Abstract:
A hip prosthesis constructed to transfer forces to the femur without relative movements that cause failures. The prosthesis stem has an open sided central bore. The proximal end of the prosthesis is configured to seat against interior surfaces of the femoral canal and support a femur head on a prosthesis shoulder that seats on the stem proximal end while maintaining the neck of the femur intact.

Description:
BACKGROUND OF THE INVENTION 
     Anatomically, the neck of the femur is the strongest part of the human skeleton. For this reason, it made sense to preserve it in hip joint replacements. This led to various attempts to cap the head of the femur neck. However, this application of force beneath the cap soon resulted in rocking motions of the implant shell, and bone resorption as a result of the relative movement. This ultimately led to fractures of the femoral neck. Based on histopathological research on this approach to anchoring, it was concluded that a sudden change in rigidity between the implant and the bone can only be transferred to the bone in the centrifugal direction by an intramedullary implant. 
     Accordingly, the object of this invention is to achieve a uniform transfer of force from a rigid implant to the bone. 
     PRIOR ART 
     Many attempts have been made to anchor prosthesis stems in the femoral neck. For example, in the case of a thrust-plate prosthesis, Huggler and Jacob attempted—with partial success—to anchor a prosthesis stem component by means of a threaded tensioner extending through the neck. (Huggler, A. H., and Jacob, H. A. C. (1984), The Uncemented Trust Plate Prosthesis. In: Morscher, E. (ed.),  The Cementless Fixation of Hip Endoprostheses , p. 125, Berlin, Heidelberg, New York: Springer) However, the collar abutments and the rigid design led to bone atrophy and resorption, a clear indication that the transfer of force was not physiological. The problem, moreover, was not remedied by using highly porous structures in the stem element. 
     The tension anchor prosthesis developed by Nguyen involved a combination of an intramedullary straight-shaft prosthesis and the tension anchor principle. (Gold, T., Schill, S., and Menge, M. (1996), die Zugankerprothses—3 Jahre klinischer Erfahrungen [The Tension Anchor Prosthesis—Three Years of Clinical Experience]. Orthop. Praxis 3:194-197). However, it is precisely the internal structures of the femoral neck that do not allow straight stems or shafts to be anchored; these structures demand right-left opposite symmetry. Histopathological research has now revealed that intramedullary rigid load-bearing members transfer force into the bone structures in a precisely defined manner, a discovery that is exploited in the following invention. 
     The advantage of a prosthesis limited to transferring the force to the femoral neck is that, in the unlikely event of a failure of the anchorage, it is still possible to employ a normal shaft anchorage without suffering any disadvantages. 
     SUMMARY OF THE INVENTION 
     The prosthesis stem of the present invention provides uniform deformation of the neck spongiosa and thus the transfer of force into those bony structures that accept the load from the load-bearing surface of the joint. Force is transferred to the bony structures of the femoral neck and the femur diaphysis without preventing the femur as a whole from deforming. The stem structures are characterized by the so-called U-shape, which is embodied in the femoral neck in dorsal, medial and ventral locations. 
     The prosthesis thereby has a U-shaped main body, which completely fills the inner surface of the femoral neck and is hollow on the inside. 
     This completely preserves the anatomical structure of the femoral neck since the osteotomy extends from the lateral transition of the femoral neck to the major trochanter and to the medial head-neck transition; in this way, the internal structures of the femoral neck remain completely intact. The prosthesis contacts the front wall of the femur in an anatomical manner, and its ventral outer surface projects over the bone structures in a parabolic shape. 
     The axis of the prosthesis corresponds to the femoral axis, as does the lateral open hollow shaft in a parallel position. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIG. 1.1 is a frontal view of the neck slip prosthesis of the present invention shown in position on a femur and supporting a femur head; 
     FIG. 1.2 is an axial view of the non-slip prosthesis shown in FIG. 1; 
     FIG. 2 is a sectional view taken as on line  2 — 2  in FIG. 1.1; 
     FIG. 3 is a sectional view taken as on line  3 — 3  in FIG. 1.2; 
     FIG. 4 is a front view of the non-slip prosthesis of the present invention showing an insertion tool; 
     FIG. 5 is a sectional view through a trial ball used with the hip neck slip prosthesis of the present invention; and 
     FIG. 6 is an exploded view of a nut used for fastening a cone in the ball of the prosthesis. 
    
    
     DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     As shown in FIG. 1.1, the prosthesis comprises a short hollow stem or shaft  10 , a shoulder 20 , and a cone 30 . The cone  30  has a hole  11  drilled through it axially. The cone  30  can accept various heads , such as shown at  40 , for the hip joint. The stem  10  has a deep coaxial dorsal side indentation  16  (FIG. 2) in order to have the stable neck structures on the dorsal side. This gives it a kidney shape when viewed in cross section, as shown in FIG.  2 . 
     The femur bone  100  is shown in the drawings and at an upper end of the greater trochanter  124 , which is preserved, and the lesser trochanter  125  are also shown in FIG. 1.1. The osteotomy or cut shown at plane  121  extends from the lateral transition of the femoral neck  120  to the major trochanter  124 , thereby preserving the femoral neck intact. The prosthesis contacts the wall  123  of the femoral neck  123  (FIG. 1.2) by fitting the natural contour, and the natural outer surface of the prosthesis projects over the bone structures in a parabolic shape as represented by the line  13 . The axis of the prosthesis stem is also indicated at  111 , and it corresponds to the femoral axis. The laterally open hollow shaft  112 , which is shown in cross section in FIG.  2  and also in FIG. 1.1, so that the axis of the opening having surface  12  and the axis  111  are parallel. FIG. 2 is taken along line  2 — 2  with closes the femoral canal at an angle, but the view plane of FIG. 2 crosses both the surface  12  and the axis of the femoral canal. 
     The head of the femur is shown at  130 , and it can be seen that when it is in position, the neck  120  of the femur is preserved, and the supporting structure of the greater trochanter and lesser trochanter support the curved upper portions of the prosthesis, such that the stem  10  will fit closely into the femur canal. It should also be noted that the calcar femoris, the rear wall of the femoral neck, is shown at  122  in FIG. 1.2. 
     The neck-slip prosthesis is inserted axially using a guide instrument  200  shown in FIG.  4 . The guide instrument  200  has a handle  230 , with an impact member at the end  240 , which is convention. The guide instrument has a guide rod  250  that fits into a guide channel  21  along the axis  111  of the stem  10 . The medial outer surface of the prosthesis  14 , also represented by a dotted line portion  14 , to show it is parabolic, compresses the medially adjacent strong spongiosa in the Adam&#39;s bow; and likewise, the spongiosa along the wall are uniformly compressed in the dorsal position shown at  16  in FIG. 1.2 as well as in the ventral position along parabolic line  13 . The guide instrument  230  is inserted axially along the prosthesis axis  111  and, as shown in FIG. 4, screwed onto the trial prosthesis  21  using the coaxially oriented holding device  220  provided with a threaded screw  221  and a drive socket  222 . The screw  221  attaches in to bore  22  when the prosthesis is to be inserted. 
     A 4.5-mm hole  11  is drilled in the prosthesis through the cone channel after the insertion guide instrument  200  has been removed. Using the outside-in technique, a tension anchor  50  and washer  54  are inserted in hole  11  and the end of anchor  50  is screwed into the cone nut  55 . and the tension anchor  150  is then tightened to 2.5 Nm using a torque wrench. 
     A trial head  40 , shown in FIG. 5, is designed such that after the guide instrument  200  is inserted and the proper position is reached, the nut  55  cannot slide as a result of the nub  41  (FIG.  05 / 41 ) that engages the cone. 
     Example of the Invention 
     The hip joint is exposed, for example, using Bauer&#39;s methodology with the patient in the dorsal position. The dome is removed by an osteotomy along line or plane  121  in FIG. 1.1 and the femur is dislocated. The head of the femur is removed by separation along osteotomy line  121  while preserving the femoral neck  120 . The head-end (cone  30 ) of the prosthesis is prepared. For example, a press-fit ceramic head implant, such as  40 , is ground to shape and is inserted in the head or metaphysis  130 . The femur is then rotated outward and adduced, and the intermedullary canal of the femur  100  is opened up using an 11.2-mm diamond grinding wheel. 
     The intermedullary canal is probed using the guide instrument  200 , and if this can be done without meeting resistance, the axis  111  of the femoral canal  100 A has been correctly established. Then the spongiosa of the metaphysis together with the spongiosa of the femur neck  120  are ground until the trial prosthesis can be inserted into the femoral canal. The U-shape of the prosthesis projects just beyond the inner corticalis of the femoral neck on the ventral side  123  and on the dorsal side  122 . The trial prosthesis is then removed, and the corresponding prosthesis is tapped in using the insertion instrument or applicator. Then, using a 4.5 mm bit, a hole for anchor  50  is drilled posterolaterally of the cone to align with hole  11  through the dense portion of the femur, the cone lock nut  55  is inserted in the cone, and the trial head  40  is set to “s”=small. The tension anchor  50  expands along and is centered on the collum-centrum axis of the femur. 
     The leg (femur) is then repositioned gently and the drilled hole for anchor  50  is located first in the normal zero position, and then in inner rotation and abduction, and the length of the tension anchor through the drilled hole is measured using a gauge. The tension anchor  50  is inserted in cone lock nut  55  and tightened to a torque of 2.5 Nm. The joint is then dislocated, the trial head  40  is replaced with the correct head having the proper length, after the correct head has been definitively identified by trial positioning using the correct trial head. 
     After repositioning the prosthesis with a definitive ceramic head of the proper length, using the tension anchor in the cone, drainages are inserted and the wound is closed. 
     In FIG. 6, an exploded view of the cone lock nut  55  is shown. The cone lock nut has cylindrical guides  55 . 3 , as can be seen, and a head or end cap  55 . 1  shown rotated 90° from its position. As can be seen, the head or end cap has a recess that receives the cone nub  41  so that the nub  41  will retain the nut  55  from rotating. A bore  55 . 4  goes across the head portion, as well, and there is a central bore also shown at  55 . 4  in the cover for the nut head. The nut has a central shaft  55 . 2  between the cylindrical guide members. 
     Also, in FIG. 4, a washer  54  is illustrated, and the threads on the tension anchor are shown at  52 . The head  51  of the tension anchor is also illustrated. 
     The shoulder  120  and the cone seat on a surface of the proximal end of the stem and as a modular system and can be configured to accept heads of different design, either concentrically with the axis of the cone or eccentrically. The tension anchor  50  is centered on the cone. Additional tension anchors comprising rods, cables or tension wires can be used. 
     The thrust rod is configured such that the thrust rod is locked at its terminal thread to prevent it from rotating relative to the cone nut, for example by means of one to four HDPE stoppers. 
     The femoral component is configured such that the femur stem axis  111  coincides with the femur canal axis, and in the frontal plane, the collum-centrum axis of the femur forms an angle of between 125° and 145°, preferably about 135° in the diaphysis axis (CCD angle)  18  as shown in FIG. 1.1. In the axial view, the collum-centrum axis defines an angle between the diaphysis and the femoral neck axis from 5° to 15°, generally in the range of 7°. As shown, the proximal end is configured such that the outer surface of the proximal end is curved on the ventral side in an axially convex shape, or it can be formed in a convex and then a concave shape, and perpendicular thereto it is curved in a concave parabolic shape, such that the center point of surface curvature is on the ventral side and such that the diameter decreases continuously parabolically toward the proximal position. 
     Further, the medial outer surface of the proximal end of the stem has a convex curvature in the axial position and perpendicular thereto along the medial contour has a concave curvature such that the surface curvature center point is in the medial position, and its radius decreases continuously parabolically in the proximal direction. It also is noted that the one or more of the outer ventral, medial, lateral and dorsal surfaces can be provided with ribs. 
     The dorsal outer surface of the stem proximal end in the axial position can have a concave or convex-concave-convex shape in the form of a breaking wave or a rounded “3” having asymmetric halves and a rounded transition, and perpendicular thereto, the dorsal outer surface can be straight or concave with a center point of curvature located on the dorsal side. The surface is preferably parabolic so it has a continuous decrease in the radius in the proximal direction. 
     The stem of the prosthesis makes a transition to the cone by shoulder  20  and the cone, as a modular system, can accept various heads in a concentric or eccentric manner and has the central hole for holding a tension anchor. The implant can have a surface roughness of between 50 and 250 μm, preferably between 70 and 150 μm.