Scapula

1. Overview

The scapula, commonly known as the shoulder blade, is a large, flat, triangular bone that forms the posterior part of the shoulder girdle. It serves as the site of articulation for the humerus and clavicle and acts as an attachment platform for numerous muscles involved in upper limb movement and stabilization. The scapula plays a central role in linking the upper extremity to the axial skeleton.

2. Location

The scapula is located on the posterior thoracic wall:

• Position: Between the second and seventh ribs, lying over the posterolateral chest wall.

• Medial border: Parallels the vertebral column.

• Lateral angle: Articulates with the humeral head at the glenoid cavity to form the glenohumeral (shoulder) joint.

• Superiorly: Connects to the clavicle at the acromioclavicular (AC) joint.

3. Structure

The scapula is a flat, triangular bone with several important anatomical features:

• Three borders: Superior, medial (vertebral), and lateral (axillary).

• Three angles: Superior, inferior, and lateral.

• Anterior surface: Features the subscapular fossa for the subscapularis muscle.

• Posterior surface: Divided by the prominent spine into the supraspinous and infraspinous fossae.

• Spine of scapula: Ends laterally as the acromion, which articulates with the clavicle.

• Coracoid process: A curved anterior projection for muscle and ligament attachment.

• Glenoid cavity (fossa): A shallow socket that articulates with the head of the humerus to form the shoulder joint.

The scapula is made of compact bone on the outer surfaces and spongy bone within, supporting muscular and ligamentous attachments.

4. Function

The scapula serves multiple vital functions in upper limb biomechanics:

• Muscle attachment: Serves as the origin or insertion site for 17 muscles, including the deltoid, trapezius, rotator cuff group, and serratus anterior.

• Articulation: Forms part of the shoulder joint (glenohumeral joint) and the shoulder girdle (with the clavicle).

• Mobility platform: Moves in multiple directions (elevation, depression, protraction, retraction, rotation) to enable full arm movement.

• Force transmission: Helps transfer forces from the upper limb to the thorax via muscle and ligament connections.

5. Physiological role(s)

Although passive, the scapula plays key roles in physiological and neuromuscular coordination:

• Scapulohumeral rhythm: Coordinates with the humerus during arm elevation, contributing to range of motion and preventing impingement.

• Stabilization: Serves as a stable base for rotator cuff and other shoulder muscles to act effectively.

• Respiratory support: Anchors muscles like the serratus anterior that assist in forced respiration.

• Proprioception: Through associated musculature, provides sensory feedback to the nervous system for coordinated upper limb movement.

6. Clinical Significance

Disorders and injuries involving the scapula can affect upper limb function and posture:

• Scapular fractures:

  • Often occur due to high-energy trauma; may involve the body, neck, or glenoid fossa.

• Scapular winging:

  • Caused by paralysis of the serratus anterior (long thoracic nerve injury); results in prominence of the medial border during arm movement.

• Shoulder impingement syndrome:

  • Poor scapular positioning can lead to compression of rotator cuff tendons under the acromion.

• Snapping scapula syndrome:

  • Characterized by audible and palpable clicking due to bursitis or abnormal bone contact during scapular movement.

• Muscle imbalances or postural dysfunction:

  • Rounded shoulders or kyphosis can affect scapular mobility and shoulder kinematics, leading to pain or dysfunction.

• Surgical landmark:

  • The spine of the scapula is a palpable and reliable landmark in surgical and clinical assessments.