Musculoskeletal System
Serratus anterior and winged scapula
Injury to the long thoracic nerve can weaken serratus anterior, the muscle that holds the scapula against the chest wall, so the medial border of the scapula lifts away.
Remember
Long thoracic nerve injury can weaken serratus anterior, so the medial border of the scapula lifts away from the chest wall, often more prominent when pushing against a wall.
Serratus anterior is a broad muscle on the side of the chest wall. It arises as strips from the upper ribs and attaches to the medial border of the scapula, which is the edge nearest the spine. It holds the scapula, or shoulder blade, against the chest wall.
It protracts the scapula, which means it draws the scapula forwards around the ribcage, for example in a push. It also rotates the scapula upwards, turning the shoulder socket upwards, and this lets the arm be raised above shoulder height.
The long thoracic nerve is the motor nerve to the muscle. It arises from the roots C5 to C7 of the brachial plexus, the network of nerves running from the neck into the arm.
Injury to this nerve can weaken serratus anterior. The nerve is usually damaged by a violent stretch of the arm or during surgery in the armpit. The muscle holds the scapula against the ribs, so when it is weakened the medial border lifts away from the chest wall. This is medial scapular winging, and it is often more prominent when the patient pushes against a wall. Loss of upward rotation also explains the difficulty in lifting the arm above shoulder height.
Winging has other causes too. Weakness of trapezius gives lateral winging instead, with the scapula displaced outwards. Paralysis of the rhomboids (deep muscles of the upper back) is described as giving lateral winging too, but it is a less usual cause than weakness of serratus anterior. Trapezius is supplied by the spinal accessory nerve (cranial nerve XI).
Memory aid
a clamp that keeps the shoulder blade flat on the ribs
The muscle acts like a clamp holding the shoulder blade to the ribs; when the clamp fails, the blade lifts away like a wing.
Key facts
- Attachments
- from the upper ribs to the medial border of the scapula, the edge nearest the spine.
- Actions
- protracts and upwardly rotates the scapula, holding it against the chest wall.
- Nerve
- the long thoracic nerve, from roots C5 to C7 of the brachial plexus.
- Sign
- the medial border of the scapula lifts away from the chest wall, often most obvious when pushing against a wall.
- Typical cause
- traction injury, where the arm is stretched violently.
- Lateral winging
- weakness of trapezius displaces the scapula outwards instead; rhomboid paralysis is described as doing the same, though it is a less usual cause than weakness of serratus anterior.
Exam traps
- Trapezius can also cause a winged scapula, but its weakness gives lateral winging with a weak shrug, not the medial border lifting off the chest wall.
- The dorsal scapular nerve supplies the rhomboids and levator scapulae, the deep back muscles that move the scapula, and it tempts because it too arises from the upper roots of the brachial plexus (C5), but serratus anterior has a different nerve supply.
- The axillary nerve supplies deltoid and teres minor (a small muscle that rotates the arm outwards), so its injury weakens abduction and lateral rotation of the arm; it does not supply serratus anterior and does not cause medial scapular winging.
- Retraction pulls the scapula back towards the spine and is the opposite of the forward protraction in a push, so losing it does not explain the medial winging seen on a wall push.
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