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If a patient cannot abduct the arm above 90°, which muscles and nerves are commonly involved?
Answer:
- Serratus anterior (long thoracic nerve, C5-7).
- Trapezius (spinal accessory nerve, CN XI).
Explanation:
Abduction of the arm beyond 90° requires upward rotation of the scapula, produced mainly by the serratus anterior and trapezius working together.
Injury to either the long thoracic nerve or spinal accessory nerve can therefore impair abduction above 90°.
High-yield point:
- 0–15°: supraspinatus.
- 15–90°: deltoid.
- >90°: upward rotation of scapula by serratus anterior + trapezius.
