a hollow shaft 1 in the preferred embodiment of an endoprosthesis in fig1 consists of two pressed or forged semi - tubular parts 2 and 3 that are welded together . one of the semi - tubular parts 2 makes up the inner , medial part and the other semi - tubular part 3 the outer , lateral part of the shaft 1 . the later part extends out beyond the inset medial part at the end furthest away from the joint by about half a shaft length , the inner and outer lateral parts jointly defining a hollowed out portion or elongated notch 18 . this also simplifies construction of a prosthesis as the welding seam 4 which connects the two semi - tubular parts 2 and 3 does not need to extend over the whole length of the shaft 1 . a disc 5 is welded to the shaft 1 at the end nearest to the joint . a cone 6 is connected to the disc 5 onto which ball joints with corresponding internal discs can be attached . the outer form of the shaft 1 is like that of a curved conical tube and is , as such , adapted to coincide with the shape of the natural marrow zone in the femur . a great number of computer - tomographical measurements were taken to enable the confectioning or rafting of the shaft . it has been proven that it is favorable to take the differing geometries of the left and right marrow zones into account , and to produce left and right prostheses with their well - known characteristic additional formations , as these can carry the torsional load better . the embodiments shown in the figures are symmetric and can be implemented for both left and right sides . fig2 illustrates a longitudinal section through the hollow shaft 1 of the prosthesis according to fig1 . it can be seen that the wall - thickness of the tubular shaft diminishes in the direction towards the end furthest away from the joint . the constant and continual variation of the wall - thickness means that the longitudinal rigidity of the prosthesis shaft can be adapted well to coincide with that of the bone , whereby stress peaks can be avoided and the area of contact between the shaft and the bone remains uninfluenced . a further preferred embodiment of the prosthesis according to the invention is illustrated in fig3 . the shaft has large openings 7 . these openings enhance and augment the effect described hertofore . in general , openings should not be inserted in the convex outer side of the shaft , as this outer part of the shaft is greatly strained . as notch strains , which can occur at bores or other openings , can be problematic with regard to the durability of the prosthesis , it is suggested that only a small number of larger sized openings are formed . these lower the resistance to bending of the prosthesis in the area of its largest curvature . such openings can lead to bone material growing into the hollow area of the shaft , which makes a possible explantation much more difficult . it is therefore better , either to try to prevent the bone from growing into the hollow area of the shaft , or to construct the prosthesis with a joint part which is separable from the shaft . the latter enables the bone to be removed from the inside of the shaft , especially by way of drilling . the growth of spongiosa into the shaft illustrated in fig3 is prevented by filling the hollow area with polyethylene 14 and covering the polyethylene surface in the openings 7 with a coating 15 of apatite . this variation of the preferred embodiment is enlarged and illustrated in fig7 . another means to weaken the shaft wall locally can be carried out by inserting recesses 8 into the inner side of the shaft wall , as can be seen illustrated in fig4 . these recesses 8 can be inserted if the inner surface of the shaft wall is accessible . this is especially the case when the prostheses shafts are produced in a dual - stage production process . in this process the two pressed or forged semi - tubular parts are connected together at a later stage . these semi - tubular parts can , if they have not already been formed during moulding and forging , be additionally worked on with cutting devices , e . g . using chemical milling . the insertion of recesses 9 in the outer surface of the prosthesis -- see fig5 -- is also possible . the recesses 9 are filled with a known material 10 such as porocoat to prevent bone material from growing into the recesses . porocoat is a trademark of de puy , inc . an indiana corporation , and is used in connection with sintered porous metal layer which permits ingrowth of live tissue , sold only as applied to a medical prosthesis . in another further embodiment of the invention , as illustrated in fig8 the recesses are not completely filled with porocoat - coating 10 , so that there still exists a small recess in relation to the rest of the prosthesis surface . the bone material 16 which grows into these remaining recesses , together with the procoat - coating 10 with which it is contact , ensure that the shaft 1 sits tightly . if , however , it is necessary at a later date to remove shaft 1 , the in - grown bone material 16 shears away and the prosthesis is free . this is because the cracking plane of the bone material 16 does not form a barrier as it conforms approximately with the surface plane of the adjacent shaft surface . the extremely tight connection between bone 16 and porocoat material 10 does not have to be severed . it is also possible the allow the bone material 16 to grow in through the openings 7 in the prosthesis shaft , in which case it then has to be drilled away from the inside of the prosthesis if an explantation becomes necessary due to a renewed operation . this is only possible , if the hollow area of the shaft 9 is accessible when required . in this case , a disconnectable connection is provided between the hollow shaft 1 and the joint part 11 , as illustrated in fig6 . the disc 5 of the joint part 11 is connected to the flange - shaped end of the hollow shaft 1 and tightened with an outer bracing ring 12 during implantation . the bracing ring 12 has a predetermined breaking point by way of an area of material weakness 13 so that the joint part 11 can be easily removed , requiring only a small amount of force . it will be understood that the above description of the present invention is susceptible to various modifications , changes and adaptations , and the same are intended to be comprehended within the meaning and range of equivalents of the appended claims .
endoprosthesis , in particular a hip joint prosthesis , provided with a hollow shaft being implantable in a hollow space of a bone , being of a tubular and curved shape and diminishing with respect to its diameter and material cross - cut , whereby , from the end that is near to the joint towards the end that is far away from the joint , the bending rigidity of the shaft diminishes to a relatively larger extent than the longitudinal rigidity of the shaft .