Shoulder Joint: Ligaments of the Glenohumeral, AC, and SC Joints

What are the ligaments in the shoulder joint?

The shoulder joint, renowned for its exceptional mobility, relies heavily on a complex network of ligaments to provide crucial static stability, preventing excessive movement and dislocation while allowing for a wide range of motion.

Understanding Shoulder Joint Stability

The shoulder is not a single joint but a complex of several articulations working in concert: the glenohumeral (GH) joint, the acromioclavicular (AC) joint, and the sternoclavicular (SC) joint. While muscles (especially the rotator cuff) provide dynamic stability, ligaments are essential for passive, static stability, holding bones together and limiting extreme movements.

Ligaments of the Glenohumeral Joint

The glenohumeral joint, a true ball-and-socket articulation between the head of the humerus and the glenoid fossa of the scapula, is the most mobile and least inherently stable joint in the body. Its primary ligamentous stabilizers include:

• Glenohumeral Ligaments (GHLs): These are thickenings of the anterior joint capsule and are crucial for preventing anterior and inferior dislocation of the humeral head. They are typically described as three distinct bands:

  • Superior Glenohumeral Ligament (SGHL): Originates from the superior glenoid rim and inserts onto the anatomical neck of the humerus. It resists inferior translation of the humeral head when the arm is adducted and contributes to external rotation stability.
  • Middle Glenohumeral Ligament (MGHL): Originates from the superior glenoid rim, anterior to the SGHL, and inserts onto the anatomical neck of the humerus. It helps resist anterior translation of the humeral head, particularly when the arm is abducted to 45 degrees, and limits external rotation.
  • Inferior Glenohumeral Ligament (IGHL) Complex: This is the most significant anterior stabilizer, especially when the arm is abducted and externally rotated (the "apprehension position"). It consists of three parts:
    • Anterior Band: Originates from the anterior-inferior glenoid rim and inserts onto the anterior aspect of the humeral neck. It is the primary restraint to anterior translation when the arm is abducted and externally rotated.
    • Posterior Band: Originates from the posterior-inferior glenoid rim and inserts onto the posterior aspect of the humeral neck. It resists posterior translation.
    • Axillary Pouch: The hammock-like tissue connecting the anterior and posterior bands, which provides inferior stability when the arm is abducted.

• Coracohumeral Ligament: Originates from the base of the coracoid process of the scapula and inserts onto the greater and lesser tuberosities of the humerus, blending with the superior capsule. It helps resist inferior translation of the humeral head, especially when the arm is adducted, and limits external rotation. It also forms the roof of the rotator cuff interval.

Ligaments of the Acromioclavicular (AC) Joint

The AC joint connects the distal clavicle to the acromion of the scapula. While small, it is critical for shoulder girdle motion and stability. Its ligaments include:

• Acromioclavicular Ligament: Surrounds the AC joint capsule, reinforcing it superiorly and inferiorly. It primarily resists anterior-posterior separation of the clavicle and acromion.

  • Superior AC Ligament: Stronger and thicker, it is a primary restraint to posterior translation of the clavicle on the acromion.
  • Inferior AC Ligament: Thinner and less defined.

• Coracoclavicular Ligaments: These are exceptionally strong and crucial for the vertical stability of the AC joint, preventing superior displacement of the clavicle relative to the acromion. They also play a role in limiting clavicular rotation. They consist of two distinct parts:

  • Trapezoid Ligament: Located more laterally, originating from the superior surface of the coracoid process and inserting onto the trapezoid line on the inferior surface of the clavicle. It resists posterior displacement and medial translation of the clavicle and provides some resistance to compression forces.
  • Conoid Ligament: Located more medially and posteriorly, originating from the base of the coracoid process and inserting onto the conoid tubercle on the inferior surface of the clavicle. It primarily resists superior displacement and posterior rotation of the clavicle.

Ligaments of the Sternoclavicular (SC) Joint

The SC joint connects the medial end of the clavicle to the manubrium of the sternum. It is the only direct bony attachment of the upper limb to the axial skeleton and is surprisingly stable due to its strong ligaments:

• Anterior Sternoclavicular Ligament: Reinforces the joint capsule anteriorly, preventing anterior displacement of the clavicle.
• Posterior Sternoclavicular Ligament: Reinforces the joint capsule posteriorly, preventing posterior displacement of the clavicle.
• Interclavicular Ligament: Connects the superior aspects of the two clavicles across the jugular notch, providing stability to both SC joints and preventing excessive superior displacement of the clavicles.
• Costoclavicular Ligament: A very strong, short ligament connecting the inferior surface of the medial clavicle to the superior surface of the first rib and its costal cartilage. It is a primary stabilizer, resisting superior, anterior, and posterior displacement of the clavicle and limiting clavicular elevation.

Related Stabilizing Structures

While not strictly "ligaments of the shoulder joint" in the same sense as the above, other structures are critical for shoulder stability and function:

• Glenoid Labrum: A fibrocartilaginous ring attached to the rim of the glenoid fossa. It deepens the shallow glenoid cavity, increasing the contact area with the humeral head and providing an attachment point for the glenohumeral ligaments.
• Joint Capsule: A fibrous sac enclosing the glenohumeral joint, providing a general layer of containment. The glenohumeral ligaments are thickenings within this capsule.
• Rotator Cuff Tendons: The tendons of the supraspinatus, infraspinatus, teres minor, and subscapularis muscles. While muscles provide dynamic stability, their tendons blend with the joint capsule, contributing to static stability, especially during movement.

Understanding the intricate interplay of these ligaments is fundamental for comprehending shoulder biomechanics, assessing injuries, and designing effective rehabilitation and strengthening programs. Each ligament plays a specific role in maintaining the delicate balance between mobility and stability that defines the remarkable function of the human shoulder.