a bottom - loading multi - axial bone anchor apparatus is disclosed . the apparatus includes a receiver member , a crown member , a bone anchor and a retaining member . the receiver member defines an upper opening and a lower opening , which may form part of the same opening , a channel , and a groove . the crown member and bone anchor are loaded into the lower opening of the receiver member , and the retaining member fits around the bone anchor and into the groove in the receiver member . the bone anchor is capable of multi - axial positioning with respect to the receiver member . an elongated member is placed in the channel of the receiver member , contacting the crown member , and a compression member is applied via the upper opening . the compression member presses down on the elongated member , which presses down on the crown member and locks the bone anchor between the crown member and the retaining member .