Shoulder Blade Ligaments: Connections, Functions, and Clinical Significance

What are the ligaments in the shoulder blade?

While technically no ligaments are "inside" the shoulder blade itself (as ligaments connect bones), several crucial ligaments connect to the scapula (shoulder blade), stabilizing its articulations with the clavicle and humerus, and others span across features of the scapula to form critical anatomical structures.

Understanding the Scapula's Role in Shoulder Stability

The scapula, or shoulder blade, is a flat, triangular bone that forms the posterior aspect of the shoulder girdle. It serves as an anchor point for numerous muscles involved in arm movement and shoulder stability. Unlike other appendicular bones that articulate directly with the axial skeleton through robust ligamentous connections, the scapula's primary connection to the trunk is muscular, allowing for its wide range of motion. However, it forms two key synovial joints where ligaments play a critical role: the acromioclavicular (AC) joint with the clavicle and the glenohumeral (GH) joint with the humerus. Additionally, some ligaments are intrinsic to the scapula, spanning between its bony processes.

Ligaments Connecting the Scapula to the Clavicle

These ligaments are vital for the stability of the acromioclavicular (AC) joint, which connects the acromion process of the scapula to the lateral end of the clavicle.

• Acromioclavicular (AC) Ligaments: These are fibrous bands that reinforce the AC joint capsule.

  • Superior AC Ligament: Located on the superior aspect of the joint, it blends with the aponeuroses of the deltoid and trapezius muscles.
  • Inferior AC Ligament: Found on the inferior aspect, it is weaker and often less distinct.
  • Function: They primarily resist anterior and posterior displacement of the clavicle relative to the acromion.

• Coracoclavicular (CC) Ligaments: These are exceptionally strong ligaments providing the primary vertical stability for the AC joint, preventing superior displacement of the clavicle. They connect the coracoid process of the scapula to the clavicle.

  • Conoid Ligament: Medial and cone-shaped, it originates from the base of the coracoid process and inserts onto the conoid tubercle on the inferior surface of the clavicle. It primarily resists posterior rotation and superior displacement of the clavicle.
  • Trapezoid Ligament: Lateral and quadrilateral, it originates from the superior surface of the coracoid process and inserts onto the trapezoid line on the inferior surface of the clavicle. It primarily resists medial displacement and posterior rotation of the scapula.
  • Function: Together, the conoid and trapezoid ligaments are crucial for the integrity of the AC joint and the transmission of forces from the upper limb to the axial skeleton.

Ligaments Intrinsic to the Scapula

These ligaments are located entirely within the scapula or connect two processes of the scapula itself.

• Superior Transverse Scapular Ligament (Suprascapular Ligament): This ligament bridges the suprascapular notch, converting it into the suprascapular foramen.

  • Location: It stretches from the base of the coracoid process to the medial side of the suprascapular notch.
  • Function: The suprascapular nerve passes through this foramen, while the suprascapular artery passes over it. It protects the nerve and can be a site of nerve compression if hypertrophied or ossified.

• Inferior Transverse Scapular Ligament (Spinoglenoid Ligament): Less consistently present and often described as a fibrous band rather than a true ligament.

  • Location: Extends from the lateral border of the spine of the scapula to the posterior rim of the glenoid.
  • Function: When present, it forms the spinoglenoid notch, through which the suprascapular nerve and vessels pass to supply the infraspinatus muscle.

• Coracoacromial Ligament: This strong, triangular ligament connects the coracoid process to the acromion process of the scapula.

  • Location: Spans between the lateral border of the coracoid process and the medial aspect of the acromion.
  • Function: It forms the coracoacromial arch (along with the acromion and coracoid process), which acts as a protective "roof" over the glenohumeral joint, preventing superior displacement of the humeral head and protecting the underlying rotator cuff tendons and subacromial bursa. It is not part of a joint capsule but rather an extrinsic ligament providing structural support.

Ligaments of the Glenohumeral Joint (Associated with the Scapula's Glenoid Fossa)

The glenohumeral (shoulder) joint is a ball-and-socket joint formed by the articulation of the head of the humerus with the glenoid fossa of the scapula. The glenoid fossa is deepened by the glenoid labrum, a fibrocartilaginous rim. The following ligaments are integral to the stability of this highly mobile joint, connecting the humerus to the scapula.

• Glenohumeral Ligaments (Superior, Middle, Inferior): These are thickenings of the fibrous capsule of the glenohumeral joint.

  • Location: They originate from the anterior aspect of the glenoid labrum and neck of the scapula and course to the anatomical neck of the humerus.
  • Function: They are crucial for anterior stability of the glenohumeral joint, particularly when the arm is abducted and externally rotated, preventing anterior dislocation of the humeral head. The Inferior Glenohumeral Ligament complex (anterior and posterior bands with an intervening axillary pouch) is the most important stabilizer when the arm is abducted.

• Coracohumeral Ligament:

  • Location: Arises from the base of the coracoid process of the scapula and inserts onto the greater and lesser tubercles of the humerus.
  • Function: It strengthens the superior aspect of the joint capsule, reinforces the rotator cuff interval, and helps prevent inferior displacement of the humeral head.

The Scapula's Unique Stability Mechanism

It is important to emphasize that while ligaments provide static stability to the joints involving the scapula, the scapula itself does not articulate directly with the thoracic cage via ligaments. Instead, its position and movement are predominantly controlled by a complex network of muscles (e.g., serratus anterior, rhomboids, trapezius, levator scapulae). This muscular stabilization, known as the "scapulothoracic articulation" (a physiological rather than anatomical joint), allows for the wide range of motion characteristic of the shoulder complex. Ligaments, in this context, serve to secure the scapula's connections to the clavicle and humerus, enabling the muscles to function effectively.

Clinical Significance and Injury

Ligaments associated with the scapula are frequently involved in shoulder injuries:

• AC Joint Sprains: Often result from a direct fall onto the shoulder or a direct blow to the acromion, causing tearing of the AC and/or coracoclavicular ligaments. These are graded based on the severity of ligamentous damage.
• Glenohumeral Instability and Dislocation: Tears or stretching of the glenohumeral ligaments, often combined with labral tears (e.g., Bankart lesion), can lead to recurrent shoulder dislocations.
• Suprascapular Nerve Entrapment: The suprascapular nerve can be compressed as it passes through the suprascapular foramen (under the superior transverse scapular ligament) or the spinoglenoid notch (under the inferior transverse scapular ligament), leading to pain and weakness in the supraspinatus and infraspinatus muscles.

Conclusion

While the phrase "ligaments in the shoulder blade" might initially suggest intrinsic structures, a detailed anatomical understanding reveals that ligaments either connect the scapula to adjacent bones (clavicle, humerus) or span across its bony processes. These ligaments are indispensable for maintaining the structural integrity and stability of the shoulder girdle, enabling the complex movements of the upper limb while also being vulnerable to injury in high-impact scenarios. Understanding their specific roles is fundamental for comprehending shoulder biomechanics and pathology.