Modular screw head assemblies

An orthopedic fixation assembly comprising:

• a tulip head having two arms defining a rod slot therebetween, each of the arms defining an interior threaded portion along an inner surface, the tulip head having a bore extending therethrough;
• a uni-planar bone fastener including a screw head receivable in the tulip head and a shaft configured for engaging bone,
• a saddle having an upper surface defining a rod seat aligned with the rod slot and a lower portion configured for receiving the screw head;
• and
• a retaining clip positioned in a groove in the tulip head and around a bottom of the screw head, wherein the screw head has a plurality of flats aligned with corresponding flats in the lower portion of the saddle to restrict movement of the shaft relative to the tulip head to one direction.