Bone anchor receiver upright arms and closures having helically wound threads to apply thrust with controlled splay

A spinal fixation structure intended for use with an elongate rod, the spinal fixation structure comprising:

• a receiver housing having a central bore defining an inner surface in an upper portion of the central bore with an open channel configured to receive the elongate rod, and upright arms having a discontinuous helically wound receiver thread formed therein, the discontinuous helically wound receiver thread having a receiver cylindrical root surface, a base height adjacent the receiver cylindrical root surface measured as an axial distance extending parallel with respect to a longitudinal axis of the receiver housing, a receiver linear load flank, and an opposed receiver linear stab flank;
• and
• a closure having a closure central axis, a cylindrical body including a top end having a top surface, a bottom end having a bottom surface opposite the top surface, a central internal drive structure extending into the cylindrical body centered along the closure central axis, and an outer complementary mating helically wound closure thread formed thereon and including a continuously wound closure linear load flank having a radially outwardly extending surface portion that angles upwardly toward the top end of the closure or extends perpendicularly relative to the closure central axis, a closure linear stab flank having a radially outwardly extending linear clearance surface portion, and a closure cylindrical root surface defining an outer surface of the cylindrical body and a base height adjacent the closure cylindrical root surface measured as an axial distance extending parallel with respect to the closure central axis with the base height of the receiver thread being substantially equal to the base height of the closure thread,
• wherein the closure thread has a thread pitch within a range of 0.040 inches and 0.070 inches,
• wherein the closure thread is configured to engage the receiver thread to secure the elongate rod within the open channel of the receiver housing in a locked configuration with an applied torque of at least 75 inch pounds to apply a locking thrust or a downward force of the closure against the rod;
• wherein threaded engagement of the closure and the receiver housing is such that axial loading between the receiver load flank and the closure load flank up to the applied torque of at least 75 inch pounds causes the upright arms to splay outwardly relative to each other, wherein the outward splay of the upright arms initially increases with increasing torque, and then the outward splay is ultimately constrained by a rate of outward splay decreasing with further increasing torque, and
• wherein a clearance gap exists between the receiver linear stab flank and the closure linear stab flank when the spinal fixation structure is in the locked configuration.