Polyaxial bone fixation element

A fastener device configured to secure a spinal rod relative to a bone, the fastener device comprising:

• an anchor body including an anchor body housing, the anchor body defining a through hole that extends through the anchor body housing along a longitudinal axis, the anchor body further defining a rod-receiving channel that is configured to receive the spinal rod and that is offset with respect to the through hole, the anchor body further including an upper end, a lower end, and an inner surface, the inner surface defining a least a portion of the through hole;
• and
• a fastener including a head, a threaded shaft that extends out with respect to the head in a distal direction, and a neck that extends between the head and the threaded shaft, the head including an outer surface configured to articulate relative to the inner surface when the head is inserted in the through hole, at least a portion of the outer surface being convex and defining a portion of a sphere, the sphere defining a first diameter, and the fastener including a concave surface that extends along both the head and the neck, the neck defining a second diameter less than the first diameter and measured:
• 1) in a direction perpendicular to the distal direction, and 2) at a location spaced in the distal direction of an entirety of the concave surface,
• wherein the head configured to rotate within the anchor body, when the head is in the through hole, as the threaded shaft angulates between a first position, in which the threaded shaft is aligned with the longitudinal axis, and a second position,
• wherein the second position is angularly offset from the first position by at least 45 degrees before the anchor body contacts the neck so as to prevent further angulation of the threaded shaft.