Bone anchor receiver fastener structure with splay control

A threaded locking closure mechanism for a bone anchor assembly, the threaded locking closure mechanism comprising:

• a receiver, the receiver including a body, a central receiver axis extending through the body, two upwardly extending arms, an open channel defined between the two upwardly extending arms and for receiving a rod therein, and an inwardly extending thread formed therein and comprising a downwardly facing linear loading mating surface and a cylindrical root surface portion;
• and
• a fastener including a top side, a bottom side, a central fastener axis extending through the top side and the bottom side, a cylindrical core body having an exterior surface defining a fastener outer thread formed thereon, and a cylindrical root surface portion, the fastener outer thread having a thread pitch, an upwardly facing linear loading mating surface extending outwardly from the cylindrical root surface portion, and a cylindrical crest surface portion defining an outermost point of the fastener, wherein the thread pitch is between 0.039 inches and 0.045 inches,
• wherein the fastener is configured for rotatable positioning in the receiver to:
• i. secure the rod in the open channel with the cylindrical crest surface portion of the fastener outer thread and the cylindrical root surface portion of the inwardly extending thread form of the receiver being spaced apart by a horizontal distance;
• and
• ii. allow outward splaying of the two upwardly extending arms to within a range of about 0.004 inches to 0.007 inches relative to each other near a top of each of the upwardly extending arms, thereby reducing torque applied to the fastener when securing the rod in the open channel.