An instrument holder system comprising: a base comprising: a disk including a central bore; an annular post extending from the disk along an axis to form an annulus surrounding the central bore; and a plurality of guide slots, each of the plurality of guide slots comprising: a disk portion extending in a radial direction along the disk; and a post portion extending an axial direction along the annular post; a plurality of teeth positioned in the plurality of guide slots, respectively, each tooth comprising: a rail for movement in the disk portion; a spoke extending from the rail for movement in the post portion; and a tab extending from the rail; and a cap comprising: a cover portion configured to cover the base, the cover including an aperture to receive the annular post; and a plurality of positioning slots (304/336) disposed in the cover configured to receive the tabs of the plurality of teeth, respectively; wherein each positioning slot is disposed oblique to the radial direction such that rotation of the cap causes the rail of each tooth to move in a respective one of the disk portions of the plurality of guide slots so that the spoke of each tooth moves relative to the annulus and the system further comprises a biasing mechanism to bias the cap into a position where the plurality of teeth is withdrawn from the annulus to an inward position in order to constrict around the instrument. a base comprising: a disk including a central bore; an annular post extending from the disk along an axis to form an annulus surrounding the central bore; and a plurality of guide slots, each of the plurality of guide slots comprising: a disk portion extending in a radial direction along the disk; and a post portion extending an axial direction along the annular post; a disk including a central bore; an annular post extending from the disk along an axis to form an annulus surrounding the central bore; and a plurality of guide slots, each of the plurality of guide slots comprising: a disk portion extending in a radial direction along the disk; and a post portion extending an axial direction along the annular post; a disk portion extending in a radial direction along the disk; and a post portion extending an axial direction along the annular post; a plurality of teeth positioned in the plurality of guide slots, respectively, each tooth comprising: a rail for movement in the disk portion; a spoke extending from the rail for movement in the post portion; and a tab extending from the rail; and a rail for movement in the disk portion; a spoke extending from the rail for movement in the post portion; and a tab extending from the rail; and a cap comprising: a cover portion configured to cover the base, the cover including an aperture to receive the annular post; and a plurality of positioning slots (304/336) disposed in the cover configured to receive the tabs of the plurality of teeth, respectively; a cover portion configured to cover the base, the cover including an aperture to receive the annular post; and a plurality of positioning slots (304/336) disposed in the cover configured to receive the tabs of the plurality of teeth, respectively; wherein each positioning slot is disposed oblique to the radial direction such that rotation of the cap causes the rail of each tooth to move in a respective one of the disk portions of the plurality of guide slots so that the spoke of each tooth moves relative to the annulus and the system further comprises a biasing mechanism to bias the cap into a position where the plurality of teeth is withdrawn from the annulus to an inward position in order to constrict around the instrument.