Joints of the Shoulder Complex

Glenohumeral joint

This joint has the largest range of motion in the body. It is essentially part-spherical. In order to allow the wide range of motion, the concavity of the glenoid has a shallow rim, to delay impingement by the humerus as it approaches the limits of motion, that is augmented by a soft labrum. The glenoid labrum deepens the shallow concavity of the glenoid fossa thereby enhancing the stability of the shoulder joint. The shallow articulation means that the joint has little inherent stability, and so it depends on the coordinated actions of the muscles to maintain stability, particularly those which constitute the rotator cuff. The ligaments of the glenohumeral joint capsule only act to stabilize the joint when tightened, which only occurs at the limits of the ranges of motion.

The lack of restraint in the glenohumeral joint means that although it has a spherical geometry, there is usually some inherent laxity. The concave surface of the glenoid often has a greater radius of curvature (i.e. it is relatively flat) than does the articular surface of the head of the humerus. This leads to a relatively small area of contact between them. This has two side effects: firstly, it causes the joint contact pressure to rise; secondly, it allows some joint mobility. This is seen as a linear translation movement that is a slight subluxation of the humeral head relative to the glenoid as the head rotates. If the head were located in a glenoid with matching radius of curvature, then it would not be free to translate, and so it would be forced to spin in one place as the humerus rotates, and this causes a relative sliding motion between the articular surfaces. This is the case for the hip joint. The glenohumeral joint has some rolling motion mixed in with the sliding as the humerus starts a movement of external rotation. For example, the humeral head will initially roll posteriorly over the surface of the glenoid, perhaps for 3 mm. as it encounters increasing restraint from the slope of the labrum, so it tends to stop rolling and start sliding in one place.  Noting this arrangement for natural joints, some joint replacements have been made with their metal humeral head with a smaller radius than the polyethylene glenoid surface, in order to allow these small natural secondary movements. The downside to this, however, is the possibility of increased wear due to the raised contact pressure. The glenoid is somewhat egg shaped, with the narrower part superiorly, and it is typically 35 mm in superior-inferior and 25 mm in anterior-posterior directions. Sometimes, the face of the glenoid is angled away from the normal orientation, and this can be exaggerated by arthritic erosions, and leads to a tendency to instability, as the head of the humerus tends to slip off.

The articular surface of the humeral head is nearly hemispherical and is angled superiorly approximately 45 degrees and posteriorly approximately 30 degrees. This allows greater ranges of abduction and external rotation before impingement occurs. When transverse sections of the proximal humerus are viewed, the articular surface is seen not to be centered on the long axis of the shaft: it is offset posteriorly. The combination of offset and angulation is variable, and this is allowed for by some shoulder prosthesis designs.

There is little data available about forces acting across the glenohumeral joint, but a relatively simple analysis can show that it will reach three times body weight, and perhaps higher, during abduction to 90 degrees while supporting a load of 12% body weight in the hand.  This moment is shared by the deltoid with other muscles, which act to elevate the humerus and also to stabilize the head of the humerus into the glenoid, especially the supraspinatus. It is possible to apportion muscle forces using schemes which allow for the number of muscle fibers (the ‘physiological cross-sectional area) and the degree of muscle activation, that can be assessed by electromyography. However, all other muscles have a humeral elevation moment arm which is smaller than that of the deltoid, and this means that the simplification leads to a lower bound for the muscle force acting across the glenohumeral joint. The force vector diagram shows the relative magnitudes of the muscle and joint forces, and also their directions in the plane of the scapula. It is clear that the majority of the joint force is caused by the muscle action. The resultant joint force shown will cause the head of the humerus to press against the central superior rim of the glenoid. Activities such as shoulder flexion or abduction while holding a weight of 2kg in the hand lead to joint forces of approximately 1.5 body weight. These forces act centrally and superiorly or superiorly and anteriorly onto the glenoid, giving a tendency for the head of the humerus to sublux superiorly. When this occurs, the subluxation is limited by the head of the humerus impinging against the coracoacromial ligament, which passes in an anterior-posterior direction above the head of the humerus.

Articular cartilage
Hyaline articular cartilage covers the joint surfaces of synovial joints. It has two major functions: to distribute joint loads widely in order to minimize local stresses, and to minimize friction and wear between opposing surfaces. Articular cartilage is highly specialized for these functions and generally achieves them throughout life without failing despite the rigorous mechanical environment. Cartilage lacks blood vessels, lymphatics, and nerves. The synovial membrane of a joint is usually attached around the margins of the articular cartilage of the joint. Collagen and proteoglycans together with water account for the mechanical properties of collagen. Cartilage allows joints to self lubricate and the loss of this ability following damage may be an important factor in the etiology of osteoarthritis.

The glenoid fossa is lined by hyaline cartilage, thinner in the middle of the fossa and thicker towards the rim.

Sternoclavicular joint

A shallow saddle shaped joint between the manubrium and first costal cartilage medially and the medial end of clavicle laterally.  The articular surface of the clavicle is larger than that of the sternum, and is convex vertically and slightly concave in the sagittal plane.  The clavicular notch of the sternum is reciprocally curved, although the surfaces are not completely congruent.  The joint is lined with fibrocartilage, and a fibrocartilaginous disc divides it into medial and lateral compartments, each lined with its own synovial membrane.  The fibrous capsule surrounds the articular surfaces, thickened in front and behind but thin above and below.
The brachiocephalic vein is formed posterior to the sternoclavicular joint.

Acromioclavicular joint

The acromioclavicular (AC) joint is a plain synovial joint between the small oval facet on the lateral end of the clavicle and the facet at the medial aspect of the acromion. The articular surfaces are lined with fibrocartilage.
A wedge-shaped articular disc is often found in the AC joint, partially separating the joint surfaces. The capsule is attached to the articular margins and reinforced by superior and inferior acromioclavicular (AC) ligaments. The coracoclavicular ligament, with its conoid and trapezoid components provides major stability to the joint.

Biomechanics of the shoulder complex

The shoulder complex is a term which encompasses three bones: the clavicle, scapula, and humerus, the two joints which link these bones: the acromioclavicular joint and the glenohumeral joint, and the two joints which attach the shoulder complex to the rest of the body: the sternoclavicular joint and the scapulothoracic joint. It is clear that the wide range of shoulder mobility, which is greater than that of any other joint in the body, depends on these joints working together synchronously, in order to allow the arm and hand to be positioned where desired in space around the body. Not only can the humerus be positioned in a space exceeding a hemisphere, it can also be rotated while in this range of positions. This requires the glenohumeral joint to be spherical, in order to maintain congruent articulation.

Movement of the humerus away from the side of the body is known as shoulder elevation, and this is given different names depending on the direction that the arm is moved. Anterior movement in the sagittal plane is shoulder flexion, and is usually 180 degrees, while posterior motion, shoulder extension in the sagittal plane, can reach 70 degrees. If the arm is elevated to the side, this can be either true lateral abduction, or, more functionally, abduction in the plane of the scapula, which is angled approximately 30 degrees anteriorly. Abduction typically reaches 170 degrees, but some subjects are more mobile and can both flex and abduct their shoulder more than 180 degrees. Abduction with the humerus in neutral rotation is limited at the glenohumeral joint by impingement of the greater tuberosity of the humeral head against the coracoacromial complex of the scapula, which occurs at approximately 120 degrees abduction. In order to obtain the full normal range of motion, there is a coupled secondary external rotation of the humerus which occurs automatically as abduction progresses further, that carries the greater tuberosity posteriorly to avoid impingement. Ranges of motion tend to decrease with advancing age.

If the shoulder is slightly flexed, so that the humerus is brought anterior to the thorax, the shoulder can be adducted, bringing the arm towards the centerline of the body, by approximately 60 degrees. The arm can also be moved in a horizontal plane after abducting the shoulder 90 degrees, and this allows the arm to move across the front of the body to approximately 140 degrees range of motion, and 45 degrees posteriorly from the lateral direction. Thus, the shoulder complex allows an overall range of motion of 180 degrees in almost all directions. This range of motion of the humerus in space is accomplished by motion of the scapula relative to the thorax, in addition to the dominant glenohumeral motion. This sharing of mobility has been studied most  extensively for humeral abduction in the plane of the scapula. Different researchers have found variations in motion, but overall it seems that 60% of this motion arises at the glenohumeral joint and 40% at the scapulothoracic joint. Similarly, shoulder motion with the humerus in a horizontal plane includes scapular protraction and retraction movement. This scapular mobility is essential for maintaining some reduced shoulder function if arthritis with loss of bone requires an arthrodesis of the glenohumeral joint. Scapular motion is linked to the clavicle, which controls the position of the acromioclavicular joint in relation to the thorax at the sternoclavicular joint.

Rotation of the humerus can also compound the mobility of the shoulder complex. When the arm is alongside the thorax with the elbow flexed 90 degrees, internal rotation is limited by the hand reaching the body, while external rotation is limited to 90 degrees by tightening of the soft tissues crossing the anterior aspect of the shoulder, such as the anterior glenohumeral ligaments and subscapularis muscle. Similar ranges of humeral rotation are found when the humerus is rotated with the shoulder abducted 90 degrees. If the elbow is extended when rotation occurs, the hand rotates further than the humerus, due to the additional mobility of the radioulnar joints in the forearm.

The great mobility of the shoulder complex allows the hand to be positioned in space; it also allows the hand to reach to a particular point via different paths of motion. This adaptability has led to some apparently paradoxical observations on complex shoulder motion, most notably that known as Codman’s paradox. This paradox poses the question of whether the humerus is in a position of internal or external rotation when the forearm is held horizontally above the head.  If the humerus is returned to the side of the body by flexion of the glenohumeral joint in the sagittal plane, the forearm ends up across the front of the body, so the humerus is in internal rotation. Conversely, if the humerus is lowered by rotating in an arc of adduction in the coronal plane, the forearm ends up pointing away from the body – the humerus clearly in external rotation. This paradox arises because the path taken through space, and the final destination, are affected by the sequence in which rotations occur about perpendicular axes. This phenomenon is well known among engineers, who refer to the three mutually perpendicular rotations as pitch, yaw and roll, and conventions are used to ensure uniformity of the sequence of application of the rotations and, therefore, of the motion described.

Sternoclavicular joint

This joint has a relatively flat saddle shaped articulation to accommodate movements of the clavicle, with a meniscus, that acts to spread the load over the joint surfaces. During shoulder abduction/adduction, the clavicle is elevated/depressed; during anterior/posterior movements of the shoulder (protraction/retraction), the clavicle swings with the shoulder. Thus, the sternoclavicular joint allows the clavicle to move to any point within a cone shaped volume in space. Although this motion may appear to cause a rotation of the clavicle, this is not so: the motion is primarily circumduction, in which the circular path of the lateral end of the clavicle is attained by differing combinations of elevation/depression and protraction/retraction, with approximately 30 to 40 degrees for each range of motion.  There is sufficient mobility here for the clavicle to rotate about its long axis to a limited extent. The major load on this joint arises from the clavicle pressing towards the centerline of the body. This causes a tendency for the joint to sublux, and this is resisted by the compressive joint force and the strong sternoclavicular ligaments which connect the medial clavicle inferiorly to the first rib and sternum.

Acromioclavicular joint

This joint is loaded primarily in compression by the actions of the muscles which pull the shoulder towards the centerline of the body, such as the anterior pectoral muscles. There is also a superior-inferior shearing load, due to muscles which tend to elevate the clavicle, but this is counteracted by the inferior component of the tensions in pectoralis major and anterior deltoid, which pass to the humerus from the clavicle. Since the acromioclavicular joint surfaces are oblique, the loads applied tend to cause the lateral end of the clavicle to sublux superiorly if the joint is disrupted: the lateral end of the clavicle tends to slide ‘uphill’ across the slope of the joint surface on the acromion. This tendency is resisted by the coracoclavicular ligaments, damage to which allows the characteristic  superior prominence of the end of the clavicle. The acromioclavicular ligaments which surround the lateral end of the clavicle are slack enough to allow motion between the clavicle and the scapula, with a relative scapular rotation of approximately 20 degrees during humeral abduction, and shearing actions at the joint during shoulder protration/retraction, when the motion is centered at the conoid part of the coracoclavicular ligaments. Similarly, shoulder abduction-adduction motion causes the scapula to rotate about a horizontal anterior-posterior axis relative to the clavicle (which is itself rotating similarly about the sternoclavicular joint), that is centered amidst the coracoclavicular ligaments. Since these attach away from the end of the clavicle, this motion also causes the end of the clavicle to slide across the surface of the acromion as the scapula rotates.

Scapulothoracic joint

The scapulothoracic joint is not a true articular joint, but simply a plane of separation of the scapula and the subscapularis muscle from the thorax (rib cage), that allows relative motion. The thoracic surface here is the superficial aspect of the serratus anterior overlying the ribs. It allows the scapula to slide anteriorly/posteriorly around the rib cage with shoulder protraction/retraction. As it does so, the scapula rotates about a vertical axis, since the ribs are curved, and so shoulder protraction causes the glenoid to face more anteriorly, and more posteriorly with retraction. The scapula also rotates in its own plane as the humerus is abducted/adducted. In shoulder abduction, approximately 40% of the humeral abduction motion occurs at the scapulothoracic articulation. There is typically some 75 degrees of scapular upward rotation in 180 degrees abduction. By rotating the scapula, the glenoid is angled superiorly as the humerus is abducted, and so superior impingement is delayed. Further, the superior angulation of the glenoid allows the compressive joint force to remain within the articular concavity, which is stable. If the scapula did not abduct with the humerus, then the muscles would pull the head of the humerus into inferior (caudal) subluxation. Rotation of the scapula in its own plane is caused by coordinated actions of the serratus anterior and the upper and lower parts of the trapezius muscles: their actions can create a rotation moment acting onto the scapula without a resultant force tending to move it linearly. Scapular protraction and retraction are due to actions of serratus anterior, and rhomboid plus trapezius actions, respectively.

Movement at the scapulothoracic joint is essential to patients who have their glenohumeral joint arthrodesed (artificial surgical ankylosis), as some upper limb mobility still remains.



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