A 17-year-old boy is brought to the emergency department due to a lower extremity injury. The patient was playing football with his friends when he was tackled roughly and "felt something snap" in his right leg. He avoids bearing weight on the right foot. Examination shows a large contusion and tenderness over the right proximal fibula. X-ray of the right leg is shown in the exhibit. Which of the following is most likely to be impaired on physical examination?
Show Explanatory Sources
The sciatic nerve branches into the common fibular (peroneal) nerve and the tibial nerve posteriorly in the thigh just proximal to the popliteal fossa. After coursing around the posterolateral head and lateral neck of the fibula, the common fibular nerve divides into superficial and deep branches:
The superficial branch innervates the muscles of the lateral compartment of the leg, which function primarily to evert the foot. It also provides sensory innervation to the dorsum of the foot.
The deep branch innervates the muscles of the anterior compartment of the leg, which act mainly as dorsiflexors of the foot and toes. It also provides sensory innervation to the region between the first and second toes.
This patient has a fracture of the neck of the fibula. The common fibular nerve's proximity to the neck of the fibula makes fractures in this region a common cause of injury to the nerve. Damage to the nerve at this level causes loss of dorsal foot sensation, as well as impaired dorsiflexion and eversion, resulting in foot drop.
(Choice A) The tibialis posterior (innervated by the tibial nerve) is the major muscle involved in foot inversion. The tibialis anterior (innervated by the deep fibular nerve) is less important, and patients with fibular nerve injuries usually have normal foot inversion.
(Choices B and E) The tibial nerve courses through the popliteal fossa and innervates the skin of the posterior calf, lateral foot, and sole; tissues of the knee joint; and muscles providing plantar flexion and inversion (eg, gastrocnemius, soleus, tibialis posterior). Injury to the tibial nerve would cause weakened plantar flexion and inversion of the foot, as well as decreased sensation at the sole of the foot.
(Choice D) Loss of sensation at the medial aspect of the lower leg would result from a lesion of the saphenous nerve, the largest pure sensory branch of the femoral nerve.
Educational objective:
The common fibular (peroneal) nerve is vulnerable to injury where it courses around the neck of the fibula. It can be injured when the fibular neck is fractured, resulting in loss of sensation over the dorsum of the foot, as well as weakness of foot dorsiflexion (deep fibular nerve) and eversion (superficial fibular nerve).