Foot and ankle surgery (+/– saphenous nerve block).
The sciatic nerve arises from the lumbar and sacral plexus (L4- S3). It enters the thigh between ischial tuberosity and the greater trochanter and then descends the posterior thigh lying superficial to adductor magnus and deep to biceps femoris, semitendinosus and semimembranosus muscles. As it enters the popliteal fossa it is joined by the popliteal artery. The sciatic nerve divides into the tibial nerve and common peroneal nerve within 10 cm of the popliteal crease in 90% of individuals.
Popliteal vessels (vein superficial to the artery), biceps femoris.
This block can be performed with the patient in either:
1. The Sims’ position with the operative side up (lateral decubitus position, with the upper leg slightly flexed at the hip and knee).
2. The prone position.
3. The supine position with the hip and knee flexed and the heel resting on the bed.
Figure 1: Patient, transducer and needle positioning for popliteal fossa block in the a) supine position with in-plane b) lateral position with in-plane and c) prone position with an out-of-plane needle technique.
Place the transducer transversely across the popliteal fossa, just above the popliteal crease. Identify the popliteal artery. Gentle pressure will identify the popliteal vein lying superficial to the artery. The tibial nerve will be seen as a very bright echogenic structure superficial to the vein. Tilting the transducer caudally / cephalad will improve visualisation. Scanning laterally, the common peroneal nerve can be found underneath the medial border of biceps femoris (Figure 2). Scanning proximally, follow the tibial nerve towards the apex of the popliteal fossa. Here you will see the common peroneal nerve joining the tibial nerve to form the sciatic nerve (Figure 3).
Figure 2: a) ultrasound image of the tibial and common peroneal nerve just proximal to the popliteal crease b) annotated image (CP=common peroneal nerve; TN=tibial nerve; PV=popliteal vein; PA=popliteal artery; BF=biceps femoris muscle).
Figure 3: a) Ultrasound image of the tibial and common peroneal nerves in the proximal part of the popliteal fossa b) annotated image (BF=biceps femoris muscle; CP=common peroneal nerve; TN=tibial nerve; PV=popliteal vein).
Bright hyperechoic structures with distinct speckled internal architecture. The common peroneal nerve is seen lateral to the tibial nerve and is often more hypoechoic. The nerves lie superficial and lateral to the popliteal vessels. Depending on the level scanned, either the sciatic nerve (one nerve) or the tibial and common peroneal (two nerves) will be seen. The division can be identified by scanning up and down the popliteal fossa.
For an in-plane technique the needle should be inserted from the lateral aspect of the thigh. Identify the needle position beside sciatic nerve. The block should be performed at the level where the nerves are best identi ed either as a single (one nerve) or double (two nerves) injection technique. Inject 2 ml of local anaesthetic and observe spread surrounding the nerve. Complete the injection with 10 ml per individual nerve or 20 ml for sciatic nerve.
1. The sciatic nerve can display significant anisotropy. Caudal and cephalad angulation of the transducer can significantly improve the ultrasound view if it is initially poor.
2. The sciatic nerve is a large nerve and the time to onset of the block can take much longer than for a small peripheral nerve.
3. The sciatic nerve is comprised of two distinct anatomical entities (the tibial and common peroneal nerves), enclosed within a common fascial sheath (mesoneurium). A needle placed beneath the mesoneurium of the sciatic nerve will still be outside the epineurium of the tibial and common peroneal nerves. Injection within the mesoneurium gives a very distinct scan appearance and is associated with ‘rapid onset, low volume block’. This technique is commonly used for catheter placement
4. Be careful not to pierce the biceps femoris tendon when using the lateral to medial in-plane needle approach as this can be very painful. Palpate the tendon before needle insertion and decide whether to pass anterior or posterior to it.
Plan B/C/D blocks for the lower limb-foot and ankle include: Ankle blocks and proximal sciatic nerve blocks.
More information about the popliteal block and analgesia for knee surgery can be found at the excellent ATOTW article here
Text and images have been reproduced from the 2nd edition of the RA-UK Handbook, which can be purchased on Amazon, or is received on joining RA-UK. This excellent resource also contains practical descriptions of all of the advanced blocks referenced in the editorial.