Force Feedback In Surgery: Physical Constraints and Haptic Information

author:Christopher Robert Wagner
adviser:Robert D. Howe
degree: Ph.D.
institution: Harvard University

Force feedback is widely assumed to enhance performance in robotic surgery, but its benefits have not yet been systematically assessed. Further, the implementation cost of force sensors that allow force feedback is prohibitive, due to the stringent design requirements imposed by the surgical environment. In this dissertation, we address both sides of this cost/benefit analysis of force feedback. We proposed a novel force sensor design targeting force feedback in surgery, as well as investigate the specific benefits allowed by force feedback.

We demonstrate the benefit of force feedback in surgery through a series of psychophysical experiments. By investigating performance on tasks with and without force feedback, we find that the primary benefit of force feedback is that interaction forces are reduced. This results in an increase in patient safety, because high forces correlate directly with tissue trauma. Two mechanisms enable force reduction and other benefits: 1) force feedback transforms environmental interaction forces into mechanical constraints and 2) forces act as a source of information to the surgeon. Mechanical constraints passively reduce intrusions into environmental structures (and, thereby, forces) due to the interactions of the compliance of the hand and the stiffness in the environment. Because this benefit is completely passive, it happens without cognitive response by the user. Accordingly, these benefits occur instantaneously, on the time scale of mechanical interactions. Force feedback also allows additional manipulation strategies that take advantage of these physical interactions, potentially reducing mental workload of the surgeon. We also find that force feedback provides information to the surgeon. While the number of ways that forces can potentially inform surgeons is large, the interaction between training and other sources of information is complex. We find that training is necessary, in some contexts, to take full advantage of the informational benefits of force feedback in surgery.

We propose a three axis force sensor design using strain gages and the Shape Deposition Manufacturing (SDM) technique, where components are embedded inside a pourable epoxy. The performance of the SDM based sensor (0-2 N range, 0.15 RMS calibration error, 0.15 N drift over five minutes) is comparable to a similarly sized metal element sensor built using standard strain gage design techniques. The resulting three axis sensor is small enough for minimally invasive surgical tasks, waterproof, and insensitive to temperature changes. Adapting the SDM design for mass production is straightforward because no machining is required of the SDM sensors (sensors are cast in reusable molds), resulting in a high performance, low cost, disposable sensor.

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