Prosthesis (i.e. – artificial body members) – parts thereof – or ai – Implantable prosthesis – Bone
Patent
1992-08-04
1994-05-24
Green, Randall L.
Prosthesis (i.e., artificial body members), parts thereof, or ai
Implantable prosthesis
Bone
623 18, 403112, A61F 244
Patent
active
053144775
DESCRIPTION:
BRIEF SUMMARY
The invention concerns a prosthesis for intervertebral discs designed to replace fibrocartilaginous discs to connect the vertebra and the spinal column, and instruments for implanting it.
It is well known that the intervertebral discs often become settled, deformed, shifted or just simply wear out. The result is multiple pathological symptoms causing intense pain and some anxiety to the patients.
For a long time, the only ways surgeons could intervene, mainly to relieve the patients' pain, was simple ablation of the defective disc or intervertebral synostosis, which temporarily relieved the patient, but at times gave him/her a certain functional handicap. Over the last twenty years, recourse has been made, more or less successfully, to prostheses designed to replace intervertebral discs that were totally or partially ablated.
The research in this area has taken two routes; one is to replace the defective disc with a kneecap-type disc made of a ductile, incompressible material, placed between two corresponding plates with spherical impressions, as is described in French patent 2.372.622; the other consists of replacing the defective disc with an artificial disc made of a composite material with practically the same mechanical characteristics as the natural disc, as is described in French patent 2.124.815, namely a disc made of an elastomer reinforced with a textile material.
A combination of these two research routes produced a prosthesis, as described in the application for European patent 0.042.271, whose main goal is a two-part prosthesis that has a hemispheric pad and a hemispheric impression, respectively; one part is metal and the other synthetic (polyethylene, polymethacrylate).
While these disc prostheses make it easy to restore and maintain over time a normal separation between the vertebrae (7 to 14 mm), keeping them in a transverse position leaves something to be desired, for the relative movement of the vertebrae concerned causes progressive misalignment of the prosthesis, causing pathological symptoms requiring fast surgical intervention. To attempt to solve this problem, certain disc prostheses have been equipped, on the outside of the pressure plates, with small clamps designed to be implanted in each of the vertebra as soon as the surgeon releases the separating force initially applied in order to separate the vertebra sufficiently so that the prosthesis may be put into position. However, this lateral joining method is effective only when the vertebra remain parallel and, consequently, when the spinal column is not subject to flexure. It is easy to see that this cannot happen in the opposite case, for then the clamps remain supported in their impressions on the side of the flexure, while they escape from the opposite side; which causes gradual staggering of the impressions when the flexure is accompanied by a certain rotation. Consequently, the prosthesis gradually moves away from its ideal location and is ultimately expelled, causing serious problems.
Furthermore, it is difficult to reconcile, in terms of the kneecap, the characteristics of flexibility and incompressibility, which are often accompanied by a certain sensitivity to wear and to creep; which has thus far caused certain manufacturers of prostheses for intervertebral discs to resort to three-part units, permitting the introduction of a dual kneecap joint composed of two spherical caps with large radii, opposite the base. However, this concept, although it allows the contact surfaces in particular to be increased, has the disadvantage of substantially reducing the thickness of the dual kneecap joint and making it easier to dislocate it, by ejection, particularly when the spinal column flexes near the vertebrae concerned, since this attraction is translated into a pinching of the dual kneecap joint in the direction of the flexure, accentuating the separation in the other direction.
The purpose of this invention is to remove these disadvantages. This invention, as it is characterized, solves the problem of creating an intervertebral
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Green Randall L.
J.B.S. Limited Company
Willse D.
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