The term “sciatic” block is a bit of a misnomer: usually the sciatic nerve (L4-S3) has already split into the common peroneal (L4-S2) and tibial nerves (L4-S3) well above the level of the popliteal fossa, though there’s a lot of variation in where this actually occurs [1] (a large minority will have it divide as high as the pelvis).
A regional block catching both nerves at this level can provide anaesthesia to the distal calf, ankle, and foot. It’s ideal for use in a penetrating injury to the lower leg, and learning to block the nerves here (or at the ankle) is an important skill, because foreign bodies or open wounds to the sole of the foot are very hard to anaesthetise in any other way, and when using long-acting local anaesthetic agents it can provide pain relief for around eight hours.
Here you can see the cutaneous sensory supply of the foot - each nerve is a branch of one of either the common peroneal (marked P) or the tibial nerve (marked T) - with the exception of the saphenous nerve (coloured purple), which is a branch of femoral nerve. A PSNB is therefore ideal for anaesthetising the majority of the foot, and when used in combination with a femoral or fascia iliaca block (which will anaesthetise the saphenous nerve) the entirety of the leg below the knee can be rendered insensate, allowing the anaesthesia of everything from fractured ankles to landmine injuries.
Anatomy:
After emerging from the greater sciatic foramen the sciatic nerve descends between gluteus maximus and the rest of the lateral rotators, then into posterior compartment of the thigh, where (usually) it will formally split at the apex of the popliteal fossa.
Above is an image of a right-sided popliteal fossa (adapted from Last’s Anatomy 12th Ed.). In the proximal half of the popliteal fossa, the textbook orientation of neurovascular structures (going from medial to lateral and conveniently also from deep to superficial) is:
The popliteal artery,
the popliteal vein,
the tibial nerve, and then
the common peroneal nerve.
Bear in mind the popliteal vein will sometimes appear as a double structure, running as a pair of venae comitantes on either side of the popliteal artery.
Preparation:
1. Do a thorough neurological exam first. This won’t be possible after the block is placed.
2. Consent them for the risks: there’s a roughly 3-10% chance of nerve injury - this is usually transient and usually only sensory (96.5%), but in rare cases can be permanent and can rarely involve motor function (3.5%) [2,3]. Otherwise remember to list the standard battery of risks including bleeding, infection, pain, failure of the procedure (the success rate is only 69-88% for ultrasound-guided PSN blocks performed by anaesthetists [4]), and of course - local anaesthetic systemic toxicity.
3. Roll the patient prone in order to expose the popliteal fossa (some alternative methods have been proposed to access the nerves from the supine position [5], though we freely admit we have no experience with this technique).
4. Prep the area with iodine or chlorhexidine (though remember, a single injection of local anaesthetic without the placement of a nerve block catheter is no different to any other intramuscular injection – where you would prepare the skin with nothing more than a 70% isopropyl alcohol wipe. We endorse using an aseptic non-touch technique, whereby the area is prepped appropriately, but we feel that sterile gloves, US-probe covers, drapes, and the other accoutrement of surgical sterility are probably unnecessary for the placement of a single block under emergency circumstances (an elective theatre setting where a catheter is placed is a different kettle of fish altogether however).
5. Draw up your long-acting local anaesthetic. We typically use 10-20mL of 0.75% ropivacaine (7.5mg/mL) for a total dose of 75-150mg which should be effective and safe for use in the average adult patient - do your own calculations if the patient is smaller than this. One study demonstrated that the dose required to achieve an effect 50% of the time (ED50) was 6mL of 0.5% (30mg), and 95% of the time (ED95) was 16mL of 0.5% (80mg) [4] - though again, this was in trained anaesthetists using ultrasound, and so we may need to rely on a using slightly higher doses and volumes for similar rates of success if the sonoanatomy is not clear.
Sonoanatomy:
This image was obtained by the author using a Butterfly iQ+ on the “nerve” preset.
For an excellent resource for the sonoanatomy of this block (and indeed all sonoanatomy for regional anaesthesia) check out the other ultrasound images published by the New York School of Regional Anaesthesia: https://www.nysora.com/ultrasound-guided-popliteal-sciatic-block/
Procedure:
1. Using POCUS, place the transducer transversely and optimise your view of the anatomical structures. The best place to actually perform the block is about 10cm above the popliteal crease [1], but you may want to first track the structures up and down to identify where they merge or bifurcate to better orient yourself. If struggling, first identify the pulsating anechoic popliteal artery. The tibial nerve will be superficial and lateral to this, and the common peroneal more superficial and lateral again. They will appear hyperechoic, as in the image above. If possible, trace them proximally until they form a peanut-like shape next to one another.
2. Standing or kneeling next to the side I am aiming to block, I usually use my dominant hand to insert the needle - this is easy when blocking one limb, and harder when blocking the other. If you are right-handed like me, one technique to get around this is to orient yourself more to the head-end of the patient when blocking the proned right leg (allowing you to hold the probe with your left hand and the needle in your right, inserting the needle into the ultrasound’s sight-picture from the right upper corner of the screen) but orienting yourself toward the foot-end of the patient when blocking the proned left leg (this allows you to always keep the same orientation of structures on the screen, and avoids ever needing to cross your hands over). Note that when the patient is supine (i.e. for a fascia iliaca block) you can do the same thing, but the sides will be reversed.
3. When infiltrating the local anaesthetic, use gentle negative pressure first to ensure no aspiration of blood, and stop if there is resistance to injection, the nerve appears to swell, or the patient complains of paraesthesia – these may indicate intra-neural injection. Aim to inject within Vloka’s sheath, utilising hydrodissection to identify what space your needle tip is in.
References:
[1] Vloka JD, Hadzić A, April E, Thys DM. The division of the sciatic nerve in the popliteal fossa: anatomical implications for popliteal nerve blockade. Anesth Analg. 2001;92(1):215-7.
[2] Park YU, Cho JH, Lee DH, et al. Complications after multiple-site peripheral nerve blocks for foot and ankle surgery compared with popliteal sciatic nerve block alone. Foot Ankle Int. 2018;39(6):731-5.
[3] Lauf JA, Huggins P, Long J, et al. Regional nerve block complication analysis following peripheral nerve block during foot and ankle surgical procedures. Cureus. 2020;12(7):e9434.
[4] Jeong JS, Shim JC, Jeong MA, Lee BC, Sung IH. Minimum effective anaesthetic volume of 0.5% ropivacaine for ultrasound-guided popliteal sciatic nerve block in patients undergoing foot and ankle surgery: determination of ED50 and ED95. Anaesth Intensive Care. 2015;43(1):92-7.
[5] Mistry T, Sonawane K, Keshri V, Balavenkatasubramanian J, Sekar C. Ultrasound-guided CAPS (crosswise approach to popliteal sciatic) block: a novel technique for supine popliteal fossa block. Cureus. 2022;14(1):e20894.