of the adapter in which the two eccentric conical surfaces of the adapter and spherical cap cooperate in such a way that the spherical cap comes to rest at the desired point is quite difficult. In practice, the eccentricity of the conical surface on the adapter is therefore oriented with the aid of a manipulation head in such a way that the cone axis is located as close as possible to the axis estimated to be optimal, and after that the cap is put in place. Upon alignment of the center of the cap in the anterior/posterior direction, however, there is necessarily also a shift in the cranial or caudal direction.
OBJECT OF THE INVENTION
It is therefore the object of the invention to propose an endoprosthesis for a shoulder joint in which the advantages of the prior art cited are preserved, while the aforementioned disadvantages of ball joints are avoided. In particular, the greatest possible forces of friction should be attained between the individual parts that are displaceable and rotatable counter to one another. Moreover, it should be possible to define the direction and position of the joint axis independently of one another. Compared to the prior art, the calibration of direction and location of the spherical cap axis should be simplified. The sensitivity of the settings of the axial direction, in terms of inclination and rotation, to unintended changes in the inclination position or rotation position should be minimized as much as possible by adjustment of the other setting. Also, the direction of the axis should have the slightest possibly vulnerability to influence from a

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