The implant (1) has an expandable outer shell (2), expanded radially by an insert (3) which is forced inside the shell and fixed in place by a non-return assembly (12) of interlocking components (13, 14) with annular ridges. The outer component (13) of the two has sufficient elasticity to pass over the inner one (14) and enable their two facing ridged surfaces to engage. Once the implant has been inserted and locked in place the surplus end of its inner rod (40) is cut off.