A bone anchor assembly includes a bone anchor having a proximal head and a distal shaft configured to engage bone, a receiver member for receiving a spinal fixation element to be coupled to the bone anchor, an insert positioned in the proximal end of the receiver member having a pair of spaced apart insert arms defining a recess therebetween, and a closure mechanism positionable between and engaging the insert arms to capture a spinal fixation element within the recess of the insert and fix the spinal fixation element with respect to the receiver member.
