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Technical Briefs

# Steerable Mechanical Joint for High Load Transmission in Minimally Invasive Instruments

[+] Author and Article Information
Tin Yan Nai

Department of Biomechanical Engineering, Faculty of Mechanical, Maritime and Materials Engineering,  Delft University of Technology, Delft, The Netherlandstimnai@gmail.com

Just L. Herder

Department of Biomechanical Engineering, Faculty of Mechanical, Maritime and Materials Engineering,  Delft University of Technology, Delft, The Netherlandsj.l.herder@tudelft.nl

Gabriëlle J. M. Tuijthof1

Department of Biomechanical Engineering, Faculty of Mechanical, Maritime and Materials Engineering, Delft University of Technology, Delft, The Netherlands;  Orthopedic Research Center Amsterdam, Department of Orthopedic Surgery, Academic Medical Centre, Amsterdam, The Netherlandsg.j.m.tuijthof@tudelft.nl

1

Corresponding author.

J. Med. Devices 5(3), 034503 (Sep 30, 2011) (6 pages) doi:10.1115/1.4004649 History: Received December 16, 2010; Revised June 21, 2011; Published September 30, 2011; Online September 30, 2011

## Abstract

As minimally invasive operations are performed through small portals, the limited manipulation capability of straight surgical instruments is an issue. Access to the pathology site can be challenging, especially in confined anatomic areas with few available portals, such as the knee joint. The goal in this paper is to present and evaluate a new sideways-steerable instrument joint that fits within a small diameter and enables transmission of relative high forces (e.g., for cutting of tough tissue). Meniscectomy was selected as a target procedure for which quantitative design criteria were formulated. The steering mechanism consists of a crossed configuration of a compliant rolling-contact element that forms the instrument joint, which is rotated by flexural steering beams that are configured in a parallelogram mechanism. The actuation of cutting is performed by steel wire that runs through the center of rotation of the instrument joint. A prototype of the concept was fabricated and evaluated technically. The prototype demonstrated a range of motion between −22° and 25° with a steering stiffness of 17.6 Nmm/rad (min 16.9 – max 18.2 Nmm/rad). Mechanical tests confirmed that the prototype can transmit an axial load of 200 N on the tip with a maximum parasitic deflection of 4.4°. A new sideways steerable mechanical instrument joint was designed to improve sideways range of motion while enabling the cutting of strong tissues in a minimally invasive procedure. Proof of principle was achieved for the main criteria, which encourages the future development of a complete instrument.

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Copyright © 2011 by American Society of Mechanical Engineers
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## Figures

Figure 1

(a) The circular shaft (transparent) with handle and instrument tip demonstrates the concept of the sideways steerable instrument using a parallelogram configuration and two tape springs (flexural steering beams (FSBs)). (b) Cross section of one compliant rolling element (CORE) that is flanked by two FSBs and is surrounded by a spring (the steel wire is omitted). (c) Cross section B-B is the transverse cross section of the shaft showing a stacked pair of monolithic layers in the center which are flanked by the two FSBs, and the steel wires; and surrounded by the circular shaft. All relevant dimensions are indicated (values are available in Table 2).

Figure 2

Exploded view of all parts of the prototype. Photographs top left: Fully assembled prototype where two steel tubes fixate a spring mounted around a CORE. Two assembled monolithic layers zoomed in at one CORE, where two FSBs are welded to the sides and the steel wire runs through the holes in the clamps. An existing instrument tip mounted on the prototype for demonstration. Photograph bottom right: Magnified photograph of two stacked COREs with crossed flexures tcore of 0.028 mm.

Figure 3

Experimental set up in two configurations. (a) Set up to measure RMtip , K, and Klocked . The instrument tip is fixed within in the semicircular disk. The steel cable is attached to the outer surface of the disk and to the force sensor. A piston pulls the steel cable. Simultaneously displacement is measured with the magnetostrictive position sensor. (b) Set up to measure ϕp . The mass runs along a pulley and is connected to the steel wire, which runs superiorly through the prototype and runs back inferiorly. The prototype is clamped in its greatest steering angle ϕtipmax . The displacement between unloaded and loaded instrument tip is measured with a laser displacement sensor (see enlarged drawing where ϕp is indicated).

Figure 4

Results of the moment- angular deflection measurements from which K and Klocked where determined. Angular deflection of the instrument tip caused by an applied moment in (a) a free mode and (b) a locked mode.

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