Prosthesis (i.e. – artificial body members) – parts thereof – or ai – Implantable prosthesis – Bone
Reexamination Certificate
1997-08-06
2001-05-29
Willse, David H. (Department: 3738)
Prosthesis (i.e., artificial body members), parts thereof, or ai
Implantable prosthesis
Bone
C623S023180
Reexamination Certificate
active
06238436
ABSTRACT:
BACKGROUND OF THE INVENTION
The invention relates to a modular joint prosthesis having a head part and at least one shaft part which are insertably connectable with one another by means of a cone connection.
Such joint prostheses are produced in the most various shapes and sizes, to enable the most precise possible adaptation to the shape of the bone of a particular patient, as well as the current status of the bony tissue, taking the clinical picture into account.
By means of a multi-part embodiment with nonpositive connection of the corresponding individual parts in the proximal region, the adaptation can be accomplished optimally. At the same time, positioning of the joint head is possible regardless of the shaft diameter.
From European Patent Disclosure EP-B1 0 243 298, a kit for a shaft prosthesis is known, which has a head part that can be provided with a joint ball, an end part anchored in the bone, and an intermediate part that can be positioned between the two. All the parts have conical bores or complementary pegs, and as a result the prosthesis can be assembled by making conical insert connections. The head part and intermediate part each have an axial through bore.
The end part is provided with releasable engagement means, for the sake of transmitting a force in the axial direction. When the individual parts are put together, the corresponding bores are aligned axially in the direction of the shaft. The individual parts of the prosthesis are put together using a tie rod that transmits a force in the axial direction and that penetrates both the head part and succeeding shaft parts and can be screwed into the threaded bore of the end part. As a result, the head part, or the intermediate part, and the end part are firmly tightened against one another, so that loosening of the individual parts of the prosthesis from the mechanical strain during use need not be feared.
The above-described embodiment has the disadvantage that in such a prosthesis, the insert connection between the individual parts cannot be undone or released in the implanted state, even though this is sometimes necessary for medical reasons—for instance for resetting or replacing the head part. The conical insert connections, even after prior removal of the tie rod provided in the interior of the shaft, are held together so firmly that, if disassembly becomes necessary, damage to the femur or at least undesired loosening of the shaft is sometimes unavoidable, which is detrimental to the patient.
SUMMARY OF THE INVENTION
In view of the deficiencies in the prior art, it is the object of the invention to create a modular joint prosthesis of the generic type described at the outset, which in a simple way allows the individual elements to be loosened from one another, even from the outside.
This object is attained according to the invention by providing a bore that extends essentially coaxial to the longitudinal axis of a shaft part of a modular joint prosthesis and through the head part of the modular joint prosthesis and at least one section of the shaft part, the bore having a first segment with a first diameter and a first thread in the head part and a second segment in the shaft part with a second thread part and a second diameter, where the first diameter is greater than the second diameter so that an insert connection between the head part and the shaft part can be released.
The invention encompasses the recognition that with a screw bolt, considerable pressure forces can be generated in the axial direction if the end of the bolt is braced against an abutment. To that end, the edge of the bore of a succeeding shaft element is also suitable, on the condition that the outer diameter of a bolt to be screwed into the threaded bore is greater than the inside diameter of the succeeding element. Since replacement, especially of the head element, may become advisable (in advanced disease or for other indications), the succeeding elements may have a bore of constant diameter; in that case, the last (distal) element optionally has a thread that has a tie rod for connecting the shaft elements or setting the cone elements. The exertion of force for loosening two successive elements can be achieved all the more conveniently, the less its pitch is selected to be.
It is especially advantageous in the invention that without removing the lower part of the shaft, which remains in the bone space, the head part near the joint can be removed and replaced if needed. This is associated with substantially less stress on the patient than in conventional prostheses. Moreover, with the head part removed the corresponding interior of the bone is also accessible for other therapeutic purposes. “Head part” in the present afflication is understood to mean the part of the prosthesis near the joint, a part that can also be advantageously used in the case of joint replacement in other regions of the body.
Prosthesis shafts embodied as a modular system have, at least as the head part and end part, a system of various individual elements of different sizes, which are preferably joined together by putting together their proximal or distal ends that have corresponding conical pegs or recesses. The requisite stability of the insert connection is assured by screw means—preferably embodied as a tie rod. The end part is embodied as a hollow shaft, and the axially extending longitudinal bore of the carrier part has a thread on its proximal end portion. The tie rod is guided through a cylindrical channel, located on the same axis as the longitudinal bore in the carrier part, in the head part and screwed into the end part, which is provided with a thread.
According to the preferred embodiment of the invention, at least part of the cylindrical channel in the head part of the modular hip prosthesis shaft is embodied as a threaded bore. This threaded bore has a diameter which is greater than the diameter of the threaded bore, receiving the tie rod, in the succeeding shaft part. This diameter relationship advantageously assures that when the prosthesis parts have been put together, a free area remains on the proximally provided, frustoconical end of the succeeding shaft part, this free area being an essentially circular-annular top face of the truncated cone. This face region is available as an abutment for a screw bolt, which can be screwed into the head part once the tie rod is removed. The bolt screwed into the head part is braced on the substantially circular-annular abutment. Upon a further screwing motion, an axial pressure force is favorably generated, which releases the firm conical insert connection (despite the absence of the tie rod) between the head part and the succeeding shaft part enough that the head part can be removed for replacement purposes, or can be swiveled to the desired extent with regard to the succeeding shaft part on its frustoconical peg provided on the proximal end. To assure an adequately large abutment area for force transmission when the connection between the head part and the succeeding shaft part is loosened or released, a diameter ratio between the bores in the head part and the succeeding shaft part in the range from 1.5 to 2.5 is favorable.
In another advantageous further feature of the invention, the thread provided for the screw bolt extends in the head part of the modular hip prosthesis shaft over the entire length of the available cylindrical channel. For both the threaded bore in the head part of the hip prosthesis shaft and for the threaded portion on the proximal end of the succeeding shaft part, a metric thread is provided. The thread in the head part preferably has a lesser pitch than the thread on the proximal end of the succeeding shaft part and thereby makes it easier to generate the pressure force, by means of the screw bolt, that is required to loosen the firm conical insert connection.
In a further advantageous feature of the invention, the described provisions can be employed, using various graduations, multiple times in successive elements with one and the same modular shaft. In the descriptio
Fischer Hans-Joachim
Kranz Curt
Lob Gunter
Steur Gerd
Biomet, Merck Deutschland GmbH
Kinberg Robert
Venable
Voorhees Catherine M.
Willse David H.
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