Shoulder joint

Type: The shoulder joint (or glenohumeral joint from Greek glene, eyeball, + -oid, 'form of', + Latin humerus, shoulder) is a multiaxial synovial ball and socket joint.

Articular surface: The joint is formed by articulation of the scapula and the head of the humerus. Therefore, it is also known as the glenohumeral articulation.

Fig: Bones of Shoulder joint

Maintenance of stability:

1.      The coracoacromial arch or secondary socket for the head of the humerus.

2.      The musculotendinous cuff of the shoulder.

3.      The glenoidal labrum helps in deepening the glenoid fossa. Stability is also provided by the muscles attaching the humerus to the pectoral girdle, the long head of the biceps brachii, the long head of the triceps brachii, and atmospheric pressure.

Ligaments:

As the articular capsule is opened the three glenohumeral ligaments are noticeable on the anterior part of the capsule.

1.      The capsular ligament: It is very loose and perits free movements. It is least supported inferiorly where dislocations are common. Such a dislocation may damage the closely related axillary nerve.

·         Medially, the capsule is attached to the scapula beyond the supraglenoid tubercle and the margins of the labrum.

·         Laterally, it is attached to the anatomical neck of the humerus with the following exceptions.

·         Inferiorly, the attachment extends down to the surgical neck.

·         Superiorly, it is deficient for passage of the tendon of the long head of the biceps brachii.

·         Anteriorly, the capsule is reinforced by supplemental bands called the superior middle and inferior glenohumeral ligaments. The capsule is lined with synovial membrane. An extension of this membrane forms a tubular sheath for the tendon of the long head of the biceps brachii.

2.      The coracohumeral ligament: It extends from the root of the coracoid process to the neck of the humerus opposite the greater tubercle. It gives strength to the capsule.

3.      Transverse humeral ligament: It bridges the upper part of the biciptal groove of the humerus (between the greater and lesser tubercles). The tendon of the long head of the biceps brachii passes deep to the ligament.

4.      The glenoidal labrum: It is a fibrocartilageinous rim which covers the margins of the glenoid cavity, thus increasing the depth of the cavity.

Bursa related of the joint:

1.      The subacromial (Subdeltoid) bursa.

2.      The subscapularis bursa, communicates with the joint cavity.

3.      The infraspinatus bursa, may communicate with the joint cavity.

Relations:

·         Superiorly: Coracoacromial arch, subacromial bursa, Supraspinatus and deltoid.

·         Inferiorly: Long head of the triceps brachii.

·         Anteriorly: Subscapularis, coracobrachialis, short head of biceps brachii and deltoid.

·         Posteriorly: Infarspinatus, teres minor and deltoid.

·         Within the joint: Tendone of the long head of the biceps brachii.

Blood supply:

1.      Anterior circumflex humeral vessels.

2.      Posterior circumflex humeral vessels.

3.      Suprascapular vessels.

4.      Subscapular vessels.

Nerve supply:

1.      Axillary nerve.

2.      Musculocutaneous nerve.

3.      Suprascapular nerve.

Movements:

The muscles which produce movements at the glenohumeral joint are principally deltoid, pectoralis major, latissimus dorsi and teres major. These long muscles all converge on the humerus, acting at mechanical advantage on a joint which, as a result of glenoid shallowness and capsular laxity, is relatively unstable. The long muscles are counteracted by the rotator cuff, a group of short muscles (subscapularis, supraspinatus, infraspinatus and teres minor) which are attached nearer to the joint, and which centre the head of the humerus in the glenoid fossa through the midrange of motion, when the capsuloligamentous structures are lax.

1.      Flexion: Normal flexion is about 90°. Pectoralis major (clavicular part), deltoid (anterior fibres) and coracobrachialis assisted by biceps. The sternocostal part of pectoralis major is a major force in flexion forwards to the coronal plane from full extension.

2.      Extension: Normal extension is about 45°. Deltoid (posterior fibres) and teres major, from the dependent position. When the fully flexed arm is extended against resistance, latissimus dorsi and the sternocostal part of pectoralis major act powerfully until the arm reaches the coronal plane.

3.      Abduction: Deltoid. Initially its effect is mainly upward and, unless opposed, this would displace the humerus upwards. Subscapularis, infraspinatus and teres minor exert downward traction and so apply an opposing force: together with deltoid they constitute a ‘couple' to produce abduction in the scapular plane. Supraspinatus assists in effecting and maintaining this movement, but its precise role is controversial. Supraspinatus 0-1 controversial, Deltoid 1-9, Serratus anterior 9-18, Upper and lower fibers of trapezius 9-18.

4.      Adduction: Normally, the upper limb can be swung 45° across the front of the chest. This is performed by the pectoralis major, latissimus dorsi, teres major, and teres minor muscles.

5.      Medial rotation: Normal medial rotation is about 55°. Pectoralis major, deltoid (anterior fibres), latissimus dorsi, teres major and, with the arm pendent, subscapularis.

6.      Lateral rotation: Normal lateral rotation is 40° to 45°. Infraspinatus, deltoid (posterior fibres) and teres minor. Lateral rotation is important for clearance of the greater tubercle and its associated tissues as it passes under the coracoacromial arch, as well as for relaxation of the capsular ligamentous constraints.

7.      Circumduction: This is a combination of the above movements.

References:

1.     https://en.wikipedia.org/wiki/Shoulder_joint

2.     Clinical Anatomy By Regional (Richard S.Snell, MD, PhD) [Part-9: Page 462] Edition 8th

3.     Gray’s Anatomy By Susan Standring , Edition 40^th

4.     Atlas of human Anatomy By Mark Nielsen, Shawn Miller [Page 131-133]

5.     BD Chaurasia’s Human Anatomy ,Volume 01, Section 01 (10.Joints of upper limb)