Abstract:
The invention relates to a uncemented prosthesis having a tension anchor embodied as a thrust rod that is solely anchored internally in the femural neck by means of a conically axial tension anchor in the dorsolateral corticalis. The femural neck is completely preserved in this prosthesis.

Description:
BACKGROUND OF THE INVENTION  
         [0001]    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 (Wagner, Freeman). However, the punctiform application of force beneath the cap soon resulted in rocking motions of the shell, bone resorption as a result of the relative movement, and ultimately to fractures of the femural 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.  
           [0002]    Accordingly, the object of this invention it is to achieve a uniform transfer of force from a rigid implant to the bone.  
         PRIOR ART  
         [0003]    Many attempts have been made to anchor prosthesis stems in the femural neck. For example, in the case of a thrust-plate prosthesis, Huggler and Jacob (Juggler, 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) attempted—with partial success—to anchor a prosthesis stem component by means of a threaded tensioner extending through the neck. 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.  
           [0004]    The tension anchor prosthesis developed by Nguyen (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), involved a combination of an intramedullary straight-shaft prosthesis and the tension anchor principle. However, it is precisely the internal structures of the femural neck that do not allow straight 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.  
           [0005]    The advantage of a prosthesis limited to transferring the force to the femural 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.  
         DESCRIPTION OF THE INVENTION  
         [0006]    An essential element of the prosthesis stem is the uniform deformation of the neck spongiosa and thus the transfer of force into those structures that accept the load from the load-bearing surface of the joint, transferring it to the bony structures of the femural neck and the femur diaphysis without keeping the femur as a whole from deforming. The structures are characterized by the so-called U-shape, which is embodied in the femural neck in dorsal, medial and ventral locations. The prosthesis thereby has a U-shaped main body, which completely fills the inner surface of the neck and is hollow on the inside (U-shape: FIG. 01).  
           [0007]    This completely preserves the anatomical structure of the femural neck since the osteotomy extends from the lateral transition of the femural neck to the major trochanter (FIG. 01. 1 / 121 ) to the medial head-neck transition; in this way, the internal structures of the femural neck remain completely intact (FIG. 01. 1 / 120 ). 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 (FIG. 01. 2 / 13 ).  
           [0008]    The axis of the prosthesis (FIG. 01. 1 / 111 ) corresponds to the femural axis (FIG. 01. 1 / 111 ), as does the lateral open hollow shaft (FIG. 01. 1 / 12 ) in a parallel position. The prosthesis comprises a short hollow shaft (FIG. 01. 1 / 10 ), the shoulder (FIG. 01. 1 / 20 ), and the cone (FIG. 01. 1 / 30 ), which has a hole drilled through it axially (FIG. 01. 1 / 11 ) and can accept various heads (FIG. 01. 1 / 40 ). The stem has a deep coaxial indentation in order to have the stable neck structures on the dorsal side. This gives it s kidney shape when viewed in cross section.  
           [0009]    The neck-slip prosthesis is inserted axially using a guide instrument (FIG. 04/ 230 ). The medial outer surface (FIG. 01. 1 / 14 ) 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 (FIG. 01. 2 / 16 ) as well as in the ventral position (FIG. 01. 2 / 13 ). The guide instrument is inserted axially along the prosthesis axis and screwed onto the trial prosthesis (FIG. 04. 21 ) using the coaxially oriented holding device (FIG. 04/ 220 ) provided with a thread (FIG. 04. 221 ) and a socket (FIG. 04. 222 ). Using a 4.5-mm bit, a hole is drilled through the cone channel above the cone (FIG. 04/ 11 ) in the prosthesis that is inserted in this manner after the insertion instrument has been removed. Using the outside-in technique, the tension anchor (FIG. 04/ 50 ) and washer (FIG. 04/ 54 ) are inserted and screwed into the cone nut (FIG. 06/ 55 ), and then tightened to 2.5 Nm using a torque wrench.  
           [0010]    The trial head (FIG. 05/ 40 ) is designed such that after the guide is inserted and the proper position is reached, the nut cannot slide a as a result of the nub (FIG. 05/ 41 ) that engages the cone. 
       
    
    
     EXAMPLE OF THE INVENTION  
       [0011]    The hip joint is exposed, for example, using Bauer&#39;s methodology with the patient in the dorsal position. The dome is removed and the femur is dislocated. The head of the femur is removed while preserving the femural neck, and the head-end of the prosthesis is prepared. For example, a press-fit head implant is ground to shape, and a ceramic insert is inserted. The femur is then rotated outward and adduced, and the intermedullary canal is opened up using an 11.2-mm diamond grinding wheel.  
         [0012]    The canal is probed using the guide instrument, and if this can be done without meeting resistance, the axis of the femural canal has been correctly established. Then the spongiosa of the metaphysis together with the spongiosa of the neck are ground until the trial prosthesis can be inserted. The U-shape of the prosthesis projects just beyond the inner corticalis of the femural neck on the ventral side and on the dorsal side. The trial prosthesis is then removed, and the corresponding prosthesis is tapped in using the applicator. Then, using a 4.5 mm bit, a hole is drilled posterolaterally through the cone through the dense portion of the femur, the cone lock nut is inserted in the cone, and the trial head is set to “s”=small.  
         [0013]    The leg is then repositioned gently and the drilled hole is located in the normal zero position, and then in inner rotation and abduction, the length of the tension anchor through the drilled hole is measured using a gauge, and the tension anchor is inserted and tightened to a torque of 2.5 Nm. The joint is then dislocated, the trial head 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.  
         [0014]    After repositioning with a definitive ceramic head of the proper length, drainages are inserted and the wound is closed.