**2. Biomechanics**

Understanding rotator cuff functions and shoulder biomechanics, it is crucial to understand shoulder disorders and their pathogenesis. The rotator cuff allows the stabilization of the glenohumeral joint, compressing the humeral head on the glenoid of the scapula [5, 6]. This mechanism is due to the equal and opposite action of the subscapularis anteriorly and infraspinatus and teres minor muscles posteriorly [7–10].

Scientific studies about shoulder biomechanics have explained precisely the contribution of every ligaments, tendons and muscles to shoulder stability. The action of rotator cuff is to compress the humeral head against scapular glenoid giving stabilization to the joint and allowing concentric rotation [6, 11–13]. Rotator cuff muscles play an important role in stabilizing glenohumeral joint through this compressive mechanism, in particular during midranges motion in which ligaments are lax [14]. Concavity compression mechanism is also important at end-ranges of motion, during which rotator cuff muscles protects ligaments by limiting the range of motion [15, 16] and decreasing strain, usually increased when shoulder reaches maximum abduction and extrarotation [14, 17, 18]. When shoulder joint is in neutral position, rotator cuff muscles contribute equally in providing anterior stability [19]. However, with glenohumeral joint in end-range abduction, subscapularis is a less effective stabilizer than other muscles, while the biceps brachii starts to play a role in joint stability [20].

A lot of factors and conditions may contribute to shoulder pain. The most common cause is rotator cuff tendinopathy; its importance is linked not only to its high prevalence rate, but also

As mentioned, rotator cuff injury is one of the most common shoulder disorders. Among these, the most common are tendinosis, partial thickness tear, and complete rupture. The incidence of the cuff injuries varies from 5 to 39%; it increases in the elderly population, being

In this context, physical medicine and rehabilitation plays a fundamental role. The conservative approach consists of several interventions. The aim of these is to decrease shoulder pain and to regain shoulder function, with the goal to reduce the degree of impingement, decreasing swelling and inflammation, and minimizing the risk of further injuries. Many studies have shown that conservative therapy is the first-line treatment for shoulder disorders, in fact rehabilitative approach allows a reduction in pain feeling and symptoms within few weeks [4].

In literature, several studies have proposed conservative treatment for shoulder diseases, such as non-steroidal anti-inflammatory drugs (NSAIDs), cortisone injections, stretching and strengthening exercises, manual therapy, and physical energies (cryotherapy, extracorporeal shock wave therapy (ESWT), laser therapy, ultrasounds, etc.) to reduce pain feeling and

The purpose of this chapter is to give an overview about shoulder disorders and their conservative treatment by means of physical therapy, reviewing scientific researches, and merging it with our experience in this field. Rehabilitation of shoulder disorders is not easy, because its complex function, which involves not only local structures integrity, but also biomechanics contribution, form other body subsystems. For this reason, it is important to highlight the

Understanding rotator cuff functions and shoulder biomechanics, it is crucial to understand shoulder disorders and their pathogenesis. The rotator cuff allows the stabilization of the glenohumeral joint, compressing the humeral head on the glenoid of the scapula [5, 6]. This mechanism is due to the equal and opposite action of the subscapularis anteriorly and infra-

Scientific studies about shoulder biomechanics have explained precisely the contribution of every ligaments, tendons and muscles to shoulder stability. The action of rotator cuff is to compress the humeral head against scapular glenoid giving stabilization to the joint and allowing concentric rotation [6, 11–13]. Rotator cuff muscles play an important role in stabilizing glenohumeral joint through this compressive mechanism, in particular during midranges motion in which ligaments are lax [14]. Concavity compression mechanism is also important at end-ranges of motion, during which rotator cuff muscles protects ligaments by limiting the range of motion [15, 16] and decreasing strain, usually increased when shoulder

approximately 6 and 30%, respectively in patients aged below and above 60 years [3].

because it is a disabling condition, causing high costs for health service [2].

restore shoulder range of motion (ROM) and function.

need of establish a global rehabilitative approach.

spinatus and teres minor muscles posteriorly [7–10].

**2. Biomechanics**

208 Advances in Shoulder Surgery

The "concavity compression" is an important stability mechanism. The compression of the humeral head (convex), exercised by muscles of the rotator cuff, on the glenoid cavity (concave), maintains stable humeral epiphysis, in relation to translational forces. Resistance to joint subluxation is directly proportional to the depth of the articular concavity and to the compression force exerted by the muscles of the rotator cuff [15]. Concavity compression, providing stability of glenohumeral joint, also depends on the extension of glenoid's articular surface (glenoid arc) available to accommodate humeral head [21].

Stabilization function is made evident when the disease is established; in fact, a rotator cuff tear results in the inefficiency of this mechanism with the consequent sliding of the humeral head upward. This is due to the action of the upper fibers of the deltoid muscle, resulting in a subacromial impingement [22].

The action of the rotator cuff muscles must be highly coordinated to be able to perform a specific movement. The muscles must work in coordination, as the rotation of the glen humeral joint does not have a fixed axis [23].

Tension loads acting on rotator cuff tendons can be divided into two types: concentric and eccentric. Concentric ones are generated when humerus has the same direction of cuff muscle, as it happens during abduction against resistance. These loads are better tolerated by the cuff insertion, which clears the acromion at low angles of elevation, protecting it from impingement by the coracoacromial arch. Eccentric tension loads are produced when the arm movement is opposed to the direction of cuff muscles direction. This occurs in example during active resistance to a downward force applied on the humerus. In fact, when the humeral head rotates with respect of the scapula, bending loads stress cuff tendons; cuff elasticity permits to resist these loads. Rotator cuff tendons are also subjected to compressive loads; an upwardly load presses the cuff between the humeral head and the coracoacromial arch [24]. Furthermore, it has demonstrated a morphologic adaptation of the supraspinatus tendon with fibrocartilaginous areas in regions of compression, due to mechanical forces [25].

Every rotator cuff muscle origins on the scapula, which therefore influences the activity of these muscles. Therefore, rotator cuff performance is strictly related to the functional state of the scapula. When this bone is well stabilized, thus presenting the proper position in both static and dynamic tasks, it permits rotator cuff to work at an optimal level. However, alterations in scapular kinematics produce an unstable support to rotator cuff and consequently it affects the biomechanics of the shoulder. Scapular dysfunctions may be causative in rotator cuff disorders or may be the result of rotator cuff injuries, increasing the alteration [26].

Many authors have studied scapula kinematics in patients with rotator cuff diseases; alterations in scapular function have been found in most studies. In subjects with clinical symptoms or imaging demonstrating rotator cuff disorders, studies have demonstrated biomechanics alterations, especially scapular dyskinesia [27]. Possible alterations are not consistent, with various combinations of changes like increase in upward rotation, decrease in posterior tilt and increase in internal rotation. However, the exact relationship between scapular dyskinesia and rotator cuff disorder is not completely clear; is dyskinesis a cause, an effect or a compensation? [26]

shoulder kinematic alterations can cause a dynamic reduction of the subacromial space (compressing rotator cuff tendons) due to a superior shift of the humeral head [31] or an altered

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Recent studies suggested that subacromial bursa is a pro-inflammatory membrane responsible for shoulder pain and other subacromial disorders. Blaine [33] demonstrated that inflammatory cytokines, such as Tumor necrosis factor (TNF), Interleukin 1 (IL-1), Interleukin 6 (IL-6), cyclooxygenase-1 (COX-1) and cyclooxygenase-2 (COX-2), increase in subacromial bursal in subjects suffering from bursitis and rotator cuff syndrome. It should also be pointed

Peritendinous alterations in rotator cuff disorders are thought to be a secondary phenomenon. Chandler [35] showed that the increased tension in the coracoacromial ligament, due to tendinopathy, stimulates the neoformation of bone on the underside of the acromion, which

Acromial lateral sloping or glenoid version is structural features which play an important role in rotator cuff pathology as extrinsic factors. As mentioned in the previous paragraph, acromion shape influences supraspinatus tendon as it passes under the coracoacromial arch. Even a forward scapula posture, caused by forward head posture and increased kyphosis in

Intrinsic factors, causing rotator cuff tendinopathy, affect tendon morphology and performance. There is growing evidence in literature supporting the fundamental role of these

Intrinsic mechanisms, such as aging processes, poor vascularity and altered biology, lead to

A reduction in vascular supply of tendons is implicated in rotator cuff tears pathogenesis. In

of supraspinatus tendon at greater tubercle and myotendinous junction) which represents the

Tendon degeneration is the expression of an increased production by tenocytes of metalloproteinase enzymes (MMP); this means that tendon tears are an active and cell-mediated process. The hypothesis is that rotator cuff tears are the results of an imbalance between tendon synthesis and degradation, maybe due to the failed regulation of MMP activity in response of repeated mechanical strains. Tendon degeneration is further evident as it has demonstrated an increase in sulfated glycosaminoglycans (GAG) in supraspinatus tendinosis [41]. Sulfated GAG are associated with acute inflammation and new matrix formation, as well as amyloid production. A study conducted by Cole [42] demonstrated that supraspinatus chronic tears were characterized by 70% amyloid deposition on tendon context, unlike only 25% in patients suffering from acute traumatic injuries. Another feature that may lead to shoulder disorders is genetics; it seems to be related to the polymorphism of genes which regulates collagen synthesis, like the one found in Achilles tendinopathy. However, this is just a hypothesis since no genes were identified till now as risk

area between the insertion

tendon degradation also influencing tensile forces and altering loads [37–40].

1934, Codman described for the first time the "critical zone" (1 cm<sup>2</sup>

most common site for tendon injury due to its reduced vascularity [4].

out that IL-1 and IL-6 play an important role as mediators of collagen catabolism [34].

scapula biomechanics that leads acromion sliding downwards [32].

may result in an impingement syndrome leading to enthesopathy.

combination, can reduce the subacromial space [36].

mechanisms in shoulder disorders.

factors for rotator cuff diseases [43–45].
