Here’s what’s on tap for issue #68 of the USMLE Times:
Step 1 MSK Question
Video & podcast of the week
Tweet for thought
This week’s training links
This week’s reading recommendation
Question deep-dive & breakdown
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A 42-year-old female presents to your office with a two-week history of right hip pain and difficulty walking. She underwent right-sided total hip replacement surgery fifteen days ago. Physical examination shows normal strength in the hip flexors and extensors. However, she explains that she cannot cross her right leg over her left while seated. Sensation is diminished over the medial aspect of the right thigh and grossly intact in the left leg. Based on her clinical presentation, which of the following is most likely injured?
A. Femoral nerve
B. Superior gluteal nerve
C. Sciatic nerve
D. Inferior gluteal nerve
E. Obturator nerve
The answer & question breakdown is at the bottom of the post.
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ANSWER + QUESTION BREAKDOWN
The patient is a 42-year-old female with right hip pain and difficulty walking following a recent right-sided total hip replacement surgery. The physical examination findings suggest weakness in hip adduction and an inability to cross the right leg over the left while seated. There is also diminished sensation over the medial aspect of the right thigh. Based on these findings, you should consider the nerve supply to the affected muscles and the sensory distribution of the involved nerves.
ANSWER CHOICES:
ANSWER CHOICE A:Femoral nerve
The femoral nerve innervates the hip flexors (iliopsoas) and knee extensors (quadriceps). It provides sensory innervation to the anterior and medial aspects of the thigh. Injury to the femoral nerve would result in weak hip flexion and knee extension, inconsistent with the patient's presentation.
ANSWER CHOICE B: Superior gluteal nerve
The superior gluteal nerve innervates the gluteus medius and minimus muscles, which are primarily responsible for hip abduction. Injury to this nerve would cause weakness in hip abduction, not adduction, and would not affect sensation in the medial thigh.
ANSWER CHOICE C: Sciatic nerve
The sciatic nerve innervates the hamstrings and all muscles below the knee. It provides sensory innervation to the posterior thigh, lower leg, and foot. Injury to the sciatic nerve would cause weakness in knee flexion, all foot and ankle movements, and sensory deficits in the posterior thigh and lower leg. These findings do not reflect those seen in this patient.
ANSWER CHOICE D: Inferior gluteal nerve
The inferior gluteal nerve innervates the gluteus maximus and aids in hip extension. Injury to this nerve would cause weakness in hip extension, not adduction, and would not affect sensation to the medial thigh.
ANSWER CHOICE E: Obturator nerve
The obturator nerve innervates the adductor muscles of the thigh (adductor longus, adductor brevis, adductor magnus, and gracilis). It provides sensory innervation to the medial aspect of the thigh.
Injury to the obturator nerve would cause weakness in hip adduction. This would result in difficulty crossing the affected leg over the unaffected leg - as is seen in this patient. It would also cause sensory deficits in the medial thigh - another finding consistent with the patient's presentation. This is the correct answer.
THE VERDICT: Based on the patient's clinical presentation, the obturator nerve is most likely injured. The obturator nerve originates from the lumbar plexus (L2-L4) and passes through the obturator foramen to enter the medial thigh. It innervates the adductor muscles of the thigh, which are responsible for hip adduction (moving the thigh towards the midline). The obturator nerve also provides sensory innervation to the medial aspect of the thigh.
In this case, the patient exhibits weakness in hip adduction and an inability to cross her right leg over her left while seated, which suggests dysfunction of the adductor muscles. Additionally, the diminished sensation over the medial aspect of the right thigh is consistent with the sensory distribution of the obturator nerve. The recent history of right-sided total hip replacement surgery further supports the possibility of obturator nerve injury, as this nerve is vulnerable during such procedures.
Therefore, based on the scenario's overall findings, the most likely cause is an injury to the obturator nerve.
FINAL ANSWER:E: Obturator nerve
That’s all for this week’s issue of The USMLE Times!
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