DragonFlex – Smart Steerable Laparoscopic Instrument

Abstract

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 dependent on the tensile strength of the cables and stiffness of the rigid bodies within the tip. Since both the steel cables and the rigid bodies are highly stiff, EndoWrist can be considered stiff as well. Despite its high stiffness, EndoWrist's lifespan is limited to only ten procedures [3]. The limiting factor is the fatigue resistance of the steel cables, which are guided over pulleys whose diameter is smaller than half of the tip's width. Hence, after repeatedly bending and rolling over a very small bending radius, the steel cables inevitably fail resulting in expensive replacement and maintenance costs. Moreover, the tip features a complex design of numerous miniature elements, including pulleys and rivets, leading to high manufacturing and assembly costs as well as potential sterilization issues [1].Taking into consideration the scope of the potential improvements, one would desirably aim for a design of a structurally simple steerable laparoscopic tip free from cable fatigue, while attaining sufficient bending stiffness for the surgical environment and possibly even improving on EndoWrist's maneuverability and dimensions. Considering all these requirements, at TU Delft we designed a smart laparoscopic instrument prototype named DragonFlex.In order to improve the reliability and the lifespan of cable-driven joints, their bending radius has to be maximized to the achievable limits. One straightforward approach, implemented in conventional flexible endoscopes, is guiding the cables smoothly along a series of hinged elements, together creating a sufficiently large bending radius as to avoid fatigue. Yet, such mechanisms are complex, bulky and lack stiffness due to underactuated DOF.The opposite approach, implemented in EndoWrist and in our DragonFlex design, is decreasing the number of steerable elements to the bare minimum i.e. two mutually rotatable elements per one planar rotational DOF. Once a 90 deg bend is made with only two elements, the cable bending radius gets critically small resulting in cable fatigue. However, this can be easily prevented, provided the cables are leaning over pulleys, like in EndoWrist, yet of a sufficiently large radius. The question of maximizing the pulley diameter, restricted by a constant instrument width can be resolved by splitting the circular pulley profile along the instrument's longitudinal axis into two equal axisymmetric non-tangential arcs, thus creating a leaf or drop-like shape, whose diameter can be increased beyond the instrument's width, as illustrated in Fig. 2.In our DragonFlex design, each tip element features four of these arced guides, two inner and two outer, in order to support both driving cables during the joint rotation to either side. The steerable tip elements interface in a rolling joint [4], Fig. 3, providing a moving point of rotation and thus moment arms for both cables anywhere within ±90 deg. In order to equalize the tensile forces in both cables, their moment arms were equalized as well at both straight and bent positions. A fully constrained SolidWorks drawi