Sciatic block

sciatic1

 

Introduction

SS sciatic nerve block (popliteal block) provides long lasting postoperative analgesia (24 hrs or longer) following foot and ankle surgery. Sciatic block can also be used as the primary anesthesia for this surgery; however, an onset time of up to 60 mins often precludes sciatic block use for this indication. Our preference is to perform ultrasound guided sciatic block proximal to the knee just proximal to where the nerve forms the tibial and common peroneal nerves i.e. the so called popliteal block. Compared to more proximal approaches (e.g. subgluteal block), popliteal nerve block minimises hamstring weakness, and thus patients retain the ability to flex the knee (and therefore keep the foot off the ground when mobilising). Furthermore, compared to more proximal approaches, at the popliteal block level, the nerve contains relatively more connective tissue between fascicles, 1 so inadvertent intraneural needle placement/injection should theoretically be better tolerated.


Skill level: Moderate due to the difficulty often encountered visualising the sciatic nerve.


Procedure time: 5-10 mins (5 mins for experienced practitioners)


Common indications

Common:

1.   Foot/ankle surgery (usually combined with saphenous nerve block). NB. for less painful procedures distal to the ankle, consider the shorter duration ankle block, which is not associated with significant motor block.

2.    Knee surgery (e.g. TKJR, ACLR) to control posterior knee pain and pain secondary to the hamstring donor site (usually combined with femoral +/- obturator block).


Sedation: e.g. midazolam 2 mg + alfentanil 250-500 mcg (caution not to oversedate as patients need to be placed prone).


Anatomy: The sciatic nerve arises from the lumosacral plexus (L4-S3) and runs through the buttock down the lower limb. It is the longest and widest single nerve in the human body.

Branches: Tibial and common peroneal nerve, which arise 5-10 cm proximal to the knee crease.

The sciatic nerve supplies nearly the whole of the skin of the leg, the muscles of the back of the thigh, and those of the leg and foot: the skin on the posterior aspect of the thigh and gluteal regions, as well as the entire lower leg (except for its medial aspect, which is supplied by the saphenous nerve).


Surface landmarks: mark the side to be blocked.


Needle: 8 cm 18G Tuohy epidural needle. Consider stimulating Tuohy needle if US visualisation of the nerve is difficult. Prepare a multi-orifice catheter if necessary (see perineural catheter technique).

 

Setup

1.    Linear probe e.g. Sonosite L38.

2.    8-10 cm Tuohy needle.

3.    30 mL LA filled syringe


Procedure (Video 1 and 2)

  1. First turn patient prone
  2. Under US guidance, infiltrate the skin, subcutaneous fat, and muscle along the intended needle tract down to the sciatic nerve using an in-plane needle-probe alignment, a 22-25G hypodermic needle and 10 mL lidocaine 1%.
  3. Standing on the patient's right side, needle entry point is: medial for the right leg and lateral for the left leg (Fig. 1).

 

ultrasound guided Sciatic block at the distal thigh level.

Fig. 1. Sciatic block at the distal thigh level.


4.   Nerve stimulation may be helpful during ultrasound guided sciatic block in patients in whom sciatic nerve imaging proves difficult. Set to 1-1.5 mA, pulse width 0.3 ms and 2 Hz. Aim to briefly elicit a plantar motor response or calf twitch to confirm sciatic nerve visualisation.

5.   Place the probe in the popliteal crease and visualise the popliteal artery. Look for a hyperechoic and often fasciculated area just superficial to the artery. This is the tibial nerve. Trace the tibial nerve proximally until the common peroneal becomes visible (laterally) and more proximally, the two nerves join to form the sciatic nerve. Trace the sciatic nerve proximally until the best image is obtained. 2 Optimal sciatic nerve visualisation is particularly probe orientation dependent i.e. the probe being exactly 90 degrees to the nerve, and often requires probe angulation towards the foot rather than 90 degrees to the skin as the sciatic nerve travels distally from deep to superficial at this location.

6.   With a 30 ml LA filled syringe attached directly to the Tuohy needle, advance the needle towards the deep (anterior) aspect of the sciatic nerve (injecting first deep to the nerve will move the nerve closer, rather than away from the skin).

7.   With the Tuohy needle bevel facing the nerve (i.e. facing the probe), penetrate the fascia immediately surrounding the sciatic nerve (anterior to the nerve). Contrary to popular belief, this layer is a complex fascial layer rather than the epineureum. With an 18G Tuohy needle, penetration of this fascial sheath requires a short, sharp deliberate needle movement once the needle tip is against the fascia, and is usually associated with a distinct "pop". Injection of LA should reveal a clear demarcation between injectate and nerve and/or visible separation of the surrounding fascia and nerve (Fig. 2, 3). Deposit 15 mL LA at this point.


Sciatic nerve block sonography

Fig. 2. Sciatic nerve block (popliteal block) sonographic image. LA=local anesthetic. Blue arrow=needle shaft.


sciatic block ultrasound after local anaesthetic injection

Fig. 3. Sciatic block (popliteal block) utrasound image after LA injection. Illustrates the fascial sheath and the clear demarcation between the fascia and the underlying nerve.

 

8.   Reposition needle to the posterior/superficial aspect of the sciatic nerve, preferably without necessitating a second penetration of the fascial nerve covering. This can be achieved by hydrodissecting a tract between the anterior and posterior nerve surface. Deposit the remaining 10-15 mL of LA at this point.

For sciatic catheter placement

9.   Once an appropriate needle tip position has been confirmed just posterior to the artery, but inside the fascial sheath, transfer the hand previously holding the probe to the needle hub. Disconnect syringe and advance the catheter with the catheter advancing piece sited within the needle hub. Advance catheter 3 cm beyond needle tip.

10.   Withdraw the needle over the catheter. Being stabilised by the surrounding muscle, sciatic catheters do not carry the same risk of dislocation as interscalene catheters; therefore, stabilisation of the catheter prior to fixation (by pressing catheter against the skin with finger at skin entry point) is generally not necessary.

11.   Withdraw the catheter to 1.5 cm beyond the original needle tip position (the proximal catheter of a triple orifice catheter is 1.4 cm proximal to the catheter tip) e.g. needle depth 6cm, then fix at 7.5 cm.

12.   Carefully apply medical cyanoacrylate (e.g. dermabond) to skin entry site.

13.   Apply tincture of benzoin to a 2 cm radius of skin around the catheter puncture site (improves Lockit-skin adhesion).

14.   Apply Lockit catheter fixation device

15.   It is a good idea to protect the catheter from the thigh tourniquet using gauze and tape.

16.   Dress the catheter after surgery as described in catheter fixation.


Local anesthetic regimen: 30 mL local through the needle e.g ropivacaine 0.50%.


Specific Complications: None of note.


Clinical PEARLS

1.    Needle shaft/tip visualisation during popliteal block is usually not an issue as the lateral needle entry point ensures the shaft is relatively perpendicular to the US beam. That said, "jiggling" the needle back and forth and injecting 1 mL aliquots of injectate can assist needle tip visualisation. Also consider using an echogenic Tuohy needle (e.g. Pajunk echogenic Tuohy needle).

2.    For sciatic catheter placement:

a.    This is one nerve where LA is injected through the needle prior to catheter advancement. This is because for a high success rate for popliteal block, LA needs to be placed around the nerve in at least two positions, preferably circumferentially. 3

b.    Consider using concomitant neurostimulation.

c.    Follow the general recommendations under perineural catheter technique.

d.   In our experience, a 2 mL/hr background infusion with PRN hourly 5 mL boluses of ropivacaine 0.2% (e.g. via a PainBuster elastomeric pump) provides potent analgesia.

e.    An alternative to the in-plane approach for sciatic block is the out-of-plane approach. This may be a preferable approach for deliberately injecting LA between the tibial and common peroneal nerves as a needle approach from caudad to cephalad provides a direct needle path to the space between the respective nerves. It may also be a better approach for perineural catheter placement as it facilitates orientation of the needle more in line with the sciatic nerve; and therefore, catheter advancement several cm should not result in the catheter  deviating away from the nerve (compared to the in-plane perpendicular to nerve approach). Using a cephalad to caudad needle approach will place the catheter fixation point on the distal posterior thigh away from the awkward popliteal area.

 

Videos

Video 1. Sciatic nerve block



Video 2. Ultrasound-guided sciatic nerve block – sonography.

 


References

 

1.         Moayeri N, Groen GJ. Differences in quantitative architecture of sciatic nerve may explain differences in potential vulnerability to nerve injury, onset time, and minimum effective anesthetic volume. Anesthesiology. 2009;111:1128-34.

2.         Barrington MJ, Lai SL, Briggs CA, Ivanusic JJ, Gledhill SR. Ultrasound-guided midthigh sciatic nerve block-a clinical and anatomical study. Reg Anesth Pain Med. 2008;33:369-76.

3.         Brull R, Macfarlane AJ, Parrington SJ, Koshkin A, Chan VW. Is circumferential injection advantageous for ultrasound-guided popliteal sciatic nerve block?: A proof-of-concept study. Reg Anesth Pain Med. 2011;36:266-70.