A bone anchor assembly includes a bone anchor, a receiver member for receiving a spinal fixation element to be coupled to the bone anchor, and a closure mechanism. The closure mechanism is positionable between the arms of the receiver member to capture a spinal fixation element within the receiver member and fix the spinal fixation element with respect to the receiver member. The closure mechanism includes an outer thread engageable with an inner thread on the arms of the receiver member. The inner thread and the outer thread have a thread angle between 5° and 25°.