Shoulder Anatomy
The shoulder is an incredibly complex structure. Though it is called a joint, the shoulder complex is actually made up of FOUR joints, SIX important ligaments, FIVE tendons, over a dozen muscles, FOUR nerves and a few other important structures that can contribute to shoulder pain.
Bones
Bones are the foundation of our body. They provide the framework for ligaments, tendons and muscles to connect to. When working together, these structures help to move a body part- in this case, helping our arm move and rotate.
Humerus (upper arm bone):
This is the long bone of the upper arm. It is made up of the head, neck, and shaft. The near end connects with different bones to make the shoulder joint, whereas the far end connects with the bones of the forearm to make the elbow joint.
- The head is ball shaped, which makes it easy to move and rotate. There are also different areas (the greater and lesser tuberosities) where the rotator cuff attaches.
- The neck is the transition zone where the head meets the shaft. This is a vulnerable area where breaks in the shoulder can occur.
- The shaft is the long, straight part of the bone
Clavicle (collar bone):
This is an S-shaped curved bone that connects the arm to the thorax. It articulates with the acromion of the scapula and the sternum (breast bone). It helps to provide stability in elevation of the arm.
Scapula (shoulder blade):
This is a flat bone that glides on the back portion of the rib cage. It also has prominences called the acromion and coracoid process that provide places where ligaments can attach to provide stability to the shoulder joint. The glenoid of the shoulder is more commonly known as the “socket” and is where the head of the humerus connects to make the “ball and socket” joint of the shoulder.
Joints
There are three major bones that make up the shoulder complex: The HUMERUS (upper arm bone), the SCAPULA (shoulder blade), and the clavicle (collar bone).
These three bones come together to make four separate joints
The Glenohumeral Joint (GH Joint):
Known as a “ball and socket joint”, this is the connection between the upper end of the humerus (humeral head) and the glenoid of the scapula. This is the most recognizable joint of the shoulder and allows free movement in nearly all directions.
The Acromioclavicular Joint (AC Joint):
Made up of the connection of the acromion of the shoulder blade and the far end of the collar bone.
The Sternoclavicular Joint (SC Joint):
Made up of the breastbone and the near side of the collar bone.
The Scapulothoracic Joint:
Made up of the flat surface of the shoulder blade and rib cage.
Ligaments
Ligaments connect the bones in these joints to provide stability so the shoulder stays in place without dislocating:
- Glenohumeral ligaments
- Made up of the superior, middle and inferior GHL, these provide stability to the head of the humerus and form the joint capsule.
- Coraco-acromial ligament
- Coraco-clavicular ligament
- Transverse humeral ligament
Other Important Structures
Glenoid Labrum:
The labrum of the shoulder is a fibrocartilage ring that circles the glenoid of the shoulder blade
It creates a deeper socket in the shoulder to provide more stability to the relatively shallow bone
The labrum also serves as a shock absorber of sorts.
Biceps Brachii:
Though this is not part of the rotator cuff, it attaches deep in the shoulder
This muscle has two heads (tendons) that connect it to the shoulder.
The long head of the biceps connects inside the shoulder to the labrum. The constant tension and pulling can cause tears of the superior labrum.
Rotator Cuff
The rotator cuff is made up of a group of muscles and tendons that attach to the head of the arm bone. Together, these muscles provide the power to move the arm. They turn into tendons which attach the muscle to bone.
Infraspinatus:
This is the muscle/tendon that comes across the back of the shoulder and helps to turn the arm outwards (external rotation). Starts on the middle part of the back of the shoulder blade and attaches to the middle portion of the greater tuberosity on the head of the humerus.
Subscapularis:
This muscle/tendon of the rotator cuff attaches to the front of the shoulder and helps to turn the arm inwards (internal rotation).
Starts on the front of the shoulder blade and attaches to the lesser tuberosity on the head of the humerus.
Supraspinatus:
This muscle/tendon comes over the top of the shoulder and helps to move the arm in abduction (in a sideways arc). This is the most commonly torn tendon in the rotator cuff.
Starts at the top of the shoulder blade and attaches to the top portion of the greater tuberosity on the head of the humerus
Teres minor:
This muscle/tendon also comes across the back of the shoulder and helps to turn the arm outwards when in abduction.
Starts on the lower part of the back of the shoulder blade and attaches to the lowest portion of the greater tuberosity on the head of the humerus
