Shoulder Joint

Overview

The shoulder is physically and functionally complex since it is one of the most freely mobile parts of the human body due to the glenohumeral joint articulation. The shoulder girdle, which joins the upper limb to the axial skeleton via the sternoclavicular joint, is located within it. The shoulder's tremendous range of motion comes at the tradeoff of decreased joint stability, making it prone to dislocation and injury.

The human shoulder is the body's most movable joint. This mobility gives the upper extremity a great range of motion in the sagittal plane, including adduction, abduction, flexion, extension, internal rotation, external rotation, and 360° circumduction. The shoulder also allows for scapular protraction, retraction, elevation, and depression. The shoulder joint is additionally unstable due to its vast range of motion.

Shoulder joint anatomy

The clavicle and scapula form the shoulder girdle, which articulates with the upper limb's proximal humerus. The shoulder has four joints: the sternoclavicular (SC), acromioclavicular (AC), scapulothoracic, and glenohumeral.

The sternoclavicular joint is the only synovial saddle joint that joins the upper limb to the axial skeleton. It connects the clavicle to the sternum's manubrium and is stabilized by the costoclavicular ligament. The acromioclavicular joint is a planar synovial joint that joins the scapula's acromion to the clavicle.

It is principally stabilized by the coracoclavicular ligament, with auxiliary stabilizers being the super and inferior acromioclavicular ligaments. The scapulothoracic joint is an articulation of the scapula gliding over the posterior thoracic cage rather than a genuine joint.

The glenohumeral joint is a highly mobile ball-and-socket synovial joint that is supported by the rotator cuff muscles, which attach to the joint capsule, as well as the biceps and triceps brachii tendons. The glenoid fossa of the scapula articulates with the humeral head. The fossa accommodates less than one-third of the humeral head, resulting in a shallow articulation.

The labrum is a fibrocartilaginous ring that adheres to the glenoid fossa's outside rim and adds depth and support to the humeral head. Bursae are tiny fluid-filled sacs that surround the capsule and aid in movement. The subacromial, subdeltoid, subscapular, and subcoracoid bursae are the most common.

The major movements at the glenohumeral joint are:

Abduction is defined as the upward lateral movement of the humerus out to the side, away from the body, in the plane of the scapula.
Adduction is the downward movement of the humerus in the plane of the scapula from abduction.
Flexion is the straight anterior movement of the humerus.
Extension is the movement of the humerus straight backwards.
External rotation: Lateral movement of the humerus around its long axis away from the midline.
Internal rotation is defined as the movement of the humerus medially around its long axis toward the midline.
Horizontal adduction (transverse flexion): the movement of the humerus toward and across the chest in a horizontal or transverse plane.
Horizontal abduction (transverse extension): the displacement of the humerus away from the chest in a horizontal or transverse plane.

Blood Supply and Lymphatics

The axillary artery is the main blood vessel in the shoulder, and several of its branches supply it. The superior thoracic artery, thoracoacromial artery, lateral thoracic artery, subscapular artery, anterior humeral circumflex artery, and posterior humeral circumflex artery are among these branches. The subclavian artery has branches that nourish the area of the shoulder before it becomes the axillary artery after passing beyond the lateral margin of the first rib.

The suprascapular artery and the transverse cervical artery are added to the subclavian artery by the thyrocervical trunk. The dorsal scapular artery usually branches off the subclavian artery, but it can also branch off the transverse cervical artery.

Muscles of the shoulder

The intrinsic muscles of the shoulder connect the scapula and/or clavicle to the humerus. These include:

Deltoid

Function:

The anterior aspect is in charge of the arm's flexion and medial rotation.
The middle aspect is in charge of arm abduction (up to 90 degrees).
The posterior aspect is in charge of the arm's extension and lateral rotation.

Origin: Lateral clavicle, acromion and scapular spine
Insertion: Deltoid tuberosity
Innervation: Axillary nerve (C5, C6)

Teres major

Function: Adduction and medial rotation of the arm
Origin: Posterior surface of the scapula at its inferior angle
Insertion: Intertubercular groove of the proximal humerus on its medial aspect
Innervation: Lower scapular nerve (C5, C6)

Supraspinatus (Rotator Cuff)

Function: Initiation of arm abduction (first 15 degrees), stabilize glenohumeral joint
Origin: Posterior scapula, superior to the scapular spine/supraspinous fossa
Insertion: Top of the greater tubercle of the humerus
Innervation: Suprascapular nerve (C5, C6)

Infraspinatus (Rotator Cuff)

Function: Lateral rotation of the arm, stabilize glenohumeral joint
Origin: Posterior scapula, inferior to the scapular spine/Infraspinous fossa
Insertion: Greater tubercle of the humerus, between the supraspinatus and teres minor insertion
Innervation: Suprascapular nerve (C5, C6)

Teres minor (Rotator Cuff)

Function: Lateral rotation of the arm, stabilize glenohumeral joint
Origin: Inferior angle of the scapula
Insertion: Inferior aspect of the greater tubercle
Innervation: Axillary nerve (C5, C6)

Subscapularis (Rotator Cuff)

Function: Adduction and medial rotation of the arm, stabilize glenohumeral joint
Origin: Anterior aspect of the scapula
Insertion: Lesser tubercle of the humerus
Innervation: Subscapular nerves (C5, C6, C7)

Other muscles that affect movement at the shoulder joint include:

Trapezius

Function:

Upper fibers elevate the scapula and rotate it during abduction of the arm (90 to 180 degrees)
Middle fibers retract the scapula
Lower fibers pull the scapula inferiorly.

Origin: Skull, nuchal ligament and the spinous processes of C7 to T12
Insertion: clavicle, acromion and the scapular spine
Innervation: Accessory nerve (C5, C6)

Latissmus dorsi

Function: Extends, adducts and medially rotates the upper limb
Origin: Spinous processes of T6 to T12, iliac crest, thoracolumbar fascia, and the inferior three ribs
Insertion: Intertubercular sulcus of the humerus
Innervation: Thoracodorsal nerve (C6, C7, C8)

Levator scapulae

Function: Elevates the scapula
Origin: Transverse processes of the C1 to C4 vertebrae
Insertion: Medial border of the scapula
Innervation: Dorsal scapular nerve (C5)

Rhomboid major

Function: Retracts and rotates the scapula
Origin: Spinous processes of T2 to T5 vertebrae
Insertion: Inferomedial border of the scapula
Innervation: Dorsal scapular nerve (C5)

Rhomboid minor

Function: Retracts and rotates the scapula
Origin: Spinous processes of C7 to T1 vertebrae
Insertion: Medial border of the scapula
Innervation: Dorsal scapular nerve (C5)

Serratus anterior

Function: fixes the scapula into the thoracic wall
Origin: Surface of the upper eight ribs at the side of the chest
Insertion: Along the entire anterior length of the medial border of the scapula
Innervation: Long thoracic nerve (C5, C6, C7)

Pectoralis major

Function:

Clavicular head flexes and adducts arm
Sternal head adducts and medially rotates the arm
Accessory for inspiration

Origin:

Clavicular head: medial half clavicle
Sternocostal head: Lateral manubrium and sternum, six upper costal cartilages and external oblique aponeurosis

Insertion: Intertubercular groove of the proximal humerus on its lateral aspect
Innervation: Medial and lateral pectoral nerves (C6, C7, C8)

Pectoralis minor

Function: Depression of the shoulder, protraction of the scapula
Origin: Third, fourth, fifth ribs close to their respective costal cartilages
Insertion: Coracoid process
Innervation: Medial pectoral nerve (C8, T1)

Subclavius

Function: Depression and stabilization of the clavicle
Origin: First rib medially
Insertion: Middle of the clavicle, inferiorly
Innervation: Nerve to subclavius (C5, C6)

Coracobrachialis

Function: Flexion and adduction of the arm
Origin: Coracoid process
Insertion: Middle of the humerus, on its medial aspect
Innervation: Musculocutaneous nerve (C5, C6, C7)

Biceps brachii

Function: Resists dislocation of the shoulder, flexion of the forearm, supination of the forearm
Origin:

Short head: coracoid process
Long head: supraglenoid tubercle of the scapula and superior labrum

Insertion: Radial tuberosity of radius and forearm fascia (as bicipital aponeurosis)
Innervation: Musculocutaneous nerve (C5, C6)

Triceps brachii

Function: Resists dislocation of the shoulder, major extensor of the forearm
Origin:

Lateral head: above the radial groove of the humerus,
Medial head: below the radial groove of the humerus
Long head: infraglenoid tubercle of the scapula

Insertion: Olecranon process of ulna and forearm fascia
Innervation: Radial nerve (C6, C7, C8)

Shoulder joint capsule

A loose fibrous capsule surrounds the shoulder joint. It runs from the scapula to the humerus, completely encircling the joint. A synovial membrane lines the capsule's inside surface.

The capsule is attached to the anatomical neck of the humerus. Only extends to its medial edge, where the fibers protrude by around 1 cm. The capsule contains two attachment lines on the scapula.

The first is on the capsule's anterior and inferior sides, where it inserts into the scapular neck posterior to the glenoid labrum. The second is on its superior and posterior sides, where the capsular fibers merge with the glenoid labrum directly. The capsule arches over the supraglenoid tubercle and its long head of the biceps brachii muscle attachment here, forming these intra-articular structures.

The capsule stays flexible to allow for upper limb motion. Its reinforcement is provided by ligaments and muscular tendons. The three glenohumeral ligaments thicken the anterior capsule, and the tendons of the rotator cuff muscles stretch across the capsule, merging with its exterior surface.

The rotator capsule is a continuous covering formed by these tendons. Supraspinatus superiorly, infraspinatus and teres minor posteriorly, subscapularis anteriorly, and the long head of triceps brachii inferiorly make up this muscle group.

This reinforced capsule has two weak points. The first is the rotator interval, which is a gap in the unreinforced capsule between the subscapularis and supraspinatus tendons. The second is the inferior capsular aspect, which is the weakest area in the capsule. When the arm is in the anatomical position, the loose inferior capsule forms a fold. When the arm is abducted, it gets stretched and the least supported.

The capsule has two openings:

The tendon of the long head of the biceps brachii crosses between the larger and lesser tubercles of the humerus.
The subscapular bursa connects with the glenohumeral joint cavity via the superior and middle glenohumeral ligaments.

Bursae packed with synovial fluid aid in joint movement. The subdeltoid-subacromial (SASD) bursa is found between the joint capsule and the deltoid muscle or acromion. Subcoracoid bursae are also located between the capsule and the coracoid process of the scapula. The subscapular bursa is positioned between the capsule and the subscapularis tendon, whereas the coracobrachial bursa is found between the subscapularis and coracobrachialis muscles. These bursae allow the shoulder joint components to move readily over one another.

Spaces

Significant joint spaces are:

The normal glenohumeral space is 4–5 mm.

Supraspinatus outlet view X-ray, showing subacromial space measurement.

In shoulder radiographs, the usual subacromial space is 9–10 mm; this gap is significantly larger in men, with a modest decrease with age. A subacromial gap less than 6 mm in middle age is problematic and may signal a rupture of the supraspinatus tendon.

The axillary space is an anatomic space between the related shoulder muscles. The subscapular artery and axillary nerve pass via this area.

Bursae

A number of small fluid-filled sacs known as synovial bursae are located around the capsule to aid mobility:

The subacromial-subdeltoid bursa is located between the joint capsule and the deltoid muscle.
The subacromial bursa is located between the capsule and the acromion.
The subcoracoid bursa is located between the capsule and the scapula's coracoid process.
The coracobrachial bursa is located between the subscapularis muscle and the coracobrachialis tendon.
The subscapular bursa, also known as the scapularis subtendinous bursa, is located between the capsule and the tendon of the subscapularis muscle.

The supra-acromial bursa does not normally communicate with the shoulder joint.

Natural Variants

There are some small anatomic differences in the labrum's attachment locations, size, and histologic composition. These differences are not considered pathologic.

Variations in clavicle shape are considered natural and are not frequently pathogenic. These differences can range from a nearly straight bone to one with pronounced curvature. The canalis nervi supraclavicularis is another clavicular variant that affects 6-10% of the population. A foramen arises through the clavicle in this variety, and the medial supraclavicular nerve flows through this supplementary osseous canal.

The size and shape of the scapular notch vary. The superior transverse scapular ligament spans the notch. In 10% of patients, this ligament ossifies, resulting in a bone foramen for the suprascapular nerve.

Shoulder joint pain

Clavicle Fracture

The clavicle is important for shoulder stability, strength, and range of motion. Clavicle fractures, on the other hand, are quite prevalent, accounting for 5% of all fractures in adults. Clavicle fractures are classified into three kinds:

Group 1: A clavicle fracture in the middle; the most common type of clavicle fracture.
Group 2: Fracture of the clavicle's lateral one-third; osteoarthritis frequently develops following a group 2 fracture if the fracture impacts the acromioclavicular (AC) joint.
Group 3: Clavicle fracture on the medial one-third of the clavicle; the most uncommon type of clavicle fracture.

Proximal Humerus Fracture

The humerus' anatomic neck is located at the juncture of the humeral head and the tubercles. Fractures of the humeral head's anatomic neck are uncommon and have a bad prognosis because the fracture frequently affects blood supply to the humeral head. The humeral surgical neck is located distal to the tubercles. The surgical neck fracture is more prevalent and has a better prognosis.

Rotator Cuff Injuries

Every year, nearly 18 million Americans suffer from shoulder pain, the majority of which is caused by rotator cuff injuries. Tears can be asymptomatic or cause significant discomfort and restricted movement due to a combination of trauma, overuse, or age-related degeneration. According to research, smoking, hypercholesterolemia, and a family history of tears all predispose to tears. Even in the case of tiny full-thickness tears, conservative, non-surgical treatment is the first line of defense and may be helpful.

When it isn't, or when the rip is significant enough, surgical repair is a good option. In most cases, rotator cuff tendonitis/impingement causes pain during overhead movements and is caused by the tendon of the supraspinatus muscle being squeezed down by the acromion, which can cause inflammation around the tendon and in the fluid-filled bursae around it.

Dislocations

The glenohumeral joint is the most often displaced joint in the body, accounting for up to 45 percent of all dislocations. Anterior dislocation accounts for 96% of all occurrences and is frequently caused by a force directed to the shoulder joint while the arm is in abduction and external rotation. Posterior dislocation is the second most prevalent type of dislocation, accounting for 2% to 4% of all occurrences. Posterior dislocations are typically caused by indirect causes such as electric shock or convulsions, which cause constriction of the shoulder's relatively stronger internal rotators.

Because it is the most mobile joint in the body, the glenohumeral joint lacks stability and is prone to dislocation. Anterior dislocations are the most prevalent, accounting for 97% of all dislocations. A blow to an abducted, externally rotated, and extended extremity is the most common cause.

Anterior dislocation may cause axillary nerve injury, resulting in deltoid paralysis and diminished cutaneous sensation over the shoulder, as well as ligament tears and fractures. With a decrease of the humeral head back into the glenoid fossa, patients frequently regain axillary nerve function.

Although posterior dislocations are less common, they are connected with seizures. With posterior dislocations, there is a greater risk of rotator cuff and ligament rupture than with anterior dislocations. Inferior dislocations are extremely uncommon and are caused by hyperabduction. They are the most prone to axillary nerve and artery injury.

The axillary nerve runs near to the glenohumeral joint and wraps around the neck of the humerus, and it can be injured during dislocation or later attempts to reduce the displaced joint. When the axillary nerves are injured, there is a loss of sensation over the lateral shoulder as well as deltoid paralysis.

Following anterior dislocation, Hill-Sachs lesions (impaction fracture of the posterolateral humeral head on the anteroinferior glenoid) and Bankart lesions (detachment of the anteroinferior labrum with or without an avulsion fracture) might occur. The recurrence rate of glenohumeral joint dislocation is roughly 50% on average; however, the risk of reoccurrence increases significantly with younger age of initial dislocation.

Adhesive capsulitis

Adhesive capsulitis, often known as frozen shoulder, affects 2 to 5% of the population, with the majority of patients being females over the age of 55. According to the theory, inflammation in the shoulder capsule area generates acute pain as well as capsular fibrosis and adhesions, which lead to a restricted range of motion in all planes.

Adhesive capsulitis is strongly linked to endocrine problems such as diabetes and hypothyroidism. The treatment is conservative, with the majority of cases resolving on their own. Surgical intervention, which involves releasing the fibrotic capsule, is reserved for refractory cases.

Osteoarthritis

The shoulder joint, like other joints that are heavily used, is prone to wear and tear degeneration of the articular cartilage within the joint. Age, female gender, obesity, anatomical characteristics, muscle weakness, and joint damage are all risk factors for osteoarthritis. Patients experience moderate to severe discomfort as a result of bone-on-bone contact.

Treatment is often conservative, with NSAIDs serving as the first line of defense. In order to reduce inflammation in refractory osteoarthritis, intra-articular corticosteroid injections may be required. Surgery, in the form of arthroplasty, is reserved for severe instances in whom medication has failed to relieve symptoms.

Conclusion

The shoulder is particularly vulnerable to damage because it includes the most movable joint in the body. Repair or replacement of bones, joints, or tendons may necessitate surgical intervention. In cases of impingement, techniques such as arthroscopy, complete arthroplasty, and bone shaving are employed.