A Review - Basic Anatomy

Here are a few links that offer pictures of the many of the discussed structures:

From the University of Newcastle upon Tyne, "Shoulder anatomy made easy by [email protected]"

The scapula as seen from the fron of the body.

The scapula as seen from the back.

The front of the humerus

Skeletal structures

The shoulder complex is intricately connected with the spine. The spine consists of different type of vertebrae. From top to bottom they are: cervical (there are 8), thoracic (12), lumbar (5), sacral (5, but they're fused together) and coccygeal (4 are fused together). For the shoulder only the cervical and the first thoracic vertebrae play a role. When they are mentioned they are labeled as such: C5-C6. This indicates the 5th and 6th cervical vertebrae.

The shoulder consists of three bones: the clavicle, the humerus and the scapula. The humerus forms a joint with the glenoid fossa (part of the scapula), the clavicle joins with the acromion of teh scapula (forming the AC joint) and the coracoid process of the scapula floats over the entire structure.

The clavicle acts as a strut keeping the shoulder and arm braced away from the thorax. The clavicle also forms a canal between the neck and the arm. Th clavicle sits above this canal, protecting the vessels running to and from the arm.

The scapula is built like a big triangle, made from smaller triangles. The spine of the scapula divides the scapula into top and bottom parts.

1. The infraspinous fossa - the lower triangle of the scapula provides attachment sites for the large muscles
2. The spine of the scapula - the divider of the scapula, it runs from the medial edge to the lateral edge. At the lateral edge it forms the acromion. The acromion is an arch that turns toward the front of the body. The clavicle attaches to the acromion at the top of the shoulder. This structure can be felt bu following the collarbone out to where it enters the shoulder and forms the A-C(acromion/clavicular) joint.
3. The glenoid fossa - this is found on teh front side on the lateral edge of the spine of the scapula. The glenoid forms the flat part (facing laterally) that provides the rigid joint for the head of the humerus. This is where the arm joins the rest of the body.
4. The coracoid process - this beak-like projection is found above the glenoid from the lateral edge of the upper triangle. The arm rotates below the coracoid and several ligaments attach here.

The humerus is the large bone of the upper arm. Despite the appearant singularity of this bone, knowledge of its structure is essential in understanding the working of the shoulder. The four structures of importance for the shoulder are:

1. The head - This is the top of the upper arm. It forms a ball like structure. The head of the humerus fits into the shallow glenoid fossa.
2. The intertubercular groove of the humerus - is the groove between the greater and lesser tubercles.
3. The greater tubercle - A tubercle is an elevated rough spot on bone to which muscles attach. Found at the lateral margin of the humerus, the greater tubercle is the point of attachment of some of the rotator cuff muscles.
4. The lesser tubercle - is found at the front of the humerus. This anchors one of the muscles of the rotator cuff as well as forming a border for the intertubercular groove.

Musculature

The muscles at the front of the shoulder

The pectoralis muscles (major and minor) run from the front of the chest to the shoulder. The pectoralis major muscle is the large muscle of the chest. It helps to bring the arm toward the body on the side and in front of the body from the sides. The pec. major attaches to the clavicle and to the intertubercular groove from the sternum; it acts as an ADductor and performs internal rotation.

The pec. minor sits underneath the pectoralis major. It runs from the middle of the chest to the ***************. ; these muscles depress the scapula and pull it anteriorly.

The subclavius runs immediately below the clavicle; it acts to lower the clavicle.

The serratus anterior runs along the side of the body to inside of the shoulder bladeattaches. This muscle externally rotates the scapula.

The posterior muscles of the shoulder are:

• The trapezius is the biggest muscle of the neck. This muscle runs from the neck and spine out to the shoulder. This large muscle moves the shoulder blade up and down, as well as pointing the shoulder blade toward the back.¹.
• The latissimus dorsi are the large muscles along the body under the armpit. This muscle extends, adducts, internally rotates the arm. It also raises the body toward the arm while climbing.
• The levator scapulae runs from the C1-C4 cervical vertebrae to the top of the scapula. This muscle elevates the scapula and rotates is, pointing the glenoid fossa down.
• The deltoid is the biggest muscle in the shoulder. This is the muscle that runs over the shoulder joint down to the arm. Parts of the muscle come from the back, the top and the front of the shoulder and run to the top of the outer part of the arm. The deltoid has many functions: it flexes, it extends, it medially rotates, it laterally rotates, and it abducts the arm.
• The teres major runs from the bottom of the scapula to the intertubercular groove. This muscle adducts and medially rotates the arm.

The rotator cuff muscles are commonly referred to as the SITS muscles. As a group these muscles keep the head of the humerus in the glenoid fossa.

1. The supraspinatus attaches from the scapula to the greater tubercle and initiates and assists the deltoid muscle in abduction.
2. The infraspinatus muscle runs from the scapula to the greater tubercle and laterally rotates the arm.
3. The teres minor attaches from the scapula to the greater tubercle and serves the same function as the infraspinatus.
4. The subscapularis attaches from the scapula to the lesser tubercle of the humerus. This muscle medially rotates the arm and adducts it.