Abstract:
A prosthetic part includes an anchorage section that is introducable into a bone recess, the anchorage section comprising at least partially a cage structure, a plate section that extends in a lateral direction beyond the anchorage section, and a supporting section for a joint part. The plate section is provided with at least one opening or at least one indentation, through which a severing instrument is introducable to cut a connection between the anchoring section and corticalis substance of a bone. The plate section has a marginal region having a circumferential arrangement of indentations, and the at least one opening or the at least one indentation is elongated and extends parallel to an edge of the plate section.

Description:
CROSS-REFERENCES TO RELATED APPLICATIONS  
       [0001]    This application is a Divisional of U.S. Patent application Ser. No. 09/155,837, filed Jun. 1, 1999, of the same inventor.  
     
    
     
       STATEMENT REGARDING FEDERALLY SPONSORED RESEARCH OR DEVELOPMENT  
         [0002]    Not applicable.  
         BACKGROUND OF THE INVENTION  
         [0003]    Technical Field  
           [0004]    The invention relates to a prosthetic part to be implanted into the end of a thighbone adjacent to the hip joint and an anchorage section that is introducable into a bone-recess.  
           [0005]    A prosthetic part of this type is described in DE 37 07 518 A1.  
           [0006]    After such a prosthetic part has been implanted, spongy substance grows between the anchorage pillars of the cage structure, as a result of which the anchorage section of the prosthetic part is dynamically and firmly connected to the end of the thighbone. After they have healed into the bone, prosthetic parts of this type which can be implanted without the use of cement are so firmly connected to the bone that removal which may possibly become necessary often necessitates opening of the sections of the cortical substance of the bone adjacent to the anchorage section.  
         SUMMARY OF THE INVENTION  
         [0007]    By means of the present invention a prosthetic part having a plate section having a substantially rectangular marginal contour seen from above is developed further in such a way that removal of the prosthetic part from the thighbone is possible without removing parts of the cortical substance.  
           [0008]    In accordance with the invention this object is achieved by means of a prosthetic part wherein two longitudinal edges of the plate section are each provided with at least one opening or at least one indentation, and the at least one opening or the at least one indentation is elongated and extends parallel to an edge of the plate section.  
           [0009]    The object of the invention also is achieved by a prosthetic part having an anchorage section that is introducable into a bone recess, said anchorage section comprising at least partially a cage structure; a plate section that extends in a lateral direction beyond the anchorage section; and a supporting section for a joint part. The plate section is provided with at least one opening or at least one indentation in the vicinity of the edges of the plate section through which a severing instrument is introducable to cut a connection between the anchoring section and corticalis substance of a bone. The at least one opening or that at least one indentation is elongated and extends parallel to an edge of the plate section.  
           [0010]    The plate section comprises a marginal region having a circumferential arrangement of indentations.  
           [0011]    That the circumferential arrangement of indentations comprises a succession of indentations of differing depths.  
           [0012]    In the case of the prosthetic part according to the invention, passages are present or can easily be created by removing webs of material located beneath indentations or by drilling the supporting plate free, through which passages a chisel or a slender milling cutter can be guided. In this way a slit can be produced in the section of the spongy substance adjacent to the cortical substance, as a result of which the cohesion between the spongy substance that has grown into the cage structure and the cortical substance is weakened. The bridges of spongy substance leading to the cortical substance that cannot be removed by the chisel or by the milling cutter can then be broken up by mechanical loading of the prosthetic part (e.g., impact and/or twisting, with application of considerable force). After this, the prosthetic part can be taken out of the bone.  
           [0013]    Further features of the invention are specified in the specification.  
           [0014]    The volume of spongy substance that has grown in is inseparable from the cortical substance of the bone on the two principal surfaces. A further development of the invention permits an incision to be produced in the volume of spongy substance, which may extend practically over the entire surface of contact with the cortical substance. In this way it is possible for the prosthetic part to be removed in a straightforward manner.  
           [0015]    A further feature of the invention is with regard to particularly rapid opening of the depression, whereby a secure and load-bearing connection of the edge of the plate section to the interior of the plate section is guaranteed by virtue of the depressions having only slight depth. In the depressions having only slight depth the web of material can be removed by means of a milling cutter, whereby the walls of the depression may serve as a guide for the milling cutter.  
           [0016]    A further feature of the invention permits the setting angle of the chisel or milling cutter, which is introduced into an opening or into an opened depression to be varied.  
           [0017]    In the case of a prosthetic part according to the invention a drill for drilling free the points of attachment of the anchorage pillars pertaining to the cage structure can be simply and precisely aligned with the axis of an anchorage pillar.  
       
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0018]    The invention is elucidated in more detail below on the basis of embodiments with reference to the drawings. Shown in the latter are:  
         [0019]    [0019]FIG. 1: a lateral view of a prosthetic part for an artificial hip joint;  
         [0020]    [0020]FIG. 2: a view of the right side of the prosthetic part shown in FIG. 1;  
         [0021]    [0021]FIG. 3: a transverse section through a depression which is provided in a near-edge region of a plate section of a modified prosthetic part;  
         [0022]    [0022]FIG. 4: a longitudinal section through the depression provided in a near-edge region of a plate section of the modified prosthetic part according to FIG. 3;  
         [0023]    [0023]FIG. 5: a similar view to FIG. 4 but wherein a depression having little depth is reproduced;  
         [0024]    [0024]FIG. 6: a top view of the plate section of a further modified prosthetic part which is provided in the marginal region of the plate section with a succession of depressions of differing depths; and  
         [0025]    [0025]FIG. 7: a view similar to FIG. 1, wherein a further modified prosthetic part is shown.  
     
    
     DETAILED DESCRIPTION OF EMBODIMENTS  
       [0026]    Designated overall by  10  in FIG. 1 is a prosthetic part which serves for implantation into the upper end of a thighbone  12  indicated by a dashed line. To this end the upper section of the thighbone  12  which bears the condyle is resected and a recess  14  corresponding to the clear outer contour of the prosthetic part  10  is produced in the thighbone.  
         [0027]    The prosthetic part  10  has supporting plate  16  which comprises a horizontal plate section  18  and a plate section  20  ascending obliquely upwards to the trochanter major. Molded onto the plate section  18  is a trunnion  22  ascending obliquely upwards, onto which a condyle  24  indicated by a dashed line is capable of being mounted.  
         [0028]    Cast onto the underside of the supporting plate  16  is a cage structure which is designated overall by  26 . This cage structure consists of four curved longitudinal anchorage pillars  28 ,  30 ,  32 ,  34  which substantially predetermine the shape of a curved pyramid having a rectangular transverse cross-section. From the upper end of the plate section  18  there emanate two curved additional anchorage pillars  36 ,  38 , the configuration of which is adapted to the trochanter major of the thighbone (femur) and which have a smaller spacing from one another than the adjacent anchorage pillars  32  and  34 .  
         [0029]    In the interior of the clear contour of the cage structure  26  predetermined by the anchorage pillars  28 - 38  additional stiffening pillars are provided which extend in oblique directions between the anchorage pillars  28 - 38 . Three such stiffening pillars  40 ,  42 ,  44  are shown.  
         [0030]    The anchorage pillars and the stiffening pillars each have a cylindrical core  46  with axially spaced anchorage collars  48 .  
         [0031]    The cores  46  of the anchorage pillars typically have a diameter of 2.0-4.5 mm, the diameters of the anchorage collars are each about 0.8-1.5 mm larger than the diameter of the cores. The axial extent of the anchorage collars amounts to about 0.6-1.2 mm, their spacing from one another amounts to about 3-6 mm.  
         [0032]    For the stiffening pillars the cores may have a diameter of about 1.5-3.0 mm, the diameters of the anchorage collars may be about 0.6-1.2 mm larger that the core diameter, and the axial dimension of the anchorage collars may amount to about 0.5-1.0 mm, their axial spacing to about 2.0-4.0 mm.  
         [0033]    Cast onto the lower end of the cage structure  26  is a shaft section  50 . The latter has an olive-shaped end section  52 . A drill hole  54  extends through the interior of the shaft section  50 .  
         [0034]    With a view to implanting the prosthetic part described above, the end of the thighbone is resected as described above, the upper edge of the cortical substance being given a contour corresponding to the marginal contour of the supporting plate  16 . The bone material removed from the end of the bone in the course of preparation of the recess  14  is ground in a bone mill and poured into the cage structure  26 . The outer surface of the shaft section  50  is coated with a cement. The prosthetic part  10  is then inserted into the end of the thighbone  12 , so that the supporting plate  16  now forms a new upper termination for the bone. The prosthetic part  10  is then temporarily fixed with wire.  
         [0035]    After the cement has hardened, the prosthetic part  10  is connected to the upper end of the thighbone in partially loadable manner. In the further healing process the ground spongy substance coalesces and fills out the cage structure, at the same time growing up against the cortical substance and residues of spongy substance that have remained on the cortical substance.  
         [0036]    Alternatively, the shaft section  50  may not be coated with cement, a possibility which is advantageous with regard to easier later removal of the prosthetic part.  
         [0037]    After the healing process has been concluded an adhesive connection consequently exists between the outer surface of the shaft section  50  and the cortical substance of the thighbone, whereas in the region of the cage structure  26  there is an uninterrupted volume of spongy substance which has grown around the cage structure  26 .  
         [0038]    After the spongy substance has brown into the cage structure  26  a very firm loadable connection exists between the thighbone and the prosthetic part. If for some reason the prosthetic part should have to be removed from the thighbone at any time, this would probable necessitate removing a part of the cortical substance surrounding the cage structure  26  and chiseling the cage structure  26  free again with a chisel through the window that has been created this way. This would considerably restrict the options for a replacement by means of a prosthetic part that has to be cemented in.  
         [0039]    In order to enable the cage structure  26  to be chiseled free without producing a window in the spongy substance, in the prosthetic part shown in FIGS.  1  and  2  elongated openings  60 ,  62  are provided in the vicinity of the longitudinal edges of the supporting plate  16 . A thin, slender chisel can be guided through said openings after the prosthetic part  10  has healed in, in order in each case to chisel free one of the two principal surfaces of the cage structure  26 . After such an operation to chisel the cage structure  26  free, bridges of spongy substance remain, via which the cage structure remains connected to the cortical substance in the vicinity of the upper comers of the supporting plate  16  and on the narrow sides of the cage structure. These remaining bridges can be broken open by rotating the prosthetic part  10  about the axis of the recess  14  and/or by inflicting severs blows on the supporting plate  16 .  
         [0040]    If it is desired that the supporting plate  16  normally seals the upper end of the thighbone  12  completely but that access to the two principal surfaces of the cage structure  26  should nevertheless be possible, instead of openings it is possible, according to FIGS. 3 and 4, to work with indentations  64  in the supporting plate  16  which, seen in top view, have the same geometry as the openings  60 ,  62  shown in FIGS. 1 and 2. However, the indentations  64  have a thin bottom wall  66 , which has the desired barrier effect, but on the other hand they can easily be mechanically destroyed by the chisel which serves to chisel the cage structure  26  free.  
         [0041]    As is evident from FIG. 3, the lateral walls of the indentations  64  or of the openings that are later created from said indentations or of openings that are present from the very beginning are convex, as represented at  68 . This permits the chisel or milling cutter which is used for chiseling the cage structure  26  free to be inclined also in the perpendicular direction relative to the longitudinal direction of the indentation or opening.  
         [0042]    In a further modification, according to FIG. 5 use may be made of such indentations in which the bottom wall  66  makes up an appreciable part of the thickness of the supporting plate  16 , so that the indentations  64  do not have a negative influence on the mechanical strength of the marginal region of the supporting plate  16 , so that the indentations  64  do not have a negative influence on the mechanical strength of the marginal region of the supporting plate  16 . This design of the indentations  64  permits several such indentations  64  permits several such indentations to be distributed along the edge of the supporting plate  16 , as a result of which a larger part of the interface between cage structure  26  and cortical substance of the thighbone  12  is made accessible. With such a design of the indentations  64  removal of the bottom wall  66  is effected by means of a milling cutter of suitable width, said milling cutter being guided by the walls of the indentation  64  parallel to the edge of the supporting plate  16 .  
         [0043]    In a further modification of the invention, according to FIG. 6 an arrangement of indentations  64   a  and  64   b  may be provided on the upper side of the supporting plate  16  continuously following the edge, the bottom walls  66   a  and  66   b  of said indentations being thin and thick, respectively. The bottom wall of the entire indentation arrangement can then be opened rapidly by means of a milling cutter which once again is positively guided parallel to the edge of the supporting plate  16  by the walls of the indentations, but under usage conditions a very loadable connection formed by the bottom walls  66   b  arms exists between the edge of the supporting plate  16  and the principal section thereof.  
         [0044]    In the case of the prosthetic part  10  according to FIG. 7 all the base-points of anchorage pillars which emanate from the supporting plate  16  are marked by a conical marker  70  on the upper side of the supporting plate  16 . These markers serve at the same time as centering holes for the point of a drill with which the material pertaining to the supporting plate located above these anchorage pillars can be drilled away. Once this has been done for all the anchorage pillars the supporting plate  16  comes free from the cage structure (thin webs of material possibly left standing can be broken in the course of drilling by progress of the drill), so that said cage structure can be simply chiseled or milled out of the recess  14 .