2. Foot Flashcards
Basic anatomy and Peripheral Nerve Blocks
forefoot. Five nerves
need to be blocked before local anaesthesia is complete. Concentrations may need to be
reduced if the patient is frail or if the procedure is bilateral
- Saphenous
- Posterior tibial nerve:
- Deep peroneal nerve
- Sural nerve:
- Superficial peroneal nerve
Saphenous nerve
Saphenous nerve:
this supplies a variable portion of the medial border of the foot
and ankle. It is a terminal branch of the femoral nerve and is anaesthetized
immediately anterior to the medial malleolus where it is superficial, close to the
saphenous vein. It is blocked with subcutaneous local anaesthetic, for example,
levobupivacaine 0.5% 5 ml.
Posterior tibial nerve:
this supplies the plantar surface of the foot.
It is a branch of the sciatic nerve
(which divides into tibial and common peroneal branches in the popliteal fossa)
and is blocked behind the medial malleolus where it lies posterior to the posterior tibial artery.
The needle is gently directed perpendicular to the skin
until it encounters bone, and is then withdrawn 1–2 mm prior to injection of 3–5 ml
levobupivacaine 0.5% on either side of the artery.
This nerve can also reliably be
blocked in the popliteal fossa.
Deep peroneal nerve
: this supplies only a small area of skin on the dorsum of the
foot between the first and second toes.
It passes beneath the extensor retinaculum at
the front of the ankle joint and is most readily blocked between the tendons of
extensor hallucis longus and extensor digitorum longus where it lies lateral to the
dorsalis pedis artery.
It is blocked with a total of 3–5 ml levobupivacaine 0.5% placed
on either side of the artery and deep to the fascia.
Sural nerve:
Sural nerve: this supplies sensation to the fifth toe and the lateral border of the foot.
It is a branch of the tibial nerve; at the level of the ankle it lies superficially behind the
lateral malleolus. Subcutaneous infiltration of levobupivacaine 0.5% 5 ml between
the lateral malleolus and the tendo Achilles usually provides effective analgesia.
Superficial peroneal nerve
Superficial peroneal nerve: this supplies much of the dorsum of the foot (excepting
the small area supplied by the deep peroneal nerve, and the lateral foot which is
supplied by the sural nerve).
It is a branch of the common peroneal nerve, which
divides further into terminal branches at the level of the malleoli.
It is blocked with a
ring of superficial infiltration of levobupivacaine 0.5% 10 ml between the anterior
tibia and the lateral malleolus.
Possible local anaesthetic techniques: foot
include subarachnoid (spinal) block,
lumbar extradural (epidural) block, sacral extradural (caudal) block, sciatic nerve
block at the hip, sciatic nerve block in the popliteal fossa, intraosseous nerve block
between the metatarsals (for procedures in the distal foot which cannot be performed
under digital nerve [ring] block), intravenous regional anaesthesia (Bier’s block,
which needs high compression pressures and high volumes to obtain satisfactory
analgesia), and local infiltration (this is unlikely to be satisfactory for awake surgery
but is included for completeness).
Complications:
Complications: these are largely generic and include failure and partial failure, local
anaesthetic toxicity (you may need to modify the concentrations quoted above to
reduce the total dose), nerve and vessel damage, intravascular and intraneural
injection, and complications related to the lower limb arterial tourniquet (see under
‘The Arterial Tourniquet’ in Chapter 3)
General considerations for regional
anaesthesia techniques
Popliteal fossa sciatic nerve block
The popliteal artery (pulsatile) and vein (compressible) are identified and confirmed using Doppler at the
level of the knee crease. The TN is seen lying just posterior to
these structures. Scanning proximally, the CFN moves from
lateral (just deep to the biceps femoris tendon) to medial to join
the TN. Using dynamic scanning (proximal/distal movements
of the transducer) helps visualise the CFN and TN
Block conduct
The patient can be positioned supine with the leg raised,
lateral or prone. Keep the patient’s leg (and therefore the
SN) straight to aid visualisation.
(ii) Perform the block just distal to the SN bifurcation, where the two branches lie in a common paraneural sheath. This allows injection of local anaesthetic both around and between the CFN and TN, ensuring that local anaesthetic is within the paraneural sheath.
(iii) A ‘ComptoneCruveilhier septum’ lies between the two branches; this is pierced to achieve spread of local anaesthetic around both branches.14 Careful ‘hydrodissection’ and observation of spread around the CFN and TN branches ensures successful blockade.
iv The needle is inserted laterally and passed medially.
Choosing an insertion point slightly anterior to the
transducer results in better needle visualisation owing to
a more parallel transducer/needle orientation.
(v) Deposit up to 20 ml local anaesthetic around and between the branches.
Ankle block
Tibial nerve
e plantar aspect of the foot. The TN is identified
posterior to the medial malleolus adjacent to the posterior
tibial artery
Position the patient laterally with the side to be blocked
underneath or supine with the leg to be blocked crossed
in a figure of 4 shape to allow a posterior needle approach.
(ii) Classically the TN was blocked at the medial malleolus
using a landmark technique.
(iii) The ultrasound approach allows more proximal
blockade (so expect to cover the heel). An approach
5e10 cm proximal to the medial malleolus helps with
good skin to transducer contact and moves the bulk of
the Achilles tendon (AT) away from an in-plane needle
path.
(iv) A posterior to anterior in-plane needle approach avoids
TP and FDL tendons.
(v) Gentle ‘hydrodissection’ is required to achieve circumferential spread with up to 5 ml local anaesthetic.
(vi) This nerve should be blocked first because it is the largest
of the ankle nerves and therefore has the longest onset
time
Deep fibular nerve
The DFN supplies cutaneous innervation to the first webspace
on the foot. The nerve is visualised in the distal leg proximal to
the ankle joint, usually lying lateral or anterior to the anterior
tibial artery (ATA),
i) Use a lateral to medial in-plane needle approach,
depending on the nerve’s relationship with the ATA.
(ii) Use careful ‘hydrodissection’ to deposit 1e3 ml local
anaesthetic around the nerve.
Superficial fibular nerve
The SFN supplies the majority of the cutaneous innervation to
the dorsal aspect of the foot. The SFN has no vasculan
Position the patient with the leg internally rotated.
(ii) Scan proximally from the lateral malleolus. As the fibula
moves deeper, visualise the FL and FB overlying the fibula and the EDL anteriorly. Look at the intersection between the muscle groups and for the nerve passing
through the muscular fascia overlying the muscles.
(iii) Dynamic scanning will help to visualise the nerve as it
crosses the fascia.
(iv) Aim to block the nerve superficial or deep to the
muscular fascia (depending on best view) with 1e3 ml
local anaesthetic.
(v) The SFN may give rise to branches (that supply the foot)
more proximally after emerging through the fascia, so the
block should be placed either just after or before it passes
through the fascia to both block any early branches.
Sural nerve
The sural nerve provides cutaneous innervation to the lateral
malleolus and the fifth phalanx in both the plantar and dorsal
aspects. The sural nerve enters the foot posteriorly to the
lateral malleolus. Scanning the lower leg distally towards the
lateral malleolus, the nerve and short saphenous vein (SSV)
are seen to ‘roll of
i) The patient can be positioned laterally or with the leg
internally rotated.
(ii) Scan proximally just posterior to the lateral malleolus so
that the FB lies anteriorly and the AT posteriorly.
(iii) Identify the sural nerve and SSV ‘rolling off’ the AT when
scanning distally into the space just anterior to the AT.
(iv) Use an anterior to posterior in-plane needle approach to
block the nerve with 1e3 ml local anaesthetic.
(v) A perivenous injection may be effective if it is difficult
to identify the sural nerve. Using ‘hydro-dissection’,
adva-nce the needle tip into fluid to avoid nerve
contact.
