A method of attaching an extension to a bone anchor for use in percutaneous spinal surgery is provided that comprises the steps of: providing a bone anchor including a first portion for attachment to a vertebra and a second portion for attachment to a connecting element, the bone anchor including an extension coupling surface； providing an elongate hollow extension having a distal end and a proximal end, the extension having a rotationally movable anchor engaging member projecting radially from the distal end thereof； placing the distal end of the extension adjacent the bone anchor； and rotating the anchor engaging member to engage the extension coupling surface on the anchor and couple the extension and the bone anchor.
