A spinal fixation device is provided including a modular head assembly and a bone screw. The modular had assembly includes a housing, an anvil, an insert, and a snap ring. The housing defines a proximal surface and an opposite, distal surface, and the proximal and distal surfaces define a throughbore therethrough. The anvil is configured to be slidably received within a portion of the throughbore. The insert defines a proximal surface and an opposite, distal surface, and the distal surface defines a first counterbore therein that terminates at a first annular surface. The first annular surface defines a second counterbore that terminates at a second annular surface. The snap ring is configured to be disposed within the first counterbore of the insert when in a first configuration, and within the second counterbore of the insert when in a second configuration.
