Bottom loading bone anchor assemblies with drag retaining ring and related methods

A method of assembling a bone anchor assembly, comprising:

• radially compressing a retaining ring located around a shank, the shank including a head and a bone-engaging portion and the retaining ring including a base and a walled portion extending proximally from the base, the walled portion having a maximum outer diameter that is less than a maximum outer diameter of the base;
• passing the shank and the retaining ring in the compressed state proximally through an opening at a distal end of a receiver member;
• advancing the retaining ring proximally within the receiver member such that at least a portion of the base aligns with a first annular recess of a groove formed in the receiver member;
• expanding the retaining ring to hold at least a portion of the base within the first annular recess of the groove;
• and
• seating the head of the shank in the base of the retaining ring such that a maximum diameter of the head of the shank contacts the walled portion of the retaining ring and the bone engaging portion of the shank extends distally from the retaining ring.

A bone anchor assembly, comprising:

• a receiver member having proximal and distal ends with a central longitudinal axis extending therebetween and a longitudinal bore with an opening at the distal end of the receiver member;
• a retaining ring disposed in a groove formed in the receiver member, the retaining ring having a base and a walled portion that extends proximally from the base, the base having a maximum outer diameter that is greater than a maximum outer diameter of the walled portion;
• and
• a shank having a head portion seated within the base of the retaining ring and a bone engaging portion that extends distally from the receiver member,
• wherein the walled portion of the retaining ring is configured to exert a drag force on the head portion of the shank to resist rotation thereof, and
• wherein the base further comprises a proximal-facing shelf from which the walled portion extends, the shelf extending substantially perpendicular to the walled portion.