Robotic instrument systems, apparatus, and methods for controllably rotating a tool or adapter coupled to a distal portion of a medical instrument such as a catheter. An interface, which may be integral with the medical instrument or a component of a separate rotatable apparatus or adapter, is operably coupled, e.g. fixedly coupled, to the distal end of the instrument. A tool, such as a rotatable portion of a collar or tool base, or a working instrument operably coupled thereto, is rotatable relative to the interface. The interface and collar have guide channels. A control element extends through the medical instrument and respective guide channels such that the tool or collar is controllably rotatable about the instrument axis by axial movement of the control element relative to the instrument.