Tissue motion due to needle deflection.

Various concepts of steerable needles have been developed in order to reduce placement errors during insertions and to enable complex procedures through curved trajectories in minimally invasive surgery. When inserted into soft tissue, motion of the targeted location ahead of the needle tip has to be taken into account for controlling such tools. This paper investigates the motion caused by flexible bending needles in comparison to rigid straight ones when inserted into a homogeneous tissue phantom. A laser based experimental setup is used to measure displacements of the substrate around the needles. Displacements are transformed into the local frame in order to quantify the relative substrate motion. It is shown that the radial contribution of the displacements is higher for bending needles and that this effect increases with higher path curvatures. This motion must be taken into account for controlling steerable needles along curved trajectories to reduce placement errors in applications such as multi-targeting or reinsertions in soft tissue.

Highly resolved strain imaging during needle insertion: Results with a novel biologically inspired device.

Percutaneous needle insertions are a common part of minimally invasive surgery. However, the insertion process is necessarily disruptive to the substrate. Negative side effects are migration of deep-seated targets and trauma to the surrounding material. Mitigation of these effects is highly desirable, but relies on a detailed understanding of the needle-tissue interactions, which are difficult to capture at a sufficiently high resolution. Here, an adapted Digital Image Correlation (DIC) technique is used to quantify mechanical behaviour at the sliding interface, with resolution of measurement points which is better than 0.5mm, representing a marked improvement over the state of the art. A method for converting the Eulerian description of DIC output to Lagrangian displacements and strains is presented and the method is validated during the simple insertion of a symmetrical needle into a gelatine tissue phantom. The needle is comprised of four axially interlocked quadrants, each with a bevel tip. Tests are performed where the segments are inserted into the phantom simultaneously, or in a cyclic sequence taking inspiration from the unique insertion strategy associated to the ovipositor of certain wasps. Data from around the needle-tissue interface includes local strain variations, material dragged along the needle surface and relaxation of the phantom, which show that the cyclic actuation of individual needle segments is potentially able to mitigate tissue strain and could be used to reduce target migration.