An instrument for establishing orientation of a pelvic prosthesis comprisesa tripod having an angularly adjustable guide rod on it. The tips of the legs definea plane, and the guide rod is set by the surgeon to a defined orientation with respectto this plane on the basis of preoperative studies. In use, two of the legs of theinstrument are positioned by the surgeon at defined anatomical locations onthe pelvis (e.g., a point in the region of the posterior/inferior acetabulumand a point on the anterior superior iliac spine). The third leg then lands on thepelvis at a point determined by the position of the first two points, as well asby the separations between the third leg and the other two legs. The separationsare adjustable, but are preferably fixed percentages of the separation betweenthe first and second legs. The position of the guide rod then defines with respectto the actual pelvis the direction for insertion of a prosthesis.
