Fracture of surgical neck of humerus leads to loss of abduction movement of the corresponding shoulder joint due to injury of:
The surgical neck is located just below the head of the humerus. The axillary nerve and the anterior and posterior circumflex humeral arteries pass close by. The axillary nerve is crucial here because it innervates the deltoid muscle. If the deltoid isn't working, shoulder abduction is impaired since the deltoid is the main muscle for that movement. Also, the axillary nerve gives a sensory branch to the skin over the shoulder.
Looking at the options, the correct answer should be the axillary nerve. Let me check the distractors. If the options include other nerves like the musculocutaneous, radial, or median, those are incorrect because they don't innervate the deltoid. The musculocutaneous nerve supplies the biceps, which is more involved in elbow flexion. The radial nerve is for extension, and the median nerve is for forearm and hand functions. The suprascapular nerve innervates the supraspinatus and infraspinatus, but those are more for shoulder stabilization than abduction. So the key point is that the axillary nerve injury leads to deltoid paralysis, causing loss of abduction. The clinical pearl here is that the axillary nerve is at risk in surgical neck fractures, affecting both motor and sensory functions. That's the high-yield fact to remember.
**Core Concept**
Fractures of the surgical neck of the humerus often injure the **axillary nerve**, which innervates the **deltoid muscle**—critical for shoulder abduction. The axillary nerve also provides sensory innervation to the shoulder. This injury disrupts motor function, leading to loss of abduction.
**Why the Correct Answer is Right**
The axillary nerve (C5-C6) wraps around the surgical neck of the humerus. Injury here paralyzes the **deltoid** (primary abductor of the shoulder) and **teres minor** (rotator cuff muscle). Without deltoid function, active abduction beyond 15° is impaired. Sensory loss over the lateral shoulder ("regimental badge" area) may also occur. The mechanism involves direct trauma or traction during fracture displacement.
**Why Each Wrong Option is Incorrect**
**Option A:** *Musculocutaneous nerve* innervates biceps and brachialis, controlling elbow flexion—not abduction.
**Option B:** *Radial nerve* controls wrist/forearm extension; injury causes wrist drop, not shoulder dysfunction.
**Option C:** *Median nerve* affects hand and forearm; unrelated to shoulder abduction.
**Option D:** *Suprascapular nerve* innervates supraspinatus and infraspinatus, aiding shoulder stability but not primary abduction.
**Clinical Pearl / High-Yield Fact**
Remember: **"Axillary nerve = Deltoid + Shoulder Sensation"**. In surgical neck fractures, always assess deltoid strength and shoulder sensation. Differentiate from rotator cuff injuries (suprascapular nerve) by noting the specific loss of abduction and sensory deficits.
**Correct Answer: C. Axillary nerve**