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SHOULDER ANATOMY

The shoulder is one of the most sophisticated and complicated joints of the body:

This section will hopefully explain some of the terminology you might hear and relate this to disorders of the shoulder complex. Understanding how the different layers of the shoulder are built and connected can help you understand how the shoulder works and is affected by injury and overuse.

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BONES & JOINTS OF THE SHOULDER

The bones of the shoulder consist of the humerus (the upper arm bone), the scapula (the shoulder blade), and the clavicle (the collar bone).

The clavicle is the only bony attachment between the trunk and the upper limb. It forms the front portion of the shoulder girdle and is palpable along its entire length with a gentle S-shaped contour.The clavicle articulates at one end with the sternum (chest bone) and with the acromion of the scapula at the other. This articulation between the acromial end of the clavicle and the acromion of the scapula forms the roof of the shoulder.

The scapula is a large, flat triangular bone with three processes called the acromion, spine andcoracoid process. It forms the back portion of the shoulder girdle. The spine (which is located at the back of the scapula) and the acromion can be readily palpated on a patient.

anterior view

The flat blade of the scapula glides along the back of the chest allowing for extended movement of the arm. The coracoid process is a thick curved structure that projects from the scapula and is the attachment point of ligaments and muscles.

right lateral view

The scapula is also marked by a shallow, somewhat comma-shaped glenoid cavity, which articulates with the head of the humerus.

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The top end of the humerus consists of the head, the neck, the greater and lesser tubercles, and the shaft. The head is half-spherical in shape and projects into the glenoid cavity. The neck lies between the head and the greater and lesser tubercles. The greater and lesser tubercles are prominent landmarks on the humerus and serve as attachment sites for the rotator cuff muscles.

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There are four joints making up the ‘shoulder joint’:

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Note how the ball (head) of the humerus fits into a shallow socket on the scapula called the glenoid. One can see that this ball does not fit into the glenoid cup at all; this allows for the wide range of movement provided by the shoulder, at the cost of skeletal stability. Joint stability is provided instead by the rotator cuff muscles, related bony processes and glenohumeral ligaments.

SHOULDER LIGAMENTS

Ligaments are soft tissue structures that connect bones to bones. There are several important ligaments in the shoulder.

Glenohumeral ligaments (GHL)
A joint capsule is a watertight sac that surrounds a joint. In the shoulder, the joint capsule is formed by a group of ligaments that connect the humerus to the glenoid. These ligaments are the main source of stability for the shoulder. They are the superior, middle and inferior glenohumeral ligaments. They help hold the shoulder in place and keep it from dislocating.

Coraco-clavicular ligaments (CCL)
These two ligaments (trapezoid and conoid ligaments) attach the clavicle coracoid process of the scapula. These tiny ligaments (with the acomioclavicular joint) play an important role in keeping the scapula attached to the clavicle and thus keeping your shoulder ‘square’. They carry a massive load and are extremely strong.
A fall on the point of the shoulder can rupture these ligaments with dislocation of the AC Joint.

Transverse Humeral ligament (THL)
THL holds the tendon of the long head of biceps brachii muscle in the groove between the greater and lesser tubercle on the humerus (intertubercular sulcus).

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SHOULDER TENDONS

Tendons are extensions of muscles that attach muscles to bone. Muscles move the bones by pulling on the tendons.

Biceps tendons

The biceps muscle has two tendons at the shoulder, called the Long Head and Short Head.

The Long Head of Biceps (LHB) is a very important tendon that travels through the shoulder joint (glenohumeral joint). The biceps tendon begins at the top of the shoulder socket (the glenoid) and then passes across the front of the shoulder to connect to the biceps muscle. (The biceps is the muscle that weightlifters are always showing off).

The LHB is a common source of shoulder pain and can rupture.

ROTATOR CUFF

The rotator cuff tendons are a group of four tendons that connect the deepest layer of muscles to the humerus. They are the tendons of the rotator cuff muscles.
These are (from front to back):

The supraspinatus is the most commonly affected tendon, both by overuse and trauma. It is the muscle that lifts your arm out to the side (a very important movement for most daily tasks). Injury to the tendon can result in a Rotator Cuff Tear. Overuse can lead to Subacromial Impingement.

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NERVES OF THE SHOULDER

All of the nerves that travel down the arm pass through the axilla (the armpit) just under the shoulder joint and are known as the Brachial Plexus before dividing into the individual nerves. These nerves carry the signals from the brain to the muscles that move the arm. The nerves carry signals back to the brain about sensations such as touch, pain, and temperature.

The Brachial Plexus is made up of a large number of nerves that supply the arm with it’s ability to function and feel.

Nerve problems around the shoulder are rare, but the most commonly affected of these nerves are:

SHOULDER BURSAE

Sandwiched between the rotator cuff muscles and the outer layer of large bulky muscles is a structure known as the Subacromial BursaBursae are everywhere in the body. They are found wherever two body parts move against one another and there is no joint to reduce the friction. A bursa is simply a sac between two moving surfaces that contains a small amount of lubricating fluid.

Think of a bursa like this: If you press your hands together and slide them against one another, you produce some friction. In fact, when your hands are cold you may rub them together briskly to create heat from the friction. Now imagine that you hold in your hands a small plastic sack that contains a few drops of salad oil. This sack would let your hands glide freely against each other without a great deal of friction.

Inflammation and swelling of the subacromial bursa is known as bursitis and associated with Subacromial Impingement of the Shoulder

 
subacromial bursa

GLENOID LABRUM

The shoulder joint is considered a ‘ball and socket’ joint, however, the ‘socket’ (the glenoid fossa of the scapula) is quite shallow and small, covering at most only a third of the ‘ball’ (the head of the humerus). The socket is deepened by the glenoid labrum.

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The glenoid labrum is similar to the meniscus of the knee. It is a fibro-cartilaginous rubbery structure which encircles the glenoid cavity deepening the socket providing static stability to the glenohumeral joint. It acts and looks almost like a washer, sealing the two sides of the joint together.

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The labrum is described like a clock face with 12 o’clock being at the top, 3 o’clock at the front, 6 o’clock at the bottom, and 9 o’clock at the back. Clinicians may reverse the 3 o’clock and 9 o’clock for left shoulder describing 3 o’clock at the back. This can be confusing, so the European Society of Shoulder & Elbow Surgeons (SECEC) has agreed to keep 3 o’clock at the front for either shoulder.

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An injury to the shoulder with shear forces either in the anterior or posterior or superior directions leads to a labral tear in the affected area. An injury between 3 and 6 o’clock is known as Bankart tear. superior labral injury is known as a SLAP tear (superior labral antero). A posterior tear of the posterior labrum is known as a posterior labral tear of reverse Bankart lesion.

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Sublabral foramen are anatomical variants, which is where the labrum can be ‘lifted up’ between 12 and 3 o’clock. It should not be confused with a labral tear, as its edge is clearly round and smooth and not red and ragged.

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SHOULDER MUSCLES

The muscles of the shoulder either connect the scapula and clavicle to the trunk, or connect the clavicle, scapula and body wall to the proximal (top) end of the humerus. The trapezius, levator scapulae, and rhomboids originate from the base of the skull and/or spine and connect the scapula and clavicle to the trunk of the body. The pectoralis major, pectoralis minor, latissimus dorsi, teres major and deltoid connect to the proximal end of the humerus and anchor it to the body.

The most important shoulder muscles are the four rotator cuff muscles – the subscapularis, supraspinatus, infraspinatus and teres minormuscles – which connect the scapula to the humerus and provide support for the glenohumeral joint.

Muscles of the arm that enter into the shoulder complex are separated into anterior (flexor) and posterior (extensor) compartments. These includebiceps brachii, triceps brachii andcoracobrachialis.

These shoulder muscles can be separated into three important groups:

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Superficial Muscles

FRONT (ANTERIOR) MUSCLES OF THE SHOULDER

Pectoralis Major provides movement and support in the front of the shoulder. The muscle has two heads; the clavicular head originates from the more midline half of the clavicle, and the sternocostal head originates from the manubrium and sternum (chest bone). This muscle inserts into the lateral lip of the intertubercular sulcus on the humerus. When the two heads of the pectoralis major act together, they flex, adduct and medially rotate the arm at the glenohumeral joint.

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BACK (POSTERIOR) MUSCLES OF THE SHOULDER

The trapezius muscle has an extensive origin, which includes back of the skull, and most of the vertebrae of the spine. It inserts on to the clavicle, acromion and crest of the spine of scapula. The trapezius muscle is a powerful elevator of the shoulder and also rotates the scapula to extend the reach upwards

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Latissimus dorsi muscle originates from the spinous process of the lower six thoracic vertebrae, lumbar vertebrae, sacral vertebrae, the iliac crest of the hip bone and the lower three or four ribs. It finally inserts on to the bottom of the intertubercular groove. Latissimus dorsi extends, adducts and medially rotates the arm. It also draws the shoulder downward and backward and keeps the inferior angle of the scapula against the chest wall.

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SIDE (LATERAL) MUSCLES OF THE SHOULDER

The large deltoid muscle provides the characteristic contour of the shoulder and is the largest, strongest muscle of the shoulder. It originates in three portions, the anterior, middle and posterior portions. The anterior portion arises from the anterior border and superior surface of the clavicle. The middle portion from the acromion process and the posterior portion from the lower border of the crest of the spine of the scapula.

These three portions come together and insert into the deltoid tuberosity on the shaft of the humerus. Each portion has a different action on the body. The anterior portion flexes and medially rotates the arm, while the middle portion abducts the arm. Finally the posterior portion extends and laterally rotates the arm.

The deltoid muscle takes over lifting the arm once the arm is:

Deep Muscles

FRONT (ANTERIOR) MUSCLES OF THE SHOULDER

The pectoralis minor muscle is a small triangular shaped muscle that lies deep to pectoralis major muscle and passes as three muscular slips from the thoracic wall (ribs III to V) to the coracoid process of the scapula. Pectoralis minor draws the scapula forward and downward, and raises the ribs in forced inspiration.

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The subclavius muscle is a small muscle that lies deep to pectoralis major muscle. It passes from rib I at the junction between the rib and its costal cartilage to a groove on the inferior (lower) surface of the clavicle. It depresses the clavicle, draws the shoulder forward and downward, and steadies the clavicle during movements of the shoulder girdle.

BACK (POSTERIOR) MUSCLES OF THE SHOULDER

The levator scapulae muscle originates from the transverse processes of the cervical vertebra and descends to attach to the superior (uppermost) and medial (near the midline) corner of the scapula. This muscle elevates the scapula.

Rhomboid major and minor muscles attach from the vertebral column to the medial border of the scapula, slightly below the levator scapulae muscles. These muscles retract and elevate the scapula.

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Teres major muscle originates from posterior surface of the inferior angle of the scapula and attaches the medial lip of the intertubercular sulcus which lies on the anterior surface of the humerus. This muscle extends and medially rotates the humerus.

SIDE (LATERAL) MUSCLES OF THE SHOULDER

Serratus anterior muscle originates as a number of muscular slips from the outer surfaces and superior borders of the first eight or nine ribs, and fascia covering the first intercostal spaces (spaces between each rib). They then form a flattened sheet which passes around the thoracic wall and attaches to the anterior (costal surface, that glides over the ribs) of the medial border of the scapula. The serratus anterior pulls the scapula forward of the the thoracic wall and rotates the scapula for abduction and flexion of the arm.

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The rotator cuff tendons attach to the deep rotator cuff muscles. These 4 muscles are involved in raising the arm from the side and rotating the shoulder in the many directions. The rotator cuff mechanism also helps keep the shoulder joint stable by holding the humeral head in the glenoid socket.

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Subscapularis forms the largest component of the posterior wall of the axilla (area on the body directly under the joint where the arm connects to the shoulder). It originates from and fills the subcapular fossa on the anterior surface of the scapula and inserts on the lesser tuberosity of the humerus, and part of the capsule of the shoulder joint. This muscle medially rotates the arm, and stabilizes the glenohumeral joint.

MUSCLES OF THE SHOULDER & ARM

Biceps brachii muscle originates as two head. The long head appears as a tendon from the supraglenoid tubercle of the scapula and passes over the head of the humerus, deep to the joint capsule and enters the intertubercular sulcus where it is held in position by the transverse humeral ligament. The short head of biceps brachii arises from the coracoid process of the scapula and joins the long head further down the humerus. The biceps brachii inserts primarily as a tendon into the radial tuberosity of the forearm and into the fascia on the medial part of the forearm. Biceps brachii is the primary flexor of the forearm; it also supinates the forearm and weakly flexes the arm at the shoulder.

Coracobrachialis muscle, together with the short head of biceps brachii muscle, originates from the tip of the coracoid process. It inserts on to the middle third of the medial surface and border of the humerus. Coracobrachialis weakly adducts the arm and aids in stabilizing the humerus.

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Triceps brachii muscle has three heads. The long head originates from the infraglenoid tubercle of the scapula; the lateral head from the upper half of the posterior surface of the shaft of the humerus and the medial head from the posterior surface of the lower half of the shaft of the humerus. All of these heads pass vertically down the arm to insert on the olecranon of the ulna. The triceps is the primary extensor of the forearm at the elbow joint. Because the long head crosses the glenohumeral joint, it can also extend and adduct the humerus.

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