the prosthesis consists of a stem 1 , terminating at the top in a neck bearing 2 , of a neck 3 and of a head 4 , which contains a conical bore for attachment to the cone 5 which terminates the prosthesis at the top . as can be seen from comparing fig1 to 3 , the heads 4 have a different bore diameter , the result of which is that the distance a , which is the distance between the bottom surface of the neck bearing 2 and the center of the head 4 , is different . the stem 1 of the prosthesis is strongly curved in the projection on the plane including the lateral and medial contours , in which it is shown in the drawing . in the example illustrated in fig4 the lateral and medial stem boundary lies between two concentric circular arcs 14 and 15 whose spacing corresponds to the maximum stem thickness . the curvature of its centre line in the upper two thirds ( to approximately the cutting plane c ) follows a circular arc . the radius of the center line , indicated by dot - and - dash lining , preferably lies between 7 and 15 cm , more preferably between 9 and 13 cm . it is preferably concentric to the circular arcs 14 , 15 in the upper two thirds of the stem length . in the lower section on the other side of the cutting line c , the stem is likewise curved , but with a greater radius of curvature . it can also be straight here if it is correspondingly short , that is to say expediently no longer than half the uniformly curved stem length . however , it is preferably also slightly curved and can then also be correspondingly longer in relation to the upper stem section . in the case shown , its length measured along the centre line represents approximately two thirds of the uniformly curved section above the cutting line c . in cross - section the stem is preferably of elongate shape , with the longer axis in the lm direction . its extent in the ap direction in the central area is approximately constant in the upper third of the stem length ( between the cutting lines a and b ) and from there tapers approximately uniformly to the tip . in this way , in the projection on a horizontal plane , it provides in the upper third a comparatively large surface area for the transmission of vertical forces . the cross - sectional development can best be seen from fig5 in which the cross - sectional shape in cutting plane a is shown with full lines , the cross - sectional shape in cutting plane b is shown with broken lines , and the cross - sectional shape in cutting plane c is shown with dot - and - dash lines . finally , fig6 shows the configuration of cutting plane d . it will also be seen that the stem is grooved . the grooves 6 run parallel to the centre line of the stem and are separated by a rib 7 whose height ( corresponding to the extent of the stem in the ap direction ) remains constant between the cutting lines a and b and decreases only slightly towards the cutting line c . by contrast , the extent of the lateral and medial cross - sectional boundaries 8 , 9 decreases comparatively more sharply . in the upper two thirds of the stem length , the groove base keeps approximately the same distance from the center plane 10 of the stem . from these features it follows that the groove flanks in the upper two thirds of the stem length provide a large surface area for the transmission of medially directed forces . in addition , particularly in the upper third , a large surface area is available for transmission in the vertical direction . the stem surface is expediently designed for intimate connection with the natural bone tissue , for example roughened , or provided with a hydroxyapatite coating . this applies principally to the upper two thirds of the stem length , whilst the tip can , if appropriate , have a smooth design . as is known , the neck bearing is used for the additional transmission of forces to the resection surface . the grooves 6 can be continued through the area of the neck bearing in order to permit packing of bone material during implantation , or insertion of a releasing instrument in the case of implant removal . the total curvature of the prosthesis stem corresponds approximately to the total directional change between the femoral neck 11 and the diaphysis 12 . the almost uniform curvature permits insertion into a correspondingly pre - shaped bone cavity . the stem shape thus permits considerable freedom in the choice of the resection length l of the femoral neck , because the stem can be inserted to a greater or lesser extent before its lower end sets a limit by coming to bear against the hard cortical bone 13 of the diaphyseal or metaphyseal area . in the operation , before insertion of the prosthesis stem , a rasp of the same shape can be used to prepare a cavity of the same shape . it is generally more expedient first to use a thinner tool to hollow out a narrower channel into which the prothesis stem is pressed , displacing the relatively soft , spongy bone material . by this means , in the case of cementless implantation , a more secure fit is achieved immediately after the operation . in addition , bone material can also be subsequently packed in by way of the grooves . in the projection on the ap plane , the stem is expediently curved in accordance with the natural course of the bone . in the upper area , it exhibits the anteversion of the femoral neck . fig7 shows the determining of the stem contour of the prosthesis . the dot - and - dash lines show those circular arcs of which the contour is ideally composed . in fig7 the lateral contour is designated la and the medial contour is designated m . different sizes of prosthesis will of course be provided within a series . however , since the stems are essentially identical to each other , the ideal shape can be represented for all of them together and can be adapted by a conversion factor k which specifies the size ratio of the respective prosthesis in relation to the ideal shape and accordingly should in the present case be between 0 . 8 and 1 . 1 . the proximal section p is delimited on the medial side by a circular arc 20 whose radius expediently lies between 6 and 8 cm . in the illustrated example , it is 7 . 2 cm . the radius of the lateral arc 21 expediently lies between 11 and 14 cm , and , according to the example illustrated , is more preferably 12 . 4 cm . at the crossover point s , the proximal section ends and merges into the distal section d . at this point the arcs 20 , 21 merge in the same direction into arcs with 22 , 23 . this means that the arcs 20 and 22 at the crossover point coincide in the shape of the arc 24 , while the arcs 21 and 23 coincide in the shape of the arc 25 . the arcs 24 and 25 deviate ( corresponding to the wedge shape of the stem ) at an angle α from one another . the angle of deviation is expediently 3 to 5 ° and , in the illustrated example , 3 . 8 °. the length of arc 22 on the lateral side of the distal section is expediently between 22 and 35 cm . in the illustrated example it is 27 cm . the radius 23 is expediently between 18 and 25 cm . in the illustrated example it is 21 cm . the distance between the lateral contour and the medial contour is most readily determined at the crossover point s . it is expediently between 1 . 7 and 2 . 1 cm , preferably between 1 . 8 and 2 . 0 cm . in the illustrated example it is 1 . 9 cm . in practice , small deviations from the ideal contour are to some extent unavoidable , and are to some extent desirable for adaptation to particular requirements . however , they should not be over 3 mm , preferably not over 2 mm . if , at some point , the true contour deviates from the ideal contour considerably in one direction , it should deviate from the ideal contour in the same direction on the opposite side . as has already been mentioned above , the given measurements are to be corrected according to the freely selectable factor k in accordance with the desired size of the prosthesis . the prosthesis can be delivered with different stem sizes . the length of the prosthesis stem , measured in a straight line from the center of its attachment on the neck bearing to the tip , is expediently between 10 and 13 cm . several sizes are expediently offered , for example with 105 , 115 and 125 mm stem lengths . the arc of the upper two thirds of the stem length expediently extends over approximately 35 to 45 degrees , the curvature in the lower third over 2 to 10 degrees , preferably about 5 degrees . although the neck emerges approximately in the same direction from the upper end of the stem , it may however enclose , with the latter , an acute angle of preferably 0 to 20 degrees . the sum of these angles forms the complementary angle to the so - called ccd angle , which is expediently between 110 and 130 degrees .
a hip - joint endoprosthesis with a stem to be anchored in the femur , and with a joint head supported by the stem via a neck . in order , upon implantation , to be able to use one and the same prosthesis with a different choice of resection plane within the neck of the head , the stem of the prosthesis is curved in a substantially uniform circular arc ; the neck adjoins the upper end of the stem approximately in the same direction ; the length of the stem is not greater than 150 mm ; and the hip - joint endoprosthesis , while keeping the same stem shape , is available with different neck lengths , of which several are smaller than the length of an average natural femoral neck .