[Dimensional changes of polyethylene acetabuli in Müller's hip endoprosthesis. Report on measurement methods and their clinical significance].

The dimensional changes of hip sockets of total endoprosthesis of the Müller Type are subject of this article. Regular-anterior/posterior radiographs of the pelvis were taken to determine the orientation of the center of the prosthetic head in relation to the wire marker of the poly(ethylene) cup. Further measurements were done using casts of worn acetabular cups gained at revisional surgery. The possible errors of measurements of both methods are discussed. Both, creep and wear contribute to the dimensional changes of the hip sockets, the proportional amount of each mechanism is not known. Data from laboratory examinations suggest a relatively high rate of creep in the early time of function. With longer periods the dimensional changes are predominantly caused by wear. In the beginning of joint function our measurements show a high rate of the yearly dimensional change. The shift of the head amounted 0.5 mm per year and diminished after five years to rates of 0.15 to 0.2 mm. All dimensional changes that exceed a shift to the head of 0.2 mm per year are considered to be unfavourable and to contribute to loosening.