Shoulder
In human anatomy, the shoulder is the part of the body where the arm meets the trunk. It is formed by the conjunction of the ends of three bones: the clavicle, the scapula and the humerus; as well as muscles, ligaments and tendons. The main joint of the shoulder is the one that joins the head of the humerus with the scapula, it is called the glenohumeral joint and it has two articular surfaces, one of them corresponds to the head of the humerus that has a hemispherical shape and the other is the glenoid cavity of the scapula, these surfaces are covered by cartilage that allow smooth and painless movement. Externally, a soft tissue envelope surrounds the whole, it is the so-called joint capsule that is reinforced by several ligaments that give it stability and prevent the bones from moving beyond their physiological limits. A set of muscles and their tendons are attached to the surfaces of the bones and make the mobility of the joint possible, among them the rotator cuff is very important, formed by four muscles that provide mobility and stability to the shoulder. Several transparent, sac-like structures called serous bursas allow the smooth sliding of the different mobile components. The shoulder is the joint with the greatest range of motion in the human body.
Joints
The shoulder is an joint complex, made up of several joints. These can be classified into two groups:
First group:
- Gloenohumeral Articulation: It is also known by joint of the shoulder and is composed of the head of the humerus and the glenoided cavity, the latter in turn sits on the scapula. It would be considered as a true articulation from the anatomical point of view, since it is two cartilageous surfaces that slide together. It is a diartrosis type joint and within it is in turn classified as enartrosis.
- Subdetoid ticking: Also considered as a second shoulder joint. Anatomically it would not be considered as a joint so it would be considered a false joint or sisarcosis, although physiologically it would. They are two surfaces that slide between themselves; the deep face of the deltoids and the cuff of the rotators, here we can find a serosa bag that facilitates the slip. Subdeltoid articulation is mechanically linked to the glenohumeral articulation: any movement of the glenohumeral generates a movement in the subdeltoid. Other anatomy books describe instead the joint called subacromial, which would be located between the shoulder roof and the cuff of the muscle rotators, which compacts the head of the humerus. The entire space is made up of synovial bags.
Second group:
- Escapulotoric Articulation: physiological and non-anatomical Articulation is a false joint or sisarcosis. Perform a scapula slide over the rib grill. It is considered the most important articulation of the group even though it cannot act without the other two since they are mechanically joined.
Two glide planes appear between cells, we can see it in the cross-sectional image of the thorax.
On the left side of the image we see the thorax, with the oblique part of the ribs and the intercostal muscles. The bones shown are the humerus, with attachment of the pectoralis major, the deltoid muscle on the outside surrounding it. In the section of the shoulder blade we see the following muscles covering it: in front the subscapularis muscle, and in the back the infraspinatus, teres minor and teres major. From the medial border of the scapula to the lateral thorax extends the serratus anterior. In this way, two sliding stations are created between the fibers:
- A space between the subscapularis (which covers the scapula) and the serratus anterior, the omoserratic space.
- A space between the chest wall and the serratus anterior (muscle), known as the thoracic or parietoserratic space.
Appreciating the right half of a section, we observe the functional structure of the shoulder girdle:
-The scapula forms part of a plane that will form an angle of 30º with the plane corresponding to the dorsal support, remaining parallel to the frontal plane
This angle represents the physiological plane of abduction of the shoulder joint.
-The clavicle has an italic S shape, and despite this it is not going to be a perfect S, but rather it is oblique outwards and backwards. To do this, it follows a direction whose angle with the frontal plane is 30º. It is articulated in front and also behind with the sternum, with the sternocostoclavicular joint in the middle part and it will form an angle of 60º with the scapula.
-This angle between the clavicle and the scapula will be open inwards in the anatomical position and will undergo modifications depending on the movements of the scapular ilium.
-If we look at the thorax and the shoulder girdle in its posterior part, the scapula is represented in the frontal plane, despite what it may seem to the naked eye. The scapula extends from the 2nd to the 7th rib, in physiological position. The inner edge of the spine of the scapula is at a distance of 5-6 cm from the line of the spinous processes. Its lower angle is 7 cm from the line of the spinous processes, therefore, as can be seen, the inner edge is not straight, but rather oblique, so that as one descends along this edge, greater separation is observed..
- Articulation acromioclavicular: True articulation between the outer portion of the collarbone and the acromion. It's an arthrodia type joint. It is a flat joint with powerful ligaments, which causes its amplitude of movement to be very limited.
- Stereoclavicular Articulation: True joint, it is located between the inner portion of the collarbone and the upper part of the breastbone. It is a toroid type joint, reciprocal lace or mounting chair. The lack of congruence between the sternal joint and the clavicular is compensated with the presence of a fibrocartilaginous joint disc.
In each of the groups, the joints are mechanically linked (they act together). The two groups also act simultaneously and in variable proportions.
Muscles
Muscles help support the shoulder and allow the joint to move in all directions. The 11 most important muscles for shoulder function are:
- The supraspine muscle. Perform the abduction or separation of the arm.
- The subscapular muscle. It performs internal rotation.
- The infraspine muscle. Make the external rotation.
- The lower round muscle. It contributes to external rotation.
- The biggest round muscle. Contribute to the extension.
- The muscle deltoids. It performs the extension, bending and abduction.
- The major pectoral muscle. It intervenes in aduction or approximation, bending and extension.
- The wide dorsal muscle. It intervenes in extension and aduction.
- Coracobrachial muscle. Drive to the humerus.
- The trapeze muscle. Lift, remove and externally rotate the escapula.
- The previous serrato muscle. Rotate the escapula and transact it earlier against the chest wall
Rotator cuff
The group formed by the supraspinatus, infraspinatus, teres minor, and subscapularis muscles, as well as their corresponding tendons, is called the rotator cuff. These four muscles start from the scapula and are inserted into the head of the humerus very close, although in reality each one of them is independent. In this area, injuries frequently occur, especially tendinitis that is sometimes generically called rotator cuff tendinitis, while on other occasions the specific muscle affected is specified, naming in such a case supraspinatus tendinitis, which is the most common or of any of the other three muscles that make up the cuff.
Biomechanics
The shoulder joint has great capacity for movement in all directions (in all three axes, and all three planes of space). The main movements are listed below with their limits under normal conditions.
Elementary movements around the sagittal, coronal and longitudinal axes:
- Flexion. It is done by lifting the arm forward. Its range is 0° to 180°. The main muscles that run is action are the major deltoids and pectoral. The accessories are the coracobrachial, subscapular and biceps.
- Extension. Movement contrary to the previous one. Width from 0° to 50°. The main muscles that execute it are the largest pectoral (from the bending), wide and round dorsal. Accessories are deltoids and triceps.
- Abduction or separation. It is done by moving the arm outward, its width is 0° to 90°. The main muscles are deltoid and supraspine. The accessories are subscapular and biceps.
- Aduction or approximation. It is the anti-previous movement and has equal amplitude. If the subject is in a reference position, that is to say with the arm next to the trunk, the aduction will be impossible. The main muscles are larger pectoral, subscapular, wide dorsal. The accessories are the coracobrachial, subscapular, biceps and triceps.
- Internal rotation. This movement can be executed by holding your hand inside with the 90° bending elbow. The main muscles are coracobrachial, wide dorsal, larger round, larger pectoral. Accessories are deltoids, supraespinoso and biceps.
- External rotation. Inverse to the previous one, it is carried the hand out with the bend of 90°. The main muscles are lower infraspine and round. The accessory is the deltoid.
Combined movements, combine several axes:
- Circumduction: The movement combines the three axes by describing a cone of circumduction.
- Horizontal bending: It is part of a position of 90° abduction associating the bending with the aduction of 140° of amplitude.
- Horizontal extension: It is part of a movement of 90° abduction associating the extension with the aduction of a width of about 30°-40°.
Joint biomechanics of the shoulder girdle
1- Glenohumeral joint: It consists of the head of the humerus, which is similar in shape to a hemisphere but is far from regular, and the glenoid cavity in the shoulder blade. The latter is oriented outwards, forwards and slightly upwards, likewise its concave surface is irregular. On the other hand, it is delimited by the glenoid rim and has a smaller surface than that of the humeral head. Considering the disparity between the size of the surface between the articular facets of the humerus and the glenoid cavity and its irregularity, it is concluded that there is not a good congruence between the two. To improve this congruence there is a fibrocartilaginous ring called the glenoid labrum located on the glenoid rim.
2- Subdeltoid joint: In its operation, it constitutes a glide plane between the deep face of the deltoid muscle and the rotator cuff. Between these two structures is a serous bursa. In the abduction movement this serous bursa slips and the inferior capsular recess is put into tension. On the other hand, the coracobrachialis muscle constitutes the anterior guard of this joint while the brachial triceps constitutes its inferior guard.
3- Scapulothoracic joint:
Currently, in addition to the basic movements of this joint, other movements involving this joint have been described. These are abduction or flexion of the arm, which are combined to different degrees. With the help of several X-rays (performed by Caffinière) made during abduction, which were compared with the scapula, the actual components of this movement were studied. Thus it was deduced that during active abduction the scapula must perform four movements:
1) It ascends between 8 and 10 cm, without performing an anteriorization (as was believed in previous studies).
2) It moves in the shape of a bell, practically linear. It is about 38° when the abduction goes from 0 to 145°. Angular rotation becomes equal at the scapulothoracic and glenohumeral joints after 120° in abduction.
3) A tilting movement in which the tip of the scapula will move forwards and upwards and the upper portion will move backwards and downwards. The amplitude of this movement is 23° during abduction from 0° to 45°. The movement takes place around a transversal axis, oblique from the inside out and from the back to the front.
4) A “pivot” movement. Its main characteristic is that it will be a diphasic movement, it will have two times:
- At first the glenoid tends to be oriented backwards and for this it will follow an angle of 10°. This occurs during abduction from 0º to 90º.
- In a second stage, the glenoid tends to return to an upward orientation. The angle in this case is 6°. This means that the glenoid does not recover its initial orientation in the anteroposterior plane.
4- Acromioclavicular joint: This joint is requested in flexion-extension movements of the shoulder, due to the bascule movements of the shoulder blade that subject the clavicle to a torsion.
5- Sternoclavicular joint: This joint corresponds to what is called cardan in mechanics, it is shaped like a saddle. It has two degrees of freedom but by combining both, movements can be made on the longitudinal axis or rotations.
Ligaments
- Superior glenohumeral ligament. Unite the glenoided round of the scapula with the anatomical neck of the humerus.
- Medium glenohumeral ligament. From the glenoiding round of the scapula to the lower tuberosity of the humerus or troquin.
- Lower glenohumeral ligament. It extends from the glenoid round of the scapula to the humerus below the troquín.
- Acromioclavicular ligament. Join the collarbone with the acromion.
- Coracohumeral ligament. It is a very powerful ligament that extends from the coracoid apophysis of the scapula to the greater and lesser tuberosities of the humerus.
- Coracoacromial ligament. From the apophysis to the acromion.
- Traceoid ligament. It extends from the lower edge of the collarbone to the coracoid apophysis of the scapula.
Serous sacs
Serous bursae are small sac-shaped structures filled with synovial fluid that serve to cushion friction between different structures. The main ones of the shoulder joint are:
- Underground bag. It is located in the space between the scapula and the handle of the rotators. It has the function to prevent the tendons of the muscles that make up the cuff from scratching against the acromion.
- Subdeltoid bag. Situada bajo el deltoides muy próxima a la bolsa subacromial con la que se comunican, por lo que en ocasiones se denominan indistintamente a ambos.
- Subscapular bag. This bag is positioned under the subsecapular muscle, communicating with the shoulder joint cavity through the Weitbrecht foramen.
Frequent injuries
Shoulder Dislocation
Dislocation is the injury in which the bone segments that form a joint are separated. Shoulder dislocation, or scapulohumeral dislocation occurs frequently due to the great range of motion that this joint presents. It is generally an anterior dislocation, that is, the head of the humerus comes out of its normal position in the glenoid cavity of the scapula and is placed in front, making any type of movement in which these bony elements must be displaced very difficult and painful. It usually occurs due to trauma, being very frequent in sports practice. The treatment requires the placement by means of special maneuvers of the humerus in its correct place, this action should never be attempted by people who are not health professionals.
Acromioclavicular Dislocation
Acromioclavicular dislocation or acromioclavicular separation should not be confused with true shoulder dislocation or scapulohumeral dislocation. It is produced by separation of the bony ends of the acromion and clavicle. It generally occurs due to direct trauma, for example accidental falls in which the shoulder hits the ground, very frequent in the practice of certain sports activities. Under normal conditions, the acromioclavicular joint is supported by two ligaments that give it stability, the acromioclavicular ligament and the coracoclavicular ligament. The severity of the condition is highly variable, as it can range from a simple ligament strain to a complete rupture of the same with injury to some of the nearby muscles, sometimes the end of the clavicle pierces the trapezius muscle.
Supraspinatus Tendonitis
Supraspinatus tendinitis is caused by inflammation of the tendon of the supraspinatus muscle, and is one of the most common causes of shoulder pain. When the arm is raised overhead, this tendon has a great tendency to rub against the lower edge of an area of the scapula called the acromion that lies just above. The repeated friction causes inflammation of the tendon and sometimes even tears and ruptures of the same.
Biceps Tendonitis
It is due to inflammation of the tendon of the long head of the biceps brachii muscle in the shoulder region. It causes pain in the anterior part of the joint. Sometimes, after a long evolution, tendon rupture can occur.
Bursitis
Bursitis is inflammation of a serous bursa, structures that are generally found near joints and serve to facilitate the sliding of moving parts, particularly bones, muscles, and tendons. Subacromial bursitis is common in the shoulder region, caused by inflammation of the serous bursa of the same name that protects the supraspinatus muscle from rubbing against the acromion. Inflammation of the bursa is generally due to repetitive microtrauma and causes pain in the external anterior and lateral region of the shoulder, also causing mobility deficits. It occurs frequently in athletes who throw repetitively.
Adhesive capsulitis
Adhesive capsulitis (also called "frozen shoulder" or "retractile capsulitis"), consists of a retraction of the joint capsule, accompanied by pain, inflammation, and severely decreased mobility of the scapulohumeral joint, both active and passive. It can be primary, when there is no apparent cause, and secondary, in which case it is caused by prolonged immobilization after trauma or other inflammatory disorders that affect the joint. Adhesive capsulitis is a long-standing process that occurs most frequently in patients between 40 and 70 years of age. It can be treated with rehabilitation measures and physiotherapy. It has a tendency to improve after a period of around 15 months, although mobility deficits often persist.
Fractures of the Neck of the Humerus
The neck of the humerus is located close to the scapulohumeral joint. These types of fractures are common in children and patients over 50 years of age and may affect the anatomical neck of the humerus or the surgical neck, depending on their location. In most cases they occur as a consequence of an indirect mechanism, for example a fall in which the subject rests his hand on the ground with the upper limb extended. Less frequently, the mechanism is direct due to a direct blow to the area, since the shoulder region is well protected by different muscles, such as the deltoid, which act to cushion the impacts.
Osteoarthritis
Osteoarthritis is a painful, degenerative disease characterized by progressive destruction of the cartilage that lines the joints, the presence of bony protrusions called osteophytes, and joint deformities. The shoulder is less prone to osteoarthritis than other joints such as the knee and hip. When it exists, it is usually due to various factors that have favored its appearance: repeated trauma, old fractures, shoulder stiffness, repeated dislocations or long-standing rotator cuff tendinitis.
Arthritis
Arthritis is an inflammatory process that affects a joint, it must be distinguished from osteoarthritis, which is a degenerative process. Osteoarthritis is sometimes called osteoarthritis, terminology that tends to cause confusion, since it is not a true arthritis.
Shoulder arthritis can be caused by various diseases, some of the most frequent are rheumatoid arthritis, polymyalgia rheumatica, septic arthritis due to infectious agents and microcrystalline arthritis due to uric acid deposits (gout). Rheumatoid arthritis when it affects the shoulder causes effusion inside the joint and intense pain with limitation of mobility, it can involve both the scapulohumeral and acromioclavicular joints.
Referred pain
Referred pain is pain that is experienced in an area far from where the injury or problem that causes it is located. In the shoulder it is relatively common and can be caused by cardiac causes such as acute myocardial infarction, lung causes such as pneumothorax or Pancoast tumor, and abdominal causes such as gallbladder and bile duct stones.
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