Optimization of the flexural rigidity of a concentric pipe used in transmitting rotation for arc-shaped forceps

Arc-shaped forceps are important in pharyngeal cancer surgery, endoscopic sinus surgery, and single port surgery. Rotating the gripper of an arc-shaped forceps with respect to its shaft axis can improve the manipulability of the forceps. Concentric pipe structures consisting of inner and outer pipes are often used for this purpose. The inner pipe transmits rotations from its proximal end to distal end. However, the shape of the bent inner pipe is generally different from that of the outer pipe, and this causes large friction between the outer and inner pipes. To reduce friction, a plastic tube is often used for the inner pipe. However, since its torsional rigidity is low, this results in a large twist angle. This paper proposes a procedure to fit a metal bent inner pipe to an outer pipe by adjusting the flexural rigidity of the inner pipe with slits to reduce the friction. The procedure minimizes the twist angle between the proximal and distal end of the arc-shaped forceps by reducing the torque required to rotate its metal inner pipe. We develop a 5 mm forceps with a 3.5 mm inner pipe. The twist angle is 2.8° when a load torque of 20 Nmm is applied, which is a typical load torque for suturing, whereas the twist angle of a PEEK (polyetheretherketone) inner pipe is 24.3° in simulation. Ex vivo experiments show that the required forceps torque is small in simulated clinical scenarios.