A 69-year-old hospitalized woman is evaluated for new-onset weakness of the right foot. The patient has a history of chronic hip pain due to osteoarthritis. Two days ago, she underwent left total hip arthroplasty under general anesthesia while in the right lateral decubitus position. Since the surgery, she has had difficulty moving her right foot. Other medical conditions include lumbar spinal stenosis, hypertension, type 2 diabetes mellitus, and a small ischemic stroke with no residual deficit. On examination of the right lower extremity, the patient is unable to dorsiflex the ankle, evert the foot, or extend the toes. Sensation is decreased over the dorsum of the right foot. Injury to which of the following structures is the most likely cause of this patient's current symptoms?
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The common fibular (peroneal) nerve originates in the popliteal fossa as the lateral branch of the sciatic nerve. From there, it courses around the posterolateral head and lateral neck of the fibula before dividing into the deep and superficial fibular nerves:
The superficial fibular nerve innervates the muscles of the lateral compartment of the leg, which primarily function to evert the foot. It also provides sensory innervation to the lateral lower leg and dorsum of the foot.
The deep fibular nerve innervates the muscles of the anterior compartment of the leg, which function primarily to dorsiflex the foot and toes. It also provides sensory innervation to the region between the first and second toes.
The common fibular nerve's superficial location at the level of the proximal fibula and the bony prominence of the fibular head leave the nerve particularly susceptible to compression from a leg cast or prolonged positioning in the lateral decubitus position (eg, during surgical procedures). Common fibular neuropathy characteristically presents with deficits involving the deep fibular (eg, weakness of foot dorsiflexion and toe extension) and superficial fibular (eg, weakness of foot eversion, sensory loss over the dorsal foot) nerves (Choice B).
(Choice C) Although the femoral nerve is rarely affected during hip replacement, this injury would result in weakness on hip flexion and knee extension, along with sensory loss over the anterior thigh.
(Choice D) The lateral sural cutaneous nerve is a branch of the common fibular nerve that provides sensory innervation to the posterolateral lower leg; it does not provide motor innervation.
(Choice E) Irritation of the lumbosacral spinal nerve roots can cause variable sensorimotor deficits due to involvement of multiple myotomes and dermatomes. This patient's sensorimotor findings are specific for common fibular nerve injury.
(Choice F) The most common cause of proximal sciatic nerve injury is trauma or surgery involving the hip joint. However, sciatic nerve injury causes deficits in the common fibular, tibial, and sural nerve distributions, with loss of sensation over the posterior leg and weakness involving the hamstrings and most of the muscles of the lower leg.
(Choice G) The tibial nerve can be injured at the knee by trauma or compression (eg, Baker cyst in the popliteal fossa). Proximal tibial neuropathy usually causes weakness of foot plantar flexion and decreased sensation over the sole.
Educational objective:
The common fibular nerve is superficially located at the proximal fibula, leaving it vulnerable to injury by compression by the fibular head (eg, prolonged lying on the affected side). Patients typically have weakness of foot dorsiflexion (ie, footdrop), foot eversion, and toe extension, along with sensory loss over the lateral lower leg and dorsal foot.