Scapula: The Anchor of the Shoulder Girdle

1. Overview

The scapula (shoulder blade) is a flat, triangular bone situated on the posterolateral aspect of the thorax, overlying the 2nd to 7th ribs. It acts as the primary attachment point for muscles of the arm and back.

2. Surfaces and Fossae

• Costal Surface: Contains the Subscapular Fossa, where the subscapularis muscle originates.
• Posterior Surface: Divided by the Spine of the Scapula into two regions:
  • Supraspinatous Fossa: (Origin of Supraspinatus).
  • Infraspinatous Fossa: (Origin of Infraspinatus).

3. Processes

• Acromion: The lateral continuation of the spine. It articulates with the clavicle (AC joint).
• Coracoid Process: A beak-like projection anteriorly. It serves as an attachment for the short head of the biceps brachii, coracobrachialis, and pectoralis minor.
• Glenoid Cavity: A shallow socket that articulates with the humerus. Its depth is slightly increased by a fibrocartilaginous rim called the Glenoid Labrum.

4. Scapulohumeral Rhythm

For every 180° of arm abduction, 120° occurs at the glenohumeral joint, and 60° occurs due to the rotation of the scapula. This 2:1 ratio is essential for full range of motion.

5. Clinical Correlates

• Winged Scapula: Resulting from paralysis of the Serratus Anterior muscle (due to long thoracic nerve injury). The medial border of the scapula protrudes posteriorly like a wing.
• Bankart Lesion: An injury to the anterior-inferior aspect of the glenoid labrum, often following shoulder dislocation.

6. Questions

1. Which nerve injury causes a "winged scapula"?
2. What are the three muscles that attach to the coracoid process?
3. What is the functional importance of the Glenoid Labrum?