Longitudinally-adjustable bone anchors and related methods

Longitudinally-adjustable bone anchors and related methods are disclosed herein. The ability to adjust a bone anchor longitudinally can allow the surgeon to bring an implanted bone anchor up to the rod instead of or in addition to bringing the rod down to the bone anchor, which can simplify or eliminate the rod contouring step and reduce or eliminate reduction forces. For example, the surgeon can use a pre-bent rod or put "ideal contours" into a rod, lay the rod across a series of bone anchors, and adjust each bone anchor longitudinally to meet the rod. As another example, coarse adjustment of the fixation system can be achieved by contouring the rod and then fine adjustments can be made by bringing each bone anchor up or down to the rod. Various adjustment mechanisms are disclosed, including bone anchors with telescoping portions and bone anchors with risers or spacers.