Examples of a suture anchor (100) having a non-expandable body (105) and expendable cap (110) are described herein. In response to an axial insertion force that brings the non-expandable body and expendable cap together inside a bone hole, the expendable cap expands radially. This creates a radial force of expansion that advantageously augments an interference fit between the suture anchor and bone hole, which leads to higher fixation strength. Other examples include retention features that inhibit the expendable cap from returning back into its unexpanded state. This advantageously maintains the radial force of expansion against the surrounding pressure of the walls of the bone hole pressing back on the suture.