A simple insertion technique to reduce the bending of thinbevel-point needles

Abstract

Objective: Needle insertion is a common component of most diagnostic and therapeutic interventions. Needles with asymmetrically sharpened points such as the bevel point are ubiquitous. Their insertion path is typically curved due to the rudder effect at the point. However, the common planned path is straight, leading to targeting errors. We present a simple technique that may substantially reduce these errors. The method was inspired by practical experience, conceived mathematically, and refined experimentally. Methods: Targeting errors are reduced by flipping the bevel on the opposite side (rotating the needle 180° about its axis), at a certain depth during insertion. The ratio of the flip depth to the full depth of insertion is defined as the flip depth ratio (FDR). Based on a model, FDR is constant 0.3. Results: Experimentally, the ratio depends on the needle diameter, 0.35 for 20Ga and 0.45 for 18Ga needles. Thinner needles should be flipped a little shallower, but never less than 0.3. Conclusion: Practically, a physician may expect to reduce ∼80% of needle deflection errors by simply flipping the needle. The technique may be used by hand or with guidance devices.

Keywords: Needle deflection; needle insertion; needle model; needle steering.

Figure 1

(a) Bevel point needle inserted to a depth, rotated 180°, and inserted further, (b) needle path that returns the needle point on the straight trajectory at the depth of the target, (c) the needle flip depth ratio (FDR) exhibits very little variation (0.293 – 0.300) for a wide range of its two parameters x₂ and R.

Figure 2

The setup: (a) experiment box and (b) targeting error measurement example, (c) box with ex-vivo tissues fixed in gelatin (deepest side clear for measurements).

Figure 3

Two photos of an 18Ga needle inserted in gelatin: (a) at the flip position x₁, and (b) at the final depth of insertion x₂.

Figure 4

FDR coefficient as a function of the needle cross section moment of inertia, $I_y=\frac{\pi d^4}{64}$, where d is the needle diameter.