Kinesiology of the shoulder

Side view of flexion in the near sagittal plane of the right glenohumeral joint.

A point on the head of the humerus is shown spinning around a point on the glenoid fossa. Stretched structures are shown as long thin arrows; and they are PC, posterior capsule; ICL, inferior capsular ligament; CHL, coracohumeral ligament. Stretched posterior capsule results in slight anterior translation of humerus. (important!)

Rotator Cuff and Biceps Brachii Long Head

Anteriorly - subscapularis Superiorly -supraspinatus Posteriorly - infraspinatus, teresminor - Rotator cuff muscles form a cuff over humeral head to actively stabilizes GH joint during all dynamic activities - Rotator cuff muscle tendons blend into joint capsule - This unique anatomic arrangement helps explain why the mechanical stability of the GH joint is so dependent on the innervation, strength, and control of the rotator cuff muscles - 'Weak' areas - the rotator cuff fails to cover two regions of the capsule: inferiorly, and a region between the supraspinatus and subscapularis known as the rotator interval. (IMPORTANT) - Rotator interval (Superior GH ligament and Coracohumeral ligament) may be reinforced by biceps brachii long head tendon, coracohumeral ligament and parts of the GH capsular ligaments.

Factors contributing to mobility

Articulating surfaces - Not pure ball-and-socket joint - Contribution from scapular movements - Head of the humerus and the glenoid fossa are both lined with articular cartilage, lubricating the movement Joint capsule - Volume is about 2x humeral head size - Synovial membrane lines the inner wall of the capsule - The loose-fitting and expandable capsule provides extensive mobility to the GH joint. - This mobility is evidenced by the amount of passive translation available at the GH joint. - The humeral head can be pulled away from the fossa a significant distance without causing pain or trauma to the joint. In the anatomic or adducted position, the inferior portion of the capsule appears as a slackened or redundant recess called the axillary pouch.

Scapulothoracic elevation

Both SC and AC joint contributes to scapulothoracic joint - cannot just treat scapulothoracic joint, but rather all 4 articulations Scapular elevation occurs as a composite of SC and AC joint rotations. - For the most part, the motion of shrugging the shoulders is a direct result of the scapula following the path of the elevating clavicle around the SC joint. - Slight downward rotation of the scapula at the AC joint allows the scapula to remain nearly vertical throughout the elevation - Downward rotation of scap to help elevate more

ST upward and downward rotation

Complete upward rotation of the scapula occurs by a summation of clavicular elevation at the SC joint and scapular upward rotation at the AC joint.\ - These coupled rotations are essential to the full 60 degrees of upward rotation at the scapulothoracic joint. - The scapula may rotate upwardly and strictly in the frontal plane as in true abduction but more often follows a path closer to its own "scapular" plane.

Upward rotation of the scapula (important)

Contributes to one-third of shoulder elevation (flexion or abduction). Why? - Projects glenoid fossa upward and antero-laterally to prevent humerus from hitting the glenoid fossa to continue the upward journey - Preserves optimal length-tension relationship of supraspinatus and middle deltoid muscles - Preserves volume of subacromial space - A reduced subacromial space during abduction may lead to a painful and damaging impingement of the residing tissues, such as the supraspinatus tendon

Describe the arthrokinematics of clavicular elevation and depression at the sternoclavicular joint.

Convex clavicle on concave sternum, roll and slide in opposite direction Anterior view of a mechanical diagram of the arthrokinematicsof roll and slide during elevation (A) and depression (B) of the clavicle around the right sternoclavicular joint. The axes of rotation are shown in the anterior-posterior direction near the head of the clavicle. 1. CCL, costoclavicular ligament 2. ICL, interclavicular ligament 3. SC, superior capsule. stretched costoclavicular ligament produces a downward force on the clavicle in the direction of the slide during clavicular elevation (ICL is taut) stretched Interclavicular ligament and superior capsule during clavicular depression (CCL is taut)

Posterior view of the right shoulder complex after the arm has abducted 180 degrees

External rotation is to prevent greater tubercle of humerus from hitting the arch!!

ideal posture of the shoulder girdle, and what is rounded shoulders

Ideal posture of the shoulder girdle : - incorporates a slightly elevated and relatively retracted scapula, with the glenoid fossa facing slightly upward. - The upper trapezius, by attaching to the lateral end of the clavicle, provides excellent leverage around the SC joint for maintenance of this ideal posture. - With long-term paralysis of the trapezius, the glenoid fossa loses its upwardly rotated position, allowing the humerus to slide inferiorly. - The downward pull imposed by gravity on an unsupported arm may strain the capsular ligaments at the GH joint and eventually lead to an irreversible dislocation. Rounded shoulders : - Both scapulas are slightly depressed, downwardly rotated, and protracted. In principle, this posture can lead to similar (but usually far less damaging) biomechanical stress on the SC and GH joints as described for the girl with actual muscle paralysis. - As evidenced by the position of the medial border and inferior angle in the subject in Figure 5-41, B, both scapulas are also slightly internally rotated and anteriorly tilted—postures believed to predispose to impingement of the tissues within the subacromial space

Structures contributing to AC joint stability

Ligaments : 1. Superior and 2. inferior acromioclavicular joint ligaments 3. Coracoclavicular ligament (conoid and trapezoid ligament) - The coracoclavicular ligament provides an important extrinsic source of stability to the AC joint. - As a whole, the entire ligament is stronger and absorbs more energy at the point of rupture than most other ligaments of the shoulder. - These structural features, in conjunction with the coracoclavicular ligament's near-vertical orientation, suggest an important role in suspending the scapula (and upper extremity) from the clavicle. - Articular disc (contribute to stability) Muscles : 1. Deltoid 2. Upper trapezius - The superior capsular ligament is reinforced through attachments from the deltoid and trapezius. Distinct differences exist in the function of the SC and AC joints. - The SC joint permits extensive motion of the clavicle, which guides the general path of the scapula. - The AC joint, in contrast, permits more subtle movements between the scapula and lateral end of the clavicle.

GH Designed for mobility, not stability

Longitudinal diameters -humeral head 1.9 times glenoid fossa Transverse diameters - humeral head 2.3 times glenoid fossa Result: glenoid fossa covers only one-third of humeral head (important) Side view of right glenohumeral joint with the joint opened up to expose the articular surfaces. Note the extent of the subacromial space under the coracoacromial arch. Normally this space is filled with the supraspinatus muscle and its tendon, and the subacromial bursa. The longitudinal and horizontal diameters are illustrated on both articular surfaces.

Kinematic relationships at the GH joint

Often, a fourth motion is defined at the GH joint: - horizontal flexion and extension (also called horizontal adduction and abduction). - The motion occurs from a starting position of 90 degrees of abduction. The humerus moves anteriorly during horizontal flexion and posteriorly during horizontal extension. Abduction and adduction : - External rotation of the GH joint naturally accompanies abduction—a point easily verifiable by palpation. (internal rotation for adduction) Flexion and extension : - the spinning of the humeral head draws most of the surrounding capsular structures taut. - Tension within the stretched posterior capsule may cause a slight anterior translation of the humerus at the extremes of flexion. - At least 120 degrees of flexion are available to the GH joint. - Flexing the shoulder to nearly 180 degrees includes an accompanying upward rotation of the scapulothoracic joint. - Full extension of the shoulder occurs to a position of about 65 degrees actively (and 80 degrees passively) behind the frontal plane. - The extremes of this passive motion likely stretch the capsular ligaments, causing a slight anterior tilting of the scapula. This forward tilt may enhance the extent of a backward reach. Internal and external rotation : - The humeral head simultaneously rolls posteriorly and slides anteriorly on the glenoid fossa (for ER), opp for IR - if without concurrent anterior slide, the head displaces posteriorly, roughly 38 mm

ST protraction and retraction

Protraction of the scapula occurs through a summation of horizontal plane rotations at both the SC and AC joints. - The scapula follows the general path of the protracting clavicle around the SC joint - The AC joint can amplify, offset, or otherwise adjust the total amount of scapulothoracic protraction by contributing varying amounts of internal rotation. - Because scapulothoracic protraction occurs as a composite of motions at the SC and AC joints, a decrease in motion at one joint can be partially compensated for by an increase at the other. - Consider, for example, an individual with severe degenerative arthritis and decreased motion at the AC joint. - The SC joint may compensate by contributing a greater degree of protraction, thereby limiting the functional loss associated with forward reach of the upper limb.

1. Sternoclavicular Joint : What is it made up of? Types of joint

The clavicle, through its attachment to the sternum, functions as a mechanical strut, or prop, holding the scapula at a relatively constant distance from the trunk. Located at the lateral end of the clavicle is the acromioclavicular joint. This joint, and associated ligaments, firmly attaches the scapula to the clavicle. The anterior surface of the scapula rests against the posterior-lateral surface of the thorax, forming the scapulothoracic joint. - This articulation is not a true anatomic joint; rather, it is an interface between bones. Movements at the scapulothoracic joint are mechanically linked to the movements at both the sternoclavicular and acromioclavicular joints. 1. Clavicle 2. Manubrium 3. Cartilage of first rib What types of joints are sternoclavicular joints? A. Ball-and-socket B. Saddle (Almost interlocking) Clavicular elevation : Convex on Concave (the convex joint surface slides in the direction opposite to the bone segment's rolling motion) More mobile hence less stable

Humerus

The head of the humerus, nearly one half of a full sphere, forms the convex component of the glenohumeral joint The head faces medially and superiorly, forming an approximate 135-degree angle of inclination with the long axis of the humeral shaft. Relative to a medial-lateral axis through the elbow, the humeral head is rotated posteriorly about 30 degrees within the horizontal plane, this rotation is know at retroversion. - aligns the humeral head within the scapular plane for articulation with the glenoid fossa

Mobility of AC joint : 1. What planes are involved in these movements? 2. Which bone moves?

The motions of the AC joint are described by the movement of the scapula relative to the lateral end of the clavicle. 1. - Horizontal plane : scapular internal and external rotation - Sagittal plane : Anterior/posterior tipping - Scapular plane : scapular upward/downward rotation (30 degrees, important) 2. The scapula moves (A) Posterior view showing the osteokinematicsof the right acromioclavicular (AC) joint. The primary motions of upward and downward rotation are shown in purple. Horizontal and sagittal plane adjustment motions, considered as secondary motions, are shown in blue and green, respectively. Note that each plane of movement is color-coded with a corresponding axis of rotation. Images (B) and (C) show examples of rotational adjustment motions at the AC joint: internal rotation during scapulothoracic protraction (B), and anterior tilting during scapulothoracic elevation (for the scapula to follow the convex thorax) !!! (C).

acromiohumeral distance during active shoulder abduction in the scapular plane

The orange bar (along horizontal axis, 20-35°) indicates the arc of abduction where the HUMERAL HEAD is closest to the undersurface of the acromion. The red bar (35-70°) indicates the arc of abduction where the distal attachment of the SUPRASPINATUS is closest to the undersurface of the acromion. The blue bar indicates the arc of abduction where the PROXIMAL SHAFT is closest to the undersurface of the acromion. (Shoulder abduction angle is defined as the angle between a vertical reference and the long axis of the humerus.)

The arthrokinematics of the right glenohumeral joint during active abduction.

The supraspinatus is shown contracting to direct the superior roll of the humeral head. - thereby protecting it from being pinched between the humeral head and undersurface of the acromion process. - The muscular force also adds to the dynamic stability of the joint. (Dynamic stability refers to the stability achieved while the joint is moving. - The taut inferior capsular ligament (ICL)is shown supporting the head of the humerus like a hammock. Note that the superior capsular ligament(SCL) remains relatively taut because of the pull from the attached contracting supraspinatus. (important) - Stretched tissues are depicted as long black arrows.

scapula

The triangular-shaped scapula has three angles: inferior, superior, and lateral. Palpation of the inferior angle provides a convenient method for following the movement of the scapula during arm motion. The scapula also has three borders. With the arm resting by the side, the medial or vertebral border runs almost parallel to the spinal column. The lateral or axillary border runs from the inferior angle to the lateral angle of the scapula. The medial end of the spine diminishes in height at the root of the spine. - In contrast, the lateral end of the spine gains considerable height and flattens into the broad and prominent acromion (from the Greek akros, meaning topmost, highest). The acromion extends in a lateral and anterior direction, forming a horizontal shelf over the glenoid fossa. The clavicular facet on the acromion forms part of the acromioclavicular joint

Paralysis of the Upward Rotators of the Scapulothoracic Joint

Trapezius Paralysis : - Complete paralysis of the trapezius usually causes moderate to marked difficulty in elevating the arm overhead. - The task typically can still be accomplished through full range of motion as long as the serratus anterior is fully innervated. - Elevation of the arm in the pure frontal plane is particularly difficult with trapezius paralysis because this action requires that the middle trapezius generate a strong retraction force on the scapula Serratus Anterior Paralysis : - Paralysis of the serratus anterior muscle causes significant disruption in normal shoulder kinesiology. - As a rule, persons with complete paralysis of the serratus anterior have great difficulty actively elevating the arm above the head. - This difficulty exists even though the trapezius and glenohumeral abductor muscles are fully innervated. Attempts at shoulder abduction, especially against resistance, typically result in limited elevation of the arm, coupled with an excessively downwardly rotated scapula. - Normally, contraction of a normal serratus anterior strongly upwardly rotates the scapula, thus allowing the contracting middle deltoid and supraspinatus to rotate the humerus in the same rotary direction as the scapula. - In cases of paralysis of the serratus anterior, however, the contracting middle deltoid and supraspinatus dominate the scapular kinetics, producing a paradoxic (and ineffective) downward rotation of the scapula. - The combined active motions of downward rotation of the scapula and partial elevation of the arm cause the deltoid and supraspinatus to overshorten rapidly. - As predicted by the force-velocity and length-tension relationships of muscle, the rapid overshortening of these muscles reduces their maximal force potential. - This reduced force potential, in conjunction with the downward rotation position of the scapula, reduces both the range of motion and torque production of the elevating arm - Can cause winging (Anteriorly tilted and internally rotated) - Such a position can eventually cause adaptive shortening of the pectoralis minor muscle—a direct antagonist to the serratus anterior. Increased passive tension in the pectoralis minor would further promote an anteriorly tilted and internally rotated position of the scapula

Clavicle

With the arm in the anatomic position, the long axis of the clavicle is oriented slightly above the horizontal plane and about 20 degrees posterior to the frontal plane. The rounded and prominent medial or sternal end of the clavicle articulates with the sternum. The costal facet of the clavicle rests against the first rib. Lateral and slightly posterior to the costal facet is the distinct costal tuberosity, an attachment for the costoclavicular ligament

Dislocations of the sternoclavicular joint, which account for how many % of joint dislocation and shoulder dislocation? Good to know

- 1% of joint dislocations in the body - 3% of shoulder girdle dislocations This suggests that the SC joint is very stable

Shoulder Abduction in the Frontal Plane versus the Scapular Plane

- Abducting the shoulder in the scapular plane is a more natural movement and generally allows greater elevation of the humerus than abducting in the pure frontal plane - This abduction movement can usually be performed with greater ease and with less external rotation, at least in the early to mid ranges of shoulder motion. - Impingement is avoided because scapular plane abduction places the apex of the greater tubercle under the relatively high point of the coracoacromial arch - Abduction in the scapular plane also allows the naturally retroverted humeral head to fit more directly into the glenoid fossa.

What is the function of Roll-and-Slide Arthrokinematics

- Allows a larger convex surface (large humeral head) to roll over a smaller concave surface (shallow and small glenoid fossa), WITHOUT RUNNING OUT of articular surface. - Clinical example of frozen shoulder (adhesive capsulitis) where inferior capsular ligament may become thickened excessively. - Limited inferior slide of humeral head -> roll of humeral head jams it against coracoacromial arch - An adult-sized humeral head that is rolling up a glenoid fossa without a concurrent inferior slide would translate through the 10-mm subacromial space after only 22 degrees of GH joint abduction (A) A model of the glenohumeral joint depicting a ball the size of a typical adult humeral head rolling across a flattened (glenoid) surface. Based on the assumption that the humeral head is a sphere with a circumference of 16.3 cm, the head of the humerus would translate upward 1 cm after a superior roll (abduction) of only 22 degrees. This magnitude of translation would cause the humeral head to press against the contents of the subacromial space. (B) Anatomic representation of the model used in (A). Note that abduction without a concurrent inferior slide causes the humeral head to impinge against the arch and block further abduction.

Concave convex for SC joint

- Although highly variable, the medial end of the clavicle is usually convex along its longitudinal diameter and concave along its transverse diameter. - Hence during protraction/retraction of clavicle (concave transverse diameter of clavicle over convex transverse diameter of sternum), clavicle rolls and slides in the same direction - whereas for elevation and depression, roll and slide in opposite direction - The clavicular facet on the sternum typically is reciprocally shaped, with a slightly concave longitudinal diameter and a slightly convex transverse diameter.

Structures contributing to SC joint stability

- Articular surfaces Ligaments : 1. Anterior and posterior sternoclavicular joint ligaments 2. Interclavicular ligament 3. Costoclavicular ligament Joint capsule : - Anterior disc Muscles : 1. Sternocleidomastoid 2. Sternothyroid 3. Sternohyoid 4. Subclavius When active, muscles add further stability to the joint: anteriorly by the sternocleidomastoid, posteriorly by the sternothyroid and sternohyoid, and inferiorly by the subclavius

2. Acromioclavicular Joint

- Clavicle, acromion - Plane synovial joint with three rotational and three translational degrees of freedom - The AC joint is a gliding or plane joint, reflecting the predominantly flat contour of the joint surfaces - Because of the predominantly flat joint surfaces, roll-and-slide arthrokinematics are not described - The right acromioclavicular joint. (A) An anterior view showing the sloping nature of the articulation. (B) A posterior view of the joint opened up from behind, showing the clavicular facet on the acromion and the fragmented disc

Glenoid Labrum

- Fibrocartilage (unlike the joint capsule which is hyaline cartilage) - Covering the rim of glenoid fossa - Deepening the depth of glenoid fossa (attributing about 50% of the depth) - By deepening the concavity of the fossa, the labrum increases contact area with the humeral head and therefore helps stabilize the joint - Prone to injury: - Superior part loosely attaches to the rim of glenoid fossa - 50% of the fibres of biceps brachii long head tendon are direct extensions of superior glenoid labrum; excessive tension (e.g. overhead movt) can tear superior glenoid labrum - Result : Superior Labrum Anterior posterior - Exceedingly large or repetitive forces within the biceps tendon can partially detach the loosely secured superior labrum from its near-12 o'clock position on the glenoid rim. - The relatively high incidence of superior labral tears in throwing athletes, such as baseball pitchers, is likely related to the forces produced within the biceps during this activity. - The long head of the biceps is stressed (along with the anterior and inferior capsule) during the "cocking" phase of pitching, and again as the muscle rapidly decelerates the arm and forearm during the follow-through phase of the pitch. This stress is transferred directly to the superior labrum. - A weakening of the proximal attachment of the long head of the biceps likely limits the muscle's ability to restrain anterior translation of the humeral head

About the shoulder complex

- Not a single joint - Mobility versus stability - Collectively these joints provide extensive range of motion - Interplay of muscles affects the kinematics and functions of the shoulder complex - Kinematic concepts to understand shoulder scaption(scapular plane elevation) - 30°anterior to coronal plane

3. Scapulothoracic joint

- Not a true joint, rather an articulation - The 2 surfaces do not make direct contact; rather, they are separated by muscles, such as the subscapularis, serratus anterior, and erector spinae. The relatively thick and moist surfaces of these muscles likely reduce shear within the articulation during movement. An audible clicking sound during scapular movements may indicate abnormal contact within the articulation. - Cannot perform shoulder elevation if this joint is tight (important) - Scapula, posterior-lateral thoracic wall - No joint capsule, separated by muscles - What are these muscles? - Positioned ribs 2 to 7 in anatomical position - 'Resting' posture of the scapula (scapular plane, IMPORTANT) : - 10°of anterior tilt - 5-10°of upward rotation - 30-40°of internal rotation "The movements at the scapulothoracic joint are fundamental components of shoulder kinesiology."

An example of force-couple synergism

- Rhomboids and lower trapezius retract scapula. - Rhomboids are also scapular elevators whilst lower trapezius scapular depressors. They work synergistically to neutralize their antagonistic actions especially during forceful scapular retraction.

4. Glenohumeral Joint

- Scapula, humerus - Ball-and-socket Does the humeral head fit perfectly into the glenoid fossa of the scapula? Why not? In anatomical position: - Glenoid fossa -upward rotated and articular surface projects antero-laterally in scapular plane - Humeral head -projects medially, superiorly and posteriorly (due to retroversion)

Scapulothoracic Posture and its effects on static stability

- Slightly upwardly rotated - Normally when one stands at complete rest with arms at the sides, the head of the humerus remains stable against the glenoid fossa. This stability is referred to as static because it exists at rest. One passive mechanism for controlling static stability at the GH joint is based on the analogy of a ball compressed against an inclined surface - At rest, the superior capsular structures (SCS) provide the primary ligamentous support for the humeral head. These structures include the superior capsular ligament, the coracohumeral ligament, and the tendon of the supraspinatus. - Combining the resultant capsular force vector with the force vector due to gravity yields a compressive locking force, oriented at right angles to the surface of the glenoid fossa. The compression force (CF) stabilizes the GH joint by compressing the humeral head firmly against the glenoid fossa, thereby resisting descent of the humerus.86,87 The inclined plane of the glenoid also acts as a partial shelf that supports part of the weight of the arm. (A) The rope indicates a muscular force that holds the glenoid fossa in a slightly upward-rotated position. In this position the passive tension in the taut superior capsular structure(SCS) is added to the force produced by gravity (G),yielding the compression force(CF). The compression force applied against the slight incline of the glenoid "locks" the joint. (B) With a loss of upward rotation posture of the scapula (indicated by the cut rope), the change in angle between the SCS and G vectors reduces the magnitude of the compression force across the GH joint. As a consequence, the head of the humerusmay slide down the now vertically oriented glenoid fossa.

Factors contributing to stability

- Stability at the GH joint is achieved by a combination of passive and active mechanisms. Active mechanisms rely on the forces produced by muscle. These forces are provided primarily by the embracing nature of the rotator cuff group. Passive mechanisms, on the other hand, rely primarily on forces other than activated muscle. At the GH joint the passive mechanisms include : (1) restraint provided by capsule, ligaments, glenoid labrum, and tendons; (2) mechanical support predicated on scapulothoracic posture; and (3) negative intracapsular pressure. 1. Ligaments - Glenohumeral capsular ligaments - Coracohumeral ligament 2. Joint capsule - Fibrous capsule strengthened by GH capsular ligaments 3. Glenoid labrum 4. Muscles - Rotator cuff (subscapularis, supraspinatus, infraspinatus and teres minor) - Long head of biceps brachii - Unlike the capsular ligaments, which produce their greatest stabilizing tension only when stretched at relatively extreme motions, muscles generate large, active stabilizing tensions at virtually any joint position. - The rotator cuff muscles are considered the "dynamic" stabilizers of the GH joint because of their predominant role in maintaining articular stability during active motions. 5. Biomechanics of scapulothoracic posture

Scapulohumeral rhythm

- Synchronous upward rotation of the scapula with humeral flexion or abduction. - Ratios vary among studies, generally GH:ST motion is about 2:1 - The scapula and humerus move in a systematic and coordinated rhythm. - The exact ratio of GH to scapulothoracic motion may vary according to the plane of motion and the location within the ROM. - the exact ratio of GH to scapulothoracic motion during active ROM is likely to depend on muscle activity.

Coracoacromial Arch

- The coracoacromial arch is formed by the coracoacromial ligament and the acromion process of the scapula. - The coracoacromial ligament attaches between the anterior margin of the acromion and the lateral border of the coracoid process. - Functional 'roof' of GH joint The space between the coracoacromial arch and the underlying humeral head is the subacromial space : - Arm at rest - 1 cm height in the healthy adult - It contains : 1. Subacromial bursa 2. Supraspinatus muscle and tendon 3. Biceps brachii long head 4. Superior capsule Functions of bursae : - protects muscle and tendons from friction with bone - The subacromial bursa lies within the subacromial space above the supraspinatus muscle and below the acromion process. - This bursa protects the relatively soft and vulnerable supraspinatus muscle and tendon from the rigid undersurface of the acromion. - The subdeltoid bursa is a lateral extension of the subacromial bursa - it limits frictional forces between the deltoid and the underlying supraspinatus tendon and humeral head

Mobility : What planes are involved in these movements? Which bone moves? What are the limiting structures? What is the purpose of these movements?

- The osteokinematics of the clavicle involve a rotation in all three degrees of freedom. - Each degree of freedom is associated with one of the three cardinal planes of motion: sagittal, frontal, and horizontal. - The clavicle elevates and depresses, protracts and retracts, and rotates around the bone's longitudinal axis. - The primary purpose of these movements is to place the scapula in an optimal position to accept the head of the humerus. - Essentially all functional movements of the glenohumeral joint involve some movement of the clavicle around the SC joint. - As described later in this chapter, the clavicle rotates in all three degrees of freedom as the arm is raised overhead Frontal plane : Elevation/depression Horizontal plane : Protraction/Retraction Sagittal plane : Posterior rotation

Upward rotation of the scapula (by upward rotators)

- The pull of the lower fibers of the serratus anterior on the inferior angle of the scapula rotates the glenoid fossa upward and laterally. These fibers are the most effective upward rotators of the force-couple, primarily because of their larger moment arm for this action - The upper trapezius upwardly rotates the scapula indirectly by its superior-and-medial pull on the clavicle. - The lower trapezius upwardly rotates the scapula by its inferiorand-medial pull on the root of the spine of the scapula

Describe the arthrokinematics of clavicular rotation at the sternoclavicular joint.

- The third degree of freedom at the SC joint is a rotation of the clavicle around the bone's longitudinal axis. - During shoulder abduction or flexion, a point on the superior aspect of the clavicle rotates posteriorly 20 to 35 degrees. - As the arm is returned to the side, the clavicle rotates back to its original position. - The arthrokinematics of clavicular rotation involve a spin of its sternal end relative to the lateral surface of the articular disc. - Axial rotation of the clavicle is mechanically linked with the overall kinematics of abduction or flexion of the shoulder and cannot be independently performed with the arm resting at the side.

Ligaments of GH joint

- To generate stabilizing tensions across the joint, the inherently loose capsular ligaments must be elongated or twisted to varying degrees; the resulting passive tension generates mechanical support for the GH joint and limits the extremes of rotation and translation. - By reinforcing the walls of the capsule, the capsular ligaments also assist with maintaining a negative intra-articular pressure within the GH joint. Middle GH ligament : - The ligament blends with the anterior capsule and broad tendon of the thick subscapularis muscle, then attaches along the anterior aspect of the anatomic neck. Inferior GH ligament : - The hammock-like inferior capsular ligament has three separate components: an anterior band, a posterior band, and a sheet of tissue connecting these bands known as an axillary pouch Coracohumeral ligament : - The coracohumeral ligament also blends with the superior capsule and supraspinatus tendon

The mechanics of posterior rotation of the right clavicle

- the relatively slackened coracoclavicular ligament while at rest in the anatomic position. - At the early phases of shoulder abduction, the scapula begins to upwardly rotate at the AC joint, stretching the relatively stiff coracoclavicular ligament. - The inability of this ligament to significantly elongate restricts further upward rotation at this joint. - Tension within the stretched ligament is transferred to the conoid tubercle region of the clavicle, a point posterior to the bone's longitudinal axis. - The application of this force rotates the crank-shaped clavicle posteriorly. (A) At rest in the anatomic position, the acromioclavicular (AC) and sternoclavicular (SC) joints are shown with the coracoclavicular ligament represented by a slackened rope. (B) As the serratus anterior muscle rotates the scapula upward, the coracoclavicular ligament is drawn taut. The tension created within the stretched ligament rotates the crank-shaped clavicle in a posterior direction, allowing the AC joint to allow full upward rotation.

arthrokinematics of clavicular protraction and retraction at the sternoclavicular joint.

1. ACL, anterior capsular ligament 2. CCL, costoclavicular ligament 3. PCL, posterior capsular ligaments. Superior view of a mechanical diagram of the arthrokinematicsof roll and slide during retraction of the clavicle around the right sternoclavicular joint. The vertical axis of rotation is shown through the sternum. Sternoclavicular joint contributes to overall scapulothoracic protraction and retraction

6 Kinematic Principles of Shoulder Abduction

1. Active shoulder abduction of 180°occurs as a result of simultaneous 120°GH abd and 60° scapulothoracic upward rotation (based on 2:1 scapulohumeral rhythm). 2. The 60° scapulothoracic upward rotation is the result of simultaneous elevation at the SC joint and upward rotation at AC joint. 3. The clavicle retracts at the SC joint, if abd in frontal plane. - Recall that in the anatomic position the clavicle lies approximately horizontal, about 20 degrees posterior to the frontal plane. - During shoulder abduction the clavicle retracts about another 15 degrees. - The retracting clavicle assists the AC joint with optimal positioning of the scapula within the horizontal plane. 4. The upwardly rotating scapula tilts posteriorly (and sometimes externally rotates slightly) at the AC joint. 5. The clavicle posteriorly rotates around its own axis. 6. The GH joint externally rotates if abdin frontal plane.

4 Joints that make up the shoulder complex

1. Sternoclavicular joint 2. Acromioclavicular joint 3. Scapulothoracic joint 4. Glenohumeral joint Before the kinematics of the sternoclavicular and acromioclavicular joints are described, the movements at the scapulothoracic joint must be defined. Primary movements at the scapulothoracic joint are traditionally described as elevation and depression, protraction and retraction, and upward and downward rotation.