Referring to FIGS. 9A and 10A, the portion of the longitudinal bore 104 of the IM implant 102 that receives the second component 340 of the locking device 300, along section 10A-10A of FIG. 9A, for example, can be shaped to have an inner wall having a closed periphery 120 in the form of a non-circular, elongated curve, such as an ellipse or oval or other elongated shape, as shown in FIG. 10A. The