Patent Publication Number: US-2007123993-A1

Title: Trapezal or trapezo-metacarpal implant

Description:
FIELD OF THE INVENTION  
      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.  
      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  
      There exist several types of trapezal and trapezo-metacarpal implants.  
      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.  
      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  1 C show a trapezal implant  1 , whose stem  1   a  is mounted in the metacarpus  2  and the head  1   b  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.        

      So as to increase the stability of this implant and avoid partial dislocation as shown in  FIG. 1B  or full dislocation as shown in  FIG. 1C , it has been proposed to perforate its head from one side to the other so as to pass therethrough a tendinous tongue adapted to limit its movements, or to ligate it. These techniques complicate the task of the surgeon and limit the mobility of the implant, causing trouble for the patient.  
      Another solution is described in the article entitled “L&#39;implant trapézien de Swanson dans le traitement de l&#39;arthrose péri-trapézienne” (“The Swanson trapezal implant in the treatment of peri-trapezal arthritis”), by Y. Allieu et al., which appeared in the review “Annales de Chirurgie de la Main” (“Annals of Surgery of the Hand”), Volume 3, No. 2, 1984. It consists in first carrying out an oblique cut at the proximal end of the metacarpus, and fixing the implant in the varus position, which is to say fixing it in an inclined manner relative to the axis of the metacarpus by orienting it inwardly of the hand, as shown in  FIG. 2 . Thus inclined, the implant is held in contact with the scaphoid, and has less tendency to dislocate. This solution thus significantly improves the stability of the implant. 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.  
      There are also known trapezo-metacarpal implants, such as the Swanson condylar implant, whose head is to be disposed in the space provided on the one hand by forming a cut on the proximal end of the metacarpus and on the other hand by removing a portion of the trapezium opposite this proximal end. These implants are also made of silicone. A titanium version of the Swanson condylar implant however has been proposed.  
      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.  
      Moreover, these implants have a relatively thick and narrow head, adapted to be disposed in a cavity of matching shape previously formed in the trapezium. This co-action between a thick and narrow head and a deep and narrow cavity gives stability to the implant—which is the intended purpose—because the head is retained in the cavity, but does not take account of the anatomical curvature of the trapezo-metacarpal articulation, which is suitable for ample relative movements between the trapezium and the proximal end of the metacarpus. By way of illustration,  FIG. 3  shows the Swanson condylar implant with its stem  5   a  inserted in the metacarpus  2  of the thumb and its head  5   b  disposed in a deep and narrow cavity in the trapezium  6 . The small radius of curvature of the surface portion of the head in contact with the trapezium, made necessary by the thickness and narrowness of the head, limits the angular swing of the implant, which hinders movements of the thumb.  
     OBJECTS AND SUMMARY OF THE INVENTION  
      The present invention seeks particularly to provide a trapezal or a trapezo-metacarpal implant which will be at least as stable as the implants described above and which will be relatively easy to emplace.  
      To this end, there is provided a trapezal or trapezo-metacarpal implant comprising a stem terminating in a head, the stem and the head being adapted respectively to be inserted in the metacarpus of the thumb and to be disposed in a space obtained particularly by removing at least a portion of the trapezium, the surface of the head being constituted by a base connected to the stem and adapted to rest on the proximal end of the metacarpus, a distal surface portion opposite the base and serving as a contact surface with the bone located facing the proximal end of the metacarpus, and a connection portion connecting the base and the distal surface portion of the head, characterized in that the distal surface portion of the head is inclined relative to a longitudinal axis of the stem by a predetermined angle, such that, when the head is mounted straight in the metacarpus, the distal surface portion of the head can be located in varus. Said bone facing the proximal end of the metacarpus is the scaphoid in the case of a trapezal implant, and the remaining portion of the trapezium in the case of a trapezo-metacarpal implant.  
      Thus, according to the invention, the stability of the implant is given by the varus position of the distal surface portion of the head, as in the mentioned article by Y. Allieu et al. “L&#39;implant trapézien de Swanson dans le traitement de l&#39;arthrose péri-trapézienne” (“The Swanson trapezal implant in the treatment of peri-trapezal arthritis”). However, unlike this article, it is not the entire implant which is positioned in varus, but only a portion of the implant, including the distal surface portion of the head.  
      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.  
      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.  
      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.  
      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&#39;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. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
      In the accompanying drawings:  
       FIGS. 1A, 1B  and  1 C, already discussed, show a portion of a hand in which has been implanted a trapezal implant according to the prior art, respectively in the rest position, in pinching position with the implant partially dislocated, and in pinching position with the implant fully dislocated, this portion of the hand being seen on the dorsal side, with the thumb and trapezo-metacarpal articulation in profile and the other fingers seen from above;  
       FIG. 2 , already discussed, shows, in the same type of view as that of  FIGS. 1A, 1B  and  1 C, a trapezal implant according to the prior art mounted obliquely in the metacarpus of the thumb;  
       FIG. 3 , already discussed, shows, in the same type of view as that of  FIGS. 1A, 1B  and  1 C, a trapezo-metacarpal implant according to the prior art mounted in the metacarpus of the thumb;  
       FIG. 4A  shows in profile view a trapezo-metacarpal implant according to a first embodiment of the invention;  
       FIG. 4B  shows in profile view a modification of the implant according to the first embodiment of the invention;  
       FIG. 5  shows, in the same type of view as those of  FIGS. 1A, 1B  and  1 C, the implant according to the first embodiment of the invention emplaced in the hand of a patient;  
       FIG. 6  shows in profile view a trapezo-metacarpal implant according to a second embodiment of the invention;  
       FIG. 7  shows in profile view a trapezal implant according to a third embodiment of the invention; and  
       FIG. 8  shows in profile view a trapezal implant according to a fourth embodiment of the invention. 
    
    
     DETAILED DESCRIPTION OF SEVERAL EMBODIMENTS OF THE INVENTION  
       FIG. 4A  shows an implant according to a first embodiment of the invention.  
      The implant according to this first embodiment is a trapezo-metacarpal implant, in the general form of a nail. It comprises a stem  10  terminating, at a so-called proximal end, in a head  20  and whose opposite end, called the distal end, is constituted by a point  101 .  
      Stem  10  is adapted to be inserted in the metacarpus of the thumb, and the head  20  to be disposed in a space obtained on the one hand by providing a cut or resection on the proximal end of the metacarpus and on the other hand by removing a portion of the trapezium facing the proximal portion of the metacarpus.  
      The head  20  has a flattened shape. Its longitudinal section is circular. Its surface is constituted by an annular and plane base  201  connected to the proximal end of the stem  10  and adapted to rest on the proximal portion of the metacarpus, a distal surface portion  202  opposite the base  201  and serving as a contact and friction surface with the remaining portion of the trapezium, and a rounded and annular lateral edge  203  connecting the distal surface portion  202  to the base  201 . Among the surface portions  201 ,  202  and  203 , only the distal portion  202  is liable to be in contact with the remaining portion of the trapezium. The distal surface portion  202  has a convex shape, and preferably consists in a spherical cap.  
      The stem  10  comprises a truncated conical portion  102  whose smaller diameter end is connected to the point  101  and larger diameter end is prolonged by a divergent connecting portion  103  connecting the base  201  of the head  20  to the truncated conical portion  102  of the stem  10 .  
      According to the invention, the distal surface portion  202  of the head  20  is inclined relative to a longitudinal axis  104  of the stem  10  by a predetermined angle α, which is to say that the mean plane  204  tangent to the distal surface portion  202  forms with the longitudinal axis  104  of the stem  10  an angle β=90°−α different from 90°. In the embodiment shown in  FIG. 4A , the head  20  in its totality is inclined relative to the axis  104  of the stem  10 . Thus, in particular, the base  201  is inclined relative to the axis  104  in the same direction as the distal surface portion  202 , and by the angle α. The angle of inclination α is typically at least equal to 10° and preferably equal to 15°.  
      The diameter D of the head  20  and the radius of curvature R of the distal surface portion  202  are selected to be sufficiently great to match as much as possible the anatomical curvature of the trapezo-metacarpal articulation. Typically, the implant according to the invention is designed in four different sizes. In each of these sizes, the ratio between the diameter D of the head, measured along a longitudinal axis of the head inclined relative to the longitudinal axis  104  of the stem in the same direction as the distal surface portion  202  and by an angle equal to the angle α, and the length L of the stem, measured along the longitudinal axis  104  of the stem between the distal end  101  and the point of intersection I between the plane  205  defined by the base  201  and the axis  104 , is preferably equal to at least 0.84. The ratio between the radius of curvature R of the distal surface portion  202  and the length L of the stem is preferably equal at least to 0.6.  
      The thickness E of the head can be relatively small. Typically, the ratio between the thickness E of the head  20 , measured perpendicularly to the base  201 , and the length L of the stem, is at most equal to 0.42.  
      Preferably, the distal surface portion  202  of the head  20  is not only inclined, but also offset relative to the longitudinal axis  104  of the stem  10  to the side of the implant where the angle between the distal surface portion  202  and the axis  104  is smaller, which is to say the side designated in  FIG. 4A  by the reference C 1  and corresponding to the angle β=90°−α. In this first embodiment, all of the head  20  is offset relative to the axis  104  to the side C 1 . Thus, the geometric center of the head  20 , like the center of the distal surface portion  202 , is located beyond the axis  104 , on the side C 1 .  
      In the example shown in  FIG. 4A , in which the distal surface portion  202  has a spherical shape, the axis of symmetry  206  of the distal surface portion  202  intersects the longitudinal axis  104  of the stem  10  at the point I of intersection between the plane  205  defined by the base  201  of the head  20  and the axis  104 . However, as a modification, the distal surface portion  202  could be further offset relative to the stem  10 .  FIG. 4B  shows such an arrangement, in which the axis of symmetry  206  of the distal surface portion  202  intersects the longitudinal axis  104  of the stem  10  at a point located between the distal end  101  of the stem  10  and the point I of intersection between the plane  205  defined by the base  201  and the axis  104 .  
      The implant according to the invention, constituted by the stem  10  and the head  20 , or at least the distal surface portion  202  of the head  20 , is preferably made of pyrocarbon. According to a preferred variant, the implant is constituted by a graphite substrate clad with a layer of pyrocarbon.  
      Referring to  FIG. 5 , the implant according to the invention is emplaced in the following manner:  
      An oblique varus cut is provided in the proximal end  2   a  of the metacarpus  2  and a portion of the trapezium  6  located facing this proximal end  2   a  is removed, to provide a space adapted to accommodate the head  20 . Then a straight hole is formed in the shaft of the metacarpus, along the longitudinal axis  2   b  of this latter. The stem  10  of the implant is then force-fitted into the hole until the inclined base  201  of the head  20  comes into contact with the inclined surface  2   a  of the metacarpus, such that the head  20  will be disposed in the mentioned space, in a varus position.  
      The stem  10  of the implant is thus positioned straight in the central soft portion of the metacarpus, delimited in  FIG. 5  by broken lines  2   c,  and is surrounded by the hard peripheral portion of this latter. The emplacement of the implant is thus not more complicated than for the Swanson trapezal implant shown in  FIGS. 1A, 1B  and  1 C. Moreover, once emplaced as described above, the implant is more stable than the Swanson trapezal implant. This stablity of the implant according to the invention is due principally to the fact that the surface of the implant in contact with the trapezium is increased by the varus position of the distal surface portion  202 .  FIG. 5  shows the balance of forces exerted between the implant according to the invention and the trapezium when the patient exerts a pinching effort by securely holding an object between the thumb and index finger. In this arrangement, the implant exerts on the trapezium a force F having a component T 1  tangential to the contact surface and a component N 1  perpendicular to this contact surface. The large contact surface between the trapezium and the head of the implant permits the trapezium to exert in response, a reaction force R having a tangential component T 2  sufficiently great to counterbalance the tangential component T 1  of the force F. This balancing of forces holds the implant in contact with the trapezium and thus significantly reduces the risk of dislocation.  
       FIG. 6  shows a second embodiment of the implant according to the invention. The implant according to this second embodiment is a trapezo-metacarpal implant which differs from the implant according to the first embodiment essentially in that only the distal surface portion  202   a  is inclined relative to the stem  10   a,  and not the entire head  20   a.  Thus, the base  201   a  of the head  20   a  is perpendicular to the longitudinal axis  104 a of the stem  10   a.  The connection portion  203   a  of the head  20   a  is thus higher on the side C 2  of the implant where the angle between the distal surface portion  202   a  and the axis  104   a  is greater, than on the opposite side C 1 .  
      As in the first embodiment, the distal surface portion  202   a  is preferably offset relative to the axis  104   a  of the stem to the side C 1 .  
      The ratios indicated above between the diameter D of the head and the length L of the stem, and between the radius of curvature R of the distal surface portion of the head and the length L of the stem, also hold true for this second embodiment. On the other hand, the thickness of the head is necessarily greater.  
      When it is emplaced in the patient, this implant rests on a straight cut previously provided in the proximal end of the metacarpus, and not on an oblique cut as in the first embodiment.  
       FIG. 7  shows a third embodiment of the invention. The implant according to this third embodiment is a trapezal implant, which is to say that the head  20   b  has a shape and size suitable to replace all the trapezium. The distal surface portion  202   b  of the implant has a concave shape which permits it to co-act with the distal portion of the scaphoid. The head  20   b,  including the base  201   b,  the distal surface portion  202   b  and the connecting portion  203   b,  is inclined similarly to the first embodiment.  
       FIG. 8  shows a fourth embodiment of the invention. The implant according to this fourth embodiment is a trapezal implant whose head  20   c  comprises a distal surface portion  202   c  inclined relative to the axis  104   c  of the stem  10   c  and a base  201   c  perpendicular to the axis  104   c.  As in the second embodiment, this implant is adapted to rest on a straight cut previously provided in the proximal end of the metacarpus.