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To the Editor:—
The sciatic nerve is one of the first nerves reported to be scanned with ultrasound.1However, its sonographic visibility can be challenging in many patients, especially those with advanced age or obesity.2In the popliteal fossa, differentiation of the sciatic nerve from adjacent muscle and adipose tissue, as well as the detection of separation into its tibial and common peroneal components, demands substantial skill and experience. Although generally considered static structures, peripheral nerves can move within the body. Nerves normally adapt to changes in bed length resulting from limb movement by path straightening and fascicle stretching.3,4Although nerve motion associated with upper extremity movement has been described with ultrasound,4,5there are no such reports for the lower extremity. Here we utilize dynamic scanning (ultrasound imaging during extremity movement) to identify the tibial and common peroneal components of the sciatic nerve in the popliteal fossa. Rotational and rocking motions of the sciatic nerve and its more distal components in the popliteal fossa greatly enhance the sonographic differentiation of these nerves from their surroundings.
With institutional review board approval, we reviewed ultrasound clips of the popliteal fossa. We imaged the sciatic nerve using a compact (26-mm footprint) linear transducer (15L8s at 14 MHz) and an Acuson Sequoia C256 ultrasound machine (Siemens Medical Solutions, Mountain View, CA). In short axis (transverse cross-sectional) scanning, we observed a reproducible external rotation of the sciatic nerve during active or passive dorsiflexion of the foot with the knee in full extension (35 ± 21 degrees, mean ± SD, n = 10 legs; fig. 1). During dorsiflexion of the foot, the tibial component of the sciatic nerve moved towards the posterior surface of the leg (fig. 1A). During plantarflexion, the common peroneal component of the sciatic nerve moved towards the posterior surface (fig. 1B). We noted these observations in 10 individuals, imaged with and without foot movement (table 1). Foot movement improved visibility of the sciatic nerve (P < 0.01, Wilcoxon signed-rank test). We call the component movements the “seesaw” sign because of the alternating tilt motion.
Movement of the tibial and common peroneal components of the sciatic nerve occurs independently of adjacent structures and proximal to the origins of muscles that control foot movement. Nerve movement relates to the position of these nerves relative to the axis of joint movement. The tibial nerve lies dorsal to the axis of the talocrural joint and is therefore stretched during dorsiflexion. The end branches of the peroneal nerves lie ventral to the axis of the talocrural joint and are therefore stretched during plantarflexion. Additional hip flexion during foot flexion intensifies the seesaw sign but does not alone result in sciatic nerve movement. In long axis (longitudinal) imaging, the tibial component of the sciatic nerve clearly stretches towards the foot during dorsiflexion in movement that corresponds to nerve sliding and elongation.5
The easiest patient position to elicit the seesaw sign is prone, with feet hanging over the end of the operating table. Inversion and eversion of the foot provoke similar, but less pronounced, nerve motions as those obtained with dorsiflexion and plantarflexion. Neither isometric foot flexion nor foot flexion in the presence of a flexed knee provoke the seesaw sign. Color Doppler imaging does not improve sonographic visualization of the seesaw sign because the velocity of nerve movement is near the lower limit of detection for this technique.4
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The surface movements of the tibial and common peroneal nerves occur in an antagonistic way, thereby causing the seesaw sign. In the thigh, the two components of the sciatic nerve are analogous to two adjoining ropes: one being strained while the other relaxes. Imaging the proximal popliteal fossa shows these nerves move in concert with more rotational direction, presumably indicating the presence of a common epineural sheath.6The tibial nerve component moves more extensively than the common peroneal nerve component, causing a stronger rotation of the sciatic nerve during dorsiflexion.
In the upper extremity, straightening of the median nerve path from wrist extension induced strain results in movement to the anterior surface of the forearm.7No transmission of stretching forces to the upper arm occurs until the median nerve is under significant tension in the forearm.4Consistent with this observation, the seesaw sign is seen only with the knee in full extension.
Nerve stimulation is a common approach to sciatic nerve block in the popliteal fossa. These blocks have a variable execution time and success rate. Anatomic variation in the level at which the sciatic nerve divides into its two components is a possible cause of incomplete blockade with this blind technique,6,8leading to efforts to improve sciatic nerve blockade using ultrasound imaging.9,10
Independent nerve movement may be valuable in identifying neural structures with ultrasound. Using dynamic scanning, we find that both the tibial and common peroneal components of the sciatic nerve can clearly be identified in real time. Although the seesaw sign may be of limited value in patients with reduced mobility of the foot at the ankle joint, it can be applied to other patients requiring foot and ankle surgery. Sciatic nerve movement is thus a useful tool to improve the feasibility and success of sciatic nerve blockade.
* University of California, San Francisco, San Francisco General Hospital, San Francisco, California. email@example.com