Prosthesis (i.e. – artificial body members) – parts thereof – or ai – Implantable prosthesis – Bone
Reexamination Certificate
1998-05-04
2001-05-15
Willse, David H. (Department: 3738)
Prosthesis (i.e., artificial body members), parts thereof, or ai
Implantable prosthesis
Bone
C623S022380
Reexamination Certificate
active
06231612
ABSTRACT:
BACKGROUND OF THE INVENTION
The present invention relates to an acetabular implant attached without cement.
An acetabular implant usually comprises a cup of hemispherical shape, delimiting an interior cavity, and an insert that can be fitted into this cavity. The cup is intended to be fitted into the acetabular cavity and to be attached to the pelvis, and the insert delimits a spherical cavity intended to accommodate, with a possibility of pivoting, the corresponding spherical head of a femoral shaft.
The cup is generally made of a metal, especially titanium, and the insert is made of a material which encourages the head of the femur to slide, for example an alumina ceramic or a high-density polyethylene.
Some implants are attached to the innominate bone using polymerizable synthetic cement; others are attached without cement using mechanical anchoring means such as screws.
Among these, there are so-called “press-fit” implants which are intended to be inserted forcibly into the acetabular cavity. The cup of an implant of this kind has an outside diameter which is slightly larger than that of the acetabular cavity, especially in its peripheral region, and has lumps or roughnesses which dig into the bone when the cup is impacted in the cavity.
These lumps or roughnesses are generally not enough, by themselves, to attach the cup in a way which is completely reliable over time. This is why these implants also comprise screws inserted into the bone through the cup to provide additional anchorage.
Some cups also comprise slots, and possibly openings, which give them a certain amount of radial flexibility so that they can be anchored in the acetabular cavity by expanding.
Existing implants which are attached without cement anchor the cup securely and reliably in the acetabulum, but do, however, have the major drawback of allowing debris from the wearing of the material of which the insert is made to diffuse into the acetabular cavity and into the body.
This debris is the result of the rubbing of the head of the femur in the insert and of micro-movements of the insert within the cup. It is also the result of the rubbing of the insert against the somewhat sharp edges which delimit the aforementioned recesses, holes, slots or openings in the cup, this being exacerbated by the fact that the insert tends to creep into these recesses, holes, slots or openings under the effect of the repeat stresses to which the prosthesis is subjected.
Furthermore, the aforementioned holes, slots or openings allow particle-laden synovial fluid to flow towards the bone, this flow being the result of a “pumping effect” caused by the successive application and release of pressure on the implant in the cavity as the patient walks.
This polyethylene debris causes osteolysis, which is detrimental to the firm anchorage of the screws over time, and is not well tolerated by the body.
Another drawback of “press-fit” cups is the risk of an inadequate bond between the external surface of the cup and the ilium, especially at the catching reliefs in the equatorial region of the cup, which means that it is often necessary to use screws for fastening into the bone tissue, even in the case of cups which are coated with a material which assists with osteo-integration, such as calcium hydroxyapatite.
SUMMARY OF THE INVENTION
The present invention aims to overcome these drawbacks by providing an acetabular implant which can be anchored to the innominate bone without cement and without resorting to the fitting of screws.
The implant to which it relates comprises a cup and an insert like the aforementioned, the cup being inserted by impaction and for this purpose, comprising anchoring roughnesses formed on its exterior face.
According to the invention, the cup comprises, formed in its exterior face, coaxial with it, two grooves in the form of helical threads of opposite hand which intersect one another over a large proportion of this external face, which are inclined towards the opening of the cup and which have a depth which increases progressively towards the opening of the cup, these grooves, by intersecting, delimiting a collection of contiguous and imbricated diamond-shaped roughnesses.
Advantageously, these roughnesses are uniformly distributed over the exterior surface of the cup and their dimensions decrease progressively towards the polar region of the cup.
The inclination of these grooves and the increase in their depth along the face of the cup allow the roughnesses to dig progressively into the bone over the entire periphery of the cup, at the time of impacting. At the same time, the number, arrangement and sharp nature of these roughnesses ensure perfect anchorage of the cup in the bone, making the fitting of additional anchoring screws unnecessary.
It is actually on the mechanically strongest part of the natural acetabulum that these roughnesses catch and are the most effective. This is why the depth of the grooves is greatest over that part of the cup which is contiguous with its equatorial plane, opposite which the mechanical strength of the bone wall is highest.
As a consequence of this, the cup is therefore able not to comprise any holes for the passage of such screws, or any slots or openings. The absence of such holes, slots or openings makes it possible to eliminate the risk of creep of the insert and to eliminate any somewhat sharp edge against which the insert may wear away.
The wear on the insert, and therefore the emission of particles, thus remains extremely low in the implant according to the invention.
In addition, the absence of such holes, slots or openings makes it possible for any particles which might be generated by the wearing of the insert to be trapped inside the cup, and therefore makes it possible for the risk of these particles migrating towards the bone, through the cup, to be eliminated.
In a particularly preferred embodiment, the cup has an exterior surface which is treated or coated in such a way as to encourage binding by osteo-integration onto this surface. To achieve this, it receives a coating based on calcium hydroxyapatite. Thanks to a coating with this biomaterial on the external surface of the cup, it becomes possible, after an initial attachment via the mechanical effect of the so-called “press-fit” means to the bony edge of the acitabulum, to achieve a secondary attachment of a physico-chemical nature which strengthens the primary attachment in the first few months after the operation. This attachment is obtained by absorption of the various constituents of the bone tissue onto the calcium hydroxyapatite deposit covering the external face of the cup.
As a preference, the grooves are inclined towards the opening of the cup by an angle of about 45°, for perfect anchorage of the roughnesses in the bone, with no possibility of the said cup springing back out.
Advantageously, the flanks of two roughnesses delimited by one and the same groove are asymmetric, the flank situated on the same side as the opening of the cup being more steeply inclined towards the equatorial plane relative to the axis of the cup than the flank situated on the same side as the closed end of the cup. This asymmetry encourages anchoring.
In actual fact, thanks to this advantageous feature of the invention, the groove flank situated on the same side as the opening has a larger area than the other flank, and this area is not very steeply inclined with respect to the plane tangential to the sphere, which means that the maximum stresses exerted at this point on the bone tissue are mainly compressive stresses, which encourages bone growth and the osteo-integration of the prosthesis in this delicate region.
For the same reason, the threads advantageously, in cross-section, have a “hooked-beak” shape, that is to say have a pointed end edge which is curled down slightly towards the opening of the cup.
It is therefore preferable for the exterior face of the cup to be covered with a coating which encourages osteo-integration, particularly with a coating of calcium hydroxyapatite. The thickness of this coating may,
Balay Bruno
Cartillier Jean Claude
Charlet Claude
Machenaud Alain
Semay Jean Marc
Stewart Alvin
Sughrue Mion Zinn Macpeak & Seas, PLLC
Willse David H.
LandOfFree
Acetabular implant fixed without cement does not yet have a rating. At this time, there are no reviews or comments for this patent.
If you have personal experience with Acetabular implant fixed without cement, we encourage you to share that experience with our LandOfFree.com community. Your opinion is very important and Acetabular implant fixed without cement will most certainly appreciate the feedback.
Profile ID: LFUS-PAI-O-2509826