An anchor assembly for use with a spinal rod for stabilizing bones or bone fragments, the anchor assembly comprising: a bone anchor including an enlarged, curvate head portion; a body having a longitudinal axis, an exterior side wall, an interior wall, an upper end and a lower end spaced from the upper end along the longitudinal axis, the upper end having an upper opening and the lower end having a lower opening, and the body further having a bore and a first rod receiving channel (27, 29), said bore extending substantially between said upper opening and said lower opening, and said first rod receiving channel configured and arranged to receive the spinal rod; an insert member including a first end, a second end, a lower opening, and an interior cavity for receiving at least a portion of the head portion of the bone anchor, the insert member further having an outer surface, the insert member being movably positionable within the bore of the body; and a locking cap assembly (92; 190) including a saddle (70; 70') and a locking element or locking assembly (60, 90), the saddle having two saddle arms defining a second rod receiving channel, wherein the saddle is operatively associated with the locking element or locking assembly (60, 90) as well as the insert member, the locking element or assembly being engagable with the body and being movable from an unlocked position to a locked position, wherein, when the locking element or locking assembly (60, 90) is in the unlocked position, the bone anchor is poly-axially pivotable with respect to the body, wherein movement of the locking element or locking assembly (60, 90) from the unlocked position to the locked position applies pressure to the spinal rod to secure the spinal rod with respect to the body, and causes a distal end of each of the saddle arms to translate toward the lower end of the body along the longitudinal axis to at least a lower half of the spinal rod, viewed in cross-section in a plane perpendicular to the rod's central axis, and characterized in that the insert member includes at least one slot extending from the second end, the slot permitting the insert member to have at least a portion that is expandable and collapsible, and in that movement of the locking element or locking assembly (60, 90) from the unlocked position to the locked position further causes the distal end of each of the saddle arms to apply a force to the insert member so as to urge the insert member against the curvate head portion, thereby securing a position of the bone anchor with respect to the body, when 1) the locking cap assembly (92; 190) is at least partially received in the bore, and 2) the spinal rod is received in the first and second rod receiving channels. a bone anchor including an enlarged, curvate head portion; a body having a longitudinal axis, an exterior side wall, an interior wall, an upper end and a lower end spaced from the upper end along the longitudinal axis, the upper end having an upper opening and the lower end having a lower opening, and the body further having a bore and a first rod receiving channel (27, 29), said bore extending substantially between said upper opening and said lower opening, and said first rod receiving channel configured and arranged to receive the spinal rod; an insert member including a first end, a second end, a lower opening, and an interior cavity for receiving at least a portion of the head portion of the bone anchor, the insert member further having an outer surface, the insert member being movably positionable within the bore of the body; and a locking cap assembly (92; 190) including a saddle (70; 70') and a locking element or locking assembly (60, 90), the saddle having two saddle arms defining a second rod receiving channel, wherein the saddle is operatively associated with the locking element or locking assembly (60, 90) as well as the insert member, the locking element or assembly being engagable with the body and being movable from an unlocked position to a locked position, wherein, when the locking element or locking assembly (60, 90) is in the unlocked position, the bone anchor is poly-axially pivotable with respect to the body,