Abstract:
An intramedullary nail made of a metal or a metal alloy comprises a longitudinal axis, a connecting part whose cross section has an area F orthogonal to the longitudinal axis, and a shaft part to be inserted into the medulla whose cross section has an area f also orthogonal to the longitudinal axis where f&lt;F. The metal or metal alloy of the shaft part has greater mechanical strength than the metal or the metal alloy of the connecting part.

Description:
CROSS-REFERENCE TO RELATED APPLICATION  
       [0001]     This is a continuation of pending International Application No. PCT/CH2004/000389, filed Jun. 24, 2004, the entire contents of which are expressly incorporated herein by reference thereto.  
       FIELD OF THE INVENTION  
       [0002]     The invention relates to an intramedullary nail made of a metal or metal alloy.  
       BACKGROUND OF THE INVENTION  
       [0003]     U.S. Pat. No. 4,875,474 A (BORDER) discloses an intramedullary nail whose central section has a lower wall thickness due to machining and therefore also has a lower strength, which is a disadvantage, in comparison with the proximal and distal end portions of the intramedullary nail.  
         [0004]     U.S. Pat. No. 6,261,290 B1 (FRIEDL) discloses a hollow intramedullary nail whose distal shaft part has a smaller wall thickness than the proximal connecting part. Therefore, the shaft part is more flexible than the connecting part, but here again there is the disadvantage of the lower strength of the shaft part. This is a disadvantage that is further potentiated by the various transverse bores in the shaft part.  
       SUMMARY OF THE INVENTION  
       [0005]     The present invention seeks to remedy this situation. The object of the invention is to create an intramedullary nail shaft part that is more flexible in comparison with the connecting part while nevertheless having the greatest possible strength.  
         [0006]     The invention achieves this object with an intramedullary nail having a connecting part and a shaft part. The shaft part is intended to be inserted into the intramedullary canal and has greater mechanical strength than the connecting part.  
         [0007]     A high mechanical strength is achieved despite the flexibility imparted to the shaft part. Due to this flexibility, the introduction of the intramedullary nail into the intramedullary canal is facilitated and the greater strength of the material reduces the risk of nail breakage.  
         [0008]     Another advantage is obtained due to the lower total weight of the nail due to the smaller wall thickness, approx. 30% lower weight versus a nail having a constant wall thickness. Finally, this also yields a more economical manufacturing process for the intramedullary nail because it can be manufactured more rapidly in comparison with the state of the art and no material is lost due to machining by cutting. This is achieved on the one hand due to the fact that it is possible to start with a prefabricated tube and its wall thickness may still be changed while on the other hand the method is faster on the whole than traditional techniques of metal working.  
         [0009]     In another embodiment of the intramedullary nail, the tensile strength of the shaft part shows an increasing gradient in the radial direction—from the longitudinal axis or from the wall of the existing cannulation to the surface of the shaft part. This yields the advantage that it is not necessary to insert a mandrel into the cannulation during the cold forming, thus permitting a simpler manufacturing process.  
         [0010]     In yet another embodiment, the tensile strength of the shaft part at first shows a declining gradient and then again an increasing gradient in the radial direction—from the outer surface of the shaft to the longitudinal axis or to the wall of the cannulation. In comparison with a method without inserting a mandrel into the cannulation during cold forming, a greater strength of the material can be achieve here on the whole due to the increase in the tensile strength on the inside of the intramedullary nail.  
         [0011]     Depending on the embodiment of the intramedullary nail: 
        the axial length of the connecting part amounts to at most 30%, preferably at most 10% of the total length of the intramedullary nail;     the surface of the shaft has a maximum roughness R a  of 1.6 μM, preferably max. 0.8 μm;     the metal or the metal alloy of the shaft part has at least a 5% higher mechanical strength than the metal or the metal alloy of the connecting part;     the metal or metal alloy of the shaft part has a greater strength than the metal or metal alloy of the connecting part;     the metal or metal alloy of the shaft part has a greater bending strength than the metal or metal alloy of the connecting part;     the metal or metal alloy of the shaft part has a greater torsional strength than the metal or metal alloy of the connecting part;     the metal or metal alloy of the shaft part has a greater fatigue strength than the metal or metal alloy of the connecting part.        
 
         [0019]     In another embodiment of the intramedullary nail, the connecting part and the shaft part has the same composition in terms of materials, so the intramedullary nail can be manufactured in one piece.  
         [0020]     In yet another embodiment, the higher mechanical strength of the shaft part is created by cold forming of the metal or metal alloy. The advantage here is essentially the simple way of manufacturing the intramedullary nail.  
         [0021]     In another embodiment, the outside diameter D tube  of the connecting part is larger than the outside diameter D shaft  of the shaft part so that only the shaft part need be manufactured by cold forming.  
         [0022]     In yet another embodiment, the outside diameter D tube  of the connecting part is equal to the outside diameter D shaft  of the shaft part. The advantage of this embodiment is the essentially constant mechanical strength of the intramedullary nail over the entire length.  
         [0023]     In another embodiment, the intramedullary nail comprises a cannulation that is concentric with the longitudinal axis, preferably in the form of a cylindrical cavity, whereby the wall thickness “W” of the connecting part is preferably greater than the wall thickness “w” of the shaft part. The wall thickness w conforms to the condition 0.60 W&lt;w&lt;0.85 W. 
     
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0024]     The invention and further embodiments of the invention are explained in greater detail below on the basis of the partially schematic diagrams of several exemplary embodiments.  
         [0025]      FIG. 1  shows a longitudinal section through an unmachined tube for a first manufacturing variant;  
         [0026]      FIG. 2  shows a cross section along line II-II in  FIG. 1  and  FIG. 3 ;  
         [0027]      FIG. 3  shows a longitudinal section through an inventive intramedullary nail having a tapering distal portion;  
         [0028]      FIG. 4  shows a cross section along line III-III in  FIG. 3 ;  
         [0029]      FIG. 5  shows a longitudinal section through an unmachined tube for a second manufacturing variant;  
         [0030]      FIG. 6  shows a longitudinal section through the partially machined tube according to  FIG. 5  having a tapered shaft part;  
         [0031]      FIG. 7  shows a longitudinal section through the completely machined tube according to  FIG. 6  with a constant outside diameter as the second variant of an inventive intramedullary nail;  
         [0032]      FIG. 8  shows a diagram of the mechanical strength of the shaft of an inventive intramedullary nail; and  
         [0033]      FIG. 9  shows a diagram of the mechanical strength of the shaft for a variant of an inventive intramedullary nail. 
     
    
     DETAILED DESCRIPTION OF THE INVENTION  
       [0034]     In  FIGS. 1 and 5 , an unmachined hollow cylindrical or hollow prismatic tube  10  with an outside diameter D tube  is shown, serving as the starting piece for the intramedullary nail  1 . The tube  10  has a cannulation  5  that is coaxial with the longitudinal axis  4  and is surrounded by the tube wall  11 . The cross-sectional area F of the unmachined tube  10  orthogonal to the longitudinal axis  4  is shown in  FIG. 2 .  
         [0035]      FIG. 3  shows the intramedullary nail  1  after cold forming. After cold forming, the intramedullary nail  1  is constricted diametrically on the outside from its distal end  8  on a section A of its length forming the shaft part  3  in comparison with the tube  10  as a starting piece and optionally, depending on the diameter of the mandrel inserted into the cannulation  5  during the shaping, its diameter on the inside is also reduced or unchanged. The surface  6  of the shaft part  3  opens with a conical transition into the surface  12  of the connecting part  2 . The cold formed section A of the intramedullary nail  1  has a cross-sectional area f orthogonal to the longitudinal axis  4  ( FIG. 4 ) which is smaller than the cross-sectional area F. Unformed section B of the intramedullary nail  1  adjacent to the proximal end  9  of the intramedullary nail  1  forms the connecting part  2  of the intramedullary nail  1  and has the outside diameter D tube  of the unformed tube  10  ( FIG. 1 ). An inside thread  15  is cut into the cannulation  5  in the connecting part  2  from the proximal end. Furthermore, at least one transverse bore  16  with a bore axis running across the longitudinal axis  4  is provided on the connecting part and on the shaft part  3 , whereby the angle between the longitudinal axis  4  and the bore axes is typically between 30° and 90°.  
         [0036]      FIG. 6  shows a blank produced from the tube  10  ( FIG. 5 ) and having an outside diameter D tube  for another embodiment of the intramedullary nail  1 . Here again, the tube  10  ( FIG. 5 ) has been constricted only on the section A with a length of up to an outside diameter D shaft &lt;D tube , as measured from the distal end  8  of the intramedullary nail  1 , forming the shaft part  3 . The section B of the length of the intramedullary nail  1  as measured from the proximal end  9  is unformed and also has the outside diameter D tube . The cannulation  5  of the blank is optionally either constricted or unchanged in section A, whereby the design of the cannulation  5  after cold forming depends on the diameter of the mandrel inserted into the cannulation  5  during the cold forming process.  
         [0037]      FIG. 7  shows the blank depicted in  FIG. 6  after a second cold forming which is performed after the shaping of the shaft part  3 , which is performed only on section B that forms the connecting part  2 . The connecting part  2  (section B) was compressed radially until its outside diameter corresponded to the outside diameter D shaft  of the shaft part  3 . The cannulation tapers in the transition from the shaft part  3  to the connecting part  2  and has a smaller diameter here in the connecting part  2  than in the shaft part  5 . Furthermore, an inside thread  15  is cut in the cannulation  5  in the connecting part  2  from the proximal end  9 .  
         [0038]      FIG. 8  shows a plot of the tensile strength R m  in the tube wall  11  of the cold-formed shaft part  3 . The tensile strength R m  increases in this case in the radial direction from the wall  7  of the cannulation  5  to the surface  6  of the shaft part  3 . Such a plot of the tensile strength R m  in the tube wall  11  after cold forming is characteristic of cold forming without the insertion of a mandrel into the cannulation  5 .  
         [0039]      FIG. 9  shows another plot of the tensile strength R m  after the cold forming is concluded. The tensile strength R m  in this case has a maximum at the wall  7  of the cannulation  5  and at the surface  6  of the shaft part  3  while a minimum tensile strength R m  prevails at the center of the tube wall  11 . This plot of the tensile strength R m  in the tube wall  11  after cold forming is characteristic of cold forming with insertion of a mandrel into the cannulation  5 .  
         [0040]     Two different manufacturing methods for the inventive intramedullary nail are given below.  
       EXAMPLE 1  
       [0041]     The present example corresponds to  FIGS. 1 through 4 .  
         [0042]     A hollow cylindrical or hollow prismatic tube  10  made of stainless steel with a length of typically 100 to 400 mm, an outside diameter of typically 10 to 14 mm and a wall thickness between 1.5 and 4.0 mm is machined over a section A of 70% to 90% of the tube length on the outside by cold forming, section A corresponding approximately to the shaft part  3  of the intramedullary nail  1 , so that its outside diameter is reduced to values between 8 and 12 mm and thus the tube  10  is lengthened by 20% to 40%, i.e., is brought to a final length of 120 to 500 mm. By inserting a mandrel with an outside diameter of 5 mm to 10 mm into the cannulation  5  of the tube  10  during the cold forming, the wall thickness of the tube  10  is reduced to 1 to 3 mm.  
         [0043]     In comparison with the strength values (R m  between 500 and 800 MPa) of the unmachined tube  10 , the tube  10  machined according to this invention has 5% to 20% higher strength values (R m  values between 600 and 1000 MPa). The blank obtained in this way is processed by applying transverse bores  16  in the shaft part  3  and in the unmachined remainder of the tube, i.e., in the connecting part  2  and a coaxial inside thread  15  in the cannulation  5  in the connecting part  2  to form an intramedullary nail  1 .  
       EXAMPLE 2  
       [0044]     The present example corresponds to  FIGS. 5 through 7 .  
         [0045]     A tube  10  made of stainless steel with a length of typically 100 to 400 mm, an outside diameter of typically 11 to 17 mm and a wall thickness between 1.5 and 4.0 mm is machined by cold forming over a section A of 70% to 90% of the tube length corresponding approximately to the shaft part  3  of the intramedullary nail  1 , so that its outside diameter is reduced to values between 11 and 15 mm and thus the tube  10  is lengthened by 20% to 40%, i.e., brought to a final length of 120 to 500 mm.  
         [0046]     By inserting a mandrel into the interior of the tube  10 , its wall thickness is also reduced to values between 1 and 3 mm. In comparison with the strength values (R m  between 500 and 800 MPa) of the unmachined tube  10 , the tube  10  machined according to the present invention has 5% to 20% higher strength values (R m  values between 600 and 1000 MPa).  
         [0047]     In another method step, the mandrel in the interior of the tube  10  is removed and the previously unmachined connecting part  2  of the tube  10  is shaped so that the entire tube  10  has a constant outside diameter. A lower increase in strength occurs in this connecting part because the material can move freely on the inside of the tube. The blank obtained in this way is processed to yield an intramedullary nail  1  by creating transverse bores  16  in the shaft part  3  and the connecting part  2  as well as a coaxial inside thread  15  in the cannulation  5  of the connecting