Mechanical Follow-the-Leader motion of a hyper-redundant surgical instrument: Proof-of-concept prototype and first tests

Abstract

One of the most prominent drivers in the development of surgical procedures is the will to reduce their invasiveness, attested by minimally invasive surgery being the gold standards in many surgical procedures and natural orifices transluminal endoscopic surgery gaining acceptance. A logical next step in this pursuit is the introduction of hyper-redundant instruments that can insert themselves along multi-curved paths referred to as Follow-the-Leader motion. In the current state of the art, two different types of Follow-the-Leader instruments can be distinguished. One type of instrument is robotized; the movements of the shaft are controlled from outside the patient by actuators, for example, electric motors, and a controller storing a virtual track of the desired path. The other type of instrument is more mechanical; the movements of the shaft are controlled from inside the patient by a physical track that guides the shaft along the desired path. While in the robotized approach all degrees of freedom of the shaft require an individual actuator, the mechanical approach makes the number of degrees of freedom independent from the number of actuators. A desirable feature as an increasing number of actuators will inevitably drive up costs and increase the footprint of an instrument. Building the physical track inside the body does, however, impede miniaturization of the shaft's diameter. This article introduces a new fully mechanical approach for Follow-the-Leader motion using a pre-determined physical track that is placed outside the body. This new approach was validated with a prototype called MemoFlex, which supports a Ø5 mm shaft (standard size in minimally invasive surgery) that contains 28-degrees-of-freedom and utilizes a simple steel rod as its physical track. Even though the performance of the MemoFlex leaves room for improvement, especially when following multiple curves, it does validate the proposed concept for Follow-the-Leader motion in three-dimensional space.

Keywords: Follow-the-Leader; hyper-redundant; minimally invasive surgery; natural orifices transluminal endoscopic surgery; pathway surgery; snake-like; surgical instruments; tendon-driven.

Figure 1.

Follow-the-Leader motion. (a) Artistic representation of a Follow-the-Leader instrument used to avoid obstacles. (b) Snake navigating through cluttered environment by steering its head and maneuvering its body along the created path using Follow-the-Leader motion.

Figure 2.

Schematic representation of a new approach for a Follow-the-Leader instrument. (a) The instrument containing a master that follows a physical track located outside the patient’s body, and a slave that copies the movement of the master and propagates along the path inside the patient’s body. The ‘Released’ and ‘Pulled’ arrows indicate the tension in the cables of the first segment. (b) Length behaviour of antagonist cables L1 and L2. If the length of the segment S is fixed and the cables are presumed to bend with a constant radius, the absolute length changes|ΔL1| and|ΔL2| of antagonistic cables are equal. (c) Hypothetical instrument. The cables in the master are coupled to their antagonists in the slave. A U-shaped shaft connects the master to the slave, and guides the cables.

Figure 3.

Straight configuration. (a) Improved configuration of Figure 2(c). With a straight shaft, the track and path face in the opposite direction. To allow the master to move forward over the track while the slave moves forward over the path, the shaft and cables need to extend. (b) Improved configuration of Figure 3(a) avoiding the extension of the shaft. Master and slave rigidly connected by the shaft. The track is given double the speed of the instrument to compensate for the opposite direction between track and path. (c) A mechanism of gears and gear racks pulls the track through the master at double the speed of the instrument.

Figure 4.

Three track-followers. (a)–(c) Structures to capture the shape of a track. For further explanation, see text. (d) Track-follower based on three helices conforming to the shape of a Ø3 mm track represented by a bent steel rod. 3D printed from the flexible material ABFlex on an Envisiontec Perfactory 4.

Figure 5.

Computer-aided design (CAD) drawings of PoC instrument. (a) The master consisting of the helical track-follower and an axial and torsion stiff exoskeleton that fixates and guides the cables. (b) The construction of the Ø5 mm, 112 mm long slave containing 14 segments providing a total of 28-DOFs. (c) CAD drawing of the completed MemoFlex prototype. The cable ring diameter in the master (Ø18 mm) is four times larger than the cable ring diameter in the slave (Ø4.5 mm). This results in an amplification factor four between the motion of master and slave, for example, a 5° bend in the master will result into a 20° bend in the slave.

Figure 6.

Proof-of-concept prototype; the MemoFlex. All parts were fabricated out of aluminium, except for the wheels (nylon), gear rack (stainless steel) and gears (brass). The outside of the shaft is transparent to allow visual feedback of the cables during their assembly.

Figure 7.

Evaluation of the MemoFlex showing the movement of the slave. (a) From left to right, the slave following a C-curve with a radius of ±58 mm and a footprint of ±3d_s, and S-curve with a step of ±35 mm and a footprint of ±8d_s. The orange striped line indicates the path. (b) The slave following a S-curve while supported by the insert guide with a step of ±35 mm and a footprint of ±5d_s.