scapulohumeral muscles And its components

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scapulohumeral muscles، In the resting position, the glenoid cavity and the proximal epiphysis of the humerus are oriented upwards, inwards and behind (retroversion). There is a fibrocartilage rim to increase the joint surface by 75% (although it may be absent). Not all the head is in contact in all the amplitude and type of movement with the glena to avoid the continuous clash of the cartilage.

For the correct mechanics, the capsule-ligament and muscle integrity is indispensable: the movements are fundamentally of rolling and sliding (translation and rotation).

scapulohumeral muscles

Stabilizers: Static (glenoid rim, joint capsule, superior glenohumeral ligament, middle or anterior and inferior and suspensory ligament which is the association of the coracohumeral, glenohumeral superior and Gordon-Brodie ligaments [fibers that jump from one to another]) and dynamic (internal and external rotator cuff that makes common insertion in the humeral head).

Musculature: 5 muscles are considered primary motors (deltoid, supraspinatus, infraspinatus, minor round and subscapular).

Sternocostoclavicular joint

Saddle but with an intra-articular disc that makes it not a reciprocal lace but the surfaces slip. The disc divides the joint into 2 cavities with different functions: superior (ascending and descending movements of the clavicle in the frontal plane [30-40º]) and inferior (sliding movements in the anteroposterior direction in the sagittal plane [30º]). Actually they are 3 degrees of movement because the disc produces a rotation on itself of the clavicle [40-50º].

Joint stability is achieved by the action of:

– Powerful joint capsule.

– costoclavicular or rhomboid ligament: if it breaks it involves dislocation. It also directs the movements of the clavicle thanks to the fact that it divides into 2 sheets (medial and lateral) that control each movement of the anterior chamber.

– Subclavian muscle (acts as an active ligament).

Acromioclavicular joint

Interrelated with the sternocostoclavicular joint. Theoretical arthrodia with a disc of fibrocartilage that increases the amplitudes. It is covered by a dense capsule and a superior and inferior acromioclavicular ligament system that only stabilize. Mechanically, the coracoclavicular ligaments serve as axis of rotation during the movements of the scapula: the trapezoid and conoid ligaments, which form a 90º angle to each other to control clavicular ascending-descending and anteroposterior movements.

Movements of the scapula

– Protraction (abd) and retraction (add) [30-50º]: around the vertical axis. Limited by the conoid ligament.

– Scale in and out [60º]: on the frontal plane. Limited by the trapezoid axis.

– Elevation and depression [30º]: limited by muscles antagonistic to movement and not by the trapezoid and conoid ligaments.

Subdeltoid joint

It only acts on the separation and flexion of the shoulder. Located under the deltoid. Formed by the humeral head covered by the supraspinatus (convex) muscle and the acromion, coracoids and acromiocoracoid ligament (concave). Between both segments there is a serous pocket that prevents contact and shearing. If it is injured, scarring occurs and adjacent structures join, leaving the joint fixed and preventing the entrance of the humerus into the second segment (frozen shoulder).

Anatomical variations of the acromion according to Bigliani:

Type I: flat acromion (the most frequent).

Type II: curved acromion.

Type III: hooked acromion (associated with 70% of supraspinatus tears and rotator cuff).

Subacromial entrapment (“impigement”): the abduction and rotation of the humeral head repeatedly cause the suppression of compression in the supraspinatus and the bursa. The latter is damaged and the compression passes to the periosteum that responds by producing subacromial osteophytes.

Scapulothoracic joint

There is a synergic movement of the clavicle to be dragged by the shoulder blade through the acromioclavicular joint so that for every 60º of scapulothoracic movement 20º belong to the acromion and 40º to the sternocostoclavicular joint.

Muscles and movements of the scapula:

– Protractors: pectoral major and minor and serratus.

– Retractors: rhomboid major and minor, trapezius (middle fasciculus) and latissimus dorsi (at its scapular insertion).

– Elevators: trapezius (upper fibers), angular scapula and rhomboid major and minor.

– Scale outwards (glena oriented upwards and outwards): serratus anterior and trapezoid (upper and lower fibers

Adduction of the shoulder

From 70-80º can not be performed in the frontal and scapular planes (orientation of the 30º glena). An adduction of the 45º humerus is usually associated with a flexion of the shoulder.

Agonist muscles: pectoralis major, latissimus dorsi, triceps brachii, greater round and subscapular. These muscles need fixation of the scapula through the simultaneous action of trapezius, rhomboids, angular scapula, pectoralis minor and subclavicular. Muscle pairs are established between the triceps brachii and the latissimus dorsi (the long head of the triceps nullifies the descending luxating component of the humerus that has the latissimus dorsi) and between the greater round and rhomboids (the internal rotation of the greater round is annulled by the action of the rhomboid becoming in adductor).

Stabilizing muscles of the humerus during adduction (synergists and fixators): internal deltoid fibers, short portion of the biceps brachii, coracobrachial and infraspinatus.

Shoulder flexion

Joint participation (maximum responsibilities):

0-60º: scapulohumeral joint.

60-120º: scapulothoracic joint.

120-180º: spinal column (homolateral and contralateral muscles that increase lumbar lordosis.In pathologies of the lumbar spine it is recommended not to finish the flexion movement).

Agonist muscles: deltoid (anterior fasciculus), coracobrachial, and pectoralis major (clavicular fasciculus).

Synergistic muscles: subscapular and brachial biceps (short and long portion).

The coracohumeral ligament slows movement at 60-80º of flexion, thus blocking the scapulohumeral joint. From here the serratus major and trapeze act, taking the glena upwards, outside and in front. The movement at 120º is slowed down by the wide dorsal and infraspinatus tension.

Shoulder extension

An adduction of the scapula is given in the frontal plane, pivoting it downwards and inside (internal scale).

Agonist muscles: rhomboids, latissimus dorsi, trapezius (middle fasciculus), greater round and triceps brachii (long head). [In the last 10º the spinal column also participates, for that reason there is a kinetic chain. The limit is the anterior fasciculus of the coracohumeral ligament.]

Synergistic muscles: infraspinatus and minor round.

Joint participation:

– Scapulohumeral articulation: deltoid (spinal), latissimus dorsi, greater round, triceps brachii (long head). Infraspinatus and minor round as synergistic muscles.

– Scapulothoracic articulation: rhomboids, latissimus dorsi and trapezius (middle fasciculus).

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