Surgical robotics and instrumentation

Purpose Microsurgery requires elaborate and precise operation under a microscope. We previously designed a master–slave robotic system to assist in micro-neurosurgery [1]. Anastomosis of 0.3 mm artificial blood vessels, which is very challenging with manual operation, was successfully demonstrated with the robotic system. However, the task completion time was longer than that of manual microsurgery because of the high motion scaling ratio (1/7) and poor usability of the master manipulators. Master–slave robotic systems developed for microsurgical applications employ a high motion scaling ratio (1/7–1/20) to perform fine tasks, but such a high motion scaling ratio requires large motions of the operator’s arms and hands. Such systems cannot sufficiently benefit from microsurgeons’ skills because they move their fingers instead of their arms during surgery. To enhance system usability and shorten task completion time, we designed a master manipulator that can be operated without moving of the operator’s arms and at a low motion scaling ratio of 1/3. Methods In this paper, we define the usability of master manipulators as the intuitiveness of the user interface and motion stability. The intuitiveness of the user interface is essential for surgeons to utilize their manual surgical skills in robotic surgery. Motion stability, which also refers to the robustness against hand tremors and other involuntary hand motions, can also play a major role in master manipulator usability. The slave manipulator of our microsurgical system [1] was designed to achieve a positioning accuracy of 20 lm, which is sufficient for the anastomosis of 0.3 mm blood vessels. To fully benefit from the slave manipulator’s accuracy at a motion scaling ratio of 1/3, the positioning accuracy of the master manipulator needs to be 60 lm or smaller. The positioning accuracy of the master manipulator developed in [1] was 300 lm because it was designed for use at a high motion scaling ratio (1/10–1/20). Thus, the master manipulators were replaced with new master manipulators having higher positioning accuracy: two sets of PHANTOM Premium 1.0 (Sensable, USA) platforms, each with a customized stylus. The stylus was designed to provide a similar operational feeling as manual surgery and assure motion stability while being held. We first analyzed how surgeons hold tweezers in manual surgery and designed the stylus to replace the finger position. The designed stylus can be gripped with three digits (thumb, index finger, and middle finger) with additional support by the thumb cleft, which provides surgeons with a similar operational feeling as in manual surgery. The grasping motion of the robotic forceps can be commanded by grasping the stylus handle with the thumb and middle finger. The rod of the stylus was determined to be 15 mm based on preliminary test results; it allows for steady contact with fingertips and facilitates stable pose and position changes while a secure grip is maintained. Figure 1a shows the designed stylus. The new master manipulator was developed by attaching gimbals and a stylus to the PHANTOM Premium 1.0. The gimbals were designed for pose measurement (three degrees of freedom), and encoders with an accuracy of 0.005 were mounted. The master system overview is shown in Fig. 1b. Results To validate the proposed master manipulator, we designed two experimental tasks: pointing and tracing. The subjects were one expert neurosurgeon and seven engineering students. Each subject used the master manipulator to move the micropipette attached to the distal end of the slave manipulator, and the motion of the micropipette’s tip was measured. The magnification of the microscope was set to view a 2 mm 9 2 mm square. Each task was performed under three conditions: the previous master manipulator with the motion scaling ratios of 1/3 and 1/7 and the new master manipulator with the scaling ratio of 1/3. The previous master manipulator was tested to demonstrate the effects of the design improvement and positioning accuracy enhancement. To compare the performances of robotic and manual operations, the same tasks were manually performed using the same experimental setup (Fig. 2). Pointing task Each subject was asked to position the micropipette at an intersectional point of crossed lines printed on paper and maintain its position for 10 s. The RMS error was measured to evaluate the motion stability. The results showed that the motion stability was improved by 52 % compared to the manual operation; thus, the motion stability was improved. Compared to the previous master manipulator, motion stability was maintained while the motion scaling was changed to 1/3. This was primarily due to the improved positioning accuracy of the master manipulator. Fig. 1 Proposed master system for microsurgical robotic system. a Designed stylus. b Master system overview