US Patent 9155580 Multi-threaded cannulated bone anchors

A cannulated 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 first threaded section distal to a proximal head, a second threaded section distal to and adjacent the first threaded section, and a third threaded section distal to and adjacent the second threaded section. The first threaded section having a constant major and minor diameter. The second threaded section having a tapering major and minor diameter. The third threaded section having a tapering major diameter and a constant minor diameter.