Anatomy of the shoulder

Contents

Anatomy of the shoulder bones

The shoulder consists of 3 bones: shoulder blade, collarbone and humerus, which connect the arm to the chest and allow movements in all directions.
This bone is very complex and serves as the origin and approach of many muscles that move and stabilize the shoulder. The shoulder blade is a triangular bone plate located on the back surface of the rib cage.

Shoulder blade

It extends from the second to the seventh rib and is inclined forward at an angle of 30°.
The shoulder blade has a shallow pit on one side, the so-called joint socket, in which the humeral head attaches to form the shoulder joint (glenohumeral joint).

Rear surface
The back surface of the shoulder blade points outwards. It is the attachment surface for most of the rotator muscles of the shoulder.

Spina (shoulder bone) – the most prominent part of the posterior shoulder blade. It runs across the shoulder blade and divides the surface into two parts.
Fossa infraspinata – convex pit located below the spina. The infraspinatus muscle originates in this zone.
Fossa supraspinata – it lies above the spina, is much smaller than the infraspinata fossa and has a more convex shape.
It originates from the supraspinatus muscle.
Acromion (bone corner) – is the continuation of the shoulder bone above the shoulder joint and jointed to the collarbone.

Ventral facies (costalis)
The front of the shoulder blade is defined as “facies ventralis or costalis” because it faces the chest.
This side of the shoulder blade is relatively insignificant. Over much of its surface it has a concave depression called fossa subscapularis.
The subscapularis muscle is a rotator muscle that originates on the ventral side of the shoulder blade.
On the upper side and on the lateral shoulder blade originates the processus coracoideus.
It is a hook-shaped bone process (raven-billed process), which is located directly below the collarbone.

Clavicle

The collarbone is an S-shaped bone and the main connection between the arm and the rest of the axial skeleton, especially with the sternum.
The collarbone is also a zone of important muscle attachments.
The collarbone meets the shoulder blade in the upper shoulder area, where it connects to the acromial process, with which it then forms the “acromioclavicular joint”.

Humerus

The humerus is the bone of the upper arm. The humeral head has the shape of a ball, sits in the joint socket of the shoulder blade and forms the shoulder joint with it.
The proximal (close to the chest) limb is formed from the head, the anatomical neck and the bone protrusions tuberculum majus and minus.
The tuberculi are separated by a depression, the sulcus intertubercularis.

The head is almost hemispherical.
The anatomical neck (collum anatomicum) originates from the lower part of the head, the tuberculum majus is located laterally and beyond the acromion.
The majus tuberculum is covered by the deltoid muscle, which gives the shoulder the curve.
The tuberculum minus is located in the anterior area.

The fossa between the two tuberculi (majus and minus) contains the long biceps tendon.

Shoulder joints

The shoulder is a joint with a large range of motion that allows various activities in daily life.
There are three main joints of the shoulder girdle, namely:

Shoulder joint (glenohumeral joint)
Shoulder joint (acromioclavicular joint)
Sternum-collarbone-joint (sternoclavicular joint)

Another very important joint in the movement of the shoulder is the shoulder blade-thoracic joint (scapulothoracic joint).

Glenohumeral joint
The shoulder joint is a ball joint that allows most movements of the shoulder girdle.
The humerus bone head forms a joint connection with the joint socket of the shoulder blade. The humeral head is larger than the joint socket, which adheres entirely to the surface of the humerus. The shoulder socket increases its contact surface with the humerus bone head thanks to the socket lip, a frame made of fibrocartilage.
Both articular surfaces are covered by cartilage, which protects the bone.

Acromioclavicular joint
The shoulder or acromioclavicular joint is formed by the lateral end of the collarbone and the anterior medial roof of the shoulder.
This joint plays an important role in transmitting forces from the upper limb to the axial skeleton. The mobility of the acromioclavicular joint is minimal due to the supportive ligaments.
The acromioclavicular ligament is formed by strong upper and lower ligaments, as well as weak anterior and posterior ligaments, and limits the movement of the collarbone and acromion.
The ligamentum coracoclaviculare consists of the fibrous ligamentum trapezoideum and ligamentum conoideum.
It forms a strong band to prevent vertical movement.

Sternoclavicular joint
The sternum-collarbone or sternoclavicular joint is formed from the end of the collarbone (extremitas sternalis) directed towards the middle of the body, the cartilage of the first rib and the upper and lateral part of the “handle” of the sternum (manubrium sterni).
This joint is the only one that connects the upper end of the axial skeleton to the upper limb via the collarbone.
Joint function includes all movements of the upper limb.

Scapulothoracic joint
This joint connection is mainly an insertion zone of the surrounding muscles. The main muscles that control the sternum-collarbone-joint are:

The anterior saw muscle that holds the upper shoulder blade angle against the thoracic wall.
The trapezius, which turns and lifts the shoulder blade, raising its arm.

Rotator cuff

The rotator cuff is a complex group of muscles and tendons that surround the shoulder joint.
The muscle contracts to move the bones and the tendon is the connecting piece between muscle and bone.
Their most important function is that of stabilizing the shoulder joint to prevent shoulder dislocation.
The 4 muscles of the rotator cuff are important for the rotational movements of the shoulder.

The rotator cuff is important for many routine activities. If it is injured, it can cause severe pain.
If the tendons of the rotator cuff rupture, it is called a lesion or rupture (the most common is the supraspinatus lesion).
In most patients who present to the doctor for shoulder pain, the cause is tendonitis or injury to the rotator cuff.

Shoulder joint capsule

The capsule that surrounds the shoulder joint has the shape of a ball and contains synovial fluid.
The ligaments are parts of the joint capsule and serve to keep the shoulder joint in the correct position.
If a shoulder dislocation occurs, the ligaments may rupture. The capsule has a very important intra-articular tension of its own to maintain good functionality.
If this tension of the capsule decreases, a situation arises, which is also called “unstable shoulder”. If the shoulder is blocked, “adhesive capsulitis” may be present.

Ligaments of the shoulder

Glenohumeral ligaments:
The joint capsule is a waterproof sleeve that surrounds the joint.
On the shoulder, the joint capsule is also formed by a group of ligaments that connect the humerus to the joint socket of the shoulder blade.
These ligaments are the main source of stability of the shoulder.
These are the glenohumeral ligaments (upper, middle and lower ligament) that originate from the articular socket and attach to the front of the humerus.
They serve to hold the shoulder in its position and prevent dislocation.

Korakoakromiales Band:
This band connects the processus coracoideus with the shoulder roof. The korakoakromial ligament can thicken and cause impingement syndrome.

Coracoclavicular ligaments:
These two ligaments (ligamentum trapezoideum and ligamentum conoideum) fix the processus coracoideus of the shoulder blade on the collarbone. These small ligaments play an important role in keeping the collarbone and shoulder blade together.
They can carry an enormous load and are extremely strong.

Transverse ligament of the humerus:
It holds the tendon of the caput longum of the biceps in the groove between the two cusps of the humerus (tuberculum majus and minus).

Pan lip of the shoulder joint

The socket lip or limbus is a cartilage ring that surrounds the joint socket of the shoulder joint. The lip allows an additional deepening of the socket for better stabilization of the shoulder. In addition, the lip is the starting point of the long biceps tendon.

Injuries of the pan lip are classified according to their position. They cause pain and instability of the joint.
The most common injuries are related to shoulder dislocation (called Bankart lesion) and problems with the long biceps tendon.

Subacromial or subdeltoideal bursa

The subacromial bursa is located on the upper surface of the supraspinatus tendon.
The bursa is used for cushioning and reduces friction in the movement between the bone that lies above the shoulder roof and the underlying muscles of the rotator cuff.
Often the bursa extends laterally and unites with the subdeltoideal bursa.

Nerves of the shoulder

The brachial plexus is a nerve plexus that originates from the spine and innervates the entire arm. It passes through the shoulder and armpit.

These nerves carry the signals from the brain to the muscles and enable the arm movements. They also transmit sensitivity signals, such as touch, pain and temperature, to the brain.
The brachial plexus is formed by a large number of nerves that give sensitivity to the arm and allow motor function.
Nerve complaints around the shoulder area are rare, most affected are:

Axillary nerve, which innervates the deltoid muscle and is pulled during a shoulder dislocation.
Long thoracic nerve, which innervates the anterior saw muscle and causes the “scapula alata” in case of injury.
Suprascapular nerve, it innervates the supraspinatus and infraspinatus muscles.
Musculocutaneous nerve, which innervates the biceps brachii muscle.

Neuralgic shoulder amyotrophy (also known as parsonage-Turner syndrome) is a rare condition in which there is inflammation of the brachial plexus with muscle atrophy and shoulder weakness.

Deep shoulder muscles

Anterior muscles of the shoulder

– The pectoralis minor muscle (small pectoral muscle) is a small triangular muscle that lies under the large pectoral muscle, originates from the 3rd, 4th and 5th ribs and is inserted on the coracoid process of the shoulder blade. The pectoralis minor muscle pulls the shoulder blade forward and downwards and raises the ribs with forced inspiration.

– The subclavius muscle is a small muscle located under the large pectoral muscle. It originates at the first rib and advertises at the bottom of the collarbone. The subclavius muscle pulls down the collarbone and brings the shoulder forward and down. In addition, it stabilizes the collarbone during movements of the shoulder girdle.

Posterior muscles of the shoulder

The shoulder blade lifter originates from the transverse processes of the cervical vertebrae and advertises at the medial shoulder blade angle. This muscle lifts the shoulder blade.
The small and large rhomboid muscles originate from the spine. Both advertise on the medial edge of the shoulder blade. These two muscles work together, lifting the shoulder blade and turning it inwards.
The large round muscle originates at the back surface of the lower angle of the shoulder blade and inserts at the medial lip of the bone groin of the humerus. This muscle stretches and turns the upper arm inwards.

Lateral muscles of the shoulder

– The anterior sawtooth muscle originates at the front of the first 10 ribs and forms a flattened “leaf” that passes over the chest wall and advertises at the front of the medial edge of the shoulder blade. The Serratus muscle pulls the shoulder blade towards and away from the chest wall and turns it outwards.

The four muscles of the rotator cuff are involved in raising the arm sideways and turning the shoulder in all directions. These muscles increase the stability of the shoulder by fixing the humeral head in the socket.

The supraspinatus muscle (upper bone muscle) originates from the supraspinata fossa of the shoulder blade. The muscle runs with a tendon under the roof of the shoulder, is separated from the subacromial bursa by bone tissue, runs over the shoulder joint and inserts on the upper facet of the tuberculum major. Its function is to help the deltoid muscle spread the arm, to keep the humeral head in the joint socket and to prevent the deltoid from pressing the humeral head against the acromion.
The infraspinatus muscle originates from the infraspinata fossa of the shoulder blade. The tendon of this muscle runs behind the shoulder joint and inserts at the tuberculum majus of the humerus. The infraspinatus holds the humerus in the socket and is responsible for the external rotation of the arm.
The teres minor muscle (small round muscle) originates from the lateral edge of the shoulder blade and inserts on the lower facet of the majus tuberculum of the humerus. This muscle causes the external rotation of the arm and supports the cohesion of the shoulder joint heads.
The subscapularis muscle originates from the subscapularis fossa at the front of the shoulder blade and inserts at the tuberculum minus the humerus. This muscle is important for the internal rotation of the arm and stabilizes the shoulder joint.

Superficial shoulder muscles

Anterior shoulder muscles

– The large pectoral muscle moves and stabilizes the front shoulder. The muscle consists of three parts, the pars clavicularis, which originates from the medial half of the collarbone, the pars sternocostalis, whose origin lies in the sternum, and the pars abdominalis, which originates at the anterior leaf of the tendon plate of the straight abdominal muscle. Although these three muscle parts have a different origin, they unite and advertise on the lateral lip of the sulcus intertubercularis of the humerus. The coordinated work of the three parts allows the muscle to flex, adduce and internally rotate the arm in the shoulder joint.

Posterior shoulder muscles

– The trapezius muscle has a broad origin, which involves most of the vertebrae of the spine and the back of the skull. The trapezius advertises at the level of the collarbone, at the medial edge of the shoulder roof and the spina scapulae. Its function is lifting, adduction, external rotation and lowering of the shoulder blade.

– The large back muscle with its extensive muscle belly originates from the spinous processes of the lowest 6 thoracic vertebrae, the fascia thoracolumbalis, the crista iliaca and the lowest 4 ribs. This muscle inserts at the level of the posterior edge of the sulcus intertubercularis (or biceps groove) of the humerus. The large back muscle includes the adduction and internal rotation of the arm.

In addition, he pushes the shoulder down and holds the lower part of the shoulder blade against the thoracic wall.

Lateral shoulder muscles

– The deltoid has three surfaces of origin, one anterior, central and posterior. The front part originates from the front edge and top of the collarbone. The central section originates from the acromion and the posterior part originates from the lower edge of the bone groin of the shoulder blade. These three parts converge and advertise at the tuberosity deltoidea. These three fascia exert a different action on the body.
The anterior fascia bends and turns the arm inwards, the central fascia abducts the arm, while the back part stretches the arm and turns outwards.

– The two-headed upper arm muscle has two origins, one at the level of the coracoid process of the shoulder blade (caput breve), while the other origin is at the level of the supraglenoid tuberculum (caput longum). The two fascia unite and advertise at the level of the tuberosity of the spoke. The two-headed upper arm muscle is the strongest flexor muscle of the forearm and is also able to supinate the forearm and bend the arm at shoulder height.

– The coracobrachialis muscle has its origin in the coracoid process of the shoulder blade and advertises in the middle third of the medial area of the humerus. This muscle weakly induces the arm and supports the stabilization of the humerus.

-The triceps brachii muscle is a three-headed muscle. The caput longum originates from the infraglenoidal tuberculum of the shoulder blade.
The caput laterale has its origin in the upper half of the posterior side of the humeral shaft.
The medial caput originates from the back surface of the lower half of the humerus. All these fascia run vertically down the arm to advertise at the level of the olecranon at the end of the ulna (ulna). The triceps is the main extensor of the forearm. In addition, the caput longum can stretch and bring in the humerus because it crosses the shoulder joint.

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