A prosthetic humeral device (1) having at least two guide holes (11) located at predetermined locations thereon for the acceptance of a fastening means (29), which is preferably a screw (16) and a predeterminedly-shaped washer (6) with at least one projection (17) located on the underside. After inserting the elongated stem (14) of the prosthetic humeral device (1) into the humerus (2), a tuberosity grasping tool (19) is used to grasp a tuberosity (4) and (7). The tuberosity (4) and (7) is then positioned to an appropriate level on the prosthetic humeral device (1), a hole is drilled into the tuberosity (4) and (7), the screw (16) with washer (6) is inserted, the grasping tool (19) is removed and the screw (16) and washer (6) are tightened.