To the Editor:
We read with interest the first human randomized, controlled trial of the “Suture-method versus Through-the-needle Catheters for Continuous Popliteal-sciatic Nerve Blocks” by Finneran et al.1 The technique was developed and first demonstrated in cadavers by Rothe et al. for popliteal sciatic block in prone position.2,3 The method relies on the curvature of the needle and enough space available for it to reliably exit and not in close proximity to the surgical field or other vital structures.
Unfortunately, a successful new catheter system needs to have a design that fits all the nerve block locations and all patient sizes, and this one suffers from numerous drawbacks (pertaining to infection risk, challenging nerve locations, etc.), which the authors have already pointed out in great detail.1–3
However, the ultrasound visualization of a curved needle has not been addressed. The authors mention that curved needles are better visualized than straight needles, which in our opinion is an inaccurate statement. The current ultrasound systems utilized for placement of nerve blocks are able to provide two-dimensional cross-sectional images of the tissues. As a result, so long as the needle is inside this imaging plane, it will be visible.4 However, various factors such as lateral forces exerted on straight needle tip leading to curvature may lead to an unpredictable needle trajectory which cannot always be assumed to be straight or even in a single two-dimensional plane.4 The needle might then be viewed in bits such that the tip at the end of the high-contrast shaft may be obscured by shadows or poor reflection back to the probe.
Complex needle segmenting/needle enhancing algorithms are available to visualize straight needles, and more advanced algorithms exist which can enable curved needles visualization with two-dimensional ultrasounds but not available routinely.5 These algorithms, even if available, often fail if the curvature is excessive.5,6 Similarly, the echogenic catheter mostly curved may pose similar challenges to visualize and reposition. Rothe et al. describe that manipulating the curved needle facilitates the tip to move in an arc somehow facilitating needle visualization, but they provide no evidence of this being true.3 It’s no wonder, then, that Finneran et al. struggle to visualize the needle clearly at meager depth of possibly just a few (2 to 4) centimeters as evident in their figure 3.1
We commend the authors for successfully demonstrating the utility of a somewhat simpler nerve catheter system; however, this system is far from perfect in design, ergonomics, or ease of placement.
The authors declare no competing interests.