A screw system includes a screw and a washer assembly captive to the screw. The washer assembly is polyaxially pivotable relative to the screw. The screw may be freely rotated in one direction relative to the washer assembly, but frictionally binds with the washer assembly when rotated in a second direction. A cannula insertion system includes an instrument and a cannula. The instrument may include a dilator. The cannula insertion system may have a locked setting in which the dilator is fixed relative to the cannula, and an unlocked setting in which the dilator is slidable relative to the cannula. A surgical method uses the cannula insertion system to implant the screw system.
