The Scapula: Associated Ligaments, Function, and Injuries

How Many Ligaments Are In Scapula?

While the scapula, or shoulder blade, does not contain ligaments within its structure, it serves as a crucial anchor point for numerous ligaments that stabilize the complex shoulder girdle, connecting it to the humerus (upper arm bone) and clavicle (collarbone) to facilitate a wide range of motion and provide essential stability.

Understanding the Scapula: The Shoulder Blade's Role

The scapula is a large, triangular, flat bone positioned on the posterior aspect of the rib cage. It's a cornerstone of the shoulder girdle, providing a mobile base for the upper limb. Unlike many bones that articulate directly with the axial skeleton, the scapula's primary connection to the trunk is indirect, mainly through its articulation with the clavicle and the extensive network of muscles that attach to it. This unique arrangement allows for remarkable mobility, essential for the diverse functions of the arm, from reaching overhead to intricate fine motor tasks.

The Scapula's Articulations: Where Ligaments Come Into Play

Ligaments are strong, fibrous connective tissues that primarily connect bone to bone, providing stability to joints and guiding joint motion. The scapula participates in several key articulations, each stabilized by a specific set of ligaments:

• The Glenohumeral (Shoulder) Joint: This is the primary articulation between the scapula (specifically, its glenoid cavity) and the humerus (head of the upper arm bone). It is a highly mobile ball-and-socket joint, making it inherently less stable and heavily reliant on a capsulo-ligamentous complex for integrity.
• The Acromioclavicular (AC) Joint: This articulation occurs between the acromion process of the scapula and the lateral end of the clavicle. It allows for scapular rotation and contributes to the overall stability of the shoulder complex.
• The Scapulothoracic "Joint": While often referred to as a joint, the scapulothoracic articulation is not a true anatomical joint in the same sense as the glenohumeral or AC joint. It is a physiological articulation formed by the scapula gliding over the posterior rib cage, separated by muscle layers (serratus anterior and subscapularis). There are no direct ligaments connecting the scapula to the ribs in this region; its stability and movement are entirely dependent on surrounding musculature and the integrity of the AC and sternoclavicular joints.
• Intrinsic Scapular Ligaments: The scapula also possesses ligaments that span across its own processes, contributing to the internal architecture and function of the bone itself, particularly in forming foramina for nerves and vessels.

Key Ligaments Associated with the Scapula

Given the scapula's role as a foundational component of the shoulder, it is associated with a significant number of ligaments that provide static stability to its various articulations. Here are the primary ligaments directly connecting to or spanning across the scapula:

• Glenohumeral Ligaments (GHLs): These are thickenings of the anterior joint capsule of the shoulder, providing critical stability to the glenohumeral joint. There are typically three distinct bands:
  • Superior Glenohumeral Ligament: Limits inferior translation of the humeral head.
  • Middle Glenohumeral Ligament: Limits anterior translation, especially with the arm in abduction and external rotation.
  • Inferior Glenohumeral Ligament Complex: The most important stabilizer against anterior and posterior dislocation, particularly when the arm is abducted. It consists of an anterior band, posterior band, and an axillary pouch.
• Coracohumeral Ligament: Originating from the coracoid process of the scapula and inserting onto the humerus, this ligament strengthens the superior aspect of the shoulder joint capsule and helps prevent inferior displacement of the humeral head.
• Coracoacromial Ligament: Spanning between the coracoid process and the acromion of the scapula, this ligament forms the coracoacromial arch, which protects the superior aspect of the glenohumeral joint and prevents superior displacement of the humeral head. It does not connect two separate bones but rather two processes of the same bone (scapula).
• Acromioclavicular Ligament: This ligament strengthens the joint capsule of the AC joint, connecting the acromion of the scapula to the clavicle. It provides anterior-posterior stability.
• Coracoclavicular Ligaments: These are exceptionally strong ligaments, crucial for the stability of the AC joint and the suspension of the scapula from the clavicle. They consist of two distinct bands:
  • Trapezoid Ligament: The more lateral and flatter of the two, resisting superior displacement of the clavicle and limiting excessive medial movement of the scapula.
  • Conoid Ligament: The more medial and conical, preventing superior displacement of the clavicle and controlling scapular rotation.
• Superior Transverse Scapular Ligament (STSL): This ligament spans the suprascapular notch of the scapula, converting it into a foramen (suprascapular foramen) through which the suprascapular nerve passes. The suprascapular artery typically passes over the ligament.
• Inferior Transverse Scapular Ligament (Spinoglenoid Ligament): Less consistently present and often described as a fibrous band, it spans the spinoglenoid notch, creating a passage for the inferior branch of the suprascapular nerve and vessels.

Counting these primary, distinct structures, there are approximately 9 key ligaments (3 Glenohumeral, 1 Coracohumeral, 1 Coracoacromial, 1 Acromioclavicular, 2 Coracoclavicular, 1 Superior Transverse Scapular Ligament) that are directly attached to or span across the scapula and its processes, contributing to the stability and function of the shoulder girdle.

Functional Significance of Scapular Ligamentous Support

The intricate network of ligaments associated with the scapula is vital for several reasons:

• Joint Stability: They provide passive restraint, preventing excessive movement and dislocation of the glenohumeral and AC joints.
• Movement Guidance: Ligaments help guide the precise kinematic movements of the scapula and humerus, ensuring efficient and safe range of motion.
• Force Transmission: They contribute to the efficient transmission of forces from the trunk through the shoulder girdle to the upper limb, crucial for activities requiring strength and power.
• Protection of Neurovascular Structures: Ligaments like the superior transverse scapular ligament form protective passages for nerves and vessels, safeguarding them from compression or injury.

Clinical Relevance and Injury

Due to its complex role and high mobility, the ligamentous structures around the scapula are susceptible to injury. Common examples include:

• AC Joint Separations: Often resulting from a direct fall onto the shoulder, these injuries involve tears of the acromioclavicular and/or coracoclavicular ligaments, leading to varying degrees of clavicular displacement relative to the acromion.
• Shoulder Dislocations: The glenohumeral ligaments are frequently stretched or torn during shoulder dislocations, particularly anterior dislocations, which are the most common type.
• Impingement Syndromes: While primarily muscular, the coracoacromial ligament can contribute to the subacromial space narrowing, potentially exacerbating impingement of the rotator cuff tendons.
• Suprascapular Nerve Entrapment: The superior transverse scapular ligament can become taut or ossified, leading to compression of the suprascapular nerve as it passes through the suprascapular notch, causing pain and weakness in the shoulder.

Conclusion

While the question "how many ligaments are in scapula?" might imply an internal count, the scapula itself is a bone, not a joint containing ligaments. Instead, it serves as a critical attachment site and component of multiple joints, primarily the glenohumeral and acromioclavicular joints. The approximately nine key ligaments directly associated with the scapula's stability are fundamental to the intricate biomechanics of the shoulder, ensuring both its remarkable mobility and essential structural integrity. Understanding these ligaments is crucial for comprehending shoulder function, injury mechanisms, and effective rehabilitation strategies.