Components of the Shoulder Complex The osseous segments of the shoulder complex are the clavicle, scapula, and humerus (Fig. 7-1). These three segments are joined by three interdependent linkages: the SC joint, the acromioclavicular (AC) joint, and the GH joint. The articulation between the scapula and the thorax is often described as the scapulothoracic (ST) “joint,” although it does not have the characteristics of a fibrous, cartilaginous, or synovial union. Instead, scapular motion on the thorax is directly a function of SC, AC, or combined SC and AC joint motion. The ST joint is frequently described in the literature as a “functional” joint. An additional functional articulation that is, at times, considered to be part of the shoulder complex is the subacromial (or suprahumeral) “joint.” This functional joint is formed by movement of the head of the humerus below the coracoacromial arch. Although the movement between these two components plays an important role in shoulder function and dysfunction, we will refer to it as the suprahumeral space and consider it a component of the GH joint rather than a separate linkage. The joints that compose the shoulder complex in combination with trunk motion can contribute as much as 180 of elevation to the upper extremity. Elevation of the upper extremity refers to the combination of scapular, clavicular, and humeral motion that occurs when the arm is raised either forward or to the side (including sagittal plane flexion, frontal plane abduction, and all the motions in between). Motion of the scapula on the thorax normally contributes about one third of the total motion necessary for elevation of the arm through the linked SC and AC joint motions, whereas the GH joint contributes about two thirds of the total motion. Although integrated function of all three joints is of primary interest, each of the articulations and components of the shoulder complex must be examined individually before integrated dynamic function can be appreciated. Sternoclavicular Joint The SC joint serves as the only structural attachment of the clavicle, scapula, and upper extremity to the axial skeleton. Movement of the clavicle at the SC joint inevitably produces movement of the scapula under conditions of normal function, because the scapula is attached to the lateral end of the clavicle. In order for the scapula to not move with the clavicle during SC motion, equal and opposite motions would have to occur at the AC joint; this is not typical with an intact claviculoscapular linkage. Similarly, any motions of the scapula must result in motion at the SC joint (unless scapular motions are isolated to the AC joint—which is, again, unlikely under normal circumstances). The SC joint is a plane synovial joint with three rotatory and three translatory degrees of freedom. This joint has a synovial capsule, a joint disk, and three major ligaments. ■ Sternoclavicular Articulating Surfaces The SC articulation consists of two saddle-shaped surfaces, one at the sternal or medial end of the clavicle and one at the notch formed by the manubrium of the sternum and first costal cartilage (Fig. 7-2). Because tremendous individual differences exist across people and the saddle shape of these surfaces is very subtle, the SC joint is often classified as a plane synovial joint. The sternal end of the clavicle and the manubrium are incongruent; that is, there is little contact between their articular surfaces. The superior portion of the medial clavicle does not contact the manubrium at all; instead it serves as the attachment for the SC joint disk and the interclavicular ligament. At rest, the SC joint space is wedge-shaped and open superiorly.2 Movements of the clavicle in relation to the manubrium result in changes to the areas of contact between the clavicle, the SC joint disk, and the manubriocostal cartilage. ■ Sternoclavicular Disk As is generally true at an incongruent joint, the SC joint has a fibrocartilage joint disk, or meniscus, that increases congruence between joint surfaces. The upper portion of the SC disk is attached to the posterosuperior clavicle. The lower portion is attached to the manubrium and first costal cartilage, as well as to the anterior and posterior aspects to the fibrous capsule.3 The disk diagonally transects the SC joint space (Fig. 7-3) and divides the joint into two separate cavities.1 Given its attachments, the disk acts like a hinge or pivot point during clavicle motion. In elevation and depression of the clavicle, the medial end of the clavicle rolls and slides on the relatively stationary disk, with the upper attachment of the disk serving as a pivot point. In protraction/retraction of the clavicle, the SC disk and medial clavicle roll and slide together on the manubrial facet, with the lower attachment of the disk serving as a pivot point.1 The disk, therefore, is considered part of the manubrium in elevation/depression and part of the clavicle in protraction/retraction. As the disk switches its participation from one articular segment to the other during clavicular motions, mobility between the segments is maintained and stability is enhanced. The resultant movement of the clavicle in both elevation/depression and protraction/retraction is a fairly complex set of motions, with the mechanical axis for these two movements located not at the SC joint itself but at the more laterally located costoclavicular ligament (see Fig. 7-3). The SC disk serves an important stability function by increasing joint congruence and absorbing forces that may be transmitted along the clavicle from its lateral end. In Figure 7-3, it can be seen that the unique diagonal attachment of the SC disk will check medial movement of the clavicle that might otherwise cause the large medial articular surface of the clavicle to override the shallow manubrial facet. The disk also has substantial contact with the medial clavicle, permitting the disk to dissipate the medially directed forces that would otherwise cause high pressure at the small manubrial facet. Although one might think that medially directed forces on the clavicle are rare, we shall see that this is not the case when we examine the function of the AC joint, the upper trapezius muscle, and the coracoclavicular ligament. Continuing Exploration: Three-Compartment SC Joint Anatomic examination of the SC articulation has led to the proposal that there are three, rather than two, functional units of the SC joint: a lateral compartment between the disk and clavicle for elevation and depression; a medial compartment between the disk and manubrium for protraction and retraction; and a costoclavicular joint for anterior and posterior long axis rotation. Anterior and posterior rotation are thought to occur between a portion of the disk over the first rib and a “conus” on the anteroinferior edge of the articular surface of the medial clavicle.4 ■ Sternoclavicular Joint Capsule and Ligaments The SC joint is surrounded by a fairly strong fibrous capsule but must depend on three ligaments for the majority of its support. These are the sternoclavicular ligaments, the costoclavicular ligament, and the interclavicular ligament (Fig. 7-4). The anterior and posterior SC ligaments reinforce the capsule and function primarily to check anterior and posterior translatory movement of the medial end of the clavicle. The costoclavicular ligament is a very strong ligament found between the clavicle and the first rib. The costoclavicular ligament has two segments or laminae. The anterior lamina has fibers directed laterally from the first rib to the clavicle, whereas the fibers of the posterior lamina are directed medially from the rib to the clavicle.3,5 Both segments check elevation of the lateral end of the clavicle and, when the limits of the ligament are reached, may contribute to the inferior gliding of the medial clavicle that occurs with clavicular elevation.6 The costoclavicular ligament is also positioned to counter the superiorly directed forces applied to the clavicle by the sternocleidomastoid and sternohyoid muscles. The medially directed fibers of the posterior lamina will resist medial movement of the clavicle,7 absorbing some of the force that would otherwise be imposed on the SC disk. The interclavicular ligament resists excessive depression of the distal clavicle and superior glide of the medial end of the clavicle. The limitation to clavicular depression is critical to protecting structures such as the brachial plexus and subclavian artery that pass under the clavicle and over the first rib. In fact, when the clavicle is depressed and the interclavicular ligament and superior capsule are taut, the tension in the interclavicular ligament can support the weight of the upper extremity.8 ■ Sternoclavicular Motions The three rotatory degrees of freedom at the SC joint are most commonly described as elevation/depression, protraction/retraction, and anterior/posterior rotation of the clavicle. Motions of any joint are typically described by identifying the direction of movement of the portion of the lever that is farthest from the joint. The horizontal alignment of the clavicle (rather than the vertical alignment of most of the appendicular levers of the skeleton) can sometimes create confusion and impair visualization of the clavicular motions. The motions of elevation/depression (Fig. 7-5) and protraction/retraction (Fig. 7-6) should be visualized by referencing movement of the lateral end of the clavicle. Clavicular anterior/posterior rotation are long axis rolling motions of the entire clavicle (Fig. 7-7). Three degrees of translatory motion at the SC joint can also occur, although they are very small in magnitude. Translations of the medial clavicle on the manubrium are usually defined as occurring in anterior/posterior, medial/lateral, and superior/inferior directions (see Figs. 7-5 and 7-6). Elevation and Depression of the Clavicle The motions of elevation and depression occur around an approximately anteroposterior (A-P) axis (see Fig. 7-5) between a convex clavicular surface and a concave surface formed by the manubrium and the first costal cartilage. With elevation, the lateral clavicle rotates upward, and with depression, the lateral clavicle rotates downward. The cephalocaudal shape of the articular surfaces and the location of the axis indicate that the convex surface of the clavicle must slide inferiorly on the concave manubrium and first costal cartilage, in a direction opposite to movement of the lateral end of the clavicle. The SC joint axis is described as lying lateral to the joint at the costoclavicular ligament. The location of this functional (rather than anatomic) axis relatively far from the joint reflects a large intraarticular motion of the medial clavicle. The range of available clavicular elevation has been described as upto 48 , whereas passive depression is limited, on average, to less than 15 . 9 The full magnitude of the available range of elevation is generally not utilized during functional ranges of arm elevation.10,11 Protraction and Retraction of the Clavicle Protraction and retraction of the clavicle occur at the SC joint around an approximately vertical (superoinferior) axis that also appears to lie at the costoclavicular ligament (see Fig. 7-6). With protraction, the lateral clavicle rotates anteriorly, and with retraction, the lateral clavicle rotates posteriorly. The configuration of joint surfaces in this plane is the opposite of that for elevation/depression; the medial end of the clavicle is concave, and the manubrial side of the joint is convex. During protraction, the medial clavicle is expected to slide anteriorly on the manubrium and first costal cartilage. There is about 15 to 20 protraction and 20 to 30 retraction of the clavicle available.9,11,12 Anterior and Posterior Rotation of the Clavicle Anterior/posterior, or long axis, rotation of the clavicle (see Fig. 7-7) occurs as a spin between the saddleshaped surfaces of the medial clavicle and manubriocostal facet. Unlike many joints that can rotate in either direction from resting position of the joint, the clavicle rotates primarily in only one direction from its resting position. The clavicle rotates posteriorly from neutral, bringing the inferior surface of the clavicle to face anteriorly. This has also been referred to as backward or upward rotation rather than posterior rotation.1 From its fully rotated position, the clavicle can rotate anteriorly again to return to neutral. Available anterior rotation past neutral is very limited, generally described as less than 10 . 1 The range of available clavicular posterior rotation is cited to be as much as 50 . 10 The axis of rotation runs longitudinally