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Bursa of Shoulder Girdle/Shoulder Girdle is structurally a ball-and-socket joint and functionally is considered a diarthrodial, multiaxial, joint. The glenohumeral articulation involves the humeral head with the glenoid cavity of the scapula, and it represents the major articulation of the shoulder girdle.[rx] The latter also includes minor articulations of the sternoclavicular (SC), acromioclavicular (AC), and scapulothoracic joints.[rx][rx][rx] The glenohumeral joint ranks as the most mobile joint of the human body.[rx][rx] The static and dynamic stabilizing structures allow for extreme degrees of motion in multiple planes of the body that predisposes the joint to instability events.
Bursa of Shoulder Girdle
In addition to the synovial fluid reducing friction within the joint, there are multiple synovial bursae present as well. These bursae functionally act as a cushion between joint structures, such as tendons. The most clinically significant are the subacromial and subscapular bursae. There are numerous, including:
- Subacromial/subdeltoid bursa – This structure lies between the deltoid muscle and joint capsule in the superolateral aspect of the joint. It is superficial to the supraspinatus tendon. This bursa reduces friction underneath the deltoid muscle, allowing an increased range of motion. This bursa, excluding anatomic variants, does not usually communicate with the shoulder joint itself.
- Subcoracoid bursa – This bursa is between the coracoid process and the subscapularis.
- Subscapular bursa – is located between the tendon of the subscapularis muscle and the capsule. It functions to reduces frictional damage to the subscapularis muscle during movement of the glenohumeral joint, particularly during internal rotation.
Ligament of Shoulder Girdle
Static stabilizing structures include the osseous articular anatomy and joint congruity, the glenoid labrum, the glenohumeral ligaments, joint capsule, and negative intraarticular pressure [rx]:
- Glenohumeral ligaments– Composed of a superior, middle, and inferior ligament, these three ligaments combine to form the glenohumeral joint capsule connecting the glenoid fossa to the humerus. Due to their location, they protect the shoulder and prevent it from dislocating anteriorly — this group of ligaments functions as the primary stabilizers of the joint.
- Coracoclavicular ligament – This ligament is composed of the conoid and trapezoid ligaments and spans from the coracoid process to the clavicle. It functions to maintain the position of the clavicle in conjunction with the acromioclavicular ligament. Strong forces can rupture these ligaments during acromioclavicular joint injuries.
- Coracohumeral ligament – This ligament supports the superior aspect of the joint capsule. It is a dense fibrous structure connecting the base of the coracoid process to the greater and lesser tuberosities. At its origin, the ligament is thin and broad, measuring about 2 cm in diameter at the base of the coracoid. Laterally, the CHL separates into two distinct bands that envelope the Long Head Biceps tendon at the proximal extent of the bicipital groove.
Dynamic stabilizing structures include the Long head biceps tendon, rotator cuff muscles, the rotator interval, and the periscapular muscles.
- The subscapularis has superficial and deep fibers that envelope the bicipital groove, creating the “roof” and “floor,” respectively. These fibers also coalesce with those from the supraspinatus and superior glenohumeral ligament/coracohumeral ligament complex. These structures attach intimately at the lesser tuberosity to create the proximal and medial aspect of the pulley system, with soft tissue extensions serving to further envelope the LHBT in the bicipital groove. Once the LHBT exits the groove, it takes a 30- to 40-degree turn as it heads toward the supraglenoid tubercle and glenoid labrum. Thus, the proximal soft tissue elements of the groove are especially critical for the overall stability of the entire biceps complex.
Movement of Shoulder Girdle
The glenohumeral joint possesses the capability of allowing an extreme range of motion in multiple planes.[rx]
- Flexion – Defined as bringing the upper limb anterior in the sagittal plane. The usual range of motion is 180 degrees. The main flexors of the shoulder are the anterior deltoid, coracobrachialis, and pectoralis major. Biceps brachii also weakly assists in this action.
- Extension—Defined as bringing the upper limb posterior in a sagittal plane. The normal range of motion is 45 to 60 degrees. The main extensors of the shoulder are the posterior deltoid, latissimus dorsi, and teres major.
- Internal rotation—Defined as rotation toward the midline along a vertical axis. The normal range of motion is 70 to 90 degrees. The internal rotation muscles are the subscapularis, pectoralis major, latissimus dorsi, teres major, and the anterior aspect of the deltoid.
- External rotation – Defined as rotation away from the midline along a vertical axis. The normal range of motion is 90 degrees. Primarily infraspinatus and teres minor are responsible for the motion.
- Adduction – Defined as bringing the upper limb towards the midline in the coronal plane. Pectoralis major, latissimus dorsi, and teres major are the muscles primarily responsible for shoulder adduction.
- Abduction – Defined as bringing the upper limb away from the midline in the coronal plane. The normal range of motion is 150 degrees. Due to the ability to differentiate several pathologies by the range of motion of the glenohumeral joint in this plane of motion, it is essential to understand how different muscles contribute to this action.[rx]
Range Of Motion
- I. The supraspinatus is responsible for the first 0 to 15 degrees of abduction[rx]
- II. The middle fibers of the deltoid are responsible for approximately 15 to 90 degrees of abduction following[rx]
- III. Scapular rotation due to the actions of the trapezius and serratus anterior allow for abduction beyond 90 degrees