one of the substantive elements of the prosthesis stem is the straight - line opening of the medullary canal through the greater trochanter ( fig0 / 110 ). the entire neck of the femur ( fig0 / 120 ) is preserved through the osteotomy . the design of the stem ( fig0 / 100 ) allows it to be inserted in a straight line , provided that a guide instrument in the opening canal can be inserted into the medullary canal without encountering resistance . this is the case when the opening has been executed correctly along the dorsal wall of the neck in a straight extension of the canal axis through the greater trochanter . the dorsal wall of the neck forms a straight line with the dorsal wall of the medullary canal . the design of the prosthesis takes this anatomy into account and utilizes a cylinder ( fig0 / 109 ) as a design element located around the canal axis ( fig0 / 130 ) around which the stem is designed to coincide with the anatomical clearances ( fig0 . 1 ), which results in symmetrically opposite right - left versions ( fig0 . 2 ). this results in a symmetrically opposite cross section ( fig0 ), which has as its most striking element a flat to deep indentation that is located along the dorsal side of the stem ( fig0 / 104 ) and that extends in a straight line into the tip of the prosthesis . this basically produces a convex - concave - convex curve ( fig0 / 109 — fig0 / 104 — fig0 / 110 )= s - shaped curve ( fig0 / 108 ). the reconstruction of the center of rotation takes two parameters into account : first , it was found that there is a close relationship between the so - called metaphysis opening plane ( draenert et al ., 1999 , manual of cementing technique , berlin , heidelberg : springer ) and the center of rotation ( rz ), namely that the center of rotation is , as a rule , located 25 mm above this plane . second , it was found that a prosthesis offset — the distance from the design axis or canal axis to the center of rotation of the head of the femur — of 45 mm was too long ( charnley , j ., 1989 , low friction arthroplasty of the hip : theory and practice , berlin , heidelberg , new york : springer ). based on clinical experience , a lever arm having a 40 - mm offset was considered to the optimal . the design is based on this figure ( fig0 / 101 ). this clinical experience also teaches that a physiological center - collum - diaphysis angle of 126 ° causes long - term anchorages to experience an increased failure rate . an angle of 135 ° ( fig0 / 102 ) has proven to be effective over many years , since the force is applied to the tubular portion of the femur bone as a direct compression load . one of the unique characteristics of this prosthesis is that the axis of the cone does not need to be identical to the ccd angle and instead can be steeper by 3 ° to 15 °— as a rule , 5 ° to 6 ° ( fig0 / 103 ), in order to intensify this effect . the length of the stem takes the s - shaped curve in the lateral projection into account . it ranges from 14 cm to 22 cm , and is generally about 15 cm in order to achieve reliable blocking between these curvatures ( fig0 . 2 / 105 , fig0 . 1 / 106 , and fig0 . 1 / 107 ). the unique feature of this prosthesis is its straight - line , anatomically symmetrically opposite design : the outer dorsal surface has an undulating shape in the form of a rounded “ 3 ,” the halves of which are uneven ( fig0 / 108 ). this divides the body of the prosthesis into a lateral cylindrical portion ( fig0 / 109 ) and a neck portion ( fig0 / 110 ) joined by connecting portion ( fig0 / 111 ). the dorsal indentation in the cross - section extends in a straight line along the entire length of the stem , although its depth decreases ( fig0 / 112 ). the cross sections are kidney - shaped but are not uniformly identical ( fig0 / 112 ). the exterior surfaces are also structured to include coaxially aligned longitudinal grooves and ribs , generally in the ventral and dorsal positions . the design curvature is parabolic , resulting in a u - shaped force transfer ( fig0 . 1 / 114 ). this parabolic curve is superimposed on the bone structure ( fig0 . 1 / 115 ). the curvature of the exterior surfaces defines the load / deflection diagram in the load test ( fig0 ). the insertion hole ( fig0 / 201 ) for positioning the insertion instrument ( fig0 / 140 ) is located on the extension of the axis , as is a corresponding abutment surface ( fig0 / 202 ) for the force fit or thread used to fix the position of the insertion instrument with trial prostheses ( fig0 / 203 ). both structures are located on the prosthesis shoulder ( fig0 / 200 ), which forms the transition to the cone ( fig0 / 300 ) that carries the variable head ( fig0 / 150 ). once the hip joint has been exposed via the standard technique of dorsal or lateral access , the head of the femur is removed along the planned height while preserving the femoral neck and taking the respective metaphysis opening plane into account . after the socket has been inserted the femur is positioned is such a way that the medullary cavity can be opened . along the extension of the medullary canal axis , the femur is opened at the “ knee ” of the greater trochanter using a 11 . 2 - mm diameter ( dm ) diamond hollow grinding wheel , and the straight guide instrument , having a diameter dm of 11 mm and a tip that narrows to 6 mm , is inserted ( fig0 ). the femoral metaphysis is then ground open using a hollow diamond - faced grinding wheel , and a trial prosthesis is mounted on the insertion instrument ( fig0 ). the tip of the trial prosthesis is carefully guided in via the opening hole , and then driven into the femur metaphysis via gentle hammering . if this is not possible , the medullary canal of the femur is ground open dorsally while carefully preserving the rear wall of the femural neck , so that the cylindrical component of the stem fits in neatly behind the femural neck . the size of the prosthesis is measured in two ways : the guide instrument has a scale for measuring length that indicates the size of the prosthesis . the measuring instrument must be securely restrained in the s - shaped curve of the femur . the size can be read at the transition from the neck to the greater trochanter ( fig0 ). a sliding caliper can be used in the sagittal projection to determine the maximum diameter of the inner contour of the neck . the sliding caliper is also used to determine the size of the prosthesis . the final prosthesis is driven in in the same fashion until it is anchored in the conical opening in the neck in such a way that it is absolutely tight and secure . then the prosthesis is repositioned using a trial head , and the luxation tendency is determined . if there is no risk of luxation , the final socket insert and the final head can be installed , whereupon the prosthesis is repositioned , and the work is documented using an imaging device . the muscles are then reattached layer by layer , and the wound is closed up . [ 0024 ] fig0 ap view of neck - preserving prosthesis ( nps ) having the following characteristics : [ 0031 ] fig0 / 114 u - shape of force transfer : dorsomedial , medial and anteromedial surface , parabolically curved outer surface [ 0036 ] fig0 . 1 axial view of an nps having the following characteristics [ 0038 ] fig0 . 1 / 104 dorsal indentation , necking , axially to the tip of the stem [ 0042 ] fig0 . 1 / 105 to fig0 . 1 / 107 blocking of the shaft in the sagittal projection [ 0043 ] fig0 . 1 / 114 projecting u - shape with parabolic curvature of outer surface [ 0044 ] fig0 . 1 / 115 recessed bone structure along the osteotomy [ 0058 ] fig0 / 202 abutment surface , or threaded hole ( 203 ) of trial prosthesis [ 0064 ] fig0 load / deflection diagram from load test of nps in human femur with two different outer surface curvatures
the present invention essentially relates to a hip joint endoprosthesis stem for cement - free or cemented anchoring in bones that is anchored in the femural neck and in the proximal metaphysis and preserves the internal spongiosa and compact structures that reinforce the femur , that gives the design element axial access to the medullary canal , and possesses parabolically curved outer surfaces to optimize the transfer of force to the bone .