Shoulder Complex

  SHOULDER COMPLEX

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SHOULDER COMPLEX

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The shoulder complex is composed of the scapula, clavicle, humerus, and the joints that link these bones into a functional entity. These components constitute one – half of the weight of the entire upper limb. The scapula, clavicle, and humerus that forms the shoulder complex are responsible for movement of the hand through space. Four interdependent linkages that control the three segments are as follows: y Scapulothoracic (ST) Joint. y Sternoclavicular (SC) Joint. y Acromioclavicular (AC) Joint. y Glenohumeral (GH) Joint.

U Scapulothoracic Joint:

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The scapulothoracic (ST) joint is formed by the articulation of the scapula with the thorax on which it sits. It is not a true anatomic joint because it has none of the usual joint characteristics (union by fibrous, cartilaginous, or synovial tissues). In fact, the articulation of the scapula with the thorax depends on the anatomic acromioclavicular (AC) and sternoclavicular (SC) joints. The SC and AC joints are interdependent with the ST joint because the scapula is attached by its acromion process to the lateral end of the clavicle via the AC joint; the clavicle, in turn, is attached to the axial skeleton at manubrium of the sternum via the SC joint. Any movement of the scapula on the thorax must result in movement at either the AC joint, SC joint, or both. That is, the functional ST joint is part of a true closed chain with the AC and SC joints.

Ö Scapulothoracic Position:

Normally, the scapula rests at a position on the posterior thorax approximately 2 inches from the midline, between the second through seventh ribs. The scapula also lies 30° to 40° forward of the frontal plane and is tipped anteriorly approximately 10° to 20° from vertical with a good deal of individual variability.

Ö Scapulothoracic Motions:

The motions of the scapula at scapulothoracic joint are as follows: y Elevation / Depression. y Protraction / Retraction (Abduction / Adduction). y Upward Rotation / Downward Rotation (Medial Rotation / Lateral Rotation). Elevation and depression of the scapula are described as translatory motions in which the scapula moves upward (cephalad) or downward (caudally) along the rib cage from its resting position. Protraction and 2

retraction of the scapula are described as translatory motions of the scapula away from or toward the vertebral column, respectively. Upward and downward rotations are rotatory motions that tilt the glenoid fossa upward or downward, respectively. Upward and downward rotation can also be described by referencing movement of the inferior angle away from the vertebral column (upward rotation) or movement of the inferior angle toward the vertebral column (downward rotation).

Ö Scapulothoracic Stability:

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Stability of the scapula on the thorax is provided by the structures that maintain integrity of the linked AC and SC joints. The muscles that attach to both the thorax and scapula maintain contact between these surfaces while producing the movements of the scapula. The ultimate function of scapular motion is to orient the glenoid fossa for optimal contact with the maneuvering arm, to add range to elevation of the arm, and to provide a stable base for the controlled rolling and sliding of the articular surface of the humeral head.

USternoclavicular Joint:

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™ Articulation: This occurs between the sternal end of the clavicle, the manubrium sterni, and the first costal cartilage. ™ Type: Synovial double – plane joint. ™ Capsule: This surrounds the joint and is attached to the margins of the articular surfaces. ™ Articular Disc: This flat fibrocartilaginous disc lies within the joint and divides the joint’s interior into two compartments. Its circumference is attached to the interior of the capsule, but it is strongly attached to the superior margin of the articular surface of the clavicle above and to the first costal cartilage below. ™ Ligaments: Following are the ligaments of the sternoclavicular joint: 1) Sternoclavicular (SC) Ligaments. 2) Costoclavicular Ligament. 3) Interclavicular Ligament. The anterior & posterior SC ligaments reinforce the capsule. The SC ligaments serve primarily to check anterior and posterior movement of the head of the clavicle. The costoclavicular ligament is a strong ligament that runs from the junction of the first rib with the first costal cartilage to the inferior surface of the sternal end of the clavicle. Costoclavicular ligament checks elevation of the clavicle. Interclavicular ligament is present between the two clavicles and it presents excessive depression or downward glide of the clavicle. 3

™ Synovial Membrane: This lines the capsule and is attached to the margins of the cartilage covering the articular surfaces. ™ Nerve Supply: Supraclavicular nerve & nerve to subclavius.

Ö Sternoclavicular Motions:

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The motions that occur at the SC joint are as follows: y Elevation / Depression. y Protraction / Retraction. y Anterior Rotation / Posterior Rotation. The motions of elevation and depression occur around an anterior – posterior axis between a convex clavicular surface and a concave surface formed by the manubrium and first costal cartilage. When movement of the clavicle is plotted, the SC joint axis appears to lie lateral to the joint at the costoclavicular ligament. The range of clavicular elevation averages about 45°, while there is about 15° of depression. Protraction and retraction of the clavicle occur at the SC joint around a vertical axis that also appears to lie at the costoclavicular ligament. The configuration of joint surfaces in this plane is that the medial end of the clavicle is concave and the manubrial side of the joint is convex. There is about 15° protraction and 15° retraction of the clavicle. Rotation of the clavicle occurs as a spin between the saddle-shaped surfaces of the clavicle and manubriocostal facet. Unlike many joints that can rotate in either direction from resting position of the joint, the clavicle rotates in only one direction from its resting position. The clavicle rotates posteriorly from neutral, bringing the inferior surface of the clavicle to face anteriorly. From its fully rotated position, the clavicle can rotate anteriorly again to return to neutral. The axis for rotation runs longitudinally through the clavicle, intersecting the SC joint. The range of clavicular rotation is cited to be anywhere from 30° to as much as 55°.

U Acromioclavicular Joint:

™ Articulation: This occurs between the acromion of the scapula and the lateral end of the clavicle. ™ Type: Synovial plane joint. ™ Capsule: This surrounds the joint and is attached to the margins of the articular surfaces. ™ Articular Disc: From the capsule, a wedge-shaped fibrocartilaginous disc projects into the joint cavity from above. ™ Ligaments: The capsule of the AC joint is weak and cannot maintain integrity of the joint without reinforcement by following ligaments: 1) Acromioclavicular Ligaments. 4

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2) Coracoclavicular Ligaments. Superior and inferior acromioclavicular ligaments reinforce the joint capsule. The superior acromioclavicular ligament assists the capsule in apposing articular surfaces and in controlling horizontal joint stability. Coracoclavicular ligament is divided into a lateral portion, the trapezoid ligament, and a medial portion, the conoid ligament. Both portions of the coracoclavicular ligament prevent upward rotation of the scapula at the AC joint. ™ Synovial Membrane: This lines the capsule and is attached to the margins of the cartilage covering the articular surfaces. ™ Nerve Supply: Suprascapular Nerve.

Ö Acromioclavicular Motions:

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The primary motions that take place at the AC joint are as follows: y Medial Rotation / Lateral Rotation. y Anterior Tipping / Posterior Tipping. Medial and lateral rotation of the scapula occurs around a vertical axis through the AC joint. Medial and lateral rotation brings the glenoid fossa medially (or anteriorly) and laterally (or posteriorly), respectively. These motions must occur to maintain the contact of the scapula with the horizontal curvature of the thorax as the scapula slides around the thorax in protraction and retraction. If protraction of the ST joint occurred as a pure translatory movement, the scapula would move directly away from the vertebral column and the glenoid fossa would face laterally. Only the vertebral border of the scapula would remain in contact with the rib cage. In reality, full scapular protraction results in the glenoid fossa facing anteriorly with the full scapula in contact with the rib cage. The scapula follows the contour of the ribs by rotating about a vertical axis at the AC joint, with the vertebral border of the scapula moving posteriorly and the glenoid fossa moving anteriorly. The anterior orientation of the glenoid fossa is also important in flexion of the arm to keep the fossa behind the humeral head and prevent posterior dislocation. The anterior and posterior tipping of the scapula occurs around a coronal axis through the joint. Anterior tipping moves the superior border of the scapula anteriorly and the inferior angle posteriorly. Posterior tipping is of course, the opposite motion. Scapular tipping, like medial and lateral rotation of the scapula, occurs to maintain the contact of the scapula with the contour of the rib cage. As the scapula moves upward or downward on the rib cage in elevation or depression, the scapula must adjust its position to maintain full contact with the vertical curvature of the ribs. Elevation of the scapula requires anterior tipping. More significant anterior tipping of the scapula occurs during posterior rotation of the clavicle. If the clavicle and scapula were one piece (no AC joint), attempted posterior rotation of the clavicle at the SC joint would 5

force the inferior angle of the scapula into the rib cage and the motion would be stopped. Instead, the AC joint absorbs the clavicular rotation, effectively allowing the scapula to remain in place by counter-rotating.

U Glenohumeral Joint:

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™ Articulation: This occurs between the rounded head of the humerus and the shallow, pear-shaped glenoid cavity of the scapula. The articular surfaces are covered by hyaline articular cartilage, and the glenoid cavity is deepened by the presence of a fibrocartilaginous rim called the glenoid labrum. ™ Type: Synovial ball-and-socket joint. ™ Capsule: This surrounds the joint and is attached medially to the margin of the glenoid cavity outside the labrum; laterally it is attached to the anatomic neck of the humerus. The capsule is thin and lax, allowing a wide range of movement. It is strengthened by fibrous slips from the tendons of the subscapularis, supraspinatus, infraspinatus, and teres minor muscles (the rotator cuff muscles). ™ Ligaments: The following ligaments reinforce the capsule: 1) Glenohumeral (GH) Ligaments. 2) Transverse Humeral Ligament. 3) Coracohumeral Ligament. 4) Coracoarcomial Ligament. The Glenohumeral ligaments are three weak bands of fibrous tissue namely, superior, middle and inferior glenohumeral ligaments that strengthen the front of the capsule. However, a thin area of the capsule between the superior and middle GH ligaments (known as the foramen of Weitbrecht) is a particular point of weakness in the capsule. Superior GH ligament contribute most to stability when the arm is at the side (0°), and the inferior GH ligament complex contributes most to stability when the GH joint is at 90° or more. The transverse humeral ligament strengthens the capsule and bridges the gap between the two tuberosities. The coracohumeral ligament strengthens the capsule above and stretches from the root of the coracoid process to the greater tuberosity of the humerus. The coracoacromial ligament extends between the coracoid process and the acromion. Its function is to protect the superior aspect of the joint. ™ Synovial Membrane: This lines the capsule and is attached to the margins of the cartilage covering the articular surfaces. It forms a tubular sheath around the tendon of the long head of the biceps brachii. ™ Nerve Supply: Axillary and Suprascapular nerves. ™ Bursae: Several bursae are associated with the shoulder complex in general and the GH joint specifically. Although all contribute to function, the most important are the subacromial and subdeltoid bursae. These bursae separate 6

the supraspinatus tendon and the head of the humerus from the acromion, coracoid process, coracoacromial ligament, and deltoid muscle. The bursae may be separate but are commonly continuous with each other. Collectively the two are known as the subacromial bursa. The subacromial bursa permits smooth gliding between the humerus and supraspinatus tendon and its surrounding structures.

Ö Coracoacromial Arch:

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The coracoacromial (or suprahumeral) arch is formed by the coracoid process, the acromion, and the coracoacromial ligament that spans the two bony projections. The coracoacromial arch forms an osteoligamentous vault that covers the humeral head and forms a space within which the subacromial bursa, the supraspinatus tendon, and a portion of the tendon of the long head of biceps lie. The coracoacromial arch protects the structures beneath it from direct trauma from above. Such trauma is common and can occur through such simple daily tasks as carrying a heavy bag slung over the shoulder. The arch also prevents the head of the humerus from dislocating superiorly, because an unopposed upward translatory force on the humerus would cause the head of the humerus to hit the coracoacromial arch. Paradoxically, the impact of the humeral head into the arch (while beneficially preventing dislocation) simultaneously can cause painful impingement of the structures lying in the suprahumeral space. When the suprahumeral space is narrowed, the likelihood of impingement of the supraspinatus tendon and subacromial bursa increases.

Ö Glenohumeral Motions:

< Glenohumeral Osteokinematics: The GH joint is usually described as having 3° of freedom: y Flexion / Extension. y Abduction / Adduction. y Medial Rotation / Lateral Rotation. The joint is generally, though not universally, considered to have 120° of flexion and about 50° of extension. The range of medial/lateral rotation of the humerus varies with position. With the arm at the side, medial and lateral rotation may be limited to as little as 50° of combined motion. Abducting the humerus to 90° frees the arc of rotation to 120°. The restricted arc of medial/lateral rotation when the arm is at the side is due to the impact of the lesser tubercle on the anterior glenoid fossa with medial rotation and the impact of the greater tubercle on the acromion with lateral rotation. When the arm is abducted, these bony restrictions play little role, so checks of motion become capsular and muscular. The range of abduction of the humerus in the frontal plane will be diminished if the humerus is maintained in neutral or medial rotation. 7

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When the humerus is medially rotated, the humerus will not abduct on the glenoid fossa beyond 60°; at neutral rotation, 90° of GH abduction can be obtained. The restriction to abduction is caused by the impingement of the greater tubercle on the coracoacromial arch. When the humerus is laterally rotated 35° to 40°, the greater tubercle will pass under or behind the arch so that abduction can continue unimpeded. < Glenohumeral Arthrokinematics: The glenoid fossa and humeral head are incongruent surfaces; the convex humeral head is a substantially larger surface and may have a different radius of curvature than the shallow concave fossa. Given this incongruence, rotations of the joint around its three axes do not occur as pure spins, but have changing centers of rotation and shifting contact patterns within the joint. Elevation of humerus requires that the humeral head glide inferiorly (caudally) in a direction opposite to movement of the shaft of the humerus. For example, abduction of the humerus would cause a superior (cephalad) rolling of the humeral head on the fossa. The large humeral head would soon run out of glenoid surface and head of humerus would impact on the overhanging coracoacromial arch. However, if the head of the humerus glides inferiorly while it rolls up the fossa, full ROM can be achieved. Although inferior glide of the humeral head is necessary to minimize upward roll of the humeral head, it would appear that the center of rotation of the head still moves superiorly on the glenoid even though the magnitude of reported shift differs. Additionally, the humeral head may glide anteriorly or posteriorly and medially or laterally on the fossa.

Ö Static Stabilization of the Dependent Arm: Given the incongruence of the GH articular surfaces, the bony surfaces alone cannot maintain joint contact in the dependent position (arm hanging at the side). As the humerus head rests on the fossa, gravity acts on the humerus parallel to the shaft in a downward direction (negative translatory force). This would appear to require a vertical upward pull to restore equilibrium. Muscles such as the anterior deltoid or brachii could only supply such a vertical force. Scientists have shown that all muscles of the shoulder complex are electrically silent in the relaxed, unloaded limb and even when the limb is tugged vigorously downward. The mechanism of joint stabilization, therefore, must be passive. Gravity acting on humerus is a pure translatory force but lies at a distance from the eccentrically located center of rotation of the humeral head. Given the axis and the line of pull, gravity creates an adduction moment on the humerus. Gravity must be offset by a force that can apply a torque of equal magnitude in the direction of abduction. Such a force can be applied by the structures of the rotator interval capsule (superior 8

capsule, superior glenohumeral ligament, and coracohumeral ligament) that are taut when the arm is at the side. Given the attachment of rotator interval capsule structures on the greater tubercle, the moment arm (MA) of this passive force is nearly twice that of the more centrally located force of gravity. The action line of the rotator interval capsule is upward (offsetting the downward translatory component of gravity) and into the glenoid fossa (compressing joint surfaces). When the passive force of the rotator interval capsule is inadequate for static stabilization, as it may be in the heavily loaded arm, activity of the supraspinatus is recruited.

Ö Dynamic Stabilization of the Glenohumeral Joint:

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< The Deltoid & Glenohumeral Stabilization: Deltoid muscle is a prime mover (along with the supraspinatus) for GH abduction. The anterior deltoid is also considered the prime mover in GH flexion. Both abduction and flexion are elevation activities with many biomechanics similarities. When the muscle action line of deltoid (FD) is resolved into its translatory (ftd) and rotatory (frd) components, the translatory component is by far the larger. That is, the majority of the force of contraction of the deltoid causes the humeral head to translate superiorly; only a some proportion of force causes rotation (abduction) of the humerus. The component forces of the deltoid provide an example in which a translatory force applied in the direction of the joint is not a stabilizing influence. The articular surface of the humerus is not in line with the shaft of the humerus; therefore, a force parallel to the bone creates a dislocating rather than a stabilizing (compressive) effect. The superior (caudal) translatory force of the deltoid, if unopposed, would cause the humeral head to impact the coracoacromial arch before much abduction had occurred. Once the inferiorly directed force of the coracoacromial arch is introduced by humeral head contact, rotation of the humeral head could, theoretically, continue against the leverage provided by the arch. However, but pain from impinged structures would prevent much motion. The inferior translatory pull of gravity cannot offset ftd, because the resultant force of the deltoid must exceed that of gravity before any rotation can occur. Another force or set of forces must be introduced. This is a major function of the muscles of the rotator, or musculotendinous, cuff. < The Rotator Cuff & Glenohumeral Stabilization: The supraspinatus, infraspinatus, teres minor, and subscapularis muscles compose the rotator or musculotendinous cuff. These muscles are considered to be part of a “cuff” because the inserting tendons of each muscle of the cuff blend with and reinforce the GH capsule. When the force of any one muscle of infraspinatus, subscapularis, and teres minor (or all three taken together) is resolved into its components, it can be seen that the 9

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rotatory force (fr) not only tends to cause at least some rotation of the humerus, but fr also compresses the head into the glenoid fossa. Although the muscles of the rotator cuff are important GH joint compressors, equally critical to the stabilizing function of rotator cuff muscles is the inferior translatory pull (ft) of the muscles. The sum of the three negative translatory components of the rotator cuff nearly offsets the superior translatory force of the deltoid muscle. In addition to their stabilizing role, the teres minor and infraspinatus muscles contribute to abduction by providing the lateral rotation necessary to prevent the greater tubercle from impacting the acromion. The infraspinatus and subscapularis added to the abduction torque, whereas the teres minor added to the lateral rotatory torque. The medial and lateral rotatory forces also help center the humeral head, with increased anterior and posterior displacements evident when rotator cuff forces are reduced. Saha referred to these forces as “steerers.” A steering muscle causes a changeover of surfaces within the joint, usually by gliding, and directs the articular surfaces to the appropriate points of contact. He noted that muscles serve both as vertical steerers and, later in the elevation range, as horizontal steerers. He particularly credits the subscapularis with being able to posteriorly steer the humeral head, thus offsetting anterior dislocation forces. The action of the deltoid along with the combined actions of the infraspinatus, teres minor, and subscapularis form a force couple. In a force couple, the divergent pulls of the forces create a pure rotation. In this case, the divergent pulls create an almost perfect spinning of the humeral head around a fixed axis of rotation. < The Supraspinatus & Glenohumeral Stabilization: Although the supraspinatus muscle is also par of the rotator cuff, the action line of the supraspinatus muscle has a superior (cephalad) translatory component, rather than the inferior (caudal) component found in the other muscles of the cuff. Given its line of pull, the supraspinatus is of no use in offsetting the upward dislocating action of the deltoid. The supraspinatus still is effective as a stabilizer of the GH joint because, like the other cuff muscles, its rotatory component generates a strong compressive force. Unlike the other cuff muscles, the rotatory component of the supraspinatus has a large enough moment arm (MA) that it is capable by itself of producing a full or nearly full range of GH joint abduction and, with the assistance of gravity, stabilizing the joint. Gravity acts as stabilizing synergist to the supraspinatus by offsetting the small upward translatory pull of the muscle. Gravity and the supraspinatus, using Saha’s terminology, act as vertical steerers; the resultant of the two forces causes an inferior gliding of the humeral head during abduction of the shaft, allowing full articulation of the surfaces, and preventing abnormal superior displacement. 10

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< The Long Head of the Biceps & Glenohumeral Stabilization: The long head of the biceps runs superiorly from the anterior shaft of the humerus through the bicipital groove between the greater and lesser tubercles to attach to the supraglenoid tubercle and superior labrum. It enters the GH joint capsule through an opening between the supraspinatus and subscapularis muscles where it penetrates the capsule but not the synovium. Within the bicipital groove, the biceps tendon is enveloped by a tendon sheath and tethered there by the transverse humeral ligament that runs between the greater and lesser tubercles. The long head of the biceps, because of its position at the superior capsule and its connections to structures of the rotator interval capsule, is sometimes considered to be part of the reinforcing cuff of the GH joint. The biceps muscle is capable of contributing to the force of flexion and can, if the humerus is laterally rotated, contribute to the force of abduction and anterior stabilization. Although elbow and shoulder position may influence its function, the long head appears to contribute to GH stabilization by centering the head in the fossa, and by reducing vertical (superior & inferior) and anterior translations.

Ö Scapulohumeral Rhythm:

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The combination of concomitant GH and ST motion is most commonly referred to as scapulohumeral rhythm. The complete range of shoulder flexion and abduction is 180° out of which 120° of movement occurs at GH joint and 60° of movement occurs at ST joint. Thus, overall ratio is 2° of GH motion to 1° of ST motion. Scapulohumeral motion involves motion of the SC and AC joints, as well as ST and GH joints. Because the ST joint is part of a closed chain, movement of the scapula occurs only with motion at both the AC and SC joints. The 60° arc of upward rotation through which the scapula moves during elevation of the arm can be attributed primarily to SC and secondarily to AC motion produced by the force couple of the trapezius and serratus anterior muscles. These two muscles are the only muscles capable of upwardly rotating the scapula. < Phase One: The upper portion of the trapezius muscle elevates the clavicle; the lower portion of the trapezius muscle combines with the upper and lower portions of the serratus anterior muscle to produce an upward rotatory force on the scapula. The middle trapezius may also contribute to upward rotation. Although upward rotation of scapula would appear to occur at the AC joint, the coracoclavicular ligament prevents this AC movement because the ligament binds the coracoid process of the scapula to the clavicle. The upward rotatory force on the scapula from the contracting muscles, 11

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therefore, must produce movement at the next available joint: the SC joint. The pull of the trapezius and the serratus anterior muscles on the scapula force the clavicle to elevate. Clavicular elevation carries the scapula through 30° of upward rotation as the scapula rides on the lateral end of the rising clavicle while maintaining a relatively fixed scapuloclavicular angle. Elevation of the clavicle is checked when the costoclavicular ligament becomes taut. Because the ST upward rotation and clavicular elevation occur concurrently with GH motion, the GH joint can be expected under normal conditions to simultaneously flex or abduct about 60°. Given 30° of ST upward rotation and 60° of GH flexion or abduction, the arm will be elevated approximately 90° to 100° from the side of the body. During the initial 30° of ST motion, the AC joint maintains a relatively fixed relation between the scapula and clavicle, although allowing 10° of medial rotation and some anterior tipping of the scapula to maintain the scapula against the changing contour of the rib cage. [Contraction of trapezius and serratus anterior causes the scapula to rotate upwards at AC joint, but the coracoclavicular ligament prevents this movement. So, this force is transferred to SC joint, which causes 30° of elevation of the scapula. This results in 30° of upward rotation of scapula. Excessive elevation of the clavicle is prevented by costoclavicular ligament.] < Phase Two: As the lower trapezius and serratus anterior continue to generate an upward rotatory force on the scapula, upward rotation at the AC joint is still restrained by the coracoclavicular ligament while the SC joint is now constrained by tension in the costoclavicular ligament (which checked further clavicular elevation). With no other available motion to dissipate the upward rotatory force being created by the trapezius and serratus muscles, tension in the coracoclavicular ligament builds as the coracoid process of the scapula gets pulled downward. The tensed conoid ligament draws its posteroinferior clavicular attachment forward and down as the coracoid process drops, causing the clavicle to posteriorly rotate. Posterior rotation of the clavicle around its longitudinal axis will flip the lateral end of the crank-shaped clavicle up without causing further elevation at the SC joint and while still maintaining a relatively fixed scapuloclavicular angle. The magnitude of posterior rotation of the clavicle may be anywhere from 30° to 55°. The scapula that is attached to the lateral end of the rotating clavicle, however, will be carried through an additional 30° of upward rotation. As 12

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the scapula finds its final position on the rib cage, the AC joint absorbs varying amounts of anterior/posterior tipping and medial/lateral rotation. [As both the coracoclavicular and costoclavicular ligaments have become taut, it causes the clavicle to rotate posteriorly. Because of the irregular shape of the clavicle, the lateral end of the clavicle flips up causing 30° of upward rotation of the scapula at AC joint. This completes the remaining amount of flexion of GH joint.] If 180° is accepted as the maximal range of flexion and abduction of the humerus, raising the arm to the horizontal involves 60° of GH motion and 30° of ST motion, with the scapular contribution produced by clavicular elevation at the SC joint. Raising the arm from the horizontal to vertical position involves an additional 60° of GH movement and 30° of ST movement produced by clavicular rotation and AC motion. For the clavicle to rotate about its longitudinal axis, it would appear to require mobility of both the SC and AC joints. Thus, SC joint is of primary importance both for the first 30° of ST upward rotation and for the second 30° of ST upward rotation, with the AC joint playing a supporting role.

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U Muscles of Elevation:

Elevation activities are described as those requiring muscles to overcome or control the weight of the limb and its load, and usually involve components of GH flexion or abduction, and scapular upward rotation. The completion of normal elevation depends not only on freedom of movement and integrity of the joints involved but also on the appropriate strength and function of the muscles producing and controlling movement.

Ö Deltoid Muscle:

The deltoid is at resting length (optimal length-tension) when the arm is at the side. When at resting length, the deltoid’s angle of pull will result in a predominance of superior translatory pull on the humerus with an active contraction. With an appropriate synergistic downward pull from the infraspinatus, teres minor, and subscapularis, the rotatory components of the anterior and middle deltoid are effective primary movers for flexion and abduction, respectively. The anterior deltoid can assist with abduction after 15° of GH motion. When the humerus is in the plane of the scapula, the anterior and middle deltoid are optimally aligned to produce elevation of the humerus. The action line of the posterior deltoid has too small a moment arm (MA) to contribute effectively to abduction; it serves primarily as a joint compressor and functions such as horizontal abduction. 13

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As the humerus elevates, the translatory component of the deltoid diminishes its superior dislocating influence and shifts its line of pull increasingly toward the glenoid fossa. At the same time, the rotatory component of the deltoid must counteract the increasing torque of gravity as the arm moves toward horizontal. Analysis by EMG shows gradually increasing activity in the deltoid, peaking at 90° of humeral abduction and plateauing for the remainder of the motion (Saha found a peak at 120° with a drop off to moderate activity at 180°). The peak activity in flexion does not occur until the end of the range and there is less total activity. Although the MA of the deltoid gets larger as the humerus elevates and the torque of gravity diminishes once the arm is above horizontal, the high activity level of the deltoid continues. The deltoid’s shortening fibres are approaching active insufficiency. As a result of the loss of tension due to extreme shortening, a greater number of motor units must be recruited to maintain even equivalent force output. The multipennate structure and considerable cross section of the deltoid help compensate for the relatively small MA, low mechanical advantage, and less-than-optimal length/tension. Maintenance of appropriate length/tension of the deltoid is strongly dependent on simultaneous scapular movement. When the scapula is restricted, the deltoid becomes actively insufficient and can only achieve and barely maintain 90° of GH abduction. With complete derangement of the cuff, a contraction of the deltoid results in a shrug of the shoulder rather than in abduction of the humerus. Stimulation of the Axillary nerve produces approximately 40° of abduction. Partial tears in or partial paralysis of the cuff will weaken the rotation produced by the deltoid.

Ö Supraspinatus Muscle:

The supraspinatus muscle is considered an abductor of the humerus. The pattern of activity of the supraspinatus is essentially the same as that found in the deltoid. The moment arm (MA) of the supraspinatus is fairly constant throughout the ROM and is larger than that of the deltoid for the first 60° of shoulder abduction. When the deltoid is paralyzed, the supraspinatus alone can bring the arm through most if not all of the GH range, but motion will be weaker. With a Suprascapular nerve block that paralyzes the supraspinatus and the infraspinatus, the strength of elevation in the plane of the scapula is reduced by 35% at 0° and by 60% to 80% at 150°. The secondary functions of the supraspinatus are to compress the GH joint, to act as vertical steerers for the humeral head, and to assist in maintaining the stability of the dependent arm. With isolated and complete paralysis of the supraspinatus muscle, some loss of abduction force is evident, but remaining musculature can perform most of its functions. 14

Ö Infraspinatus, Teres Minor, & Subscapularis Muscle:

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When scientists assessed the combined actions of the infraspinatus, teres minor, and subscapularis muscles, electromyographic activity indicated a nearly linear rise in action potentials from 0° to 115° elevation. Activity slightly dropped between 115° and 180°. Total activity in flexion was slightly greater than that in abduction. In abduction an early peak in activity of these muscles appeared at 70° of elevation. Steindler hypothesized that early peak was a response to the need for depression of the humeral head, whereas the latter peak at 115° was a result of increased activity of these muscles in producing lateral rotation of the humerus. The medial rotatory function of the subscapularis acts to steer the head of the humerus horizontally, while continuing to work with the other cuff muscles to compress and stabilize the joint.

Ö Upper & Lower Trapezius & Serratus Anterior Muscles:

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The upper trapezius and upper serratus anterior muscles form one segment of a force couple that drives the scapula in elevation of the arm. These two muscle segments, along with the levator scapula muscle, also support the shoulder girdle against the downward pull of gravity. The lower trapezius and lower serratus anterior muscles form the second segment of the force couple. When EMG monitored activity of the upper and lower trapezius and serratus anterior muscles during humeral elevation, the curves were similar and complementary. Activity in the trapezius rises linearly to 180° in abduction, with more undulating activity in flexion. The serratus anterior shows a linear increase in action potentials to 180° in flexion, with undulating activity in abduction. The middle trapezius is also active during elevation and may contribute to upward rotation of the scapula. In abduction of the arm, the force of the trapezius seems more critical to the production of upward rotation of the scapula than the force of the serratus anterior. When the trapezius is intact and the serratus anterior is paralyzed, abduction of the arm can occur through its full range although it is weakened. When the trapezius is paralyzed, abduction of the arm is both weakened and limited in range to 75°. Without the trapezius (with or without the serratus anterior), the scapula rests in a downwardly rotated position due to the unopposed effect of gravity on the scapula. When abduction of the arm is attempted, the middle and posterior fibres of the activated deltoid increase the downward rotatory pull on the scapula. Although the deltoid can still achieve the 90° of GH motion attributed to it when the scapula has been immobilized, the 90° occurs on a downwardly rotated scapula; the net effect is that the arm will rise from side only about 60° to 75°. In flexion the anterior orientation of the scapula is important in that this can be produced only by the serratus anterior. If the serratus anterior is intact, 15

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trapezius paralysis results in loss of force of shoulder flexion but there is no range deficit. If the serratus anterior is paralyzed (even in the presence of a functioning trapezius), flexion will be both diminished in strength and limited in range to 130° to 140° of flexion. When scapular retraction component of the trapezius is unopposed by the serratus anterior, the trapezius is unable to upwardly rotate the scapula more than 20° of its potential 60°. Although the serratus anterior and trapezius are the prime movers for scapulothoracic upward rotation, these muscles serve an equally important function as stabilizing synergists for the deltoid acting at the GH joint. All muscles pull on both proximal and distal attachments equally. When both ends are free to move, the lighter end will usually move first. In most instances, the lighter of the two segments of the joint is the distal segment. Rather, uniquely, the lighter segment of the GH joint is the proximal scapular segment. If the deltoid acted on its lighter proximal segment rather than the heavier humerus, the scapula would rotate downward before the humerus would elevate. The deltoid muscle would become actively insufficient before much humeral elevation was produced. The trapezius and serratus anterior muscles, as upward scapular rotators, prevent the undesired downward rotatory movement of the scapula during deltoid contraction. The trapezius and serratus anterior maintain optimal length-tension in the deltoid and permit the deltoid to carry its heavier distal lever through full ROM. Thus, the role of the scapular force couple of the trapezius and the serratus anterior is both agonistic to scapular movement and synergistic to GH movement. The trapezius and the serratus anterior produce desired scapular upward rotation, while preventing undesired movement by the deltoid as it elevates the GH joint.

Ö Rhomboid Muscles:

The rhomboid major and minor muscles are active in elevation of the humerus, especially in abduction. These muscles serve a critical function as stabilizing synergists to the muscles that rotate the scapula. They contract eccentrically to control the change in position of the scapula produced by the trapezius and the serratus anterior. Paralysis of these muscles causes disruption of the normal scapulohumeral rhythm and may result in diminished ROM.

U Muscles of Depression:

Depression involves the forceful downward movement of the arm in relation to the trunk. If the arm is fixed by weight bearing or by holding on to an object, depression is then the forceful movement of the trunk upward in relation to the arm. In depression activities, the scapula tends to rotate downward and adduct 16

during the humeral motion, but there is not a consistent ratio of one segment to the other.

Ö Latissimus Dorsi & Pectoral Muscles:

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When the upper extremity is free to move in space, the latissimus dorsi muscle serves an important function in adduction and medial rotation of the humerus, as well as in extension of the humerus. When the latissimus pulls on its attachment to the scapula and on its humeral attachment, it can also adduct and depress the shoulder girdle. When the hand is fixed in weight bearing, the latissimus dorsi muscle will pull its caudal attachment on the pelvis toward its cephalad attachment on the scapula and humerus. This results in lifting the body up as in a seated pushup. When the hands are bearing weight on the handles of a pair of crutches, a contraction of the latissimus will unweight the feet as the trunk rises beneath the fixed scapula; this will allow the legs to swing forward through the crutches. Activity of latissimus dorsi may contribute to joint stability because it causes compression of the GH joint when the arm is above the horizontal. The clavicular portion of the pectoralis major can assist the deltoid in flexion of the GH joint but the sternal and abdominal portions are primary depressors of the shoulder complex. The combined action of the pectoralis major’s sternal and abdominal portions parallels that of the latissimus dorsi, although the pectoralis is located anterior rather than posterior to the GH joint as is the latissimus. In activities involving weight bearing on the hands, both the pectoralis major and the latissimus can depress the shoulder complex, while anterior/posterior movement of the humerus and protraction/retraction of the scapula are neutralized. The depressor function of these muscles is further assisted by the pectoralis minor muscle, which acts directly on the scapula to depress and rotate it downward.

Ö Teres Major & Rhomboid Muscles: The teres major muscle, like the latissimus dorsi, adducts, medially rotates, and extends the humerus. The teres major is active primarily during resisted activities, but may also be active during unresisted extension and adduction activities behind the back. Function of the teres major muscle is strongly dependent on activity of the rhomboid muscles. The teres major muscle originates on the scapula and attaches to the humerus. Consequently, the segment it attaches to proximally is lighter than the segment it attaches to distally. The proximal scapula must be stabilized to permit the teres major to act effectively as an extensor and adductor of the humerus. Without stabilization, the teres major would upwardly rotate the lighter scapula rather than move the heavier humerus. The rhomboids muscles, as downward rotators of the scapula, offset 17

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the undesired upward rotatory force of the teres major. By fixing the scapula as the teres major contracts, the rhomboids allow the teres major to move the heavier humerus.

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