A 7-year-old boy is brought to the emergency department after injuring his right knee in a bicycle accident. Physical examination shows a swollen right knee with abrasion of the overlying skin. Distal motor and sensory innervation are intact, and popliteal and pedal pulses are normal. No other significant trauma is noted. X-ray reveals a nondisplaced patellar fracture. The patient is referred to orthopedics and placed in a leg cylinder cast. On follow-up a week after cast placement, he has paresthesia and numbness of the dorsum of the right foot along with weakness in foot dorsiflexion and eversion. Which of the following is the most likely site of nerve compression in this patient?

Question

A 7-year-old boy is brought to the emergency department after injuring his right knee in a bicycle accident.  Physical examination shows a swollen right knee with abrasion of the overlying skin.  Distal motor and sensory innervation are intact, and popliteal and pedal pulses are normal.  No other significant trauma is noted.  X-ray reveals a nondisplaced patellar fracture.  The patient is referred to orthopedics and placed in a leg cylinder cast.  On follow-up a week after cast placement, he has paresthesia and numbness of the dorsum of the right foot along with weakness in foot dorsiflexion and eversion.  Which of the following is the most likely site of nerve compression in this patient?

Quizplus · Accepted Answer

11ec160b_35c0_77d6_9249_3d38745038e7_MD0001_00 The sciatic nerve branches into the common fibular (peroneal) nerve and the tibial nerve just proximal to the popliteal fossa. After coursing around the neck of the fibula, the common fibular nerve divides into deep and superficial branches. The deep branch innervates the anterior compartment muscles of the leg (foot and toe dorsiflexion) and provides sensation to the webspace between the first and second toes. The superficial branch innervates the lateral compartment muscles (foot eversion) and provides sensation to the dorsum of the foot and lateral shin. The common fibular nerve is frequently injured by trauma (eg, lateral blows to the knee, tightly applied plaster casts) due to its superficial location at the neck of the fibula. Common fibular nerve injury can cause weakness on foot dorsiflexion (ie, foot drop) and eversion, as well as impaired sensation over the lateral shin and dorsal foot and between the first and second toes. (Choice A) Compartment syndrome can occur with fracture/casting of the extremities because increased pressure in a confined space can compromise blood flow. Patients initially have severe pain out of proportion to the degree of injury, followed by weakness and sensory loss. Anterior compartment syndrome can injure the deep branch of the fibular nerve, causing impaired foot dorsiflexion and sensory loss between the first and second toes, but it would not affect foot eversion or sensation to the dorsal surface of the foot. (Choice C) Lateral compartment syndrome can injure the superficial branch of the fibular nerve, causing impaired foot eversion and sensory loss over the lateral shin and dorsum of the foot; weakness in foot dorsiflexion would not be expected. (Choices D and E) After coursing through the popliteal fossa, the tibial nerve innervates the posterior compartment muscles of the leg. It then passes through the tarsal tunnel below the medial malleolus and provides sensory innervation over the sole and motor innervation to the intrinsic foot muscles. Tibial nerve injury at the popliteal fossa can cause weakness on foot plantar flexion, foot inversion, and toe flexion, whereas injury at the tarsal tunnel can cause sensory loss over the sole with intrinsic foot muscle weakness. Educational objective: Trauma/sustained pressure to the neck of the fibula can injure the common fibular nerve, causing weakness on foot dorsiflexion and eversion along with impaired sensation over the lateral shin, dorsal foot, and between the first and second toes. __________ References: Evaluation and treatment of peroneal neuropathy. (http://www.ncbi.nlm.nih.gov/pubmed/19468889)

A 7-Year-Old Boy Is Brought to the Emergency Department After