Laparoscopic instruments can be differentiated into rigid and steerable. The rigid ones comprise an inflexible shaft equipping them with four degrees of freedom (DOF): axial sliding, axial rotation and pivoting in two perpendicular planes around the incision point. This fulcrum effect greatly restricts the range of motion and limits the surgeon mainly to frontal or sideways approach to the tissue. In contrast, steerable instruments have additional degrees of freedom due to one or more joints in the tip, enabling the surgeon to reach behind or over obstacles [1].

A state-of-the-art example of a steerable instrument is Intuitive Surgical's EndoWrist made for the da Vinci surgical robot and favorably used in prostatectomy procedures, Fig. 1. EndoWrist has two rotational DOF acting in two perpendicular planes in a form of pulleys, each individually controlled by a looped cable. The stiffness of the tip is...

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