Scapulothoracic Articulation

Normally there is considerable soft tissue flexibility, allowing the scapula to slide along the thorax and participate in all upper extremity motions.

Motions of the Scapula
Motions of the scapula are:

• Elevation, depression, protraction (abduction), and retraction (adduction), with clavicular motions at the SC joint are also component motions when the humerus moves.

• Upward and downward rotation, seen with clavicular motions at the SC joint and rotation at the AC joint, occurs concurrently with motions of the humerus. Upward rotation of the scapula is a necessary component motion for full ROM of flexion and abduction of the humerus.

• Winging and tipping, seen with motion at the AC joint concurrently with motions of the humerus. Winging is a transverse plane motion where the medial border lifts away from the rib cage; it normally occurs with horizontal adduction of the humerus. Forward tipping of the scapula occurs in conjunction with internal rotation and extension of the humerus when reaching the hand behind the back.

Scapular Stability
Postural relationship. In the dependent position, the scapula is stabilized primarily through a balance of forces. The weight of the arm creates a downward rotation, abduction, and forward tipping moment on the scapula. The downward rotation is balanced by the dynamic support of the upper trapezius and serratus anterior. The forward tipping and abduction is balanced by the dynamic support of the rhomboids and middle trapezius.

Active arm motions. With active arm motions, the muscles of the scapula function in synchrony to stabilize and control the position of the scapula so the scapulohumeral muscles can maintain an effective length-tension relationship as they function to stabilize and move the humerus. Without the positional control of the scapula, the efficiency of the humeral muscles decreases. The upper and lower trapezius with the serratus anterior upwardly rotate the scapula whenever the arm abducts or flexes, and the serratus anterior abducts (protracts) the scapula on the thorax to align the scapula during flexion or pushing activities. During arm extension or during pulling activities, the rhomboids function to downwardly rotate and adduct (retract) the scapula in synchrony with the latissimus dorsi, teres major, and rotator cuff muscles. These stabilizing muscles also eccentrically control acceleration motions of the scapula in the opposite directions.

Faulty posture. With a faulty scapular posture, muscle length and strength imbalances occur not only in the scapular muscles but also in the humeral muscles, altering the mechanics of the glenohumeral joint. A forward tilt of the scapula (seen with a forward head posture and increased thoracic kyphosis) is associated with decreased flexibility in the pectoralis minor, levator scapulae, and scalenius muscles and weakness in the serratus anterior or trapezius muscles. This scapular posture changes the posture of the humerus in the glenoid, which assumes a relatively abducted and internally rotated position with respect to the scapula. The glenohumeral internal rotators may become less flexible, and external rotators may weaken, affecting the mechanics of the joint.

Focus on Evidence

A study by Borstad and Ludewig, which looked at the effect of pectoralis minor resting length on scapular kinematics in subjects without shoulder pain, documented that those individuals with a short pectoralis minor (n = 25) had greater scapular internal rotation (protraction) and less posterior tipping during arm elevation in flexion, abduction, and scaption than those with a longer pectoralis minor (n = 25), thus providing evidence for altered pectoralis minor muscle length and altered scapular movement. In a related study by the same author, a correlation between the postural impairments of increased thoracic kyphosis, scapular internal rotation and forward tipping, and decreased pectoralis minor length was found to be significant, thus further supporting the relationship between muscle length and posture.

Suprahumeral (Subacromial) Space
The coracoacromial arch, composed of the acromion and coracoacromial ligament, overlies the subacromial/subdeltoid bursa, the supraspinatus tendon, and a portion of the muscle. These structures allow for and participate in normal shoulder function. Compromise of this space from faulty muscle function, faulty postural relationships, faulty joint mechanics, injury to the soft tissue in this region, or structural anomalies of the acromion lead to impingement syndromes. After a rotator cuff tear, the bursa may communicate with the glenohumeral joint cavity.

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