Multi-axial bone screw mechanism

A bone fixation system comprising:

• a coupling member comprising a lower surface and a channel opposite the lower surface, the channel being formed by two arms extending from a base portion;
• a hole extending through the coupling member from the base portion of the channel to the lower surface of the coupling member;
• a spacer within the hole of the coupling member, the spacer having a top surface adjacent to the base portion of the channel and a bottom surface generally opposite the top surface;
• a hollow retaining ring within the hole of the coupling member, the retaining ring comprising an inner surface, an outer surface, a top surface spaced from the bottom surface of the spacer, and a slot extending from the inner surface of the ring to the outer surface of the ring;
• a bone screw comprising a head portion having an outer surface adjacent to the inner surface of the retaining ring and a threaded portion extending from the head portion;
• an elongated rod extending through the channel of the coupling member;
• and
• a locking member positioned for securing the rod within the channel of the coupling member.

A method of assembling a bone screw assembly comprising the steps of:

• providing a coupling member having a lower surface, a channel opposite the lower surface, and a hole through the coupling member from the channel to the lower surface;
• inserting a spacer having a top surface and a bottom surface into the hole through the lower surface of the coupling member so that the top surface of the spacer is adjacent to the channel;
• inserting a hollow retaining ring into the hole through the lower surface of the coupling member so that a top surface of the retaining ring is spaced from the bottom surface of the spacer;
• and
• inserting a bone screw comprising a threaded portion extending from a head portion into the hole through the lower surface of the coupling member so that a top portion of the head portion is adjacent to the bottom surface of the spacer and the entire threaded portion is external to the coupling member.