Implantable Joint Prosthesis

Abstract

Implantable prosthesis of a joint comprising a central flexible hollow body portion, in the form of a tube of silicone elastomer, into the ends of which are inserted medullary pins of the same material, to extend outwardly from the body portion for insertion into the medullary ducts of the bones to be joined. The pins may be adhesively secured to the bones, or at least partly surrounded by a textile sheath colonizable by bone tissue. The cavity may be filled with sterile air or liquid.

Description

The present invention relates to an implantable prosthesis of a joint.

It has already been proposed to replace deficient or destroyed natural joints by flexible prostheses for insertion in the medullary ducts and comprising a flexible central mass extended by two medullary pins, the prosthesis consisting of a single piece of silicone elastomer or equivalent material (i.e., simultaneously offering resistance to alternating flexion, mechanical stability and suitable innocuousness).

In use it has been found that such a prosthesis is not entirely satisfactory since the central mass presents a couple of excessive resistance to flexion relative to the resistance of the terminal pins. Consequently the prosthesis tends to bend at the level of the pin rather than at the level of the central mass, even when this mass is weakened by a groove parallel to the bending axis. Moreover, if the pin is not sufficiently anchored (notably by colonizable of bone tissue) it tends to slide longitudinally in the medullary duct and to pull out partially during the flexion.

According to the present invention there is provided implantable prosthesis of a joint comprising a central flexible hollow body portion, defining a central unobstructed cavity enabling the central portion to be bent with flattening of the opposite walls, and two medullary pins extending outwardly from said central portion, each dimensioned effective to be inserted into the medullary ducts of the bones to be joined.

The flexible portion preferably is of substantially elliptic transverse cross-section and of a longitudinal section of rectangular to diamond shaped configuration; more particularly the longitudinal section may be an ellipse or more simply a rectangle, optionally ending in arcs of a circle, an ellipse or a parabola.

Generally the ratio of the major axis to the minor axis of the internal and external transverse cross-sections may be between 1 and 2, the major axis being parallel to the bending axis. Good results have been obtained with a ratio of 1.5 to 1.2.

The wall thickness of the central body portion is a function of the elasticity of the material employed and of the result desired. With silicone elastomers of medicinal quality of 85 to 110 kg/cm.sup.2 breaking strength, 380 to 550 percent elongation (AFNOR T 46 002), 47 to 60 percent recovery (DIN 53 512), the wall thickness may vary between 1/10 and 1/3 and preferably between 1/6 and 1/4 of the outer diameter for a tube of circular section. This thickness is not necessarily the same over all of the periphery, it can in particular be greater at the side of the centre of flexion, a ratio of wall thicknesses of up to 2:1 being possible.

The length of the central body portion depends on the desired effect, an average length of 1 to 21/2 times (preferably 1 to 11/2 times) the diameter generally being suitable in the case of a body portion having a cavity which is a cylindrical. Most often a body portion of the shortest possible length is chosen, the portion of joint which is to be replaced being itself the shortest possible.

At either side of the central transverse plane of the body portion, and preferably beyond the above defined length, the wall of the tube may gradually thicken toward the interior until at the two ends of the cavity a conical or curved wall as indicated above is formed.

The tubular portion may initially be filled with sterile air. After implantation water may diffuse through the wall into the tubular portion. It is thus possible initially to introduce into the cavity a sterile, physiologically compatible liquid, for example an isotonic saline solution containing 1 percent of zinc sulphate.

The medullary extremities may be of any material and shape currently utilized for implantable prostheses. They may be cylindrical or slightly conical, flexible or rigid, preferably have a surface which is colonizable or adapted to be glued by the well-known surgical adhesives. They may, for example, be of silicone elastomer sheathed in a tricot of polyester fibre which has been adhesively affixed or of potentially porous ceramic material (described for example in Chemical Engineering, June 16, 1969, p. 42 or in Chemical and Engineering News, Mar. 15, 1971, p. 15).

Implanting of the prosthesis is accomplished according to customary techniques.

The prosthesis according to the invention enables the bending line to be well localized, it only offers a weak resistance to flexion and in particular permits bending of large amplitude. Moreover, in contrast to solid, flexible prostheses, it has its maximum restoring couple in a position near the extension which is an important advantage when the muscle or extensor tendon are deficient. It is believed that these advantages are due to the fact that the flexion of the prosthesis causes a two-fold deformation, first flattening of the tube and then bending of the flattened walls, and to the fact that this latter bending affects two layers which are partly movable relative to one another. It is to be noted that filling the cavity with liquid imposes a constant volume on the cavity which limits the tube being crushed and enhances the bending resistance. Moreover, the presence of the cavity improves absorption of terminal shocks.

In order that the invention will more readily be understood, the following description is given, merely by way of example, reference being made to the accompanying drawing, in which the sole FIGURE is a cross-section through one embodiment of prosthesis according to the invention.

The example of joint prosthesis illustrated in the drawing comprises a flexible central tubular portion 1 and two medullary pins 2 and 3. The portion 1 is a tube of circular section of silicone elastomer, outer diameter 8 mm, inner diameter 4 mm, length 14 mm. The pins 2 and 3 are rods of the same kind of elastomer, diameter 4 mm, length 20 mm, embedded 5 mm. in the tube. At the time of assembly the surfaces and junction angles are coated with a moisture-curable silicone adhesive which results in integrating the assembly and in suppressing abrupt sectional variations. The cavity 4 may be filled with isotonic saline solution with the aid of a syringe, prior to total curing of the adhesive. As illustrated the surface of the medullary pins 3 is covered with a sheath 5 of material colonizable by the bone tissue.

The articulation may, of course, be obtained from two preformed pieces, one corresponding for example to the whole of portions 1 and 2, or it may even be obtained directly as a single piece, for example in a revolving mould.

The elastomer may contain certain radio-opaque charges which facilitate the post-surgery control.

Claims

I claim:

1. Implantable prosthesis of a joint comprising a central flexible hollow body portion, defining a central unobstructed cavity, enabling the central portion to be bent with flattening of the opposite walls and two medullary pins extending outwardly from said central portion, each dimensioned effective to be inserted into the medullary ducts of the bones to be joined.

2. Implantable prosthesis of a joint as claimed in claim 1, wherein said central body portion is in the form of a tubular sleeve, having ends, and wherein said medullary pins are inserted into said ends.

3. Implantable prosthesis of a joint as claimed in claim 1, wherein said central body portion is formed of silicone elastomer.

4. Implantable prosthesis of a joint as claimed in claim 1, wherein said central flexible hollow body portion is of substantially elliptical transverse cross-section, the ratio of the major axis length to minor axis length being between 1 and 2.

5. Implantable prosthesis of a joint as claimed in claim 1, wherein said pins are interconnected solely by said flexible hollow body portion.