The Memo Slide: An explorative study into a novel mechanical follow-the-leader mechanism

Abstract

Follow-the-leader propagation allows for the insertion of flexible surgical instruments along curved paths, reducing the access required for natural orifice transluminal endoscopic surgery. Currently, the most promising follow-the-leader instruments use the alternating memory method containing two mechanical memory-banks for controlling the motion of the flexible shaft, which reduces the number of actuators to a minimum. These instruments do, however, require concentric structures inside the shaft, limiting its miniaturization. The goal of this research was, therefore, to develop a mechanism conforming the principles of the alternating memory method that could be located at the controller-side instead of inside the shaft of the instrument, which is positioned outside the patient and is therefore less restricted in size. First, the three-dimensional motion of the shaft was decoupled into movement in a horizontal and vertical plane, which allowed for a relatively simple planar alternating memory mechanism design for controlling planar follow-the-leader motion. Next, the planar movement of the alternating memory mechanism was discretized, increasing its resilience to errors. The resulting alternating memory mechanism was incorporated and tested in a proof-of-concept prototype called the Memo Slide. This prototype does not include a flexible shaft, but was fully focused on proving the function of the alternating memory mechanism. Evaluation of the Memo Slide shows the mechanism to work very well, being able to transfer any planar path that lays within its physical boundaries along the body of the mechanism without accumulating errors.

Keywords: Natural orifices transluminal endoscopic surgery; follow-the-leader; minimally invasive surgery; pathway surgery; surgical instruments.

Figure 1.

Flexible surgical instrument and follow-the-leader propagation. (a) The MultiFlex, an Ø5-mm flexible instrument with 10 degrees of freedom. The instrument is controlled by manually adjusting the shape of the handle, which is mimicked by the flexible part of the shaft. (b) Snake moving through a cluttered environment by transferring a curved path initiated by its head along its body. This motion is referred to a follow-the-leader propagation.

Figure 2.

Schematic 2D drawing showing the functioning of the HARP consisting of two concentrically placed flexible arms which can both be independently locked.^, (a) Inner-arm locked and outer-arm made flexible, (b) Flexible outer-arm advanced forward while steering, (c) Outer-arm locked and inner-arm made flexible, (d) Inner-arm catches up with outer-arm. For explanation, see text.

Figure 3.

Cable-ring mechanism. Left: schematic cross-sectional view of a cable-ring mechanism. Cables are enclosed in a ring by an inner-spring and an outer-spring. Right: schematic side view of a cable-ring segment in bent position. Assuming that the centerline (S) does not change length, the absolute length change (|ΔL|) of antagonist cables is equal.

Figure 4.

Two-dimensional schematic representation of cable control for follow-the-leader (FTL) propagation. Arrows with an “S” stand for a steering action. Arrows with an “A” stand for an advancing action. Labels (a) to (f) inclusive indicate a sequence of configurations, which are further explained in the text. The brace is used in Figure 5.

Figure 5.

The instrument of Figure 4 drawn without the shaft with the addition of an alternating memory (AM) mechanism. The figure zooms in on the advancing action above the brace of Figure 4. For explanation, see text.

Figure 6.

The proof-of-concept prototype MemoSlide: (a) schematic representation with the control-point now supported by the leader and follower-elements. (b) Close-up of the control-points and wedge-shaped grooves in the MemoSlide. (c) Top view of the MemoSlide showing all its components. The steering wheel is used for the steering action of Figure 4, moving the leader-element to the left of right. The crank is used for the advancing action of Figure 4, rotating the cams that regulate the locking of the elements and the sliding motion of the memory-bank. The sliding motion of the memory-bank is initiated by the height profile on the circumference of the left cam pushes the left locking-bar downward. (d) 3D View of the cam-driven discrete locking system. (e) The oval-shaped grooves in the cams regulate the lowering and raising of the locking-bars, respectively, resulting in the locking and releasing of the corresponding elements.

Figure 7.

Finished MemoSlide: (a) close-up of the interaction between the memory-elements (left) and the follower-elements (right), each follower-element containing two ball-bearings, one at the top and one at the bottom, the bottom one interacting with the wedge-shaped grooves. (b) Photo of the entire prototype with the top ball-bearings of the follower-elements guided through slots in a bronze plate and displaying—as an example—a triangular path. (c) Top view of the MemoSlide showing a triangular path propagating through the mechanism. A video of the functioning MemoSlide can be found alongside the digital version of this article.

Figure 8.

The MemoSlide as a surgical instrument: (a) schematic representation of how the AM mechanism can be integrated into an instrument. The AM mechanism and shaft are placed on a cart, which runs over a gear rack that is fixed to the ground. Cables connect the segments of the flexibles shaft to the leader and follower-elements as was illustrated in Figure 4 (cables not drawn). At the top, the leader segment is steered using a joystick. In the middle, the entire instrument is pushed one segment length forward while the memory-bank remains stationary to the ground (illustrated by the cross) and updates the shape of the shaft. At the bottom, the memory-bank slides back and the instrument regains its initial configuration. The transitions between the images represent steps 1–4 of Figure 5. (b) Artistic impression of an instrument capable of follow-the-leader propagation in 3D. Based on the MemoSlide, the instrument incorporates two 2D alternating memory (AM) mechanisms that separately control the movements in the vertical and horizontal plane. Steering the instrument is realized by a joystick, which connects to both AM mechanisms.