Prosthesis (i.e. – artificial body members) – parts thereof – or ai – Implantable prosthesis – Bone
Reexamination Certificate
2000-01-03
2001-07-24
Philogene, Pedro (Department: 3732)
Prosthesis (i.e., artificial body members), parts thereof, or ai
Implantable prosthesis
Bone
C623S020140, C623S020260, C623S020270
Reexamination Certificate
active
06264696
ABSTRACT:
TECHNICAL FIELD
The invention comprises a knee endoprosthesis of the kind comprising a tibial part comprising a tibial rod, one end of which is for inserting into the patient's tibia, and a tibial plate fixed to the other end of the tibial rod; a tibial insert disposed on the tibial plate; and a femoral part comprising a femoral rod and condyles borne by the femoral rod and each comprising a first sliding surface corresponding in shape to a second sliding surface on the tibial insert, the condyles being at least partly separated by an intercondylar space;
the condyles and the tibial insert co-operating via a coupling means such as a journal which extends through the tibial insert via an opening formed in the insert and is fixed to the tibial part; and a coupling head extending into the intercondylar space and a coupling shaft fixed to the condyles and extending into an opening in the coupling head, so that the femoral part can pivot relative to the coupling shaft and thus move from an unbent position (total extension of the knee) to a bent position (approximately 120° bend of the knee).
A knee prosthesis of this kind is known e.g. from the document EP-0 791 343.
Knee prostheses of this kind have a number of disadvantages.
BACKGROUND OF THE INVENTION
The tibial insert, usually of polyethylene, is mounted so as to be freely rotatable relative to the tibial plate (generally of metal) so as to permit clearance in proper rotation of the femur relative to the tibia, the proper rotation occurring via the condyles (usually of metal) which slide on the corresponding sliding surfaces of the tibial insert. In the unbent position (knee bending angle &agr;=0°) of the knee, rotation of the femur relative to the tibia is blocked by the condyles abutting on the polyethylene insert. As soon as the knee bends, however, (&agr; becomes greater than 0) the block is completely relaxed and the femur can rotate over a very wide angle before the knee has appreciably bent. This is not a true reproduction of the anatomical knee and may result in undesirable jamming.
SUMMARY OF THE INVENTION
The invention aims to obviate these disadvantages by providing a prosthesis in which, when the knee moves from a completely unbent position, the femur part is locked to the tibial part progressively during extension (unbending) of the knee.
This is achieved by providing a progressive locking element adapted to limit the proper rotation of the condyles relative to the tibial plate, starting from a knee-bending angle &agr;
0
determined in advance and up to a zero angle, the amplitude of proper rotation decreasing from the angle &agr;
0
to zero amplitude (total locking) at a zero angle (unbent knee).
According to an advantageous improvement of the invention, the progressive locking element comprises a second tibial insert fixed to the tibial plate and having a shape such that when the knee is in the extended position the second fixed tibial insert projects into an intercondylar space formed between the condyles and fits into the seat formed by the intercondylar space so as to block any axial rotation of the femur relative to the tibia, and such that when the knee bends and the second insert comes out of its seat, a clearance between the walls of the intercondylar space and the second insert occurs progressively up to a bending angle (&agr;
0
) determined in advance, after which the movements in axial rotation are free.
This special arrangement prevents the rotary tibial insert from being used for progressively blocking the condyles and thus wearing out as quickly as in the case where the blocking element is an integral part of the tibial insert, in which case the tibial insert, which can rotate freely relative to the tibial plate, will be stressed both by bending and by axial rotation or in varus/valgus. In the present case, the movable insert is stressed only during bending, since the stresses during axial rotation or in valus/valgus, inter alia in the locked position, are borne entirely by the fixed second tibial insert.
In an improvement according to the invention, the coupling means is fixed relative to the tibial plate in order still further to limit the possibility of proper rotation and thus additionally lock the femur when in the unbent position relative to the tibia.
REFERENCES:
patent: 4268920 (1981-05-01), Engelbrecht et al.
patent: 4358859 (1982-11-01), Schurman et al.
patent: 5411555 (1995-05-01), Nieder
patent: 5766257 (1998-06-01), Goodman et al.
patent: 5800552 (1998-09-01), Forte
patent: 5824102 (1998-10-01), Buscayret
patent: 5954770 (1999-09-01), Schmotzer et al.
patent: 35 29 894 A1 (1987-03-01), None
patent: 0 724 868 A1 (1996-08-01), None
Biegun Jean-François
Marceaux Pascal
Reigner Bernard
Aesculap
Philogene Pedro
Renner Kenner Greive Bobak Taylor & Weber
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