The lower digitations of serratus anterior, arising from ribs 5-9, are the most powerful portion of the muscle and are critical for scapular upward rotation during arm elevation. They interdigitate with the external oblique on the chest wall, forming a visible serrated pattern in muscular individuals. The lower slips are the primary cause of winging when lost, as they rotate the inferior angle away from the thorax.
| Origin | Outer surfaces and superior borders of ribs 5-9 (lower digitations) |
|---|---|
| Insertion | Inferior angle and costal surface of the scapula (lower fibres insert into the inferior angle) |
| Nerve Supply | Long thoracic nerve (C5, C6, C7) |
| Blood Supply | Lateral thoracic artery |
| Actions |
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They upwardly rotate and protract the scapula, essential for raising the arm overhead and for holding the scapula against the chest wall.
Selective wasting of the lower serratus slips is the most visually dramatic sign of long thoracic nerve palsy, producing medial winging of the inferior angle that is accentuated by wall push-ups. The lower slip fibres are the first to recover after long thoracic nerve injury due to their more distal position. Strengthening the lower serratus slips is the cornerstone of scapular dyskinesis rehabilitation in overhead athletes.
The lower digitations are visible and palpable on the lateral chest wall between the latissimus dorsi and pectoralis major in lean individuals. Wasting is confirmed by comparing the serrated contour bilaterally during shoulder protraction.
Paralysis of the lower serratus slips from long thoracic nerve injury producing medial scapular winging, loss of arm elevation above 90 degrees, and a characteristic inferior angle protrusion that worsens with forward flexion.
Musculotendinous injury to the lower digitations from sudden load with the arm overhead, producing lateral chest wall pain exacerbated by deep breathing and protraction.
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