Trapezal or trapezo-metacarpal implant

Abstract

The implant comprises a stem (10) terminating in a head (20). The stem is adapted to be inserted in the metacarpus of the thumb and the head to be disposed in a space obtained particularly by removing at least a portion of the trapezium. The surface of the head is constituted by a base (201) connected to the stem and adapted to rest on the proximal end of the metacarpus, a distal surface portion (202) opposite the base and serving as a contact surface with the bone located facing the proximal end of the metacarpus, and a connecting portion (203) connecting the base and the distal surface portion of the head. The distal surface portion (202) of the head is inclined relative to a longitudinal axis (104) of the stem by a predetermined angle (α), such that, when the stem (10) is mounted straight in the metacarpus, the distal surface portion (202) of the head can be located in varus.

Description

FIELD OF THE INVENTION [0001] The present invention relates to a trapezal or a trapezo-metacarpal implant, comprising a stem terminating in a head. The stem is adapted to be inserted in the metacarpus of the thumb, and the head to be disposed in a space obtained particularly by removing all the trapezium, in the case of a trapezal implant, or a portion of the trapezium, in the case of a trapezo-metacarpal implant. [0002] The implant according to the invention can be used to treat rhizarthritis, a very frequent affliction consisting in an arthritis of the base of the thumb, essentially trapezo-metacarpal. STATE OF THE ART [0003] There exist several types of trapezal and trapezo-metacarpal implants. [0004] Among the most widely known are the so-called Swanson trapezal implant, constituted by a stem terminating in a head, the entirety made of silicone. In this implant, the head has a suitable size and shape to replace all the trapezium. [0005] This implant has two principal drawbacks: ...

AI Technical Summary

Benefits of technology

[0016] The inclination of the distal surface portion of the head relative to the stem permits inserting the stem straight into the metacarpus, which is to say along the longitudinal axis of this latter. The stem can thus be placed exclusively in the soft central portion of the bone, which facilitates the work of the surgeon who does not have to cut into the hard peripheral portion. Such a positioning of the stem moreover avoids rendering the bone excessively fragile.

[0017] There is no need, according to the invention, to ligate the implant or to pass a tendon through it. On the other hand, contrary to the trapezo-metacarpal implants of the prior art, the head can be wide and its distal surface portion can have a large radius of curvature corresponding substantially to the radius of curvature of the trapezo-metacarpal articulation. The head can also be flatter. The surgeon thus does not need to make a large cut on the proximal end of the metacarpus, nor to cut deeply into the trapezium, for which the removal of a small cap of large radius of curvature can suffice.

[0018] The stability of the implant according to the invention can be further increased by designing this latter such that the distal surface portion of the head will be not only inclined but also offset relative to the longitudinal axis of the stem to the side of the implant where the angle between the distal surface portion of the head and the longitudinal axis of the stem is smaller.

[0019] Preferably, at least the distal surface portion of the head is made of pyrocarbon. Pyrocarbon has a very good coefficient of friction with bone, which permits it to slide without adherence over the bones with which it is in contact and without giving rise to wear. In contrast to silicone, which is too soft, and titanium, which is too hard, pyrocarbon has a modulus of elasticity, also called Young's modulus, near that of bone. The reciprocal forces exerted on the implant and the neighboring bones thus distribute themselves evenly, thus reducing the risk of pain for the patient.

Problems solved by technology

This implant has two principal drawbacks: the material from which it is made is unsuitable: silicone gives rise in certain patients to an allergy known as siliconite; moreover, because of its insufficient hardness, the silicone wears in contact with the adjacent bones, giving rise to deterioration of the implant; it is not stable and, in particular, has a tendency to dislocate, fully or partially; FIGS. 1A, 1B and 1C show a trapezal implant 1, whose stem 1a is mounted in the metacarpus 2 and the head 1b completely replaces the trapezium; in FIG. 1A, the implant is normally positioned relative to the adjacent bones and particularly relative to the scaphoid 3 with which it is in contact; FIG. 1B shows a typical configuration of partial dislocation of the implant during a pinching effort involving the thumb and at least one other finger of the hand; FIG. 1C shows a typical arrangement of full dislocation of the implant after such a pinching effort.

These techniques complicate the task of the surgeon and limit the mobility of the implant, causing trouble for the patient.

Nevertheless, fixing the implant in an inclined manner relative to the axis of the metacarpus requires providing an oblique hole in the shaft of the metacarpus, a delicate surgical operation to perform, which requires hollowing out the hard peripheral portion of the bone and which moreover renders this latter fragile.

In the same way as for the Swanson trapezal impant, the material from which these trapezo-metacarpal implants are made is unsuitable.

As previously explained, silicone is too soft.

As for titanium, it is too hard and wears the bones with which it is in contact.

It also makes the implant painful for the patient.

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