This application describes surgical instruments and implants, including a rod reduction instrument, for building a posterior fixation construct across one or more segments of the spinal column. Extension guides are provided that attach to bone anchors implanted within the spine. The extension guides have a guide channel that align with a rod channel in the anchor to help direct the rod to the anchor. The rod reducer may be passed through the interior of the guide assemblies with a distal end arranged in a first position allowing the distal end to pass through the guide. Once through the guide the distal end may be expanded to provide dual contact on the rod at each end of the bone anchor rod channel.