Prosthesis (i.e. – artificial body members) – parts thereof – or ai – Implantable prosthesis – Bone
Reexamination Certificate
2001-06-07
2003-06-24
Huson, Gregory L. (Department: 3751)
Prosthesis (i.e., artificial body members), parts thereof, or ai
Implantable prosthesis
Bone
C623S017120, C623S011110, C623S017120, C623S018110, C606S064000, C606S08600R, C606S087000, C606S075000, C606S075000
Reexamination Certificate
active
06582468
ABSTRACT:
BACKGROUND OF THE INVENTION
The invention relates to intervertebral disc prostheses. The document EP-0 356 112 discloses an intervertebral disc prosthesis comprising two generally flat plates and an elastomeric body interposed between the plates and fixed to them by its flat end faces. The mechanical behavior of this prosthesis is quite similar to that of a natural healthy intervertebral disc, especially when the body is compressed between the two plates.
SUMMARY OF THE INVENTION
One object of the invention is to provide a disc prosthesis of a different type that more closely approximates to the mechanical properties of a natural healthy intervertebral disc.
To achieve this object, the invention provides an intervertebral disc prosthesis comprising two plates and a compressible body interposed between the plates, at least one end of which has an area of contact with one of the plates, in which prosthesis the body and the plate are such that the area of contact increases when a stress on the plate in the direction of the body is increased.
Thus, when the compressive stress on the body increases from a moderate value or zero, the height of the body, measured from one plate to the other, reduces relatively quickly. Subsequently, as the contact area increases, the reaction of the body becomes greater and a comparatively greater stress has to be applied to reduce the height by an equivalent amount. In other words, for the lowest compression values the mechanical reaction of the body during the compression varies very little as a function of the change of height. Consequently, the curve representing the applied stress as a function of the variation of height is inclined only a small amount from the horizontal for low compression values. At the beginning of the stroke, therefore, the load supplied is small. This property is also that of a natural healthy disc. The behavior of the body can therefore be adapted not only by the choice of material but also by the shape of the end face or faces with a contact area which varies so as to approximate as closely as possible to the mechanical properties of a natural healthy intervertebral disc.
Advantageously, the area of contact is defined by a face of the plate and a face of the end of the body, one of the faces of the plate or of the body, notably the face of the body, being curved and convex, and the other face being flat.
Advantageously, the area of contact is defined by a face of the plate and a face of the end of the body, both faces being curved in at least one common direction, and one being concave and the other convex, the concave face having at least one radius of curvature greater than a corresponding radius of curvature of the convex face.
This configuration therefore enables the variations of mechanical reaction to be introduced as indicated above. Furthermore, when the body is free to move sideways with respect to the plate, as will be seen later, this configuration ensures the relative centering of the two faces. For example, after the two faces have been displaced relative to each other, these curvatures enable them to recenter themselves automatically.
Advantageously, the body has at least one end in contact with one of the plates and free to move with respect to the plate in a direction parallel to the plate.
This arrangement therefore reduces the risk of excessive stress developing between the two vertebrae in a direction perpendicular to the longitudinal direction of the spine, that is to say in shear.
Advantageously, the body has an end housed in a depression in one of the plates that forms a lateral stop for this end.
Lateral displacements of the body relative to the plate can therefore be limited or even prevented.
Advantageously, the body comprises a viscoelastic material, notably silicone.
With such a material it is possible to give a hysteresis form to the curve representing the stress on the compressed body relative to its changing height. Since this curve is also that of a natural healthy disc, its mechanical properties are approximated to even more closely.
The prosthesis advantageously comprises a fluid interposed between the plates.
The addition of a fluid therefore increases the hysteresis form, especially when the fluid is compressible such as a gas or a mixture of a liquid and a gas which is partially soluble in the liquid.
Advantageously, the fluid is in contact with the plates.
Advantageously, the fluid is around the periphery of the body.
Advantageously, the prosthesis comprises an enclosure containing the fluid and constructed in such a way that it has a cross-sectional area parallel to the plates that is effectively invariable when a stress pushing the plates toward each other varies.
Advantageously, the prosthesis is intended for the lumbar region of the spine.
Other features and advantages of the invention will also appear in the following description of a preferred embodiment and two variants given by way of non-limiting examples.
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Huson Gregory L.
Kokabi Azy
Lerner David Littenberg Krumholz & Mentlik LLP
Spryker Spine
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