Surgical instrument and positioning method

A surgical instrument having an anchoring member securable to an anatomical structure and an implement portion which is adjustably repositionable relative to the anchoring member. The implement portion is selectively translatably adjustable and selectively rotatably adjustable relative to the anchoring member. The implement portion may be selectively translatable along and/or selectively rotatable about at least two substantially perpendicular axes. The implement portion may be selectively translatable about three substantially perpendicular axes and/or selectively rotatable about three substantially perpendicular axes. The instrument may also include at least one reference element to facilitate the registration of the implement portion in a computer implemented guidance system. The reference element may be three non-linearly positioned elements disposed on a member which is removably mounted to the instrument. The implement portion may be a resection guide and the instrument may be used in the resection of a tibia when implanting a prosthetic knee joint.