A spinal fixation device, comprising: a bone screw including head and a shank extending from the head; a modular head assembly, comprising: a housing having a proximal surface, a distal surface, and a throughhole formed through the housing from the proximal surface to the distal surface; an anvil slidable within a portion of the throughhole; an assembly cap secured to the housing, the assembly cap including an inner surface defining a cavity having a first portion and a second portion, the first portion having a first diameter and the second portion having a second diameter smaller than the first diameter; a retaining ring positioned at least partially within the cavity of the assembly cap, the retaining ring transitionable between a first configuration in which the retaining ring is sized to receive the head of the bone screw and a second configuration in which the retaining ring is compressed about the bone screw, wherein movement of the retaining ring from the first portion to the second portion compresses the retaining ring from the first configuration to the second configuration and secures the bone screw relative to the housing, characterized in that the modular head assembly further comprises a biasing member circumferentially surrounding an outer surface of the anvil and arranged to impart a distally directed force on the head of the bone screw as the modular head assembly is passed over the head of the bone screw and the head of the bone screw is received within the throughole. a bone screw including head and a shank extending from the head; a modular head assembly, comprising: a housing having a proximal surface, a distal surface, and a throughhole formed through the housing from the proximal surface to the distal surface; an anvil slidable within a portion of the throughhole; an assembly cap secured to the housing, the assembly cap including an inner surface defining a cavity having a first portion and a second portion, the first portion having a first diameter and the second portion having a second diameter smaller than the first diameter; a retaining ring positioned at least partially within the cavity of the assembly cap, the retaining ring transitionable between a first configuration in which the retaining ring is sized to receive the head of the bone screw and a second configuration in which the retaining ring is compressed about the bone screw, a housing having a proximal surface, a distal surface, and a throughhole formed through the housing from the proximal surface to the distal surface; an anvil slidable within a portion of the throughhole; an assembly cap secured to the housing, the assembly cap including an inner surface defining a cavity having a first portion and a second portion, the first portion having a first diameter and the second portion having a second diameter smaller than the first diameter; a retaining ring positioned at least partially within the cavity of the assembly cap, the retaining ring transitionable between a first configuration in which the retaining ring is sized to receive the head of the bone screw and a second configuration in which the retaining ring is compressed about the bone screw, wherein movement of the retaining ring from the first portion to the second portion compresses the retaining ring from the first configuration to the second configuration and secures the bone screw relative to the housing, characterized in that the modular head assembly further comprises a biasing member circumferentially surrounding an outer surface of the anvil and arranged to impart a distally directed force on the head of the bone screw as the modular head assembly is passed over the head of the bone screw and the head of the bone screw is received within the throughole.