A bone anchor assembly , comprising: a bone anchor having a proximal head portion and a distal shank portion; a receiver member comprising opposed arms defining a recess , the arms comprising shelves extending radially inward from inner walls of the arms , the receiver member having a polyaxial seat formed therein and configured to polyaxially seat the head portion of the bone anchor; and a compression cap disposed within the recess of the receiver member and having a distal end configured to engage the head portion of the bone anchor and a pair of spaced-apart arms defining a U-shaped seat for receiving a spinal fixation element , the compression cap including wings extending radially from outward-facing surface of the arms of the compression cap, the wings being configured to mate with the shelves of the receiver member such that the compression cap is retained within the receiver member and exerts a frictional force on the head portion of the bone anchor to prevent free movement of the bone anchor relative to the receiver member while still allowing a user to move the receiver member relative to the bone anchor; wherein the wings of the compression cap each comprise a proximal surface that is upwardly sloped at an angle to a longitudinal axis (L 2 ) of the compression cap such that a radially outermost edge of the wings is proximal to a radially innermost edge of the wings; and wherein the proximal-facing surfaces of the wings are configured to mate with the cut-outs formed on distal-facing sides of the shelves of the receiver member upon rotation of the compression cap with respect to the receiver member from an unsecured position to a secured position. a bone anchor having a proximal head portion and a distal shank portion; a receiver member comprising opposed arms defining a recess , the arms comprising shelves extending radially inward from inner walls of the arms , the receiver member having a polyaxial seat formed therein and configured to polyaxially seat the head portion of the bone anchor; and a compression cap disposed within the recess of the receiver member and having a distal end configured to engage the head portion of the bone anchor and a pair of spaced-apart arms defining a U-shaped seat for receiving a spinal fixation element , the compression cap including wings extending radially from outward-facing surface of the arms of the compression cap, the wings being configured to mate with the shelves of the receiver member such that the compression cap is retained within the receiver member and exerts a frictional force on the head portion of the bone anchor to prevent free movement of the bone anchor relative to the receiver member while still allowing a user to move the receiver member relative to the bone anchor; wherein the wings of the compression cap each comprise a proximal surface that is upwardly sloped at an angle to a longitudinal axis (L 2 ) of the compression cap such that a radially outermost edge of the wings is proximal to a radially innermost edge of the wings; and wherein the proximal-facing surfaces of the wings are configured to mate with the cut-outs formed on distal-facing sides of the shelves of the receiver member upon rotation of the compression cap with respect to the receiver member from an unsecured position to a secured position. <EOS>