Generality
The shoulder is the even region of the trunk, situated in a latero-superior position, on which three important bones meet: the clavicle, the scapula and the humerus.
The joints and muscles of the shoulder allow the human being to perform a wide variety of gestures: from launching an object to lifting a weight, from writing to drawing an ideal circle with the arm.
The shoulder can suffer different types of injuries: bone fractures, dislocations of the glenohumeral joint and injuries to the muscular tendons.
Definition
The shoulder is the even region of the body that marks the meeting between three very important bones: the clavicle, the scapula and the humerus.
These three bone elements - all strictly equal - work together to hook each upper limb to the trunk.
Seat of two fundamental joints, of muscles, ligaments and tendons, the shoulder occupies the latero-superior section of the trunk, from the base of the neck to the origin of the arm.
Brief review of the concepts: sagittal plane, medial position and lateral position
In anatomy, medial and lateral are two terms with the opposite meaning. However, to fully understand what they mean, it is necessary to take a step back and review the concept of the sagittal plan.
Figure: the plans with which the anatomists dissect the human body. In the image, in particular, the sagittal plane is highlighted.
The sagittal plane, or median plane of symmetry, is the antero-posterior division of the body, a division from which two equal and symmetrical halves are derived: the right half and the left half. For example, from a sagittal plane of the head derive a half, which includes the right eye, the right ear, the right nasal nostril and so on, and a half, which includes the left eye, the left ear, the left nasal nostril etc.
Returning to the medial-lateral concepts, the word media indicates a relationship of proximity to the sagittal plane; while the word side indicates a relationship of distance from the sagittal plane.
All anatomical organs can be medial or lateral with respect to a reference point. A couple of examples clarify this statement:
First example. If the reference point is the eye, it is lateral to the nasal nostril of the same side, but medial to the ear.
Second example. If the reference point is the second toe, this element is lateral to the first toe (toe), but medial to all the others.
Shoulder anatomy
The purpose of this article is to describe the most important anatomical elements of the shoulder, ie the bones, the joints with their ligaments, muscles, blood vessels and nerves.
BONES
As already mentioned above, the shoulder skeleton includes three bones: the clavicle, the scapula and the humerus.
The clavicle is the S-shaped bone, located in the antero-posterior part of the thorax, which connects the sternum to each scapula (to be precise to the so-called acromion of each scapula).
Figure: clavicle.
It belongs to the category of long bones, is convex medially and concave laterally and represents the only bone element of the human body completely horizontal.
Subdividable into three portions - sternal extremity, body and acromial extremity - the clavicle participates in the formation of the shoulder with the acromial extremity and the body ; the achromial end is the most lateral portion, that is the one closest to the point of origin of the upper limb; the body, on the other hand, is the central portion, compressed between the achromial end and the sternal end.
The scapula is the even bone, posteriorly posterior to the rib cage, which connects the trunk to the upper limb of each side of the body.
Figure: scapula.
Flat and triangular in shape, it has some anatomical features that make it a truly unique bone element. In fact, on its surfaces, it presents two bone processes ( acromion and coracoid process ) - which guarantee the union between the scapula and the clavicle - and a cavity ( glenoid cavity ) - which houses the head of the humerus and hooks the upper limb to the trunk .
The humerus is the even bone that constitutes the skeleton of each arm, that is, the upper limb section between the shoulder and the forearm.
Belonging to the category of long bones, it contributes to the formation of two important joints of the human body: the glenohumeral joint (commonly known as the shoulder joint) and the elbow joint .
Figure: humerus
The glenohumeral joint has as its protagonists the head of the humerus, located on the proximal end, and the glenoid cavity of the scapula. The elbow joint, on the other hand, involves the anatomical elements of the distal end of the humerus (the so-called trochlea and the so-called capitulum ) and the distal extremities of the bones of the forearm, ulna and radium.
As far as the shoulder is concerned, the portions of the humerus which are part of it are the proximal end, that is the one assigned to the formation of the glenohumeral joint, and the first part of the so-called body (or diaphysis) of the humerus.
JOINTS
According to most anatomists, there are five proper shoulder joints: the glenohumeral joint (or shoulder joint or scapulohumeral joint), the acromioclavicular joint, the sternoclavicular joint, the scapulothoracic joint and the subdeltoid joint.
Among these joint elements, both for the role they hold and for the structural complexity, the glenohumeral joint and the acromioclavicular joint deserve particular mention.
- Glenohumeral joint . As previously stated, the shoulder joint is the result of the interaction between the head of the humerus and the glenoid cavity of the scapula.
However, limiting ourselves to this description would indeed be very simplistic, since the glenohumeral articulation includes many other structural elements and many other peculiarities, without which it could not exist.
First aspect: the shoulder joint is a diarthrosis and, like all diarthroses, is contained in a fibrous-connective sleeve, called the joint capsule . With an extension that goes from the so-called anatomical neck of the humerus (inferiorly) to the edges of the glenoid fossa (superiorly), the articular capsule holds the humerus and scapula together and has a particular membranous layer, which takes the name of synovial membrane .
The synovial membrane has the task of producing a fluid, the so-called synovial fluid, which reduces friction between the articular surfaces. For articular surfaces, we mean the head of the humerus and the hollow of the glenoid fossa.
Second aspect: in some strategic points external to the joint capsule, the synovial membrane forms pockets (or bags ) filled with synovial fluid: the three most important are the subacromial bursa, the subscapular bursa and the suboracoid bursa .
Acting as anti-friction and anti-rubbing pads, these three bags prevent the joint surfaces from rubbing against neighboring muscles (or their tendons), causing damage during joint movements.
The subacromial bursa resides under the deltoid muscle and the acromion of the scapula and above the tendon of the supraspinatus muscle (NB: one of the four muscular elements of the rotator cuff). Its function is to preserve the deltoid and supraspinatus muscles from repeated rubbing and possible injuries.
The subscapular bursa takes place between the subscapularis muscle tendon (NB: another muscular element of the rotator cuff) and the scapula, thus avoiding direct contact.
Finally, the suboracoid bag locates in front of the subscapular muscle and below the coracoid process. Its specific task is to preserve the coracobrachial and subscapularis muscles and tendons of the biceps brachialis muscle.
Third aspect: to stabilize the relationship between the humerus and scapula within the joint capsule, there are a series of ligaments and tendons . A ligament is a formation of fibrous connective tissue that joins together two distinct bones or two different parts of the same bone; a tendon is a very similar structure but with the substantial difference that unites a muscle to a bone element.
The ligaments of the shoulder joint are: the glenohumeral ligaments, the coracomeric ligament and the transverse humeral ligament . The tendons, on the other hand, are: the tendon of the long head of the biceps brachialis muscle and the tendons of the muscles constituting the rotator cuff (subscapularis, supraspinatus, small round and sub-spinal).
- Acromioclavicular joint . The acromioclavicular joint is the result of the communication between the acromion of the scapula and the acromial (or lateral) end of the clavicle.
The acromion of the scapula is a hook-shaped bone process, which derives from the scapular spine and projects in the upper-lateral direction.
Where acromion and clavicle unite, they have two bony surfaces, more properly called facet joints, suitable for the purpose.
To increase the stability of the acromioclavicular joint, there are the two coracoclavicular ligaments : the conoid ligament and the trapezoid ligament .
The really curious aspect of these two elements is that they perform a stabilizing action, even though they are not directly linked to the acromion. In fact, they run from the lower edge of the acromial extremity of the clavicle to the coracoid process of the scapula.
They owe their effectiveness to the strength and resistance with which they are endowed.
Figure: the glenohumeral joint. As in all diarthrosis, the articular surfaces are covered with hyaline cartilage. The hyaline cartilage makes the joint surfaces particularly smooth and facilitates the movements of the joint.
To make it less likely that the head of the humerus slips out of the glenoid fossa, is a fibrous cartilage formation, located on the edges of the fossa itself (lip). Although it is always cartilaginous tissue, this fibrous cartilage has a consistency quite different from the hyaline cartilage: it is much rougher.
Figure: ligaments of the glenohumeral joint. The glenohumeral ligaments are, in fact, three bands that run from the glenoid fossa to the anatomical neck of the humerus. Their function is to stabilize the anterior region of the joint.
The coracomeric ligament combines the coracoid process with the greater tubercle of the humerus. Its task is to guarantee solidity to the upper portion of the joint.
The transverse humeral ligament runs from the greater tubercle of the humerus to the lesser tubercle (always of the humerus). Its function is to stabilize the tendon of the long head of the biceps femoris muscle, within a groove of the humerus known as the intertubercular sulcus .
The most attentive readers will have noticed that the image shows another ligament: the so-called coracoacromial ligament, interposed between the scapular acromion and the scapular coracoid process. Despite not having direct contact with the structures of the glenohumeral joint, it contributes to its stability, in particular to keep the head of the humerus in place.
In the image, synovial bags are also highlighted.
From the site: //www.studyblue.com/notes/note/n/kinesiology-review/deck/1011482
Does the sternoclavicular joint belong to the shoulder?
The sternoclavicular joint is the result of the union between the sternal end of the clavicle and the handlebar of the sternum .
The sternal end of the clavicle is the medial portion of the clavicle. The handlebar of the sternum is the upper region of the flat bone located in the center-upper part of the thorax (NB: the sternum also has a central region, called the sternal body, and a lower region, known as the xiphoid process ).
Anatomy experts have long debated whether to include the sternoclavicular joint in the topic "shoulder joints" for several reasons. Surely, the main motivation is that the sternal extremity of the clavicle occupies a somewhat central position in the human body and is more a part of the thorax than of the shoulder.
MUSCLES
Premise: the muscles have two extremities, one of origin and one terminal, which bind to the skeleton through tendons.
Numerous muscles are placed in the shoulder.
Some of these muscle elements bind to the skeleton of the shoulder with both ends (initial and terminal), while others (the remaining ones) have only one end (or initial or terminal).
To simplify the study of the muscular structure of the shoulder, the anatomists have thought to exploit the aforementioned characteristic of the muscles and distinguish the muscles in two categories: intrinsic, which correspond to those with both ends of insertion on the shoulder, and extrinsic, which are those having only one insertion end at the level of the shoulder skeleton.
The intrinsic muscles of the shoulder are a total of 6: the deltoid, the supraspinatus, the sub-term, the small round, the subscapularis, the large round.
The extrinsic muscles of the shoulder, on the other hand, are 11 in all: the anterior dentate, the subclavian, the small pectoralis, the sternocleidomastoid, the scapula elevator, the large rhomboid, the small rhomboid, the trapezium, the coracobrachialis, the biceps brachialis (both the long head and the short head) and the brachial triceps (only the long head).
In the table below, the reader can consult, with a few more details, the muscular scaffolding of the shoulder: the care was taken to report, for each muscle, the point of origin and the terminal zone.
Intrinsic muscles | Seat of the initial end | Seat of the terminal end |
| Deltoid muscle | It has three points of origin:
| Deltoid tuberosity of the humerus body |
| Supraspinatus muscle | Axillary (or lateral) border of the scapula | Greater tubercle of the humerus (located on the proximal end) |
| In spindle muscle | Subpianted pit of the scapula (back surface) | Greater tubercle of the humerus |
| Small round muscle | Supraspinous fossa of the scapula (back surface) | Greater tubercle of the humerus |
| Subscapularis muscle | Subscapular fossa of the scapula (anterior surface) | Greater tubercle of the humerus |
| Big round muscle | Lower corner and lateral edge of the scapula | Intertubercular furrow of the humerus |
Extrinsic muscles | Which end takes place on the shoulder? | Seat of the other end |
| Anterior serrated muscle | Terminal extremity, on medial border of the scapula | Initial end, on the external surface of the first 8-9 ribs of the rib cage |
| Subclavian muscle | Terminal extremity, on the lower face of the clavicle body | Initial end, on first rib |
| Pectoralis minor muscle | Terminal extremity, on the coracoid process of the scapula | Starting end, on a space between 3rd and 5th rib |
| Sternocleidomastoid muscle | Initial end, on the medial portion of the clavicle body (NB: it has a second initial end, which originates from the handlebar of the sternum) | Terminal extremity, on the mastoid process of the temporal bone (skull bone) |
| Elevating muscle of the scapula | Terminal end, on the lateral edge of the scapula | Initial end, on transverse process of the first four cervical vertebrae (C1-C4) |
| Rhomboid large muscle | Terminal end, on the lateral edge of the scapula | Initial end, on spinous process of thoracic vertebrae T2, T3, T4, T5 |
| Rhomboid small muscle | Terminal end on the side edge of the scapula | Initial end, on spinous process of the seventh cervical vertebra (C7) and of the first thoracic vertebra (T1) |
| Trapezius muscle | Terminal extremities, on clavicle (lateral body portion), scapular spine and acromion | Initial end, on spinous process of all cervical vertebrae and all thoracic vertebrae |
| Coracobrachial muscle | Initial end, on a coracoid process | Terminal extremity, on the middle portion of the humerus body |
| Biceps brachialis muscle | The long head has the initial end on the supraglenoid tuberosity. The short head has the initial end on the coracoid process of the scapula | Terminal extremity, on radial tuberosity of the radius (forearm bone) |
| Long head of the triceps brachialis muscle | Starting extremity, on subglenoid tuberosity of the scapula | Terminal extremity, on olecranon of the ulna |
* In this list of muscles, only the muscles that somehow also reside in the anatomical region of the shoulder appear.
However, it is good to remind readers of the existence of two muscles - the large dorsal and the large pectoral - which, although not properly seated on the shoulder, respond to the characteristics of intrinsic muscle elements and participate in numerous movements of the arm (see chapter dedicated to the functions).
Figure : site of the shoulder muscles of the scapula, small rhomboid, large rhomboid and large dorsal.
The large dorsal muscle has various initial extremities: on the spine segment T7-L5, on the iliac crest, on the lower corner of the scapula, on the thoracolumbar fascia and on the last 3-4 ribs. On the contrary it has a unique terminal end, located at the level of the intertubercular sulcus of the humerus.
SPRAY IRRORATION
The anatomical region of the shoulder receives arterial blood (thus rich in oxygen) from numerous direct and indirect branches of the axillary artery .
As far as venous blood is concerned, this flows towards the so-called deep venous system, with axillary and subclavian veins, and towards the so-called superficial venous system, with the cephalic vein .
INNERVATION OF THE SHOULDER
Among the nervous structures of the shoulder region, there are some that pass only through this anatomical district and others, instead, that innervate the local anatomical elements (muscles, skin, etc.).
Both the nervous structures that only cross the shoulder and those that innervate the muscles, the skin and the other local anatomical elements derive from the brachial plexus, precisely from some of its collateral and terminal branches.
The brachial plexus is an important reticular formation of different spinal nerves (ie nerves of the peripheral nervous system ), which have the task of innervating not only the shoulder, but also the entire upper limb (therefore arm, forearm and hand).
Functions
Thanks to its numerous muscles and its important joints (glenohumeral in particular), the shoulder has such a mobility that it allows the human being to perform an enormous variety of gestures: from the simplest ones, like to greet with a gesture of the hand or writing, to the more complex ones, like throwing an object or lifting a weight.
During their research, physiology and biomechanics experts have studied all possible shoulder movements and have come to the conclusion that there are at least 13 different types:
- The scapular adduction movement. It is the gesture by which the two shoulder blades tend to get as close as possible to the sagittal plane.
- The scapular abduction movement. It is the opposite gesture to the scapular adduction, hence the one in which the shoulder blades tend to move as far away as possible from the sagittal plane.
- The elevation of the shoulder blades . It is the gesture of raising the shoulder blades.
- Depression of the shoulder blades . It is the lowering movement of the shoulder blades.
- Upward rotation of the shoulder blades . It is the gesture carried out by the shoulder blades, when they raise their arms towards the sky.
- The downward rotation of the shoulder blades . It is the gesture carried out by the shoulder blades, when the arms are carried from above along the body.
- The true abduction of the arm . It consists of raising the arm from a position that is along the sides of the body to one that is perpendicular to the spine.
When the arm is raised further (therefore it goes beyond the plane of perpendicularity), the upward movement of the shoulder blades is exploited.
- The true adduction of the arm . It is the opposite movement to the true abduction of the arm and serves to bring the arm back from perpendicular to the vertebral column to parallel to the sides of the body (ie the one that is the starting position, in case of true abduction).
As in the previous case but on the other hand, if the arm starts from a higher position than the perpendicularity plane, the ability to rotate the shoulder blades downwards is used (N: B only up to the perpendicular plane).
- Flexion of the arm . It consists of raising the humerus forward, from a starting position that is parallel to the trunk. The correct gesture requires that the palm of your hand is facing upwards.
- The extension of the arm . It consists of lifting the humerus backwards, from a starting position that is parallel to the trunk. The correct movement requires the palm of your hand to look at the floor.
- Internal rotation of the arm . It consists of rotating the arm inwards, with the elbow bent at 90Â ° and the hand parallel to the ground (NB: the palm is facing upwards).
- External rotation of the arm . It consists of rotating the arm outwards, with the elbow bent at 90Â ° and the hand parallel to the ground (NB: the palm is facing downwards). In fact, it is the movement opposite to the internal rotation of the arm.
- The circling of the arm . It consists in moving the arm, with the elbow and the hand extended, in a circular way. In other words, it's like drawing a circle with the entire upper limb.
Figure: some shoulder movements.
Figure : site of the shoulder muscles of the scapula, small rhomboid, large rhomboid and large dorsal.
The large dorsal muscle has various initial extremities: on the spine segment T7-L5, on the iliac crest, on the lower corner of the scapula, on the thoracolumbar fascia and on the last 3-4 ribs. On the contrary it has a unique terminal end, located at the level of the intertubercular sulcus of the humerus.
| The main movements of the shoulder and the muscles that participate in it. | |
| Shoulder movement | Muscles involved |
| Scapular adduction | Large rhomboid, small rhomboid and trapezoid |
| Scapular abduction | Front tooth, small pectoral and large pectoralis |
| Elevation of the scapula | Scapula lift and upper trapezium fibers |
| Depression of the scapula | Small pectoralis, trapezium (lower fibers), subclavian and large dorsal |
| Upward rotation of the shoulder blades | Trapezoid and serrated front |
| Downward rotation of the shoulder blades | Small breastplate, large pectoralis, subclavian and large dorsal |
| True abduction of the arm | Supraspinatus and deltoid |
| True adduction of the arm | Small round and lower deltoid fibers |
| Flexion of the arm | Great pectoralis, coracobrachialis, biceps brachialis and deltoids (only anterior fibers) |
| Arm extension | Large dorsal, small round, long head of the brachial and deltoid triceps (posterior fibers) |
| Internal rotation of the arm | Undular, large dorsal, small round, large pectoral and deltoid (anterior fibers) |
| External rotation of the arm | Sub-post, small round and deltoid (back fibers) |
| Surrounding the arm | Great pectoralis, subscapularis, coracobrachialis, biceps brachialis, supraspinatus, deltoid, large dorsalis, large, round, small, round, and long end of the brachial triceps |
Shoulder Diseases
Figure: anterior muscles that participate in shoulder movements. Among these is also highlighted the pectoralis major muscle, which originates from the clavicle body and from the sternum and ends at the level of the intertubercular sulcus of the humerus.
The problems that can affect the shoulder, in some cases very considerably, are numerous. Certainly, bone fractures of the shoulder skeleton, shoulder dislocations and rotator cuff lesions deserve special mention.
FRACTURES OF THE SKELETON OF THE SHOULDER
Fractures of the shoulder skeleton include: fracture of the clavicle, fracture of the scapula and fracture of the proximal portion of the humerus .
Clavicle rupture is a very common circumstance. According to some surveys, in fact, the clavicle is one of the bones of the human body that breaks more easily.
Fracture of the scapula is a very rare condition, which usually occurs as a result of severe trauma to the chest. It does not require special treatments, but only a period of absolute rest.
Finally, the fracture of the proximal portion of the humerus has a fairly high incidence and mainly concerns victims of falls or traumas directed to the arm. In some unfortunate cases, it could be accompanied by a lesion of the axillary nerve and the posterior circumflex artery of the humerus.
SHOULDER RELEASES
In medicine, the term dislocation indicates a traumatic event that causes the loss of mutual relations between the bony surfaces involved in a joint.
It can be of two types: front and rear . In anterior dislocations of the shoulder, the humerus moves forward; in the posterior dislocations of the shoulder, instead, the humerus exits backwards.
Rarely, shoulder dislocation is an isolated phenomenon. In fact, it is often associated with damage to ligaments, bones, cartilage and muscles.
Shoulder dislocations have a high incidence, especially among young and active people.
INJURIES HEADSET OF ROTATORS
The rotator cuff is a muscle-tendon complex, located on the scapula, in which 4 muscles participate with their corresponding tendons: in the upper compartment, the supraspinatus tendon takes place; anteriorly, the subscapularis muscle tendon; finally, posteriorly, the tendons of the muscles underpins and small round .
The rotator cuff lesion consists of the partial or total tearing of one or more of the tendons that join the aforementioned muscles to the bone structures.
It can be caused by traumas directed to the shoulder, but also by repeated movements, which cause the progressive deterioration of the tendon structure. The latter is, for example, the case of the great swimmers who suffer from the so-called "swimmer's shoulder", due to the continuous movement of the arms.
In general, the most affected muscle is the supraspinatus, which resides on the posterior surface of the scapula, above the so-called scapular spine.
Rotator cuff injuries are shoulder injuries that mainly affect people aged 40 and over.
