Pectoral Girdle or Shoulder Girdle connects the free part of the upper limb with the trunk. It consists of scapula and clavicle on each side. A series of anatomical organization makes the girdle extremely mobile. The mobility of scapula is essential for free movement of upper limb. The clavicle acts as a strut which holds the scapula laterally allowing the upper limb to swing freely from the side of the trunk.
Pectoral girdle consists of three joints: i) Sternoclavicular joint, ii) Acromioclavicular joint and iii) Glenohumeral joint. The scapula is connected to the clavicle and humerus (bone of the arm) by acromioclavicular and glenohumeral joints respectively. The upper limb and pectoral girdle are connected to the trunk (axial skeleton) only through the sterno-clavicular joint. Scapula does not make any anatomical joint with axial skeleton rather it is connected to the vertebral column and thorax only by the muscles. This connection of scapula with the thorax is sometimes referred as conceptual scapula-thoracic joint or physiological scapula-thoracic joint. Thus the extremely mobile pectoral girdle is supported and stabilized by the muscles that are attached to the ribs, sternum and vertebrae.
Joints of Pectoral Girdle
Type: Saddle type of synovial joint.
The sternal end of the clavicle articulates with the manubrium sterni and the first costal cartilage.
Capsule: This surrounds the joint and is attached to the margins of the articular surfaces.
Synovial membrane: This lines the inside of capsule and extends to the margins of the cartilage covering the articular surfaces.
Anterior and Posterior sternoclavicular ligaments reinforce the capsule from the front and behind.
Interclavicular ligament strengthens the capsule superiorly. It extends from sternal end of one clavicle to the sternal end of the other.
Costoclavicular ligament connects the inferior surface of sternal end of clavicle to the 1st rib and its costal cartilage.
Articular disc: It is a fibrocartilaginous disc which divides the interior of the joint into two compartments. The peripheral margin of the disc is attached to the interior of the capsule. It is also strongly attached to the superior margin of the articular surface of the clavicle above and to the first costal cartilage below.
It is very mobile to allow movements of the pectoral girdle and upper limb. The movements that occur in SC joint are: forward and backward movements and elevation and depression. During full elevation of upper limb the clavicle is raised to approximately a 600 angle.
Muscles producing the movement:
Forward movement: Serratus anterior
Backward movement: trapezius and rhomboids
Elevation: trapezius, sternocleidomastoid, levator scapulae and rhomboids
Depression: pectoralis minor and subclavius
Nerve supply: The supraclavicular nerve and the nerve to the subclavius muscle.
Blood supply: by internal thoracic and suprascapular arteries.
Type: Plane synovial joint
Articulation: lateral end of clavicle articulates with the acromion of the scapula.
Capsule: Capsuleis relatively loose and is attached to the margin of the articular surfaces. From the capsule an incomplete wedge shaped fibrocartilaginous disc projects into the joint cavity from the above.
Ligaments: Superior and inferior acromioclavicular ligaments reinforces the capsule. The integrity of the joint is maintained by the extrinsic ligament called coracoclavicular ligament. It consists of a pair of strong bands that connect the coracoid process of scapula to the clavicle. The coracoclavicular ligament consists of two parts: conoid and trapezoid part.
The conoid ligament is cone shaped with its apex directed downward and attached to the root of the coracoid process and its base to the conoid tubercle on the undersurface of the clavicle. The trapezoid ligament is extends laterally for its attachment from the superior surface of coracoid process to the trapezoid line on the inferior surface of the clavicle. The coracoclavicluar ligament is also responsible for suspending the scapula and free limb from clavicular strut.
Movement: A gliding movement takes place when the scapula rotates or when the clavicle is elevated or depressed. The thoraco-appendicular muscles that attach to and move the scapula cause the acromion to move on the clavicle.
Nerve supply: Supraclavicular, lateral perctoral and axillary nerves
Blood Supply: Suprascapular and thoracoacromial arteries
Glenohumeral joint is also called shoulder joint.
Type: Ball and Socket synovial joint
Articulation: occurs between the large rounded head of the humerus and a shallow glenoid cavity of the scapula. The articular surfaces are lined by the hyaline cartilage. The shallow glenoid cavity is deepened by a fibrocartilaginous tissue called glenoid labrum. The cavity receives more than a third of humeral head which is held in position by the tonus of the musculotendinous rotator cuff muscles.
The fibrous capsule surrounds the joint and is relatively thin and lax allowing the greater range of movement. Superiorly it is strengthened by the fibrous slips from the rotator cuff muscles. Inferiorly it is relatively weaker and is common site for dislocation of the joint.
Attachment of the capsule:
Medially – it is attached to the margin of the glenoid cavity outside the labrum. It also enclosed the origin of long head of biceps brachii.
Laterally – it is attached to anatomical neck of humerus except at two places, a) upper end of bicipital groove to allow the passage of tendon of long head of biceps brachii and b) inferomedially, the line of attachment extends downward for about 1 cm to the surgical neck.
Synovial Membrane: it lines the inside of the capsule and reflects from it onto the glenoid labrum and humerus as far as the articular margin of the head. It forms a tubular sheath around the long tendon of biceps brachii. It passes through the anterior wall of the capsule to form the subscapular bursa beneath the subscapularis muscle.
Transverse humeral ligament
Coracoacromial ligament (accessory ligament)
Glenohumeral and coracohumeral ligaments are the thickening of joint capsule that strengthens the anterior and superior aspects of the capsule respectively.
Glenohumeral ligaments are three fibrous bands, evident only on the interior of the joint capsule. The superior, middle and inferior bands of the glenohumeral ligaments are attached medially to the superomedial margin of glenoid cavity and blends with glenoid labrum. Laterally, all three bands radiate and are attached to the upper part of lesser tubercle, lower part of lesser tubercle and lower part of anatomical neck respectively.
The coracohumeral ligament extends from the root of the coracoid process to anatomical neck opposite the greater tubercle of the humerus.
The transverse humeral ligament bridges the gap between the two tubercles and strengthens the capsule.
Coracoacromial ligament is an accessory ligament which along with the inferior aspects of acromion and coroacoid process of scapula forms coracoacromial arch and protects the superior aspect of the joint as it prevents the superior displacement of humeral head. A subacromial bursa intervenes between the arch above and the tendon of supraspinatus and greater tubercle below.
It is made up of fibrocatilaginous tissue and helps deepening the glenoid cavity. It is attached to the margin of the glenoid cavity except above from where the long head of biceps brachii arises.
Movements at shoulder joint:
The shoulder joint has greater mobility. (Its stability has been compromised at the cost of the mobility as compared to hip joint which has greater stability but has limited range of movements). The freedom of movement is mainly due to the thin and lax capsule and the large size of humeral head compared to the receiving socket provided by the glenoid cavity. The glenohumeral joint produces movements around three axes and permits flexion-extension, adduction-abduction, medial-lateral rotation and circumduction.
Flexion and Extension:
Flexion and extension take place right angle to the plane of the body of scapula around an axis that passes through the humeral head and center of glenoid cavity. Flexion moves the arm forward and medially and is about 900. Extension moves the arm backward and laterally and is about 450.
Abduction and adduction take place parallel to the plane of the body of scapula around an axis which passes through the head of humerus and is parallel to the glenoid cavity. The abduction carries the arm laterally and adduction medially. During adduction the arm can be swung 450 across the front of the chest.
Mechanism of Abduction:
Abduction of the arm is accomplished by movement at shoulder joint as well as by the rotation of scapula at conceptual scapula-thoracic joint. The abduction is initiated for the first 150 by supraspinatus muscle which also holds the head of the humerus against the glenoid cavity. The later action then allows deltoid to take over and complete the further abduction. The limb can be elevated by 1800 during abduction. Except for the first 30 degree of abduction which occur due to movement at shoulder joint only, in every 150 elevation, 100 occurs at shoulder joint and 50 by rotation of scapula at conceptual scapula-thoracic joint in the ration of 2:1. When the arm is abducted to 900, the further elevation is prevented as the articular surface is exhausted and the greater tubercle impinges on the lateral edge of acromion. The further elevation is then accomplished by the lateral rotation of humerus by 1800 which brings the greater tubercle posteriorly thus providing more articular surface to continue elevation. From 1200-1800, the abduction is accomplished by the rotating the scapula.
Medial and lateral rotations:
The plane producing these movements is a vertical axis passing through center of the humeral head to the center of the capitulum. In a semi-flexed elbow, the medial rotation at shoulder joint carries the hand medially and lateral rotation moves the hand laterally.
It is a combination of above mentioned movements so that the lower end of the humerus defines the base of the core and the humeral head forms apex of the cone.
Muscles producing the movements at shoulder joint:
Flexion: Anterior fibers of Deltoid, Pectoralis Major and Coracobrachialis (weak flexor)
Extension: Posterior fibers of Deltoid, Latissimus Dorsi and Teres Major
Abduction: Supraspinatus (initial 150) and Deltoid
Adduction: Pectoralis Major, Latissimus Dorsi, Teres Major and Subscapularis
Medial Rotation: Subscapularis, Latissimus dorsi, Teres Major, Pectoralis Major, Anterior Fibers of Deltoid
Lateral Rotation: Infraspinatus, Teres Minor and Posterior Fibers of Deltoid
Factors stabilizing the shoulder joint:
As noted earlier, the shoulder joint is relatively weaker joint owing to its thin and lax joint capsule, greater mobility and shallow glenoid fossa to receive larger head of the humerus.
Rotator cuff: The tendons of small muscles namely subscapularis, supraspinatus, Infraspinatus and teres minor on their way to insertion on humerus flatten and blend with each other and with the fibrous capsule of shoulder joint. The tone of these muscles support and strengthen the shoulder joint from the front, above and from the behind.
Glenoid labrum: this fibrocartilaginous tissue deepens the glenoid cavity.
Glenohumeral and Coracohumeral ligaments support the joint from anterior and superior aspects respectively.
The coracoacromial arch prevents the upwards displacement of the joint. Similarly, the tendon of long head of biceps brachii also holds the head of the humerus in position during abduction. Similarly, the downward displacement of humerus during abduction is prevented by the long head of triceps brachii.
Anterior circumflex humeral artery, branch of third part of axillary artery
Posterior circumflex humeral artery, branch of third of axillary artery
Branches of suprascapular artery, branch of thyrocervical trunk of subclavian artery or directly from subclavian artery
Axillary nerve and suprascapular nerve
Following resources are used while preparing this post (readers are strongly recommended to go through them for more details):
K. L. Moore's Clinically Oriented Anatomy
R. Snell's Clinical Anatomy