Index
Tendons are thick fibrous cords that connect the muscles to the bone.
Stress or excessive joint use can inflame the tendons and cause tendonitis.
The shoulder ( 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 a single joint, the shoulder is formed by two separate joints: the glenohumeral and acromioclavicular.
These two joints work together to circumscribe the arm 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 shoulder joint’s muscles, ligaments, and cartilages 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 additional shoulder 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 quickly weaken the shoulder.
The shoulder joint is vulnerable to dislocations due to sudden arm jerking, especially in children, before strong muscles develop.
The shoulder dislocation 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 harrowing 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, typical of a synovial joint.
The head of the humerus is much larger than the glenoid fossa, which gives the inherent joint instability.
The glenoid fossa is deepened with a fibrocartilage border called the glenoid labrum to reduce the surface disproportion.
Joint capsule and Bursa
The joint capsule is a fibrous covering that encloses the standard structures.
It extends from the anatomical neck of the humerus to the edge of the glenoid fossa.
The joint capsule is loose, allowing 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.
Several synovial bags are present to reduce this friction in the shoulder joint.
The Bursa is a structure filled with synovial fluid and acts as a buffer between the tendons and other joint structures.
The clinically essential bags 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 crucial role in stabilizing bone structures.
Most ligaments are thickenings of the joint capsule:
- Glenohumeral ligaments (upper, middle, and lower): consists of three bands, 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.
- 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.
- 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.
- The other major ligament is the coracoacromial ligament: Unlike the others, it does not thicken 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 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:
- The extension (upper extremity backward in the sagittal plane): is produced by the posterior deltoid, the latissimus dorsi, and the more excellent round.
- Flexion (upper extremity forward in the sagittal plane): produced by the biceps brachii (both heads), the pectoralis major, anterior deltoid, and the coracobrachialis.
- 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 following 15 to 90 degrees.
- Adduction (upper extremity towards the midline in the coronal plane): produced by the contraction of the pectoralis major, the latissimus dorsi, and the more excellent round.
- Medial rotation (rotation towards the midline): directed by contraction of the subscapular, pectoralis major, latissimus dorsi, more excellent round, and anterior deltoid.
- Lateral rotation (outside the midline) is produced by the contraction of the infraspinatus and the minor game.
Mobility and stability
The shoulder joint is one of the most mobiles 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 sizeable humeral head; there is a disproportion of 1: 4 on the characters.
- 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 fibrocartilaginous protrusion borders the glenoid cavity and reduces the risk of dislocation.
- Ligaments: the ligaments 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 harrowing 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 about 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 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 rotator cuff muscles play a vital role in stabilizing the glenohumeral joint.
They are often subjected to intense pressures, so injuries to these muscles are relatively frequent.
Tendinitis is 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.
