Background: Delft University of Technology has developed two working prototypes of flexible instruments for surgery, together with a master-slave system. The master-slave system is compatible with the simple parallel cable configuration and the more complex parallel and diagonal cable configuration. It is assumed that the instrument with a complex cable configuration requires fewer segments, compared to an instrument with a simple cable configuration, to cover a complex path, with the same accuracy.
Study design: research
Methods: The two different cable configurations are modeled. The model will first determine the shapes one segment can form. Afterwards, the possible shapes of the segments are combined to judge the performance of the flexible instruments with an algorithm. The algorithm is purely kinematic and does not take forces into account.
Results: The results show a strong preference for the cable configuration with parallel and diagonal cables. This configuration needs fewer degrees of freedom to reach the same error.
Conclusion: Therefore it can be concluded that the cable configuration with parallel and diagonal cables is more promising in combination with the predesigned master-slave system.

Flexible steerable needles can follow curved paths inside the human body, allowing to steer around objects, to adjust for mispunctures, and to reach multiple targets. However, the designs of existing ultra-thin (diameter < 1 mm) steerable needles require axial rotation of the needle to steer in 3D space causing needle twisting, which results in difficulties in needle trajectory control. Therefore, the goal of this paper is to design and experimentally evaluate an ultra-thin steerable needle, which can steer omnidirectional without the need of axial rotation.This study describes a new way of ultra-thin needle steering by a retractable steering mechanism inside a flexible cannula. The steering mechanism prototype consists of three Nitinol wires with a small custom-made curvature at the tip to increase the internal bending moment. Pulling one or multiple elements results in reorientation of the tip , thereby allowing to steer without axial rotation of the needle. An experimental prototype was made, which includes the needle together with a control unit. The final needle prototype has a total of three elements to steer in three-dimensional space and has an outer diameter of 0.5 mm.The prototype was tested in porcine gelatine phantoms (10 wt%) to evaluate the performance of the needle in terms of steering direction and curvature. The needle was able to steer towards the direction of the retracted element. Increasing the level of element retraction resulted in a higher steering curvature.The maximal median deflection was 62.76 mm for an insertion of 65 mm into the gelatine. The proposed needle showed good potential in omnidirectional steering with high curvature and it is able to follow multi-curved trajectories. Future improvements can be done to refine the design and make it user-friendly. Possibly, this will allow the proposed needle to compete with ultra-thin standard bevel-tip and pre-curved needles.