A balloon dilator device, comprising an annularly shaped, cylindrical type structure having walls that are expandable from a radially collapsed state to a radially expanded state by inflation of a balloon inserted within the annular structure. Once the walls have been expanded, they remain in the expanded state even if the balloon is deflated, because the radially expanded state is a state of minimum mechanical potential energy, and in order to return to the collapsed state, the structure would have to pass a state of higher potential energy. The device walls require sufficient stiffness in their longitudinal direction to enable the device to be pushed into a minimally invasive incision made in the subject. This device stiffness can be achieved either by its mechanical material properties, or by its substantially closed wall structure, or by use of a stiff protector sheath used to protect the walls during insertion.