The marrow-cavities of several human femora were cleaned and filled with plastics; the femoral component design was developed based on these moulds. Different sizes of the stem were obtained by scaling down the biggest mould in steps of 10%. The stem has an oval cross-section and is twisted similar to the form of the marrow-cavity; therefore different designs for the left and right femur are necessary. As the marrow-cavity of the femur tapers down to the middle of the shaft the length of the prosthetic stem cannot be selected arbitrarily. The stem must end above the narrowest site of the cavity. Data are presented. To avoid disadvantageous frictional stresses between the collar of the prosthesis and the plane of resection of the femoral neck both must be aligned perpendicular to the most common direction of the load of the hip joint. Therefore a step-like osteotomy of the femoral neck becomes necessary without disturbing the calcar femorale instead of an inclined osteotomy. A firm contact between the femoral wall and the collar, which forms an angle of 64 degrees with the axis of the femoral shaft, guarantees that only small frictional stresses occur between collar and femoral cortex if the load of the hip joint varies within the physiological range. A set of ceramic femoral heads with three different conical borings yield different lengths of the neck of the prosthesis. Independent of which femoral head and which size of prosthesis are chosen the direction of the maximum hip load in any case thrusts the contact area between collar and femoral wall. Thus dangerous tilting moments round the medial calcar femorale do not occur, the incidence of a fracture of the prosthesis shaft is therefore reduced. After implantation of the anatomically designed femoral component both remodeling and resorption of the calcar femorale are observed. This reaction is independent of the kind of fixation, i.e. if a smooth stem was fixed with cement or a stem with a porous, cancellous bone-like metallic surface was implanted without cement fixation.(ABSTRACT TRUNCATED AT 250 WORDS)

one hundred and thirteen re-operations necessitated by loosening of the prosthesis shaft were analyzed with regard to surgical stress, technique and results. Of these operations to replace the prosthesis shaft, the surgical technique employed depended on the individual situation in 94 cases. In some, the implant and cement cone were removed, in some the medullary space was foragad, and in others single or multiple fenestration was performed. The surgical investment in these 94 operations proved to be especially high. The main disadvantage of individually oriented, partly improvised procedures is that granular tissue remains in the former contact zone between cement cone and bone. Uncovering of the shaft of the femur in a planned procedure employing the fenestration/chiseling method (19 patients) represents a way of cleaning the medullary cavity completely prior to fitting the replacement implant, with a low level of surgical stress. Implants whose shape corresponds as closely of the shaft are preferred to the so-called long-shaft implants. The authors see a considerable improvement in the departure from individually oriented and partially improvised procedures in replacement interventions: the surgical stress imposed by the operation is easier to assess and the safety of the implant renders the chances of success greater.