SHOULDER ANATOMY

The shoulder is the most flexible joint in the human body, allowing a wide range of movement. This mobility comes at the cost of stability, making the shoulder particularly vulnerable to injury. Its complex structure is made up of bones, cartilage, ligaments, muscles, tendons, nerves, and blood vessels working together to enable smooth and controlled motion.

What bones form the shoulder joint?

The shoulder is a ball-and-socket joint formed by three bones: the humerus, scapula, and clavicle.

The head of the humerus (upper arm bone) forms the “ball” of the joint. This fits into a shallow socket in the scapula called the glenoid cavity. Together, they form the glenohumeral joint, which is the main joint of the shoulder.

The scapula (shoulder blade) is a flat, triangular bone that provides attachment for many muscles responsible for shoulder movement and stability. It includes important bony structures such as the acromion, spine, coracoid process, and glenoid cavity.

The clavicle (collarbone) connects the scapula to the sternum (breastbone). It forms two important joints: the acromioclavicular joint (with the scapula) and the sternoclavicular joint (with the sternum). The clavicle also helps protect important nerves and blood vessels as they travel from the neck to the arm.

What is the function of cartilage and the joint capsule?

The ends of the bones are covered with smooth articular cartilage, which allows the joint surfaces to glide smoothly with minimal friction. This cartilage also acts as a shock absorber during movement.

The glenoid labrum is a ring of fibrocartilage that surrounds the glenoid cavity. It deepens the socket and improves joint stability by helping the humeral head stay securely in place.

The joint capsule is a strong but flexible fibrous sleeve that surrounds the joint, extending from the humerus to the edge of the glenoid. Inside the capsule, the synovial membrane produces synovial fluid, which lubricates the joint and reduces friction during movement.

What are bursae and what do they do?

Bursae are small fluid-filled sacs that reduce friction between moving structures in the shoulder, such as tendons, muscles, and bone. They act as protective cushions and allow smooth, pain-free movement.

Common bursae in the shoulder include:

Subacromial bursa – located beneath the acromion, reducing friction between the rotator cuff tendons and the overlying structures

Subscapular bursa – located near the subscapularis tendon, reducing friction during shoulder movement

What ligaments support the shoulder?

Ligaments are strong fibrous tissues that connect bone to bone and provide stability to the shoulder joint. Key ligaments include:

Glenohumeral ligaments – strengthen the joint capsule and help prevent dislocation of the humeral head

Coracoclavicular ligaments – connect the clavicle to the coracoid process of the scapula and help stabilise the shoulder girdle

Acromioclavicular ligament – stabilises the connection between the clavicle and the acromion

Coracoacromial ligament – forms an arch that helps prevent upward displacement of the humeral head

What muscles make up the shoulder?

The shoulder is powered and stabilised by several muscle groups, with the rotator cuff being the most important for joint stability.

The rotator cuff consists of four muscles that surround the shoulder joint and help keep the humeral head centred in the socket while allowing movement. These muscles work together to provide both stability and controlled motion.

The deltoid muscle forms the outer layer of the shoulder and is responsible for the main movements of the arm, including lifting and overhead activity.

What tendons are important in the shoulder?

Tendons connect muscles to bones and allow movement of the shoulder joint.

Important tendon groups include:

Rotator cuff tendons – connect the rotator cuff muscles to the humerus and provide stability and movement

Biceps tendons – connect the biceps muscle to the shoulder and assist with arm lifting and stability

How do nerves supply the shoulder?

Nerves carry signals between the brain and the shoulder, controlling movement and sensation such as pain, touch, and temperature.

The main nerve network in the shoulder is the brachial plexus, which originates from the neck and travels through the shoulder into the arm. Major nerves include the axillary, radial, ulnar, median, and musculocutaneous nerves.

How is blood supplied to the shoulder?

Blood supply to the shoulder is delivered by arteries that originate from the subclavian artery.

As it travels downward, the subclavian artery becomes the axillary artery in the armpit and then the brachial artery in the upper arm. These vessels ensure oxygen and nutrients are delivered to the shoulder and arm.

Venous drainage returns blood to the heart through veins such as:

Axillary vein – drains into the subclavian vein

Cephalic vein – runs along the outer arm and drains into the axillary vein

Basilic vein – runs along the inner arm and also drains into the axillary vein

The shoulder is a highly complex and finely balanced joint that relies on the coordinated interaction of bones, cartilage, ligaments, muscles, tendons, nerves, and blood vessels to function effectively.