When the Virtual Fixture concept was proposed by Rosenberg to the U.S. Air Force in 1991, augmented surgery was an example use case, expanding the idea from a virtual ruler guiding a real pencil, to a virtual medical fixture guiding a real physical scalpel manipulated by a real surgeon. The objective was to overlay virtual content upon the surgeon's direct perception of the real workspace with sufficient realism that it would be perceived as authentic additions to the surgical environment and thereby enhance surgical skill, dexterity, and performance. A proposed benefit of virtual medical fixtures as compared to real hardware was that because they were virtual additions to the ambient reality, they could be partially submerged within real patients, providing guidance and/or barriers within unexposed tissues.