A bone anchor is provided bone anchor comprising an anchor member (1) having a first end (11) and a second end (12) and an tubular section between the first end and the second end, a longitudinal axis (L) extending from the first end to the second end, and a bone engagement structure (16) for engaging the bone in at least a portion of the tubular section wherein the tubular section (13) is expandable in a radial direction by reducing a distance between the first end and the second end and wherein the anchor member (1) is made of a first material a core member (2, 2) having a first end (21) and a second end (22) and being insertable into the anchor member and connectable thereto, the core member (2, 2) being configured to cooperate with the anchor member (1) such that by actuating the core member with respect to the anchor member the tubular section (13) is expandable characterized in that the core member (2, 2) is made of a second material that is different from the first material.
