Prosthesis (i.e. – artificial body members) – parts thereof – or ai – Implantable prosthesis – Bone
Reexamination Certificate
2002-09-04
2004-11-02
Philogene, Pedro (Department: 3732)
Prosthesis (i.e., artificial body members), parts thereof, or ai
Implantable prosthesis
Bone
C623S022380, C623S022240, C623S022280
Reexamination Certificate
active
06811569
ABSTRACT:
BACKGROUND OF THE INVENTION
The subject of the present invention is an expansible acetabular prosthesis with double mobility.
The technical field of the invention is the production of surgical equipment implantable in the human body.
The main application of the invention is the production of a hip prosthesis for replacing the cotyloid cavity of the hip bone, the whole prosthesis being intended to be anchored without cement.
Total hip prostheses comprise two elements:
1. The femoral stem: this component replaces the head and neck of the femur. A stem, forming a continuation of the neck, ensures anchoring thereof in the femoral bone, with or without cement.
2. The acetabular component: this replaces the osseous acetabulum and articulates with the prosthetic femoral head. A hemispherical cavity forms the sliding surface. This element can be made in one piece from metal, polyethylene or ceramic. However, to allow it to be fixed without cement, many models are made up of the following two elements:
a) a metal cup fixed in the bone,
b) a plastic insert, in particular of polyethylene, fixed in the cup and articulating with the prosthetic head.
The problems encountered in surgical practice are the wear between the femoral head and the acetabular component, and also the fixation of the metal cup in the bone and the wear observed between the insert and the metal cup.
A number of ways of fixing the metal cup are presently employed, in particular sealing (a), impaction by force (b), screwing (c), or fixation by expansion (d), all of which are discussed below.
a) Sealing involves interposing an acrylic-type cement between the bone and the cup, which anchors in the bone and ensures the fixation of the component. However, this technique has disadvantages associated with the characteristics of the cement and of the bone and can lead in the medium term or long term to loosening.
b) Impaction by force involves press-fitting the metal cup into a bone cavity prepared with the aid of hemispherical reamers which calibrate the osseous acetabular cup to a diameter which is slightly smaller than that of the metal cup.
A coating or roughening of the outer surface of this cup improves the immediate mechanical stability of the cup and permits secondary stabilization by regrowth of bone. However, this initial mechanical stability is uncertain because it largely depends on the way in which the bone is prepared and especially on its hardness.
Most of the models therefore propose a complementary fixation recommended in cases where the press-fit retention is deemed insufficient. This fixation is ensured by screws which are placed in the bone via holes formed in the metal cup.
This technique of impaction by force has the following disadvantages:
the possible micro-movements between the screw and the metal cup can release particularly damaging metal products of wear, and
the possible contact between the screw head and the insert is a possible source of wear and creep. Another problem is the creep of the insert (if it is made of polyethylene) relative to the screw holes, since this produces debris from wear which can migrate through the screw holes and lead to progressive osteolysis, which promotes mobilization of the implant.
c) Screwing of the acetabular cup involves using a metal cup with a threading on its outer surface, which threading is screwed into the bone. This method of fixing is little used at present because the position of the component is uncertain at the time of fitting. This is because the press-fit or screw-in acetabular cups cannot be fitted with precision because their positioning can be deviated by the bone during fitting.
d) In fixation by expansion, a metal cup is used which is designed to be opened by an intermediate element of metal or polyethylene situated between the cup and the insert, for example a ring.
Different types of acetabular prostheses fixed by expansion are known, and these comprise three elements, which are:
an artificial acetabular cup which is a cup provided with expansion slots which are symmetrically and uniformly distributed at angles over its periphery and which define segments or portions of a spherical cap sector;
a threaded intermediate element which is:
a) a ring intended to be screwed into the acetabular cup, as is described in patent application EP 486 403, or
b) an intermediate core which comprises a threaded finger forming an appendix on its pole and which cooperates with a tapped hole in the artificial acetabular cup, as is described in patent application FR 2 700 946;
a friction insert which bears or engages on said intermediate element having a sliding surface in the shape of a hemispherical cavity, and in which an artificial head ball engages to restore the articulation of the hip.
The present invention concerns expansible acetabular prostheses with three elements, as are described above. More particularly, the present invention concerns acetabular prostheses with three elements substantially of spherical cap shape.
With the expansible acetabular prostheses which are presently available, the plastic insert which has to take up all the loading on the hip may creep and thus deform inside the acetabular cup.
In the case of an intermediate element with a continuous surface which better supports the insert, the acetabular cup is fixed by screws and this again does not ensure good overall stability: the reason being that the components of the prosthesis retain a certain mobility and elasticity which are also incompatible with good stability over time.
SUMMARY OF THE INVENTION
The object of the present invention is to remedy these disadvantages in particular, and the present invention aims to make available a prosthesis having:
improved mechanical stability,
easier and more precise fitting in the bone,
reduced wear of the components,
the possibility of permitting double mobility by virtue of an insert which is movable in the intermediate element.
To do this, the subject of the present invention is an acetabular prosthesis comprising:
a hollow artificial acetabular cup, preferably with an outer surface of the spherical cap type, in particular hemispherical, having expansion slots distributed over its periphery;
an intermediate cup-shaped element, with an inner surface in the shape of a spherical cap, in particular hemispherical;
a hollow insert with outer and inner surfaces of spherical cap shape, preferably hemispherical, fitting in the intermediate element;
said acetabular cup and said intermediate element comprise screwing means so that the outer surface of said intermediate element can come into congruent contact, by screwing, against at least one internal meridian generatrix of each segment delimited by two of said consecutive slots,
said means of screwing said intermediate element into said acetabular cup comprise a peripheral threading formed on the inner surface of the acetabular cup near its opening, in which threading said intermediate element is screwed with the aid of a complementary threading on its outer surface near its opening,
the internal diameter of the opening of said acetabular cup, in the standby position before screwing of said intermediate element, is smaller than the external diameter of said intermediate element, and
said acetabular cup is equipped with bone-anchoring teeth on its outer face.
This configuration of the acetabular prosthesis according to the invention in which the intermediate element bears in contact on at least one internal meridian generatrix of the internal cavity of the artificial acetabular cup permits a good distribution of the forces and stresses across the entire surface of the artificial acetabular cup.
In addition, the peripheral threading according to the present invention ensures progressive opening of the slotted element, so that the positioning of the acetabular cup at the time of fitting is made more precise.
More particularly, the intermediate element is a hollow core with a hemispherical outer surface and is able to come into contact against at least one internal meridian generatrix of each segment defin
Afriat Jacques
Bensadoun Jean-Louis
Bonderer David A
Philogene Pedro
Proconcept SA
Young & Thompson
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