VH Dissector: Upper Limb: Scapular Region

In the scapular region we have 2 sets of muscles we should consider. 

1. Those that move the scapula on the thorax, known as the scapular muscles. The majority of the scapular muscles are also the superficial back muscles.   Note the difference in this list from the listed steps in the superficial back muscle section.

2. Those that stabilize the humerus on the glenoid fossa known as the rotator cuff muscles.  

The Trapezius Muscle is a large diamond shape muscle of the superficial back that has three sections; Upper, Middle, Lower. Each section of the trapezius muscle generates individual motions of the scapula on the rib cage, but together the each assist with upward rotation of the scapula on the thorax. Refer to your assigned textbook for the Origin, Insertion, Innervation, Action, and Blood supply.

Rotate - the cadaver to visualize this muscle.

The Rotator Cuff consists of four muscles that join the scapula to the humerus (scapulohumeral muscles): supraspinatus, infraspinatus, teres minor, and subscapularis.

These muscles cover all of the glenohumeral joint, except the inferior aspect. All of them are consider rotators of the humerus, however their primary responsibility is stabilizing the humerus in the center of the glenoid fossa. The rotator cuff provides dynamic stability by the concave-compression mechanism, compressing the round head of the humerus into the concave glenoid fossa of the scapula.

Other muscles attached to the scapula that are not considered scapulothoracic muscles or rotator cuff muscles and  help to move the shoulder joint are the Deltoid and the Teres major.

The Deltoid has three portions; anterior, middle, and posterior. It is able to move the humerus in all 3 planes for 8 total motions, flexion/extension, abduction/adduction, medial rotation/lateral rotation, and horizontal abduction/horizontal adduction.

Spaces of the Shoulder

There are three spaces around the shoulder girdle. These are the Subacromial Space, Quadrangular Space, and the Triangular Space..

Subacromial Space - aka suprahumeral space. It is the space between the Acromion process and the humeral head.  Within this space there are 4 structures running from superficial to deep, the subacromial bursa, the supraspinatus tendon, the superior capsule, and the long head of the biceps tendon.

Quadrangular Space - it is an area of the posterior shoulder in which the posterior humeral circumflex artery and the axillary nerve pass. Lateral border of the space is the surgical neck of the humerus, Medial border is the long head of the triceps, Superior border is teres minor, and the inferior border is teres major. Because the lateral border of the space is the surgical neck of the humerus a fracture may put the axillary nerve and posterior humeral circumflex artery at risk.

Triangular Space- it is an area in which the circumflex scapular artery and radial nerve passes. The medial border is long head of triceps, superior border is teres major/latissimus dorsi, and lateral border is the humerus. 

Joints of the Shoulder Complex

Sternoclavicular Joint - It is a synovial joint that is the only connection between the upper extremity and the trunk. The proximal end of the clavicle articulates with the manubrium of the sternum. The two articulating surfaces are saddle shape. Congruence and stability of the two surfaces is improved by a complete fibrocartilage intraarticular disc. The disc's primary responsibility is to resist medially directed (compressive) forces. The disc attaches to the entire joint capsule separating the joint into two distinct joint spaces. There is a weak joint capsule around the joint that is reinforced by the anterior and posterior sternoclavicular ligaments. There is also an interclavicular ligament that passes from the superiomedial aspect of each clavicle to attach on the upper border of the manubrium. Helps resist inferior displacement of the lateral clavicle. The costoclavicular ligament runs from the first rib superior laterally to the clavicle, resisting superior displacement of the lateral clavicle.

Injuries to the sternoclavicular joint are uncommon, particularly dislocations. Dislocations generally are anterior or posterior in nature. Posterior dislocations are considered a medical emergency because of the potential impingement of the mediastinal and cervical structures that run in this area. Compression of the trachea and/or esophagus is also a potential serious complication of a posterior dislocation.

Refer to your assigned textbook for further information.

Acromioclavicular Joint - Is a synovial joint that is made up of the lateral end of the clavicle articulating with the medial border of the acromion process. There is a partial fibrocartilage disc within the joint. The AC joint is enclosed with a relatively loose joint capsule. This capsule is reinforced with the superior and inferior acromioclavicular ligaments. 

The coracoclavicular ligament is an extra-capsular ligament that is the primary stabilizing structure to maintain the orientation of the clavicle and scapula. This ligament is separated into two distinct portions:

1. The Conoid- which is a cone shape ligament that runs from the coracoid process to the conoid tubercle of the clavicle. This portion of the ligament also plays a vital role in the biomechanics of the shoulder. 

2. Trapezoid- this portion arises from the coracoid process and inserts on the trapezoid line of the clavicle, just lateral to the conoid tubercle. It plays a vital role in the biomechanics of the shoulder. 

The coracoacromial ligament runs from the superior, lateral coracoid process to the acromion, just anterior to the AC joint completing the roof of the superohumeral space. This ligament does not play a role in AC joint stability. It acts as a counter balance to the pulling forces placed on the coracoid process by the pectoralis minor and coracobrachialis muscles. The most significant role of this ligament traditionally has been described in the pathology of impingement syndrome of the supraspinatus tendon. However, contemporary biomechanics finds fault in this description of non-specific shoulder pain as the supraspinatus tendon is no longer under the acromion process and coracoacromial ligament beyond 70 degrees of elevation.

Refer to your assigned textbook for further information.

Scapulothoracic Articulation - The gliding of the concave shaped scapula on the convex shaped rib cage is commonly called the scapulothoracic joint, however, it does not demonstrate all the traditional components of a joint. In fact, the only relation it has to a joint is it articulates and moves among 2 fascial planes, therefore, it is more correctly referred to as pseudojoint.

This articulation is extremely important in maintaining, and allowing, stability of the upper extremity. The scapulothoracic articulation contributes largely to the overall range of motion (ROM) of the shoulder. The movement of the scapulothoracic articulation along with the glenohumeral joint in contributing to shoulder ROM has been termed the “scapulohumeral rhythm. Scapulothoracic rotation contributes approximately 60° of motion out of the 180° of motion available at the shoulder.  Scapulothoracic motion for normal scapulohumeral rhythm requires normal strength and timing of the force couple for upward rotation and scapular retraction for stability.

Refer to your assigned textbook for further information.

Glenohumeral Joint - This is what is usually thought of as the “shoulder joint”. It consists of the head of the humerus articulating with the glenoid fossa of the scapula. It is a diarthrodial joint that has a high degree of mobility. The joint is inherently unstable because of the shape and contour of the two articulating surfaces. The humeral head is approximately twice the size of the glenoid fossa. This means there has to be a good combination of both static and dynamic stabilizers to prevent chronic dislocation of the glenohumeral joint.

Static stability of the glenohumeral joint is provided by a joint capsule which has redundancies that form the glenohumeral ligaments. There are three glenohumeral ligaments:

1) Superior Glenohumeral - The superior glenohumeral ligament (SGL) is the smallest and least understood of the glenohumeral capsular structures, and is reported to be present in 90% to 97% of shoulder studies. It originates from the upper pole of the glenoid cavity and the base of the coracoid process, and is attached to the MGL, to the biceps tendon, and to the labrum. It inserts just superior to the lesser tuberosity in the region of the bicipital groove. There is a normal foramen or opening between the SGL and MGL, allowing communication with the subscapularis bursa. The SGL is closely related to the extraarticular coracohumeral ligament. The coracohumeral ligament originates in the lateral aspect of the coracoid and inserts on the greater tuberosity. The SGL and the coracohumeral ligament contribute to the stabilization of the glenohumeral joint and prevent posterior and inferior translation of the humeral head. When present and well-formed (developed), the SGL represents the primary capsuloligamentous restraint to inferior translation of the unloaded, abducted shoulder joint.

2) Middle glenohumeral ligament. The middle glenohumeral ligament (MGL) attaches to the anterior aspect of the anatomic neck of the humerus, medial to the lesser tuberosity. It arises from the glenoid by way of the labrum and scapular neck. Of the three glenohumeral ligaments, the MGL demonstrates the greatest variation in size and thickness. It may present as thin ligamentous tissue or appear cord-like and as thick as the biceps tendon. Wall and coworkers found that it was absent in up to 27% of specimens. When present, the MGL can be identified between the subscapularis tendon (as it passes across the subscapularis) and the anterior labrum or anterior band of the IGL. The MGL demonstrates a more vertical orientation with internal rotation and a horizontal orientation (elongation) with external rotation of the shoulder. The MGL functions in the stability of the shoulder joint from 0º to 45º of abduction. Along with the subscapularis tendon and the superior part of the IGL, the MGL contributes to anterior stability at 45º of abduction. In the lower and mid-ranges of abduction, it limits external rotation. The MGL has been shown to have a secondary role in anterior stability of the shoulder in 90º of abduction when the anterior band of the IGL is cut. Inferior translation of the abducted and externally rotated shoulder is limited as a secondary restraint function of the MGL

3) 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.

Coracohumeral ligament- it is a thick band that spans from the base of the coracoid process to the capsular ligament near the greater tuberosity. It is the ligament that becomes bound down during immobilization of the shoulder and thus restricting external rotation.

Glenoid Labrum - The glenoid labrum is made up of fibrocartilage. The labrum is a continuous structure surrounding the glenoid rim. Despite the continuous structure there are distinct differences in the morphology when comparing the superior and inferior portions. The superior and anterior portions have loose attachments to the glenoid rim. The inferior labrum, however, has a much firmer attachment. This may be because of the attachment of the glenohumeral ligaments in this area.

The main function of the labrum is to deepen the glenohumeral joint to increase the stability of the joint. The labrum doubles the depth of the glenohumeral joint. It also acts as a “bumper” against humeral head translation and as a shock absorber. The superior labrum also serves as an attachment site for the long head of the biceps tendon. This tendon has about 50% attachment on the labrum and 50% attachment on the supraglenoid tubercle. There are serious clinical implications here which will discuss later, with the biceps tendon itself.

The vascularity of the labrum is limited to the periphery and is more prevalent in the inferior and posterior portions. The blood flow comes from branches of the suprascapular artery, circumflex scapular artery, and the posterior humeral circumflex artery

Bursae

There are several that lie within the vicinity of the shoulder joint. They are flattened fluid filled sacs that lie between tendons and bone. They help reduce the friction of a moving tendon on a bone or skin rubbing on a bony surface.

Subacromial Bursa- lies between the deltoid muscle and the supraspinatus tendon and the fibrous capsule of the shoulder. It is located inferior to the acromion process and coracoacromial ligament. This is bursa is susceptible to trauma via repetitive overhead actions leading to subacromial bursitis (a form of impingement syndrome).

Credits:

VH Dissector steps modified for Drexel Dissector by Dr. Haviva Goldman from original website activity created by Jeffrey Fahl, MD, Kyle Petersen, PhD, Richard Drake, PhD, Alesha Petitt, MA, Claira Ralston, MS and Kim Price, MA and modified by Jeffrey Fahl, MD, Michael Smith, PhD, Albany Medical College.