Introduction
· Five major nerves, which are responsible for motor and sensory innervation of the foot and the ankle, are:
o Tibial nerve
Medial and lateral plantar branches
o Superficial peroneal (also known as fibular) nerve
o Deep peroneal (fibular) nerve
o Sural nerve
o Saphenous nerve
· Tibial Nerve
o Sciatic nerve (spinal cord segments: L4-S3) is the main nerve that innervates the posterior compartment of the thigh
o Sciatic nerve bifurcates into two terminal branches in the popliteal fossa just proximal to the knee joint:
Tibial nerve,
Common peroneal (fibular) nerve
o After the site of bifurcation, tibial nerve continues its vertical trajectory to enter the posterior compartment of the leg and moves down to the sole of the foot.
o It innervates all the muscles of the posterior compartment of the leg.
o Enters the foot through the tarsal tunnel and lies posterior to the medial malleolus.
o Within the tunnel, the tibial nerve gives medial calcaneal branches that go through the flexor retinaculum to supply the heel.
o In between the medial malleolus and the heel, the tibial nerve bifurcates into two braches: medial plantar nerve(larger) and lateral plantar nerve(smaller).
o These two branches follow the trajectory of their corresponding arteries.
o Medial plantar branch of the tibial nerve (MPN):
§ It is the major sensory nerve in the sole of the foot:
· Skin of the anteromedial 2/3 of the plantar side of the foot
· Both sides and the tips of the first three phalanges and the medial side of the fourth phalanx.
§ Medial plantar nerve also has motor braches that innervates the following muscles:
· Abductor hallucis,
· Flexor digitorum brevis,
· Flexor hallucis brevis, and,
· First lumbrical
§ Medial plantar nerve gives a medial digital branch that innervates the skin on the medial side of the great toe: proper plantar digital nerve.
§ Medial plantar nerve tri-furcates after giving its medial branch. These branches move anteriorly to innervate the lateral side of the great toe, both sides of the second and third phalanges and the medial side of the fourth digit. These branches are referred as the common plantar digital nerves.
o Lateral plantar branch of the tibial nerve (LPN):
§ It is mainly the motor branch of the tibial nerve.
§ It supplies all intrinsic muscles in the sole, except the ones that are innervated by the medial plantar nerve listed above.
§ It innervates the anterolateral 1/3 of the plantar skin, and carries sensations form lateral side of the fourth digit and both sides of the fifth digit.
§ After the bifurcation, LPN moves laterally and anteriorly across the sole until it reaches the base of the fifth metatarsal bone where it divides into superficial and deep braches.
§ Superficial branch:
· Its proper plantar digital branch supplies the lateral side of the fifth digit. It also carries motor fibers that innervate the flexor digiti minimi brevis, and the dorsal and plantar interossei muscles in between the fourth and the fifth metatarsals
· Its common plantar digital branches give ride to proper digital nerves to supply the adjacent sides of the fourth and the fifth digits
§ Deep branch:
· Motor innervation of the second to fourth lumbricals, adductor hallucis, and all interossei muscles except the ones between the 4th and the 5th metatarsals.
· Superficial and Deep Peroneal (Fibular) Nerves:
o Common peroneal (fibular) nerve is one of the two terminal branches of the sciatic nerve.
o It is first seen in the popliteal fossa just proximal to the posterior aspect of the knee joint along with the tibial nerve
o After it gives its branch to the short head of the biceps femoris, it continues its downwards trajectory but deviates anterolaterally in order to get into the lateral and anterior compartments of the leg
o Common peroneal(fibular) nerve makes an anterior turn around the fibular neck, and as it passes under the peroneus longus muscle, it divides into two branches:
§ Superficial peroneal(fibular) nerve
§ Deep peroneal(fibular) nerve
o Superficial Peroneal (Fibular) Nerve
§ Starts at the level of the fibular neck
§ Moves down in between the peroneus longus and peroneus brevis muscles, and gives motor branches to these two muscles.
· It is necessary for eversion of the foot
§ In the distal 1/3 of the lower leg, it penetrates the deep crural fascia to become subcutaneous, and enters the dorsal aspect of the foot in the superficial fascia.
· Gathers sensory information from the distal anterolateral 1/3 side of the lower leg.
§ As it reaches the anterior aspect of the ankle, it gives out two sensory branches on the extensor retinaculum: medial and intermediate dorsal cutaneous nerves. Later, they give out dorsal digital nerves.
§ It carries sensory information from skin on the dorsal aspect of the foot and toes, except:
· Skin on dorsomedial side of the great toe and dorsolateral side of the second toe (innervated by the deep peroneal nerve)
· Skin on the lateral side of the foot and the fifth toe (innervated by the sural nerve)
o Deep Peroneal (Fibular) Nerve
§ Starts at the level of the fibular neck, then moves anteriorly to enter the anterior compartment.
§ Moves down through the lower leg in between these three muscles within the anterior compartment: tibialis anterior, extansor hallucis longus, extensor digitorum longus.
· Gives motor branches to these muscles, thus, it is important for ankle dorsiflexion and extension of the phalanges.
§ It enters the dorsal aspect of the foot by passing under the extensor retinaculum along with the dorsalis pedis artery and extensor hallucis longus muscle tendon
· Tendon lies medially to the deep peroneal nerve
§ As the nerve exits the extensor retinaculum, just distal to the ankle joint, it gives its lateral branch to innervate the extensor digitorum brevis muscle.
§ Its medial branch continues to move anteriorly on the dorsal aspect of the foot and penetrates the deep fascia between the first two metetarsophalangeal joints to give out sensory dorsal digital branches to the adjacent dorsal surfaces of toes I and II.
§ Before it passes the deep fascia, it gives out small motor branches to supply the first two dorsal interossei muscles.
· Sural Nerve
o It is the pure sensory branch of the tibial nerve
o Most of the time, the sural nerve branches out from the proximal segments of the tibial nerve high in the leg.
o Moves down through the lower leg in between the two heads of the gastrocnemius muscle.
o It is subcutaneously located in the posterolateral aspect of the lower leg
o It enters the foot in the superficial fascia posterior to the lateral malleolus
§ This is the site where surgeons prefer to take out sural nerve grefts, and where the nerve is most commonly injured.
o It continues as the lateral dorsal cutaneus nerve and innervates the lateral aspect of the heel, the foot and dorsolateral surface of the fifth digit.
· Saphenous Nerve
o The saphenous nerve is the longest and pure sensorial branch of the femoral nerve. It branches out in the proximal portion of the thigh
o The saphenous nerve enters the adductor canal (Hunter Canal). It does not give any motor branches.
o As it exits the canal, the saphenous nerve becomes superficial under the sartorius muscle
§ Gives out an infrapatellar branch to innervate skin on the anteromedial aspect of the knee
o Continues its subcutaneous trajectory down to the ankle on the medial side of the lower leg
§ Gathers sensory information from the medial lower leg and medial aspect of the ankle.
o Enters the foot by passing in front of the medial malleolus.
o Its terminal branches innervate skin on the medial side of the proximal foot (down to the level of the first metatarsophalangeal joint)
· Clinical Correlations
o Tarsal Tunnel Syndrome
§ It is a fibrous-and-osseous canal that lies at the medial side of the ankle.
· Roof: Flexor retinaculum
· Medial: calcaneus and talus
· Inferior: abductor hallucis
§ Neurovascular bundle and muscle tendons pass through the tarsal tunnel.
· Tibial nerve
· Posterior tibial artery and vein
· Flexor hallucis longus tendon
· Flexor digitorum longus tendon
§ Tarsal tunnel syndrome is compression of the tibial nerve as it passes under the flexor retinatulum, through the tarsal tunnel.
§ Most common reason for tarsal tunnel syndrome is a fracture or dislocation after a trauma involving the medial malleolus, calcaneus and/or talus. Scar tissue, bone and cartilage fragments following a fracture can cause impingement.
§ Other reasons for tibial nerve impingement:
· Intrinsic: ganglion cysts, tendinopathy, tenosynovitis, lipoma/tumor, peri-neural fibrosis, osteophytes
· Extrinsic: shoes, anatomic deformity, systemic inflammatory disease, lower extremity edema
§ Common symptoms are;
· Pain with prolonged standing and walking
o Has a sharp and burning characteristic.
· Numbness and tingling in the sole of the foot, the distal foot, toes and sometimes the heel.
§ Physical examination findings are;
· Tenderness over the area posterior to medial malleolus – Tinel’s sign is positive.
· Pes planus can be observed
· Muscle wasting of intrinsic foot muscles
· Pain with foot dorsiflexion and eversion
· Compression test: plantarflexion + inversion of the ankle and digital pressure on the tibial nerve. If pain is elicited, it is highly sensitive and spesific.
§ Obtain weight-bearing plain radiographies of the foot and ankle to evaluate bony structures, MRI for possible soft tissue pathologies
§ Electromyography (EMG) study should be performed
· Distal motor latencies of 7 msec or more
· Prolonged sensory latencies
· Decreased amplitude of motor action potentials of abductor hallucis and abductor digiti minimi are suggestive EMG findings.
§ First line of treatment is conservative, such as lifestyle modifications and non-steroid anti-inflammatory durgs(NSAIDs).
§ If 3-6 months of conservative approach fails to resolve the symptoms, and if there is a positive EMG result, then surgical tarsal tunnel release is indicated.
· Recurrence is due to insufficient release.
o Common peroneal nerve palsy
§ Sciatic nerve bifurcates into two terminal branches in the popliteal fossa just proximal to the knee joint:
· Tibial nerve,
· Common peroneal (fibular) nerve
§ Common peroneal(fibular) nerve makes an anterior turn around the fibular neck and as it passes under the peroneus longus muscle, it divides into two branches:
· Superficial peroneal(fibular) nerve
· Deep peroneal(fibular) nerve
§ At this level, it becomes superficial and open to trauma and compression
§ Patients with common peroneal nerve palsy most typically present with “drop foot” due to loss of ankle dorsiflexion.
§ Most common causes are fibular head fracture following a trauma, surgery near the knee joint, prolonged compression (cast or brace), crossing legs(especially if the patient is thin), rapid weight loss (loss of protective fat layer around the nerve)
§ Physical examination findings are;
· Motor findings: foot drop, steppage gait, weakness in eversion
· Sensory findings: numbness and paresthesia predominantly on dorsum of the foot and first web space(deep peroneal nerve).
§ L5 radiculopathy should also be considered among the differential diagnoses in patients with drop foot. Suggestive signs/findings for L5 radiculopathy are;
· Weakness in inversion
· Lower back pain
· Decreased lower extremity deep tendon reflexes
§ Plain radiographies should be obtained to evaluate bony structures.
§ EMG study is necessary to detect the lesion level.
§ MRI and USG can be asked to check for any soft tissue masses that cause compression. Lumbar MRI should be asked to evaluate the spinal cord and nerve roots.
§ First line of treatment is conservative.
· Lifestyle modifications to relieve external compression
· Physiotherapy
· Vitamin supplements
· Ankle foot orthoses
§ If 3-6 months of conservative treatment fails, if there is a mass causing impingement, if there is traumatic nerve injury, then surgery is indicated.
o Other nerve palsies
§ Saphenous nerve palsy:
· Purely sensorial deficits
· Numbness/paresthesias over the anteromedial skin of the knee, medial side of the lower leg and medial side of the foot up to the first metatarsophalangeal joint.
· No motor deficits
§ Sural nerve palsy:
· Purely sensorial deficits
· Numbness/paresthesias over the posterolateral portion of distal lower leg, lateral side of the foot, 5th phalanx.
· No motor deficits