Functional anatomy of the shoulder

 

The shoulder is an articular assembly to  be considered as a SAMA (mechanically controlled articular system) which includes the scapulo-thoracic (acromio and sterno-clavicular joints) and its omo-thoracic sliding space and the scapulo-humeral: gleno-humeral joint and its subdeltoid sliding space. This shoulder complex is both the  most mobile with its 3 axes of work and its 3 degrees of articular freedom, but also potentially the most unstable of the organism which is used to orient the hand in the 3 planes of the body. space for the purpose of gripping and climbing. Scapulo-thoracic and scapulo-humeral function not successively, but concomitantly thanks to the combined action of the 19 muscles that compose it (out of the 54 that comprise the upper limb). 
For the Montpellier resident François Bonnel , these muscles act in the form of 25 rotational couples, functioning as a single muscle, so that the anatomical terminology distinguishing movements of flexion-extension, abduction-adduction, elevation-lowering and internal and external rotation turns out to be tendentious, rotational mobility constituting the only mechanical substrate, while the mechanical stresses to which the shoulder is subjected are such that many unstable shoulders find their explanation in muscular desynchronization.

This knowledge of the functioning of the shoulder has greatly advanced the techniques of rehabilitation and surgery and especially the concept of three-dimensional articular dynamic rotation stabilization. Total scapular mobility allowing the grasping of objects in all directions without, however, overshadowing the need for highly developed proximal stabilization structures, correlated with the visual field. This correlation makes it possible to understand that the limitation of the rotation of the head has repercussions on the possibilities of prehension. 
For Fick , the participation of the different joints is 50% for the scapulohumeral, 40% for the acromioclavicular and 10% for the sternoclavicular.

Anatomically and functionally, the shoulder is made up of:

I- 3 bones forming an unstable osteo-articular chain:

                               

The three bones: humerus, clavicle, scapula (scapula) composing the armature of the shoulder are in a position of maximum instability during the various movements, their articular surfaces of connection between them and with the axial skeleton being as we will see it eminently discordant, the spatial arrangement of the 3 bone segments being oriented in a divergent manner will favor the three-dimensional rotational instability of Bonnel. The very slender clavicle is the only connecting bone with the axial skeleton, while the scapula is attached by its  supero-external angle to the acromioclavicular and scapulohumeral joints. To fulfill the mobility contract, the joint surfaces are not very retentive and have very small joint contact surfaces: 6 cm2 for the glenoid surface, 3 cm2 for the acromioclavicular surface and 4 cm2 for the sternoclavicular.

II- 3 true joints: the glenohumeral, the acromioclavicular and the sternoclavicular

                                  

The glenohumeral joint

          

The glenohumeral joint belongs to the group of enarthrosis with a spherical humeral surface (1/3 of a sphere). Its retroversion orientation ensures a minimum of antero-posterior stability, thus limiting the risk of dislocation. We note the presence of 2 tuberosities: the major (Trochiter) and minor (Trochin) tubercles and an inter-tuberostal bicipital gutter. The cartilage that covers the glenoid cavity makes it particularly flat; glenoid enlarged however by a fibro-cartilage: the glenoid bead (labrum) whose function is to improve joint concordance but also, alas, to undergo significant constraints which will lead to its progressive destruction. The angular value of the cartilaginous surface of the humeral head from 150° to 160° and that of the glenoid from 60° reflects the importance of mobility. This anatomical arrangement predisposes to great mobility at the expense of stability.

The glenohumeral joint capsule and its anterior continuity solutions, the glenohumeral ligaments, the coracohumeral ligament which participates in the suspension of the humerus and plays a role of brake in the movements of rotations and flexion-extension of humerus (capsulo-ligamentary detachment of Broca in pathology), intervene for François Bonnel only very little in the stability of the glenohumeral, this stability being devolved almost exclusively on the dynamic level to the peri-articular musculature and to the anatomical integrity of the labrum on the static level (Bankart lesion). 

The acromioclavicular joint

                                  

One of the characteristics of the acromioclavicular is to be subjected to sudden and significant pressures and to be a flexible joint whose shape and especially the orientation of the articular surfaces are intended to prevent the clavicle from moving downwards. low, (frequency of upper dislocations). 

The presence of the acromioclavicular joint allows flexible transmission of stresses and prevents the scapula from being in a horizontal plane propelled away from the rib cage (Bonnel). The articular facets are separated at their upper half by a prismatic crescent or by a complete meniscus firmly anchored to the acromion and more loosely to the clavicle. It is the site of degenerative lesions from the second decade of life (De Palma).

Stability is provided incidentally by the capsule reinforced by the acromioclavicular ligament. The main ligaments are extrinsic: trapezoid ligament, thick and quadrilateral, located in the frontal plane and triangular conoid ligament, less powerful, more vertical than the previous one, and positioned sagittal. They limit the range of motion of the scapula and thus ensure the mechanical coupling. The medial and lateral coraco-clavicular ligaments are only thickenings of the clavi-pectoral aponeurosis with which they merge.

With variable figures depending on the authors (Fischer, Carret, Muller, Conway), 40% of the anterior elevation of the arm, 60% of the posterior elevation and 13% of the abduction of the arm, return on average to the acromioclavicular. In ante and retropulsion, this acromioclavicular mobility is accompanied by a movement of opening and closing of the scapuloclavicular angle limited by the conoid and trapezoid ligaments. Thanks to the curved "S" shape of the clavicle, the acromioclavicular joint is also (Inman) the site of axial rotational movements of the clavicle in the meniscoclavicular joint socket controlled by the acromioclavicular and coraco ligaments -clavicular.

The dynamic coaptation of the articular surfaces is the fact of the different muscles: the trapezius tends to press the acromion under the clavicle and thus reinforces the closeness of contact between the articular surfaces present, the sterno-cleido-mastoid tends to spread the articular surfaces by elevating the clavicle, the pectoralis major and the subclavius ​​tend to press the clavicle on the acromion, the latissimus dorsi and the serratus anterior act indirectly by pressing the articular surfaces against each other.

Attached to this acromioclavicular joint is the syndesmosic assembly formed by the coracoid process and the clavicle: indeed, in certain movements of the shoulder, the underside of the clavicle comes into contact with the coracoid process and the two bones slide one on top of the other until sometimes developing a coracoclavicular neo-joint with fibro-cartilaginous encrustation. 

The sternoclavicular joint

                             

The sternoclavicular joint is the unique articular junction element of the upper limb to the axial skeleton. It belongs to the group of saddle joints. In fact, the articular surface of the clavicle extending at its lower part by the first costal cartilage, it is more accurate to speak of a sterno-costo-clavicular joint whose stability is increased by the interposition of a fibro - intra-articular cartilage whose role is to absorb the stresses transmitted to the sternum by the upper limb. The joint capsule is thickened by the anterior and posterior sternoclavicular and interclavicular ligaments. The costoclavicular ligament extrinsic to the capsule remains the most effective means of restraint. 

The angular displacements are variable and according to the authors go from 32° to 40° during the abduction of the arm (de La Caffinière, Carret and Fischer). The costo-clavicular ligament by its position plays the role of axis vis-à-vis the clavicle which can be compared to the beam of a balance. The movements carried out are: on the one hand those of elevation, of lowering with an amplitude of 8 to 10 cm, and on the other hand of antepulsion and retropulsion. The association of these movements results in a combined rotation of which the trapezius, deltoid, pectoralis major, subclavius ​​and to a lesser degree sterno-cleido-mastoid muscles are the main motors. The costo-clavicular ligament is the brake during elevation, while the inter-clavicular ligament brakes the lowering. 

Dumontier likens the sternal-clavicular joint to the foot of a windsurfing mast which bars access to the noble vascular-nervous and visceral elements of the neck-trunk junction, with potential seriousness of posterior dislocations and potential danger of retro dislocations - sternals of the S/C, because of the noble retro sterno-clavicular elements:

- large vessels: arterial and venous brachiocephalic trunks = innominate vessels 

- nerves: vagus (Xth cranial pair) and phrenic for the diaphragm.

- internal jugular vein

- esophagus, and very far behind the less exposed trachea.

The various noble vascular-nervous and visceral elements of the neck-trunk junction

III- Two sliding spaces:

1- The subdeltoid

                             

                                     

 Subdeltoid sliding bursa

2- The omo-thoracic

It is divided into two spaces by the serratus anterior muscle: the inter-serrato-scapular space and the inter-serrato-thoracic space. The mechanical justification of this space is due to the fact that in the first phase of abduction (0° to 90°), the responsible muscles reach their shortening limit. To achieve an elevation of 180°, the solution adopted is that of tilting the scapula (bell movement) thanks to a frontal rotation movement of 25° to 50° according to the authors. The end of the elevation movement being completed by a spinal inflection (Kapandji). The amplitude of the displacements of the scapula is greater during abduction and flexion.

                        

IV- The 2 truncated muscular pyramids of spinoscapulo-humeral connection 

The upper limb hanging from the axial skeleton by the shoulder girdle with discordant and reduced articular contact surfaces is obliged to ensure a certain stability, to have extra-articular muscular complementary means of connection arranged in the form of a truncated pyramid at spinal medial axial base and axillary lateral apex. The walls of this pyramid are inside the serratus anterior muscle, behind the trapezius and rhomboid muscles, in front the pectoralis major muscle, above the supraspinatus and levator muscle of the scapula (angular of the scapula), below the latissimus dorsi muscle. These muscles ensure the fixation of the scapula allowing the action of the glenohumeral joint which can then move on a relatively stable base. 

At the level of the scapulo-humeral joint, we find the same stabilization system in the form of a truncated pyramid with a scapular base and a humeral apex. The anterior, external and posterior surfaces are limited by the deltoid, the internal surface is limited by the coraco-brachialis muscle.

V- Two distinct functional entities that complement each other: 

1- the scapulo-humeral with its subacromial space 

2- the scapulo-thoracic and its muscular planes.

The subacromial space

     

The articular cavity of this subacromial space is practically as wide there as that of the knee and on its floor are located the rotator cuff and the TLB, while the vault is represented by the acromio-coracoid arch: antero-inferior edge of the acromion, coracoacromial ligament and underside of the acromioclavicular joint. It is in this space, particularly stressed in elevation and rotation, that the degenerative pathology of the rotator cuff will develop.

The scapulo-thoracic and its muscular planes:

                         

VI- peri-articular and articular structures represented essentially

1- by the tendons of the rotator cuff (supra and infra-spinous, under scapular, TLBiceps.

                          

2- by the glenohumeral, acromio-coracoid, acromio-clavicular ligaments; but also by the vast and loose glenohumeral capsule, the glenoid bead or labrum, the 2 tuberosities of the humeral head (trochiter and trochin), the coracoid process...

                                  

3- by the nerve structures involved: 

 - C4
nerve rootprojecting into the anterior scapulo-thoracic (anterior branch of C4) and posterior (posterior branch of C4) 

- C5
nerve rootwith metameric points (external deltoid, delto-pectoral furrow and supra lateral epicondylar) at the origin of referred shoulder pain. 
- Charles Bell's suprascapular 

VII- Codman's paradox is the perfect illustration of three-dimensional rotation torques

It is in the realization of the complex movement of Codman that the function of three-dimensional rotational stabilization of the muscles of the shoulder dear to Bonnel is best appreciated. Its objective analysis is based on a successive movement of internal and external rotation described by Mac Conaill as "diadochodal" (Mac Conaill MA: Studies in the mechanics of the synovial joints: displacement on articular surfaces and significance of saddle joints. Irish J M. Sci., 223-235, 1946).

                                             

 It's in his book The Shoulder: Rupture of the Supraspinatus Tendon and Other Lesions In or About the Subacromial Bursa. Boston: Thomas Todd Co., 1934 that Codman speaks of his paradox, and describes it thus:

If, starting from the reference position of the upper limb, hanging vertically along the body, the palm of the hand applied against the thigh, the thumb pointing forwards. 

 We first perform, with the upper limb extended, a 180° abduction in the frontal plane, bringing it to the vertical.

Then, from this position, the limb is brought down, in the sagittal plane, that is to say forwards, (two red arrows) by performing a relative extension of 180°.

The upper limb then returns to its original position, but, importantly, the palm “looks out” and the thumb is directed backwards.

There has therefore occurred, without our being aware of it, an internal rotation of 180° of the upper limb on its longitudinal axis: this is what, according to Codman, constitutes the paradox.

If, starting thumbs forward, you try to do the course backwards: first flex 180° forwards, then descend to the side in the frontal plane, you cannot return along the body thumb in back, by insufficiency of external rotation, due to ligament blockage.

On the other hand, it is possible to go thumb back, in first flexion, then go down on the side to return to the initial position, thumb forward: this is the opposite movement to that described first.

 It is impossible to begin an abduction a second time if the thumb is pointing backwards, again, due to ligament blockage.

VIII- Functions of the shoulder muscles

                                     

                           

 Although the muscles of the shoulder girdle represent only a third of the total number of muscles, they constitute by their weight more than half of all the muscle mass of the upper limb and fulfill two functions which require the development of significant forces:

- the function of suspension-elevation of the whole body by the upper limbs

- the function of supporting and transporting heavy loads.

Among other functions, these muscles ensure the angular displacement of the articular surfaces with a large amplitude allowing the positioning of the hand in all directions at the same time as their stabilization under constraint. 
The motor muscles of the scapula are 6 in number with the rhomboid, the levator scapulae, the trapezius, the serratus anterior, the pectoralis minor and incidentally the omohyoid. 

The motor muscles of the clavicle are 2 in number with the subclavius ​​and sterno-cleido-mastoid muscles.

The motor muscles of the humerus are 11 in number with the deltoid, pectoralis major, sub scapularis, supraspinatus, infraspinatus, teres minor, teres major, latissimus dorsi, coraco-brachialis, long portion of the biceps and long portion of the triceps. 

To achieve maximum efficiency of the rotation torques, good stabilization of the humeral head is necessary and all the muscles participate in it, but a certain number of them in contact with the humeral head: the supraspinatus, infraspinatus, small round, sub scapular and long portion of the biceps constituting the rotator cuff, are essential. 

25 centering muscle couples will act successively at the level of the scapulo-humeral, acromio-clavicular, sterno-clavicular joints, and the scapulo-thoracic sliding space. that Milch defines in three cones whose vertices are centered on the humerus, a conception which only takes account of the glenohumeral joint. Bonnel taking into account all the joints distinguishes 5 muscle groups: suspensors, depressors, adductors, internal and external rotators, antepulsors and retropulsors but does not recognize on the analytical level specifically abductor muscles, abduction being the result of the actions of muscular couples antagonists. 

The circumduction movement is the result of several displacements in the three planes of space, with in the frontal plane: abduction-adduction, in the sagittal plane: flexion-extension (antepulsion-retropulsion) and in the horizontal plane : internal and external rotation.

Abduction
The conservation of abduction movement is fundamental in the function of the entire upper limb and is based on a rotation in the frontal plane first of the humerus and then of the scapula. 

This complex movement uses, on the one hand, the glenohumeral joint from 0° to 90° and, on the other hand, from 90° to 180°, the scapulothoracic sliding space and the acromioclavicular joints. and sternoclavicular.

The abduction of the humerus from 0° to 90° is a complex movement which involves all the structures of stability and mobility. During passive abduction, the humerus, via the greater tubercle, abuts against the acromion and the inferior glenohumeral ligament is stretched, preventing any progression. External rotation of the humerus erases the rigid abutment and allows elevation.

Active abduction, under the sole contraction of the deltoid muscle, is not sufficient for Bonnel to understand the phenomenon and contrary to the classical conception, it would not be an abductor. In fact, during paralysis of the circumflex nerve, we can observe conservation of abduction and during perforation or rupture of the muscles of the rotator cuff, despite an intact deltoid muscle, it is not possible to achieve an abduction movement.

 The mechanical principles of abduction obey the law of three-dimensional rotational dynamic centering and can be schematized as follows: a beam being placed parallel to a wall, with a traction rope at one end, it is not possible to put this beam at 90° from the vertical, because we obtain an ascent. To achieve this, it is necessary to add a stop to the upper part which will stabilize the beam. Stability should not impede mobility, the stop will be elastic. 
This approach to the abduction movement involves only 2 muscles out of the 19 that surround the joint: the isolated action of the deltoid leads to an ascent of the humeral head, the suspensory muscle, but does not cause an abduction movement. To oppose this ascent, two elements intervene, one active represented by the  supraspinatus muscle and the other passive by the osteo-ligamentous acromio-coracoid vault considered as a so-called subacromial joint. It is a sliding space with a conjunctiva bursa which in certain circumstances becomes fibrotic and will limit the amplitude of the abduction movement. This elastic stop adapts to all planes of space without entailing limitations. The abduction movement is only possible by the additional muscular stabilization of the subscapularis and infraspinatus short depressor muscles, and of the long depressors with the pectoralis major and the latissimus dorsi.

Beyond 90°, the trapezius and serratus anterior cause the scapula to tilt using two types of muscle couples: on the one hand a couple of agonist muscle action and on the other hand a couple of action of antagonistic muscles. The scapula as an intermediate bone is subject to muscular actions which act as fixators or rotators in the three spatial planes. The scapula is permanently held together by shorter muscles, the levator scapulae, the rhomboid and the pectoralis minor. In the elevation to the zenith, the spine undergoes a lateral inclination.

Internal rotation, external rotation

In the group of agonist muscles resulting in a movement of internal rotation, we distinguish the pairs: rhomboid - levator of the scapula, small pectoral - subclavius, rhomboid - superior beam of the trapezius.

Among the group of antagonistic muscles resulting in a movement of external rotation, the couple of the upper and lower bundles of the trapezius, the couple of the upper and lower bundles of the serratus anterior are distinguished.

Adduction

In current gestural dynamics, adduction only activates a few muscles insofar as gravity tends to keep the arm in contact with the thorax. True adduction is only achieved in climbing with the entry into action of long and powerful muscles: pectoralis major and latissimus dorsi.

The fixation of the scapula is the first phase of adduction during climbing, with the simultaneous contraction of the trapezius, the rhomboid, the levator scapulae, the pectoralis minor and the subclavius. The scapula thus fixed, the arm will be able to be brought back effectively against the thorax under the actions of the teres major, the pectoralis major, the infraspinatus and the subscapularis. To avoid lower dislocation of the humeral head, the upper muscles of the humerus (deltoid, short portion of the biceps, coraco-brachialis, long portion of the triceps) come into action in the form of recentering rotation torques. The action of climbing requires both force and large amplitude displacement that the latissimus dorsi is the only one to achieve with the help of the long head of the triceps which prevents dislocation of the humeral head.

The final positioning of the hand in addition to the movements of pronation and supination of the forearm require for better performance an integrity of the horizontal rotation movement of the scapulo-thoracic joint which greatly compensates for a stiffness of the scapulo-joint -humeral. The movement of external rotation is obtained by the isolated action of the infraspinatus and the teres minor. Due to joint discordance, there is a risk of posterior dislocation or at least posterior instability. To avoid this eventuality, the anterior muscles, sub scapularis and pectoralis major, favor the recentering of the humeral head. The range of motion is complemented by the trapezius and rhomboid. The angular distribution is 30° at the scapulo-humeral,

 The grip-oriented internal rotation function requires a larger number of muscles with the subscapularis, latissimus dorsi, pectoralis major and teres major. To these muscle groups, we must attach the action of the tendon of the long head of the biceps which limits the amplitude of the movement of external rotation of the humerus and behaves like an internal rotator.

Antepulsion Flexion - Retropulsion extension - elevation - lowering

In the sagittal plane, displacements take place at two levels, displacements take place at two levels, that of flexion-extension for the scapulo-humeral joint and that of antepulsion-retropulsion for the acromio-clavicular joint assembly, sternoclavicular and scapulothoracic sliding space.

Raising or lowering movements of the shoulder represent the second type of movement of the shoulder complex subjected to significant stress during climbing.

For the flexion movement, the amplitude is 30° with the active muscle being the anterior bundle of the deltoid, the coraco-brachialis, the long head and the short head of the biceps and the vertical fibers of the subscapularis muscle.

For the extension movement, the active muscles are represented by the posterior bundle of the deltoid, the teres major, the latissimus dorsi and the long head of the triceps.

The stability of the humeral head in relation to the glenoid cavity of the scapula is ensured thanks to the action of muscular rotation torques of flexion and extension.

Much more useful than retropulsion, the antepulsion involves the scapulo-humeral, the scapulo-thoracic sliding space and the sterno-costoclavicular.

However, as with other movements, muscle torques are essential to stabilize bone structures. The complementarity of movement via the thoracic sliding space is supported by the trapezius, serratus anterior, rhomboid and latissimus dorsi couples.

The muscles inserting on the scapula and the clavicle ensure by their direction of the movements of elevation, of lowering, of antepulsion and retropulsion. These same muscles when they contract in couple involve a stabilization of the scapula (fixator) or a rocking fundamental phenomenon during the abduction.

During elevation, the displacement of the scapula (scapula)-clavicle assembly is 8 to 12 cm. The muscles involved are the superior clavicular head of the trapezius, the rhomboid and the levator scapulae. The lowering is brought about by the lower head of the trapezius, the pectoralis minor and the subclavius.

 For the antepulsion, the displacement of the scapula is 8 to 15 cm. Active muscles are serratus anterior, pectoralis minor, subclavius, and superior trapezius. For retropulsion, the active muscles are the trapezius as a whole and the rhomboid.

long biceps

The true function of the long head of the biceps brachii is to be considered in the assembly formed by the deltoid muscle on the outside and by the coraco-brachialis muscles and short portion of the biceps brachii on the inside. This tendon is located in the middle of the angle formed by the two muscles, hence its name of vector bisector. When the deltoid and coraco-brachialis muscles, a short portion of the biceps brachii, contract, the humeral head rises. To protect the rotator cuff from this mechanical overload, the long head of the biceps brachii via its intra-articular path will counterbalance this action by a dynamic lowering effect.

long triceps 

The action of the triceps brachii is to be considered at 3 levels: the arm along the thorax (zero position), the contraction of the triceps brachii leads to an ascent of the humeral head. At 90° abduction the triceps brachii has a coaptation action. Beyond 90°, its coaptation function is constant and is reinforced by the subglenoid position of the tendon which prevents inferior dislocation of the humeral head. 

In the 0° position, the latissimus dorsi and teres major muscles counterbalance the dislocating action of the long head muscle of the triceps. 
At 90° the three muscles have a coaptation function. From this angulation, the latissimus dorsi and teres major muscles have a luxating action which is counterbalanced by the triceps brachii muscle which constitutes the second vector bisector of recentering.

Anatomically, the two vector bisectors of recentering with the tendon of the long head of the biceps brachii at the upper part of the head (supraglenoid tubercle) and that of the long head of the triceps brachii at the lower part (infraglenoid tubercle) have for Bonnel an essential role in the inferior or superior instabilities of the humeral head.

Scapulohumeral rhythm

                               

The humeroscapular and scapulothoracic articular elements are tightly coupled by a muscular continuity which transmits the mechanical stresses. There is a fundamental distinction between morphological anatomy and functional anatomy. 

Thus the entity of the supraspinatus muscle belongs to the morphological anatomy and the muscle-deltoid, supraspinatus and levator of the scapula assembly, to the functional anatomy. 

The coraco-brachialis and short portion of the biceps muscles are continuous with the pectoralis minor muscle. According to the same principle, the teres major muscle is continuous with the rhomboid muscle. 

These muscles constitute the stato-dynamic diamond of the scapula. The deltoid muscle continues through its anterior bundle with the pectoralis major muscle and through its posterior bundle with the trapezius muscle. The purpose of these muscular arrangements is to ensure the mechanical coupling between the effector organ of the upper limb and the axial skeleton via the bones.

The coracoid process 

                                 

It is an essential element of both static and dynamic connection of the whole shoulder with these 4 tendinous insertions (conoid, trapezoid, acromio-coracoid, coraco-humeral ligaments) and these 3 muscular insertions (small pectoral, coraco- brachialis,  short head of the biceps). It is truly the mixed central core of static and dynamic stabilization of the articular complex of the shoulder.

The clavicle is limited in its range by the trapezoid and conoid ligaments which insert at the superior edge of the coracoid process. Inside we find the coraco-clavicular ligament which passively completes the stability of the clavicle.

The acromiocoracoid ligament has a dual function: on the one hand it participates in the stabilization of the humeral head during abduction as an elastic stop, on the other hand the acromion and the spine of the scapula are subjected to very high tensile stresses by the trapezius muscle. To counterbalance this action of tension, the acromiocoracoid ligament intervenes as an element of neutralization. 

The humerus is the third element connected to the coracoid process with the coraco-humeral ligament which suspends the humerus and limits the movement of external rotation. 

The pectoralis minor muscle, which attaches to the coracoid process, is the only anterior muscle of the scapula preventing the opening of the scapulo-thoracic angle. It is aided posteriorly by the serratus anterior muscle. The coracoid process actively suspends the humerus with the coracobrachialis and the short head of the biceps.

Centers of rotation and shoulder

         

It is important to know the instantaneous center of rotation to fully understand the modes of muscle action. The analytical studies made it possible to define the instantaneous centers of rotation. For the glenohumeral joint, the instantaneous center was defined at the level of the anatomical neck (Fick), inside and below the greater tubercle (De Luca).

It seems that the kinematic analyzes of Carret, Fisher and Dimnet are closer to reality. The results showed that during abduction from 0° to 50° the instantaneous center is located in the lower half of the humeral head, and from 50° to 90° in the upper half of the humeral head. 

For the sternoclavicular joint, the instantaneous center of rotation during abduction is located at the level of the inner quarter of the clavicle. For the scapulo-thoracic unit, the instantaneous center of rotation is located for abduction in the frontal plane at the level of the spinal border and in the horizontal plane, in the middle of the spine of the scapula (Carret). 

For the acromioclavicular joint, the instantaneous center of rotation is above the outer quarter of the clavicle. Taken as a whole, the center of rotation was defined by Provins as variable from the coracoid process to the spinal border of the scapula then the axillary border. The concentration of inserting elements on the coracoid process are tangible evidence of its role as the center of the entire shoulder complex. It is an essential connecting element, both static and dynamic, of the shoulder.

IX- Clinical incidence according to Bonnel 

The mechanical organization of the shoulder complex induces, in the event of dysfunction, a certain number of well systematized degenerative and unstable lesions.

1- The overload degenerative shoulder 

Mechanical overloads are expressed in the form of tendinopathies which anatomically can be classified into 4 stages: 

- the first stage is that of edematous tendinopathy which can regress if the mechanical stress ceases. 

- the second stage is that of fissure tendinopathy. 

- the third stage is that of tendinopathy with area of ​​necrosis and mucoid appearance. 

- the fourth stage is, depending on the intensity, that of micro-breaks or clean breaks. 

Whatever the evolutionary stage, on histological analysis of the tendon we find lesions belonging to several stages with lesions of hyper-vascularization or fibrosis which reflect nature's efforts to achieve repair. In some cases, calcifications are found in the tendon. Under mechanical stress, the tendon is subject to micro-ruptures with attempts at healing and depending on the local physico-chemical conditions of calcium precipitation. These calcifications are to be distinguished from calcifications of the subacromial bursa. 

New conception of the subacromial impingement 

                                            

The functioning of the shoulder complex is essentially based on muscular synergies. In light of such mechanical organization and arthroscopic findings of the glenohumeral joint, Bonnel considers a different design for rotator cuff injuries:

1- The long portion of the biceps

In a first phase, the imbalance between the ascending deltoid, coraco-brachialis, short portion of the biceps brachii muscles and the depressor muscle represented by the vector bisector of recentering with the long head of the biceps brachii will lead to a mechanical overload of the tendon of the long biceps; overload responsible for tendinopathy of varying intensity with cracks, micro-ruptures and ultimately, rupture. This first phase with lesion of the long head of the biceps brachii is random and depends on the type of muscular asynchrony.

2- The rotator cuff

In the second phase, the imbalance between the deltoid muscle and the muscles of the rotator cuff will cause an overload by distension between the three subscapular, infraspinatus and supraspinatus muscles which leads to fissural lesions and endo-micro-ruptures. joints visible by arthroscopy. Gradually, the cap becomes thinner, sometimes ending in rupture. The location and the extent of the rupture are functions of the axes of imbalance.

3- The conflict

The third phase, which involves the acromion-greater tubercle conflict, would only be the consequence of the muscular asynchrony which results in an ascent of the humeral head in contact with the lower surface of the greater tubercle and the acromio-coracoid ligament. This phase of conflict is perfectly reversible, if thanks to rehabilitation, we manage to rebalance the actions between the ascending and lowering muscles. 

Neer's acromioplasty removes the impingement but allows the muscle imbalance to persist. In the rupture phase, rehabilitation is less effective, because the rotator cuff lowering strap can no longer play its role effectively.

2- The unstable shoulder

Asynergies during muscle contraction will lead to joint instability which can be uni-directional or multi-directional.

X- Functional assessment of the shoulder

It is proposed, beyond the anatomical classifications, to make an assessment which will endeavor to collect functional elements:

- analytical passive and active mobility: EA, EL, RP, RI and RE, complex hand-neck and hand-back movements.

- the Constant functional score out of 100 points and the isokinetic evaluation.

XI- Some definitions

1- The plane of the scapula: this is the plane of physiological mobility of the scapula between 20 and 40° of antepulsion. Sliding planes of the scapula: the serratus anterior is separated from the thoracic cage and the subscapularis by cellulo-fatty planes. These are essential for the smooth sliding of the scapula on the thorax and are considered part of the articular complex of the shoulder.

2- The Zero-position of Saha corresponds in rehabilitation, to the angle of elevation of 150° to be reached passively in the plane of the scapula at 20-40° of antepulsion, before attacking the active mobilizations. It is a position that allows good muscular awakening of the deltoid, while protecting the rotator cuff and with good centering of the humeral head. It is a position of equilibrium where the agonist and antagonist forces cancel each other out.

3- Sohier's pathway in the antero-superior conflict of the cuff; it is an S trajectory which avoids the trochiter-subacromial vault impingement, without causing any mechanical impingement.

NB: Sohier described 3 ways of passage:

- the anterior way by abduction-internal rotation and antepulsion

- the posterolateral route by external rotation-elevation

- the posterior approach by internal rotation-retropulsion and adduction.

In conclusion
The shoulder is indeed the most complex and exciting of the joints; it manages to reconcile 2 contradictory requirements of mobility and stability, at least as long as the periarticular musculature remains perfectly synchronous because it is not polluted by uni- or multi-dimensional laxity, a lesion of the labrum or tendinopathy of the rotator cuff with or without conflict under acromial. With its operation in three-dimensional rotational muscular couples François Bonnel, after IA Kapandji, renews the ideas on biomechanics but also after Neer, Jobe and some other great orthopedists, the pathophysiology of the conflicting degenerative shoulder and reinforces rehabilitation medicine as a key asset in the therapeutic management of painful and/or unstable shoulders,

In order to take full advantage of the article above, we recommend a careful prior reading of the works of Pr François Bonnel and IA Kapandji.