Lower Extremity Innervation

Snapshot

A 27-year-old woman gives birth to a healthy baby boy. She had an uncomplicated vaginal delivery and was placed in obstetric stirrups during the delivery. Soon after the birth, she complains of numbness and weakness in her right foot. On exam, she has decreased sensation to light touch along the dorsum of her foot including in the first webspace. Ankle dorsiflexion is 3/5 and ankle plantarflexion is 5/5. She walks with a steppage gait. (A compression neuropathy of the common peroneal nerve)

Overview

Introduction

Clinically relevant nerves to the lower extremity
- hip/thigh
  - motor
    - obturator nerve
    - femoral nerve
    - sciatic nerve
    - superior gluteal nerve
    - inferior gluteal nerve
  - sensory
    - lateral femoral cutaneous nerve
    - obturator nerve
    - femoral nerve
- leg/foot
  - motor
    - tibial nerve
    - common peroneal
      - deep peroneal nerve
      - superficial peroneal nerve
  - sensory
    - sural nerve
    - saphenous nerve
    - deep peroneal nerve
    - superficial peroneal nerve
tibial nerve

Obturator Nerve (L2-L4)

Motor innervation
- hip adduction
  - adductor magnus, longus, and brevis
  - gracilis
Sensory innervation
- medial thigh
Terminal branches
- none
Injury
- causes
  - rare
  - anterior hip dislocation
- motor deficit
  - impaired hip adduction
- sensory deficit
sensory loss over medial thigh

Femoral Nerve (L2-L4)

Motor innervation
- hip flexion
  - iliacus
  - sartorius
  - rectus femoris (part of the quadriceps femoris)
- knee extension
  - quadriceps femoris
    - rectus femoris
    - vastus medialis, intermedius, and lateralis
Sensory innervation
- anteromedial thigh
- medial leg and foot (via the saphenous nerve)
Terminal branches
- saphenous nerve
Injury
- causes
  - trauma
    - penetrating pelvic injury
  - compression
    - prolonged hip flexion (e.g., lithotomy position)
  - iatrogenic
    - pelvic, abdominal, or spinal surgery
- motor deficit
  - impaired hip flexion
  - impaired knee extension
- sensory deficit
  - sensory loss over the anteromedial thigh
sensory loss over the medial leg

Sciatic Nerve (L4-S3)

Motor innervation
- knee flexion
  - hamstrings
    - semimembranosus
    - semitendinosus
    - biceps femoris (long head)
Sensory innervation
- no direct sensory innervation
- dorsum of the foot and anterolateral leg (via superficial peroneal nerve)
- plantar aspect of the foot (via tibial nerve)
- posterolateral leg (via sural nerve)
Terminal branches
- common peroneal nerve
- tibial nerve
Injury
- causes
  - trauma
    - lumbar intervertebral disc herniation
  - entrapment
    - piriformis syndrome
  - iatrogenic
    - hip surgery
- motor deficit
  - impaired knee flexion
  - impaired ankle dorsiflexion, plantarflexion, inversion, and eversion
- sensory deficit
  - sensory loss over dorsal and plantar surfaces of the foot
sensory loss over the lateral leg

Common Peroneal Nerve (L4-S2)

Motor innervation
- knee flexion
  - biceps femoris (short head)
Sensory innervation
- lateral knee
- anterolateral leg and dorsum of foot (via superficial peroneal nerve)
- posterolateral leg (via sural nerve, which also receives innervation from the tibial nerve)
Terminal branches
- deep peroneal nerve
- superficial peroneal nerve
Injury
- causes
  - compression
    - obstetric or anesthesia stirrups
    - lying on side during anesthesia without appropriate padding
    - tight plaster cast of leg
  - trauma
    - fracture of fibular neck
- motor deficit
  - impaired ankle and toe dorsiflexion
    - presents as a foot drop
    - patient will compensate with exaggerated hip and knee flexion (steppage gait)
  - impaired ankle eversion
- sensory deficit
  - sensory loss over anterior and lateral leg
sensory loss over dorsum of the foot including first webspace

Deep Peroneal Nerve (L4-L5)

Motor innervation
- ankle dorsiflexion
  - tibialis anterior
- toe extension
  - extensor digitorum longus
  - extensor hallucis longus
Sensory innervation
- first webspace
Terminal branches
- none
Injury
- causes
  - compression
    - tight laces or ski boots
  - trauma
    - anterior ankle trauma
- motor deficit
  - impaired ankle dorsiflexion
    - presents as a foot drop with a steppage gait
  - impaired ankle inversion
    - tibialis posterior can still invert the foot
- sensory deficit
sensory loss over first webspace

Superficial Peroneal Nerve (L4-S1)

Motor innervation
- ankle eversion
  - peroneus longus and brevis
Sensory innervation
- anterolateral leg
- dorsum of foot except for the first webspace
Terminal branches
- none
Injury
- causes
  - trauma
    - fracture of the fibular diaphysis
- motor deficit
  - impaired ankle eversion
- sensory deficit
  - sensory loss over the anterolateral leg
sensory loss over the dorsum of the foot

Tibial Nerve (L4-S3)

Motor innervation
- ankle plantarflexion
  - gastrocnemius-soleus complex
- ankle inversion
  - tibialis posterior
- toe flexion
  - flexor digitorum longus
  - flexor hallucis longus
Sensory innervation
- plantar aspect of the foot (via medial and lateral plantar nerves)
- posterolateral leg (via sural nerve, which also receives innervation from the common peroneal nerve)
Terminal branches
- medial and lateral plantar nerves
Injury
- causes
  - trauma
    - posterior knee dislocation
- motor deficit
  - impaired ankle plantarflexion
  - impaired ankle inversion
    - tibialis anterior can still invert the foot
  - impaired toe flexion
- sensory deficit
sensory loss over plantar aspect of the foot

Superior Gluteal Nerve (L4-S1)

Motor innervation
- hip abduction
  - gluteus medius and minimus
  - tensor fascia latae
Sensory innervation
- none
Terminal branches
- none
Injury
- causes
  - posterior hip dislocation
  - injection to upper medial gluteal region
- motor deficit
  - impaired hip abduction
    - presents with a positive Trendelenburg test and a Trendelenburg gait
      - Trendelenburg test
        patient is asked to lift one foot off the ground and to stand on one foot
        normally, the gluteus medius and gluteus minimus muscles contract to abduct the hip and prevent it from tipping to the unsupported side
        in a patient with an SGN injury, the pelvis will tip to the unsupported side due to impaired hip abduction
      - Trendelenburg gait
        a patient with an SGN injury will lean away from the unsupported side when walking to indirectly raise the pelvis in order to allow their leg to clear the ground
- sensory deficit
none

Inferior Gluteal Nerve (L5-S2)

Motor innervation
- hip extension
  - gluteus maximus
Sensory innervation
- none
Terminal branches
- none
Injury
- cause
  - rare
  - hip arthroplasty
- motor deficit
  - impaired hip extension
    - presents with difficulty climbing stairs, stepping onto a bus, and arising from a chair
- sensory deficit
none

Lateral Femoral Cutaneous Nerve (L2-L3)

Motor innervation
- none
Sensory innervation
- anterolateral thigh
Terminal branches
- none
Injury
- cause
  - compression as the nerve passes under the inguinal ligament (meralgia paresthetica)
    - pregnancy
    - tight clothing
    - obesity
- motor deficit
  - none
- sensory deficit
sensory loss, numbness, and paresthesias over the anterolateral thigh

Sensory Innervation to the Foot

The foot has multiple nerves carrying sensory input
Identifying areas of sensory loss can aid in the localization of specific nerve lesions
Localization
- lateral foot
  - sural nerve
    - receives branches from the tibial nerve and common peroneal nerve
- medial foot
  - saphenous nerve
    - terminal branch of the femoral nerve
- dorsal aspect of the foot
  - superficial peroneal nerve
    - terminal branch of the common peroneal nerve
- first webspace
  - deep peroneal nerve
    - terminal branch of the common peroneal nerve
- plantar aspect of the foot
  - tibial nerve (via lateral plantar, medial plantar, and calcaneal nerves)
    - terminal branch of the sciatic nerve

Lower Extremity Innervation

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