Prosthesis (i.e. – artificial body members) – parts thereof – or ai – Implantable prosthesis – Bone
Reexamination Certificate
2001-01-03
2002-03-05
Philogene, Pedro (Department: 3732)
Prosthesis (i.e., artificial body members), parts thereof, or ai
Implantable prosthesis
Bone
C623S021110, C623S021150, C623S021160
Reexamination Certificate
active
06352560
ABSTRACT:
FIELD OF THE INVENTION
The present invention relates to a joint prosthesis comprising a first part with a cylindrical socket and a second part with a cylindrical head, the cylindrical head and socket being complementary to one another, in such a manner that together they are able to form a linear hinge which extends in the longitudinal direction of the head/socket.
BACKGROUND OF THE INVENTION
A prosthesis of this nature is known, in the form of a finger joint prosthesis, from U.S. Pat. No. 3,991,425. A prosthesis of this nature, of at least a prosthesis which appears extremely similar, is also known under the name Djoa prosthesis. The Djoa prosthesis comprises a cylindrical roll surface in which a slot is formed. A hollow cylindrical socket runs over the cylindrical roll surface, with a projection which projects out of the socket fitting into the slot. The purpose of this projection is to prevent the socket from moving sideways, i.e. in the longitudinal direction of the linear hinge, with respect to the cylindrical roll surface, in order to ensure stability of the joint prosthesis in relation to loads which act in the longitudinal direction of the hinge. However, a drawback of this design is that in the event of such loads a force is exerted on the underlying bones to which the joint is attached, via the slot with the projection situated therein. As a result, the stems may be dislodged from the underlying bone or may break off, which would lead to considerable damage to the underlying bone.
GB patent specification 1,530,301 has disclosed a finger prosthesis which differs from the joint prosthesis according to the present invention in particular by the fact that the socket parts are fixed to the head parts by means of a snap-action connection, i.e. the socket parts extend over more than 180°, as is clear from the figures. Owing to the snap-action connection, this prosthesis, in the implanted position, does not allow any movement of the prosthesis parts with respect to one another apart from the pivoting movement about the longitudinal axis of the head parts/socket parts. It is not possible for the prosthesis parts to move apart from one another and to pivot with respect to one another transversely to the hinge direction.
DE-Offenlegungsschrift 2,814,752 has disclosed a two-part joint prosthesis, in which the first part comprises a fork with a socket part inside. The second part is provided with a head part which can be fixed to the socket part by means of a snap-action connection. Since the head part is of spherical or convex design, and the socket part is shaped accordingly, this joint prosthesis has two degrees of freedom. In addition to the main hinged movement, limited pivoting with respect to the main hinged movement is also possible. The prosthesis disclosed by DE-2,814,752 cannot move apart in the implanted position.
SUMMARY OF THE INVENTION
The object of the present invention is to provide a joint prosthesis which, on the one hand, provides so-called medial/lateral stability, so that it is impossible for the moving parts to move in the longitudinal direction of the hinge with respect to one another, and, on the other hand, allows transverse forces, acting in the longitudinal direction of the hinge, on the bones which are connected by the joint without this inevitably leading to excessive load on the connection between the joint parts and the respective bones to which they are attached.
According to the invention, this object is achieved by means of a special design of the joint prosthesis, and in particular by the fact that:
the cylindrical socket extends over at most 180° in the circumferential direction,
the cylindrical head is provided with a radial thickened section which extends in the circumferential direction, and
the cylindrical socket is provided with a radial recess which extends in the circumferential direction and in which the radial thickened section can be accommodated, preferably with the cylindrical head bearing inside the cylindrical socket.
In this design, since the cylindrical socket extends over at most 180°, the cylindrical head can be removed from the cylindrical socket by pulling it out in a direction which is parallel to or lies between the tangents at the free ends of the extent of the cylindrical socket. When the bone to which the cylindrical socket is attached extends in this direction, this essentially means that this bone is pulled away from the joint in its longitudinal direction. However, this also means that when, in this position, a transverse force which is directed in the longitudinal direction of the linear hinge acts on the said bone, this bone can undergo a pivoting movement, during which the cylindrical head comes out of the socket at one end of the linear hinge and is pressed against the socket at the other end of the linear hinge. Consequently, the action of such a transverse force on the connection between the prosthesis parts with the respective bones is reduced considerably. These transverse forces can then be absorbed entirely by the natural, or possibly artificial, ligaments which hold the joint together. If the cylindrical socket extends over less than 180°, the area in which such a tilting movement of the bone which is attached to the cylindrical socket with respect to the linear hinge can take place will be increased in size, as will be clear. However, an important advantage of the joint prosthesis according to the invention is that this so-called tilting freedom which has been outlined above decreases as the bone which is attached to the cylindrical head is situated further and further outside the area defined by the tangents at the free ends of the cylindrical head. Depending on the exact orientation of the cylindrical socket with respect to the underlying bone to which this socket is attached, a moment with respect to the longitudinal axis of the said bone will act on the said bone. Such a rotation can then be absorbed, in the case where there is a joint at the other end of the said bone, by the said joint or by the ligaments associated with the said joint. Generally, the cylindrical socket will be oriented in such a manner, with respect to the bone to which it is attached, that the cylindrical head can be removed from this socket when the two bones are situated essentially in line with one another. As will be clear to a person skilled in the art, such a joint prosthesis according to the invention can generally be used with very great success for finger joint prostheses, toe joint prostheses, and possibly elbow or knee prostheses.
In order to achieve a so-called pivoting freedom or tilting freedom of the joint prosthesis which is equal in both opposite directions (also known in the specialist medical field as the medial and lateral directions), it is advantageous, according to the invention, if the radial thickened section and the radial recess are arranged centrally, as seen in the longitudinal direction of the linear hinge.
In order to ensure that after a so-called tilting or pivoting of the joint parts with respect to one another, the cylindrical head moves correctly back into the cylindrical socket (i.e. with their cylinder axes essentially coinciding), it is advantageous, according to the invention, if the radial thickened section and/or the radial recess are formed so as to be self-aligning with respect to one another. It will be clear to a person skilled in the art that a self-aligning effect of this nature can be achieved in a wide variety of ways by suitable shaping of circumferential sides, which extend in particular in the circumferential direction, of the radial thickened section and/or the radial recess. According to the invention, the self-aligning effect can be achieved, inter alia, by providing the circumferential sides of the radial thickened section with bevels and/or rounded portions, optionally in combination with bevels and/or rounded portions on the circumferential sides of the radial recess.
In order, on the one hand, to provide sufficient medial/lateral stability, i.e. resistance to displacemen
Poeschmann Paul Henricus
Van Leeuwen Cornelis Marinus
Philogene Pedro
Van Straten Beheer B.V.
Young & Thompson
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