Abstract:
The invention relates to an intramedullary implant for use between two bones or two bone fragments. The implant includes a single-piece body having a generally elongate shape and having, at each end, areas for anchoring to the bone portions in question, characterized in that one of said areas has a generally cylindrical shape while the other area has a flat cross-section.

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
       [0001]    This application is continuation of U.S. patent application Ser. No. 12/918,105, filed on Oct. 29, 2012, which is a US national phase of PCT application PCT/FR2009/051658, filed Sep. 2, 2009, published Mar. 18, 2010 as 2010/029246, and claiming the priority of French Patent Application No. 0856035 itself filed Sep. 9, 2008, whose entire disclosures are herewith incorporated by reference. 
     
    
     FIELD OF THE INVENTION 
       [0002]    The invention relates to the technical field of orthopedic implants, particularly for arthrodesis and osteosynthesis. 
         [0003]    More particularly, the invention relates to an intramedullary implant for arthrodesis between two bone parts or osteosynthesis between two bone fragments, particularly in the case of the hand or foot. 
       BACKGROUND OF THE INVENTION 
       [0004]    Different solutions have been proposed to achieve these functions. 
         [0005]    For example, a solution comes from the teaching of patent application FR 2,884,406 [US 2008/0177262], of which the applicant of the present application is also the applicant. This patent describes an intramedullary osteosynthesis device constituted of an elongated body whose ends constitute anchor zones cooperating with the bone parts to be immobilized. The anchor zones are shaped and made of a material selected to enable insertion into the bone parts, then to ensure an anchor in the bone parts by preventing any rotational movement by resisting traction and by maintaining a compression force. 
         [0006]    Another solution also comes from patent application FR 07.02003 [US 2010/0131014], also from the same applicant. This document describes an implant in the form of two anchor zones connected by a central zone and whose general shape is substantially inscribed in a very elongated rectangle of X-shape, so as to form in the anchor zones two legs adapted to move apart by elastic or shape-memory effect. 
         [0007]    From this design, different criteria have been established to make the implant easy to place and efficient in order to create a primary and secondary stability for the osteosynthesis or arthrodesis site. 
         [0008]    However, these solutions are not adapted for the case of an implant made of resorptive material. 
       BRIEF SUMMARY OF THE INVENTION 
       [0009]    From this state of the art, the object that the invention proposes to attain is further improving the anchor and the stability of the implant as well as its adaptation to the morphology of the implantation site when the implant is made of resorptive material. 
         [0010]    To solve such a problem, a resorptive intramedullary implant between two bones or two bone fragments has been designed and developed; it is constituted, in a known manner, of a single-piece body having a general elongated shape with, at each end, zones for anchoring to the bone parts being considered. According to the invention, one of the zones has a cylindrical shape, whereas the other zone is flat. 
         [0011]    Advantageously, the implant is made of a resorptive material whose mechanical properties are determined to last the time necessary for the consolidation, so that the implant is resorbed after six months. For example, the implant is composed of lactic acid polymer or copolymer (PLA, PGA . . . ). 
         [0012]    Considering the specific mechanical characteristics of resorptive materials, and to solve the given problem of improving anchor and stability, the cylindrical cross-section is threaded and tapers in the direction of its free end. 
         [0013]    To solve the given problem of enabling a deformation by elasticity, thus causing an expansion adapted to the geometry of the site and to the properties of the material, the flat cross-section zone has, substantially in its median portion, an opening adapted to enable elastic deformation of the zone. The opening defines at least two anchor arms. 
         [0014]    It therefore appears that the combination of a cylindrical and threaded anchor zone and a flat-sectioned anchor zone is particularly advantageous considering the problem to be solved. 
         [0015]    To solve the given problem of resisting the shear and flexion forces susceptible of occurring in the area of the bone site, between the two anchor zones, the body has a central zone of transition adapted to resist the shear and flexion forces occurring in the area of the bone site and adapted to serve as an abutment. 
         [0016]    From this basic design of the implant, the anchor zones are either coaxial or angularly offset by between about 1° and 30° and, advantageously, by 10°. The bend between the anchor zones is located so as to substantially correspond to an arthrodesis line of the bones being considered. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0017]    The invention is explained in more detail hereinafter with reference to the attached drawings, in which: 
           [0018]      FIG. 1  is a perspective view of the implant; 
           [0019]      FIG. 2  is a front view of the implant before insertion into the bone part in question; 
           [0020]      FIG. 3  is a side view corresponding to  FIG. 2 ; 
           [0021]      FIG. 4  is a view like  FIG. 2  showing the position of the anchor arms of the flat section after insertion; 
           [0022]      FIG. 5  is a perspective view of another advantageous embodiment of the implant; 
           [0023]      FIGS. 6 and 7  show the installation of the implant into two bone parts. 
       
    
    
     DETAILED DESCRIPTION 
       [0024]    The implant according to the invention has a one-piece body  1  of elongated shape and having a first proximal zone A 1  and a second distal zone A 2 . The entire implant body is made of a resorptive material whose mechanical properties are determined for the implant to be resorbed in no less than about 6 months. In one embodiment, the implant is composed of lactic acid polymer or copolymer (PLA, PGA . . . ). 
         [0025]    As will be described later in the description, the zones A 1  and A 2  have anchor formations for the respective bone parts. Taking into account the specific characteristics of the resorptive material and to attain the given object of anchor and stability, the zone A 1  is of a cylindrical shape section whereas the other zone A 2  is flat. 
         [0026]    The zone A 1  has a generally cylindrical outer surface  1   a  with a limited taper toward its free end. The surface  1   a  has a helical rib forming a screwthread  1   a   1 . 
         [0027]    The zone A 2  is flat and has substantially in its center, an opening  1   b  adapted to enable elastic deformation of the zone A 2 . More particularly, the opening  1   b  defines at least two anchor arms  1   c  and  1   d , each having at least one outwardly projecting tooth  1   c   1 ,  1   d   1 . 
         [0028]    Advantageously, between the two zones A 1  and A 2  the body  1  has a central zone C for transition adapted to resist shear and flexion forces that can occur at the end of a bone. By way of nonlimiting example, this median zone C can have a length of about 3.5 mm and a thickness of about 2 mm, for an overall implant length comprised between about 15 and 25 mm and a diameter of about 2 or 3 mm at the zone A 1 . 
         [0029]    In the embodiment shown in  FIG. 1 , the two zones A 1  and A 2  are coaxial. 
         [0030]    To solve the problem of adaptation to the shape of the implantation site, the anchor zones A 1  and A 2  can be offset at an angle a adapted to the geometry of the bone site. This angle a is comprised between about 1° and 30° and, advantageously, on the order of 10° when the implant is for foot arthrodesis ( FIG. 5 ). 
         [0031]    In this embodiment in which the two anchor zones are angularly offset, the bend is located so as to correspond substantially to the arthrodesis line of the bone parts being fused. 
         [0032]      FIGS. 6 and 7  schematically show the positioning of the implant according to the invention between two bone parts O 1  and O 2 . After suitable holes have been made in the bone by a rasp-type tool, the operator screws the thread  1   a  into the bone part O 1  substantially up to the median zone C that serves as abutment preventing the implant from sinking too deeply into the bone ( FIG. 6 ). The operator then fits the second bone part O 2  back onto the anchor arms  1   d  and  1   c  of the zone A 2 , the anchor arms then spread and tighten by elasticity ( FIG. 7 ). 
         [0033]    The operative technique can be the following: drilling of the two holes with a conventional drill; preparation of the holes with a rasp for the flat side and a bone tap to form the inner screw thread on the cylindrical side; Use of a screwdriver with a gripper end; screwing in the cylindrical side P 1  [A 1 ] for an arthrodesis IPP of the foot; and fitting of the bone back onto the flat side [A 2 ] of the implant. 
         [0034]    The advantages are readily apparent from the description; in particular, it is to be emphasized and understood that the combination of the two anchor zones A 1  and A 2  of cylindrical and a flat shape, respectively, significantly enhances anchor and stability of the implant adapted to the geometry of the bone site and to the material properties, namely, a resorptive material. 
         [0035]    Although the invention herein has been described with reference to particular embodiments, it is to be understood that these embodiments are merely illustrative of the principles and applications of the present invention. It is therefore to be understood that numerous modifications may be made to the illustrative embodiments and that other arrangements may be devised without departing from the spirit and scope of the present invention as defined by the appended claims.