The foundation of all shoulder rehabilitation interventions is a thorough understanding of the anatomy, complex three-dimensional movement patterns, and multiple functional task demands at the shoulder girdle. The skeletal, articular, and muscular structure and resultant kinesiology support the two key requirements of mobility and dynamic stability that ultimately facilitate the functional achievement of task demands.

Optimal mobility, which is necessary to allow coordinated and sequential movements of the bones and joints to respond to varied task demands, is directly related to joint congruity, the pliability of the joint capsules and ligaments, and the flexibility of the surrounding muscles. Dynamic stability is necessary to convert the relatively unstable joint articulations, especially the glenohumeral joint, into a closed chain that allows for the transfer of forces between the trunk and the arm, minimizes intra-articular forces and loads, and creates ball-and-socket kinematics at the glenohumeral joint.

Rehabilitation interventions can affect many of these mobility and stability factors to create optimal shoulder function. This chapter will review the anatomic, physiologic, and mechanical concepts and principles to provide the foundation for postoperative rehabilitation of the shoulder.

Shoulder girdle motion is based on the complex interaction of the sternoclavicular (SC), acromioclavicular (AC), and glenohumeral (GH) joints, as well as the scapulothoracic (ST) articulation. The SC joint is a saddle-shaped synovial joint formed by the medial end of the clavicle and the superolateral aspect of the manubrium; it is the only skeletal articulation that links the upper extremity with the axial skeleton. It is supported by strong ligamentous structures, including the intra-articular disk ligament, the costoclavicular (rhomboid) ligament, the interclavicular ligament, and the capsular ligaments. Sternoclavicular joint motion is described as elevation/depression, protraction/retraction, and posterior/anterior rotation (Figure 1.1). Elevation/depression occurs around a anterior-posterior directed axis and results in raising and lowering of the lateral end of the clavicle. Protraction (forward motion of the lateral end of the clavicle) and retraction (backward motion of the clavicle) occurs around an vertically directed axis. Posterior/anterior rotation occurs along the long axis of the clavicle and results in the anterolateral aspect of the clavicle rotating up and back, and down and forward, respectively.

The AC joint is a diathrodial synovial joint formed by the distal clavicle, acromion, the surrounding capsule, and an intra-articular fibrocartilaginous meniscal disc. It is a stable pivot for coordinated movements of the clavicle and scapula. The AC capsular ligaments attach 3 to 5 mm medial to the distal end of the clavicle and provide the primary restraint of anterior-posterior translation. The coracoclavicular (CC) ligaments, the conoid and trapezoid, are the primary restraint to superior translation of the distal clavicle, and also provide some rotational control. The CC ligaments extend from the undersurface of the clavicle to the base of the coracoid process.
The conoid ligament is more medial, attaching slightly posterior and, on average, 45 mm from the AC joint; it is mainly responsible for inferior/superior joint stability. The trapezoid ligament inserts roughly 15 mm laterally and more centrally on the clavicle, and provides restraint to inferior/superior as well as lateral translation.

The AC joint is inherently very stable and has medial-lateral limited motion. Three rotary motions available at this joint include upward/downward rotation, anterior/posterior tilt, and internal/external rotation (Figure 1.2). These motions are best understood if one considers the direction in which the glenoid fossa and posterior acromion move with each motion. Upward/downward rotation occurs around an axis that is roughly perpendicular to the body of the scapula. During upward rotation, the glenoid fossa rotates in a superior direction. As the scapula downwardly rotates, the glenoid fossa moves in an inferior direction. Anterior/posterior tilting occurs around a medial-lateral axis that runs through the scapular spine. During anterior tilt the posterior acromion moves in an anterior-superior direction, and duinrg posterior tilt it moves in a posterior-inferior direction. Internal/external rotation occurs around a vertical axis. As the scapula moves into internal rotation (IR), the glenoid fossa rotates in an anteromedial direction; during external rotation (ER), the glenoid fossa rotates in a posterolateral direction.

The GH joint is formed by the shallow glenoid and articular segment of the humeral head. The glenoid, covered in hyaline articular cartilage, is surrounded by the fibrocartilaginous labrum. The labrum extends and deepens the glenoid fossa both spreading joint loads and providing restraint to humeral head translation. The capsule has a synovial lining and arises from the margin of the labrum, attaching along the humeral anatomic neck. The labrum is the site of origin of the superior, middle, and inferior GH ligaments. The superior labrum is also the origin of the long head biceps tendon.

The glenohumeral ligaments are discrete thickenings within the capsule that control rotation and translation of the humeral head, especially at the extremes of glenohumeral motion. The superior and middle GH ligaments arise from the anterior glenoid and insert laterally on the humerus. The inferior glenohumeral ligament complex is comprised of anterior and posterior bands that support the inferior capsule and provides stability in varying positions of arm elevation and rotation. The rotator cuff interval is comprised of the coracohumeral ligament, the superior glenohumeral ligament, and rotator cuff interval capsule. The coracohumeral ligament arises from the lateral coracoid, is contained in the rotator cuff interval, and inserts on the lesser and greater tuberosities. The entire capsulolabral complex is essential to glenohumeral stability.

The GH joint has six degrees of freedom with three rotary and three translatory motions. Together with ST, SC, and AC joint motion, the GH joint enables a large range of shoulder, which is important for performing a wide range of functional activities. Flexion/extension occurs about a medial-lateral axis, IR/ER about a vertical axis along the shaft of the humerus, and abduction/adduction about an anterior-posterior axis. Functional elevation typically occurs in the scapular plane. Translatory motions are generally small (1–2 mm superior/inferior, 3–5 mm anterior/posterior), and are important for normal glenohumeral motion.

The relatively shallow glenoid “socket” allows substantial mobility of the GH joint, yet provides limited stability. The bony stability can be further altered by pathology, including glenoid deficiency (bony Bankart lesions) and humeral head defects (Hill-Sachs and reverse Hill-Sachs). When the arm is positioned at the side, the superior GH ligament and coracohumeral ligament provide some resistance to inferior subluxation. The coracohumeral ligament also resists humeral ER and the superior GH ligament contributes to anterior stability. As the arm is progressively elevated, the capsular contributions to anterior stability progressively shift to the middle GH ligament, and eventually the inferior GH ligament complex. At higher angles of arm elevation, the inferior GH ligament complex functions as a “sling” or hammock, contributing to inferior stability, as well as anterior stability when the arm is externally rotated, and posterior stability when the arm is internally rotated. Capsular or labral injuries inherently reduce stability, and increase reliance on the secondary constraints of the dynamic activation of the rotator cuff.

The rotator cuff plays a critical role in maximizing glenohumeral joint stability by centering the humeral head in the glenoid and providing medial compressive force throughout motion. In the presence of capsular or labral injuries, it is even more critical to ensure optimal activation, control, and endurance from the rotator cuff muscle group. While the supraspinatus—which provides a medial compressive line of action and functions as an accessory abductor—often receives the greatest attention, the remainder of the cuff musculature (subscapularis, infraspinatus, teres minor) is more important

in offsetting the superior translatory component of the deltoid during arm elevation. The subscapularis, infraspinatus, and teres minor also play key roles in preventing excessive translation by controlling anterior and posterior humeral head motion.

The ST articulation occurs at two fascial plane interfaces between the scapula and thorax. These fascial planes are located between the subscapularis and serratus anterior muscles, and the serratus anterior muscle and posterolateral aspect of the thorax. Although ST motion describes movement of the scapula on the thorax, this motion is a composite motion controlled and constrained by the SC and AC joint motions. ST motion consists of two translatory motions (elevation/depression and protraction/retraction) and three rotary motions (upward/downward, internal/external, and anterior/posterior tilt).

Elevation and depression of the scapula are directly linked with clavicular elevation and depression respectively (Figure 1.3). As the scapula elevates and depresses on the thorax, small amounts of AC joint motion occur to ensure optimal alignment of the scapula with the thorax. Scapular protraction and retraction are linked with clavicular protraction and retraction, respectively (Figure 1.3). Additionally, during these motions, small amounts of scapular IR and ER occur at the AC joint, which facilitates optimal positioning of the scapula on the thoracic wall. ST rotations include upward/downward rotation, anterior/posterior tilt, and IR/ER. It is important to recognize that during most functional activities involving the shoulder girdle, these ST rotations do not occur as isolated motions. For example, as the arm is raised over the head, the typical pattern of ST motion includes elevation, retraction, upward rotation, posterior tilt, and ER or IR depending on the primary plane of arm elevation (ER if the arm is raised closer to the frontal plane, i.e., abduction). These ST motions are important for maintaining optimal alignment between the humeral head and glenoid fossa, optimal size of the subacromial space, ideal length tension relationship of the rotator cuff muscles, and contributing to the range of arm elevation.