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 to fix the spinal fixation element with respect to the receiver member. The bone anchor includes a distal shaft having a distal threaded section and a proximal threaded section. The distal threaded section has a first pitch and a first number of thread starts and the proximal threaded section has a second pitch less than the first pitch and a second number of thread starts greater than the first number of thread starts. The distal threaded section and the proximal threaded section have a constant lead.
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