Anatomy of Shoulder Joint

This article is prepared for undergraduate medical students. Postgraduates and practicing surgeons will find it beneficial to revise anatomy.

The shoulder joint is a ball and socket joint between head of the humerus and glenoid cavity of scapula (glenohumeral joint). Considering clinical significance, shoulder has to be considered as “shoulder complex”. It is a group of three bones, four joints, ligaments, muscles, blood vessels, nerves and other tissues. In this write-up, we will discuss mainly structure of joints including ligaments and movements.

Osseous Components of Shoulder Complex:
Humerus – proximal end
Scapula – as a whole
Clavicle – as a whole

Joints of Shoulder Complex:
Glenohumeral Joint:
It is a synovial, ball-and-socket type of joint. The head of humerus is less-than-half ball, covered by articular hyaline cartilage. The glenoid cavity is a shallow socket, much smaller as compared to humeral head. The socket is enlarged and deepened by a rim of fibro-cartilaginous structure (glenoid labrum), attached to the edge of glenoid cavity. The labrum is triangular in cut section, with base attached to glenoid cavity and edge giving attachment to capsule of the joint. Large size of humeral head and shallow glenoid cavity, result in good mobility at the cost of stability. This joint is dislocates easily as compared to other joints.
The joint is surrounded by fibrous capsule, which is attached to anatomical neck of humerus and edge of glenoid cavity and labrum. The capsule is lax and allows free movements of the joint. The capsule is strengthened by three ligaments or capsular thickenings, called superior, middle and inferior glenohumeral ligaments. These are best visualised by opening the joint posteriorly and removing the humeral head. These provide stability to joint anteriorly and prevent anterior dislocation of this joint.
The coracohumeral ligament is a broad band and strengthens the joint superiorly. It is attached to the lateral border of the root of coracoids process and passes obliquely downwards and laterally to the front of the greater tuberosity and blends with supraspinatus tendon.
The transverse humeral ligament is a broad band, attached at edges of intertubercular sulcus and converts it into a canal. Long tendon of biceps passes through this canal.
The coracoacromial ligament is a triangular strong ligament at superior aspect of glenohumeral joint. Its apex is attached to the edge of the acromion just anterior to the articular surface for clavicle and its base the whole lateral border of the coracoids process. The ligament, together with the acromion and coracoids, forms an arch which protects the joint superiorly.
The superior transverse scapular (suprascapular) ligament converts the scapular notch into a foramen. Suprascapular nerve passes through this foramen, while suprascapular vessels cross over the ligament. This ligament is sometimes ossified.

Applied Anatomy. Owing to the anatomical construction of glenohumeral joint, there is wide range of movements, however it is more frequently dislocation than any other joint. Dislocation usually occurs anteriorly, where the capsule is weakest. The head usually ruptures the capsule and comes to lie outside. After reduction of dislocated joint, re-dislocation occurs if the arm is placed in abduction and external rotation, like throwing a stone or ball. If after reduction this position is avoided for few weeks, the rupture of capsule may heal.
When the glenohumeral joint is ankylosed (fused) due to some disease or surgical arthrodesis (fusion) is done, the loss of movements is partly compensated for by increased mobility of scapula. The target of position after any fusion is 20-30 degrees each of abduction, forward flexion and internal rotation. This is a position when palm of the hand is placed behind the neck.

Acromioclavicular (AC) Joint: It is a plane synovial joint, formed by lateral end of clavicle and medial margin of acromion process of scapula. Both articular surfaces are covered by fibrocartilage.
The joint is surrounded by fibrous capsule, which is attached to the margins of articular cartilage.
An articular disc is sometimes present in the joint cavity. The disc, when present, usually occupies the upper part of the joint and partly separates the joint cavity. Rarely it separates the cavity completely.
The acromioclavicular ligament is quadrilateral and covers the AC joint superiorly. The ligament has parallel fibres which interlace with aponeurosis of deltoid and trapezius muscles.
The coracoclavicular ligament connects the coracoids process with the under surface of lateral end of clavicle. It has two parts, trapezoid and conoid, separated by fat or bursa. This is a strong ligament and most important factor to prevent superior displacement of clavicle at the AC joint.

Applied Anatomy. In cases of severe injury, acromioclavicular ligament is torn and results in upward displacement of the clavicle. The dislocation of AC joint may or may not be associated with fracture of clavicle. The torn acromioclavicular ligament needs surgical repair to stabilise the AC joint. In old untreated cases when the ligament ends are atrophic, reconstruction using a strong fascial band or a tendon is required.

Sternoclavicular Joint:
The joint is formed bysternal (medial) end of clavicle, clavicular notch of manubrium sterni and cartilage of first rib. The articular surface of the clavicle is much larger than that of sternum and is covered by fibrocartilage. It is convex vertically and slightly concave anteroposteriorly, and therefore is a type of saddle joint. The sternal notch is reciprocally curved but the two surfaces are not completely congruent. The joint is completely divided by an articular disc. Its ligaments are capsular, anterior and posterior sternoclavicular, interclavicular and costoclavicular.
The fibrous capsule is thin and surrounds the joint and is thickened anteriorly and posteriorly.
The anterior sternoclavicular ligament is broad band of oblique fibres attached to the articulating bones.
The posterior sternoclavicular ligament is a weaker band on the posterior aspect of the joint.

The supraclavicular ligament connects sternal ends of both clavicles. Its fibres are attached to manubrium and it is continuous above with deep cervical fascia. Suprasternal ossicles, if present, are in this ligament.
The costoclavicular ligament is cone like and connects upper part of medial end of first rib with medial end of clavicle.

Applied Anatomy. The strength of sternoclavicular joint depends upon its ligaments and mainly intra articular disc. The force of injury is usually transmitted along the long axis of clavicle, dislocation is rare and there is fracture of clavicle rather than displacement of the joint.

Scapulothoracic Joint:
It is not a true joint as there are no articulating bones. It can be considered a functional joint as the scapula moves over the thoracic cage with various movements of shoulder complex. The scapula is held in this position by muscles, ligaments and the two joints of clavicle, sternoclavicular and acromioclavicular.

Bursae of Shoulder Complex:
A bursa is a synovial membrane lined sac, contains synovial fluid, acts as a cushion between two structures by prevention friction.
Subscapularis bursa, lies between the subscapularis tendon and joint capsule. It communicates with the glenohumeral joint cavity through an opening between superior and middle glenohumeral ligaments.
Subacromial bursa, lies between deltoid and joint capsule, extends under the acromion and coracoacromial ligament and separates these structures from supraspinatus tendon. This bursa provides free movements of humeral head by rotator cuff muscles. Inflammation of this bursa (subacromial bursitis) results in shoulder pain on abduction.
Infraspinatus bursa, lies between the tendon of infraspinatus and joint capsule. It is sometimes present may communicate with joint cavity.

Movements:
The shoulder is a multi-axial ball and socket joint, and therefore has a wide variety of movements. The basic movements are flexion-extension, abduction-adduction and medial lateral rotation. Other movements like circumduction are combination of basic movements. The laxity of joint capsule and larger head size as compared to shallow glenoid cavity, permit wider range of movements.
When analysing shoulder movements, it is important to consider position of scapula and the glenoid cavity in normal anatomical position. When the arm is by the side in resting position, the glenoid cavity faces anterolaterally and humeral head posteromedially. Although this relationship is perfect, humerus is medially rotated according to body planes. Therefore one aspect is to analyse movements of humerus in relation to scapula rather than in anatomical sagittal and coronal body planes. As a result, flexion carries arm forwards and across the front of chest. This movement takes place around an axis which passes through the humeral head at right angles to the plane of glenoid. In extension the arm moves posteriorly and laterally. In the same context, arm will move laterally and anteriorly in abduction and back to its position in adduction. The axis of these movements passes through the humeral head and in a plane parallel to the plane of glenoid cavity.
If however the movements of the humerus are considered in relation to the trunk and not the scapula, flexion and extension occur in paramedian plane and abduction-adduction in coronal plane.

Movements of the shoulder girdle:
Movements of clavicle occur at sternoclavicular and acromioclavicular joints. These movements are always associated with movements of scapula, and movements of the scapula are usually accompanied by movements of the arm. These are movements of shoulder girdle. There are three basic movements of scapula
(1) Elevation and depression (shoulder shrugging)
(2) Forward movement (protraction) and backward movement (retraction) along the chest wall
(3) Rotation forwards or upwards and return rotation using the inferior angle as its pivot.

Muscles producing movements in body planes:
Extension (arm moving posteriorly in sagittal plane):
Deltoid (posterior fibres), Latissimus Dorsi, Teres Major.
Flexion (arm moving anteriorly in sagittal plane):
Pectoralis Major, Deltoid (anterior fibres), Coracobrachialis, Biceps Brachii (weak assistant)
Abduction (arm moving away from midline in coronal plane):
Supraspinatus – first 0-15 degrees of abduction
Deltoid (middle fibres) – next 15-90 degrees of abduction
Trapezius, Serratus Anterior rotate scapula to have more abduction past 90 degrees
Adduction (arm moving towards midline in coronal plane):
Pectoralis Major, Latissimus Dorsi,Teres Major
Internal rotation: Subscapularis, Pectoralis Major, Latissimus Dorsi, Teres Major, Deltoid (anterior fibres)
External rotation: Infraspinatus, Teres Minor

Muscles:
Muscles connection the Upper Limb and Vertebral Column
Trapezius
Rhomboid Major
Rhomboid Minor
Levator Scapulae
Muscles connection the Upper Limb and Thoracic Wall
Serratus Anterior
Pectoralis Major
Pectoralis Minor
Suclavius
Scapular Muscles
Deltoid
Supraspinatus
Infraspinatus
Subscapularis
Teres Major
Teres Minor
Muscles of the Upper Arm
Coracobrachialis
Biceps Brachii
Triceps Brachii
Brachialis

Rotator Cuff: This is a group of four muscles, all attached to the tuberosities at the upper end of humerus, in a form of a cuff. The muscles are subscapularis, supraspinatus, infraspinatus and teres minor. The attachment is partly blended with the capsule of glenohumeral joint. The muscles keep the humeral head firmly in contact with glenoid cavity. Each muscle has its role in movements of arm, depending on its origin and insertion.

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