Anatomy

Level of difficulty

  • Intermediate

Anatomy

  • The sciatic nerve is the largest nerve in the body. It is derived from the lumbosacral plexus (L4-S3 nerve roots).
  • The sciatic nerve is composed of two individual nerves bundles within a common epineural sheath (Vloka’s sheath) – the tibial nerve (L4-S3) and the common peroneal nerve (L4-S2).
  • Motor function: innervation of the muscles of the posterior thigh and adductor magnus (hamstring component).
  • Sensory function: lateral lower leg, heel, dorsal and plantar aspects of the foot.
  • The nerve exits the pelvis via the greater sciatic foramen and enters the gluteal region inferior to the piriformis muscle. It passes deep to the long head of the biceps femoris and descends within the posterior thigh in an inferolateral direction. The two component nerves separate at varying levels within the leg dependent on individual anatomy – Majority will bifurcate at the apex of the popliteal fossa but up to 11% divide in the gluteal region.
  • The popliteal fossa is a diamond shaped depression with the following boundaries
    • Superomedial – semimembranosus and semitendinosus muscles
    • Superolateral – biceps femoris
    • Inferomedial and inferolateral – the medial and lateral heads of gastrocnemius respectively
  • The tibial nerve traverses through the popliteal fossa and down the lower leg posterior to the tibia. It supplies motor function to the posterior compartment of the lower leg and sensation to the sole of the foot.
  • The common peroneal nerve runs along the medial border of biceps femoris over the lateral head of gastrocnemius. It wraps around the neck of the fibula and divides into the superficial and deep peroneal nerves. These nerves provide motor function to the anterior and lateral aspects of the lower leg and sensation over the lateral lower leg and dorsum of the foot (including toes).
  • Branches from the tibial nerve and common peroneal nerves combine to form the sural nerve which provides sensation to the skin overlying the posterolateral side of the lower leg and the lateral aspect of the foot.

Nerves blocked

  • Sciatic nerve and its two branches: tibial nerve and common peroneal nerve. (Dependent on the level of the block and the anatomical variation in the site of sciatic nerve division)

Indications

  • Perioperative
    • Surgery on the lower leg: foot, ankle, Achilles tendon, amputations
    • Analgesia for calf tourniquet
  • Postoperative
    • Analgesia below the patella
  • In combination with a saphenous nerve block for full coverage of the lower leg distal to the patella

Positioning

  • 3 possible positions:
    • Supine
    • Prone
    • Lateral decubitus

Scanning technique and Structures to identify

  • Scanning technique is the same irrespective of patient position.
  • Place the ultrasound probe in a transverse orientation immediately cephalad to the popliteal crease.
  • Survey the structures systematically:
    • Orientate the probe with the semimembranosus and semitendinosus muscles medially and the biceps femoris muscle laterally.
    • Identify the bony shadow formed by the femur.
    • Identify the pulsation of the popliteal artery which lies deep and medial to the sciatic nerve (tibial + common peroneal nerves). Colour doppler can be used to identify the artery.
    • Ensure the popliteal vein is not collapsed by pressure from the transducer.
    • The nerves can be identified in relation to the artery as either one large or two smaller round/oval hyperechoic structures.
  • Move the probe in a cephalad direction while maintaining the transverse plane in parallel to the popliteal crease.
  • Follow the nerve anatomy cephalad within the posterior thigh. Identify the point at which the sciatic nerve branches into the two component nerves. The nerves can be blocked individually or at a point cephalad to this division.

Ultrasound images of Popliteal sciatic nerve

Popliteal Sciatic Nerve

Popliteal nerve division in common paraneural sheath

Tibial and Common Peroneal nerves

Popliteal sciatic nerve in long axis

Block performance

Position/ergonomics

  • Supine
    • Have the ipsilateral knee flexed
  • Prone
  • Lateral decubitus
  • For all positions, if a nerve stimulator is being used the calf and foot need to be exposed to assess for acceptable motor responses.

Probe

  • Linear 13-6MHz transducer.
  • For larger patients, a curvilinear 5-2MHz transducer may be required.

Settings

Depth

  • Typically, within 5cm.

Needle size

  • 80mm short-bevel nerve block needle (usually for in-plane approach as the skin to nerve distance is greater than the out of plane approach where a 50mm needle can be used).

 

Needling technique

  • In-plane (lateral to medial – typical. Medial to lateral – alternative).
  • Out-of-plane (typically used for catheter insertion). One benefit of this technique is the needle path: passing though skin and adipose tissue is less painful than muscle.

Popliteal Sciatic Block

Inplane Popliteal Sciatic Block

Out of Plane Popliteal Sciatic Block

Optimal needle tip position

  • Immediately adjacent to the target nerve.
  • The sciatic sheath should be penetrated and local anaesthetic infiltrated deep to the sheath to surround both nerves. Identify circumferential spread of local anaesthetic around the sciatic nerve (or its components).

Drug choice

  • For longer acting nerve blocks, bupivacaine 0.25% to 0.5%.
  • For a more rapid onset nerve block, a mixture of lidocaine and ropivacaine.

Volume

  • 20 – 40ml.

Optimal current if NS used

  • Start at 1.0 – 1.5mA and when a twitch in the expected muscle group develops, reduce the current.
  • Aim for nerve stimulation at 0.3 – 0.5mA.

Muscle responses acceptable

  • ­Foot inversion/eversion/plantarflexion/dorsiflexion.
  • Plantarflexion is more predictive of a complete sensory response than dorsiflexion.

Pearls

  • Ensure the transducer pressure is not too firm on the thigh – this may collapse the popliteal vein and will increase the risk of unintentional intravascular injection.
  • Due to the angle of the path the sciatic nerve takes within the thigh and the effect of anisotropy, it is usually necessary to tilt the probe slightly caudally to give a better ultrasound image of the nerve. This will create an angle of incidence between the ultrasound beam and the nerve itself that is closer to 90 degrees.
  • The medial side of the lower leg has sensory innervation from the saphenous nerve which is a branch of the femoral nerve. Consider a combined block if the operative site extends into this area.
  • When identification of the nerve structures is difficult, there are several options to improve visualization:
    • Hydrodissection can separate tissue planes and make the anatomy easier to identify. This is due to the contrast between the hypoechoic fluid and the hyperechoic nerve tissue.
    • Requesting the patient to plantarflex and dorsiflex the foot will cause the tibial and common peroneal nerves to slide up and down, which should differentiate them from the surrounding tissue and structures. This ultrasound appearance is known as the “seesaw” sign.
  • Studies have been conducted looking at the differences in efficacy and onset time between the various techniques of nerve identification – nerve stimulation alone, ultrasound alone, and a combination. Dufour et al. demonstrated 16% success of sensorimotor block at 30 minutes using neurostimulation alone in comparison for 65% when using ultrasound and neurostimulation.
  • There is no difference in block efficacy when comparing a combined technique with ultrasound alone.

Complications

  • Neuropraxia/neuropathy, infection, bleeding, block failure.
  • Complications are uncommon – the reported rate is between 0 and 10%
  • A retrospective study focusing on a continuous popliteal nerve block technique reported:
    • Complete block failure (4%)
    • Partial block failure (10%)
    • Postoperative peripheral neuropathy (1.26%)

Continuous techniques

  • Indicated for foot, ankle and lower leg analgesia (e.g. amputation). A combination catheter technique with a femoral nerve catheter can be used to provide effective analgesia for lower limb amputations (i.e. BKA, AKA, and through-knee).
  • The aim is to place the catheter tip in a position to allow continuous infiltration of local anaesthetic around the sciatic nerve. This position may be through the sciatic sheath (i.e. perineural).
  • A hydrodissection technique is recommended (with local anaesthetic or 0.9% sodium chloride) to create a perineural space into which the catheter can be advanced.
  • With the catheter in position, inject local anaesthetic (or 0.9% sodium chloride) down the catheter and observe the spread on ultrasound.
  • If the catheter is in a suboptimal position, it may be possible to withdraw the tip and reassess the spread.
  • When optimal position is identified, the catheter should be secured with dermabond skin glue and sterile transparent dressings.

Contraindications

  • Absolute:
    • Non-consenting patient
    • Local anaesthetic allergy
    • Infection at the site on injection
  • Relative:
    • Coagulopathy
    • Systemic infection

Video Popliteal Sciatic catheter insertion

Video Surgical Sciatic catheter insertion

References

  • Study of variations in the divisions, course and termination of the sciatic nerve. Shewale AD, Karambelkar RR, Umarji BN. J Krishna Inst Med Sci Univ. 2013;2(1):62–68.
  • Plantar flexion seems more reliable than dorsiflexion with Labat’s sciatic nerve block: a prospective, randomized comparison. Taboada M, Atanassoff PG, Rodríguez J, Cortés J, Del Rio S, Lagunilla J, Gude F, Alvarez J. Anesth Analg. 2005 Jan;100(1):250-4.
  • Neuropathic complications after 157 procedures of continuous popliteal nerve block for hallux valgus surgery. A retrospective study. Hajek V, Dussart C, Klack F, Lamy A, Martinez JY, et al. OrthopTraumatolSurg 2012 Res 98: 327-333.
  • The “seesaw” sign: improved sonographic identification of the sciatic nerve. Schafhalter-Zoppoth I, Younger SJ, Collins AB, Gray AT. Anesthesiology. 2004 Sep;101(3):808-9.
  • Combined ultrasound and neurostimulation guidance for popliteal sciatic nerve block: a prospective, randomized comparison with neurostimulation alone. Dufour E, Quennesson P, Van Robais AL, Ledon F, Laloë PA, et al. AnesthAnalg 106: 1553-1558, 2008.
  • https://www.nysora.com/techniques/lower-extremity/ultrasound-guided-popliteal-sciatic-block/
  • http://usra.ca/regional-anesthesia/specific-blocks/lower-limb/sciaticnervepopliteal.php