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Advanced Anatomy: The Shoulder Girdle (2 CE Hours)

This course is approved for 2 hours of Continuing Education for Massage Therapists by the Texas Department of State Health Services: Approved Provider: MARK SCOTT URIDEL CE0009. Mark S. Uridel is approved by the National Certification Board for Therapeutic Massage and Bodywork (NCBTMB) as a continuing education provider. This course is approved for 2 hours of continuing education by the Yoga Alliance.

This informational article provides advanced anatomical study of the shoulder girdle. 

Learning Objectives: After reading this course, you will...

  1. be able to identify the bones, ligaments and muscles associated with the shoulder girdle.
  2. be able to explain the importance of the rotator cuff muscles in stabilizing the shoulder joint.
  3. be able to describe the movement of the shoulder joint and shoulder girdle.
  4. be able to explain the improtance of the stabilizers of the shoulder complex.
  5. be able to understand the mechanism of action of the muscles of the shoulder girdle.
  6. be able to identify the origin, insertion and innervation of the muscles of the shoulder girdle.

Introduction

The shoulder girdle is one of the most important multiple joint complexes of the human body.  Together the various joints that make up the shoulder girdle have a huge impact on function related to occupational activities and daily activities. It is important to understand the anatomy of the shoulder girdle so that we can better serve our clients and patients.

Bones of the Shoulder Girdle

The shoulder girdle is comprised of the humerus (upper arm bone), the scapula (shoulder blade) and the clavicle (collar bone).

Humerus
Proximal End (Head)
The head forms one third of a sphere and faces posteromedially to articulate with the glenoid fossa of the scapula.  Distal to the head are the greater and lesser tuberosities which merge distally into a rounded shaft.

The intertubercular or bicipital groove, which houses the long head of the biceps, is located between the greater and lesser tuberosities along the anterior surface of the humerus.

Biomechanics of the Humerus

The structure of the humerus is adapted to transmit the loads imposed by the muscles which position the arm in space and allow the hand to act forcefully on outside objects, such as pushing or pulling a lever. The hemispherical articular surface of the head of the humerus is adapted to allow the wide range of motion of the glenohumeral joint (Movie 1, 2, 3) and also to transmit the forces caused by the muscles.

The humerus can be loaded in many ways, due to the wide range of positions and loading activities encountered in daily life. Because of this, the diaphysis is formed as a cylindrical tube, a structural form which is optimal for situations requiring strength in both bending and torsion. These loading modes are often acting in combination, such as when the palms of the hands are pressed together with the elbows flexed 90 degrees. In this situation, the humerus is acted on mainly by the anterior pectoral muscles, which act both to adduct the humerus and to internally rotate it. Due to the distance from the head of the humerus to the points of insertion of the pectoral muscles, there is a significant bending action in the shaft in addition to the torsion caused by the length of the flexed forearm.

Because the head of the humerus must resist compressive joint forces from a wide range of directions, it has an extensive area of articular cartilage, to keep the contact pressure to an acceptable level. The head is retroverted from the medial direction, so that it is facing onto the glenoid when the arm is held with the humerus alongside the body and the forearm pointing anteriorly, which is the normal functional posture, since this places the central axis of the articular surface of the humeral head in the same plane as that of the scapula. The head of the humerus consists primarily of cancellous bone, with trabeculae oriented to support the subchondral plate. At the lateral superior edge of the head of the humerus, the greater tuberosity is positioned to give the greatest possible lever arm for the action of the supraspinatus muscle when abducting the humerus. Similarly, the lesser tuberosity, situated anteriorly, provides a prominent attachment for the subscapularis, which aids the muscle in internal humeral rotation actions by increasing its moment arm about the longitudinal axis of motion.

Scapula
This flat triangular bone lies obliquely over the posterior aspect of the chest wall overlying 2nd to 7th ribs.  The thin flattened body has costal and dorsal surfaces, superior, medial and lateral borders, superior, inferior and lateral angles.  The lateral angle is expanded to form the head.  This has a shallow concave articular surface, facing laterally and slightly anteriorly, for articulation with the humerus. The subscapular fossa represents the costal concave surface of the scapula.
The dorsal surface is convex, and is divided into two unequal areas by a shelf like projection, the spine, with the smaller supraspinous fossa above and the larger infraspinous fossa below. The spine is triangular, and projects horizontally from the upper part of the dorsal surface of the scapula. Its posterior border is thickened to form the crest. The acromion arises from the lateral aspect of the spine. The coracoid process arises from the upper part of the head and neck.  It projects superiorly, and then bends sharply forwards.  The scapular notch interrupts the superior border just medial to the base of the coracoid process.  

Biomechanics of the scapula
This bone acts primarily to provide extensive attachment areas for muscles that contribute to the rotator cuff: supraspinatus, infraspinatus and subscapularis. They focus their tensions across the glenohumeral joint, pulling the head of the humerus medially, thus causing the compressive joint force component that stabilizes the head of the humerus into the relatively shallow articulation of the glenoid. The blade of the scapula can be transparently thin, and it is protected from buckling failure, under the actions of the muscles which attach across it, by the spine of the scapula, that separates the attachments of the supraspinatus and infraspinatus muscles. The scapula depends on the clavicle to control its position, as the link to the thorax is somewhat tenuous. The scapula is effectively suspended below the lateral end of the clavicle by the coracoclavicular ligaments, and is also joined to it at the acromioclavicular joint, which is surrounded by acromioclavicular ligaments.

Clavicle
The clavicle connects the axial and appendicular skeletons of the upper extremity.  It has an S-shaped configuration, with a convex anterior border medially and a concave anterior border laterally.  It is flattened and narrowed laterally, and is thicker and more cylindrical medially.  The clavicle articulates with the sternoclavicular joint medially and with the acromioclavicular joint laterally.  The surfaces of the sternoclavicular joint are covered by fibrocartilage, and a fibrocartilaginous articular disk divides the joint into separate recesses.

The medial end is roughened inferiorly by the insertion of the costoclavicular ligament.  This may form a depression, the rhomboid fossa.  The lateral end is flattened, with superior and inferior surfaces, the inferior surface roughened by the coracoclavicular ligaments. 

Pectoralis major is attached to the anterior surface medially, with deltoid attached laterally.  Sternocleidomastoid is attached to the superior surface at its medial end.  Trapezius is attached to the posterior surface laterally.  Subclavius is attached to the inferior surface. 

The clavicle ossifies from two primary centers which appear in the fifth week of embryonic life.  A secondary center is formed in the cartilage at the medial end, appearing in the teens or later.  The clavicle lacks a medullary cavity.

Biomechanics of the Clavicle
This bone acts primarily to control the position of the shoulder in space, in relation to the thorax. This is because the muscles acting on the shoulder, particularly the trapezius posteriorly and the pectoral muscles anteriorly, pull the shoulder towards the centerline of the body in many actions. Because the clavicle is relatively rigid, this means that the position of the shoulder is controlled to be on a sphere in space that is centered at the sternoclavicular joint. The muscle actions cause the clavicle to act as a strut in axial compression, with loads being transmitted from the acromioclavicular joint laterally to the sternoclavicular joint medially. The clavicular attachments of pectoralis major and the anterior part of deltoid act to bend it, but it may fail by a buckling mode leading to fracture under excessive loads.      

Coracoacromial ligament

These act to prevent disruption of the acromioclavicular joint, under the influence of the muscle forces and other loads acting on the clavicle and scapula.

The two main parts of this ligament complex, conoid and trapezoid, attach to the clavicle over a wide area, and this causes different parts of them to become tight as the shoulder, and with it the clavicle, moves. Reconstruction of these ligaments is difficult because of the tensile loads which they carry in normal activities, compounded by the requirement to maintain the normal flexibility of the acromioclavicular joint, which does not allow normal methods of rigid internal fixation.

Inferior glenohumeral ligament: axillary pouch
The inferior glenohumeral ligament (IGL) forms the thickest part of the joint capsule and is the largest and most important of the glenohumeral ligaments. It consists of three components, the anterior band, the axillary pouch, and the posterior band.  The anterior and posterior bands are attached to and contribute to the formation of the anterior and posterior glenoid labrum.  In adduction, the IGL is lax.  It tightens with increasing abduction, and the anterior and posterior bands move superiorly with respect to the humeral head.  At 90º of abduction, the IGL is the primary restraint for anterior and posterior dislocations.  The axillary pouch is located between the anterior and posterior bands and attaches to the inferior two-thirds of the entire circumference of the  glenoid by means of the labrum.  Like the anterior and posterior bands, it is lax in the adducted position with the arm by the patient's side.  It extends inferior to the body of the glenohumeral joint as a redundancy of thickened capsular tissue and is best visualized on coronal oblique images. 

The inferior glenohumeral ligament complex (IGLC) originates from either the glenoid labrum or glenoid neck and inserts into the humeral neck at the periphery of the articular margin.  The anterior band forms the anterior labrum at the medial attachment of the IGL to the glenoid.  This relationship is sometimes called the inferior glenohumeral ligament labral complex, or IGLLC, because of the importance of the IGLC in forming the anterior labrum.  The posterior band contributes to the formation of the posterior labrum. The IGLC functions as a hammock, cradling the humeral head as abduction increases.  Different portions of the complex support the humeral head both anteriorly and posteriorly during 90º of abduction with internal and external rotation.

Inferior glenohumeral ligament
As well as the many muscles that stabilize the glenohumeral joint, the joint capsule includes several thickened bands which are ligamentous. The inferior glenohumeral ligament is perhaps the most important of these, being associated with anterior dislocation of the head of the humerus. When this occurs, the capsule may be pulled away from the anterior aspect of the labrum and glenoid, causing a Bankart lesion. The inferior glenohumeral ligament attaches to the anterior aspect of the labrum and from that to the glenoid, and to the humerus anteriorly and inferior to the center of the articulation, around the inferior margin of the articular cartilage. These attachments cause the ligament to be tightened by both external rotation and abduction of the glenohumeral joint, when it becomes the primary restraint against anterior humeral subluxation.

Coracohumeral ligament
The coracohumeral ligament, which originates on the lateral aspect of the base of the coracoid process inferior to the origin of the coracoacromial ligament, courses in a horizontal or transverse direction to its insertion on the greater tuberosity on the lateral aspect of the bicipital groove. 
The coracohumeral ligament lies in the interval between the anterior margin of supraspinatus tendon and the superior margin of subscapularis tendon, (the rotator cuff interval), blending with adjacent tendons and with the underlying joint capsule.  At the anterior superior aspect of the shoulder, it overlies and is superficial to the superior glenohumeral ligament.

Glenohumeral joint capsule

The loose capsule is attached proximally to the labrum and glenoid rim, distally to the articular margins of the head of humerus, except inferiorly where it extends 1-2cms onto the neck.  The capsule is lax inferiorly allowing greater mobility.  Posteriorly and inferiorly, the capsular insertion is directly onto labrum.  Superiorly, the capsule is attached to the glenoid rim at the base of the labrum, and includes the origin of long head of biceps tendon.  Anteriorly, the attachment is more complex, due to the attachment of the middle glenohumeral ligament and the synovial recesses. The capsule is reinforced by the tendons of the rotator cuff, and the tendon of long head of triceps below.   There are commonly two openings in the capsule, for the passage of long head of biceps tendon between the humeral tuberosities and an anterior opening which communicates with subscapularis bursa.  A third opening may be present posteriorly, and allows communication between the joint space and a bursa beneath infraspinatus.

 

 

 

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