A locking instrument assembly (10, 200, 400, 500) for use in conjunction with anintramedullary device (100) is disclosed. The locking instrument assembly (10,200, 400, 500) includes an inner collet (12, 70, 212, 412, 512), an outer body (30,80, 230, 430, 530), and a knob (60, 90, 260, 460, 560). The inner collet (12, 70,212, 412, 512) has a collar (20, 79, 220, 420, 520) and a fastener member (22, 222,422). The knob (60, 90, 260, 460, 560) engages the fastener member (22, 222, 422)to press the outer body (30, 80, 230, 430, 530) against the collar (20, 79, 220,420, 520). As the knob (60, 90, 260, 460, 560) mates with the fastener member (22,222, 422), the collar (20, 79, 220, 420, 520) applies a clamping force to the intramedullarydevice (100).