A 35-year-old man comes to the emergency department after being involved in a snowmobile accident. He complains of severe pain in his right groin and is unable to extend his right knee. He has no other past medical problems. He is an avid athlete and runs five miles every day. On physical examination, he is unable to extend his right knee against resistance; passive movements are within normal limits. There is a sensory loss over the anterior and medial thigh, medial shin, and the arch of the right foot. The knee jerk is markedly decreased on the right side. Which of the following nerve injuries can most likely explain the physical findings?

Question

A 35-year-old man comes to the emergency department after being involved in a snowmobile accident.  He complains of severe pain in his right groin and is unable to extend his right knee.  He has no other past medical problems.  He is an avid athlete and runs five miles every day.  On physical examination, he is unable to extend his right knee against resistance; passive movements are within normal limits.  There is a sensory loss over the anterior and medial thigh, medial shin, and the arch of the right foot.  The knee jerk is markedly decreased on the right side.  Which of the following nerve injuries can most likely explain the physical findings?

Quizplus · Accepted Answer

Explanation: This patient is most likely suffering from a traumatic femoral nerve injury, which is an uncommon lower extremity nerve injury due to the nerve's location within the pelvis and anterior thigh. On examination, patients with significant femoral neuropathies exhibit weakness involving the quadriceps muscle group, with sparing of leg adduction (which is a function of the obturator nerve). This muscle weakness is exemplified by an inability to extend the knee against resistance. In addition, sensory loss over the anterior aspect and most of the medial aspect of the thigh is typical. This sensory loss extends down the medial shin to the arch of the foot due to dysfunction of the saphenous nerve (branch of the femoral nerve). The knee jerk also decreases in amplitude or becomes absent. (Choice B) A patient with a significant sciatic nerve injury will complain of weakness affecting most of the lower leg musculature, including the hamstrings. Hip flexion, extension, abduction, adduction, and knee extension are usually normal. There is sensory loss involving the lower leg. In contrast to a femoral nerve injury, the medial calf and arch of the foot may be spared secondary to the preserved innervation by the saphenous nerve (a branch of the femoral nerve). Furthemore, sensation is spared above the knee, both anteriorly and posteriorly. The knee jerk is normal, but the ankle jerk is unobtainable. The most common cause of compression or injury to the sciatic nerve in this region is trauma, which includes hip dislocation, fracture, or replacement. Other etiologies include wayward buttock injections, compression by external sources (e.g., prolonged bed rest), and any deep-seated mass in the pelvis (e.g., hematoma). (Choice C) A patient with an obturator nerve injury will present with pain, weakness in leg adduction, and sensory loss over a small area in the medial thigh. Obturator neuropathy is often secondary to pelvic trauma or surgery. (Choice D) A patient with a common peroneal nerve injury will usually present with an acute foot drop accompanied by weakness in foot dorsiflexion and eversion. In addition, the patient may also complain of paresthesias and/or sensory loss over the dorsum of the foot and lateral shin (superficial peroneal nerve territory). The injury is usually located at the knee, on the lateral aspect of the fibular head. Educational objective: Femoral nerve injury is characterized by the following physical findings: inability to extend the knee, loss of knee jerk reflex, and sensory loss over the anterior and medial aspects of the thigh, medial aspect of shin, and arch of the foot.

A 35-Year-Old Man Comes to the Emergency Department After Being