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 is 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 with weakness in foot dorsiflexion and eversion. Which of the following is the most likely site of nerve compression in this patient?
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This patient's dorsal foot numbness and weakness of foot dorsiflexion and eversion are due to compression of the common fibular (peroneal) nerve by his cast. The common fibular nerve originates in the popliteal fossa as the lateral branch of the sciatic nerve. After coursing around the posterolateral head and lateral neck of the fibula, it divides into superficial and deep branches:
The superficial branch innervates the muscles of the lateral compartment of the leg (foot eversion) and provides sensation to the dorsum of the foot and lateral shin.
The deep branch innervates the muscles of the anterior compartment of the leg (foot and toe dorsiflexion) and provides sensation to the web space between the first and second toes.
Common fibular nerve injury from extrinsic compression (eg, pressure from cast, surgical positioning) most commonly occurs at the fibular head because of its bony prominence and the superficial position of the nerve. Fracture of the fibular neck (weakest point of the proximal fibula) is another common cause of common fibular nerve injury.
(Choices A and C) Compartment syndrome can occur with extremity fracture/casting from increased pressure (eg, swelling) within a confined space. 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 foot. Lateral compartment syndrome can injure the superficial branch of the fibular nerve, causing impaired foot eversion and sensory loss over the lateral shin and dorsal foot, but 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. Injury at the tarsal tunnel can cause sensory loss over the sole (vs the dorsum) with intrinsic foot muscle weakness. Injury at the popliteal fossa can cause additional weakness on foot plantar flexion (vs dorsiflexion), foot inversion (vs eversion), and toe flexion.
Educational objective:
Common fibular (peroneal) nerve injury can occur with extrinsic compression at the fibular head, resulting in weakness on foot dorsiflexion and eversion. Sensation will also be impaired over the lateral shin, dorsal foot, and between the first and second toes.