Regional anesthesia for foot and ankle surgery 

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Regional anesthetic techniques commonly used for foot and ankle surgery include:

1.  Ankle block
2.  Sciatic nerve block (usually combined with saphenous block)

Both the ankle and popliteal sciatic nerve block provide excellent analgesia following foot and ankle surgery. 1 The ankle block has the advantage of causing minimal motor block – if patients are given a postoperative “shoe”, which allows ankle dorsi and plantar flexion (rather than a plaster cast), mobilisation is possible without crutches. 2 However, the ankle block has a limited duration: approx. 12 hrs is the rule compared to approx. 24 hrs for sciatic nerve block. 1 The main disadvantage of sciatic block is the inevitable foot drop. Foot drop is of minimal consequence if a plaster cast is applied, as patients will require crutches to mobilise regardless of anesthetic choice. On the other hand, if only a postoperative “shoe" is to be used, foot drop can necessitate the use of crutches when they might otherwise not have been required. Foot drop also renders bilateral sciatic blocks contraindicated in ambulatory patients, whereas bilateral ankle blocks are usually well tolerated in the ambulatory setting e.g. bilateral bunion repair.

Compared to more proximal approaches, we prefer the popliteal/distal thigh approach for sciatic block ("popliteal block") for two reasons:

1.  Hamstring motor block is minimised, therefore, patients retain the ability to flex the knee. Knee flexion is essential for keeping the foot off the ground (particularly if only a postoperative “shoe” is used – foot drop can be problematic in this situation).

2.  The nerve contains more connective tissue at the politeal level; 3 therefore, inadvertent intraneural injection will theoretically be better tolerated.

Studies have yet to identify which patients benefit from continuous over SS sciatic block. While a sciatic catheter unequivocally improves pain control following major foot/ankle surgery, 4 its utility for less painful procedures is unclear. We reserve sciatic nerve catheters for ankle arthrodesis, ankle replacement and major distal lower extremity osteotomies.

Saphenous block

Anatomical textbooks depict the sensory innervation of the saphenous nerve in the foot as a narrow medial band of skin on its dorsal surface. However, the nerve is known to have significant interpatient variability. Regardless of the anatomical extent of the surgery, it is more common to administer a combined saphenous/sciatic nerve block rather than an isolated sciatic block. Saphenous block is usually conducted at the level of the medial malleolus, however, if surgery involves the proximal ankle or lower leg, the block is performed at the adductor canal, located at the distal thigh level. 5

References

1.         McLeod DH, Wong DH, Vaghadia H, et al. Lateral popliteal sciatic nerve block compared with ankle block for analgesia following foot surgery. Can J Anaesth 1995;42:765-9.

2.         Fredrickson MJ, White R, Danesh-Clough TK. Low-volume ultrasound-guided nerve block provides inferior postoperative analgesia compared to a higher-volume landmark technique. Reg Anesth Pain Med 2011;36:393-8.

3.         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.

4.         Singelyn FJ, Aye F, Gouverneur JM. Continuous popliteal sciatic nerve block: an original technique to provide postoperative analgesia after foot surgery. Anesth Analg 1997;84:383-6.5.            Manickam B, Perlas A, Duggan E, et al. Feasibility and efficacy of ultrasound-guided block of the saphenous nerve in the adductor canal. Reg Anesth Pain Med 2009;34:578-80.