Abstract
Steering of needles involves the planning and timely modifying of instrument-tissue force interactions to allow for controlled deflections during the insertion in tissue. In this work, the effect of tip shape on these forces was studied using 10 mm diameter needle tips. Six different tips were selected, including beveled and conical versions, with or without pre-bend or pre-curve. A six-degree-of-freedom force/torque sensor measured the loads during indentations in tissue simulants. The increased insertion (axial) and bending (radial) forces with insertion depth — the force-displacement slopes — were analyzed. Results showed that the ratio between radial and axial forces was not always proportional. This means that the tip load does not have a constant orientation, as is often assumed in mechanics-based steering models. For all tip types, the tip-load assumed a more radial orientation with increased axial load. This effect was larger for straight tips than for pre-bent or pre-curved tips. In addition, the force-displacement slopes were consistently higher for (1) increased tip angles, and for (2) beveled tips compared to conical tips. Needles with a bent or curved tip allow for an increased bending force and a decreased variability of the tip load vector orientation.
Highlights
It was assumed that the resultant force acting on the needle tip plays an important role in the direction and extent of needle steering, even though the needle shape is a result of loads acting along the full needle length
Current mechanics-based steering models often use a constant tip load, which relates to the tissue elasticity and rupture toughness[9], with a fixed orientation, e.g. orthogonal to the bevel[4]
Evaluating the tip load for other tip shapes would allow for extensions of current models to other steering techniques and needle designs
Summary
Results showed that the ratio between radial and axial forces was not always proportional This means that the tip load does not have a constant orientation, as is often assumed in mechanicsbased steering models. The tip-load assumed a more radial orientation with increased axial load This effect was larger for straight tips than for pre-bent or pre-curved tips. Needles with a bent or curved tip allow for an increased bending force and a decreased variability of the tip load vector orientation. The occurrence of needle deflections in clinical practice is typically attributed to unbalanced force interactions at the tip This imbalance occurs during the insertion phase and results from the needle itself, which may be asymmetric in shape, or from uneven properties or boundary conditions within the tissue
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