A pedicle screw assembly, comprising: a bone screw having a head and a shank, the head defining at least one groove extending in a length direction of the bone screw between a neck of the head and a proximal end of the head; and a modular head assembly comprising: a housing having a proximal surface and a distal surface, the housing defining a through hole extending along a longitudinal axis of the housing and between the proximal and distal surfaces of the housing, a counterbore being formed in the distal surface of the housing, extending towards the proximal surface of the housing and terminating at an annular face, an inner surface of the counterbore being formed to have a greater diameter than an inner surface of the proximal portion of the throughole; a cap coupled to the housing, the cap forming a retaining ring receiving portion having a proximal portion defining a first diameter and a distal portion defining a second diameter smaller than the first diameter; a retaining ring formed of a resilient material disposed within the retaining ring receiving portion, the retaining ring being transitionable between an expanded configuration in which the retaining ring is sized to receive the head of the bone screw and a compressed configuration in which the retaining ring prevents the head of the bone screw from passing distally through the retaining ring; an anvil received in the counterbore of the housing, the anvil including a leaf spring extending from a distal end thereof and at least one protrusion sized and shaped to be received by the at least one groove for restricting movement of the bone screw to a single plane, wherein the leaf spring is arranged to bias the retaining ring toward the distal portion of the retaining ring receiving portion after the head of the bone screw has been uploaded through the retaining ring; and a biasing member circumferentially disposed about the anvil and positioned to engage the annular face of the counterbore of the housing. a bone screw having a head and a shank, the head defining at least one groove extending in a length direction of the bone screw between a neck of the head and a proximal end of the head; and a modular head assembly comprising: a housing having a proximal surface and a distal surface, the housing defining a through hole extending along a longitudinal axis of the housing and between the proximal and distal surfaces of the housing, a counterbore being formed in the distal surface of the housing, extending towards the proximal surface of the housing and terminating at an annular face, an inner surface of the counterbore being formed to have a greater diameter than an inner surface of the proximal portion of the throughole; a cap coupled to the housing, the cap forming a retaining ring receiving portion having a proximal portion defining a first diameter and a distal portion defining a second diameter smaller than the first diameter; a retaining ring formed of a resilient material disposed within the retaining ring receiving portion, the retaining ring being transitionable between an expanded configuration in which the retaining ring is sized to receive the head of the bone screw and a compressed configuration in which the retaining ring prevents the head of the bone screw from passing distally through the retaining ring; an anvil received in the counterbore of the housing, the anvil including a leaf spring extending from a distal end thereof and at least one protrusion sized and shaped to be received by the at least one groove for restricting movement of the bone screw to a single plane, wherein the leaf spring is arranged to bias the retaining ring toward the distal portion of the retaining ring receiving portion after the head of the bone screw has been uploaded through the retaining ring; and a biasing member circumferentially disposed about the anvil and positioned to engage the annular face of the counterbore of the housing. a housing having a proximal surface and a distal surface, the housing defining a through hole extending along a longitudinal axis of the housing and between the proximal and distal surfaces of the housing, a counterbore being formed in the distal surface of the housing, extending towards the proximal surface of the housing and terminating at an annular face, an inner surface of the counterbore being formed to have a greater diameter than an inner surface of the proximal portion of the throughole; a cap coupled to the housing, the cap forming a retaining ring receiving portion having a proximal portion defining a first diameter and a distal portion defining a second diameter smaller than the first diameter; a retaining ring formed of a resilient material disposed within the retaining ring receiving portion, the retaining ring being transitionable between an expanded configuration in which the retaining ring is sized to receive the head of the bone screw and a compressed configuration in which the retaining ring prevents the head of the bone screw from passing distally through the retaining ring; an anvil received in the counterbore of the housing, the anvil including a leaf spring extending from a distal end thereof and at least one protrusion sized and shaped to be received by the at least one groove for restricting movement of the bone screw to a single plane, wherein the leaf spring is arranged to bias the retaining ring toward the distal portion of the retaining ring receiving portion after the head of the bone screw has been uploaded through the retaining ring; and a biasing member circumferentially disposed about the anvil and positioned to engage the annular face of the counterbore of the housing.