Fastening devices, systems, and methods

A fastener comprising:

• a shaft comprising:
• a minor diameter;
• a proximal end;
• a distal end;
• and
• a longitudinal axis;
• and
• a plurality of helical threads disposed about the shaft along the longitudinal axis between the proximal and distal ends of the shaft, the plurality of helical threads comprising:
• a first helical thread comprising:
• a first concave undercut surface open toward the proximal end of the shaft;
• and
• a first convex undercut surface facing toward the distal end of the shaft;
• and
• a second helical thread comprising:
• a second concave undercut surface open toward the distal end of the shaft;
• and
• a second convex undercut surface facing toward the proximal end of the shaft;
• wherein, when the fastener is viewed in section along a plane containing the longitudinal axis of the shaft:
• the first concave undercut surface and the first convex undercut surface of the first helical thread each extend from the minor diameter of the shaft to a first crest of the first helical thread;
• the second concave undercut surface and the second convex undercut surface of the second helical thread each extend from the minor diameter of the shaft to a second crest of the second helical thread;
• a first interlocking space is formed intermediate the first concave undercut surface and the second concave undercut surface;
• a second interlocking space is formed intermediate the first convex undercut surface and the second convex undercut surface;
• and
• at least one of:
• the first helical thread does not have mirror symmetry with itself across any plane perpendicular to the longitudinal axis of the shaft;
• and
• the second helical thread does not have mirror symmetry with itself across any plane perpendicular to the longitudinal axis of the shaft.

An implantable bone anchor comprising:

• a shaft comprising:
• a minor diameter;
• a proximal end;
• a distal end;
• and
• a longitudinal axis;
• a first helical thread disposed about the shaft along the longitudinal axis between the proximal and distal ends of the shaft, the first helical thread comprising:
• a first concave undercut surface open toward the proximal end;
• and
• a first convex undercut surface facing toward the distal end;
• and
• a second helical thread disposed about the shaft adjacent the first helical thread, the second helical thread comprising:
• a second concave undercut surface open toward the distal end, opposite the first concave undercut surface;
• and
• a second convex undercut surface facing toward the proximal end, opposite the first convex undercut surface;
• wherein, when the implantable bone anchor is viewed in section along a plane containing the longitudinal axis of the shaft:
• a first interlocking space is formed intermediate the first concave undercut surface and the second concave undercut surface that extends to the minor diameter of the shaft;
• a second interlocking space is formed intermediate the first convex undercut surface and the second convex undercut surface that extends to the same minor diameter of the shaft as the first interlocking space;
• and
• at least one of:
• the first helical thread does not have mirror symmetry with itself across any plane perpendicular to the longitudinal axis;
• and
• the second helical thread does not have mirror symmetry with itself across any plane perpendicular to the longitudinal axis.