Please use the following scenario for the next 2 Questions.

A ten-year-old falls off their bike and has difficulty moving their shoulder. After examination In the ER they are found to have a fracture of the surgical neck of the humerus. Following being in a cast for 6 weeks they report significant weakness in abduction and mild weakness in external rotation. Upon examination, you also find sensory loss all around the contour of the shoulder and lateral upper arm.

Based on the proximity of neurovascular structures to this fracture site, what are the most likely muscle groups responsible for this weakness?

Question

Please use the following scenario for the next 2 Questions.

A ten-year-old falls off their bike and has difficulty moving their shoulder. After examination In the ER they are found to have a fracture of the surgical neck of the humerus. Following being in a cast for 6 weeks they report significant weakness in abduction and mild weakness in external rotation. Upon examination, you also find sensory loss all around the contour of the shoulder and lateral upper arm.

Based on the proximity of neurovascular structures to this fracture site, what are the most likely muscle groups responsible for this weakness?

BlackTom AI · Accepted Answer

Contextual setup: The scenario describes a fracture at the surgical neck of the humerus with subsequent weakness in shoulder abduction and external rotation, plus sensory loss around the shoulder and lateral upper arm. This pattern points toward injury of the axillary nerve, which runs closely with the surgical neck and supplies the deltoid and teres minor muscles, as well as sensation over the lateral shoulder (the 'regimental badge' area).

Option 1: Teres minor and infraspinatus. While teres minor is innervated by the axillary nerve and can be weak with axillary injury, infraspinatus is innervated by the suprascapular nerve. If the axillary nerve were exclusively injured, you would expect weakness in teres minor but not necessarily infraspinatus. Additionally, the sensory loss described corresponds to axillary, not suprascapular, nerve territory. This makes this pairing unlikely as the primary deficit.

Option 2: Teres major and teres minor. Teres minor is plausible with axillary nerve injury, but teres major is innervated by the lower subscapular nerve (not axillary). Therefore, this combination does not align with the nerve injury pattern expected from a surgical neck fracture.

Option 3: Deltoid and infraspinatus. Deltoid weakness fits axillary nerve injury, but infraspinatus weakness points to suprascapular nerve involvement. A surgical neck fracture classically implicates the axillary nerve rather than the suprascapular nerve, so this pairing misattributes the external rotation deficit.

Option 4: Deltoid and teres minor. Deltoid weakness explains the reduced abduction after the cast period, and teres minor weakness explains part of the external rotation deficit, both of which are characteristic of axillary nerve injury. The sensory loss around the shoulder and lateral arm also matches axillary nerve involvement. This option coherently fits the clinical picture and anatomy.

Summary: The combination of deltoid and teres minor weakness with lateral shoulder sensory loss is most consistent with axillary nerve injury following a surgical neck fracture, making this the most accurate option among those listed. The other options mix muscles innervated by different nerves (suprascapular or subscapular) and do not align with the described neurovascular pattern.

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