Shoulder girdle

The shoulder girdle, also known as the pectoral girdle, is a bony structure that connects the upper limbs to the axial skeleton. It consists of two paired bones: the clavicle (collarbone) and the scapula (shoulder blade) on each side of the body. These bones provide support, stability, and articulation for the shoulder joint, enabling a wide range of movements for the arms.

Location

The shoulder girdle is located in the upper part of the torso. It extends from the base of the neck to the upper back and is positioned laterally on both sides of the thorax. The clavicle runs horizontally at the front of the chest, while the scapula lies posteriorly on the upper back, overlying the ribs.

Anatomy

The shoulder girdle, also known as the pectoral girdle, forms the skeletal foundation for the upper limb’s attachment to the axial skeleton. It is composed of two clavicles (collarbones) and two scapulae (shoulder blades), which are connected through joints, muscles, ligaments, and soft tissues. The shoulder girdle provides mobility, stability, and structural support to the shoulder and upper limbs.

Here is a detailed breakdown of the anatomy of the shoulder girdle:

Components of the Shoulder Girdle

The shoulder girdle consists of two primary paired bones:

• Clavicle (Collarbone)
• Scapula (Shoulder Blade)

Clavicle

The clavicle is an S-shaped, long bone located horizontally at the front of the shoulder:

• Location: Extends from the sternum (manubrium) medially to the acromion of the scapula laterally.
• Surface:
  • Superior Surface: Smooth and palpable beneath the skin.
  • Inferior Surface: Rougher, with attachment points for ligaments and muscles.
• Ends:
  • Medial (Sternal) End: Articulates with the manubrium of the sternum, forming the sternoclavicular joint.
  • Lateral (Acromial) End: Articulates with the acromion process of the scapula, forming the acromioclavicular joint.
• Functions: Provides structural support and acts as a strut to keep the upper limb away from the thorax.

Scapula

The scapula is a flat, triangular-shaped bone located on the posterior aspect of the thorax:

• Location: Spans from the 2nd to the 7th rib level on the posterior chest wall.
• Surfaces:
  • Anterior (Costal) Surface: Lies against the rib cage and features the subscapular fossa for the attachment of the subscapularis muscle.
  • Posterior Surface: Contains the spine of the scapula, which divides the surface into:
    • Supraspinous Fossa: Above the spine.
    • Infraspinous Fossa: Below the spine.
• Borders:
  • Superior Border: Thin edge, containing the scapular notch.
  • Medial (Vertebral) Border: Closest to the spine.
  • Lateral (Axillary) Border: Thick edge near the arm.
• Angles:
  • Superior Angle: Near the 2nd rib.
  • Inferior Angle: Marks the 7th rib level.
  • Lateral Angle: Contains the glenoid cavity for articulation with the humerus.
• Processes:
  • Spine of the Scapula: A prominent ridge that extends laterally.
  • Acromion Process: A bony projection that articulates with the clavicle.
  • Coracoid Process: A hook-like projection anteriorly for muscle and ligament attachment.^[6]
• Glenoid Cavity: A shallow, oval-shaped fossa on the lateral angle that articulates with the head of the humerus, forming the glenohumeral joint.

Joints of the Shoulder Girdle

The shoulder girdle contains three major joints and one functional joint that allow movement:

• Sternoclavicular Joint (SC Joint):
  • Articulation: Medial end of the clavicle and the manubrium of the sternum.
  • Type: Synovial saddle joint.
  • Supported by:
    • Anterior and Posterior Sternoclavicular Ligaments
    • Interclavicular Ligament
    • Costoclavicular Ligament
• Acromioclavicular Joint (AC Joint):
  • Articulation: Lateral end of the clavicle and the acromion of the scapula.
  • Type: Synovial plane joint.
  • Supported by:
    • Acromioclavicular Ligament
    • Coracoclavicular Ligament (comprising the conoid and trapezoid ligaments)
• Glenohumeral Joint (Shoulder Joint):
  • Articulation: Glenoid cavity of the scapula and the head of the humerus.
  • Type: Ball-and-socket joint.
  • Although part of the shoulder complex, this joint connects the upper limb to the scapula.
• Scapulothoracic Joint (Functional Joint):
  • Not a true anatomical joint but a sliding articulation between the anterior surface of the scapula and the rib cage (thorax).
  • Allows movements like elevation, depression, protraction, retraction, and rotation of the scapula.^[4]

Muscles of the Shoulder Girdle

The shoulder girdle serves as the attachment site for numerous muscles that control the movement of the scapula and clavicle:

Muscles Connecting the Shoulder Girdle to the Axial Skeleton

1. Trapezius: Elevates, retracts, and rotates the scapula.
2. Levator Scapulae: Elevates the scapula.
3. Rhomboid Major and Minor: Retract and stabilize the scapula.
4. Serratus Anterior: Protracts and rotates the scapula.

Muscles Connecting the Shoulder Girdle to the Upper Limb

1. Pectoralis Minor: Depresses and protracts the scapula.
2. Subclavius: Stabilizes the clavicle.
3. Deltoid: Moves the arm and stabilizes the shoulder joint.

Ligaments of the Shoulder Girdle

Ligaments provide stability to the shoulder girdle joints:

• Sternoclavicular Ligaments: Stabilize the sternoclavicular joint.
• Acromioclavicular Ligament: Reinforces the acromioclavicular joint.
• Coracoclavicular Ligament: Connects the coracoid process of the scapula to the clavicle.
• Coracoacromial Ligament: Forms an arch over the glenohumeral joint, protecting it from superior displacement.

Blood Supply and Nerve Supply

Blood Supply

• Clavicle: Supplied by branches of the suprascapular artery and thoracoacromial artery.
• Scapula: Supplied by the suprascapular artery, dorsal scapular artery, and circumflex scapular artery.^[1]

Nerve Supply

The nerves supplying the shoulder girdle arise primarily from the brachial plexus:

• Accessory Nerve (CN XI): Innervates the trapezius.
• Dorsal Scapular Nerve: Innervates the rhomboids and levator scapulae.
• Long Thoracic Nerve: Supplies the serratus anterior.

Surface Landmarks of the Shoulder Girdle

Key anatomical landmarks of the shoulder girdle include:

• Clavicle: Easily palpable across the front of the chest.
• Acromion: The bony prominence on the lateral shoulder.
• Coracoid Process: Anterior hook-like projection below the clavicle.
• Spine of the Scapula: Prominent ridge running across the posterior scapula.
• Glenoid Cavity: Depression on the lateral scapula for humeral articulation.

Function

The shoulder girdle, composed of the clavicle and scapula, plays a fundamental role in supporting and enabling the movement of the upper limb while providing stability and flexibility. Its unique anatomical structure allows for a wide range of motions, essential for various physical activities, including lifting, pushing, pulling, and precise hand movements. The shoulder girdle works in coordination with muscles, ligaments, and joints to facilitate mobility and stability.

Below is a detailed explanation of its functions:

Connection of the Upper Limb to the Axial Skeleton

The shoulder girdle serves as the link between the upper limbs and the axial skeleton:

• The clavicle articulates with the sternum at the sternoclavicular joint, connecting the shoulder girdle to the trunk.^[8]
• The scapula connects with the clavicle at the acromioclavicular joint and with the humerus at the glenohumeral joint, allowing the upper limb to remain suspended from the axial skeleton.
• The clavicle functions as a strut, holding the upper limb away from the thorax to optimize mobility and prevent compression against the chest wall.

This connection provides a strong yet flexible base for the upper limbs.

Facilitates Upper Limb Movements

The shoulder girdle allows a wide range of movements for the upper limb by acting as a mobile base:

Movements of the Scapula (Scapulothoracic Motion):

• Elevation: Raising the scapula upward (e.g., shrugging the shoulders).
• Depression: Lowering the scapula downward (e.g., pushing shoulders down).
• Protraction: Moving the scapula forward (e.g., reaching forward).
• Retraction: Moving the scapula backward (e.g., squeezing shoulder blades together).
• Upward Rotation: Rotating the scapula so the glenoid cavity faces upward (e.g., raising the arm overhead).
• Downward Rotation: Rotating the scapula so the glenoid cavity faces downward.

These movements, primarily enabled by muscles like the trapezius, rhomboids, and serratus anterior, allow the upper limb to position itself for various tasks.

Movements of the Clavicle:

• The clavicle assists in shoulder girdle movements at the sternoclavicular joint and acromioclavicular joint.
• It rotates slightly during shoulder elevation and depression, increasing the range of motion for the shoulder.^[7]

The combined motion of the clavicle, scapula, and humerus allows for activities such as reaching, lifting, and throwing.

Stability of the Shoulder Joint (Glenohumeral Joint)

The shoulder girdle plays a critical role in stabilizing the glenohumeral joint, which is inherently unstable due to its shallow glenoid cavity:

• The scapula provides the glenoid cavity, a socket for articulation with the humeral head, enabling a wide range of motion.
• Ligaments like the coracoacromial ligament and coracoclavicular ligament stabilize the joint by preventing excessive movement.
• The muscles of the rotator cuff (supraspinatus, infraspinatus, teres minor, and subscapularis) provide dynamic stability to the shoulder joint.

This stabilization is essential during movements such as lifting or supporting heavy objects.

Weight Transmission and Force Distribution

The shoulder girdle transmits forces between the upper limb and the trunk:

• Forces generated during activities such as lifting, carrying, or pushing are distributed from the humerus to the scapula and clavicle, and then to the axial skeleton via the sternoclavicular joint.
• The clavicle acts as a shock absorber, protecting the upper limb and thorax from excessive mechanical forces.

This weight transmission enables effective use of the arms and hands while maintaining balance and posture.

Protection of Neurovascular Structures

The shoulder girdle protects critical neurovascular structures passing between the neck and upper limb:

• The clavicle shields the brachial plexus (a network of nerves supplying the upper limb) and subclavian vessels (subclavian artery and vein).
• The axillary artery, vein, and nerves pass beneath the clavicle and alongside the scapula, making their protection essential for normal arm function.^[5]

This protective role prevents injury to nerves and blood vessels during movement or trauma.

Facilitates Functional Scapulohumeral Rhythm

The shoulder girdle ensures smooth and coordinated movements between the scapula and humerus:

• Scapulohumeral rhythm refers to the synchronized motion of the scapula and humerus during shoulder elevation.
• For every 2 degrees of movement at the glenohumeral joint, the scapula rotates 1 degree.
• This coordinated movement allows for a full range of motion while minimizing stress on the glenohumeral joint.

The scapula’s ability to adjust its position ensures proper alignment of the glenoid cavity for arm movements.

Muscle Attachment for Movement and Posture

The shoulder girdle serves as an anchor for many muscles involved in:

• Upper Limb Movement: Muscles like the deltoid, trapezius, latissimus dorsi, and pectoralis major attach to the scapula and clavicle to facilitate arm movement.
• Postural Support: Muscles like the levator scapulae and rhomboids stabilize the scapula to maintain upright posture.

The extensive muscle attachments enable powerful movements and controlled stabilization.

Shock Absorption and Stress Reduction

The clavicle acts as a strut to absorb and dissipate mechanical forces:

• During activities involving weight-bearing or impact (e.g., falls or heavy lifting), the clavicle transmits forces to the axial skeleton, reducing stress on the shoulder joint and thorax.

Facilitating Overhead and Precise Movements

The shoulder girdle’s mobility enables overhead activities and fine motor tasks:

• Movements like throwing, reaching, climbing, and swimming require coordinated actions between the scapula, clavicle, and humerus.^[3]
• The flexibility of the shoulder girdle allows for precise hand and arm positioning, essential for activities such as writing, painting, and tool use.

Clinical Significance

The shoulder girdle is a critical structure that provides mobility and stability to the upper limb. Its complex anatomy makes it susceptible to various clinical conditions, which can impair movement and cause significant pain.

Fractures

• Clavicle Fractures: Common in falls or direct trauma, as the clavicle absorbs forces transmitted to the upper limb.
• Scapular Fractures: Rare but occur due to high-impact trauma (e.g., vehicle accidents).

Dislocations

• Acromioclavicular Joint Dislocation: Known as a “shoulder separation,” often caused by falls or sports injuries.
• Sternoclavicular Joint Dislocation: Can affect nearby structures like the trachea or blood vessels.

Rotator Cuff Injuries

Tears or inflammation in the muscles stabilizing the shoulder joint, leading to pain and reduced mobility.

Shoulder Instability

Recurrent dislocations of the glenohumeral joint can result from ligament laxity or trauma.

Nerve Compression

Compression of the brachial plexus or surrounding nerves (e.g., in thoracic outlet syndrome) causes pain, weakness, or numbness in the upper limb.^[2]

Arthritis

Degenerative changes in the acromioclavicular or sternoclavicular joints can lead to stiffness and pain.