Scapulothoracic joint

Anatomy

Concerning that it is not a true joint, the scapulothoracic joint doesn’t have specific articular surfaces, ligaments nor the joint capsule. The classical approach to describing the anatomy of this joint is to analyze each of its three components. The articular surfaces and detailed anatomy of the acromioclavicular and sternoclavicular joints are described in their respective articles. Here we will focus on the junction between the scapula and the thoracic wall.

The scapulothoracic junction is a connection between the anterior surface of scapula and the superolateral surface of the thoracic wall. The former is slightly concave anteriorly, while the latter is accordingly convex. The surfaces don’t adhere directly to each other. Instead, they are separated by the subscapularis muscle that fills the subscapular fossa on the anterior surface of scapula, the serratus anterior muscle which attaches to the thoracic wall, and the fascial space between these two muscles. This fascial space is filled with the loose connective tissue that facilitates the gliding movements of scapula.

Scapular stability

Since it has no ligaments, the scapulothoracic junction is stabilized by the synchronized actions and passive tensions of the three functional muscle units;

• The trapezius muscle
• The serratus anterior muscle
• The medial stabilizers of the scapula; levator scapulae and rhomboid muscles.

These muscles are also the main muscles of the scapulothoracic motion, while the muscles of the chest wall and the back assist them.

Innervation

The innervation of the scapulothoracic junction refers to the nerves that supply the main muscles of the scapulothoracic motion;

• Accessory nerve (CN XI) for the trapezius muscle
• Long thoracic nerve for the serratus anterior muscle
• Dorsal scapular nerve for the levator scapulae and rhomboids.

Blood supply

Blood supply to the scapulothoracic junction is also reflected by the supply of the scapular stabilizers;

• Occipital, superficial cervical and dorsal scapular arteries for the trapezius
• Thoracodorsal artery for the serratus anterior
• Transverse cervical and ascending cervical arteries for the levator scapulae
• Dorsal scapular and the upper five to six posterior intercostal arteries for the rhomboids

Movements

The scapulothoracic junction enables the mobility of the scapula around the fulcrum of the acromioclavicular joint. Scapular mobility is important for the proper positioning of the shoulder joint in space. The scapula moves by gliding against the chest wall in three degrees of freedom;

• Elevation (range of motion 40°) - depression (RoM 10°)
• Protraction (20°) - retraction (15°)
• External rotation (60°) - Internal rotation (30°)

During these movements, the scapula is stabilized by the muscles that attach to it and by the ligaments of the AC joint. The protraction and retraction of the scapula happens around a vertical axis passing through the lateral end of clavicle. They are limited by the conoid and trapezoid ligaments of the AC joint, respectively. Elevation and depression of the scapula occur around the sagittal axis that passes through the center of the acromioclavicular joint. The former is limited by the coracoclavicular ligament, while the latter goes as far as the coracoid process of scapula and touches the clavicle. External and Internal rotation of the scapula happen around a transverse fulcrum passing through the conoid ligament of the acromioclavicular joint and the AC joint itself. The rotatory movements are sustained by the coracoclavicular ligament.

Since it is not a true joint, the scapulothoracic junction doesn’t have the capsular pattern nor the close-packed position. The joint takes the loose-packed position when the arm is resting by the side of the body.

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Muscles acting on the scapulothoracic joint

All muscles which attach to scapula contribute to its mobility. The external rotation requires active contraction of the descending and transverse parts of the trapezius muscle, which pull the acromion and spine of scapula superomedially. Concurrent contraction of the levator scapulae muscle neutralizes  rotation, thus producing the elevation of the scapula.

Depression and internal rotation are the primary passive movements, happening with the relaxation of the trapezius and levator scapulae and consequential drop of the scapula by the force of gravity. However, pectoralis minor and the lowermost fibers of the serratus anterior muscle can actively depress the scapula, while the levator scapulae, rhomboids and pectoralis minor are active internal rotators.

Protraction happens with the pull of the serratus anterior and pectoralis minor muscles, while retraction is produced by the horizontal and ascending parts of the trapezius muscle, and rhomboid minor and major muscles.