Shoulder Joint : Anatomy, Movement & Muscle involvement

The shoulder is made up of three bones- the clavicle, the scapula, and the humerus as well as associated muscles, ligaments, and tendons. The articulations between the bones make up the shoulder joints. It is a ball and socket type of synovial joint.

The shoulder is composed of 4 joints-
The shoulder joint also is known as the Glenohumeral joint (articulation within the head of the humerus and the glenoid cavity of the scapula).
The sternoclavicular (SC) joint (clavicle joins the sternum).
The acromioclavicular (AC) joint (clavicle joins the acromion of the scapula).
The scapulothoracic joint (scapula meets with the ribs at the back of the chest).

The Capsule of the Shoulder Joint
The shoulder joint capsule is a fibrous sheath has a very loose articular capsule of the humerus and this can sometimes allow the shoulder to dislocate. The joint capsule is lax, permitting greater mobility.The long head of the biceps brachii muscle travels inside the capsule, it requires a tendon sheath to minimize friction.

The Synovial Bursae of the Shoulder Joint
Several synovial bursae are present friction in the shoulder joint. A bursa, which acts as a cushion between tendons and other joint structures.
Subacromial bursae located inferiorly to the deltoid and acromion, and superiorly to the supraspinatus tendon and the joint capsule. Inflammation of this bursa is the cause of several shoulder difficulties.

Subscapular bursae located between the subscapularis tendon and the scapula. It diminishes wear and tears on the tendon throughout movement at the shoulder joint.

The Ligaments of the Shoulder Joint
In the shoulder joint, the majority of the ligaments are thickenings of the joint capsule. Ligaments play a key role in stabilizing the bony structures.

• Glenohumeral ligaments – Consists of superior, middle and inferior bands, which runs joint capsule of the glenoid fossa to the anatomical neck of the humerus bone. They act to stabilize the anterior aspect of the joint.

• Coracohumeral ligament – Connects the base of the coracoid process through the greater tubercle of the humerus. Coracohumeral ligament supports the superior part of the joint capsule.

• Transverse humeral ligament – Spans the gap within the two tubercles of the humerus. Transverse humeral ligament keeps the tendon of the long head of the biceps in the intertubercular groove.

• Coracoacromial ligament runs within the acromion and coracoid process of the scapula, forming the coraco-acromial arch. This structure preventing superior displacement of the humeral head.

Other important structures of the Shoulder Joints:

Rotator cuff, a network of muscles and tendons that hold its place and enable the arm to rotate.
Labrum, a fibrous ring of cartilage to create a deeper socket for the ball to stabilize the joint.
Deltoid, the largest and strongest muscle provides the strength to lift the arm.
Biceps tendon which allows the elbow to bend and the forearm to rotate.

Movements of the Shoulder Joints

Flexion
Extension
Abduction
Adduction
Medial Rotation
Lateral Rotation
Circumduction

Flexion

Flexion-upper limb forwards in the sagittal plane.
Muscles involvement in flexion: pectoralis major, coracobrachialis, biceps brachii, anterior fibers of deltoid.

Extension
Extension-upper limb backward in the sagittal plane.
Muscles involvement in extension:latissimus dorsi, long head of triceps, teres major and posterior fibers of the deltoid

Abduction
Abduction-upper limb away from the midline in the coronal plane.
Muscles involvement in abduction: supraspinatus (initiates abduction of first 15 degrees), deltoid (up to 90 degrees), trapeziusand serratus anterior muscle (scapular rotation, for abduction beyond 90 degrees).

Adduction
Adduction-upper limb towards midline in the coronal plane.
Muscles involvement in adduction: pectoralis major and minor, latissimus dorsi, teres major, gravity (depending on the body position), and even the lowest fibers of the deltoid (making deltoid its own antagonist)

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Medial Rotation
Medial Rotation-rotation towards the midline, so that the thumb is pointing medially.
Muscles involvement in medial rotation: subscapularis, latissimus dorsi, teres major, pectoralis major, anterior fibers of the deltoid

Lateral Rotation
Lateral Rotation -rotation away from the midline, so that the thumb is pointing laterally.
Muscles involvement in lateral rotation: infraspinatus and teres minor, posterior fibers of deltoid

Circumduction
Circumduction-Combination of all movements.
Muscles involvement in circumduction: pectoralis major, subscapularis, biceps brachii, coracobrachialis, supraspinatus, deltoid, teres major and minor, latissimus dorsi, infraspinatus, long head of triceps