Washer assembly for spinal fixation screw

A bone screw assembly comprising:

• a bone screw having a shank and a head defining a maximum diameter;
• and
• a washer including;
• a body having a bottom face for engaging the surface of a bone and an opening therethrough sized to receive the shank of a bone screw extending therethrough;
• a cavity defined in said body in communication with said opening at said bottom face, said cavity having an opening at the upper face of the body opposite said bottom face, said cavity configured to receive the head of the bone screw when the shank extends through the opening at said bottom face;
• and
• a plurality of flaps distributed around said opening of said cavity, said flaps bendable from a first configuration in which the flaps define an opening sized to receive the head of the bone screw therethrough to a second configuration in which the flaps define an opening smaller than the maximum diameter of the head of the bone screw.