Shoulder Joint: Types, Structures, Movements, Stability and Associated Injuries

Tendons are thick fibrous cords that connect the muscles to the bone.

Stress or excessive use of a joint can inflame the tendons and cause tendonitis.

The shoulder joint ( glenohumeral joint ) is a spherical joint between the scapula and the humerus.

It is the main joint that connects the upper extremity with the trunk.

The shoulder is one of the joints with greater mobility in the human body.

Types of joints

Commonly considered as a single joint, the shoulder is formed by two separate joints: the glenohumeral and acromioclavicular joints.

These two joints work together to allow the arm to be circumscribed in a large circle and rotate around its axis in the shoulder.

The glenohumeral joint is a spherical joint formed between the joint of the rounded head of the humerus (the bone of the upper arm) and the cup-shaped depression of the scapula, called the glenoid fossa.

The glenoid fossa forms a very shallow alveolus, so that the muscles, ligaments and cartilages of the shoulder joint reinforce its structure and help prevent dislocations.

A ring of cartilage known as labrum, surrounds the glenoid fossa to extend the socket size and maintain flexibility.

To further strengthen the shoulder, the four muscles of the rotator cuff extend from the scapula and surround the head of the humerus, so that both rotate the arm and prevent dislocation.

The acromioclavicular joint is formed by a joint between the lateral end of the clavicle and the acromion process of the scapula.

It is a flat, sliding joint that gives the shoulder joint additional flexibility that would not be possible with just the glenohumeral joint.

Although both joints are held together by extensive ligaments and muscle joints, certain types of force can easily weaken the shoulder.

The shoulder joint is vulnerable to dislocations due to sudden jerking of the arm, especially in children before strong muscles develop.

The dislocation of the shoulder is extremely painful and may require surgical repair or even cause permanent damage.

Chronic or acute wear on the glenohumeral joint can cause the painful tearing of the rotator cuff tendons or a labral tear.

Both conditions are very painful and may require surgery to remove or reattach the torn tissue.

Shoulder joint structures

Joint surfaces

The shoulder joint is formed by the articulation of the head of the humerus with the glenoid (or fossa) of the scapula.

This gives rise to the alternative name for the shoulder joint, the glenohumeral joint.

Both articulation surfaces are covered with hyaline cartilage, which is typical of a synovial joint.

The head of the humerus is much larger than the glenoid fossa, which gives the joint inherent instability.

To reduce the disproportion on the surfaces, the glenoid fossa is deepened with a fibrocartilage border, called the glenoid labrum.

Joint capsule and Bursa

The joint capsule is a fibrous covering that encloses the structures of the joint.

It extends from the anatomical neck of the humerus to the edge of the glenoid fossa.

The joint capsule is loose, which allows greater mobility (particularly abduction).

The synovial membrane lines the inner surface of the joint capsule and produces synovial fluid to reduce friction between joint surfaces.

To reduce this friction in the shoulder joint, several synovial bags are present.

The bursa is a structure that is filled with synovial fluid, and that acts as a buffer between the tendons and other joint structures.

The bags that are clinically important are:

  • Subacromial: it is located inferiorly to the deltoid and the acromion, and superior to the supraspinatus tendon and the joint capsule. It is compatible with the deltoid and supraspinatus muscles. The inflammation of this bag is the cause of several problems in the shoulder.
  • Subscapular: located between the subscapularis tendon and the scapula. Reduces tendon wear during movement in the shoulder joint.

Ligaments

In the shoulder joint, ligaments play a key role in the stabilization of bone structures.

Most ligaments are thickenings of the joint capsule:

  1. Glenohumeral ligaments (upper, middle and lower): consists of three bands, which are executed with the joint capsule from the glenoid fossa to the anatomical neck of the humerus. They act to stabilize the anterior aspect of the joint.
  2. Coracohumeral ligament: connects the base of the coracoid process to the greater tubercle of the humerus. It is compatible with the upper part of the joint capsule.
  3. Transverse humeral ligament: it covers the distance between the two tubers of the humerus. Holds the tendon of the long head of the biceps in the intertubercular groove.
  4. The other major ligament is the coracoacromial ligament: Unlike the others, it is not a thickening of the joint capsule. It runs between the acromion and the coracoid process of the scapula, forming the coracoacromial arch.

Suministro neurovascular

The arterial supply to the glenohumeral joint is made through the anterior and posterior circumflex humeral arteries and the suprascapular artery.

These branches of the arteries make up an anastomotic network that is located around the joint.

The joint is supplied by the axillary, suprascapular and lateral pectoral nerves.

These nerves are derived from the C5 and C6 roots of the brachial plexus.

Therefore, a lesion of the upper brachial plexus (Erb Duchenne’s palsy) will affect the function of the shoulder joint.

Movements

As a spherical synovial joint, a wide range of motion is allowed:

  • Extension (upper extremity backwards in sagittal plane) : produced by the posterior deltoid, the latissimus dorsi and the greater round.
  • Flexion (upper extremity forward in sagittal plane): produced by the biceps brachii (both heads), the pectoralis major, anterior deltoid and the coracobrachial.
  • Abduction (upper extremity far from the midline in the coronal plane) : the first 0 to 15 degrees of abduction are produced by the supraspinatus. The average deltoid fibers are responsible for the next 15 to 90 degrees.
  • Adduction (upper extremity towards the midline in coronal plane): produced by the contraction of the pectoralis major, the latissimus dorsi and the greater round.
  • Medial rotation (rotation towards the midline): produced by contraction of the subscapular, pectoralis major, latissimus dorsi, greater round and anterior deltoid.
  • Lateral rotation (rotation outside the midline): produced by contraction of the infraspinatus and the minor round.

Mobility and stability

The shoulder joint is one of the most mobile in the body, at the expense of stability.

Factors that contribute to mobility:

  • Type of articulation: it is a spherical joint.
  • Bony surfaces: shallow glenoid cavity and large humeral head, there is a disproportion of 1: 4 on the surfaces.
  • Laxity of the joint capsule.

Factors that contribute to stability:

  • Rotator cuff muscles : These muscles surround the shoulder joint, adhere to the tubers of the humerus and fuse with the joint capsule. When these muscles are at rest they act to tighten the head of the humerus towards the glenoid cavity.
  • Glenoid labrum: this is a fibrocartilaginous protrusion that borders the glenoid cavity and reduces the risk of dislocation.
  • Ligaments: the ligaments act to strengthen the joint capsule and form the coracoacromial arch.

Injuries

Tear

Chronic or acute wear on the glenohumeral joint can cause the painful tearing of the rotator cuff tendons or a labral tear.

Both conditions are very painful and may require surgery to remove or reattach the torn tissue.

Dislocation of the shoulder joint

Clinically, dislocations in the shoulder are described by the position of the head of the humerus in relation to the infraglenoid muscle.

Anterior dislocations are the most frequent, although later dislocations may occur. The coracoacromial arch prevents superior movement of the humeral head.

An anterior dislocation is usually caused by excessive extension and lateral rotation of the humerus. The head of the humerus is forced anteriorly and inferiorly, in the weakest part of the joint capsule.

The tear of the joint capsule is associated with an increased risk of future dislocations.

The axillary nerve runs very close to the shoulder joint and can be damaged in the dislocation.

Axillary nerve injury causes paralysis of the deltoid and loss of sensation over the area of ​​the regimental plate.

A dislocation can also stretch the radial nerve, since it is tightly bound in the radial groove.

Tendonitis of the rotator cuff

The muscles of the rotator cuff play a very important role in the stabilization of the glenohumeral joint.

They are often subjected to strong pressures and, therefore, injuries to these muscles are relatively frequent.

Tendinitis refers to inflammation of the muscular tendons, usually due to excessive use.

Over time, this causes degenerative changes in the subacromial bursa and the supraspinatus tendon.

This increases the friction between the structures of the joint.

The characteristic sign of rotator cuff tendinitis is the “painful arch” (pain in the middle of abduction, where the affected area comes in contact with the acromion).