A ten year old falls off their bike and has difficulty moving their shoulder. After examination In the ER it is noted that they have sustained a fracture of the surgical neck of the humerus. Based on the proximity of neurovascular structures to this fracture site, it is determined that they also injured their axillary nerve. Which of the following motions of the shoulder would you expect there to be some weakness in due to the injury?

Question

A ten year old falls off their bike and has difficulty moving their shoulder. After examination In the ER it is noted that they have sustained a fracture of the surgical neck of the humerus.  Based on the proximity of neurovascular structures to this fracture site, it is determined that they also injured their axillary nerve. Which of the following motions of the shoulder would you expect there to be some weakness in due to the injury?

BlackTom AI · Accepted Answer

Question restatement: A 10-year-old with a fracture at the surgical neck of the humerus has an axillary nerve injury. Which shoulder motions would you expect to have some weakness in due to this injury?

Option 1: Adduction, Internal rotation. Adduction and internal rotation are primarily performed by muscles such as pectoralis major, latissimus dorsi, teres major, and subscapularis. The axillary nerve does not innervate these muscles (subscapularis is innervated by upper and lower subscapular nerves). Therefore, axillary nerve injury would not predict weakness in pure adduction and internal rotation as the defining deficit. This makes this option unlikely even though some incidental fatigue or compensatory strategies could influence movement.

Option 2: Flexion, Internal rotation. Flexion is driven by multiple muscles, including the anterior deltoid (axillary nerve) and pectoralis major; internal rotation involves subscapularis and teres major (the latter has neural input from the lower subscapular nerve). Since the axillary nerve mainly innervates the deltoid (and teres minor), there could be some mild impact on flexion via deltoid weakness, but internal rotation is largely unaffected because the subscapularis and teres major are not innervated by the axillary nerve. Thus this combination is not the classic deficit pattern expected with axillary nerve injury.

Option 3: Extension, Adduction. Extension involves posterior deltoid (axillary nerve) and teres major (not axillary). Adduction involves pectoralis major, latissimus dorsi, and teres major. While some components of shoulder movement could be subtly affected, the combination of extension and adduction is not the hallmark weakness from axillary nerve injury, which more classically targets abduction and lateral rotation via deltoid and teres minor.

Option 4: Abduction, External Rotation. Abduction is primarily driven by the deltoid muscle, which receives its innervation from the axillary nerve. External rotation is predominantly performed by teres minor (axillary nerve) and infraspinatus (suprascapular nerve); with axillary nerve injury, teres minor weakness contributes to reduced external rotation, while infraspinatus often preserves some external rotation but can still show weakness due to decreased overall external rotation torque. Taken together, loss of deltoid (abduction) and teres minor (external rotation) function aligns with an axillary nerve injury at the surgical neck, making this combination the most consistent deficit.

In summary, options 1–3 describe movement patterns that are not characteristically diminished by axillary nerve injury, because their primary innervation involves muscles not supplied by the axillary nerve or rely on other innervation for crucial actions. Option 4 directly corresponds to the muscles innervated by the axillary nerve (deltoid for abduction and teres minor for external rotation), explaining the expected weakness in these motions after the described injury.

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