Closure with strong balanced splay controlling thread form for bone anchor open receiver housings

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

• a bone anchor receiver, the receiver including a body, a central receiver axis extending through the body, two upwardly extending arms, and an open channel defined between the two upwardly extending arms and for receiving a rod therein, each of the two upwardly extending arms comprising a portion of a helically wound inner thread form with an inwardly extending downwardly facing linear loading mating surface and a cylindrical crest surface positioned inward of a cylindrical root surface of the helically wound inner thread form;
• 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 mating continuous helically wound outer thread form, a cylindrical root surface that is spaced at a first outer end of a first radius extending from the central fastener axis and having a first length, and the outer thread form having an upwardly facing linear loading mating surface extending outwardly from the cylindrical root surface, the outer thread form comprising a cylindrical crest surface that is spaced at a second outer end of a second radius extending from the central fastener axis and having a second length greater than the first length, the upwardly facing linear loading surface rotatably engaged with the downwardly facing linear loading mating surface over a contact surface region thereof, and the outer thread form having a thread pitch between 0.039 inches and 0.045 inches,
• wherein the fastener is configured for rotatable positioning in the receiver to secure the rod in the open channel with the cylindrical crest surface of the outer thread form of the fastener and the cylindrical root surface of inner thread form of the receiver being spaced apart by a horizontal distance,
• and wherein upon rotatably positioning the fastener in the receiver to secure the rod in the open channel, the contact surface region moves outwardly relative to the central receiver axis so as to permit an amount of splay and reduce the amount of torque applied to the fastener as the rod is being secured in the open channel of the receiver by the fastener in a final locked position.