A modular polyaxial bone screw assembly for holding a spine rod in a fixed spatial orientation relative to vertebral bone, the modular polyaxial bone screw assembly comprising: a polyaxial bone screw having a threaded shank and a globular head; a tulip head disposed on the globular head of the polyaxial bone screw and configured to swivel about the globular head of the polyaxial bone screw before the tulip head is fixed in spatial orientation relative to the polyaxial bone screw, and to receive and hold a spine rod as the spatial orientation of the tulip head is fixed, the tulip head having a bore extending from a first end of the tulip head to a second end of the tulip head and defining a generally cylindrical interior with an upper cylindrical portion defining an upper inner cylindrical wall coinciding with and within the first end of the tulip head, inner threading on the upper inner cylindrical wall for threaded receipt of a set screw, and a lower generally cylindrical portion defining a lower inner generally cylindrical wall coinciding with and within the second end of the tulip head and sized to receive the globular head of the polyaxial bone screw; and a collet disposed in the interior of the tulip head, the collet having a base section with a first end resting about the globular head of the polyaxial bone screw and configured to splay on the globular head of the polyaxial bone screw and fix a spatial orientation of the tulip head relative to the polyaxial bone screw through interference between the first end, the globular head of the polyaxial bone screw, and the tulip head when a spine rod is received in and affixed to the tulip head by the set screw, characterised in that the tulip head has an inner configuration on the lower inner generally cylindrical wall, the inner configuration consisting of a lip extending radially inward from the lower inner generally cylindrical wall, and the collet has an outer configuration comprising a ledge extending circumferentially about the body proximate the base section on an outer wall that cooperates with the inner configuration comprising the lip of the tulip head to fix axial and rotational position of the collet relative to the tulip head. a polyaxial bone screw having a threaded shank and a globular head; a tulip head disposed on the globular head of the polyaxial bone screw and configured to swivel about the globular head of the polyaxial bone screw before the tulip head is fixed in spatial orientation relative to the polyaxial bone screw, and to receive and hold a spine rod as the spatial orientation of the tulip head is fixed, the tulip head having a bore extending from a first end of the tulip head to a second end of the tulip head and defining a generally cylindrical interior with an upper cylindrical portion defining an upper inner cylindrical wall coinciding with and within the first end of the tulip head, inner threading on the upper inner cylindrical wall for threaded receipt of a set screw, and a lower generally cylindrical portion defining a lower inner generally cylindrical wall coinciding with and within the second end of the tulip head and sized to receive the globular head of the polyaxial bone screw; and a collet disposed in the interior of the tulip head, the collet having a base section with a first end resting about the globular head of the polyaxial bone screw and configured to splay on the globular head of the polyaxial bone screw and fix a spatial orientation of the tulip head relative to the polyaxial bone screw through interference between the first end, the globular head of the polyaxial bone screw, and the tulip head when a spine rod is received in and affixed to the tulip head by the set screw, characterised in that the tulip head has an inner configuration on the lower inner generally cylindrical wall, the inner configuration consisting of a lip extending radially inward from the lower inner generally cylindrical wall, and the collet has an outer configuration comprising a ledge extending circumferentially about the body proximate the base section on an outer wall that cooperates with the inner configuration comprising the lip of the tulip head to fix axial and rotational position of the collet relative to the tulip head.