FR2803739A1 - Threaded medical implant of relatively weak material e.g. for fixing ligament has lengthwise cavity with two inner projections for tool - Google Patents

Abstract

The implant (1), made from a relatively weak and especially resorbable material, has an axial cavity (4) extending from its proximal end (3), to receive the tip of a tool used to rotate the implant when it is being fitted or removed. The cavity has two inner lengthwise radial projections (6), each with two flat surfaces (6a, 6b) at an angle to one another. The two projections lie diametrically opposite one another in the same plane, and each of the flat surfaces is broad enough to allow the implant to be rotated when driven in, but is narrow in relation to the diameter of the cavity, e.g. between a fifth and a seventh of the diameter. The inner surfaces of the cavity between the projections are circular in shape, with a wall thickness less than in the region of the projections.

Description

The present invention relates to a threaded implant for medical use, made of a relatively weak material, in particular a bioabsorbable material.

The invention is particularly applicable to a so-called "interference" screw, that is to say a grub screw intended to ensure the fixation of a ligamentous plasty. The invention may, however, relate to any other type of threaded implant, in particular a threaded rod or pin.

Absorbable material interference screws are well known. A screw of this type generally comprises an axial duct opening at its two ends, for its engagement on a guide pin, and has an axial cavity opening at the proximal end of the screw, this cavity being intended to receive an instrument for screwing or unscrewing the screw.

On some known screws, this cavity has a hexagonal cross section and extends over a large portion, in the axial direction, of the screw to ensure longitudinal bearing surfaces of the instrument against the screw.

The exercise of a tightening torque by means of this instrument, however, generates relatively large and localized stresses on the screw, oriented in the radial direction thereof and outwards. These constraints can lead to bursting of the screw beyond a torque threshold.

This hexagonal section of the cavity also has the disadvantage of generating a succession of wall portions of the screw having a small thickness, that is to say at the angles of the hexagon, and wall portions having a greater large thickness, that is to say at the flats delimiting the cavity. During the degradation of the screw, the thinner wall portions will disappear more rapidly than the thicker wall portions, thereby generating a plurality of screw fragments in the form of bars. These fragments are likely to migrate into the body and cause damage to the body before complete degradation.

The present invention aims to remedy these substantial disadvantages, providing a threaded implant for medical use having improved strength, able to eliminate, or at least to greatly reduce, any risk of rupture of the implant during its screwing. The invention also aims to provide a threaded implant for medical use degrading homogeneously, that is to say without generating, or generating as little as possible, longitudinal fragments capable of migrating into the body.

The implant concerned comprises, in a manner known per se, an axial cavity opening into its proximal end, this cavity being shaped to receive, with rotational wedging, an instrument for driving the implant in rotation for screwing or unscrewing. of this implant.

According to the invention, the implant comprises two longitudinal projections protruding, radially and inward, from its wall delimiting said cavity; each of these bosses has two flat faces forming an angle between them, namely an anterior face with respect to the screwing direction of the implant and a posterior face with respect to this same direction; the two anterior faces of the two bosses are oriented radially relative to the implant and are diametrically opposed, so that they are located substantially in the same plane; each of them has a width such that it allows the driving of the implant in rotation in the direction of screwing, by taking support of corresponding faces that includes said instrument but this width is small compared to the diameter of the cavity, that is to say can go from seventh to fifth of this diameter; each posterior face has a width of the order of three times that of each anterior face, so that the two posterior faces are oblique and allow the progressive connection of said bosses to the rest of the wall of the implant.

The two anterior faces, oriented radially with respect to the implant, form two surfaces ideally arranged for the bearing of corresponding radial faces that the instrument comprises; they can therefore project only slightly inside the cavity, so that the bosses have a limited thickness.

This limited thickness makes it possible to confer a relative homogeneity of thickness on the wall of the implant, so that the latter can degrade without leaving longitudinal fragments that can migrate into the body.

These radial faces further allow the exercise in optimal conditions of the torque required for screwing the implant, which significantly reduces the risk of bursting of the implant during this screwing. Given their aforementioned form, these bosses extend angularly about 200 degrees of the circumference of the implant. The latter can therefore have, outside these bosses, large wall portions delimiting a circular inner face and having a thickness less than that of the bosses.

The two posterior faces form, for their part, bearing surfaces for unscrewing the implant.

It follows from all these provisions that the implant has a structure that is able to withstand perfectly the tightening torque exerted by the instrument during the screwing of the implant, while having a relatively homogeneous wall as to its thickness, able to degrade without generating longitudinal fragments likely to migrate into the body.

The depth of said cavity advantageously corresponds to a large part of the length of the implant, that is to say can extend over a distance ranging from half to two thirds of this length.

This same cavity may also have a section going down in the distal direction of the implant.

For a good understanding, the invention is again described below with reference to the attached schematic drawing showing, by way of non-limiting example, a preferred embodiment of the implant it relates.

Figure 1 is a longitudinal sectional view; Figure 2 is a cross-sectional view along the line II-II of Figure 1; Figure 3 is a perspective view of an instrument for the implementation of this implant; Figure 4 is an enlarged view of the end portion of this instrument for engagement with the implant; and Figure 5 is a sectional view along the line V-V of Figure 4, and scale still enlarged, this end portion.

FIGS. 1 and 2 show an interference screw 1 made of bioabsorbable material, of the type commonly used to allow fixation of a graft, bone or not, integral with a ligamentous plasty, to the bone to which this ligamentous plasty must be connected.

The screw 1 is made, according to a known technique, by molding a material based on lactic acid. It has a thread 2, a proximal end 3 without a head, an axial cavity 4 opening into the end 3 and an axial duct 5 therethrough, to allow its engagement on a guide pin.

The cavity 4 extends over approximately two-thirds of the length of the screw 1 and has a section going down in the distal direction. It is intended to come into engagement with the instrument 10 shown in FIGS. 3 to 5, allowing the screw 1 to be screwed on.

It appears more particularly in Figures 1 and 2 that the screw 1 has two longitudinal projections 6, projecting radially and inwardly from its wall which defines the cavity 4. The parts 7 of this wall lying between these bosses 6 have a circular shape.

Each of these bosses 6 is diametrically opposite with respect to the other, and has two flat faces 6a, 6b forming an angle between them, namely a front face 6a with respect to the screwing direction of the screw 1 and a posterior face 6b compared to that same meaning.

The two anterior faces 6a are radially oriented and are diametrically opposed with respect to each other, so that they are located substantially in the same plane; each of them has a width corresponding to one sixth of the diameter of the cavity 4 in a given axial location.

Each posterior face 6b has a width corresponding to triple that of each face 6a, so that these two walls 6b are oblique and allow a progressive connection of the bosses 6 to the rest of the wall of the screw 6.

As an indication, the screw 1 shown may have the following dimensions - a length of twenty-five millimeters, - a diameter of six millimeters at the base of the thread 2 and eight millimeters at the top of this thread, - a cavity 4 having seventeen millimeters in depth; at eleven millimeters from the proximal opening, the cavity 4 has a diameter of 3.5 millimeters (counted at the portions 7), each face 6a having a width of 0.643 millimeters while each face 6b has a width of 1.9 millimeter; each face 6a forms an angle of 105 degrees with the face 6b contiguous; a duct 5 having a diameter of 1.5 millimeters.

The instrument 10 has a structure similar to that of a screwdriver, with a rod 11 and a handle 12. The rod 11 ends, on the opposite side to the handle 12, by an end portion 13 having a complementary profile of that of the cavity 4, this portion 13 can be engaged without play in this cavity 4.

It follows from the foregoing that this end 13 has two radial walls 13a intended to come, during the tightening of the screw 1, bearing against the faces 6a, and two oblique faces 13b intended to come, during the loosening of the screw 1, bearing against the faces 6b.

The invention thus provides a screw 1 for overcoming the disadvantages of counterpart screws of the prior art. Indeed, this screw has an improved resistance, able to eliminate, or at least to greatly reduce, any risk of rupture during screwing, and, given the relatively homogeneous thickness of its wall, degrades evenly, that is, without generating, or generating as little as possible, longitudinal fragments likely to migrate into the body.

It goes without saying that the invention is not limited to the embodiment described above by way of example but embraces, on the contrary, all the variants. Thus, the invention is applicable to any type of threaded implant for medical use, including a rod or a threaded pin, and implants not necessarily bioresorbable material but simply a relatively weak material, such as a plastic material.

Claims (1)

<1> - Threaded implant for medical use, made of a relatively weak material, in particular a bioabsorbable material, comprising an axial cavity (4) opening into its proximal end (3), this cavity ( 4) being shaped to receive, with rotational wedging, an instrument (10) for driving the implant (1) in rotation to screw or unscrew this implant (1); implant (1) characterized in that it comprises two longitudinal projections (6) projecting, radially and inward, from its wall delimiting said cavity (4); each of these bosses (6) has two flat faces (6a, 6b) forming an angle between them, namely a face (6a) anterior to the screwing direction of the implant (1) and a face (6b) posterior in relation to this same meaning; the two anterior faces (6a) of the two bosses (6) are oriented radially relative to the implant (1) and are diametrically opposed, so that they are located substantially in the same plane; each of them has a width such that it allows the drive of the implant (1) in rotation in the direction of screwing, by taking support of corresponding faces (13a) that includes said instrument (10) but this width is small relative to the diameter of the cavity (4), that is to say can go from seventh to fifth of this diameter; each posterior face (6b) has a width of the order of three times that of each anterior face (6a), so that the two posterior faces (6b) are oblique and allow the progressive connection of said bosses (6) to the rest of the the wall of the implant. 2 - implant according to claim 1, characterized in that it has, outside the bosses (6), portions (7) of wall defining a circular inner face and having a thickness less than that of the bosses (6). 3 - Implant according to claim 1 or claim 2, characterized in that the depth of the cavity (4) corresponds to a major part of the length of the implant (1), that is to say extends over a distance of one-half to two-thirds of that length. 4 - Implant according to one of claims 1 to 3, characterized in that the cavity (4) has a section being reduced in the distal direction of the implant (1). 5 - Implant according to one of claims 1 to 4, characterized in that it comprises an axial duct (5) passing through it from one side to allow its engagement on a guide pin.